PALEOCEANOGRAPHY, VOL. 16, NO. 4, PAGES 390-404, AUGUST 2001
Numerical PaleoceanographicStudy of the Early Jurassic
Transcontinental Laurasian Seaway
Christian
J.Bjerrum
t, andFinnSurlyk
GeologicalInstitute,Universityof Copenhagen,
Copenhagen,
Denmark
John H. Callomon
ChemistryDepartment,UniversityCollegeLondon,London,England,UnitedKingdom
Rudy L. Slingerland
Departmentof Geosciences,
PennsylvaniaStateUniversity,UniversityPark,Pennsylvania
Abstract. The forcesgoverningmarinecirculationof a meridionaltranscontinental
seawayis exploredwith thePrinceton
OceanModel. The JurassicLaurasianSeaway,which connectedthe low-latitudeTethysOcean with the Arctic Sea is
modeledquantitatively.The globaloceanis foundto havea profoundinfluenceon seawaydynamics.A north-south
density
differenceand hencesea level differenceof the globaloceanwas probablythe main factor in forcingthe seawayflow.
When the Tethyswaterswere the denserwater,the net seawayflow was southward,and conversely,it was northwardfor
denserArctic waters.Marine bioprovincialboundariesandsedimentdataindicatethatthe seawayprobablywasdominated
by Borealfaunal groupsand reducedsalinitiesseveraltimes in the Jurassic.The model resultssuggestthat this can be
explainedby southwardflowing seawaycurrents,which may have been related to an oceanicthermohalinecirculation
whereno northernhigh-latitudedeepconvectionoccurred.
1. Introduction
pressuredifferencealong with an assumptionof a level of
no motionat a fixeddepthbelow the permanentthermocline
Transcontinentalseaways connectingtropical oceans
[e.g.,Stigebrandt,1984]. The resultingmeanunidirectional
with northernhigh-latitudeoceanshave existedrepeatedly
barotropicflowis differentfromtwo-waystraitflowbecause
throughgeologicaltime [Smithet al., 1994]. However,their
theoceaniclevelof no motionliesdeeperthanthestraitbotmarine dynamicsremain poorly understoodbecauseof a
tom [Whitehead,1998; Hay, 1995]. In somepresent-day
lack of modernanalogues.Only the Cretaceous
WesternInstraitsthe sealevel differenceresultsin a meanbarotropic
terior Seawayof North America has been modeledin any flow, but it remains to be demonstrated whether these results
detail, and the conclusionshavebeencontradictory[Erickcanbe extendedto ancienttranscontinental
seaways.
senand Slingerland,1990;Jewell, 1996;Slingerlandet al.,
The aim of the presentstudy is to increasethe under1996]. Slingerlandet al. [ 1996] deducedthat the circulation
standingof marinedynamicsof latitude-spanning
transconwas anticyclonic,whereasJewell [1996] concludedit was
tinental seaways. Focusis placed on the relativelyshalcyclonic.If straitsconnectingpresent-dayoceanbasinsmay
low transcontinental
LaurasianSeaway[e.g.,Ziegler, 1990]
be regardedassmall-scaleanaloguesof transcontinental
sea(Figure 1). The LaurasianSeawayis heredefinedto be the
ways(e.g.,the BeringStraitor the straitsaroundJapan),then transcontinental sum of interconnected Jurassic straits and
theirflow dynamicson the annualmeanshouldbe governed
seasbetweenthe northernmargin of the TethysOceanand
by densitydifferencesin the upperwatercolumnsandhence
theBorealArcticSeanorthof thepre-NorthAtlanticregion
by the hydrostaticsea level differencebetweenthe oceans
and Europe. The seawayis importantbecauseits deposits
on eitherside[e.g., Gill, 1982; Stigebrandt,1984; Ohshima,
constrainmuch of our currentknowledgeof the Jurassic1994;ShafferandBendtsen,1994]. The hydrostaticsealevel
Early Cretaceousclimate and becauseit recordsthe initial
differenceis derived from the depth-integratedhydrostatic
openingof the North Atlantic. Understandingof the natureandoriginof economicallyimportantLower andUpper
hydrocarbon
sourcerocksmay alsobe obtainedby
• NowatDanish
Center
forEarthSystem
Science,
University
ofCopen- Jurassic
hagen,Copenhagen,
Denmark.
investigating
the seawaypaleoceanography.
The Laurasian
Seaway differs from the CretaceousWesternInterior SeaCopyright
2001by theAmericanGeophysical
Union.
way of North Americain thatit probablywasshallowerand
contained
many islands. Further,the Arctic connectionto
Papernumber2000PA000512.
0883-8305/01/2000PA000512512.00
the paleo-Pacificwas much wider than the WesternInterior
390
BJERRUMET AL.: JURASSICTRANSCONTINENTAL SEAWAY
391
main factorcontrollingthe meanflow in this transcontinen-
•.•••Arcti
cSea
......
I'-'"'"
'"'••"'
•l•.....•'-•....v.......]:
_'• ........
•:............
: :•
................................
;:..
...................
•::::.:•..
......•
':•
•:'.•........
......
... ..
'
. . .• ::
tal seaway.
2. GeologicalSetting
. 30ON -
"..'
..... •:.::.:•.•:•.
"....
'.....
:•:'
'gTethys
Ocean
-?
..
N
•
,':.
During times of high relative sea level in the late Early
Jurassic(Toarcian),Europe and the North Atlantic regions
werecharacterized
by an extensivesystemof straitsandseas
extendingfrom an Arctic Sea northof (30øNto the Tethys
Oceansouthof 30øN paleolatitude[Hallam, 1988;Ziegler,
1990; Smith et al., 1994], in which faunal extinctions, de-
-.-.•.•
•.•..(,:".:.....::.:::::.•....:
---•:::•: •-::•
;::7
.
•;•{' . •:
. •.•
•.•;..
.•:•:•-:::"•....•..:•
:-.::•:-•.-..-•:•4•::.:.•..:-::.::•5•f•-•
: .• %
•:• -;'7•v7'.•':::
. . •.• • ..... .
.... .............
::.:...................... .................;.3•v-:..-.::.•-..:•.•::•.:•.
Figure 1. Toarcianpaleogeography.
The transcontinental
areaof
interest is framed. Paleoland areas are dotted. Modified
from Smith
et al. [ 1994].
Seawayconnection[Smithet al., 1994]. Previousstudiesof
theLaurasianSeawaycirculationwere qualitativeandbased
on the distributionof sedimentsandfauna.Ager [ 1975] proposedthat the subtropicalinfluenceof the Tethysextended
northwardover the epicontinentalshelf in southernEurope
and that seawaycurrentsoriginatingin the Arctic to a variable degreeinfluencednorthernEurope. The generallyBoreal natureof the faunaof northernEuropesuggests
that the
region at times was characterizedby reducedsalinitiesand
that borealseacurrentsprobablyflowed southwardinto the
region of the North Sea [Hallam, 1969; Ager, 1975]. The
southward
movement
of boreal faunas and the north-south
trendsin authigenicuraniumcontentsuggesta similar situation for the Late Jurassic[Miller, 1990]. The more precise
nature and ultimate
positionof highly kerogen-richshales,and isotopicexcursionsoccurred[Jenkyns,1988; Hesselboet al., 2000; Hallam and Wighall, 1997]. Hallam [1984, 1993] reviewed the
lithofaciesand faunal distributionsin the Jurassic"greenhouse"world andfound that northwestEuropewas probably
warm and humidduringthe Early Jurassic.The seasurface
temperatures
havebeen hypothesizedto haverangedfrom
26øC in the southto 5ø-10øCin the northin agreementwith
coralandpaleobotanical
data[e.g.,Hallam, 1969;Chandler,
1994; Rees et al., 2000].
The salinityin the southwasmostlikely closeto normal,
whereasfarther north the salinity might have been reduced
by up to 10 practicalsalinity units (psu) [Hallam, 1969;
Prauss et al., 1991; Scelenet al., 1998]. The presenceof
Toarcianblack organic-richshales,in part laminated,indicates that the central to southeasternpart of seaway must
havehadhighlyproductivesurfacewatersand/oranoxicbottom waters[Jenkyns,1988;R6hl et al., 2001; Schoutenet al.,
2000].
3. Model
and Formulation
of BC
cause of these currents remain to be un-
derstood.
Here we explore the circulationdynamicsof the Early
JurassicLaurasianSeawayduring timesof high relative sea
level usingthe PrincetonOcean Model (POM) [Blumberg
andMellor, 1987;Melior, 1998]. The successful
application
of POM to presentoceancirculationlendssomeconfidence
to the use of POM in paleoceanographic
studiesprovided
the boundaryconditions(BC) are correct,i.e., bathymetry,
surface,and open-boundaryforcing. Becausethe boundary
conditionsarepoorlyconstrained
for the geologicalpast,the
POM is hereusedin a processstudy.The originand nature
of the large-scalecurrentpatternsare determinedfrom a series of numericalmodel calculationsthat are constrainedby
the annualmeansurfaceforcingasdiagnosedfrom the atmosphericgeneralcirculationmodel (AGCM) calculationsof
Chandleret al. [ 1992]. The relativeimportanceof wind, inflow/outflow,andbuoyancyforcingfieldsin drivingthe circulationis investigated,
asis thesensitivityof circulationand
watercharacteristics
in the absenceof a hydrostaticsealevel
differenceandvariousconjectured
bathymetries.
Our results
demonstrate
that a north-southdensitydifferenceandresult-
inghydrostatic
sealeveldifferenceis likely to havebeenthe
The numericaldevelopmentof thePOM is givenin BlumbergandMelior [ 1983, 1987] andMelior [ 1998] (www.aos.-
princeton.edu/WWWPUBLIC/htdocs.pom). A detailed
treatmentof the governingequationsand the formulationof
the BC asusedin the presentstudyis givenby Oeyand Chen
[ 1992]. Formulationsof the numerical BC are describedbe-
low. Assumptionsaboutpoorly constrainedvaluesare presented in section 4.
