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Chemical Geology 201 (2003) 281–293
Carbonate geochemistry across the Eocene/Oligocene boundary
of Kutch, western India: implications to oceanic O2-poor
condition and foraminiferal extinction
A. Sarkara,*, S. Sarangib, M. Ebiharac, S.K. Bhattacharyad, A.K. Raye
aDepartment of Geology and Geophysics, Indian Institute of Technology, Kharagpur, West Bengal 721 302, IndiabGeochronology Laboratory, National Geophysical Research Institute, Hyderabad, 500 007, India
cFaculty of Science, Tokyo Metropolitan University, Tokyo 192-03, JapandPhysical Research Laboratory, Ahmedabad 380 009, India
eDepartment of Geology, Presidency College, Calcutta 700 073, India
Received 31 May 2001; accepted 22 July 2003
Abstract
Major, trace, and rare-earth element (REE) analyses of larger benthic foraminifera-bearing carbonates have been carried out
across the Eocene/Oligocene boundary (EOB) of Kutch, western India. REEs of these carbonates display LREE-depleted–
HREE-enriched patterns with low average Lan/Lun, and Lan/Ybn ratio ( < 1; 0.1–0.6), SREE values (average 10.4 ppm), and
high Er/Nd ratio (mol/mol; 0.12–0.28). The data along with petrographic, Sr/Ca ratio, and Mn concentration, etc. indicate a
rather pristine character of these carbonates. Depth profiles of several chalcophile elements, e.g., Fe, Ni, Mo, Co, Cr, Zn, As, V,
and U show enrichment near the EOB, suggesting a serious oxygen-deficient (suboxic/anoxic) condition in the overlying water
column with possible subsequent modification within pore water. The enrichments are almost synchronous to f 3 jC y18Ocooling of ocean water, a positive Ce/Ce* anomaly (maximum 1.1), and authigenic precipitation of glauconites and framboidal
pyrites, all indicating slowly accumulating, sediment-starved, semiconfined, suboxic to anoxic depositional environment. The
O2-poor condition at the EOB probably developed due to a lowering of sea level and consequent cessation of open-ocean
circulation. Simultaneously, several catastrophic climatic and environmental shifts occurred across the EOB, namely, decrease
in sea surface temperature (SST), a regression and consequent O2 deficiency, and reduced phytoplankton production which
perturbed the existing life processes.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Geochemistry; Carbonate; Eocene/Oligocene boundary; India; Extinction
1. Introduction
The transition from Eocene to Oligocene period at
about 33 Ma back is characterized by large-scale
0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0009-2541(03)00238-9
* Corresponding author. Fax: +91-3222-82268.
E-mail address: [email protected] (A. Sarkar).
extinction of both terrestrial and marine fauna (mol-
lusks, foraminifera, ostracods, and nannoplanktons;
Prothero, 1994) The extinction was especially severe
in the benthic foraminiferal community (e.g., Num-
mulites) inhabiting the shallow-shelf area of ocean
during the late Eocene (Brasier, 1995). Unlike earlier
geological extinctions like those at the Cretaceous–
A. Sarkar et al. / Chemical Geology 201 (2003) 281–293282
Tertiary (K-T) or Permian–Triassic (P-Tr) boundary,
Eocene/Oligocene boundary (EOB) extinction was
stepwise in nature occurring in five pulses (Donovan,
1989; Prothero, 1994) Both extraterrestrial causes like
impact of meteorite or comet (Sanfilippo et al., 1985;
Bottomley et al., 1997) and terrestrial causes like
volcanic emission (Kennett et al., 1985) or tectonic
forcing (Raymo and Ruddiman, 1992) have been
invoked to explain the EOB extinction, but each
proposition has its own limitations. For example, the
impact theory fails to explain the stepwise nature of
the extinction. According to the terrestrial proponents,
the Ethiopian Flood basalt volcanism is a potential
candidate for the EOB extinction, but recent Ar–Ar
dating of these basalts have shown that bulk of the
eruptions took place not at the boundary but slightly
later at 30 Ma (Hoffman et al., 1997).
In view of the close connection between geo-
chemical changes in the oceans and extinction events
in the earth history as observed in the case of the K-
T event (Asaro et al., 1982; Kyte et al., 1985) or the
P-Tr event (Holser, 1997a,b; Bhandar et al., 1992), it
is worthwhile to ask if any such changes took place
across the EOB. In particular, whether an oxygen-
poor condition developed in the ocean, as in the case
of P-Tr transition (Wignall and Twitchett, 1996), can
be investigated by the use of redox-sensitive ele-
ments like U, V, Co, Mo, and Ce (Sarkar et al.,
1993; Wang et al., 1986). Here, we report a com-
prehensive geochemical investigation using stable
oxygen isotope ratio and major, trace-elements, and
rare-earth element (REE) variation across a shallow-
marine carbonate section spanning the EOB in the
Kutch region of western India and discuss its impli-
cation in terms of oceanographic and climatic
changes.
