GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 15, PAGES 2853-2856, AUGUST 1, 1998
Carbon isotopes in aquatic plants, Long Valley caldera, California as
records of past hydrothermal and magmatic activity
JohnB. Reid,Jr.1 Jesse
L. Reynolds
2 Nathan
T. Connolly,
ShariL. Getz,Pratigya
J.Polissar
3,and
LawrenceJ. Winship
Schoolof NaturalScience,HampshireCollege,Amherst,Massachusetts
Laura J. Hainsworth 4
Lawrence Livermore National Laboratories, Livermore, California
Abstract. Hot andcoldspringscontribute"dead"(14Cfree)
streambedplants as indicators of present and past magmatic
CO2emissionsinto the caldera's surfacewaters.
Creek. Headwaters
aquaticplantshavemodem•4C,but live
The use of aquatic plants for this purpose came about
plants downstreamof the intracaldera springs are depleted in
accidentally.Where the Owens River skirts the northeast flank
•4C,(aslow as 19% modem,with apparentagesup to 13.3 of the caldera's resurgent dome, its course has alternated
kyrs). In an abandonedmeander of the upper Owens River,
betweentwo parallel meanderbelts, apparently in responseto
preserved streambed plants are buffed by 600 year old Inyo
tilting of its floodplain. One possible causeof the tilting is
Craterspumice.Apparent
•4Cagesof theseplantsexceedtrue the alternating inflation and deflation of the dome (Reid,
ages by- 1100 years indicating that they also incorporated 1992). We hoped to date the river's occupation of these
deadDIC as they grew. The preservedplants are downstreamof
meanderbelts by radiocarbon-dating the remains of ancient
Big Springs,whoseelevateddead DIC may representmagmatic plants buried in abandonedchannelsof both meanderbelts. For
'CO2. The buried plants incorporated -10% dead carbon, control, we also datedmodem plants in the Owens River and
although modem plants here have -50% dead carbon, Hot Creek. Modem plants throughout the caldera- and for at
suggestingthat more magmaticCO2 is now entering the upper least65 km furtherdownstream
- haveanomalous
•4Cages(up
Owens River than at the time of the Inyo Craters eruptions 600
to 13,000 years), particularly near the hot and cold springs.
dissolvedinorganic carbon (DIC) to the Owens River and Hot
years
Whilethisfindingmeantthatburiedplantscotildnotbe used
ago.
for dating the hypothetical past behavior of the dome, it
suggestedthat they might be useful in assessing modern and
past emissions of dead CO2 into the caldera's river system.
Introduction
Long Valley caldera(Fig. 1), a 17 x 32 km depression on
the eastern boundary of the Sierra Nevada, California, formed
about 760 kyr ago (Bailey, 1989). Magmatic activity since
then has produceda 500 m high resurgent dome in the westcentral caldera floor.
The most recent caldera-related
volcanism
was the Inyo Crater eruptions, -600 years ago, and phreatic
explosions on Mammoth Mountain,-700
years ago (M.L.
Sorey, pers. comm, 1997). Since 1980, there have b6en
several indications of renewed magmatic unrest. Seismic
activity and changes in fumarole composition may indicate
shallow magmatic intrusion (Hill et al., 1990; Sorey et al.,
1993). Leveling surveys along U.S. Highway 395 have shown
-60
cm of inflation
of
the
dome
between
1979
and
1984
(Castle et al., 1984). Diffuse emissions of magmatic CO2 near
Mammoth Mountain followed a swarm of seismic activity in
1989 (Farrar et al., 1995). Magmatic CO2 may also be entering
the surfacewaters of Long Valley and taken up by aquatic
Other investigatorshave seen •4C deficienciesin modern
plants due to magmatic activity (e.g.: Sveinbj6rnsd6ttir et al.,
1992; Srdocet al., 1986); As discussedbelow, Long Valley
provides the additional opportunity of comparing past and
present releases of magmatic CO2 to surface waters in a
volcanic setting.