3.1. Surface Boundary Conditions
At the free surface in the model the annual mean wind
stressvector componentsare modulatedby an oscillatory
component[Blumbergand Mellor, 1983]. This modulation
is a crudeapproximationto synopticstormsgiving a more
realisticmixed layer depth. The seasurfacetemperature
T
is restoredto a prescribedtemperatureTc [Cai and Godfrey,
1995]. Tc refersto prescribedexternalvaluesthat are essentially unknownand here takenfrom an AGCM [Chandler
et al., 1992]. A linear restoringtime of 60 daysfor the top
30 m of water is usedfor restoringT to T½. Using a restoring BC on T allowsT to be influencedby the advectionof
heat and deviate from To. A constantsalinity flux is calcu-
392
BJERRUMET AL.' JURASSICTRANSCONTINENTALSEAWAY
lated from the precipitationminusopen water evaporation
rate(P - Ev) of theAGCM.
The flux of momentumat the sea bottom is balancedby
matchingthe computedbottomvelocitywith the logarithmic boundarylayerformulation[Mellor, 1998]. A variable
roughness
z,• = 0.01+ 1.O/H (in meters),is usedasa crude
parameterization
of theeffectthatshallowwaterwaveshave
on modulatingthe roughness
lengthscale[Oey, 1998; Holloway and Barnes, 1998]. The resultingdrag coefficient
variesfrom 5.0 x 10-3 at H = 200 m to 22.5 x 10-3 in
25ø E
[ Arctic
sea
z
4.0 .:.•
............. :,•.•
.........
,..................
•:•.•
3.5 'Greenland
3.0
E[•
":i:.
'•
o
shallow water (H = 30 m).
River runofffrom continentsis parameterized
with velocities at discretegrid points[Oey, 1996]. The injectedwater is assumedto have a temperatureof Tc and a salinity
$ - 0.2 psu.Islandrunoffis parameterized
with thesurface
salinityBC onthesurrounding
"wet"gridpoints.Half of the
freshwater
(P - Et) summed
overtheislandis addedto the
P - Ev of thewetnode(seebelow).
.......
• :•.?' •NSB
1.5 :p:.;:,
•.....
;./
'
"
.....
....
?:-
•'•
8
:" •
4-
-•o
:./""
3.2. Open Ocean Boundaries
A radiationconditionon openboundaries
is usedfor the
barotropic
model-velocities
[Oeyand Chen,1992]. A Sommerfeld-type
radiationconditionis usedfor theinternalbaroclinicvelocities.The unknowntemperature
profileTcvand
salinityprofileScvareprescribed
duringinflow,whereas
a
linearizedadvectionequationis solvedduringoutflow[Oey
and Chen, 1992; Mellor, 1998].
ß
. •
ß ?
..• •.:....,•[•.,•.'•'•.
.•.•X....'
0.5 '
"
[
Tethys•ean
[
0.5
1.0
1.5
2.0
2.5
x103 km
4. Bathymetry and External Forces
4.1. Paleogeographyand Bathymetry
Figure2. Conjectured
modelbathymetry
for theLaurasian
Seawayduringmaximum
flooding
(weaklysmoothed
for numerical
stability).
Thevariations
in relative
bathymetry
areapproximated
The Early Jurassic,
Toarcian,paleogeography
usedasthe
startingpointfor themodelcalculations
is basedonthework
fromlithofacies
variations
shown
byZiegler[ 1990](seetext).Contourintervals(CI) are 40 m above200 m depthand200 m below. Coastlineuncertainties
may be as largeas 200 km later-
of Smithet al. [ 1994], with detailsfrom Ziegler [ 1988, 1990,
and referencestherein] and Bradshawet al. [1992] (Fig-
ure2). All latitudedesignations
throughout
thepaperarepaleolatitudes
from Smithet al. [1994]. In principle,detailed
mapping
of faciesandsedimentary
structures
canbe interpretedtogivegoodinformation
onwaterdepth(asbyDiem
[ 1984]andBuchem
andMcCave[ 1989]).However,
thespatiallyandtemporally
fragmented
rockrecordatplaces
in the
western
partof theseaway
necessitate
theinexact
first-order
ally,andbathymetric
errorsareof theorderof 20-100m in areaswith little data. Map coordinates
are paleolatitude
as used
throughout
thetext.ALH,Alemannic
High;AM, Armorican
Massif; BM, BohemianMassif;HP, HebridesPlatform;IBM, Iberian
Meseta;IM, IrishMassif;LBP,London-Brabant
Platform;P, Por-
cupine
Bank;SP,Shetland
Platform;
AHS,Alemannic
HighStrait;
AMS, ArmoricanMassifStrait;CB, ClevelandBasin;FR, Faeroe
Strait;KS, KochStrait;GS, GobanSpur;NSB, NorthSeaBasin;
PB, ParisBasin;RR, RockallStrait;SGB, SouthGermanBasins;
TBS,Thuringian-Brandenburg
Strait;VS, VikingStrait.
approach
adaptedhere.
The Early Jurassic
wasto a largeextentcharacterized
fromthelaminated
shale(seebebypostrift
thermal
subsidence
in theregion,
whichresulted waterdepthdetermined
to theapproximate
coastline
of thepaleoin subbasins
with isopachcontours
thatexhibita relatively low)is tapered
geography
used,suchthatbathymetry
contours
parallel
the
smooth"bull's-eyepattern"whenpost-Toarcian
erosionis
removedby interpolation
[Ziegler,1990]. Lithologymaps
smoothed
relativebathymery
contours
basedon lithofacies.
from Bradshawet al. [1992] and Ziegler [1988, 1990] are
Where limited information is available, such as between
usedtodelineate
deepershelfwater(> 30 m,shale)andshallow water(<20-30 m, sandstone-mudstone),
whichin turn,
is usedto definea conjectured
relativebathymetry.
Theabsolute
bathymetry
isconstructed
by combining
rel-
NorwayandGreenland,
waterdepths
weresubjectively
assigned,
suchthatnarrowregions
between
islands
wereassigned
asilldepthof 80m andintervening
marineareas
were
ativewater
depth
information
withtheoccurrences
ofthe
assigned
a depthof 180 m.
Offshore
laminated
mudstones
withnoindication
ofwave
common
Toarcian
laminated
offshore
shales
asfollows'The ß -'agitation
couldhavebeendeposited
at anywaterdepthbe-
BJERRUM ET AL.: JURASSIC TRANSCONTINENTAL
SEAWAY
393
low storm wave base. The maximum wave orbital velocitionalgridwitha 27 kmresolution
(101 x 165gridpoints).
tiesasa functionof depthcanbe foundfromsemiempiri- Theinitiallyconjectured
bathymetry
isweaklysmoothed
becalrelations
between
freefetchandstormwindspeed[e.g., forethemodelcalculations
[Mellor,1998]. Vertically,13
Bretschneider,
1966]. Assumingthat velocitiesmusthave gridpointsareused,closelyspacednearthe surfaceandat
been<0.1 m s- • forthefinelamination
nottobedisturbed, intermediate resolution near the bottom. Model calculations
theminimumwaterdepths
canbeestimated.
Forexample, aredonewitha timestepof 80 sforthebarotropic
modeand
givena wind fetchof 500 or 1000 km and a stormof unlim-
itedduration
witha windspeedof 20 m s-•, waterdepths
musthavebeen>90 or 220 m, respectively,
duringdepositionof laminated
offshoreshales.A maximumwaterdepth
of 200 m has been used in the model at locations of lami-
natedshalesanda largefreefetch(Figure2). Thisestimate
is thesameasotherestimates
of depthfor comparable
offshorefaciesof LateJurassic
agein southern
Europe[Picard
et al., 1998]. The Toarcianfinelylaminatedshalesalternate
with wavyto crinklylaminae,wherethelattermaybe indicativeof stabilizationby microbialmats[O'Brian, 1990;
Schieber, 1999]. The stabilization would have resultedin a
75-200%increase
in thecriticalshearvelocitynecessary
to
mobilizesediments[Patersonand Black, 1999]. However,
thefinelylaminated
shaleswereprobably
deposited
without
the involvementof microbialmats[O'Brian, 1990;Schieber,
1999]. Model sensitivitytestsfurthershowthat the results
arefairlyrobustto a 50% reductionin bathymetry.
Theconjectured
EarlyJurassic
modelbathymetry
ranges
from 30-200 m in the seawaydownto 750 m in the basins
northandsouthof theseaway(Figure2). Modeldepthsof
30-50m areusedalongmostcoastlines
in theabsence
of betterdata.Tidalamplificationandassociated
residualcurrents,
suchasoccurat depthsshallowerthan50 m in theNorthSea
andEnglishChannel,canthereforenotbe modeledhereand
will be treatedin a futurestudy.
The nameViking Strait (VS) is usedhereto refer to the
VikingGrabenandnorthernmost
NorthSeaareas(Figure2).
This usageis morerestricted
compared
to previoususage
[Callomon,1979]. The KochStrait(KS) is hereusedfor the
narrowregionat 46øN paleolatitudebetweenthe WestHal-
2000 s for the baroclinic mode.
4.2. External
Prescribed
Forces
The externalforcingimposedon the modelthroughthe
surfaceboundaryconditionsare wind stress,surfaceheat
flux, net freshwaterflux (precipitationminusevaporation),
andriverrunoff. All thesequantitiesareessentiallyunknown
for the Jurassicbut are heretakenfrom atmospheric
general
circulationmodel calculations.Additionally,externalvertical salinityand temperatureprofilesat the openboundaries
(OBC) imposeimportantbuoyancyforcesnorthandsouthof
the seaway.
4.2.1. Surface forcing. The
surface boundary
conditionsare computedfrom the AGCM of Chandleret al.
[1992]. M. A. Chandlerprovidedthe annualaveragedata
files from the Early JurassicAGCM calculations. Other
JurassicACGM's havebeenpublishedin detail [Valdesand
Sellwood, 1992; Moore et al., 1992]. The results of Valdes
and Sellwood [ 1992] were for the Late Jurassictime interval
and did not includeopenwater in the regionof the seaway.
The wind field resultsofMoore et al. [1992] are here usedin
a sensitivitytest. The remainingforcingfieldsfrom Moore
et al. [1992] are not availablein digital format and could
therefore not be used.
