Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 368–377
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Palaeogeography, Palaeoclimatology, Palaeoecology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Black shale deposition in an Upper Ordovician–Silurian permanently stratified,
peri-glacial basin, southern Jordan
Howard A. Armstrong a,⁎, Geoffrey D. Abbott b, Brian R. Turner a, Issa M. Makhlouf c,
Aminu Bayawa Muhammad b, Nikolai Pedentchouk d, Henning Peters e
a
Palaeozoic Environments Group, Department of Earth Sciences, Durham University, Science Laboratories, South Road, Durham DH1 3LE, UK
School of Civil Engineering and Geosciences, Drummond Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK
Department of Earth and Environmental Sciences, Hashemite University, Zarqa, Jordan
d
Petroleum Reservoir Group (PRG), Department of Geology and Geophysics, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4
e
Research Center for Ocean Margins (RCOM), University of Bremen, Post Box 330 440, 28334 Bremen, Germany
b
c
A R T I C L E
I N F O
Article history:
Received 1 October 2007
Received in revised form 8 May 2008
Accepted 15 May 2008
Keywords:
Black shales
Silurian
Peri-glacial
Jordan
A B S T R A C T
The Lower Palaeozoic (Upper Ordovician–Silurian) succession of North Africa contains one of the world's
most prolific black shale source rocks, yet the origin of these rocks remains contentious. The black shale of
the Batra Formation in Jordan was deposited at high palaeolatitude during rapid Hirnantian to early Silurian
deglaciation. Here we report geological and organic geochemical results that provide evidence for an increase
in photic zone primary productivity during ice melting. The decay of this organic matter through oxidative
respiration resulted in euxinia, which enhanced the potential for organic matter preservation. The occurrence
of isorenieratane in all samples indicates euxinia extended from the photic zone to the sediment water
interface. The stratified basins and fjords of east Antarctica provide a likely modern analogue.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
The deposition of marine black shale and the enhanced storage of
organic carbon (OC) in the geological record indicate fundamental
changes in the functioning of biogeochemical cycles and their
feedbacks during extreme climate modes and transitions (Beckmann
et al., 2005a; Page et al., 2007). Our understanding of the response of
the marine environment during these climate states can only be
gained from a study of deep time analogues.
The Upper Ordovician–Lower Silurian succession of North Africa
and Arabia contains thick (∼ 20 m), organic carbon (OC)-rich (up to
15% total organic carbon (TOC)) black shale, widely known as the “hot
shales,” which are the source of ∼30% of the world's oil (Lüning et al.,
2000, 2006). The origin of these deposits remains contentious
(Armstrong et al., 2005, 2006). The lower hot shale overlies glacial
and glacio-marine sediments deposited during the Hirnantian glaciation (∼445 Ma) and have been linked to either, nutrient enrichment of
shallow marine environments during coastal upwelling (Lüning et al.,
2000) or, freshening by deglacial meltwater (Armstrong et al., 2005).
Here we report data from Jordan that confirms the lower hot shale
was deposited in a stratified, ice margin basin during Hirnantian to
early Silurian deglaciation (Armstrong et al., 2005). We relate deglacial
sea level rise (at Milankovitch timescales) and melt water flux to
⁎ Corresponding author.
E-mail address: [email protected] (H.A. Armstrong).
0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2008.05.005
evidence for productivity changes, anoxia/euxinia and the increased
burial of organic matter. Bulk δ13C and total organic carbon (%TOC) are
taken to reflect productivity changes. The presence of isorenieratane
(XXIII; see Appendix A for structure), a biomarker of green sulphur
bacteria, is indicative of photic zone euxinia. Likely modern analogues
are the seasonally isolated basins of east Antarctica. A similar
interdisciplinary approach is necessary to elucidate the nature of
these deposits elsewhere on the Gondwana margin.
