a major change in monsoon-driven productivity in the tropical indian ocean during ca 1.2-1.0 myr:...

8/9/2019 A major change in monsoon-driven productivity in the tropical Indian Ocean during ca 1.2-1.0 Myr: Foraminiferal fa

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A major change in monsoon-driven productivity in the tropical Indian Ocean

during ca 1.20.9 Myr: Foraminiferal faunal and stable isotope data

Anil K. Gupta , M. Sundar Raj, K. Mohan, Soma De

Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur721 302, India

Received 16 July 2007; received in revised form 4 January 2008; accepted 7 January 2008

Abstract

Tropical climate is variable on astronomical time scale, driving changes in surface and deep-sea fauna during the PliocenePleistocene. To

understand these changes in the tropical Indian Ocean over the past 2.36 Myr, we quantitatively analyzed deep-sea benthic foraminifera and

selected planktic foraminifera from N125 m size fraction from Deep Sea Drilling Project Site 219. The data from Site 219 was combined with

published foraminiferal and isotope data from Site 214, eastern Indian Ocean to determine the nature of changes. Factor and cluster analyses of the

28 highest-ranked species distinguished four biofacies, characterizing distinct deep-sea environmental settings. These biofacies have been named

after their most dominant species such as Stilostomella lepidula Pleurostomella alternans (SlPa), Nuttallides umboniferGlobocassidulina

subglobosa (NuGs), Oridorsalis umbonatusGavelinopsis lobatulus (OuGl) and Epistominella exiguaUvigerina hispido-costata (EeUh)

biofacies. Biofacies SlPa ranges from ~2.36 to 0.55 Myr, biofacies NuGs ranges from ~1.9 to 0.65 Myr, biofacies OuGl ranges from ~1 to

0.35 Myr and biofacies EeUh ranges from 1.1 to 0.25 Myr. The proxy record indicates fluctuating tropical environmental conditions such as

oxygenation, surface productivity and organic food supply. These changes appear to have been driven by changes in monsoonal wind intensity

related to glacialinterglacial cycles. A shift at ~1.20.9 Myr is observed in both the faunal and isotope records at Site 219, indicating a major

increase in monsoon-induced productivity. This coincides with increased amplitude of glacial cycles, which appear to have influenced low latitude

monsoonal climate as well as deep-sea conditions in the tropical Indian Ocean. 2008 Elsevier B.V. All rights reserved.

Keywords: DSDP 214 and 219; Tropical Indian Ocean; Faunal and isotope data

1. Introduction

Earth's climate has evolved from a state of relative warmth

during the mid-Pliocene, with major ice sheets restricted to

Antarctica, to a cooler world during the late Pleistocene markedwith extensive bipolar ice sheets and increased pole to equator

temperature gradients (Raymo, 1997; Ravelo et al., 2004).

These changes are believed to have been driven by changes in

Earth's orbital parameters with a switch from 41-Kyr mode to a

100-Kyr rhythm after 900 Kyr BP (Raymo, 1997; Lisiecki and

Raymo, 2005). Changes in polar ice volume brought significant

changes in the tropics (Sirocko et al., 1999; Gupta et al., 2001).

This climatic transition is often known as the Mid-Pleistocene

Transition (MPT) (e.g., Raymo et al., 1997) or Mid-Pleistocene

Revolution (MPR) (e.g., Hayward, 2001; Xu et al., 2005), and is

well-documented in numerous proxy records (Mix et al., 1995;

Raymo et al., 1997; Gupta et al., 2001; Hayward, 2001; Guptaand Dhingra, 2004; Xu et al., 2005). At ~900 Kyr BP,

the mixed-layer thickness reduced and thermocline shoaled in

the eastern Indian Ocean, and a 100-Kyr component began to

dominate in the Indian monsoon variability (Gupta et al., 2001;

Gupta and Dhingra, 2004). Elongated benthic foraminifera

(including Stilostomella lepidula) suffered widespread extinc-

tion across this climatic transition (Weinholz and Lutze, 1989;

Gupta, 1993; Hayward, 2001; Kawagata et al., 2005, 2006).

