Download South Equatorial Current (SEC) driven changes at DSDP Site 237

Journal of Asian Earth Sciences 28 (2006) 276–290
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South Equatorial Current (SEC) driven changes at DSDP Site 237,
Central Indian Ocean, during the Plio-Pleistocene: Evidence from
Benthic Foraminifera and Stable Isotopes
Anil K. Gupta *, Moumita Das, K. Bhaskar
Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721 302, India
Received 9 July 2004; received in revised form 29 July 2005; accepted 5 October 2005
Abstract
This study attempts to analyse paleoceanographic changes in the Central Indian Ocean (Deep Sea Drilling Project Site 237), linked to monsoon
variability as well as deep-sea circulation during the Plio-Pleistocene. We used factor and cluster analyses of census data of the 34 most dominant
species of benthic foraminifera that enabled us to identify five biofacies: Astrononion umbilicatulum–Uvigerina proboscidea (Au–Up), Pullenia
bulloides–Bulimina striata (Pb–Bs), Globocassidulina tumida–Nuttallides umbonifera (Gt–Nu), Gyroidinoides nitidula–Cibicides wuellerstorfi
(Gn–Cw) and Cassidulina carinata–Cassidulina laevigata (Cc–Cl) biofacies. Knowledge of the environmental preferences of modern deep-sea
benthic foraminifera helped to interpret the results of factor and cluster analyses in combination with oxygen and carbon isotope values. The
biofacies indicative of high surface productivity, resulting from a stronger South Equatorial Current (Au–Up and Pb–Bs biofacies), dominate the
early Pliocene interval (5.6–4.5 Ma) of global warmth. An intense Indo-Pacific ‘biogenic bloom’ and strong Oxygen Minimum Zone extended to
intermediate depths (w1000–2000 m) over large parts of the Indian Ocean in the early Pliocene. Since 4.5 Ma, the food supply in the Central
Indian Ocean dropped and fluctuated while deep waters were corrosive (biofacies Gt–Nu, Gn–Cw). The Pleistocene interval is characterized by an
intermediate flux of organic matter (Cc–Cl biofacies).
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Plio-Pleistocene; Benthic foraminifera; Stable isotopes; DSDP Site 237; Central Indian Ocean; SEC
1. Introduction
The climate of the Indian Ocean is marked by seasonal
reversal of the wind systems—the so-called Indian monsoon
system, which is believed to have evolved in the late early
Miocene (Gupta et al., 2004). During the southwest (SW)
monsoon season, strong wet winds blow from the sea towards
land, causing precipitation on land and increasing productivity
of the northern Indian Ocean due to intense coastal and open
ocean upwelling. The weak and variable northeast (NE)
monsoon winds, on the other hand, are dry and blow from the
land towards the sea. During the NE monsoon season, the
overall productivity of the northern Indian Ocean is low
(Krey, 1973). The effect of the Indian monsoon can be seen on
marine fauna and flora in the geological record as far south as
178S in the eastern Indian Ocean and almost the same latitude
* Corresponding author. Tel.: C91 3222 83368; fax: C91 3222 755303.
E-mail address: [email protected] (A.K. Gupta).
1367-9120/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jseaes.2005.10.006
in the western Indian Ocean (Gupta, 1997; Gupta et al., 2004).
The deep water masses in the northern Indian Ocean (north of
108N) are mainly of northern Indian origin since the
northward advection of intermediate-deep watermasses of
southern origin is blocked by the Hydrochemical front at 108S
(Tchernia, 1980; Dickens and Owen, 1999). The abyssal
depths (O4000 m) are filled with the water mass of the
southern origin, the Antarctic Bottom Water (AABW)
(Tchernia, 1980).
Benthic foraminifera are an important component of the
deep-sea biomass having good fossilization and adaptation
potential to survive and grow in a wide range of marine
environments, including oligotrophic abyssal plains (Coull
et al., 1977), hydrothermal vents (Sen Gupta and Aharon,
1994), cold seeps (Rathburn et al., 2000) and deep-sea trenches
(Akimoto et al., 2001). During the last two decades, there have
been numerous efforts to use deep-sea benthic foraminifera in
reconstructing the paleoceanographic evolution of different
ocean basins, including paleoproductivity and bottom water
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
oxygenation (Herguera, 1992; Gooday, 1993; Smart et al.,
1994; Thomas and Gooday, 1996; Fariduddin and Loubere,
1997; Loubere, 1998; Gupta and Thomas, 1999; Gupta and
Thomas, 2003). As a result, they are studied intensively in
order to understand their role in the evolution of the benthic
ecosystem. More recently, some species of deep-sea benthic
foraminifera have been used to study productivity changes in
the surface water column (Schmiedl and Mackensen, 1997;
Gupta and Thomas, 1999; Loubere and Fariduddin, 1999;
Almogi-Labin et al., 2000; Gupta et al., 2001).
Corliss (1979, 1983) found different species of deep-sea
benthic foraminifera associated with distinct deep-water
masses in the southeastern and southwestern Indian Ocean as
was earlier observed by Schnitker (1974) in the North Atlantic.
Numerous studies on living and dead benthic foraminifera have
indicated that their distributions are closely tied to the organic
carbon flux (Miller and Lohmann, 1982; Caralp, 1984; Jorissen
et al., 1992; Gooday, 1993; Thomas and Gooday, 1996;
Schmiedl et al., 1997; Altenbach et al., 1999; Gupta and
Thomas, 1999; Wollenburg and Kuhnt, 2000; Smart, 2002).
However, some benthic faunal assemblages have been found
associated more closely with the oxygen level of the ambient
277
water (Hermelin and Shimmield, 1990; Bernhard, 1992; Miao
and Thunell, 1993; Gooday, 1994; Wells et al., 1994; Gupta,
1997; Jannink et al., 1998). Murray (2001) argued that benthic
foraminifera have the potential to serve as proxies for very low
values of oxygen. However, once oxygen is sufficient, they
cease to be effective proxies.
The present study is aimed at understanding the paleoceanographic and paleoclimatic (monsoonal) changes in the Central
Indian Ocean using deep-sea benthic foraminiferal and stable
isotope data from Deep Sea Drilling Project (DSDP) Site 237,
which lies in an area of maximum activity of the South
Equatorial Current (SEC) (Fig. 1). The SEC flows towards the
east between 58 and 158S latitude (average 108S) and intensifies
during the summer monsoon season (Tchernia, 1980; Schott
and McCreary, 2001). During the winter monsoon (NE) season,
the SEC moves somewhat northward at w58S and its strength
is substantially reduced. The high surface productivity above
Site 237 is driven by the SEC during the SW (summer)
monsoon season (Krey, 1973). The location of this site thus
provides an opportunity to understand changes in the organic
carbon flux and deep-sea oxygenation driven by the SEC
during the Plio-Pleistocene.