2. Geologic setting
The Tertiary sediments of the Kutch region of
Gujarat, western India, are exposed in a westerly
convex belt covering about 400 km2 area from Waga-
padhar-Ramania in the southeast to the Jamanwal
Dharmsala-Pipar in the west and Godhatad-Nareda
in the north (Fig. 1). The sediments comprise fossil-
iferous limestones, marls, clastics, and evaporites
which lie unconformably on either the Late Creta-
ceous Deccan Traps or Jurassic sediments (Sarkar et
al., 1996). The early Palaeogene section constitutes
three suites (equivalent to members, Ray et al., 1984):
Clastic Evaporite Marl (CEM), White Limestone Marl
(WLM), and Coral Limestone Marl Clastic (CLMC).
The oldest member CEM is of lower middle Eocene
to mid-middle Eocene age conformably overlain by
the WLM of Lutetian (mid-Eocene) to Priabonian
(late Eocene) age. The WLM is white to buff coloured
and highly fossiliferous. The rocks are made up of
bioclastic grains of large-size benthic foraminifers set
in a micritic matrix with occasional development of
sparitic cement. In general, the WLM is conformably
overlain by the CLMC of Oligocene age, but in
several outcrops, the boundary between the WLM
and the CLMC is accompanied by a passage bed
which is conformable to both WLM and CLMC. Most
of the Eocene species become extinct at the WLM-
CLMC contact. Interestingly, not only the diversity of
the Nummulites species becomes less but also their
size decreases across the WLM-CLMC contact which
is taken here as the EOB. The Eocene Nummulites are
characterized by large sizes and radial septal filaments
in plan view, whereas the Oligocene Nummulites are
smaller with reticulate septal filaments (Ray, 1987).
The macro- and microbiota together broadly suggest a
shallow-marine (water depth f 200 m) upper neritic
to littoral environment.
3. Materials and methods
Sampling across the EOB was carried out at
Wagapadhar section (lat. E68j45V, long. N23j31V)where it is exposed in a vertical river-cutting section.
The EOB could be identified by disappearance of
Nummulites–Discocyclina assemblage and appear-
ance of reticulate Nummulites–Heterostegina assem-
blage. Sampling interval was chosen suitably to cover
the transition region (about 10–20-cm interval for the
whole 2.5-m section) with higher resolution (f 2.5
cm) near the EOB. Geochemical analyses of bulk
carbonates were carried out employing different tech-
niques such as ICP-AES, Instrumental Neutron Acti-
vation Analysis (INAA), and ICP-MS (using model
Plasma Quad PQ 1 F1 Elemental Analyzer). This
allowed us to check the reliability of analysis in some
cases. For ICP-AES and ICP-MS, GSR-6 (carbonate)
Fig. 1. Geological map of Kutch along with sample location.
A. Sarkar et al. / Chemical Geology 201 (2003) 281–293 283
was used as the calibration standard (Balaram et al.,
1996). For INAA, about 50 mg of powdered ( < 200
mesh) sample was irradiated at the F-ring irradiation
site (nominal thermal neutron fluxf 1.5� 1012 cm� 2
s� 1 ) of theTRIGA-IIReactor at the Institute forAtomic
Energy, Rikkyo University, Japan. g-Rays, emitted by
the samples after irradiation, were measured by pure
Ge-detectors at Tokyo Metropolitan University, Japan.
Trace/REE abundance were obtained by comparing the
g-ray intensities of the samples with those of reference
standards such as JB-1 (basalt) and JG-1 (granodiorite)
supplied by the Geological Survey of Japan and ana-
lyzed by Jarosewich et al. (1987) and Ando et al.
(1989). Elemental data, their analytical methods, errors,
and detection limits are given in Table 1.