Numerous studies have recently been undertaken to under-

stand paleoclimatic and paleoceanographic evolution of the

Indian Ocean during the Plio-Pleistocene and the Holocene

Available online at www.sciencedirect.com

Palaeogeography, Palaeoclimatology, Palaeoecology 261 (2008) 234245www.elsevier.com/locate/palaeo

Corresponding author. Tel.: +91 3222 283368; fax: +91 3222 282700.

E-mail address: [email protected] (A.K. Gupta).

0031-0182/$ - see front matter 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.palaeo.2008.01.012
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using faunal and geochemical data (Gupta and Srinivasan,

1990; Kroon et al., 1991; Gupta et al., 2003). Most of thesestudies have focused on high-productivity areas of the Arabian

Sea such as the northwestern and northeastern Arabian Sea,

where surface productivity changes are driven by seasonal

reversals in the monsoon winds (e.g., Reichart et al., 1998; Jung

et al., 2001; Schmiedl and Leuschner, 2005; Schmiedl and

Mackensen, 2006). The southeastern Arabian Sea is also an

important region where low salinity surface currents from the

Bay of Bengal mix with those of the Arabian Sea.

Changes in organic flux to the seafloor due to variations in

surface productivity modulate deep-sea faunal composition

(e.g. Jorissen et al., 1995; Gupta and Thomas, 1999). The

amount of organic flux to the seafloor not only depends onsurface production but also on the nature of deep-sea column.

Well-oxygenated deep-sea circulation may cause reminera-

lization of organic carbon resulting in little organic material

reaching the seafloor (Schmiedl and Mackensen, 2006). To

understand if the changes in the surface and deep-water column

of the tropical Indian Ocean were driven by the Indian Ocean

climate (monsoon) and deep-sea circulation, we analyzed a

2.36 Myr record of deep-sea benthic foraminifera from Deep

Sea Drilling Project (DSDP) Site 219 from the southeastern

Arabian Sea and combined it with published planktic and

benthic records from Site 214, eastern Indian Ocean (Fig. 1).

The sediment accumulation rate is low to moderate at Sites 214

(0.91 cm/Kyr) and 219 (1.03 cm/Kyr) (Fig. 2).

2. Site location: materials and methods

DSDP Site 214 is located in the eastern Indian Ocean on the

Ninetyeast Ridge (11o20.21S, 88o43.08E; present water depth

1665 m). This site is presently located beneath a hydrological

Fig. 1. Locationmap of Deep Sea Drilling Program (DSDP)Sites 214 and219 in the tropical Indian Ocean.Also shown are deep-ocean currents (You, 2000). AABW =

Antarctic Bottom Water, NADW = North Atlantic Deep Water, CPDW = Circumpolar Deep Water.

Fig. 2. Numerical age versus depth at Site 219 (Gupta and Thomas, 1999).

235A.K. Gupta et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 261 (2008) 234245


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front in the core of the Indonesian Throughflow (Du et al.,2005), and is under the influence of the South Equatorial

Current (SEC), which intensifies during northern summer when

southeast trade winds are stronger and Indian summer monsoon

begins to strengthen (Fig. 1; Tchernia, 1980). Site 214 is bathed

by the North Indian Deep Water (NIDW) with a dissolved

oxygen content of ~5.8 ml/L (GEOSECS, 1983), which is a

mixture of Antarctic Bottom Water (AABW), North Atlantic

Deep Water (NADW) and deep water of the northern Indian

Ocean origin (Tchernia, 1980). DSDP Site 219 was drilled

during Leg 23 on the crest of the Laccadive-Chagos Ridge,

southeastern Arabian Sea (901.75N, 7252.67E; water depth

1764 m). At present, this site is bathed by the NIDW withdissolved oxygen content of ~4.8 ml/L (GEOSECS, 1983), and

lies within an area where surface production is higher due to

monsoon-induced upwelling (Gupta and Thomas, 1999;

Shankar et al., 2002).

