Carbonate geochemistry across the Eocene/Oligocene boundary of Kutch, western India: implications to...

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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.


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).


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.


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,


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.


(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.


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.


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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