4. Petrography and diagenesis
Before we discuss the results, it is necessary to
address the issue of postdepositional alteration of
these sediments. This being a complex question, the
argument regarding diagenetic alteration depends on a
host of supporting evidences. Petrographically, the
Nummulites foraminifers at Wagapadhar do not seem
to be affected by any late diagenetic processes like
Table 1
Geochemical data of carbonates across the EOB of Kutch
Sample
number
Depth
(cm)
CaCO3 Fe V Ni Mo Zn As Cr Co U Th Sr Mn La Ce Nd Sm Eu Tb Er Yb Lu SREE Ce/Ce*
WP-35 195 94.3 nm nm nm nm nm nm nm nm nm nm 391.9 324.6 nm nm nm nm nm nm nm nm nm nm nm
WP-32 85 84.8 1.4 nm 13.8 1.5 19.2 8.2 36 5.9 2.5 0.46 382.9 232.5 2.02 5.2 nm 0.83 0.2 0.1 nm 0.5 0.2 9.07 0.89
WP-28 45 64.3 nm nm nm nm nm nm nm nm nm nm 274.6 328.4 nm nm nm nm nm nm nm nm nm nm nm
WP-27 35 68.8 5.7 132 31.6 3.9 105 27 70 13 5 1.08 nm nm 2.67 6.7 2.94 1.18 0.2 0.2 0.8 0.7 nm 11.7 0.84
WP-25 22.5 80 nm 98 nm nm nm nm nm nm nm 0.89 321.8 254.8 2.31 5.65 2.83 0.92 0.18 0.28 0.8 0.6 0.08 13.7 0.86
WP-24 20 76 5.7 nm 23.2 2.4 174 28 117 24 5.8 0.74 nm nm 2.17 6.1 2.83 1.09 0.2 0.2 nm 0.6 0.2 10.6 0.9
WP-22 15 37.3 12 nm 61.2 2.7 200 78 289 60 7.7 1.82 nm nm 2.24 6.5 1.9 0.99 0.2 0.2 nm 0.6 0.4 11.1 0.97
WP-21 12.5 20.4 nm 238 nm nm nm nm nm nm nm 1.87 211.2 303.8 1.66 4.41 1.79 0.37 0.18 0.14 0.4 0.36 0.05 9.36 1.1
WP-20 10 56.6 8.6 nm 41.2 1.5 91.1 72 141 30 8 1.33 329 235.9 1.9 5.6 1.79 0.83 0.2 0.1 nm 0.4 9.01 1.01
WP-18 0 41.8 14 141 52.4 3.5 34.1 212 82.6 20 20 1.3 nm nm 1.97 5.4 1.54 0.89 0.2 nm 0.2 0.3 0.2 8.95 0.91
WP-17 � 2.5 90.8 2.4 81.6 13.7 1.9 5.15 34 15.9 3.5 4.7 0.28 nm nm 3.68 8.9 2.75 1.36 0.3 0.2 nm 0.8 0.4 15.5 0.87
WP-16 � 5 90.7 2 nm 14.1 3.3 10.9 12 25.9 4.5 5.3 0.32 572.2 289.4 2.89 7 4.06 1.1 0.3 0.2 1 0.6 0.3 12.3 0.86
WP-14 � 10 83 2.4 nm 21.2 3.8 11.2 11 33.1 6.2 5 0.6 nm nm 4.01 8.9 1.89 1.34 0.3 0.2 nm 0.8 0.3 15.8 0.83
WP-12 � 15 85 nm 43.2 nm nm nm nm nm nm nm 0.28 663.2 215 1.73 3.6 1.89 0.54 0.1 0.13 0.4 0.36 0.05 8.8 0.79
WP-10 � 20 86.5 1.4 nm 12.3 1.6 12.1 2.9 31.6 4.4 2.4 0.47 nm nm 2.29 4.9 2 0.67 0.2 0.1 nm 0.4 0.2 8.69 0.83
WP-9 � 30 93.3 nm nm nm nm nm nm nm nm nm nm 818.8 148 nm nm nm nm nm nm nm nm nm nm nm
WP-4 � 90 84.5 1 nm 13.6 1.2 10.8 2.2 27.1 2.9 1.5 0.36 820 117.7 1.61 3 1.2 0.41 0.1 0.1 nm 0.3 0.1 5.59 0.76
WP-3 � 110 85 nm 40.9 nm nm nm nm nm nm nm 0.31 nm nm 1.3 2.3 1.2 0.2 0.06 0.11 0.2 0.23 0.05 5.65 0.79
WP-1 � 150 82.1 nm nm nm nm nm nm nm nm nm nm 758.6 237.7 nm nm nm nm nm nm nm nm nm nm nm
CaCO3 and Fe concentrations are in %, while the rest are in ppm or Ag/g.Fe (1), Cr (2), Ni (8), Co (3), Sr (3), U (7), Th (5), La (7), Ce (3), Nd (11), Sm (5), Eu (5), Tb (9), Yb (12), and Lu (13) were measured by both INAA and ICP-MS methods while V
(3), Zn (1), and Er (2) were measured by ICP-MS; As (10) and Mo (9) were measured by INAA only. Mn (2) was measured in ICP-AES. Detection limit for all the elements are < 0.2
ng/g in ICP-MS; for INAA limits are Fe: 0.1%; Cr, Ni, U, Th, Co, As, Mo: 0.1 Ag/g; Tb and Lu: 0.01 Ag/g; and other REEs: 0.1 Ag/g.Numbers in the parenthesis above are the analytical errors in %.