Seventy one core samples of 10 cm3 volume from Site 219

were analyzed from a sediment sequence extending back in time

to ~2.36 Myr. Samples were soaked in water with baking soda

for 812 h, and washed over a 63 m size sieve. The washed

samples were dried in an electric oven at ~50 C and transferredinto glass vials. Hard sediment samples were treated with 23

drops of 2% hydrogen peroxide. We applied the age model for

Site 219 based on planktic foraminifer datums suggested by

Gupta and Thomas (1999). The sediment accumulation rate at

Site 219 shows a major increase at ~1.6 Myr and thereafter is

constant (Fig. 2). The interpolated numerical ages are updated to

the Berggren et al. (1995) time scale, with an average time

resolution of 33 Kyr per sample.

We generated benthic foraminiferal census data from an

aliquot of ~300 specimens in the N125 m size fraction (census

data are made available online as Table 1s). This size fraction

allows us to compare our results with recent results from theAtlantic and Indian Oceans (Mackensen et al., 1993, 1995;

Schmiedl et al., 1997; Gupta and Thomas, 2003; Gupta et al.,

2004). Factor and cluster analyses were performed on relative

abundance data of the 28 highest-ranked species (Table 1;

supplementary Table 1s), which are present in at least three

samples with a percentage of 2% or more in at least one sample.

The analysis performs standardization of the data and calcula-

tion of covariance matrix that helps remove noise and reduce the

Table 1

VARIMAX rotated factor scores of eight significant factors at DSDP Site 219

Species name Factor1 Factor2 Factor3 Factor4 Factor5 Factor6 Factor7 Factor8

Astrononian umbilicatulum 0.27678 0.54435 0.21522 0.12411 0.24152 0.09983 0.21964 0.03865

Bolivina pseudoplicata 0.09604 0.63124 0.14031 0.28713 0.33582 0.01515 0.26011 0.1557

Bolivina pusilla 0.18667 0.1082 0.06034 0.35088d 0.54077 0.29692 0.10807 0.06134

Bulimina aculeata 0.62835 0.34862 0.16157

0.0122

0.0527

0.00905 0.07341

0.38974c

Bulimina alazanensis 0.14442 0.2046 0.16591 0.12433 0.33426 0.43545 0.50979 0.03206

Bulimina striata 0.68119 0.10787 0.29073 0.03259 0.21666 0.30203 0.00952 0.2498

Cassidulina carinata 0.40159a 0.67055 0.24401 0.34092 0.07719 0.12485 0.03409 0.05668

Cibicides bradyi 0.41532 0.50645 0.13069 0.18526 0.17863 0.15353 0.27073 0.0007

Cibicides wuellerstorfi 0.63842 0.02374 0.28183 0.08202 0.02421 0.22133 0.30961 0.03629

Discopulvinulina bertheloti 0.19432 0.73936 0.32118 0.35622 0.14037 0.19348 0.01261 0.05632

Eggerella bradyi 0.5745 0.06103 0.14104 0.29633 0.08572 0.14829 0.04954 0.48948

Ehrenbergina carinata 0.45695a 0.27574 0.1506 0.19893 0.14098 0.17699 0.26047 0.27035

Epistominella exigua 0.29132 0.06991 0.08212 0.48336d 0.59049 0.0765 0.14786 0.0468

Evolvocassidulina bradyii 0.46502 0.31869 0.27943 0.06257 0.00804 0.36928 0.07362 0.03243

Gavelinopsis lobatulus 0.37221a 0.26279 0.29207 0.12792 0.34095 0.10859 0.15582 0.40186c

Globocassidulina pacifica 0.56338 0.2219 0.07875 0.43029 0.02424 0.10288 0.25646 0.37342

Globocassidulina subglobosa 0.08172 0.0121 0.65278b 0.33966 0.2013 0.31311 0.01501 0.04683

Gyroidinoides cibaoensis 0.47499 0.20755 0.4199b 0.00346 0.02495 0.29373 0.12424 0.15028

Hoglandulina elegans 0.51997

0.42207 0.01022 0.47281 0.24827 0.07963 0.06066

0.18306 Melonis barleeanum 0.32735 0.03988 0.66648 0.24837 0.11669 0.13728 0.15883 0.09138

Nuttallides umbonifer 0.02991 0.06983 0.68766b 0.14073 0.20614 0.07212 0.05179 0.13001