Fig. 1. Location map of Deep Sea Drilling Project (DSDP) sites in the Indian Ocean. Sites 219, 236, 237 and 241 are marked with larger solid circles. Stippled area
marks the latitudinal extent of the South Equatorial Current (Tchernia, 1980). The SEC moves to the South (158S) during the winter monsoon season, and migrates
northward (58N) in the summer monsoon season (average latitudinal position 108S).
278
Table 1
Faunal percent data of highest ranked species used in factor and cluster analyses
Depth
(mbsf)
Age
(Ma)
Cluster
Biofacies
24-237, 1-1,54-56
24-237, 1-2,48-50
24-237, 1-3,48-50
24-237, 1-4,48-50
24-237, 2-1,53-55
24-237, 2-2,48-50
24-237, 4-1,54-56
24-237, 4-2,48-50
24-237, 4-3,48-50
24-237, 4-4,47-49
24-237, 4-5,48-50
24-237, 4-6,50-52
24-237, 5-1,50-52
24-237, 5-2,48-50
24-237, 5-3,50-52
24-237, 5-4,50-52
24-237, 5-5,48-50
24-237, 5-6,46-48
24-237, 6-1,50-52
24-237, 6-2,48-50
24-237, 6-3,50-52
24-237, 6-4,48-50
24-237, 6-5,48-50
24-237, 6-6,50-52
24-237, 7-1,50-52
24-237, 7-2,48-50
24-237, 7-3,50-52
24-237, 7-4,50-52
24-237, 7-5,48-50
24-237, 7-6,50-52
24-237, 8-1,48-50
24-237, 8-2,31-33
24-237, 8-3,51-53
24-237, 8-4,50-52
24-237, 8-5,50-52
24-237, 8-6,51-53
24-237, 9-1,58-60
24-237, 9-2,43-45
24-237, 9-3,54-56
24-237, 9-4,64-66
24-237, 9-5,55-57
24-237, 9-6,55-57
0.54
1.98
3.48
4.98
7.03
8.48
25.5
27
28.48
29.97
31.48
33
35.5
36.98
38.5
40
41.48
42.96
45
46.48
48
48.48
50.98
52.5
54.5
55.98
57.5
59
60.48
62
63.98
65.31
67.01
68.5
70
71.51
73.58
74.93
76.54
78.14
79.55
81.05
0.038
0.235
0.413
0.59
0.833
1.005
3.02
3.201
3.35
3.399
3.448
3.498
3.58
3.703
3.829
3.954
4.077
4.2
4.275
4.33
4.386
4.441
4.497
4.553
4.608
4.663
4.719
4.775
4.83
4.885
4.958
5.007
5.07
5.125
5.2
5.255
5.33
5.379
5.437
5.495
5.546
5.6
1
1
1
1
1
1
2
2
2
2
1
3
3
3
3
3
3
2
2
2
2
2
4
4
4
4
4
4
4
4
4
4
4
5
4
5
5
5
5
5
5
4
Cc–Cl
Cc–Cl
Cc–Cl
Cc–Cl
Cc–Cl
Cc–Cl
Gn–Cw
Gn–Cw
Gn–Cw
Gn–Cw
Cc–Cl
Gt–Nu
Gt–Nu
Gt–Nu
Gt–Nu
Gt–Nu
Gt–Nu
Gn–Cw
Gn–Cw
Gn–Cw
Gn–Cw
Gn–Cw
Pb–Bs
Pb–Bs
Pb–Bs
Pb–Bs
Pb–Bs
Pb–Bs
Pb–Bs
Pb–Bs
Pb–Bs
Pb–Bs
Pb–Bs
Au–Up
Pb–Bs
Au–Up
Au–Up
Au–Up
Au–Up
Au–Up
Au–Up
Pb–Bs
d18O
d13C
3.42
0.15
3.32
0.17
3.3
0.32
3.01
2.7
2.39
2.63
2.71
0.14
0.38
0.44
0.52
0.41
2.67
0.25
2.64
0.21
2.63
0.25
2.56
0.17
2.49
0.15
2.38
0.24
2.37
0.25
2.3
0.26
2.51
0.35
2.44
0.3
2.41
0.43
2.46
0.31
2.55
2.57
2.9
2.43
2.51
0.42
0.34
0.01
0.41
0.47
Astrononion
umbilicatulum
0.28
0.54
0.56
0.22
0.22
0.38
2.13
4.42
1.78
1.16
1
1.73
1.56
2.05
2.13
3.73
2.89
3.92
2.34
2.9
2.5
3.38
0.74
3.73
2.83
1.55
2.63
0.79
3.04
2.52
6.42
4.76
4.87
0.86
2.3
2.31
3.07
3.83
2.55
2.32
0.98
3.96
Bolivina
pusilla
Bolivinita
pseudoplicata
0.56
0.55
0.82
0.28
0.43
1.12
0.67
0.97
0.78
1.48
0.32
2.28
0.86
0.62
0.69
0.3
0.57
1.63
1.75
1.45
1.4
0.56
5.81
5.04
5.75
3.3
3.75
4.06
7.78
8.81
6.32
3.18
5.88
4.39
1.45
1.68
4.23
Bulimina
alazanensis
1.94
0.82
0.28
0.86
0.9
1.15
8.91
3.89
8.31
4.48
1.25
4.06
2.52
3.65
2.87
4.07
0.32
2.34
0.29
1.95
7.04
5.15
7.46
2.83
5.15
10.53
1.59
0.51
0.36
8.25
2.38
0.88
1.29
4.61
0.77
2.63
9.14
4.08
1.86
7.88
2.2
B. striata
0.19
1.03
0.3
Cassidulina
carinata
C.
laevigata
0.83
0.82
1.69
1.52
5.37
3.46
0.97
18.89
12.5
12.92
11.52
8.95
3.47
5.03
8.05
7.72
1.92
10.63
10.37
4.06
2.29
4.46
2.87
6.65
0.66
4.97
1.45
1.12
1.69
0.45
0.6
0.98
0.58
0.58
0.27
5.15
0.75
2.83
1.03
5.26
0.79
1.01
1.08
3.21
1.9
2.21
3
0.92
3.09
1.75
1.18
1.53
1.39
1.97
3.52
0.94
Cibicides
bradyi
C.