Analytical error for CaCO3 is f4%.
nm: not measured.
A.Sarka
ret
al./Chem
icalGeology201(2003)281–293
284
Fig. 3. Sr/Ca ratio vs. Mn plot of Wagapadhar carbonates. Note all
the samples have nearly < 300 ppm Mn plotting in nondiagenetic
sector.
A. Sarkar et al. / Chemical Geology 201 (2003) 281–293 285
sparitisation. Contact between the fossil shells and the
matrix is very sharp. Boundary samples and the
samples of early Oligocene contain green glauconite
grains and pyrites. Glauconites are pellet shaped and
mostly occur within the voids of foraminiferal cham-
bers. Grains are homogeneous without any angularity
or cracks and indicate authigenic precipitation from
seawater (Odin and Fullagar, 1988; Hesselbo and
Hugget, 2001; Fig. 2). Pyrites occur both within foram
chambers as well as matrix. Glauconites and pyrites
together indicate their authigenic precipitation at oxic/
anoxic or at least oxic/suboxic boundary under an
extremely slow sedimentation environment where
Fe3 + is transiently available as Fe2 + in solution (op.
cit.; Odin and Matter, 1981; Ireland et al., 1983). In
general, the Kutch carbonates exhibit syndepositional
marine phreatic diagenesis (Sarkar et al., 1996). No
correlation between y18O and y13C values is found for
samples from Wagapadhar (Sarangi, 2000). This is
unlikely in case of pervasive diagenesis which pro-
duces sympathetic variation between these two vari-
ables (Veizer and Hoefs, 1976). Covariation plot of Sr/
Ca ratio and Mn concentration can also help in
delineating magnitude of diagenesis due to their di-
vergent partition coefficients, association with carbon-
ate lattice, and large compositional differences in
marine and meteoric waters. Diagenetic recrystalliza-
tion of biogenic calcite results in lowering of Sr/Ca
ratio (Mead and Hodell, 1995), and if the diagenetic
Fig. 2. Authigenic glauconite infilling of foraminiferal calc
fluid is derived from meteoric sources, the Mn con-
centration is enhanced (Brand and Veizer, 1980). The
Mg content also provides a clue since the low magne-
sian calcites are more stable, and it has been shown
that they have better preservation potential for isotopic
signatures (Brand and Veizer, 1980; Holser, 1997a).
The samples fromWagapadhar have less than 300 ppm
ites of boundary samples (ppl magnification� 2.5).
A. Sarkar et al. / Chemical Geology 201 (2003) 281–293286
Mn and high Sr/Ca (Fig. 3) with the points falling
close to the field of ideal nondiagenetic low magnesian
calcite. This is also supported by the observed REE
pattern of these carbonates (see later discussion on
REE). Additionally, the 87Sr/86Sr ratios of Wagapadhar
foraminifers (0.7077–0.7079) reported by Sarkar et al.
(2003) lie close to the range expected for unaltered
marine carbonates of late Eocene/early Oligocene age,
again proving their pristine character. Based on these
set of evidences we believe that the Wagapadhar
samples are not significantly affected by alteration
and have retained the original geochemical signatures
across the EOB transition.
5. Geochemistry of elements and chemical
stratigraphy
Fe concentration of the bulk samples vary from 1%
to 14%, Sr and Mn vary from 211–820 and 117–328
ppm, respectively. Concentration ranges of most of
the chalocophiles (in ppm) are Ni: 12–61; Mo: 1.2–
3.9; Zn: 15–200; As: 2–212; Cr: 15–289; Co: 2–60;
V: 40–238. Concentrations of U (1–7) and Th (0.3–
1.8) show that U is higher than that of average marine
carbonate, while Th is close to normal.
Fig. 4 shows foraminifer y18O variation across the
EOB of the Wagapadhar along with the variation of
Fig. 4. Plot of carbonate y18O along with chalcophile elements (on CaCO3-
enrichment in almost all elements at or near the boundary coinciding with
chalcophile elements as a function of depth. The
elemental enrichments are plotted on a CaCO3-free
basis. The y18O pattern demonstrates the characteristic
enrichment across the EOB similar to the profiles
observed elsewhere (Miller et al., 1988; Zachos et al.,
1992). The sudden increase in y18O is interpreted to
mean a decrease of sea surface temperature (SST) by
about 6 jC across the EOB (Sarangi et al., 1998).