"Dead"(•4C free) dissolvedinorganic carbon (DIC) in
hydrothermal and volcanic areas has been attributed to
magmatic inputs (e.g., Shevenell and Goff, 1993; Giggenbach,
1995; Rose and Davisson, 1996). While dissolution of
carbonate minerals at depth is a possibility, and the deep
reservoir of the Long Valley hydrothermal system may reside
in metamorphicmarine sediments(Sorey et al., 1991), CO2rich gas with magmatic He and C isotopic characterhas been
enteringthe calderaas diffuse emissions (-1200 T CO2/day) at
Mammoth Mountain since 1990 (Farrar et al., 1995; Sorey et
al., 1996). The isotopic characterof DIC in wells and springs
near Mammoth Mountain shows that the groundwater is
dissolving magmatic CO2 (Sorey et al., 1996). Hilton (1996)
examined He and C isotopes of hydrothermal gases and
• to whomcorrespondence
shouldbe sent
2 Now at Departmentof Environmental
Science,Policy and concludedthat the primary source of CO2 is magmatic gas,
Management,Universityof California,Berkeley,CA 94701
although a small input from thermal decarbonizationof marine
3 Now at Department
of Geosciences,
Universityof Massachusetts,carbonatescould not be excluded. Taylor and Gerlach (1984)
Amherst, MA 01003
4 NowatChemistry
Department,
EmoryandHenryCollege,Emory,VA found that volcanicrocksare not a sourceof hydrothermalCO2.
plants.In this report,we usethe anomalous•4Ccontentof
24327
Copyright1998by theAmericanGeophysical
Union.
In addition,wecanrule out decayof atmospheric
•4Cin the
groundwater
systemas the main reasonfor diminished•4C
Papernumber98GL01854.
comparedwith the half life of •4C) in the hydrothermal
0094-8534/98/98GL-01854505.00
reservoir (White et al., 1990).
activity since residencetime is estimated at -1200 years (small
2853
2854
REID ET AL.: CARBON ISOTOPES, LONG VALLEY
6180
' o oCrates
4830
4,,•o
/
•'.','&,,,.t,,• /
"•
•
/ ':+:....-i•:•
7.,o12
7180
, /
%
4020
1--"-[
CALDERA
results in stepwise increasesin concentration, such as those
shownfor arsenicin Fig. 2a. The highestinputsof most of the
analyzedelementsoccurat the major hot springs (especially
As, Na, Rb, Cs, W); cold springs contribute the bulk of a few
elements(eg: Mg, Ca, P, Mo, Zn). Sr, K and Si are addedfrom
both springtypesaboutequally.These data are available upon
request.
: q••.-:•':•:.'•..••:•:!
\:'rt-r• gt
/
•
•
•
•-•'
C
•----' ] HotCreek
Gore
,,
c•
I-%370 'i%w
'
_._--/-132•0/13
Carbon isotopic data for Long Valley plants and water
samplesare shownin Figs. 2b, 2c and 3. On a plot showing
•4Cand'3Cvariation
together
(Fig. 3), •4Crangeswidelyfrom
0% modernin magmaticCO2 to 113% in atmosphericCO•, and
2470•
thusmixing is the main causeof •4C variation. •5'3Cvaries
•n.,...••-•_Crc
•! ak¾.e
24••
8950 •
[
n:9o•"
o•
I
•
•
o.•
•
•
•
41901
1__2 c,•,:•_ -.-•
• Rte
6bridge
.......
.
reshopm•m
do•5•y.•
L gValley
Cald
on
era
much more narrowly, mainly due to fractionation
accompanying photosynthesis. CO• samples from waters
appearto be mixturesof atmospheric,magmaticand soil gas
(shadedtriangle in Fig. 3). Plants show typical depletion in
•5'3Cby about20-25 per mil relativeto their host waters
Figure 1. Map of Long Valley caldera(after Bailey, 1989)
showingapparentages(numbersin years) and percent"dead" C
(solid portionsof horizontalbars) for streambedplants, waters
(W) and an aquaticinsectcasingsample(B) in the Owens River
and Hot Creek.