kullenbergi
0.54
0.28
0.65
0.19
0.78
0.3
1.92
0.5
0.86
0.86
0.58
0.99
0.29
0.27
1.47
0.75
2.83
5.15
1.5
0.94
1.03
d
1.08
2.29
0.95
0.46
0.43
0.92
0.77
0.44
2.36
0.46
0.49
2.2
C.
wuellerstorfi
3.61
10.87
12.36
11.74
2.46
9.25
2.91
3.12
3.26
4.8
4.81
9.22
3.12
2.05
3.04
4.88
14.45
5.55
3.81
4.94
10.06
4.23
5.15
8.96
..94
5.15
2.63
3.97
2.54
11.51
4.13
12.85
1.77
9.44
9.21
8.1
9.65
10.32
10.71
5.11
15.27
11.89
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
Sample No.
Sample No.
0.22
0.44
0.5
0.31
0.69
0.3
0.29
0.58
1.12
0.28
1.47
2.24
4.72
4.64
2.63
4.76
2.03
0.36
1.83
2.86
1.33
0.43
0.46
1.16
1.31
1.47
2.04
2.32
1.97
0.88
Ehrenbergina
carinata
Epistominella
exigua
6.39
2.45
3.37
1.52
3.35
5.39
0.19
0.26
0.89
1.92
0.5
0.29
2.5
2.97
2.43
2.01
2.89
2.94
3.51
0.87
1.67
1.69
0.74
0.75
3.77
3.09
1.67
3.17
6.59
5.39
3.67
1.43
4.42
3.43
1.38
6.56
3.07
2.36
1.39
3.93
3.52
Favocassidulina
australis
0.28
0.19
1.13
1.19
0.96
0.19
0.86
0.75
1.15
5.72
1.14
0.57
1.73
0.66
5.55
2.32
2.79
1.47
1.55
3.17
0.51
0.72
0.48
0.43
0.46
0.44
0.88
1.53
1.39
0.98
1.47
0.27
1.13
0.74
1.49
7.55
4.64
3.95
4.76
3.04
1.8
6.19
1.33
1.72
0.92
2.7
0.88
3.06
5.11
3.94
3.52
Gavelinopsis
lobatulus
Globocassidulina subglobosa
G. tumida
Gyroidinoides
nitidula
G. orbicularis
Laticarinina
pauperata
2.22
1.09
0.56
0.22
2.91
3.08
0.78
0.26
5
7.34
6.18
10
5.14
3.47
7.75
14.28
8.6
4.8
6.32
12.1
12.5
12.13
9.11
8.9
13
9.15
10.23
15.12
1.59
11.5
5.88
5.22
1.89
5.15
10.53
10.32
10.15
8.27
6.42
7.61
9.29
4.29
5.99
5.4
7.02
6.78
7.65
8.37
5.91
11.87
2.22
0.54
1.12
5.22
0.67
1.54
5.81
1.13
3
3.51
4.81
2.3
5
3.43
6.07
6.9
3.18
1.31
4.97
4.06
5.02
2.54
3.68
3.73
4.72
3.09
1.32
3.97
4.06
1.08
0.46
2.38
1.67
5.43
5.06
7.83
3.8
2.31
4.44
2.72
1.12
1.95
0.67
0.96
2.13
1.82
0.6
1.91
1
0.29
0.62
0.82
1.68
1.3
0.44
0.96
2.71
0.78
0.3
1.6
1.26
2.3
0.64
1.5
0.29
4.06
0.22
0.3
2.3
2.65
0.58
1.16
2.23
1.13
0.74
0.75
1.89
2.58
1.32
0.79
2.54
1.08
0.46
3.54
0.43
1.93
0.59
0.51
0.46
0.49
0.43
0.46
0.38
3.51
2.06
4.59
2.32
2.95
0.3
1.87
0.69
0.6
0.86
1.73
0.65
0.58
1.13
2.94
3.73
2.83
1.03
1.32
0.79
0.51
2.52
1.83
6.66
4.42
4.72
1.84
1.16
3.95
2.94
0.46
3.45
1.47
0.87
1.63
0.58
5.52
2.23
1.13
0.69
0.6
0.86
0.29
0.33
0.29
2.62
1.67
0.56
2.21
0.75
2.83
1.03
1.32
3.97
2.54
2.52
0.46
2.86
1.77
1.72
3.22
1.54
0.88
0.59
1.02
0.93
1.47
0.44
Nuttallides
umbonifera
5.55
5.43
5.62
6.74
2.46
3.46
5.81
3.81
15.1
1.28
1.34
13.26
12.8
11.67
13.6
19.25
10.12
0.99
4.39
5.81
3.62
3.1
2.94
1.49
0.52
1.59
0.72
1.38
0.46
0.77
2.32
Oridorsalis
umbonatus
5.83
8.15
2.25
5.83
4.92
3.27
3.68
0.89
1.28
4.56
3.17
6.56
7.09
4.25
5.17
6.65
4.9
9.36
5.81
6.14
8.45
8.82
9.7
12.26
8.25
10.53
9.52
12.18
5.39
11.47
7.61
7.96
11.16
13.82
3.86
5.26
11.5
5.1
6.04
7.39
10.13
Osangularia
culter
0.27
0.28
0.22
0.38
1.16
0.52
0.89
0.32
2.28
2.02
0.62
1.14
0.58
0.98
1.46
1.16
0.56
2.94
5.22
1.89
5.15
1.32
0.79
2.03
2.87
1.38
0.48
1.33
0.43
0.92
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
24-237, 1-1,54-56
24-237, 1-2,48-50
24-237, 1-3,48-50
24-237, 1-4,48-50
24-237, 2-1,53-55
24-237, 2-2,48-50
24-237, 4-1,50-56
24-237, 4-2,48-50
24-237, 4-3,48-50
24-237, 4-4,47-49
24-237, 4-5,48-50
24-237, 4-6,50-52
24-237, 5-1,50-52
24-237, 5-2,48-50
24-237, 5-3,50-52
24-237, 5-4,50-52
24-237, 5-5,48-50
24-237, 5-6,46-48
24-237, 6-1,50-52
24-237, 6-2,48-50
24-237, 6-3,50-52
24-237, 6-4,48-50
24-237, 6-5,48-50
24-237, 6-6,50-52
24-237, 7-1,50-52
24-237, 7-2,48-50
24-237, 7-3,50-52
24-237, 7-4,50-52
24-237, 7-5,48-50
24-237, 7-6,50-52
24-237, 8-1,48-50
24-237, 8-2,31-33
24-237, 8-3,51-53
24-237, 8-4,50-52
24-237, 8-5,50-52