However, considering the recent SST estimate by
benthic foraminiferal Mg/Ca ratio and a high early
Oligocene ice volume effect (f 1x; Lear et al.,
2000; Sarkar et al., 2003), the cooling at Kutch was
at least f 3 jC. The y18O stratigraphy helps (along
with the fossil data) to identify the exact location of
the EOB in the column.
All the elements show distinct enrichment near the
EOB relative to the concentrations of either Eocene or
later part of Oligocene. The elements Fe, Ni, Cr, Sr,
Co, Th, and U were analyzed by both INAA and ICP-
MS, and there is good agreement between the two sets
of data. Based on the enrichment pattern of these
elements in the EOB region, one can infer that a major
change in the ocean-water chemistry took place at this
location during the EOB transition. However, it is
instructive to note that the exact locations of the peaks
vary, and as such, this can as well be interpreted by
diagenetic redistribution of elements within sediments
rather than change of water column chemistry. As the
free basis) as functions of depth across the EOB of Wagapadhar; note
y18O cooling and glauconite-rich horizon. Arrows indicate the EOB.
A. Sarkar et al. / Chemical Geology 201 (2003) 281–293 287
burial takes place, bacterially mediated oxidation of
organic carbon controls the Eh profile in the sediments
which dissolves and reprecipitates different elements
at different depths depending on specific Eh potential
of each element (Thomson et al., 1993, 1996). Studies
of redox control redistribution of elements from the
Atlantic Ocean show a depth profile of elements from
oxic through suboxic to near anoxic conditions se-
quentially as Mo, Fe–As, followed by V–Zn–U (op.
cit.). The depth concentration profiles of elements
across the EOB at Kutch show a sequence like Zn–
Co–Cr (f + 20 cm), Fe–As (f� 2.5 cm), Mo–V–
Ni–U (f� 5 cm). Clearly, the depth distribution
patterns at Kutch cannot be unequivocally interpreted
as diagenetic, although we do not completely rule out
some effect. More importantly, such diagenetic redis-
tribution generally occur in shallow sediments where
sedimentation and consequent burial rate being high,
pore-water oxygen rapidly exhausts due to organic
matter oxidation. This enhances both the response of
elements to rapidly changing redox conditions within
the sediments as well as visible separation of peaks.
As mentioned earlier, petrographically, the Kutch
EOB samples indicate that the boundary and early
Oligocene samples contain significant amount of in
situ glauconite grains as infilling of foraminiferal
chambers (Fig. 2). Such glauconitic horizons general-
ly result from slowly accumulating, sediment-starved,
semiconfined, suboxic to anoxic environment (Odin
and Fullagar, 1988; Amorosi and Centineo, 1997;
Hesselbo and Hugget, 2001). Extinction of most of
the larger benthic foraminifers and a rapid decrease in
foraminiferal y13C across the EOB of Kutch also
indicate a reduction in carbonate productivity (Sarkar
et al., 2003). Therefore, a rapid burial and develop-
ment of a strong postdepositional diagenetic front was
probably not the sole factor for the enrichment peaks
of various element near the EOB. We therefore
consider that there must have been significant change
in overlying water column chemistry as well.
Petrographic study shows presence of pyrites as
fine aggregates at or slightly above the EOB indicat-
ing authigenic precipitation. Pyrites also occur within
the foram chambers along with glauconites. In gener-
al, presence of authigenic pyrites is an indicator of
reducing depositional environment (Berner, 1984). In
oxic water, Fe is mainly present as particulate oxide
because Fe3 + is extremely insoluble, whereas in
reducing environment, iron undergoes reductive dis-
solution to Fe2 + species. If sulfide concentration in
ocean water becomes high (due to activity of anaer-
obic sulfate-reducing bacteria), formation of insoluble
iron sulfide (as fine pyrite crystal) takes place (Jacobs
et al., 1985). The Fe peak near the EOB may therefore
be due to the presence of authigenic pyrites, although
some contribution may come from glauconite grains
as well [glauconites contain about f 25% Fe in its
lattice (Jarrar et al., 2000) and modal mineralogy
indicates f 10% glauconites are present in these
carbonates]. Enrichment of other elements viz. Mo,
Ni, Zn, As, V, Cr, Co, and U might also be the result
of preferential concentration in such suboxic to anoxic
condition (Dean et al., 1997; Crusius et al., 1996;
Jacobs et al., 1985; Sarkar et al., 1993; Somayajulu et
al., 1994; Hastings et al., 1996b). For example, Mo
has been found to be considerably higher in environ-
ment where free H2S is present. In the Black Sea, H2S
concentration in water is about 400 Amol/l, and Mo is
enriched in sediments to the level of 20–40 ppm
(Crusius et al., 1996). The key to Mo removal from
anoxic seawater is a ‘‘geochemical switch’’ in which
MoO42� changes to MoS4
2� as the HS� increases and
gets easily scavenged by Fe sulfide or humic materi-
als. Similarly, in oxic seawater, U6 + is present as
uranyl tricarbonate species UO2(CO3)34� which are
highly soluble. Under oxygen-deficient/reducing con-
dition, U6 + is reduced to U4 + state which is immobile
and gets fixed onto the sediment particles (Klink-
hammer and Palmer, 1991). Indeed, higher U removal
has been observed in many modern anoxic oceanic
environments (e.g., Cariaco trench, Black Sea ; op.