Oridorsalis umbonatus 0.03694 0.08683 0.41545b 0.28318 0.23448 0.25462 0.05876 0.51602c

Osangularia culter 0.19279 0.47841 0.10761 0.14219 0.06176 0.07296 0.60514 0.16739

Pleurostomella alternans 0.66316a 0.0648 0.07837 0.06686 0.05568 0.18797 0.01202 0.16325

Pullenia bulloides 0.04405 0. 28612 0.01745 0.09139 0.28655 0.68298 0.3178 0.08143

Stilostomella lepidula 0.76712a 0.03078 0.14501 0.22505 0.18706 0.14994 0.2423 0.0025

Uvigerina hispido-costata 0.60918 0.27717 0.07434 0.38523d 0.17286 0.07584 0.17436 0.09885

Uvigerina proboscidea 0.57463a 0.2155 0.28727b 0.2008 0.4666 0.02655 0.15959 0.15555

% variance 27.73 15.18 11.50 8.44 7.66 6.0 5.28 5.02

aBiofacies SlPa, bBiofacies NuGs, cBiofacies OuGl, dBiofacies EeUh.

Fig. 3. Dendogram based on Q-mode cluster analysis of 71 samples from DSDP Site 219 using Ward's Minimum Variance method. Four clusters have been

identified on the basis of the number of clusters versus semi-partial R2. Each cluster corresponds to a biofacies named after the most dominant species within each

cluster.

236 A.K. Gupta et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 261 (2008) 234245


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data to more meaningful variables. The R-mode factor analysis

identifies the species associations and Q-mode cluster analysis

discerns the sample groups.

R-mode Principal Component Analysis (PCA) was performed

on the correlation matrix followed by orthogonal VARIMAX

rotation using the SAS software package (SAS Inc., 1988). Basedon a scree plot (xy plot) of eigenvalues versus the number of

factors and screening of the VARIMAX factor scores and species

associations, 8 factors were considered that account for 86.8% of

the total variance (Table 1). Factors that do not show relevant

species associations due to their low factor scores and rare spe-

cies occurrence, were not considered to constitute assemblages.

Q-mode cluster analysis using Ward's Minimum Variance meth-

od was run on a covariance matrix to identify sample groups

belonging to each biofacies (Fig. 3). Four clusters representing

four biofacies were identified on the basis of a plot of semi-partial

R2 values versus the number of clusters. We used information on

environments of modern benthic foraminifera (Table 2) to

interpret different biofacies at Site 219 (Table 3, Figs. 4 and 5).

We calculated Globigerina bulloides percentages from Site

219 and combined them with published percentU. proboscidea,

Globigerinoides sacculifer, Globorotalia menardii (complex)

and Gs. sacculifer stable isotope data from eastern Indian

Ocean Site 214 (Gupta and Dhingra, 2004; Fig. 7). G. bulloides

is a well tested monsoon proxy used in several paleomonsoonalstudies (Kroon et al., 1991; Overpeck et al., 1996; Gupta et al.,

2003).

3. Results and environmental preferences of benthic biofacies

Multivariate analysis was performed on reduced data of

benthic foraminifera from Site 219 to reduce noise from the data

set, which is induced by postmortem taphonomic changes.

The analysis involves standardization of the data and calcula-

tion of covariance matrix that reduces the aliasing. Given below

are details of each biofacies and their inferred environments

(Table 3). Each biofacies is named after the most dominant

species present (Table 4).

Table 2

Environmental preferences of characteristic benthic foraminiferal species defining different biofacies at Site 219

Species Interpretation

B. pusilla High organic carbon flux (Thomas and Gooday, 1996; Gupta and Satapathy, 2000)

B. aculeata No relation with food flux (Hermelin and Shimmield, 1990)

Calm depositional regimes with clayey and organic-rich sediments ( Mackensen et al., 1993)

Interglacial, high productivity (Almogi-Labin et al., 2000)C. carinata High food supply (Gupta and Thomas, 1999)

C. wuellerstorfi Raised epibenthic, suspension feeder, high energy (Lutze and Thiel, 1989)

Oligotrophic (Linke and Lutze, 1993)