24-237, 8-6,51-53
24-237, 9-1,58-60
24-237, 9-2,43-45
24-237, 9-3,54-56
24-237, 9-4,64-66
24-237, 9-5,55-57
24-237, 9-6,55-57
Eggerella bradyi
1.31
0.59
1.02
0.46
1.47
0.88
(continued on next page)
279
Pleurostomella alternans
Pullenia bulloides
P. quinqueloba
2.22
3.26
1.4
2.17
3.88
0.78
1.48
3.83
2.53
1.44
3.43
1.83
3.04
2.59
3.18
3.59
3.51
2.03
1.67
3.38
2.94
2.99
0.94
1.55
1.8
2.29
1.33
1.31
0.59
2.04
0.49
0.38
1.36
0.78
3
1.28
2.28
2.02
4.38
4.12
1.82
2.87
2.6
2.28
2.92
4.06
1.96
4.23
7.35
4.48
7.55
5.67
11.84
3.97
3.04
2.87
6.42
0.95
2.21
2.57
2.76
3.47
1.31
2.35
2.55
1.86
2.46
2.64
Sigmoilopsis
schlumbergeri
Siphotextularia solita
0.3
0.29
0.29
0.58
1.67
2.94
2.24
0.94
1.32
2.03
1.83
0.88
0.86
0.92
0.77
0.29
0.51
3.24
4.08
3.72
1.97
7.48
Stilostomella
annulifera
S. lepidula
S. subspinosa
Textularia
lythostrota
0.28
0.54
0.28
0.84
0.43
0.19
2.52
2.86
2.1
4.15
0.75
1.73
1.87
0.29
0.6
2.01
0.87
0.66
2.92
0.87
0.84
1.41
2.21
2.24
1.89
2.08
1.32
1.59
2.03
5.37
0.46
2.86
0.88
2.14
7.83
4.25
Sphaeroidina
bulloides
2.46
0.38
1.93
0.89
0.32
0.75
0.57
0.45
2.13
2.34
1.19
3.51
1.27
0.29
2.05
0.3
0.29
0.29
0.56
0.28
1.32
1.52
2.15
0.46
1.9
0.74
2.24
2.83
0.52
1.32
2.38
1.01
1.08
0.92
0.88
0.43
0.88
0.59
1.53
2.32
2.95
1.47
0.38
0.88
0.51
1.76
2.6
2.61
0.29
1.16
0.56
2.53
2.52
7.01
2.23
3.03
3.446
0.31
3.9
0.86
4.34
2.94
1.17
1.45
1.4
1.69
2.13
3
1.28
2.02
0.86
1.25
2.12
1.15
2.89
3.27
1.46
0.58
0.56
0.65
1.34
0.38
0.97
0.26
0.6
2.24
0.75
0.29
0.94
0.69
0.91
0.57
0.87
0.33
0.88
0.84
0.75
1.03
0.48
1.33
0.46
0.77
0.44
0.59
1.02
0.93
0.49
0.94
3.61
1.32
3.17
1.52
2.52
1.38
1.9
4.42
4.72
5.99
0.77
1.31
1.47
1.02
0.46
0.98
1.47
0.52
2.58
2.63
1.59
7.61
1.08
1.9
2.21
0.86
0.92
4.21
3.98
4.72
6.91
6.56
0.44
1.18
1.53
1.86
0.44
3.52
Uvigerina
proboscidea
3.33
1.9
1.97
0.86
3.35
3.46
10.47
13.77
11
12.7
3.03
3.17
2.81
3.9
4.25
1.44
4.34
12.74
7.01
7.85
9.73
7.04
3.68
5.22
8.49
10.31
7.89
8.73
11.67
12.3
6.42
8.09
13.72
22.31
8.75
23.93
25
18.58
16.84
19.53
13.3
5.72
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
24-237, 1-1,54-56
24-237, 1-2,48-50
24-237, 1-3,48-50
24-237, 1-4,48-50
24-237, 2-1,53-55
24-237, 2-2,48-50
24-237, 4-1,50-56
24-237, 4-2,48-50
24-237, 4-3,48-50
24-237, 4-4,47-49
24-237, 4-5,48-50
24-237, 4-6,50-52
24-237, 5-1,50-52
24-237, 5-2,48-50
24-237, 5-3,50-52
24-237, 5-4,50-52
24-237, 5-5,48-50
24-237, 5-6,46-48
24-237, 6-1,50-52
24-237, 6-2,48-50
24-237, 6-3,50-52
24-237, 6-4,48-50
24-237, 6-5,48-50
24-237, 6-6,50-52
24-237, 7-1,50-52
24-237, 7-2,48-50
24-237, 7-3,50-52
24-237, 7-4,50-52
24-237, 7-5,48-50
24-237, 7-6,50-52
24-237, 8-1,48-50
24-237, 8-2,31-33
24-237, 8-3,51-53
24-237, 8-4,50-52
24-237, 8-5,50-52
24-237, 8-6,51-53
24-237, 9-1,58-60
24-237, 9-2,43-45
24-237, 9-3,54-56
24-237, 9-4,64-66
24-237, 9-5,55-57
24-237, 9-6,55-57
280
Table 1 (continued)
Sample No.
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
2. General settings, materials and methods
We present the combined data set of benthic foraminiferal
assemblages and stable isotope analysis from Site 237, which
lies in the southeastern part of the Somali Basin, Central Indian
Ocean, at a water depth of 1623 m (lat. 07804.99 0 S, long.
58807.48 0 E). The present day in situ primary production in the
surface waters above Site 237 is w500 mg C mK2 dayK1
during the summer monsoon season, which is reduced to
w300 mg C mK2 dayK1 during the winter monsoon season
(Krey, 1973). In the present day ocean, the surface salinity
above Site 237 is relatively low due to the transport of water
from the Indonesian region by the SEC (Schott and McCreary,
2001).