cit.). High U concentration from ancient anoxic sedi-
ments deposited during the last ice age has also been
reported (Sarkar et al., 1993).
Apart from Mo and U, V can also be used as a
palaeo-redox indicator. Recent culture experiments on
foraminifers have shown that the V/Ca ratio in foram
shells are incorporated in direct proportion to that in
seawater (Hastings et al., 1996a). This, along with the
fact that the anoxic sediments also act as sink of
marine V (Breit and Wanty, 1991), makes this tracer a
potential tool to delineate redox condition in ancient
oceans. Other trace metals like Ni, Co, and Zn have
also been found to be incorporated at elevated levels
in laminated anoxic sediments (Dean et al., 1997;
Jacobs et al., 1985). Even Cr, a lithophile element,
A. Sarkar et al. / Chemical Geology 201 (2003) 281–293288
behaves like a chalcophile one under reducing condi-
tion and gets precipitated. It is to be noted that the Th/
U ratios of these samples vary between 0.1 and 0.2 all
throughout the section and are much lower than the
crustal abundance (f 4; Faure, 1976). This might
mean a restricted depositional setting where the dis-
solved oxygen was, in general, low. From an already
low-oxygen level, the basin perhaps became highly
anoxic at EOB and during the early Oligocene pro-
ducing the U spike.
It is clear from the above discussion that the
enrichment of the abovementioned elements in the
EOB carbonates can be explained by various mecha-
nisms, namely, preferential concentration in authigenic
pyrites or scavenging by organic matter. Admittedly,
the shifting of peaks around the EOB strongly indi-
cates some relocation by later diagenesis, although
their depth sequence does not allow us at the moment
to fit exactly into the scheme proposed by Thomson et
al. (1993, 1996).
6. REE geochemistry of Kutch carbonates
The REE concentrations (in ppm) of Kutch carbo-
nates vary from f 0.05 to 8.9. These values lie well
within the range of average marine carbonates (0.04–
14 ppm; Turekian and Wedepohl, 1961; Taylor and
McLennan, 1985). Fig. 5 shows North American
Fig. 5. NASC normalised REE patterns of bulk limeston
Shale Composition (NASC)-normalised REE plots
exhibiting a general enrichment of heavy REE over
the light REE. The Lan/Lun and Lan/Ybn ratios of the
samples are low (average varying from 0.1 to 0.6)
compared to marine carbonates or foraminiferal cal-
cites (Palmer, 1985). The SREE of this section is also
low (average 10.4 compared to typical marine car-
bonate value of f 28 ppm; Bellanca et al., 1997).
Since calcite lattice incorporates very small amount of
REE (Palmer, 1985) and the Kutch carbonates are
dominantly composed of foraminiferal tests probably
very little REE are contributed by the calcitic tests.
However, considerable concentration might come
from glauconite grains as well. Geochemically, glau-
conites might contain SREE from 15 to even 500
ppm. REE concentration increases with the maturity
of glauconite grains. More matured, homogeneous,
uncracked glauconites, such as those observed in
Kutch, show higher SREE (Jarrar et al., 2000).
Therefore, even with 10% modal glauconites of high
SREE can increase the SREE of bulk carbonate rock.
Other authigenic mineral that can concentrate REE
like La is biogenic apatite (Schmitz et al., 1991).
However, petrographic study did not indicate presence
of any biogenic mineral apatite in these carbonates,
thus ruling out such possibility. The LREE-depleted
and HREE-enriched patterns of these carbonates
mimic that of seawater where the LREE are preferen-
tially scavenged by the marine Mn–Fe oxyhydroxide
es. Note LREE-depleted–HREE-enriched pattern.