The AGCM annual averagewind stressfield has weak
westerly
windsnorthof 50øN(stress
< 0.03N m-2, Figure 3). Subtropicalnortheasterlytrade winds characterize
the southern domain of the model and have a maximum wind
stress
of 0.05 N m-2, whichis 0.01-0.04N m-2 lessthanto-
Basinand MOre-VOtingBasinTrend [Dreyer, 1992]. The
Arctic and TethysBasinsare highly simplifiedand are includedonlytomovetheopenboundaries
of themodelaway
day. Northwesterlywinds are presentover the westernpart
of the seawayat 35øN.
The surfacetemperatureTc usedin calculatingthe model
seais thesameasthelowerAGCM "forcing"boundarycondition usedby Chandleret al. [1992]. Tc decreasesalmost
linearlyfrom 28øC at 20øN to 12øC at 60øN.
The freshwaterannual averageof precipitationminus
from the area of interest.
open-water
evaporation
(P-Ev) of theAGCM iszonallyav-
It shouldbe emphasized
thatlocalpaleobathymetry
detailsandinterpretations
by otherauthorsmaydiffersubstantially from themapusedhere.In certainareasuncertainties
in thecoastalreconstruction
are up to 200 km andpotential errorsin bathymetryare of the orderof 50-100 m. The
mapis takenasa necessary
first-orderstartingpointin order
to identifythephysicalprocesses
operating
in theseawayat
scaleslargerthanindividualsubbasins.
The importance
of
eragedovertheseaway(Figure4). Southof 40øN, openwater experiencesnet evaporation,whereasexcessnet precipi-
tenbankenHighs (offshoremid-Norway)and East Greenland. Thus, the Koch Strait refers to the marine connection
over the area that has been called the East Greenland Shelf
errorsin theassumed
bathymetryis testedin modelcalculationsby varyingthisparameter(section7).
A transverseMercator projectionof the reconstructed
EarlyJurassic
seawayis mappedontoa Cartesian
computa-
tationdominates
farthernorth.Themagnitudes
of P - Ev
are comparablewith openoceanicconditionstoday,but the
minimumis displaced5ø-10ø northward,whereasthe maximum is displaced10ø southward[Silva et al., 1994].
4.2.2.
Rivers.
Mainland
river runoff is assumed to come
from 30 geographically
evenlydistributedriversalongthe
seawaymargins. The river volume transportis calculated
from the terrestrial? - Et overthe designated
drainageareas, assuminga 50% drainageefficiency[Holland, 1978;
Jewell, 1996]. The drainagesurfacesare assumedto be five
394
BJERRUM ET AL.' JURASSIC TRANSCONTINENTAL
ß
3.5
densities,respectively[Stigebrandt,1984; Gill, 1982]. D is
thelevelof no motion,hereassumed
to be at 750 m depthin
the Tethysand the Arctic Sea.
The prescribedopenboundarydepthprofilesof tempera-
:"
':i:'
tureTcvandsalinitySoyfor theArcticandTethysarechosen
:•'
• • •
• [ •
to be exponentialin form. Only the relativeT and S distributionsare of interestin orderto investigatethe importance
.:•
-';
of various
external
forces.A surface
salinityScvl(z=0)=
30psuisusedfortheArcticprofile,andScvl(z=o)
= 37psu
:.
.-.
3.0
.........
-•:.•..:;'"'
• ..
:Z
for the Tethys. The salinityand temperatureusedat 750 m
depthare takento be intermediatebetweenthe present-day
valuesand thoseof global oceanmodelswith Jurassic-like
.--.
2.5
"
...---
2.0
SEAWAY
....
//:-'. /•
1/
'
-.
. :.//•%
.;.....• .,,•-::-;•
"'
"7'-...,•..,.,
•:..
ß I "1 ..........
...........
/////•'
1.5
paleogeographies
suchthatTcvl(•=7so)
= 7ø and8øCand
.:
•½-
"""*
..................
•'"'";'•'.
$cpl(•-7s0)- 34.5and35.5psufortheArcticandTethys,
respectively[Kutzbach et al., 1990; Schmidt and Mysak,
1996]. The model resultspresentedhere can be regarded
as one end-memberof the rangeof possiblevaluesof sea-
wayflowbecause
theseScvandTcvprofilesproducea large
but physicallypossibleA•7.
....::...:
1.0
4.3.
Numerical
Two
::,.,•
Model
series of model
Calculations
calculations
are carried
out in or-
der to identify the contributionsof the different forces to
the circulation. The first seriesincludesthe basic spin-up
calculation and control calculation (SC0) with all surface
0.5
1 .O
1.5
2.0
and open boundaryforces active. In sevenother sensitivity tests(SC1-SC5c), successively,
a differentexternalforce
is removed(Table 1). The calculatedvelocitiesof a given
2.5
xl 03 km
Figure 3. Surfacewind stressvectorsobtainedfrom the AGCM
of Chandler et al. [1992] after spline interpolation. Maximum
stressis overtheNorthSearegion.Northeasterly
tradewindsdominatemostof the southernandeasternseaway.Weakwesterliesare
presentnorthof 45øN.
calculation
are then subtracted from the control calculation.
The velocity differencein each case givesthe contribution
from the force removed,when nonlinearterms are negligible [Wang et al., 1994]. The flow is nonlinearin the southwesternpart of the seawayowing to the wind stresscurl,
grid pointswide (north-south)and to extend500 km inland
from the seawaymargin. The seawaycirculationis found
notto be verysensitive
to uncertainties
in drainageareasand
efficiency.P - Et is foundby the empiricalThornthwaite
relation,whereevapotranspiration
Et is calculatedfrom the
AGCM surfaceair temperature
andhoursof daylightandP
is theAGCM zonallyaveraged
precipitation
overtheseaway
(Figure 4) [Chandler, 1994].
4.2.3. Open boundary conditions and external eleva-
Palcolatitude
1.0-
0.5
25ø N
35ø N
I
45ø N
55ø N
I ,•••+• I
• 0
tion. The ArcticandTethysBasinsaretakento haveopen
-0.5
boundariesto the westand east,respectively.The external
elevationdifferencebetweenArctic and Tethysseasurface
imposedon the boundariesis foundat eachtime stepfrom
-1.0
o
500
1000
1500
2000
2500
3000
3500
4000
the hydrostaticsea level differenceA•7 usingthe densitykrn
depthprofilesaveragedover the openboundaryregionand
minusevaporation
oversea(P - Ep) and
extending10 grid pointsinto the modeldomain.The hydro- Figure4. Precipitation
overland (P - Et) [Chandleret al., 1992]. A maximumof +1000
staticsealeveldifferenceis approximated
by
__
•
-- P-E•
mm/yrin the (P - Ep) balanceoccursoverthe seawayconstric-
tionat 45øN,whereas
a minimumof-800 mm/yris foundover
•
as I•=0
[ps(z)- pN(z)]dz,
(1)
the North Searegion.For (P - Et), P is from the AGCM results,
whereasthe evapotranspiration
Et is calculatedfrom land surface
temperature
anddaylighthours[Chandler,1994].Almostnoneg-
wherez is depthandPNandPs are the northernandsouthern
ative balanceis found over land, in contrastto that over the sea.
BJERRUM ET AL.' JURASSIC TRANSCONTINENTAL SEAWAY
395
In the second series of model calculations, different
Table 1. Model Runs, First Series.a
Contribution to Circulation From Various Forces.
SC0
SC 1
control calculation
no wind
wind
SC2
SC4
no rivers
no surf. flux
river buoyancy
surfacebuoyancy
boundaryconditionsareused(Table2). Resultsfrom SBC2,
SD7, andSW 1 are presentedin detail, whereasthe othercalculationsare mentionedwhereappropriate.In SBC2, Aq is
setto zero artificiallyon the boundaries,andin SD7, the water depthis setto be 150 m for all watergrid points.In SW 1
the windfieldof Moore et al. [1992] is used. Time integration is carried out over 12 model years from a stateof rest
SC5
barotropic:
T=15øC,
$=33 psu,fixed
totalbuoyancy
SC5a
fixed At/,
otherwiseasSC5
Averagesof the last40 daysare usedfor presentation.
local buoyancy
SC5b
SC5c
Aq = 0
no northOBC
Aq
Aq, inflow/outflow
Run
Parameter
Tested
ForceConsidered
b
with a T and $ distribution as obtained at the end of the SC0.
5. Results of Control Calculation (SC0)
The difference of surface elevation between the Arctic and
Tethysopenboundariesis Aq = 0.15m, only slightlydifferent from that derived from OBC forcing. The average
bForce contributions identified from differences between SC0
surfaceelevationover the seawayproperslopesdownward
and SC1 to SC5.
towardthe east. The highestelevations(0.2 m) are in the
part the seaway.The
friction, and densityeffects, so the methoddoes not give GobanSpur area of the southwestern
lowestseawayelevations(-0.25 m) arefoundjust eastof the
valid results in this area. However, elsewhere in the main
partof the seaway,themethodgivessomephysicalinsights North Sea Basin. The surfaceelevationslopewithin particat limitedcomputational
expense.The spin-upcalculationis ular straitsis not perpendicularto the straitaxis but slopes
startedfrom a stateof rest with horizontallyhomogeneous obliquelyacrossthe strait.
conditions.After 10 model years,a dynamicsteadystateis
reached. Both the controlcalculation(SC0) and sensitivity 5.1. Transport Stream Function
The seawayvolumetransport,expressedas the transport
calculations(SC1-SC5c) are subsequently
carriedout under
from the depth-averaged
vedifferentforcingsfor periodsof 180 days,startingfrom the streamfunction,is constructed
conditionsreachedat the endof the spin-up.Averagesof the locity field. There is an overall southwardflow (transport)
last 40 days of the control and sensitivitycalculationsare of 2.7 Sv (1 Sv = l x 106m3 s-I) (Figure5). The main
usedfor comparison.
flow is throughthe North Searegionbecauseof topographic
aFirstseries:
180daysintegrations
followingthe 10 yearspin-up.