(Hoefs, 1997), with plants near Big Springs containing the
mostnegatived'3C,andthusthe highestproportions
of soil
gas.Variationin •4Cand•3Cwith distance
downeachdrainage
are shown in Figs. 2b and 2c. An aquatic plant sample from
Lake Mary (headwaters
of Hot Creek) has a full complementof
•4C(113% modem•4C(pmC)),butall otherstreambed
plants
Here, we report the resultsof carbon isotopic studiesof the
aquaticplantsand springwatersof Long Valley as measuresof
present and past magmatic CO2 release in the caldera.
Hainsworth et al. (1995) used tree rings at a fumarole near
Mammoth
Mountain
to
document
local
variations
in
we havesampled
aredepleted
in '4C.Wheredatafor both exist,
plants and their watersare similar in pmC, with mismatches
due probably to incomplete mixing of water types,
photosynthesis-inducedfractionation and possibly the fact
the
emissions of magmatic CO2 over the past decade. In this
report, we suggestthat apparent radiocarbonages of preserved
aquaticplantsmay providecluesto magmaticC0• duringthe
Inyo Craterseruptions600 years ago, provided the plants can
be dated by other means.
Owens River
BS
160
A
140
120
Mammoth
HCD
:i
/ Hot Creek
FHSHS LH?
:$":½•i
::
.
140
120
1oo
80
100
6O
Methods
40
40
!
20
o
Surface waters and aquatic and terrestrial plants were
collected in June of 1994 and 1996. We measuredpH and
alkalinity for every site in 1996; stream discharge was
measuredat selectedsitesnear streamand spring junctions. We
precipitated DIC samples as SrCO3 using strontium chloride
and ammonium hydroxide. Liquid scintillation spectrometry
60
40
20
C
,
'
(BetaAnalytic,Miami,Florida)wasusedto measure
'4Cof all
plant samplesand all DIC precipitates but one. It, the conifer
charcoals and the buried plant remains were analyzed by
accelerator mass spectrometry (AMS) (Lawrence Livermore
NationalLaboratory).
Fractionation
in '4C wascorrected
using
measured
•5'3C.
We performed
•3Canalyses
on additionalplants
by combustion and mass spectrometry at the University of
Vermont Stable Isotope Laboratory. Inductively coupled
plasma-atomic emission spectroscopy (Thermal Jarrell Ash
ICAP 61) at Middlebury College and inductively coupled
plasma-mass spectrometry (Perkin Elmer Elan 6000) at
HampshireCollege were usedfor dissolved cation analyses of
the 1994 and 1996 water collections, respectively.
Results
The resultsof the elemental analyses show that the caldera
surface water chemistry is controlled by the hot and cold
springs (Fig. 2a). Most conservativeelements enter the stream
in pronounced,discreteinputs at the major springs and have
essentially constant concentrationsbetween these inputs. This
•o
1 oo
•ø25
t
::::•:?
ø35
.•:•
45, : : : • : : , ,
40
35
30
25
20
15
10
5
0
: ; :
35
30
25
, ,0
-25
-35
I • I!' I i I
20
15
10
5
0
distanceupstreamfrom Lake Crowley (kin)
--n-
Mammoth
+
FishHatcherySpring/ Creek +
/ Hot Creek
-•-
Little
-a-
Big Springs
Hot Creek
o
Hot Creek Delta
Hot Springs
+
Owens
+
Mono Tunnel
River
x streamsideterrestrialplants
Figure 2. (A) Arsenic concentrationsin spring and surface
waters; this stepped pattern is typical of most analysed
elements. Little Hot Creek = 248 ppb and Hot Springs = 19 1
ppb(off scale).(B) 14Ccontentof plantsandDIC. (C) •3Cof
plants and DIC. All plants are aquatic except (x) which are
terrestrial grasses immediately adjacent to the stream. "W"
next to a point signifiesDIC. The grey vertical lines represent
input from Big Springs (BS), the Hot Creek delta (HCD), Fish
Hatchery Springs(FHS), Hot Springsin Hot Creek gorge (HS),
and Little Hot Creek (LHC).