2. Stratigraphical and geological context
The Lower Palaeozoic succession in southern Jordan includes some
750–800 m of well exposed Ordovician siliciclastic sediments deposited
on the margins of the North African (Gondwana) in terrestrial to subtidal
marginal marine and shelf environments (Amireh et al., 2001; Makhlouf,
1995; Powell et al., 1994; Fig. 1). During the Late Ordovician Jordan was
located in a high latitude, east Gondwana setting, 60° S of the equator
(Cocks and Torsvik, 2002), less than 100 km from the margins of a terrestrial ice sheet in northwest Saudi Arabia. This ice sheet was characterised by two major phases of ice advance and retreat (Vaslet, 1990)
both marked by erosional unconformities (Vaslet, 1990; Figs. 1 and 2). The
first major glacial ice incised into permafrost-hardened and glacially
loaded, Tubeiliyat shoreface and nearshore shelf deposits, preferentially
excavating NW–SE trending major fault-controlled depressions, cutting a
steep-sided U-shaped valley (Turner et al., 2005). This ice advance
correlates with the first glacial advance in northwest Saudi Arabia (Vaslet,
1990; Miller and Mansour, 2007; Fig. 3), and was followed by deglaciation,
H.A. Armstrong et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 368–377
369
Fig. 1. Lithostratigraphy and chronostratigraphy for the Ordovician and Silurian of Jordan and Saudi Arabia, showing generalised depositional environments for outcrops in the
Southern Desert region of Jordan (redrawn from Turner et al., 2005). Subdivision of the Ammar Formation into the Lower and Upper Ammar is based on Abed et al. (1993).
Fig. 2. Generalised section of the glacial and deglacial succession in the Southern Desert region of Jordan and northwest Saudi Arabia showing the stratigraphy and sediment fill of the
glacially incised palaeovalley systems. Section A is located 0.5 km southwest of Jebel Umier (29° 34′ N, 35° 53′ E) and Section B is from Jebel Ammar (29° 34′ N, 35° 52′ E). Section C is
from northwest Saudi Arabia and is based on Vaslet (1990); reproduced with permission from Turner et al. (2005).
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H.A. Armstrong et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 368–377
the base of the Batra Formation in well BG-14) contain the graptolite
Neodiplograptus apographon typical of the middle and upper subzones of
the ascensus-acuminatus biozone (sensu Storch, 1990) and indicate an
earliest Silurian age. Graptolites collected at 42.82 m (4.02 m above base
of formation) and 41.57 m (5.27 m above the base of the formation)
contain Normalograptus parvulus. N. parvulus ranges through the persculptus to acuminatus biozones of Hirnantian to Rhuddanian (Llandovery)
age (Zalasiewicz and Tunnicliff,1994, Fig. 3). These age assignments allow
the correlation of the transgressive fill of the upper palaeovalley in Jordan
with the Hirnantian to early Llandovery (Rhuddanian) global eustatic sea
level rise (Cocks and Rickards, 1988; Loydell, 1998).
Armstrong et al. (2005) concluded the base of the black shale is
coincident with the maximum flooding of the first post-glacial highstand
(cf Lüning et al., 2000). The fill of the upper palaeovalley was considered
as an “expanding puddle” as originally defined by Wignall (1991).
3. Materials and methods
Fig. 3. Palaeogeographical reconstruction of eastern Gondwana, during the late Ordovician,
showing the ice sheet (shaded) and the location of Jordan and Saudi Arabia (redrawn from
Sutcliffe et al., 2000).
a rise in relative sea level and transgressive filling of the palaeovalley. The
latter is recorded by a thin, reworked bottom lag of glaciofluvial
sandstones, overlain by thick, transgressive, shoreface sandstones. Late
transgressive filling of the palaeovalley was interrupted by a second and
possibly a third subsidiary glacial advance producing a glacially polished
and grooved surface with intersecting glacial striations, indicating ice flow
from the west and northwest (Turner et al., 2005).
The fourth glacial advance produced a regionally extensive lowstand tunnel valley beneath the ice sheet (Turner et al., 2002). This was
subsequently preserved as a palaeovalley incised into the lower
palaeovalley-fill deposits or, where this is missing, into the top of the
Tubeiliyat Formation. This ice advance correlates with the second
major ice advance in Saudi Arabia (Vaslet, 1990), where the Sarah
Formation similarly records a complex record of ice advance and
retreat (Miller and Mansour, 2007). The pattern of “major” ice advances
interspersed by smaller-scale events has been interpreted as reflecting
eccentricity and obliquity moderated ice volume changes (Sutcliffe
et al., 2000; Armstrong, 2007). Following melting of the fourth ice
sheet the upper palaeovalley filled with transgressive glaciofluvial
sandstones, marine shoreface sandstones (Turner et al., 2005) and
finally black shale of the Batra Formation (Armstrong et al., 2005).