Table2. ModelRuns,SecondSeries:Changeof MeanSeawayFlow Q, Temperature
T andSalinity$ to DifferentBCa
Run
Parameter
Changed
PrincipalChangesversusSC0
SBC1
Aq -- 0
Q northwardandweakin KS, T increase,$ decrease/increase
north/south
of KS
SBC2
SW 1
no openboundary
alternativewindb
As SBC 1
SW2
presentwindc
SD 1
1.5/control
SD2
0.5/control
SD3
no VH and ALH
SD4
0.3H
in VS
SD5
0.3H
in FR-RR
SD6
+40 m over seaway
SD7
uniform
(section 7.1)
H=150
Q decreaseby 13% in KS andmostpassesthroughVS, frontsmoveto
Tethysmargin,stratification
decrease
slightlyin NSB/CB, PB, SGB
Q decreaseby 19% in KS andall passthroughVS, southwestgyreis stronger,
stratificationdecreasein NSB/CB, PB, SGB, 1-4øC increasesouthof 45øN
exceptin northeast
NSB (2øCdecrease),
0.5-1.5psudecrease
northwest
of SP-LBP
deeperseaway'Q increaseasin (2), verticalmixingoversillsdecrease,
stratificationincreaseslightly
shallowerseaway'Q decreaseasin (2), verticalmixingoversillsdecrease,
stratificationdecreaseslightly
moreopentoTethys:0.5øC, 0.3 psuincrease
in SGB,PB mixedlayer
VS flowrestricted:
Q mostlythroughFR, RR, 1ø-3øCincrease
eastof SP-IBM,
< 1 psuincrease
southof 40øN,stratification
decrease
in SGB,PB, CB
restrictedFR, RR flow: Q mostlythroughVS, 0.5øCdecrease
eastof SP-IM,
m
0.2 psudecreasenortheastof IM-AM
no islands:Q increasealongcentalpartof seaway,verticalmixingdecrease
slightly
Q increaseasin (2), currentstrappedalongwesternsideof seaway(section7.2)
aTwelveyearintegration
fromT and$ fieldof thecontrolrun(SC0). Abbreviations
asin Figure2 andNotation.
bWind field from Moore et al. [ 1992].
cpresent_day
zonallyaveragedannualmeanwindstress.
396
BJERRUM ET AL.' JURASSIC TRANSCONTINENTAL
SEAWAY
averageelevationdifferencebetweenthe Arctic and Tethys
Seasis 0.3 m, which givesa southwardflow of-2.9 Sv, only
slightlylargerthanthe modelvalueof-2.7 Sv.
3.5
5.2. Temperature and Salinity
3.0
The seasurfacemixed layer temperaturesare in general,
3ø-4øC lower than those expectedfrom climatic Tc alone
becauseof strongadvectionof cold watersfrom the north
2.5
and the mixing of deeperand surfacewatersover sills (Figure 6a). The surfacetemperaturesin the southernmostpart
of the seawayare morecloselycomparablewith thoseof cli2.0
maticTo. The zonally averagedtemperaturefield showsthat
deep cold water from the north penetratesinto the seaway
andthat surfacewatersbecomegraduallywarmertowardthe
1.5
south(Figures6a and 7a). A temperaturefront is present
southof the North Sea (•33øN) and a weak one southof the
German
and Paris Basins (•25øN).
1.0
The average seaway salinity is 33.4 psu south of the
Viking Strait, ranging from 33 to 34 psu, which is considerablylower than the averageTethys surfacesalinity of
0.5
1.0
1.5
2.0
2.5
35.7 psu(Figure 6b). Deep water of relativelyhigh salinity
x103 km
is advectedinto the seawayand mixed with surfacewater
Figure 5. Control run transportstreamfunction. CI = 0.5 Sv, of lower salinityover the sill in the Koch Strait (Figure 7b).
where1Sv= 106m3 s- 1 Thetransport
iscounterclockwise
along Low salinity water can be seenpenetratinginto the South
negativecontours(lows). Currentsare strongerat closelyspaced
contours.Note the overallsouthwardtransportin the Viking Strait GermanandParisBasins,with a salinityfront to the Tethys
in the south (Figure 7b). The seaway water mass has T
(contoursnegative)andthatthe contourslie slightlyobliqueto individual strait axes.
and S characteristics
with Arctic affinitybecauseof strong
southwardadvectiondrivenby the hydrostaticsealevel dif-
blockingby sillsaroundthe Hebridesplatform[Wanget al.,
1994]. The stream function contours also illustrate the an-
tisymmetricflow though the straits, which is a result of
the coastallytrappedcurrentsdue to the rotationof Earth
ference.
6. Contribution
of Different
Forces to the Flow
Eight sensitivitytestsare carriedout to identify the con-
[Ohshima, 1994].
tributions from different
The prominentanticyclonicgyre andboundarycurrentin
the southwestcornerof the seawayis generatedby a combinationof the wind stresscurl in the regionand buoyancy
controlrun (Table 1). Removingthe wind resultsonly in a
5% changein the mean seawayflow (SC1). In SC5, temperatureand salinity are set to 15øC and 33 psu, respectively, over the entire model domain in order to estimate
forces with associated baroclinic
instabilities.
The contours
of the streamfunction (currents)are approximatelyparallel to thoseof depthand elevation,which illustratesthe importanceof the variationsin relativebathymetryin controlling the local currents. Details of the model flow in subbasinsmay not be robustgiven the uncertaintiesin paleobathymetry.
The straitvolumetransports
Q canbe approximated
by
Q- gAV•
A,
fw
(2)
whereAqs is thestraitelevationdifference,g is gravitational
acceleration,f is the coriolisparameter,W is the width,
and A is the cross-sectional
areaof the strait [Toulanyand
Garrett, 1984; Ohshima, 1994]. POM flows for different
straitshave been comparedwith the flow derivedfrom (2)
(Table 3). Equation(2) is a goodapproximationthat can be
usedin simplifiedstudiesof transcontinental
seaways.The
forces to the circulation
seen in the
Table 3. POM (Qm) and Analytic (Qs) Strait Slows
Strait
Q•,
Qs,a
At/s,
A,
W,
Sv
Sv
m
x 106 mo'
x10 3 m
Seaway
b -2.7
KS c
VS c
HPS •
TBS c
AHS c
AMS c
-2.4
-1.7
-0.2
-0.7
-0.1
-0.3
-1.9
0.19
42.6
428
-2.1
-0.5
-0.1
-0.7
-0.2
-0.8
0.38
0.07
0.03
0.08
0.01
0.04
11.3
22.2
4.82
21.9
20.2
20.2
187
321
134
321
187
161
aQs from (2) with Ar/, acrossthe strait,cross-section
area A
and strait width W.
bFlowat 43øN usingthe averageAr/betweenthe Arcticand
Tethys.
•Strait's definedin Figure 2.
BJERRUMET AL.: JURASSICTRANSCONTINENTALSEAWAY
397
½-
: S at 10 rn depth
•
.- ' ,, •..
-4-:
.
......
:::
..
..
.
..•
E 2.5
.....
'/,..
x
•'..
..:,-•":"'
.....
'•"'.
:,.
•.-•'
2.0
"':4
.•:'
i; ,'•"•'
.•"::'
'•'
......
......
.'
'": -.,
?' "' ::'• •"""
' '"'-
":.
::'""'"• Z
1.5
' •'•• " '.........
•.0
....
0.5
:..
.... •
1.0
1.5
2.0
0.5
2.5
1.0
1.5
2.0
2.5
x103km
x103 km
Figure 6. Controlrun(a) temperature
and(b) salinityat 10 m waterdepth.CI -- 1.0øC(Figure6a) andCI -- 0.5 psu
(Figure6b). Relativelylow valuesof T characterize
theKochStrait,whereasT rangesfrom 18ø to 25øCsouthof
37øN. Reducedsurfacesalinitiesoccurin the KS andeasternpart of the seaway.
the buoyancyeffect on water circulation.The salt, heat and
river fluxes are also set to zero. The run is equivalentto
a barotropicrun with no horizontaldensitydifferences,in
which A•7 is also zero. The differencebetweenthe control
run and SC5 gives the barocliniccontributionto the flow
(i.e., total buoyancyeffects). The buoyancyeffects con-
25ø N
35ø N
45ø N
55ø N
tribute to a southwardtransportof 2.5 Sv, and almost the
entire flow structureof the controlrun is due to densitydifferences.
Run SC5a is carried out as above with uniform
T
and$ butnowwith A•7 artificiallyfixedto the valuefoundin
the controlrun, so that the local buoyancycontributioncan
be identified. The local buoyancyforce drivesmost of the
southwestgyre and a slight northwardtransportof 0.25 Sv
throughthe northernpart of the seaway. In run SC5b only
the A•7 parameterization
on the openboundaryconditionis
removed. A•7 is found to contributeto most of the flow in
the KS betweenGreenlandand Norway and in the VS in the
northern North Sea area.
7. Influence of A•7,Bathymetry and Wind
The circulationis determinedlargely by the north-south
densitygradient,which is a function of the open boundary
-50'
formulationfor the northernand southernoceans. Topo•'
vs
• -lOO,
graphicblocking influencescoastalcurrentseparationand
forces
flow throughthe Viking Strait. The bathymetryand
c1-150,
north-southsalinity differenceand therebyA•7 are poorly
-200,
constrainedby geologicalevidence. A seriesof 11 model
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
x10a km
runs are thereforecarriedout to investigatehow much the
circulation,salinity,andtemperaturedistributionswill be alFigure 7. Zonally averaged(a) temperature•d (b) salinityof the
tered
by modifyingthe open boundaries,wind forcing, or
controlrun (CI = 2.0øC (Figure7a), 0.25 psu(Figure7b)).T •d
S arezonallyaveragedeastwardalonglatitudesfrom the longitude bathymetry(Table 2).
0'
b
of the +1000 km grid point (i.e., excludingthe southwestern
p•t
of theseaway).Bathymetryof watergridpointsaveragedasabove
•d contouredby t•ck line. Note the low bottomwater temperatureswit•n the seawaydueto southw•d advection.L•bels •e as
in Figure 2.
7.1. Northern Open Boundary (SBC2)
An anti-cycloniccirculationdevelopswhenthereis no hydrostaticsealevel differencebecausethe northernboundary
398
BJERRUMET AL.' JURASSICTRANSCONTINENTAL SEAWAY
0i"ll25ø""'•""
N...................35øN
a
45øN
55øN !
.....
-so
':.
i::
.:: "'
.......;
-200
seawayset up a cyclonicgyre in the absenceof Ar/-driven
currents in the east.