RED ET AL.: CARBON ISOTOPES, LONG VALLEY
that plants integrate over the growing season, while water
CALDERA
2855
of all analyzed terrestrial plants (--26%0) and aquatic plants
samplesare instantaneous.
Fractionationof '4C/'2Cduring- above the major springs (--18%o) are typical (Deines, 1980).
photosynthesis,
-45 %0 (Fritz et al., 1989), is small compared
to the rangeof values among plant samples (19 to 113 pmC).
The large amountsof deadDIC at Big Spring and Fish Hatchery
Spring, the two major cold springs, indicate that they receive
dead carbonfrom magmatic gases,the hydrothermal system, or
contact
with
carbonate
minerals.
Total deadDIC leaving the calderacan be estimatedfrom
simultaneousmeasurementof streamdischarge,alkalinity, and
Two major springs, Big Spring and Fish Hatchery Springs
contributelighter carbon(lower•5•3C)to plants immediately
downstream(Fig. 2c). The lighter carbon stems mainly from
biogenic soil CO•. As one progresseson downstream,the low
•5•3Cof the plantsi•eturns
quicklyto the "baseline"valueof18%o.•4Cexcursions
areconsiderablylarger,andpersistfor
much greater distancesdownstream.
]4Ccontent(estimated
from plant isotopes).In June,1996,
-15 T/day total DIC and -6 T/day deadDIC were discharging Preserved plants below Big Spring as possible
from the Owens River into Lake Crowley. These values are records of past C02 release
probably slight underestimatessince by the time the water
Dead DIC from the springs of Long Valley causesdramatic
reachedthe lower Owens River some deadDIC may have been
in '4Cof live streamplantsin the calderaandfor
lost through photosynthesis and atmospheric mixing. The depletions
maximum dead DIC fluxes of the Owens River and Hot Creek
many kilometers downstream.If the deadDIC is magmatic in
were 4.6 and 5.7 T/day, respectively, giving an estimate of origin, preservedstream plants could serve as recordsof past
magmatic activity.
-10 T/day deadDIC enteringthe surfacewatersof the caldera.
MagmaticCO=is the likely sourceof deadDIC in the modern
Although the watersof Mammoth Creek (upper Hot Creek)
at US 395 chemically resemble those of the headwaterswith Long Valley hydrothermalsystem.As thermal waters saturated
verylow cationconcentrations,
the '4Ccontents
of two aquatic with CaCO3rise, calcite precipitatesand CO= evolves (White
and Peterson,1991). Hilton (1996) determinedthat this CO= is
plantstherearedepleted
(74 pmC).15'3C
values
primarily
magmaticin origin. Big Spring is a cold spring with
(-17.5 + 0.2%0) of these samplesare normal for aquatic plants
a
high
dead
DIC concentrationbut probably does not receive
(Deines, 1980). There are no major springs upstream from this
point, and contributions from carbonates is excluded by the chemical inputs from the hydrothermal system (M. L. Sorey,
low and nearly constant calcium and magnesium pets. comm., 1996). The possible sourcesof its deadcarbon
concentrations
fromLakeMary to US 395. This '4Canomalyis are interactions with carbonates in marine metasediments,
likely producedby fumarolic activity in a small valley -1.5 km
magmatic emissions of CO2 to its rechargearea on the north
northwestof the samplingsite, whereCO2 from hydrothermal side of Mammoth Mountain (Heim, 1992), and decomposition
fluids boiling at depth is enteringthe shallow groundwater.
of ancient organic matter buried by the moat rhyolites (M. L.
Unlike •4C,the •3Ccontentsof aquaticplantsarenot useful Sorey, pers. comm., 1996). Since the molar Ca concentration
asindicators
of paststreamcarbonisotopiccharacter.