Seasonal food supply (Loubere and Fariduddin, 1999)

AABW (Corliss, 1979, 1983)

E. carinata High organic food (Gupta and Satapathy, 2000)

Low-oxygen, high organic carbon (Nomura, 1991)

E. exigua Epibenthic, cosmopolitan, abyssal, opportunistic, phytodetritus feeder (Gooday, 1988; Gooday and Turley, 1990)

Seasonal food fluxes (Smart et al., 1994; Loubere and Fariduddin, 1999)

G. lobatulus Pulsed food supply (Gooday,1994)

Cool waters, partially pulsed food supply (Gupta and Thomas, 1999)

G. subglobosa Warmer AABW (Corliss, 1979)

Circumpolar Deep Water (Schnitker, 1980)

Phytodetritus feeder (Gooday, 1994)

Oligotrophic (Mackensen et al., 1995)Low-productivity associated with NADW (Fariduddin and Loubere, 1997)

Strong bottom currents, elevated, oligotrophic (Mackensen et al., 1995; Schmiedl et al., 1997)

G. cibaoensis Low-oxygen (Gupta and Thomas, 1999)

N. umbonifera AABW (Streeter 1973)

Corrosive bottom water (Bremer & Lohmann, 1982)

Oligotrophic (Gooday, 1994)

O. umbonatus AABW (Corliss, 1979)

High carbonate saturation, motile (Miao and Thunell, 1993)

High oxygen, low organic carbon (Mackensen et al., 1995)

Cosmopolitan, both oligotrophic and eutrophic (Schmiedl 1995; Schmiedl and Mackensen, 1997)

P. alternans Abyssal species, characteristic of variable organic flux (Gupta, 1994)

S. lepidula Low-oxygen, organic rich (Boersma, 1990)

High productivity (Gupta, 1993)

Low productivity (Kaiho, 1999)

U. hispido-costata High organic carbon (Altenbach and Sarnthein, 1989; Rathburn and Corliss, 1994)U. proboscidea Shallow infaunal, high organic carbon, variable oxygenation ( Lutze and Coulbourn, 1984)

High, year-round productivity (Gupta and Srinivasan, 1992; Gupta, 1997; Jannink et al., 1998; Gupta and Thomas, 1999;

Gupta et al., 2001, 2004; Murgese and De Deckker, 2005)

Low seasonality (Ohkushi et al., 2000)



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3.1. Biofacies Stilostomella lepidula Pleurostomella alternans

(SlPa)

BiofaciesSlPa is definedby species with a high negative score

on Factor 1, in the ~2.36 to 0.55 Myr interval ( Figs. 4 and 5). The

characteristic species of this biofacies are Stilostomella lepidula,

Pleurostomella alternans, Uvigerina proboscidea, Ehrenbergina

carinata, Cassidulina carinata and Gavelinopsis lobatulus,

indicating high and sustained organic flux (Tables 2 and 3).

3.2. Biofacies Nuttallides umboniferGlobocassidulina

subglobosa (NuGs)

Characteristic species of this biofacies are Nuttallides

umbonifer, Globocassidulina subglobosa, Gyroidinoides

cibaoensis and Oridorsalis umbonatus, with Factor 3 negative

scores. This biofacies is present from ~1.9 to 0.65 Myr,

indicating low and variable flux of organic matter (oligotrophic),

high seasonality, well-oxygenated and cold deep waters (Figs. 4

and 5, Tables 2 and 3). It has been suggested by Gupta et al.

(2006) that N. umbonifer is out-competed by species of Buli-

mina and Uvigerina when the food supply is high and sustained,unless the waters become corrosive to CaCO3. Under conditions

of relatively low, seasonal flux of food N. umbonifer may be

swamped by the opportunistic species such as E. exigua. Nut-

tallides umbonifer thus dominates those regions where other

calcareous benthic foraminiferal taxa cannot grow optimally,

either as a result of very low food supply (extreme oligotrophy),

or a high carbonate corrosivity (Gupta et al., 2006).