Surface temperatureincreaseseverywherewhen a uniform depthis usedbecauseturbulentmixingdecreases
where
sillswere previouslypresent.A temperaturedecreasein the
westernsideof the seawaycanbe attributedto turbulentmixing in the boundarycurrentand southwardadvectionof cold
water along its westernside. Bottom water temperatures
north of 40ø.Ndecreaseby •IøC , while temperatures
in
generalincreaseby 2ø-6øC in the North Searegionbecause
cold water now is advected west of the Shetland Platform
(Figure 10a). Temperaturesdecreasein the bottomwatersof
the South German
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
x103krn
Figure 8. Zonally averaged(a) temperature
and(b) salinitydifferencesbetweenrun SBC2 (closednorthernboundary)and the control run (e.g., AT = Ts}•c2- Tssc0). AT and AS are zonally
averaged
asin Figure7, CI --- 2øC and0.5 psu(Figures8a and8b,
respectively).$ and T are shadedwhen higherfor SBC2. Temperatureincreasesnotablybelow -30 m. Salinity increasessouthof
43øN and decreases north of 43øN.
is closed(not shown)(SBC2, Table 2). Only a weak flow
is found in the North Sea region and in the Koch Strait,
whereasthe anticyclonicgyre in the southwestis twice as
strongasin the controlrun.
Surfacetemperature
and salinityincreaseby 1ø-2øCand
0.5-1.5 psu,respectively,
overthe southernpart of the seaway,comparedwith the controlrun. T increases
by up to
4øC aroundthe Shetlandplatformand northwardbecause
thereare no strongstraitcurrentsand associated
turbulent
mixingof deeperandshallowwater.Thereis a salinityincreaseof 1.5 psuin the areanorthof the IrishMassifbecauserelativelyhighsalinitywateris advected
northward
by
the anticyclonic
flow. Salinitiesin the KS regiondecrease
by 1-2 psu.Removalof thehydrostatic
sealeveldifference
and Paris Basins because a reduced sill
depthto the Tethys allows deepercold water to penetrate
farther north over the shelf (as in SD3).
Water of low salinity is advectedsouthwardprimarily
alongthe westernsideof the seaway.A salinityincreaseof
0.5-1.5 psu is seenat all levelsin the North Sea, SouthGerman, and Paris Basins(Figure 10b). Greater water depths
over former sill areasreduceturbulentmixing, and bottom
watersof relativelyhigher salinity are not mixed with lowsalinitysurfacewatersin the Koch Strait.
Decreasingthe waterdepthonly eastof the ShetlandPlatform in run SD4 (Table 2) has a similar effect as described
above.Topographicblockingin the Viking Strait increases.
Conversely,a decreasein water depthonly westof the Shetland Platform blocks the western flow branch, so that virtually all the southwardflow passesthroughthe Viking Strait
(run SD5, Table 2).
ß
.
..
.-•
3.0
."•:
{:
......
;•:::::.
•Z
has a dramatic effect at levels below the wind mixed sur-
facelayer,as seenin thezonallyaveraged
temperature
and
salinityfields(Figure8). Temperatures
below30 m water
depthincrease
by 6ø-12øCsouthof 43øN.Northof 45øNthe
middepth
temperature
increases
by 4øC.Northof 43øNthe
salinityis reducedby 1-2psu,whereasfarthersouth,thereis
a generalsalinityincrease
by 1.5-2psubelow30 m depth.
•E
• 2.5
•.:•...-..•'
•:•...:.
2.0
:
-..
7.2. Changesin Bathymetry (SD7)
The seawaycirculationchangessignificantlyrelativeto
the controlrun when all watergrid pointshavea bathymetry
of 150 m andno sills are used(Figure9). Overall,the mean
southwardflow Q increasesin approximateagreementwith
\z..,/.
.,,._,
,.,,._,•o•$
1".......•.t upstream
t.r..•ro• x•xT
.....
• trapped....... t• are....
of n
vioussills,in agreementwith theresultsof Ohshima[ 1994].
The flow is concentrated
alongthe westernsideof the sea-
way becausethere is no topographicblockingaroundthe
Hebridesplatform. Trade windsin the easternsideof the
1.0
.
ß"!.'-"...
..
.½, ..
0.5
'
....• .;:.
••
•1•
.; ....
...
1.0
1.5
2.0
2.5
xi 0akm
Figure 9. Transportstreamfunction
for a seawayof unifom depth
(150 m; run SD?; CI = 1 Sv). A uniformwaterdepthremovesall
topograp•c steeringandblocking,sothatthe geostropMccu•ents
follow the westernsideof the seaway.
BJERRUM
ET AL.:JURASSIC
TRANSCONTINENTAL
SEAWAY
25• N
a
':
_
,--.
-100
E
35• N
45ø N
8.1. Summary of Model Results
55• N
......................
:.-.•i:.•'"ø-------•
....
:::
......
:.:.•!•:i
.....
•'"-0
.........
'::'
•'
'
-150
-200•
,. .......
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
x•O•
Figure 10. Zonally averaged(a) temperatureand (b) salinitydifferences between run SD7 and the control run.
AT
and AS
399
The numericalcalculations
corroborate
thehypothesis
that
the JurassicLaurasianSeaway functioneddynamicallyas
one singlestrait and that the hydrostaticsealevel difference
At/betweentheTethysOceanandtheArcticSeacoulddrive
a stronggeostrophically
controlledcurrentthroughit. Topographicblockingand steeringare importantfor local details
in circulation.At/is themostimportantcontrolon theoverall seawaycirculation,temperatureand salinitydistribution.
At/is, in turn,determined
by themeannorth-south
temperatureandsalinitydifferences
(ATto andASm). Topographic
blockingandsteeringare importantfor localdetailsin circulation. The calculatedlocalpaleocirculation,
paleotemperature andpaleosalinitydetailsare not robustresultsgiventhe
uncertaintyof the bathymetryand surfaceboundaryconditions.
are
The south-northhydrostaticsea level difference can be
obtainedapproximatelyfrom the temperatureand salinity
andT are shadedwhen higherin SD7. In Figure 10a, T increases differences,a linear equationof state,and (1) (Figure 11).
nearthesurfacenorthof 34øNbecause
of reducedturbulentmixing
Southwardseawayflow resultsfrom At/> 0 andconversely
over sills. T also increases in the North Sea basin because no cold
northwardflow resultsfor At/ < 0. Only a modestsalinwater is advectedsouthwardin the easternportion of the seaway.
In Figure10b,$ increases
at all levelssouthof 43øN anddecreases ity differenceis neededfor southwardflow to be possible
during "greenhouse"climatic periodswith relatively weak
around 45øN.
north-souththermalgradients.The salinitydifference,howzonallyaveragedasin Figure7 (CI = 2.0øC and0.5 psu,respectively). Bathymetryoverlapbetweenrunsis maskedout (white). S
10
7.3. Different Wind Forcing (SWl)
The annual mean wind field from Moore et al. [1992] is
usedin SW1 asa testof thepossibleinfluenceof a different
meanwindpattern.The midlatitudewesterlies
werestronger
and locatedfarthersouthcomparedto the controlrun wind
field becauseof the steeperT gradientin the model. The
mean southwardflow decreasesby 0.36 Sv, and most of
the southwardflow passesthroughthe Viking Strait. The
main T/S front movessouthwardto the northernmargin of
the Tethys(,-.,28øN).Surfacetemperatures
decrease
by 1ø2øC overmostof the seawaysouthof 40øN and alongthe
westernsideof the seaway.The generaldecreasein surface
temperaturesis due to the trade wind stressdecreaseover
the southernpart of the seaway,which in turn, leadsto a
decreasein the northwestadvectionof warm Tethys water.
Upwellingincreases
alongthe westernsideof the seawayat
i•4•0
IIE2
.--
•
0
-2
0
5
10
a Trn = Ts -T
o
15
20
[ C]
midlatitudes.
8. Discussion
Section7 presentresultsfrom a seriesof modelcalculationsexploringthesensitivityto changes
in thephysicalvariables that characterize the model. In section 8.1 the results
are first summarizedanddiscussedandthen synthesizedinto
four robustscenarioswith broadJurassicapplication.This
synthesisis then comparedwith Early Jurassic(Toarcian)
paleontological
data.
Figure 11. Approximatehydrostaticsealevel differenceAr/betweentheTethysOceanandArctic Seaasa functionof south-north
temperature
differenceATto andsalinitydifferenceASm (seetext
for detailson calculation).Ar/ > 0 corresponds
to southwardcurrents and CI = 0.5 m. Solid circle is the approximateaverage
for the controlmodel. Dashedlines bracketthe rangeof Ar/ for
which wind-drivenflow would be importantunderpresent-dayannual meanwind stress.The modelseawayflow would be northward
for AS• = 0 psu and unchangedATe. The modelflow could
havebeensouthwardor northwardfor a largerangeof ATe, when
AS• > 5 psu(i.e., whenthe Arctic hasnormalmarineto slightly
reducedsalinity).A northwardflow resultsif-2 < ASm < 5 psu
andthetemperature
gradientis changedto a present-day
value.
400
BJERRUM ET AL.' JURASSIC TRANSCONTINENTAL SEAWAY
ever,mustbe >5 psu for southwardflow to developwhen
AT,• > 20øC.The actualsalinitydifferencebetweenthe
JurassicTethysandArctic Sea is unlikely to havebeen >510 psubecausestenohalinenormalmarineneriticfossilsare
quitecommonat high latitudes[Hallam, 1969]. As a consequence,the temperaturedifferencemust havebeen < 15øC
for southward flow to occur.
andtheresultingresidualcurrents
will be veryimportantfor
the meanflow whenfrictionalforcesdominate.Including
tidesin the presentmodelthereforewouldonly changethe
flow slightlyin the shallowgrid pointsof the model(3050 m) [Bjerrum, 1999].