A. Sarkar et al. / Chemical Geology 201 (2003) 281–293 289
(occurring either as crusts along the mid-ocean ridges
or Mn nodules), thus producing a LREE-depleted
pattern which is incorporated in marine authigenic
minerals like calcites or glauconites. Along with low
Lan/Lun and Lan/Ybn and SREE, the REE patterns
indicate that they have not been significantly influ-
enced by any postdepositional alteration. The extent
of LREE–HREE fractionation can also be assessed by
comparing the Er/Nd (mol/mol) ratio with that of
typical seawater value (f 0.27), since addition of
detrital material or diagenesis reduces the value to
less than 0.1 due to preferential concentration of Nd
relative to Er (de Baar et al., 1988; German and
Elderfield, 1989; Bellanca et al., 1997). The Er/Nd
ratio of Kutch samples are high, varying from 0.12 to
0.28 (average 0.22), supporting its pristine character.
Fig. 6. (a) Ce* anomaly across the EOB of Kutch. Note positive
anomaly at the EOB. (b) Ce* vs. Nd scatter plot of Kutch carbonates
(for details, see text).
7. Ce anomaly across the EOB in Kutch
Shale-normalised REE distribution pattern of mod-
ern seawater shows a depletion of Ce relative to its
neighbouring REE (Palmer, 1985; Liu et al., 1988;
Wang et al., 1986; German and Elderfield, 1989). The
preferential removal of Ce from seawater takes place
due to oxidation of soluble Ce3 + to insoluble Ce4 + by
dissolved oxygen. Marine calcites precipitating from
such seawater reflects this pattern by having a negative
Ce anomaly. However, under reducing condition, Ce
remains in soluble + 3 state, and calcites precipitating
in this condition display a normal or even enriched
concentration of Ce. This property of Ce offers a
potential tool for deducing the redox condition of the
past oceans. We define the Ce anomaly following the
relationship given by Toyoda et al. (1990): Ce/
Ce* = 5Cen/(4Lan+ Smn) where n is the NASC-nor-
malized value of REE used. Calculated Ce/Ce* values
are given in Table 1 and plotted against depth in Fig.
6a. The Ce/Ce* shows a positive spike across the EOB
with the maximum occurring in the early Oligocene.
The presence of this positive Ce anomaly might be an
indication that a reducing condition developed in the
Kutch basin during the EOB and early Oligocene.
However, doubts have been raised about the universal
applicability of Ce anomaly to paleoredox study. Study
of Ce anomaly pattern of modern seawater from
Cariaco trench, Saanich inlet, and NW Indian Ocean
indicates that enriched Ce anomaly occurs in both
anoxic and suboxic condition. Furthermore, positive
Ce anomaly in many cases results from anoxic dia-
genesis in shallow ocean similar to the process dis-
cussed above for other trace-element diagenetic
relocation (German and Elderfield, 1989, 1990). Ear-
lier, we discussed that anoxic diagenesis, although
possible, might not have been very severe in Kutch
as the transition period under discussion experienced
slow sedimentation rate. On a Ce* anomaly–Nd
scatter plot, the oxic–anoxic boundary has been pro-
posed at Ce* anomaly value of f� 0.1 (Elderfield
and Pagett, 1986; Wright et al., 1987). Fig. 6b shows
similar plot for Kutch samples where the data cluster in
anoxic zone. Hence, the positive Ce anomaly in all
probability reflects some kind of change in overlying
water column with which sediments were interacting
over a long period of time. However, we are at present
unable to distinguish whether the condition was near
anoxic or suboxic.
A. Sarkar et al. / Chemical Geology 201 (2003) 281–293290
The suboxic to anoxic condition in the Kutch basin
during late Eocene–early Oligocene could have been
the result of high organic productivity consuming the
available dissolved oxygen or a decrease in oxygen
supply due to cutoff from open-ocean circulation. The
first possibility is ruled out based on y13C values of
carbonates which show a drop across the EOB (Sar-
angi et al., 1998; Sarkar et al., 2003), indicating a
decrease in productivity. In addition, the Sr/Ca ratio of
Nummulites tests (an indicator of calcification or bio-
productivity rate) at the EOB show major reduction
(Sarangi et al., 2001), implying lowered phytoplankton
and, consequently, foraminiferal shell production
(larger benthic foraminifers are mostly algal sym-
bionts). Therefore, a reduced open-ocean circulation
seems to be the probable candidate for this O2-poor
condition.