3.3. Biofacies Oridorsalis umbonatusGavelinopsis lobatulus

(OuGl)

This biofacies is defined by species with negative Factor 8

scores, across the ~1 to 0.35 Myr interval (Figs. 4 and 5). The

characteristic species of this biofacies are O. umbonatus,

G. lobatulus and Bulimina aculeata, indicating low to interme-

diate organic carbon flux and high seasonality (Tables 2 and 3).

3.4. Biofacies Epistominella exiguaUvigerina hispido-costata

(EeUh)

Epistominella exigua, Uvigerina hispido-costata and Boli-

vina pusilla are the characteristic species of biofacies EeUh,

with high negative Factor 4 scores (Figs. 4 and 5; Tables 1

and 3). This biofacies is present from ~1.1 to 0.25 Myr and is

suggestive of intermediate to high organic flux, high seasonality

and well-oxygenated conditions (Tables 2 and 3).

4. Discussion and conclusions

Deep-sea benthic foraminifera have been used to understand

changes in deep-water conditions driven by climate forcing

during the Pliocene and Pleistocene (Corliss et al., 1986;Thomas et al., 1995; Schmiedl and Mackensen, 1997; Gupta

et al., 2001). Several studies have shown the relationship be-

tween benthic faunal composition, productivity of the overlying

waters and organic flux to the seafloor (Lutze and Coulbourn,

1984; Miao and Thunell, 1993; Smart et al., 1994; Thomas and

Gooday, 1996; Gupta et al., 2004; Smart et al., 2007). Others

suggested oxygen and food supply are the main factors con-

trolling the spatial and in-sediment distribution of benthic fora-

minifera (Hermelin and Shimmield, 1990; Bernhard, 1992;

Jorissen et al., 1995; Schmiedl et al., 1997; Gupta and Thomas,

1999; den Dulk et al., 2000). This group explains seasonal

fluctuations in primary production (Thomas et al., 1995;

Thomas and Gooday, 1996; Schmiedl et al., 1997, 2000; denDulk et al., 2000; Wollenburg and Kuhnt, 2000; Gupta and

Thomas, 2003). Gooday (1994), Thomas et al. (1995) and

Thomas and Gooday (1996) classified phytodetritus species

(like E. exigua) and mobile, infaunal species (like species of

Melonis and Uvigerina) as indicators of predominantly sea-

sonally pulsed versus continuous primary production. Thus

benthic foraminifera are considered useful for estimating paleo-

fluxes in high-productivity areas and they are also more resis-

tant to diagenetic change compared to planktic foraminifera.

However, in oligotrophic areas deep-sea oxygenation plays an

important role in controlling benthic foraminifera over variable

time scales. Gupta and Thomas (1999) suggested that changesin oxygenation are linked partially to productivity and partially

to changes in deep-water ventilation.

Wind driven coastal and open-ocean surface productivity

influences organic carbon flux and oxygenation of deep waters

controlling benthic populations in the Arabian Sea. In the

northern part of the Indian Ocean, the wind regimes follow

seasonal change in circulation producing widespread upwelling

controlling surface productivity near Site 219 (Fig. 1). The in

situ (050 m) primary production above Sites 214 and 219 is

b20 mg C m3 h1 during JuneSeptember, when the summer

monsoon and trade winds are stronger. This is less than one

fifth that of the northwestern Arabian Sea primary production

(Krey, 1973). The primary production reduces significantly to

Table 3

Benthic foraminiferal biofacies and their interpreted environments at DSDP Site

219, southeastern Arabian Sea

Biofacies Environment

SlPa (Factor 1 negative scores) High and sustained organic flux

Stilostomella lepidula (0.76712)

Pleurostomella alternans (

0.66316)Uvigerina proboscidea (0.57463)

Ehrenbergina carinata (0.45695)

Cassidulina carinata (0.40159)

Gavelinopsis lobatulus (0.37221)

NuGs (Factor 3 negative scores) Low and variable flux of

organic matter, high seasonality,

well-oxygenated and cold

deep waters

Nuttallides umbonifer(0.68766)

Globocassidulina subglobosa (0.65278)

Gyroidinoides cibaoensis (0.4199)

Oridorsalis umbonatus (0.41545)