The Batra Formation is ∼40–120 m thick, and in the Southern
Desert region of Jordan it conformably overlies the Ammar Formation
or disconformably overlies the Tubeiliyat Formation (Fig. 1). The lower
part of the formation is found in many shallow exploration wells, and
was restricted to fault-bounded graben structures (Powell et al., 1994;
Turner et al., 2005). The formation is variably graptolitic and contains
sparse thin-shelled bivalves and rare trilobites (Masri, 1988).
In the type area the lower part of the Batra Formation comprises
17.44 m of monotonous, OC-rich black shale, which in thin section
comprise laminated, black siltstone to dark grey homogeneous
claystone couplets. The parallel laminated siltstones are OC-rich, and
grade upwards into mudstone, with irregular patches of siltstone
(? starved ripples) and homogeneous claystones. The couplets were
interpreted as distal turbidites by Armstrong et al. (2005). The absence of
bioturbation indicates anoxic/euxinic bottom water during deposition
(e.g. Droser and Bottjer, 1986). The laminites, characteristic of the whole
formation, contain pyrite framboids and marcasite concretions throughout, confirming a euxinic depositional environment (Wignall, 1994).
Andrews (1991) reviewed the biostratigraphy of this formation from
surface exposures and exploration wells and concluded that the entire
formation ranged in age from Ashgill (persculptus Biozone) to the midWenlock. Graptolites from the lower part of the core have been identified
(see also Loydell, 2007). Samples from a depth of 46.62 m (close to
Here we use carbon isotopic, biomarker and Rock-Eval analyses on
black shale from the lower Batra Formation (Jordan) to establish water
column redox conditions. Core samples were obtained from the
immature (average Tmax value of 419 °C), OC-rich lower 18 m section of
the Batra Formation in the type area of Wadi Batn el Ghul (well BG14;
Table 1) from the Southern Desert region of Jordan (29°30′50.4″ N
35°57′41″ E; Armstrong et al., 2005).
3.1. Total organic carbon (TOC) and Rock-Eval pyrolysis
The TOC contents of the dried samples were measured using a
calibrated LECO CS-244 elemental analyser. Each sample was analysed in
duplicate and standard material was analysed after every 10 analytical
samples to ensure that the analyser maintained its calibration. Rock-Eval
pyrolysis was carried out using a Delsi Oil Show Analyser. Each sample
was pyrolysed in duplicate so that the mean values of the amounts of
hydrocarbons detected under the S1 and S2 peaks as well as the
temperature Tmax, corresponding to the temperature at which the
maximum of the S2 hydrocarbon generation occurs during pyrolysis,
could be measured. Standard (5.51% TOC; 0.27 mg HC/g of rock S1;
13.59 mg HC/g of rock S2; and 430 °C Tmax) and then blank samples were
pyrolysed under these same conditions to ensure that the measurements of the unknown quantities were as precise as possible. The
average standard deviations with respect to S1, S2 and Tmax were
0.05 mg HC/g of rock, 0.43 mg HC/g of rock and 2.3 °C respectively.
3.2. Bulk stable carbon isotope analysis
13 12
C/ C ratios (δ13C) were measured on bulk sediments after removal
of the inorganic carbonates with dilute HCl using automated online
combustion followed by conventional isotope ratio-mass spectrometry
in a VG TripleTrap and Optima dual-inlet mass spectrometer, with δ13C
values calculated to the Vienna Peedee belemnite (VPDB) scale using a
within-run laboratory standard (cellulose, Sigma Chemical prod. no. C6413) calibrated against NBS-19 and NBS-22. Replicate analysis of wellmixed samples indicated a precision of ±0.1‰ (1 S.D.).
3.3. Gas chromatography (GC)/gas chromatography–mass spectrometry
(GC-MS)
The powdered black shales were Soxhlet extracted with dichloromethane/methanol (93:7; v/v) for 48 h. An aliquot of each total extract
was separated by thin layer chromatography (TLC, Kieselgel 60G, 0.5 mm
thickness) using light petroleum ether (boiling point is from 40 to 60 °C)
into aliphatic hydrocarbon, aromatic hydrocarbon and polar fractions.