Forty-two samples were analyzed from an 81.02 m long core
that covers the Plio-Pleistocene interval at Site 237. The interval
from 8.49–25.4 mbsf (meters below seafloor), covering w2
million years, is missing due to non-recovery of the core during
drilling. The age model used is based on planktic foraminiferal
datums from Singh and Srinivasan (1995) calibrated to the
Berggren et al. (1995) time scale. Each sample of 10 cm3, taken at
an interval of one sample per section (every 1.5 m), was soaked in
281
water with half a tea spoon of baking soda for 12 h prior to
washing over a 63-mm size mesh. The O63-mm fraction was then
oven-dried at 50 8C and dry-sieved over a 149-mm mesh. The
O149-mm fraction was split into suitable aliquots in order to
select w300 specimens of benthic foraminifera. Individuals of
each benthic species were identified, counted, and their relative
abundances were calculated.
Factor and cluster analyses were performed on the relative
abundance data of the 34 highest ranked species (Table 1),
which were selected on the basis of an occurrence of 2% or
more in at least one sample and were present in at least three
samples. R-mode Factor analysis was performed on the
correlation matrix followed by an orthogonal VARIMAX
rotation using the SAS/STAT package (SAS Institute Inc.,
1988). Scree plot of eigenvalues and screening of the factor
loadings helped to retain five factors (1–5) that account for
56.52% of the total variance (Table 2). Factor 4 does not show
any significant species association and was not used to define
any biofacies. Most of the species on Factor 4 are rare and
sporadic at Site 237. Since we use zeros for missing values,
SAS iterations show high factor loadings of rare and
insignificant species because of a built in function of the
Table 2
VARIMAX rotated factor loadings of five significant factors
Species name
Factor 1
Factor 2
Factor 3
Factor 4
Factor 5
Astrononion umbilicatulum
Bolivina pusilla
Bolivinita pseudoplicata
Bulimina alazanensis
Bulimina striata
Cassidulina carinata
C. laevigata
Cibicides bradyi
C. kullenbergi
C. wuellerstorfi
Eggerella bradyi
Ehrenbergina carinata
Epistominella exigua
Favocassidulina australis
Gavelinopsis lobatulus
Globocassidulina subglobosa
G. tumida
Gyroidinoides nitidula
G. orbicularis
Laticarinina pauperata
Nuttallides umbonifera
Oridorsalis umbonatus
Osangularia culter
Pleurostomella alternans
Pullenia bulloides
P. quinqueloba
Sigmoilopsis schlumbergeri
Siphotextularia solita
Sphaeroidina bulloides
Stilostomella annulifera
S. lepidula
S. subspinosa
Textularia lythostrota
Uvigerina proboscidea
% Variance
0.772
K0.257
K0.0255
0.281
0.425
K0.760
K0.661
0.063
0.231
0.088
0.304
K0.058
K0.009
0.303
K0.224
0.169
K0.005
K0.086
K0.439
0.012
K0.146
0.266
0.128
0.229
0.231
0.273
0.229
K0.043
0.229
0.249
0.317
0.306
0.140
0.523
12.77
K0.016
K0.350
K0.279
0.390
0.551
K0.080
K0.135
0.147
0.315
K0.401
0.485
0.023
0.046
0.330
0.063
K0.018
K0.029
K0.006
K0.133
0.048
K0.086
0.487
0.139
K0.254
0.830
0.194
K0.129
K0.086
0.488
K0.413
K0.250
K0.609
K0.003
K0.147
12.26
0.004
K0.500
K0.512
K0.164
0.134
0.133
0.128
K0.112
0.295
0.685
0.028
0.100
K0.328
0.085
K0.202
K0.147
K0.189
0.821
K0.163
0.092
K0.040
0.254
K0.006
K0.463
K0.079
0.177
K0.105
0.086
0.126
K0.603
K0.237
K0.284
K0.033
K0.222
11.22
K0.134
K0.143
K0.290
K0.360
0.102
K0.137
K0.155
0.506
K0.121
0.027
0.496
K0.058
0.016
0.618
K0.119
K0.072
0.033
0.055
K0.073
0.840
K0.319
0.128
0.093
K0.357
0.009
K0.131
0.133
0.012
0.459
K0.022
0.364
K0.128
0.357
0.122
10.23
K0.102
0.195
0.512
K0.024
K0.279
K0.143
0.021
0.279
K0.067
K0.103
0.058
K0.169
0.179
0.071
K0.005
0.053
0.848
K0.233
K0.111
0.011
0.608
K0.086
0.062
0.385
0.039
0.493
0.190
K0.024
0.127
K0.216
K0.411
0.186
K0.285
K0.407
10.04
282
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
package. This may lead to misinterpretation of the data; thus to
avoid this artifact of the SAS analysis, we also look at the
population trend of these species in order to select or omit a
particular factor.
Q-mode Cluster analysis was performed on a covariance
matrix using Ward’s Minimum Variance method to identify
sample groups. We identified five major clusters representing
five biofacies on the basis of a plot of semi-partial R-squared
values versus the number of clusters (Fig. 2). Information on
Recent benthic foraminifera was used to interpret the
environments of these biofacies as shown in Table 3 (Corliss,
1979; 1983; Gooday, 1994; Smart et al., 1994; Hermelin and
Shimmield, 1995; Mackensen et al., 1995; Schmiedl
and Mackensen, 1997; Schmiedl et al., 1997; Jannink et al.,
1998; Loubere, 1998; Gupta, 1997; 1999; Gupta and Thomas,
1999; Loubere and Fariduddin, 1999; Kurbjeweit et al., 2000;
Ohkushi et al., 2000). The five biofacies are plotted along
with the stable isotope values and inferred environments in
Fig. 3.
Stable Isotope analysis was performed on Cibicides
wuellerstorfi, an epibenthic species, at Case Western Reserve
University by S.M. Savin following the procedure given in
Woodruff et al. (1990). Results are reported relative to the Pee
Dee Belemnite (PDB) (Fig. 3).
Fig. 2. Dendrogram based on Q-mode cluster analysis of 42 Plio-Pleistocene samples from DSDP Site 237 using Ward’s Minimum Variance method. Five clusters
have been identified on the basis of the number of clusters versus semi-partial R-squared, defining the five biofacies.