Therelativebathymetric
differences
in theseawayareimportantin controllingthe localcurrentdetailsandthe principalrouteof the main A•/-drivencurrent.Only the larger
currentfeaturesarethereforerobustresultswith paleoceanographicimplications.Most of the flow leavinga basinwith
morethanone exit will follow the deepeststrait(run SD4
versusSD5). The regionalwind patternlikewisehasan in-
The POM barotropicflowsQ,• of an idealized400 km
wideand4000 km longseaway,arecalculated
for a rangeof
A•/values and sill depthconfigurations
(Figure12). Additionally,a present-day,zonally averaged,annualmeanwind
stressis appliedto give an estimateof a strongerwind influence. The barotropicmodel flow at 45øN differsonly fluence on which course the main southward flow will folslightlyfromtheflowderivedfrom (2), whichneglectswind. low. For strongermidlatitudewesterlywindsmostor all of
The relativelygoodcorrespondence
showthe limitedeffect the southwardflow is throughthe Viking Strait area(runs
of windson Q,•. For A•/ = 0 there is a weak southward SW1 and SW2).
The wind field usedin the controlrun is very different
flow due to the wind-generated
Ekman transport.For the
fromthatcalculated
by Mooreet al. [1992],presumably
beJurassic wind field, there would be a weak northward flow.
The present-dayNorth Atlantic annual mean wind stress causetheyuseda diffusivemixedlayeroceanin contrastto
usedby Chandleret al.
(rx ,-- 0.1 N m-2) cansetup a flowthatcorresponds
to the fixed seasurfacetemperatures
[1992].
The
robust
results
of
the
presentmodel are only
the flow drivenby A•/ = -0.08 m (Figures11 and 12).
of
Removingthe wind from the controlrun (SC0) only results weaklyinfluencedby the fixedseasurfacetemperatures
in a 5% changein the mean seawayflow (section6). The the AGCM since the presentresultsprimarily dependon
imposedon the OBC northand
wind-drivenflowwasthereforeof limitedimportance
for the the salinityandtemperature
meanflow, salinity,andtemperatureof the narrowJurassic southof the seaway.Seawayadvectionof T and $ due to
LaurasianSeawayif sill depthsor meanseawaydepthwere the A•/-drivenflow is moreimportant,ascanbe seenfrom
> 30m.
thesensitivity
calculations
(Figures8 and 10).
Four robust end-memberscenarioswith general JurasFrictionalforcesand seawaydepthstartto dominatethe
flow magnitude,whenmeansill or seawaydepthsareshal- sic implicationscan be synthesizedfrom the control and
low, ascanbe seenfrom the lackof sensitivityto A•/when sensitivityruns and the discussion(Figures 13a-13d). For
flowpredominates
andrelatively
sill depthis lessthan,--30m (Figure12). Tidalamplification A•/> 0, overallsouthward
cool waterof low salinityis advectedinto the Europeanre1.5
gion (runs SC0, SW1, SW2, and SD1-SD9). Becauseof
the uncertaintiesin paleogeography
and lithology west of
JurassicBritain, two alternativesare likely, dependingon
1
whetherwater depthor cross-sectional
area were greaterin
the
Viking
Strait
or
west
of
the
Shetland
Platform (Figures
0.5
r
13a and b; run SD4, SD5, SD7). The flow shownin Figure 13b could also be due to strongmidlatitudewesterlies
(runsSWl and SW2). For A•/ _< 0 the main T - S front
would have been located at or north of the main seaway
constriction,and water with TethysT and S characteristics
wouldhavedominatedEuropefor bothtopographic
alternatives(Figures13c and 13d) (runsSBC1 and SBC2).
-.-,_...........
. .-'r'"'"--""'•'•--•2•':"-';;":-:
=:
-0.5
i t.•
-1
•5'"'
"•ø"•'ø'""•'
'/:.,.'/ ,.,.,., ,.,.
-1.5
0
50
1 O0
150
Sill depth [rn]
Figure12. POM barotropic
flow(Q,• in Sv,solidlines)at 45øN
for anidealizedstraight-channel
seawayof 400 km width,3500km
length,and150m meanshelfdepth,asa functionof hydrostatic
sea
leveldifferenceAr/and sill depthat 45øN [Bjerrum,1999].CI =
1 Sv andis positivefor northwardflow. Equation(2), wherewind
isneglected,
is shownwithdash-dotted
lines.Forshallowsills,
is mostlydependent
on sill depth,whereasfor largerdepths,Ar/is
very important.
8.2. GeologicalDiscussion
Each of the four end-member
scenarios described above
would have influencedthe marinepaleoecologicalsystem
throughthe effectson the regionalpaleohydrosphere
andits
climatesystem.Observations
from thegeologicalrecordcan
be interpretedin light of the modelresults.For the present
purposesthe discussionis concernedwith regionsnorthof
France and Germany where a surveyis presentedfor the
Early Jurassic.This surveypointsto possiblewaysof testing thepresentresultswhenmorelongitudinaldatatransects
BJERRUMET AL.: JURASSICTRANSCONTINENTALSEAWAY
shales of the Lower
b
a
/wl>O
401
Jurassic Black
Jura of southern
Ger-
many (SGB). Hermatypic corals reachednorthwardto the
Hebridesin the Hettangian.The carbonate-siliciclastic
transition may be seenin Britain (northwestof LBP). Thesedistributionsimply north-southmarine connectionsboth east
and west of Britain (SP-IM-LBP) and marine circulation as
in scenarioB (Figure 13b). Arctic watersflow southward
Aq> 0
east of Britain, and warmer water flow northward west of
Britain as a result of wind-driven
•Major
current
ßß *Major S and T front
Minor S and T front
and/or constric-
tion near the Hebrides(compareSD4 and SD5 and SW1).
Various climate-sensitivesedimentaryfacies, clay mineralogy, andpollen data suggestthat the EuropeanLate Hiensbachianand Toarcian were climatically more temperateperiodscomparedwith thosebeforeand after, as would be expectedwhen sea surfacetemperatureswere reducedduring
southwardflow as in scenarioB [Delfaud, 1983].
ct,q<O
.-'
circulation
•
•
Sill
Restricted
area
For purposes
of comparison
with oceancirculationmodel
calculations,fossil distributionpatternsare intriguing. Ammonitesshowa strongbioprovincialendemismin the Mesozoic sincethe lateEarly Jurassicandreveallatitudinalranges
almostcertainlyrelatedto climaticfactorsandmarinecirculationpatterns[Hallam, 1969;Page, 1996]. The changesof
bioprovincialboundarieswith time haveimplicationsfor the
interpretationof the overall marine circulation. For many
ammonites,habitatsranged from relatively shallow shelf
watersat birth and at deathto deeperwater during growth
[Callomon,1985]. The migrationalrangesduringthe life cycle could have been on a scale of 1000 km. Other ammonites
were probablyinhabitingthe oceansand strayedinto shallow watersin pursuitof food in favorablesurfacewatersor
by postmortaldrift (e.g., Phylloceras,Lytoceras).
of ShetlandPlatform (wSP) less restrictedthan the Viking Strait.
Evidence of ammonite distributionsnorth of the Tethys
(b) A•/ > 0 but with wSP the morerestrictedand/orwith strong is goodin southernEngland(southwestof LBP) and southmidlatitudewesterlies.(c) Arctic water denserthan that of Tethys
ward. Farther north, well-preservedEarly Toarcianfaunas
(A•/ < 0) andwSP lessrestricted.(d) A•/ < 0 andthewSPmore
are recordedin northeastEngland (CB), in the Hebrides
restrictedthanthe Viking Strait area.
(east of HP), in central east Greenlandwest of KS, and on
Spitsbergen(in the Arctic Sea) [Ershovaand Repin, 1983;
with biostratigraphic
subzoneresolutionbecomeavailable. BrickstrOmand Nagy, 1985; Howarth, 1992].
The LaurasianSeawaywas to varying degreespopulated
In the following,abbreviations
in bracketsreferto thoseon
by five major long-ranginggroupswherePseudoliocerasis
Figure 2.
Presenceof marinebiogeniccarbonatesedimentsis be- the earliesttruly borealgenusfound in the Jurassicand oclievedto reflectrelativelyhightemperatures,
normalmarine curring throughoutthe Arctic, easternSiberia, and Alaska
salinityandoligotrophyof the photiczone. The European [Dagis, 1974]. Pseudoliocerasis commonly the dominant
Jurassicshowsa latitudinaltransitionfrom predominantly or sole componentof the ammonitefauna north of central
carbonates
in the southto predominantlysiliciclasticsed- east Greenland. At discretetimes, Pseudoliocerasspread
iments north and east of the Hebrides (HP). From east to
southwardinto Europe as far as the southernParis Basin
westthe transitionzone can be relatively sinuous,over sev(PB) but as a rapidly diminishingcomponentof the faueraldegrees
of latitude,andprobablyreflectsthemarinecir- nas. This indicatesdirect open marine connectionswith
culation.
the Arctic through the VS in the east. There is further
In theEarlyJurassic,
carbonate-rich
deposits
dominated a marked decreasesouthwestward,so that the genusbeor weresignificant
southwest
of a boundaryzoneextend- comesrare in southernEngland (west of LBP). This coin]ngfromtheIrish Massif(IM) southeastward
to the south- cideswith the carbonateboundaryzone,wherefine-grained,
ern marginsof the BohemianMassif(BM). The contrastis dark, distal mudstonespassinto condensed,offshoremarls
reflectedby theLowerJurassic
limestones
of theLusitanian and limestonesin southernEngland. Early Toarcianmem(,-,,IBM) andParisBasins(PB) southand westof the dark bers of Tethys groups(Harpoceratinaeand Hildoceratinae)
Figure 13. Foursimplifiedend-member
scenarios
(a-d),indicating
principalflowroutesandthepositions
of T/S watermassfronts.
(a) Tethyswater denserthan Arctic (A•/ > 0) and straitswest
402
BJERRUM ET AL.: JURASSIC TRANSCONTINENTAL
are stronglyrepresented
as far as northeastEnglandbut occur only rarely in central east Greenland((Hildaites), G.