The aliphatic and aromatic hydrocarbon fractions were analysed using a
Hewlett-Packard HP5890 gas chromatograph (GC) equipped with a
flame ionisation detector and a fused silica capillary column
H.A. Armstrong et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 368–377
Table 1
Late Ordovician black shales from a Batra Formation (southern Jordan) borehole (BG14)
giving sample # and position in the stratigraphical column
Sample
Height above base of Batra Formation (m)
Depth in core (m)
8402-77
3402-19
3402-18
3402-17
3402-16
3402-15
3402-14
3402-13
8402-75
8402-73
8402-74
8402-72
8402-68
8402-67
8402-66
8402-64
8402-63
8402-62
8402-61
8402-60
8402-59
8402-53
8402-52
16.76
15.28
15.14
14.84
14.34
12.94
11.74
10.94
9.84
9.52
9.27
8.77
7.77
7.52
7.27
6.77
6.52
6.27
6.02
5.77
5.52
4.02
3.77
30.08
31.56
31.70
32.00
32.50
33.90
35.10
35.90
37.00
37.32
37.57
38.07
39.07
39.32
39.57
40.07
40.32
40.57
40.82
41.07
41.32
42.82
43.07
(30 m × 0.25 mm i.d.) coated with either HP-1 or HP-5 stationary phase
(film thickness of 0.25 µm). The carrier gas was hydrogen, and the oven
temperature was held at 50 °C for 2 min and then heated at a rate of 4 °C/
min to 300 °C, at which it was held for 20 min (Fig. 4).
GC-MS was performed using a Hewlett-Packard HP5890II GC
coupled with a Hewlett-Packard 5972 mass spectrometer (ionising
voltage of 70 eV and with the source temperature at 160 °C). The GC
was fitted with a fused silica capillary column (30 m × 0.25 mm i.d.)
coated with either HP-1 or HP-5 stationary phase (film thickness of
0.25 µm). The carrier gas was helium, and for analysis of the aliphatic
hydrocarbons (see Fig. 4) the oven temperature was held at 40 °C for
2 min and then heated at a rate of 4 °C/min to 300 °C at which it was
held for 20 min. The following oven temperature programme was
used for the analysis of the aromatic hydrocarbons (primarily for the
assignment of isorenieratane XXIII): the oven was held at 60 °C for
2 min and then heated to 240 °C at 10 °C/min, further heated to 315 °C
at 4 °C/min where it was held at the final temperature for 50 min.
371
Steranes and hopanes were identified using published mass spectra
and relative retention times (e.g. Peters et al., 2005). Isorenieratane
(XXIII) was identified by its mass spectrum and by GC-MS co-injection
experiments on an HP-1 stationary phase (authentic standard of
isorenieratane (XXIII) was supplied courtesy of S. Schouten, Netherlands Institute for Sea Research, Den Burg, Netherlands). The GC-MS
results from the co-injection experiments are presented in Fig. 5.
4. Results
The presence of parallel laminations accompanied by an absence
of bioturbation throughout the section indicates euxinic bottom
waters during deposition. All samples typically contain acritarchs
and graptolites (Keegan et al., 1990) indicating a primarily marine
phytoplanktonic and zooplanktonic source of Type II kerogen. The
percentage of total organic carbon (% TOC) increases as a function of
height above the base of the Batra Formation (Fig. 6A). This section
is OC-rich with a stepwise increase in %TOC from ∼ 1% to ∼ 3% and
from ∼ 3% to ∼ 9% at 6.27 m and 12.94 m respectively above the base
of the section (Armstrong et al., 2005). Figs. 6B and 7 show that the
Rock-Eval hydrogen index (HI) values of the samples have a range of
156 to 402 mg HC/g TOC (mean value = 283 mg HC/g TOC). The δ13C
of bulk organic matter show a range of − 30.8 to − 29.6‰ with
fluctuations of up to 0.4‰ and a positive shift of 1.4‰ up the section
with the largest increase (∼ 1‰) apparent in the two uppermost
samples (Fig. 6D).
The regular sterane carbon number distributions are such that C29 N C27
C27 NN C28, where the average value of the C28/C29 steranes ratio is 0.27,
which agrees with previous observations that generally this particular
ratio is less than about 0.35 for samples older than Silurian (Grantham and
Wakefield, 1988). Both the sterane and 17α-hopane distributions indicate
thermally immature organic matter and do not vary significantly
throughout the section. Maxima in the regular steranes/17α-hopanes
(Frimmel et al., 2004) occur at ∼6.5 m and 12.94 m above the base of the
formation and coincide with the stepped increases in %TOC suggesting
major contributions to the organic matter from plankton (Fig. 6C).