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
283
Table 3
Benthic foraminiferal biofacies and interpreted environments
Biofacies
Environment
Biofacies Au–Up (Factor 1Cve scores)
Astrononion umbilicatulum (0.772)
Uvigerina proboscidea (0.523)
Bulimina striata (0.425)
Stilostomella lepidula (0.317)
S. subspinosa (0.306)
Eggerella bradyi (0.304)
Favocassidulina australis (0.303)
High, continuous food supply, low or no seasonality
Biofacies Pb–Bs (Factor 2 Cve scores)
Pullenia bulloides (0.830)
Bulimina striata (0.551)
Oridorsalis umbonatus (0.487)
Sphaeroidina bulloides (0.488)
Eggerella bradyi (0.485)
Bulimina alazanensis (0.390)
Cibicides kullenbergi (0.315)
High organic flux, low seasonality
Biofacies Gt–Nu (Factor 5 Cve scores)
Cool, carbonate corrosive deep water, organic flux variable (low to
intermediate)
Globocassidulina tumida (0.848)
Nuttallides umbonifera (0.608)
Bolivinita pseudoplicata (0.512)
Pullenia quinqueloba (0.493)
Biofacies Gn–Cw (Factor 3 Cve scores)
Gyroidinoides nitidula (0.821)
Cibicides wuellerstorfi (0.685)
C. kullenbergi (0.295)
Oridorsalis umbonatus (0.254)
Low to intermediate organic flux, high seasonality
Biofacies Cc-Cl (Factor 1 Kve scores)
Cassidulina carinata (K0.760)
C. laevigata (K0.661)
Gyroidinoides orbicularis (K0.439)
Bolivina pusilla (K0.257)
Intermediate organic flux, intermediate to high seasonality
To understand ocean-wide paleoceanographic changes
during the Plio-Pleistocene and to compare between sites in
the Indian Ocean, we reproduced biofacies graphs
for intermediate- to deep-water Sites 219, 236 and 241
(Figs. 4–6) (Gupta, 1997; Gupta and Thomas, 1999; Gupta
and Satapathy, 2000).
well as the mid-Pliocene warmth on the biota of the Central
Indian Basin (Raymo et al., 1996; Kim and Crowley, 2000).
The d13C values show a variable, but overall decreasing trend
from about 0.50‰ in the earliest Pliocene to as low as 0.15‰
in the latest Pleistocene (Fig. 3).
3.2. Biofacies
3. Results
3.1. Isotope data
Average d18O values of Cibicides wuellerstorfi are w2.5‰
between 5.6 and 4.5 Ma and increased by w0.9‰ from 4.5 Ma
to the latest Pleistocene (Fig. 3). The early Pliocene values are
close to those observed in other ODP sites (e.g. Hodell and
Venz, 1992). The d18O values show a major shift towards
higher values at w3.2 Ma, coinciding with a global increase in
benthic d18O values at 3.2–3.1 Ma and the beginning of the
Northern Hemisphere (NH) glaciation and deep sea cooling
(Shackleton and Hall, 1984; Keigwin, 1986; Seto, 1995). The
coarse sampling resolution and non-recovery of the interval
between 8.48 and 25.5 m (2 Ma) has precluded the understanding of the effect of further growth of the NH ice volume as
Results of factor and cluster analyses allowed us to
identify five biofacies (Tables 2 and 3; Fig. 3), characterizing
distinct deep-sea environments. Each biofacies is described
below with its species associations and environmental
preferences.
The Astrononion umbilicatulum–Uvigerina proboscidea–
Stilostomella lepidula (Au-Up) biofacies is defined by species
having high positive loadings on Factor 1 that include
Astrononion umbilicatulum, Uvigerina proboscidea, Bulimina
striata, Stilostomella lepidula, S. subspinosa, Eggerella bradyi
and Favocassidulina australis (Table 3). Most of these species
prefer the infaunal microhabitat and characterize an environment with a high and sustained food supply when seasonality is
low (Rathburn and Corliss, 1994; Gupta and Thomas, 1999).
Uvigerina proboscidea occupies a shallow infaunal
Oxygenation
High
High
End of Biogenic
Bloom (~4.5 Ma)
Small
Early
5
Low
3
High
No Recovery
4
Late
Miocene
Intermediate Food Supply
0.6
Ice volume
0.4
Au-Up
0.2
Pb-Bs
3.4 0
Large
Age (Ma)
Late
3
1
2
Pliocene
2.6
Inter–
Low
Low
mediate
Pleistocene
2.2
0
δ13 C (‰)
Gt-Nu
δ18 O (‰)
Gn-Cw
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
Cc-Cl
284
6
Fig. 3. Cibicides wuellerstorfi d18O and d13C values combined with the benthic foraminiferal biofacies (Table 1), plotted against numerical ages at DSDP Site 237.
On the right hand columns are shown the ice volume history, food supply and deep-sea oxygenation.
microhabitat in organic carbon rich sediments, independent of
oxygen content (Miller and Lohmann, 1982; Lutze and
Coulbourn, 1984; Rathburn and Corliss, 1994) and thrives in
areas of high productivity (e.g. Gupta and Srinivasan, 1992;
Gupta, 1997; Jannink et al., 1998; Gupta and Thomas, 1999),
specifically when food supply is continuous (low seasonality)
(Loubere, 1998; Loubere and Fariduddin, 1999; Ohkushi et al.,
2000; Gupta and Thomas, 2003; Gupta et al., 2004). This
species is most abundant during the late Neogene at several
ODP/DSDP Sites in the Indian Ocean (Gupta and Srinivasan,
1992; Gupta and Thomas, 1999; Gupta et al., 2001; Gupta and
Thomas, 2003; Gupta et al., 2004). Astrononion umbilicatulum
(synonymous to A. echolsi) has been found associated with
variable environmental conditions (Burke et al., 1993; AlmogiLabin et al., 2000). We do not have enough information
regarding the ecological preference of Bulimina striata. This
species resembles B. marginata morphologically, which has
been inferred to reflect a sustained flux of organic matter from
high surface productivity (Gupta, 1997). This species lives in
an infaunal microhabitat and thus appears to prefer a high
organic carbon flux (Rathburn and Corliss, 1994). Stilostomella
species have been found associated with an intermediate to
high flux of organic food and intermediate seasonality (Gupta
and Thomas, 2003). This biofacies is thus indicative of a high
and sustained flux of organic matter from year-round, high
surface productivity (low or no seasonality) and is similar to
the Up biofacies of Gupta and Thomas (1999) and Gupta et al.
(2004). This biofacies occupies an interval from the latest
Miocene-earliest Pliocene at Site 237 (Fig. 3).