The •3C at Big Spring is aboutan order of magnitudelower than that of
atmosphericCO2
120
,
ß
.
soil gas •
'
100,
0
deadDIC, only a small portion of the C is from calcium
carbonates. If Mg is included, at most 40% of C is from
carbonate minerals. Thus, there may be a large input of
magmatic CO2 into the recharge waters of Big Spring.
Forthcoming helium isotope data from the U.S. Geological
Survey shouldclarify this issue (M. L. Sorey, pers. comm.,
1996). Heim (1992) estimatedthat Big Spring water averages
-12 years old basedon tritium levels. Therefore,the '4C
zE.0.
o
,.•
•
- 3 ,-•I
40,
1o
20.
0
-40
4 -•
.
,
.
magmatic
CO2
•
-30
gr21waters
1 Big Springs
. •
-20
•5•3
C
2 Fish HatcherySpring
3 Hot Creek@ Hot Springs
4 Hot Springs
15
eruptions,600 years ago. AMS •4C dates on 11 conifer
2{)
charcoalsin the pumiceclusterbetween600 and 700 ybp (Fig.
4). Beneaththe pumice,we foundthe remains of aquaticplants
40
-10
contentsof plants downstreamof Big Spring may be sensitive
indicatorsof magmaticCO2emissions.
We have discoveredpreservedaquatic plant material that
may have recordedthe isotopic characterof DIC in the upper
OwensRiver at an important juncture in its recent past. We
have found a pumice-filled abandonedchannel of the Owens
River -3 km. upstreamof the Hot Creek confluenceapparently
filled in a single event at the time of the Inyo Craters
0
with
roots
still
attached
to
small
river
cobbles
in
anoxic
groundwaterin the thalweg stream gravel. AMS dates of these
ß
ß
O
ß
El
plants
Big Springs
OwensRiver aboveHC
OwensRiverbelowHC
Hot Creek aboveFHS
Hot Creek below FHS
plantremains
rangefrom1100to 2040ybp.Thus,whenthey
were buried by pumice, they contained --10% dead DIC,
comparedwith -50% in the plants of this sectionof the Owens
River today. The only major inputsupstreamof this site today
are Big Spring and the Mono CratersTunnel. Based on calcium
concentrations,the total upstream contribution from dissolved
Figure 3. '4CandnC variations
in watersandplantsof Long
Valleycaldera.Widevariationin •4Cis dueto mixingof CO2 CaCO3is no morethanone-thirdof the deadDIC.
from three reservoirs(shadedtriangle), while 1513C
varies
These resultsimply that (1) magmaticCO2 was entering the
mainly due to fractionation during one or more stages of
photosynthesis(see text).
upper Owens River at the time of the 600 ybp Inyo Craters
eruptions, and (2) that modern levels there (1994-1996) are
2856
REID ET AL.: CARBON ISOTOPES, LONG VALLEY
14Cage
anomaly
due
to-50%
magmatic
Ctoday
•••::::••_:::•::-:.:
..... .-,
....................
.... _• ..... • ....................................................
modernaquaticplantsin
anomalyfrom
upper
Owens
River
CALDERA
Giggenbach,
W. F., Variationsin the chemicaland isotopiccomposition
of fluids dischargedfrom the Taupo Volcanic Zone, New Zealand,
Journalof Volcanology
and Geothermal
Research,68, 89-116, 1995.
Hainsworth,L. J., C. D. Farrar,J. R. Southron,MagmaticCO2 record in
pineneedsandgrowthringsat MammothMountain,California,Eos,
76, 688, 1995.
--10%magmatic
C
Heim, K., Hydrologicstudyof theBig Springs,Mono County,California,
unpublished
bachelor'sthesis,CaliforniaStateUniversity,Fullerton,
preserved
aquaticplantsburiedby
1992.
Hill, D. P., W. L. Ellsworth, M. J. S. Johnston,J. O. Langbein, D. H.
..................................
7t }00
6000
5000
................... #: ..... .-.**•
4000
3000
2000
1000
.....