Dam coll.) andon Spitsbergen
(as"Pseudogrammoceras"
of
BackstrOm
and Nagy [1985]), whereasthe TethyanPhymatoceratidaehavenot beenfoundnorthof northeastEngland
(CB). Theseobservations
suggestthat therewas a transition
in Englandfrom southwardflowingcoldwatersin thenortheastto warmerwatersin thesouthwest
(Figure13b). The periodsof strongestsouthwardexcursionof borealPseudoliocerasapproximatelycoincidedandalternatedwith the most
frequentnortherlyexcursionsof the archetypicallyTethyan
Liostraca,(Phyllocerasand Lytoceras[Dean, 1954]). Both
the durationsof southerlyexcursions
of Pseudolioceras
and
the intervalsbetweenthem were highly variablewith wide
changesof distributionoccurringvery rapidly [Dean, 1954;
Howarth, 1992]. On the shortesttimescalethe genuscan
be concentrated
into a singlebed spanninga fractionof a
SEAWAY
canbe regardedas onesinglestraitfor modelingpurposes,
despiteits complexpaleogeography.
Hydrostaticsealevel
differencesbetweenthe Tethys Ocean and the Arctic Sea
could have driven a strong current through the seaway.
Bathymetricvariationwas importantfor local circulation
in subbasins,whereasthe minimum sill depthexcertedthe
dominantcontrolof the net magnitudeof the transcontinental seawayflow. Bathymetricuncertaintydictatesthat calculated current details of scales less than ,-• 200 km are not
significant.
North-southdensitydifferences
andhencehydrostatic
sea
leveldifferences
havea largeimpacton seawaysalinityand
temperature.
WhenTethyanwatersarethe denser(low hydrostaticsea level), the seawayflow is southward,giving
the watersin the seawayArctic characteristics.
Conversely,
when the Arctic waters are the denser, the seaway waters
haveTethyancharacteristics.
Changesin wind patternsor
standard
ammonite
subzone
(,-• 104years).Sporadic
appear- strengthshaveonly little impacton the net southward
or
ances
canextend
overoneorseveral
subzones
(• 106years), northwardseawayflow.
Net seawaycurrentdirectionsin the Jurassiccan be inandgapscanbe of similarmagnitudes.Theserapidfluctuprovationsin ammonitedistributionscould be interpretedas the ferred from latitudinalshifts in paleobiogeographic
resultof relativelyfast changesin the north-southdensity inces,in particular,of ammonites.Arctic influenceson seaway watersare indicatedby faunaldominanceand southdifferencesand henceseawaycurrents.
andcoolerclimaticcondiLithofaciesand faunallatitudinalchangesin Middle and wardspreadof borealammonites
Late Jurassictimes suggest,despitedifferencesin seaway tionsinferredfrom palynologyand clay mineralogy.Conversely,Tethys-influenced
seawaywatersmay be inferred
paleogeography,
thatseawaycurrentsmayhavereversedowing to changesin salinityat highlatitudesin theopenocean. from sub-Mediterranean faunal influences and indicators of
The Middle and Late Jurassic will be treated in more dewarmer water/climate. An increasedunderstandingof the
tail in a futurestudy.Overall,the lithofaciesandpaleobio- Jurassicocean-climatesystemshouldbe possibleif the
geographic
provincesof theJurassic
suggest
thatsouthward modelcalculationsare testedwithin the frameworkof highand geochemicalstudies.
seawaycurrentsdominatedin the Toarcian,Early Callovian, resolutionpaleobiogeographical
of the anoxic/latestCallovianto Early Oxfordian,late Middle and Late This would resultin a betterunderstanding
dysoxiceventsthatapparently
wereassociated
with thehyOxfordian, and in the Kimmeridgian. In interveningperiods the currentsflowed northwardor were blockedby very
pothesized
southward
flow in theToarcianandin theKimshallow sills or land as in much of the Middle Jurassic.
meridgian.
Southwardcurrentscorrespond
with periodswherehighlatitudeoceansalinitieswhere reducedwhich possiblypre-
ventednorthernhigh-latitudeconvection[cf. Shafferand
Notation
Bendtsen,1994; Schmidtand Mysak, 1996]. It is therefore
x•y•z
east,north,andupwardcoordinateaxis
proposedthatdeepwaterformationmighthavebeenoccur(0,0,0 in southwestmodel corner).
-1
ring in the Tethysand/orat southernhighlatitude[Schmidt
f(Y) Coriolisparameter,s
and Mysak, 1996; Bjerrum,1999]. The possiblerelationbep(x,y,x) density,
kgm-3.
tweenthe globalthermohaline
circulationandseawayflow
waterdepth,m.
directionsrequirefurther studyand imply that geological P - Ep(X,y) precipitationminusopen-waterevaporation,
--1
data from ancientmeridionalseawaysperhapscan be used
ms
to elucidate the ancient state of the thermohaline
circulation
[Bjerrum, 1999].
9. Summary and Perspective
The Princeton Ocean Model is used to advance our un-
derstanding
of thepaleoceanographic
processes
in transcontinentalseaways.The globaloceanis foundto havea profound influenceon seawaydynamics. Model calculations
show that the JurassictranscontinentalLaurasian Seaway
?- Z(x,y)
precipitationminusevapotranspiration
over
land,m s-i.
temperature,oC.
T(x,y)
seasurfaceclimatictemperature,øC.
temperature,øC.
•'•cp(Z) open-boundary
salinity,
psu(approximately
•00).
$(x,y,z)
salinity,psu.
'•cp(Z) open-boundary
AV
Q
hydrostaticsealevel difference,m.
straitvolume
transport,
m s-1.
roughnesslengthscale,m.
BJERRUM ET AL.: JURASSIC TRANSCONTINENTAL
mean south-north T difference, oC.
meansouth-north$ difference,psu.
Acknowledgments.We gratefullyacknowledge
Mark Chandler
for supplyingthe AGCM data files and thank J0rgenBendtsen,
Gary Shaffer, and SteveHesselbofor thoughtfuland stimulating
References
Ager, D. V., The Jurassicworld ocean,in Jurassic NorthernNorth Sea Symposium,editedby
K. Finstadand R. C. Selley, pp. 1-43, Nor.
Petrol.Soc.,Stavanger,1975.
B/ickstr6m, S. A., and J. Nagy, Depositional
history and fauna of a Jurassicphosphorite
conglomerate
(the Brentskardhaugen
Bed) in
Spitsbergen,
Skr.Nor. Polarinst.,185, 44 p.,
1985.
Jurassicnarrowmeridionalseaway:Transcontinentalcurrentsand globaloceanfeedbacks,
Ph.D. thesis,Univ. of Copenhagen,Copenhagen,Denmark,1999.
Blumberg,A. F., and G. L. Mellor, Diagnostic
and prognosticnumericalcirculationstudies
of the SouthAtlantic Bight, J. Geophys.Res.,
funded the final work.
gaeanclimateduringtheEarlyJurassic:
GCM
sivedissociationof gashydrateduringa Juras-
simulations and the sedimentary record of
paleoclimate,GeologicalSocietyof America
sic oceanic anoxic event, Nature, 406, 392395, 2000.
Bulletin, 104, 543-559, 1992.
Dagis, A. A., Toarskieammonity (Hildoceratidae) Severa Sibiri [Toarcian ammonites
(Hildoceratidae) of the North Siberia],
London, 1992.
Bretschneider,
C. L., Wave generationby wind,
deep and shallow water, in Estuary and
CoastlineHydrodynamics,Eng. Soc.Monogr.,
editedby A. T. Ippen,pp. 133-198, McGrawHill, New York, 1966.
Buchem,F. S. P. V., and I. N. McCave, Cyclic
sedimentationpatternsin Lower Lias mudstonesof Yorkshire(GB), Terra Nova, I, 461467, 1989.
Cai, W., and S. J. Godfrey,Surfaceheat flux parameterizationand the variability of the ther-
mohalinecirculation,J. Geophys.Res., 100,
10679-10692, 1995.
Callomon, J. H., Marine boreal Bathonian fossils
from thenorthernNorthSeaandtheirpalaeogeographicalsignificance,Proc. Geol. Assoc.,
90, 163-169, 1979.
Callomon, J. H., The evolution of the Jurassic
ammonitefamily Cardioceratidae,
Spec.Pap.
Palaeontol., 33, 49-90, 1985.
Chandler,M. A., Descriptionof modem and
Pangean deserts: Evaluation of GCM hydrologicaldiagnosticsfor paleoclimatestudies, in Pangea: Paleoclimate,Tectonics,and
SedimentationDuring Accretion,Zenith, and
Breakupof a Supercontinent,
editedby G. D.
Klein, Spec.Pap. Geol. Soc. Ant., 288, 117138, 1994.
Chandler,M. A., D. Rind, and R. Ruedy, Pan-
31-65, 1998.
151, 1988.
720, 1984.
Ericksen,M. C., and R. L. Slingerland,Numer-
vol. 4, edited by N. Heaps, pp. 1-16, AGU,
Washington,D.C., 1987.
Bradshaw,M. J., et al., Jurassic.,in Atlas of Paleogeography
and Lithofacies,
Mem. Geol.Soc.,
vol. 13, editedby J. C. W. Cope,J. K. Ingram,
and P. F. Rawson, pp. 107-129, Geol. Soc.,
waves
Delfaud, J., Les pa16oclimats
du Jurassiqueen
Europeoccidentale,Bull. Inst. Geol. Bassin
London, 1992.
Aquitaine,34, 121-135, 1983.
Diem, B., Analytical method for estimating Jenkyns,H. C., The Early Toarcian (Jurassic)
anoxic event: Stratigraphic,sedimentaryand
palaeowaveclimate and water depth from
geochemicalevidence,Ant. J. Sci., 288, 101waveripple marks,Sedimentology,
32, 705-
culation model, in Three-Dimensional Coastal
Ocean Models, Coastal Estuarine Sci. Ser ,
tion of a three-dimensional
structure of internal
on a continentalslope, Cont. Shelf Res., 18,
Howarth, M. K., The Ammonite Family Hildoceratidae in the Lower Jurassic of Britain,
Monogr.Palaeontogr.Soc., Palaeontogr.Soc.,
coastal ocean cir-
88, 4579-4592, 1983.
Holland, H. D., The Chemistryof theAtmo•I)here
and Oceans,JohnWiley, New York, 1978.
Holloway, P. E., and B. Barnes, A numerical
investigationinto the bottomboundarylayer
flow and vertical
Nauka, Moscow, 1974.
Dean,W. T., Noteson partof the Upper Lias successionat Blea Wyke, Yorkshire,Proc. York-
Dreyer, T., Significanceof tidal cyclicity for
modelling of reservoir heterogeneitiesin
the lower JurassicTilje Formation, midNorwegianshelf,Nor.Geol.Tidsskr.,72, 159-
Blumberg,A. F., and G. L. Mellor, A descrip-
403
discussions.The manuscripthas benefitedby commentsfrom J.