8. Implications
Depth profiles of major, trace, and rare-earth ele-
ments across the EOB region of Kutch indicate inter-
esting geochemical changes in the basin and dramatic
climate deterioration (ocean cooling and anoxia). En-
richment of several key elements having chalcophile
character near the EOB suggests establishment of
serious oxygen-deficient (suboxic/anoxic) condition
in the overlying water column with possible subse-
quent modification within pore water. The y18O-basedSST estimation indicates that at Kutch, the warm
Eocene SST of 30 jC decreased to 27 jC during the
early Oligocene. The warm Eocene climate enhanced
the net organic carbon productivity which reached its
acme during the latest Eocene (Sarkar et al., 2003). The
larger benthic foraminifers attained a diversity and
large size due to this prolific food supply which
decreased across the EOB and during the early Oligo-
cene (Sarangi et al., 2001). Since, in general, larger
benthic foraminiferas grow at temperatures above 25
jC summer isotherm and reproduction of Nummulites,
in particular, radically decreases at temperatures below
20 jC (Adams et al., 1990), it is possible that they were
immediately affected by the global cooling of SST
across the EOB. The environment became stressed, and
since larger benthic foraminifers are highly sensitive to
temperature range/variation, they possibly faced ex-
tinction. Globally, f 60% of the genera of larger
benthic foraminiferas suffered extinction at the EOB;
in addition, the rate of extinction of the larger benthics
is highest across the EOB in the entire Palaeogene
(Brasier, 1995; Saraswati, 1997). The present study
indicates that this cooling was coincident with an
apparent O2-poor condition in the water column pos-
sibly caused by a marine regression and lack of open-
ocean circulation. These eventually caused precipita-
tion of chalcophile elements near the EOB and positive
Ce anomaly in ocean-water column. Global sea level
variation (third-order cycles) during the Paleogene, as
compiled by Haq et al. (1987), also indicates f 50-m
fall in sea level across the EOB. The causal link of sea
level drop and extinction is not clearly known. Calcu-
lation of evolutionarymatrices of Eocene larger benthic
foraminiferas (diversity/extinction) and their compari-
son with the global sea level change does not indicate
one-to-one correspondence (Saraswati, 1997). Never-
theless, sea level can indirectly trigger extinction, for
example, by introducing local O2 deficiency as has
been observed in Kutch. Our study across the EOB of
Kutch indicates that simultaneously, several cata-
strophic climatic and environmental shifts occurred.
This means a complex, terrestrially induced multicaus-
al biogeochemical scenario could be responsible for the
terminal Eocene extinction (of at least the shelf com-
munities), a view also proposed for the catastrophic K-
T extinction about 65 million years ago (Birkelund and
Hakansson, 1982; Sarkar et al., 1992).
9. Conclusions
Major, trace elements, and REEs of the larger
benthic foraminifera-bearing carbonates have been
studied across the EOB section of Kutch, western
India. The salient conclusions of the study are listed
below.
1. Petrography and geochemical tracers, namely, Sr/
Ca ratio, Mn concentration, etc. indicate a rather
pristine character of these carbonates. NASC-
normalised REE values of these carbonates display
LREE-depleted–HREE-enriched patterns with low
average Lan/Lun and Lan/Ybn ratio ( < 1; 0.1 –0.6),
SREE values (average 10.4 ppm), and high Er/Nd
ratio (mol/mol; 0. 12–0.28), supporting the above
contention.
A. Sarkar et al. / Chemical Geology 201 (2003) 281–293 291
2. Depth profiles of several key elements having
chalcophile character, e.g., Fe, Ni, Mo, Co, Cr, Zn,
As, V, and U show enrichment near the EOB,
suggesting a serious oxygen-deficient (suboxic/
anoxic) condition in the overlying water column
with possible subsequent modification within pore
water. The enrichments are almost synchronous to
f 3 jC y18O cooling of ocean water, a positive Ce/
Ce* anomaly (maximum 1.1), and authigenic
precipitation of glauconites and framboidal pyrites,
all indicating slowly accumulating, sediment-
starved, semiconfined, suboxic to anoxic deposi-
tional environment. The O2-poor condition at the
EOB can be best explained by a lowering of sea
level and consequent cessation of open-ocean
circulation.
3. Simultaneously, several catastrophic climatic and
environmental shifts occurred across the EOB,
namely, decrease in SST, a regression and conse-
quent O2 deficiency, and reduced phytoplankton
production which perturbed the existing life
processes. The data generated in the present study
demand a complex terrestrially induced multicausal
biogeochemical scenario for the terminal Eocene
extinction.
Acknowledgements
We thank Department of Science and Technology,
New Delhi, for financial assistance in the form of a
research project to AS. SS thanks CSIR, New Delhi,
for granting a research fellowship. [LW]
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