OuGl (Factor 8 negative scores) Low to intermediate organic

carbon flux, high seasonalityOridorsalis umbonatus (0.51602)

Gavelinopsis lobatulus (0.40186)

Bulimina aculeata (0.38974)

EeUh (Factor 4 negative scores) Intermediate to high organic flux,

high seasonality, well-oxygenatedconditions

Epistominella exigua (

0.48336)Uvigerina hispido-costata (0.38523)

Bolivina pusilla (0.35088)



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b0.50 mg C m3 h1 above Site 219 and b0.10 mg C m3 h1

above Site 214 during DecemberMarch when variable and

weak winter monsoon winds blow across the Indian Ocean

(Krey, 1973; Tchernia, 1980; Schott and McCreary, 2001).

Because of the location of Sites 214 and 219 in oligotrophic

areas with higher dissolved oxygen content during the studied

interval, the changes in benthic foraminiferal population might

have been linked to the supply of organic food.

At Sites 214 and 219, the faunal and isotope data show a shift

during ~1.20.9 Myr very close to the Mid-Pleistocene

Transition (MPT), coinciding with the transition from 41-Kyr

to 100-Kyr periodicities, and an increased intensity of ice age

cycles in the Northern Hemisphere (Raymo et al., 1997). This

also corresponds to the Stilostomella extinction observed in

different ocean basins (Gupta, 1993; Gupta et al., 2001;

Hayward, 2001; Kawagata et al., 2005, 2006). During this

time, the East Asian monsoon system strengthened and contrast

between the summer and winter monsoon wind regime

increased (Xiao and An, 1999). Thus variability in the East

Asian monsoon increased (Qiang et al., 2001).

At Site 219, U. proboscidea was a dominant benthic species

from 2.36 to 1.2 Myr and G. bulloides was rare (Fig. 7). The

Fig. 4. Percent distribution of benthic foraminiferal species which are most dominant in different biofacies (panels a d) at DSDP Site 219. Each colour shade

represents a particular species as given at the bottom of the figure. Open arrows mark the shift in the foraminiferal faunal and stable isotope record.



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benthic biofaces SlPa and NuGs alternated during this time

(Fig. 6). This indicates that the surface productivity was low

(rare G. bulloides) above Site 219 during this time but there was

an increased lateral advection of nutrient-rich deep water

(NIDW) from the oxygen minimum zone (OMZ) of the

northwestern Arabian Sea to the southeastern Arabian Sea.

Dickens and Owen (1999) suggested lateral expansion of OMZ

waters to the eastern and southeastern parts of the Indian Ocean

in the late Neogene. In the northwestern Arabian Sea, thesummer monsoon winds cause widespread upwelling and high

surface productivity and thus increased organic flux to the

seafloor, intensifying oxygen minimum zone between 500 and

1500 m water depths. The faunal and isotope trends at Site 214

show similar conditions (Fig. 7). At Site 214, the mixed-layer

was thick (abundant Gs. sacculifer) and thermocline was deep

(rare Gr. menardii complex), indicating low surface productiv-

ity in the eastern Indian Ocean.

After 1.0 Myr, at Site 219 the benthic biofacies are domi-

nated by OuGl and EeUh while U. proboscidea decreased

in abundance, indicating low to intermediate organic carbon

flux to the seafloor and high seasonality conditions typicalof winter monsoon season. The percent G. bulloides increased

after 1.0 Myr, yet its abundance is not significant enough

to attribute it to summer monsoon but to winter monsoon

strengthening. We suggest strengthening of the winter mon-

soon since 1.0 Myr, driven by the onset of ice age cyclicity. It

has been observed by Overpeck et al. (1996) that G. bulloides

population remains atb45% during winter monsoon sea-

son and increases to N30% during summer monsoon season.

Fontugne and Duplessy (1986) suggested that during cold

intervals, the winter monsoon strengthens and summer mon-

soon weakens. In the eastern Indian Ocean, the productivity

has increased during the past 1.0 Myr as suggested by step-

wise decrease in 13C values and % Gs. sacculifer and in-

crease in % Gr. menardii complex (Fig. 7). This higher

surface productivity could have been linked to strong trade

winds, which would have intensified during cold intervals,

thus causing intense open-ocean upwelling and high surface

productivity. Site 214 is located beneath the core of the

Fig. 5. Cumulative percentages of benthic foraminiferal species dominant in different biofacies at DSDP Site 219. Open arrow marks a shift benthic foraminiferal

fauna, coinciding with the strengthening of the Indian monsoon.