The mass chromatogram for m/z 133 from the aromatic hydrocarbon
fraction (Fig. 7) reveals a pseudo-homologous series of aryl isoprenoids
up to C26 (see I through to X in Fig. 7 and Table 2) as well as the presence
of aryl isoprenoids with additional aromatic rings (see XI and XVI in
Fig. 7 and Table 2). The relative amounts of the different components
remains the same throughout the profile where the C17, C20, C21, C22
Fig. 4. Total ion chromatogram (TIC) of aliphatic hydrocarbon fraction from Upper Ordovician black shale from 16.76 m (sample 8402-77) above the base of the formation in BG-14
borehole. Pr = pristane, Ph = phytane; numbers denote total number of carbon atoms in the n-alkanes.
372
H.A. Armstrong et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 368–377
Fig. 5. GC-MS ion chromatogram of m/z 133 of aromatic hydrocarbon fraction from Upper Ordovician black shale (sample 3402-15) taken at 33.9 m in the core, 12.94 m above the base
of the formation. (A) after and (B) before co-injection with an authentic standard of isorenieratane (XXIII; courtesy of S. Schouten, Netherlands Institute for Sea Research (NIOZ), Den
Burg, Netherlands) showing enhancement of peak after co-injection. (C) is mass spectrum of isorenieratane (XXIII) from the sample.
members have reduced abundances. This distribution is similar to
that observed in Upper Devonian sediments (Hartgers et al., 1994) but
differs from that in much older mid-Proterozoic bitumens (Brocks
et al., 2005). There are also less abundant C32, C33 and C40 diaryl
isoprenoids both with and without an additional aromatic ring (see
XVII through to XXIII in Fig. 7 and Table 2). Isorenieratane (XXIII) was
present throughout the core and its identity was also confirmed by
co-elution with an authentic standard on a range of stationary phases
(Sinninghe Damsté et al., 2001).
5. Discussion
The deposition of sedimentary organic carbon in the lower Batra
Formation is delimited by stepped increases (approximate doubling at
H.A. Armstrong et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 368–377
373
Fig. 6. Composite plot of bulk organic carbon, biomarker and bulk stable carbon isotopic data. (A) Total organic carbon (TOC) content of the bulk sediment. (B) Hydrogen index (HI) of
the bulk sediment (mg hydrocarbons (HC)/g TOC). (C) Steranes/17α-hopanes ratio shows its highest value at 12.94 m above the base of the Batra formation. (D) δ13C values of organic
carbon (OC) versus Vienna Peedee belemnite (VPDB) in parts per mil (‰).
each step) in %TOC up succession. %TOC has been found to correlate
well with more direct measures of photosynthetic primary productivity such as total chlorophyll-a or total steryl chlorine esters (Nara
et al., 2005) and organic mass accumulation rate (Tyson, 1995;
Vilinski and Domack, 1998; Twichell et al., 2002; Meyers and
Arnaboldi, 2005). A longer-term trend towards more fractionated
values up succession reflects a progressive increase in the sedimentation of 12C-enriched organic matter.
Hydrogen index (HI) is a measure of hydrogen-richness and depends
on the nature of the original organic matter and the degree of
preservation during diagenesis (Peters, 1986; Bordenave et al., 1993).
Millimetre-scale laminations and presence of macroscopic pyrite
through the section suggests euxinia and that organic matter preservation potential was high throughout deposition. Our bulk rock HI values
are similar to those reported from Mediterranean, Pliocene–Pleistocene
(b3 myr old) immature sapropels which have HI values of ∼400 mg HC/g
TOC (Tyson, 1995). Bulk HI and organic matter δ13C values do not covary
(Fig. 6B and D); and this with the unchanging nature of kerogen type
through the section, suggests variations in HI reflect the changing extent
of organic matter preservation (see Tyson, 1995).
Peaks in steranes/17α-hopanes ratio (N1) coincide with the stepped
changes in %TOC, that provide a coherent signal of primary productivity.
We therefore conclude the changes in %TOC reflect changes in plankton
primary productivity in the photic zone. The decay of this organic matter
through oxidative respiration resulted in anoxia and euxinia, which
enhanced the potential for organic matter preservation.