Bendtsenand reviewsby P. Jewell and two anonymousreviewers.
The researchwas fundedby a Universityof Copenhagenstipend
to C. J. Bjerrumand a grantfrom the DanishNatural ScienceResearchCouncil(SNF) to F. Surlyk. Supercomputer
timewasfunded
by the SNF. Danish Centerfor Earth SystemScience(DCESS)
shire Geol. Soc., 29, 161-182, 1954.
Bjerrum,C., Numericalpaleoceanography
of a
SEAWAY
170, 1992.
ical simulation of tidal and wind-driven
cir-
culation in the CretaceousInterior Seaway of
North America, Geol. Soc. Am. Bull., 102,
1499-1516, 1990.
Ershova,E. S., and Y. S. Repin,Toarskiei aalenskie ammonityarchipelagaShpitsbergen,in
Geologiya Shpitsbergena,edited by A. A.
Krasilyishchikov
and V. A. Basov,pp. 150170, Minist. Geol. SSSR, Leningrad,Russia,
1983.
Gill, A. E., Atmosphere-Ocean
Dynamics,Academic,SanDiego,Calif., 1982.
Hallam, A., Faunal realmsand faciesin the Jurassic,Palaeontology,12, 1-18, 1969.
Hallam, A., Continental humid and arid zones
duringthe Jurassicand Cretaceous,Palaeogeogr.Palaeoclimatol.Palaeoecol.,47, 195223, 1984.
Hallam, A., A teevaluationof Jurassiceustasyin
the light of new data and the revisedExxon
curve, in Sea-Level Changes.'An Integrated
Approach,editedby C. K. Wilguset al., Spec.
Pub. SEPM, 42, 261-274, 1988.
Hallam, A., Jurassicclimates as inferred from the
sedimentaryand fossil record,Philos. Trans.
R. Soc. London. Ser. B, 341,287-296, 1993.
Hallam, A., and P. B. Wignail, Mass Extinctions
and Their Aftermath,Oxford Univ. Press,New
York, 1997.
Jewell,P. W., Circulation,salinity,and dissolved
oxygen in the CretaceousNorth American
seaway, American Journal of Science, 296,
1093-1125, 1996.
Kutzbach, J. E., P. J. Guetter, and W. M. Wash-
ington, Simulatedcirculationof an idealized
oceanfor Pangeantime, Paleoceanography,
5,
299-317,
1990.
Mellor, G., Usersguidefor a three-dimensional
primitive equation, numerical ocean model,
Programin Atmos. and OceanicSci., Princeton Univ., Princeton, N.J., 1998.
Miller, R. G., A paleoceanographic
approachto
the KimmeridgeClay Formation,in Deposition of Organic Facies,editedby A. Y. Huc,
AAPG Stud. Geol., 30, 13-26, 1990.
Moore, G. T., D. N. Hayashida,C. A. Roos,and
S. R. Jacobson,
The paleoclimateof the Kimmeridgian/Tithonian
(Late Jurassic)world, I.,
Results using a general circulation model,
Palaeogeogr.
Palaeoclimatol.Palaeoecol.,93,
113-150, 1992.
O'Brian, N. R., Significanceof laminationin the
Toarcian (Lower Jurassic)shales from Yorkshire, Great Britain, Sediment. Geol., 67, 2534, 1990.
Oey, L.-Y., Simulationof mesoscalevariability in
the Gulf of Mexico: sensitivitystudies,comparisonwith observations,and trapped wave
propagation,J. of Phys. Oceanogr.,26, 145175, 1996.
Oey, L.-Y., Subtidal energeticsin the FaroeShetlandChannel: Coarse-gridmodel experiments,J. Geophys.Res., 103, 12689-12708,
1998.
Hay, W., Paleoceanography
of marine organiccarbon-rich sediments, in Paleogeography, Oey, L.-Y., and P. Chen, A modelsimulationof
circulation in the northeast Atlantic shelves
Paleoclimate, and Source Rocks, edited by
A. Y. Huc, AAPG Stud. Geol., 40, 21-59,
andseas,J. Geophys.Res.,97, 20087-20115,
1995.
Hesselbo,S., D. GrOcke,H. Jenkyns,C. Bjerrum,
P. Farrimond, H. M. Bell, and O. Green, Mas-
1992.
Ohshima,K. I., The flow systemin the JapanSea
causedby a sealevel differencethroughshal-
404
BJERRUM
ETAL.:JURASSIC
TRANSCONTINENTAL
SEAWAY
low straits,J. Geophys.Res., 99, 9925-9940,
Kimmeridge
Clay Formations,
UnitedKing-
1994.
maticmodelfor the Kimmeridgian,Palaeo-
dom,Geology,26, 747-750, 1998.
geogr. Palaeoclimatol.Palaeoecol., 95, 47-
Page,K. N., Mesozoicammonoidsin spaceand
time, in AmmonoidPaleobiology,edited by
N. Landman,K. Tanabe,andR. A. Davis,Top.
Geobiol.,13, pp. 755-793, 1996.
Paterson, D. M., and K. S. Black, Water flow,
Schieber,
J., Microbialmatsin terrigenous
clas-
sedimentdynamicsand benthicbiology,Adv.
polewardoceanicheatflux explainthe warm
Cretaceous
climate?,Paleoceanography,
11,
Ecol. Res., 29, 155-193, 1999.
579--593, 1996.
Picard,S.,et al.,6•80 values
of coexisting
brachiopodsand fish: Temperaturedifferences
andestimatesof paleo-waterdepths,Geology,
26, 975-978, 1998.
Prauss,M., B. Ligouis, and H. Luterbacher,Organicmatterand palynomorphsin the 'Posidonienschiefer'(Toarcian,Lower Jurassic)of
southernGermany, in Modern and Ancient
Continental Shelf Anoxia, edited by R. V.
Tysonand T. H. Pearson,Geol. Soc. Special
Publ.,58, pp. 335-351, 1991.
Rees, P.M., A. M. Ziegler, and P. J. Valdes,
Jurassicphytogeography
and climates: New
data and model comparisons,in Warm Climatesin Earth History, editedby B. T. Huber,
K. G. MacLeod,andS. L. Wing,pp. 297-318,
CambridgeUniv. Press,New York, 21300.
Rthl, H.-J., A. Schmid-Rthl, W. Oschmann,
A. Frimmel, and L. Schwark, The Posidonia
Shale(Lower Toarcian)of SW-Germany:An
oxygen-depleted
ecosystemcontroledby sea
level andpalaeoclimate,
Palaeogeogr.
Palaeoclimatol. Palaeoecol., 165, 27-52, 2001.
S•elen,G., R. V. Tyson,M. R. Talbot,andN. Tel-
roes,Evidenceof recyclingof isotopically
72, 1992.
tics:The challengeof identificationin therock Wang, J., L. A. Mysak, and R. G. Ingram,
record,Palaios, 14, 3-12, 1999.
A three-dimensional numerical simulation of
Schmidt,G. A., andL. A. Mysak,Canincreased
HudsonBaysummeroceancirculation:
Topo-
Schouten,S., H. E. V. Kaam-Peters,W. I. C. Rijpstra, M. Schoell,and J. S.S. Damste, Eœ-
graphicgyres,separation,
andcoastaljets,J.
of Phys.Oceanogr.,24, 2496-2514, 1994.
Whitehead,
J.A., Topographic
controlof oceanic
flowsin deeppassages
and straits,Rev.Geo-
phys.,36, 423-440, 1998.
Ziegler, P. A., Evolution of the Arctic-North Atcarbonisotopiccomposition
of EarlyToarcian
lantic and the westernTethys,AAPG Mem.,
carbon,Am. J. Sci., 300, 1-22, 2000.
43, p. 198pp., 1988.
fects of an oceanic anoxic event on the stable
Shaffer,G., andJ. Bendtsen,
Roleof theBering Ziegler,P. A., GeologicalAtlas of Westernand
Straitin controlling
NorthAtlanticOceancirCentralEurope,ShellInt. Petrol.Maatschapculationand climate,Nature, 367, 354-357,
pij B. V., The Hague,Netherlands,1990.
1994.
Silva, A.M. D., C. C. Young,and S. Levitus,
NOA4 AtlasNESDIS,9, Atlas of surfacemarine data 1994, vol. 4, 1994.
Slingerland,R. L., L. R. Kump, M. A. Arthur,
P. J. Fawcett,B. Sageman,and E. J. Barron,
Estuarine circulation in the Turonian Western
C. J. Bjerrum,DanishCenterfor EarthSystemScience,
NielsBohrInstitute
for Astronomy,
Physicsand Geophysics,
Universityof Copenhagen,JullaneMariesVej 30, DK-2100CopenhagenO, Denmark.([email protected]).
J. H. Callomon,ChemistryDepartment,UniversityCollegeLondon,20 GordonStreet,LondonWC1H 0AJ,England,UK.
Smith, A. G., D. G. Smith, and B. M. Funnell,
Rudy L. Slingerland,Departmentof GeoAtlas of Mesozoicand CenozoicCoastlines,
sciences,
Pennsylvania
StateUniversity,UniverCambridgeUniv. Press,New York, 1994.
sity Park, PA 16802.
InteriorSeawayof North America,Geol. Soc.
Am. Bull., 108, 941-952, 1996.
Stigebrandt,
A., The NorthPacific:A globalscaleestuary,J. of Phys.Oceanogr.,14, 464470, 1984.
F. Surlyk, GeologicalInstitute, University
of Copenhagen,
OsterVoldgade10, DK-1350
Copenhagen
K, Denmark.
Toulany,B., andC. J. R. Garrett,Geostrophic
controlof fluctuatingflow throughstraits,J. (ReceivedFebruary15, 2000;
light CO2(aq) in stratifiedblackshalebasins:
of Phys.Oceanogr.,14, 649-655, 1984.
revisedFebruary22, 2001;
Contrasts
betweenthe WhitbyMudstoneand Valdes,P. J., andB. W. Sellwood,
A palaeocli- acceptedMarch 12, 2001.)