0
measured radiocarbon age (years bp)
Figure 4. Apparentagesof modemandpreserved
streambed
plants in the upperOwensRiver along with true ages for
Oppenheimer,
A.M. Pitt,P. A. Reasenberg,M. L. Sorey,and S. R.
McNutt, The 1989 earthquakeswarmbeneathMammothMountain,
California:an initial look at the 4 May through30 Septemberactivity,
Bulletin- Seismological
Societyof America,80, 325-339, 1990.
Hilton, D. R., The heliumand carbonisotopesystematics
of a continental
geothermalsystem:resultsfrom monitoringstudiesat Long Valley
Caldera,Chemical Geology,127, 269-295, 1996.
Hoefs,J. StableIsotopeGeochemistry,Springer,Berlin, 1997.
Reid, J. B. Jr., The OwensRiver as a Tiltmeter for Long Valley Caldera,
California, TheJournalof Geology,100, p. 353-363, 1992.
Rose, T. P. and M. L. Davisson, Radiocarbon in hydrologic systems
containingdissolvedmagmaticcarbondioxide,Science,273, 1367-
conifer charcoals collected in a pumice-filled abandoned
1370, 1996.
channel of the fiver. Charcoalscluster about 600 years in age, Shevenell,L. and F. Goff, Addition of magmaticvolatiles into the hot
suggesting
the pumiceis Inyo Cratersejecta. Modemplants
contain about 50% magmaticC, five times higher than the
magmaticC contentof plantsgrowingin the fiver at the time
of the 1400 AD eruptions.
springwaters of Loowit Canyon,Mount St. Helens, Washington,
USA, Bulletinof Volcanology,55, 489-503, 1993.
Sorey,M. L., B. M. Kennedy,W. C. Evans,C. D. Farrar, and G. A.
Suemnicht,Helium isotopeand gas dischargevariationsassociated
with crustal unrestin Long Valley caldera, California, 1989-1992,
Journalof GeophysicalResearch,98, 15871-15889,1993.
Sorey,M. L., B. M. Kennedy,andL. J. Hainsworth,Carbondioxideand
helium emissions from a reservoir of magmatic gas beneath
Mammoth Mountain, California (abstract),Eos Trans. AGU, 77, F830,
five times higher than those recordedby the plant remainsat
the time of the last caldera-related eruptions.
1996.
Sorey,M. L., G. A. Suemnicht,N. C. Sturchio,andG. A. Nordquist,New
evidence on the hydrothermal system in Long Valley caldera,
California, from wells, fluid sampling,electrical geophysics,and age
determinationsof hot-springdeposits,Journal of Volcanologyand
Acknowledgments.
We thank Mike Sorey and Roy Bailey for
stimulating
discussions;
SeanPack,MicahJessup,
and Carla Christensen
Geothermal Research, 48, 229-264, 1991.
for field assistance;
and Paul Bierman,Ray Coish,Dan Dawson, Darla
Srdoc,D., I. Krajcar-Bronic,N. Horvatincic,and B. Obelic, Increase of
Heil, and AndreaLini for the use of analyticalfacilities. The work was
•4C activityof dissolvedinorganiccarbon along a river course,
supported
by a NationalScience
Foundation
grantto JBR (EAR9628115)
Radiocarbon, 28, 515-521, 1986.
anda HowardHughesMedicalInstitutegrantto HampshireCollege.
Sveinbj6rnsd6ttir,
A. E.;. E., J. Heinemeier,N. Rud, and S. J. Johnsen,
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J)B.
Reid
Jr.z,J.L.Reynolds
2,N.T.Connolly,
S.L.Getz,
P.J.Polissar
3,
and L.J. Winship, School of Natural Science, Hampshire College.
Amherst, MA, 01002.
L.J. Hainsworth4, Lawrence Livermore National Laboratories,
Livermore,CA 94551. email: [email protected]
(ReceivedNovember3, 1997; revised April 7, 1998; accepted May 20,
1998)