Our bulk δ13C values fall within the range for modern phytoplanktonic
algae (Schidlowski, 1988) and for bulk organic matter in the Southern
Ocean at 0 °C (Rau et al., 1989; Bentaleb and Fontugne, 1998; Bentaleb
et al., 1998; Lourey et al., 2004). Increasingly more fractionated bulk
organic matter δ13C values occur up section with no change in kerogen
type. In ice margin basins at the present day changes in δ13C are usually
associated with a decreased CO2 availability in response to (a) decreased
supply by diffusive limitation, or (b) increased demand because of higher
Fig. 7. Typical GC-MS summed mass chromatogram of m/z 133 + 134 from the aromatic hydrocarbon fraction isolated from the shale organic extract at 11.64 m above the base of the
Batra Formation. This trace shows the C40 biomarker isorenieratane (XXIII). Roman numerals refer to compounds indicated in Appendix A and Table 2.
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H.A. Armstrong et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 368–377
Table levels
2
light
and growth rates (Laws et al.,1995; McMinn et al.,1999) and/or
Details
of mass spectral
identifications
of I–XXIII
the increased
availability
of (trace)
nutrients at the time of plankton
13
growth
et al.,
1963). Molecular
Severe fractionation
of δfragment
C is known
to
Peak label(Redfield
Molecular
formula
ion(m/z)
Major
ions (m/z)
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
XIX
XIX
XX
XXI
XXII
XXIII
C13H20
C14H22
C15H24
C16H26
C17H28
C18H30
C19H32
C20H34
?
C21H36
C21H27
?
C22H29
?
?
C26H32
C32H42
C32H50
C33H42
C33H42
C40H58
C40H58
C40H66
C40H66
176
190
204
218
232
246
260
274
226
288
280
302
294
336
350
344
426
434
448
448
538
538
546
546
133/134
133/134
133/134
133/134
133/134
133/134
133/134
133/134
133/134, 147, 220
133/134, 220
133/134
133/134, 185, 253
133/134
133/134, 239, 253
133/134, 195, 239, 253
133/134, 169/170, 253
133/134, 119, 173
133/134
133/134
133/134
133/134, 237
133/134, 173
133/134, 235
133/134
occur naturally in either intense phytoplankton blooms (Dunbar and
Leventer, 1992) or within sea ice (Dunbar and Leventer, 1992; McMinn
et al.,1999). Natural values for modern Southern Ocean phytoplankton can
be as low as −25‰ (Rau et al., 1991). While highly fractionated values as
high as −12‰ have been recorded from sea ice (Dunbar and Leventer,
1992; McMinn et al., 1999). Thus when sea ice melts it not only delivers a
pulse of sediment and nutrients, but also organic matter that has a more
fractionated δ13C signature (Gibson, 1999).
A pattern of long-term covariance of more fractionated δ13C and
increasing %TOC has also been reported in Cretaceous (Late Cenomanian/Turonian) black shale in the North Atlantic, deposited during
periods of enhanced continental run-off (Beckmann et al., 2005a,b).
Here the pattern has been interpreted to indicate organic carbon
sequestration to the sediment was cumulative through the depositional event (Kuypers et al., 2002).
We therefore consider the more fractionated δ13C values through
the lower Batra section to represent CO2 limitation and increased
productivity as nutrients and isotopically light carbon were supplied
by melt water. The long-term trend towards more fractionated values
reflects a progressive increase in the sedimentation of 12C-enriched
organic matter during progressive deglaciation.
The presence of isorenieratene derivatives in black shales has been
widely considered diagnostic of green sulphur reducing bacteria
(Chlorobiaceae and Chromatiaceae) and is used as evidence for photic
zone euxinia (e.g. Sinninghe Damsté and Köster, 1998). Further, pyrite
is common through the section. In the modern ocean (Killops and
Killops, 1993) the depth of the sulphate reduction zone depends on
the amount of organic matter influx from the euphotic zone and may
be relatively shallow (b20 m) in highly productive areas, where
sulphate is rapidly depleted (Brocks et al., 2005). The depth to the
chemocline in the Batra Basin may have been shallower than the
∼ 50 m found in the Black Sea at the present day (Murray et al., 1989).
The greatest concentration of stratified water bodies in high
latitudes, and possibly the world, is found in the Vestfold Hills of
Antarctica. Here meromictic lakes, isolated marine basins and fjords
occur (Burton,1981; Burke and Burton,1988a; Gallagher et al., 1989) and
seasonal anoxia is developed in these settings. These basins formed
following the retreat of the continental ice sheet ∼ 10 000 years ago,
when isostatic rebound occurred at a faster rate than sea level rise, and
the land emerged from the sea (Burke and Burton, 1988b). Seasonal
stratification is maintained in these basins by an increase in salinity
(Gibson, 1999) resulting from brine exclusion during sea ice formation.