Table 4

Relationship between biofacies factor scores of each species and its averageabundance in respective clusters

Biofacies Factor 1-ve Factor scores Cluster IV

(average %)

SlPa Stilostomella lepidula 0.76712 15.92655633

Pleurostomella alternans 0.66316 2.282477444

Uvigerina proboscidea 0.57463 19.06312037

Ehrenbergina carinata 0.45695 1.806140548

Cassidulina carinata 0.40159 8.077220475

Gavelinopsis lobatulus 0.37221 3.76770311

NuGs Factor 3-ve Factor scores Cluster III

(average %)

Nuttallides umbonifer 0.68766 2.40671494

Globocassidulina subglobosa

0.65278 4.369466469Gyroidinoides cibaoensis 0.4199 2.418400106

Oridorsalis umbonatus 0.41545 4.100768771

Uvigerina proboscidea 0.28727 30.41905914

OuGl Factor 8-ve Factor scores Cluster II

(average %)

Oridorsalis umbonatus 0.51602 4.257120018

Gavelinopsis lobatulus 0.40186 1.792069343

Bulimina aculeata 0.38974 10.77563706

EeUh Factor 4-ve Factor scores Cluster I

(average %)

Epistominella exigua 0.48336 12.30504722

Uvigerina hispido-costata 0.38523 7.116840533

Bolivina pusilla

0.35088 1.355918378



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Indonesian Throughflow (ITF) which has its maximum flow

in summer and minimum in winter (Gordon, 1986; Gordon

and Fine, 1996). We do not link the isotopic and faunal

changes at Site 214 during the last 1.0 Myr to the ITF flow

because during glacial times the sill depths between the

Pacific and the Indonesian Sea shoal, thus restricting the

movement of water between the two regions.

At Sites 214 and 219, oxygen does not appear to have been

a limiting factor since these sites lie in a well-oxygenated

(N3.8 ml/L) environment in the present-day abyssal setting and

Fig. 6. Distribution of benthic foraminiferal biofacies in time along with accumulative percentages of their key species at DSDP Site 219. Also plotted are percent

planktic foraminifer Globigerina bulloides and benthic foraminifer Uvigerina proboscidea (top panel). The shades mark the intervals during which a particular

biofacies dominates.



10/12

have remained so during the past 2.4 Myr. Organic carbon flux

linked to changes in the monsoon strength and trade wind

intensity appears to be the primary factor controlling changes in

benthic foraminiferal assemblages in the southeastern Arabian

Sea and eastern Indian Ocean. Our study thus supports the recent

observations by Gupta et al. (2003) and Gupta and Thomas

(2003) suggesting that changes in the tropical monsoons were

closely related to changes in the Northern Hemisphere ice vol-

ume both at orbital and suborbital time scales. The high vari-

ability in the tropical deep-sea environments occurred at a time

when the Earth's climate was experiencing large scale turnovers

due to the increased intensity of glacialinterglacial cycles.

Fig. 7. Percent Uvigerina proboscidea and Globigerina bulloides from Site 219 (panel e), plotted with percent U. proboscidea (panel d), mixed-layer species

Globigerinoides sacculifer and thermocline species Globorotalia menardii complex (panel c), Gs. sacculifer13C () (panel b) and 18O () (panel a) from Site

214. Data in panels ad are from Gupta and Dhingra (2004). A major switch in the proxy record is visible during ~1.20.9 Myr ago.



11/12

Acknowledgements

Deep Sea Drilling Project is thanked for providing samples

for the present study. This study was funded by DST, New Delhi

(No. SR/S4/ES-46/2003).

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at doi:10.1016/j.palaeo.2008.01.012.

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a major change in monsoon-driven productivity in the tropical indian ocean during ca 1.2-1.0 myr:...

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