The characteristic species of the Pullenia bulloides–
Bulimina striata (Pb–Bs) biofacies are Pullenia bulloides,
Bulimina striata, Oridorsalis umbonatus, Sphaeroidina
bulloides, Eggerella bradyi, Bulimina alazanensis and
Cibicides kullenbergi having strong positive loadings on
Factor 2. The test morphology of these species suggests that
many of them live infaunally (Corliss and Chen, 1988) under
low oxygen conditions and intermediate to high organic flux
(Gupta and Srinivasan, 1990). Pullenia bulloides has been
found living in shallow infaunal environments with high
organic carbon content in the Sulu Sea (Rathburn and Corliss,
1994). In the southeastern Arabian Sea, this species was
observed associated with an intermediate flux of organic
matter in poorly ventilated deep waters (Gupta and Thomas,
1999). Oridorsalis umbonatus is a long ranging species, which
lives in a variety of environments (Miao and Thunell, 1993;
Schmiedl and Mackensen, 1997; Gupta and Thomas, 1999).
Bulimina alazanensis is another species that occurs in low
oxygen and organic food rich environments at several Indian
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
285
Fig. 4. Adjusted benthic foraminiferal d18O and d13C values and benthic foraminiferal biofacies plotted against numerical ages at DSDP Site 219, southeastern
Arabian Sea (Gupta and Thomas, 1999). On the right hand columns are shown deep-sea temperature, ice volume history, organic carbon flux, and deep-sea
oxygenation. Overall the productivity decreased since the early Pliocene in the southeastern Arabian Sea.
Ocean DSDP/ODP sites (Gupta, A.K., 2005 personal observation). This biofacies characterizes the early Pliocene warm
interval marked by low oxygen isotopic ratios and decreasing
carbon isotope values at Site 237 (Fig. 3), and is inferred to
reflect a high organic flux. The end of the Pb–Bs biofacies
corresponds with the end of the so-called ‘biogenic bloom’
(Dickens and Owen, 1999; Hermoyian and Owen, 2001).
The Globocassidulina tumida–Nuttallides umbonifera (Gt–
Nu) biofacies is defined by Globocassidulina tumida, Nuttallides umbonifera, Bolivinita pseudoplicata and Pullenia
quinqueloba, having high positive loadings on Factor 5. We
do not have enough information on Globocassidulina tumida,
but its close resemblance with G. subglobosa suggests that this
species prefers low productivity environments (Gupta and
Satapathy, 2000; Singh and Gupta, 2004). The environmental
preferences of N. umbonifera are not well-constrained. The
species has been inferred to be an indicator of cool, carbonate
corrosive Antarctic Bottom Water (Lohmann, 1978; Corliss,
1979; 1983; Bremer and Lohmann, 1982; Mackensen et al.,
1995), but Van Leeuwen (1989) did not observe this association
in the Angola Basin. Others have interpreted this species as an
indicatior of low productivity, occurring at the greatest depths in
all the oceans (Gooday, 1994; Gupta, 1997; Loubere and
Fariduddin, 1999). We do not have much idea about the
ecological preference of B. pseudoplicata and P. quinqueloba as
well, although an infaunal microhabitat has been suggested
(Murray, 2001). This biofacies appears to reflect cool, carbonate
corrosive deep waters with variable organic flux and high
oxygenation (overall oligotrophic, high seasonality), characterizing the late early Pliocene interval at Site 237.
Gyroidinoides nitidula, Cibicides wuellerstorfi, C. kullenbergi and Oridorsalis umbonatus characterize the G. nitidula–
C. wuellerstorfi (Gn–Cw) biofacies having strong positive
loadings on Factor 3. Gyroidinoides nitidula prefers a shallow
infaunal microhabitat (Rathburn and Corliss, 1994), and is
determined to have lived in an environment with intermediate
organic flux and intermediate to high seasonality during the
Pliocene-Pleistocene (Gupta and Thomas, 2003). Cibicides
wuellerstorfi has been suggested as an epibenthic species that
prefers to live on elevated objects above the sediment surface
in high energy environments (Lutze and Thiel, 1989; Gooday
and Turley, 1990; Gooday, 1993; Linke and Lutze, 1993). In
the Indian Ocean, this species has been found associated with
an assemblage indicative of low to intermediate food supply,
286
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
Fig. 5. Benthic foraminiferal assemblages plotted against numerical ages at DSDP Site 236, Somali Basin (Gupta and Satapathy, 2000). On the right hand columns
are shown ice volume history, organic food supply, and deep-sea oxygenation. Overall the productivity decreased whereas oxygenation increased in the Somali
Basin since the early Pliocene.
high seasonality and strong deep ocean currents (Gupta, 1999;
Gupta and Thomas, 2003). Cibicides kullenbergi has been
observed in numerous environmental settings. This species was
found on the continental margin of northwestern Africa and
considered a low organic carbon flux species (Lutze and
Coulbourn, 1984). At Site 219, in the southeastern Arabian
Sea, C. kullenbergi is associated with an assemblage indicative
of high organic flux and warm waters (Gupta and Thomas,
1999). In the Pacific Ocean, this species occurs at numerous
sites and was defined as a warm water species (Woodruff,
1985). This biofacies suggests a low to intermediate organic
flux, and high seasonality that characterizes the early late
Pliocene interval at Site 237.
The Pleistocene interval at Site 237 is marked by
the Cassidulina carinata–Cassidulina laevigata (Cc–Cl)
biofacies. The characteristic species of this biofacies are
Cassidulina carinata, C. laevigata, Gyroidinoides orbicularis
(ZG. cibaoensis) and Bolivina pusilla having high negative
loadings on Factor 1. These species prefer to live infaunally
(Corliss and Chen, 1988). Cassidulina carinata, C. laevigata
and G. orbicularis dominate the low to intermediate
productivity assemblages in the Indian Ocean (Gupta and
Thomas, 2003). In other studies, C. laevigata was found
associated with high organic matter fluxes and low
oxygen concentrations (Schmiedl et al., 1997; Sen Gupta and
Machain-Castillo, 1993). Cassidulina carinata, on the other
hand, was observed preferring an opportunistic (slightly less
than Epistominella exigua) life strategy (Fontanier et al., 2003).
This biofacies is inferred to indicate intermediate organic flux,
moderate oxygenation and intermediate to high seasonality in
the late Pleistocene.