During the winter, a thermocline convection cell develops directly
beneath the ice cover and penetrates progressively deeper into the
basin throughout winter. At the end of the period of ice formation the
convection cell breaks down and stratification of the surface water
occurs. When the ice melts completely, lenses of relatively fresh
water cap the basins; this reduces the effect of wind mixing, with a
net result of stabilising the basin stratification, with anoxia developing at depth. The effect of, increasing water level in the basins or
decreasing maximum ice thickness during the summer results in a
shallowing of the mixocline and chemocline (Gibson, 1999). The
nature of the stratification in these basins (Gibson, 1999; McMinn
et al., 2001) is therefore similar to those reported from the many
permanently stratified basins around the world including the Red Sea
(Hartmann et al., 1998), the Cariaco Trench, the Black Sea, and fjords
of Scandinavia (Skei, 1983; Lindholm, 1996).
We envisage the geological history of the Batra Basin and its included
sediments to be directly controlled by ice margin processes. On ice
melting and retreat the exposed upper palaeovalley became a conduit for
ice meltwater. The basin was initially isolated from shallow marine
waters, likely silled and glaciofluvial sediments were deposited. As the
effects of isostatic rebound waned and sea level rose, the basin was
flushed by marine waters. On short timescales we envisage the basin
became stratified due to the formation and melting of sea ice (Fig. 8). The
millimetre-scale laminations in the black shales may reflect seasonal to
decadal (? millennial) changes in sea ice cover. On the longer term,
periods of increased surface primary productivity occurred when
prolonged ice free conditions prevailed and/or, fluxes of freshwater and
nutrients entered the already stratified, euxinic marine basin. Stratigraphical evidence (Armstrong et al., 2005) is consistent with modelling
results (Herrmann et al., 2003) and indicates the frequency of melting
events during the late Hirnantian deglaciation was likely to have occurred
on an obliquity (∼40 kyr) timescale. The black shales of the Batra Basin
record a few hundred thousand years of water column stratification and
basin euxinia during deglacial highstand. This represents one of the
oldest peri-glacial permanently stratified basins yet described.
6. Conclusions
Sedimentology, geological setting and organic geochemical
proxy data indicate the Batra Formation black shale in Jordan was
deposited in a permanently stratified, ice margin, marine basin that
existed for a few hundred thousand years. Euxinia extended into the
photic zone enhancing sedimentary carbon preservation and
sedimentation. Ice melting and/or, fluxes of freshwater and
nutrients resulted in enhanced photic zone primary productivity
Fig. 8. Conceptual model of the open water (ice free), stratified water column during the
deposition of the lower “hot shales” in the Batra Basin. This is based in part on that found
in modern ice margin basins in the Vestfold Hills, east Antarctica (Gibson, 1999, Fig. 7).
H.A. Armstrong et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 368–377
and organic matter sedimentation. The seasonally isolated basins and
anoxic fjords of east Antarctica provide a likely modern analogue,
though these are b10 000 years old. Black shale was patchily
deposited along the entire northern Gondwana margin during the
Silurian. Similarly detailed interdisciplinary studies are required to
test whether coastal upwelling can be invoked to explain the origin of
any of these deposits.
Acknowledgements
The National Resources Authority of Jordan provided access to the
core. We acknowledge support from our host institutions and the
Natural Environment Research Council for research funds including
Appendix A. Structures of isorenieratene derivatives
375
JREI awards. Dr David Loydell (University of Portsmouth) and Dr Mark
Williams (University of Leicester) kindly identified the graptolites. TOC
measurements were conducted by Dr D.M. Jones (University of
Newcastle upon Tyne) and isotopic analyses by Prof. M. Leng (NERC
Isotope Geosciences Laboratory). We thank Christine Jeans for the
preparation of the Figures and B. Bowler for technical input. H.P.
acknowledges the European Commission Research Directorates General for a Marie Curie Host Fellowship held at the University of
Newcastle upon Tyne. A.B.M. was supported by the Petroleum
Technology Development Fund, Nigeria. H.A.A. and B.R.T. acknowledge
funding from the Natural Environment Research Council. This is a
contribution to IGCP Project 503. We thank Lorenz Schwark and an
anonymous referee for their helpful suggestions.
376
H.A. Armstrong et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 368–377
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