4. Discussion
The Plio-Pleistocene interval has undergone significant
change from a warm mode in the early Pliocene to a glacial
mode in the late Pliocene–Pleistocene (Shackleton and Hall,
1984; Zachos et al., 2001). The early Pliocene interval has
been suggested to represent the climax of the Neogene
warmth (e.g. Kim and Crowley, 2000; Raymo et al., 1996;
Crowley, 1996). Although the cause of this climatic warmth is
still debatable, there could be several possible mechanisms
including changing CO2 levels, orographic effect due to
Cenozoic uplift and/or latitudinal heat transport (Rind and
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
287
Fig. 6. Benthic foraminiferal assemblages and Orbulina universa test diameters plotted against numerical ages at DSDP Site 241 (Gupta, 1997). Also shown in the
right hand columns are organic food supply and deep-sea oxygenation.
Chandler, 1991; Crowley, 1996). Raymo et al. (1996)
suggested that in the early Pliocene, CO2 levels were 35%
higher than the preindustrial levels, whereas Zhisheng et al.
(2001) argued for a phased uplift of the Himalaya–Tibetan
Plateau that influenced the Indian monsoon system since the
late Miocene.
The benthic foraminiferal biofacies at Site 237 reflect
changes in the organic flux and in oxygenation of deep waters.
However, there is no direct correlation between high food
supply and low oxygenation. Gupta and Thomas (1999)
suggested that the changes in productivity at southeastern
Arabian Sea DSDP Site 219 (water depth 1764 m) are linked
partly to the SW monsoon variability and partly to changes in
oxygenation (Fig. 4). Changes in oxygenation are related in
part to productivity and partly to changes in deep-water
ventilation. Since Site 237 lies in an area influenced by the
SEC, which intensifies during the Indian southwest (SW)
monsoon season (Tchernia, 1980), we expect the deep-sea
fauna to have been influenced by the organic food supply from
the surface partly or wholly linked to the SW monsoon/SEC
induced open ocean upwelling. Because of the location of Site
237 in an open ocean setting where deep water is expected to
have abundant oxygen, the oxygen may not have played a
major role in influencing the benthic fauna at this site in the
studied interval (e.g. Murray, 2001).
During the early Pliocene warm period, the productivity
was high over large parts of the Indo-Pacific region during
the biogenic bloom, which ended at about 4–3 Ma after
reaching its peak at 5–4.5 Ma in the Indian Ocean and at 4 Ma
in the Pacific Ocean (Berger and Stax, 1994; Dickens and
Owen, 1994; Farrell et al., 1995; Gupta and Thomas, 1999;
Hermoyian and Owen, 2001). The biogenic bloom ranged from
w9–3 Ma, during which time the Oxygen Minimum Zone
(OMZ) expanded over intermediate depths (w1000–2000 m)
of the Indian Ocean (Hermelin, 1992; Dickens and Owen,
1994; Gupta and Thomas, 1999). Filipelli (1997) linked this
high productivity event in the Indian Ocean (8–4 Ma) to an
increased nutrient supply due to increased weathering,
resulting in turn from increased monsoonal activity since the
early Pliocene. This was an interval characterized by a stronger
SW monsoon (e.g. Takahashi and Okada, 1997; Gupta and
Thomas, 1999; 2003). At Site 237, two biofacies, characteristic
of high organic food supplies (Au–Up and Pb–Bs), alternate
between 5.6 and 4.5 Ma, suggesting a high food supply linked
to the intense SW monsoon/SEC induced upwelling (Fig. 3).
The end of biofacies Pb–Bs corresponds with the end of the
288
A.K. Gupta et al. / Journal of Asian Earth Sciences 28 (2006) 276–290
biogenic bloom at ca 4.5 Ma. In general, productivity (biogenic
bloom) increased at several lower bathyal- to abyssal-depth
DSDP sites in the Indian Ocean during the early Pliocene,
which ended at ca 4.5–3 Ma (Figs. 4 and 5; Gupta, 1997; Gupta
and Satapathy, 2000; Gupta and Thomas, 1999). The biogenic
bloom event is also observed at low-productivity eastern Indian
Ocean Sites 757 and 758 (Gupta and Thomas, 2003; Singh and
Gupta, 2004).
Since w4.5 Ma, the deep-sea temperatures cooled (d18O
increased) and productivity fluctuated, most likely due to
weakening of the SW monsoon and strengthening of the
Northeast (NE) monsoon (Zachos et al., 2001; Gupta and
Thomas, 2003). The seasonality increased and food supply
became more variable during the late Pliocene–Pleistocene.
During this time, three biofacies (Gt–Nu, Gn–Cw, Cc–Cl),
characteristic of variable (low to intermediate) organic flux,
were dominant at Site 237 (Fig. 3). Although the interval 3–
1 Ma is missing at Site 237 owing to non-recovery of core
during drilling, overall the productivity decreased from the
early to late Pleistocene at numerous DSDP Sites in the Indian
Ocean (Figs. 4–6).
5. Conclusions
Results of multivariate and stable isotope analyses of deepsea benthic foraminifera suggest that the food supply to the sea
floor varied owing to changes in the intensity of the Indian
monsoons/South Equatorial Current over the past 5.6 Ma at
DSDP Site 237. The faunal trends suggest that the organic
carbon flux, linked to monsoon variability and/or changes in
the South Equatorial Current, played a major role in shaping
the benthic faunal population in the Central Indian Ocean. The
low seasonality and high food supply (high surface productivity) from 5.6 to 4.5 Ma (biofacies Au–Up, Pb–Bs), was
followed by a more variable and seasonal flux of organic matter
from 4.5 Ma onwards (biofacies Gt–Nu, Gn–Cw, Cc–Cl). This
change coincides with the end of the Indo-Pacific biogenic
bloom (9–4 Ma). The Oxygen Minimum Zone expanded to
large areas of intermediate depths in the Indian Ocean during
the early Pliocene when food supply was high and summer
monsoon intensity peaked (Takahashi and Okada, 1997;
Dickens and Owen, 1999). We note from the earlier studies
that a similar change occurred at a number of lower bathyal to
abyssal sites in different parts of the Indian Ocean (Figs. 4–6).
The d18O values were low in the early Pliocene (w5.6–4.5 Ma)
corresponding to an interval of global warmth.
Acknowledgements
AKG thanks the Deep Sea Drilling Project for providing
samples to carry out the present study. MD thanks the IIT,
Kharagpur for financial support. S.M. Savin, Case Western
Reserve University, supported the isotope analysis. This
manuscript has greatly benefited from thoughtful comments
from Chris W. Smart and two anonymous reviewers.
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