Carbonate and carbon fluctuations in the Eastern Arabian Sea over 140 ka: Implications on productivity changes?

ARTICLE IN PRESS

Deep-Sea Research II 52 (2005) 1981–1993 www.elsevier.com/locate/dsr2

Carbonate and carbon fluctuations in the Eastern Arabian Sea over 140 ka: Implications on productivity changes? M.V.S. Gupthaa, P. Divakar Naidua,, Birgit Gaye Haakeb, Ralf Schiebelc a National Institute of Oceanography, Dona Paula, Goa-403 004, India Institute of Biogeochemistry and Marine Chemistry, University of Hamburg, Grabenstr. 27, 20357 Hamburg, Germany c School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH, UK b

Received 25 April 2003; accepted 21 May 2005

Abstract Biological productivity in the western Arabian Sea was higher during interglacial than glacial times. In the eastern Arabian Sea productivity was higher during the glacials compared to interglacials, which is in sharp contrast to the southwest monsoon intensity variations. To examine temporal changes in productivity in the eastern Arabian Sea over the last 140 ka, oxygen isotopes, calcium carbonate and organic carbon on three cores (SL-1 & 4 and SK 129-CR05) were analyzed. Oxygen isotope records display distinct glacial and interglacial transitions. In the northeastern (Core SL1) and eastern Arabian Sea (Core SL-4) both calcium carbonate and organic carbon variations show no significant systematic relationship with glacial and interglacials periods. In the southeastern Arabian Sea (Core SK-129-CR05) calcium carbonate shows high and low values during interglacial and glacials, respectively, and temporal changes in organic carbon concentration are significant only during MIS 5. Differential variation of calcium carbonate and organic carbon concentration at the northeastern and southeastern Arabian Sea, and between glacials and interglacials, are attributed to regional differences in sedimentation rates, dilution and preservation, which modify the signal of carbonate and carbon production. r 2005 Elsevier Ltd. All rights reserved.

1. Introduction Monsoons play a dominant role in controlling the regional climate, biological productivity and Corresponding author. Tel.: +91 832 2450232;

fax: +91 832 2450602. E-mail address: [email protected] (P.D. Naidu).

particulate flux to the sediment in the northern Indian Ocean. Sediment trap experiments have demonstrated that biological productivity and terrigenous supply in the Arabian Sea is strongly linked to the intensity of the monsoons. Detailed studies have been carried out in the Arabian Sea to understand the monsoonal influence on the biological productivity and terrigeneous supply

0967-0645/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2005.05.003

ARTICLE IN PRESS 1982

M.V.S. Guptha et al. / Deep-Sea Research II 52 (2005) 1981–1993

during the Late Quaternary (Sirocko and Sarnthein, 1989; Shimmield et al., 1990; Clemens et al., 1991; Murray and Prell, 1992; Naidu, 1991; Naidu et al., 1993; Naidu and Shankar, 1999; Bhusan et al., 2001). It is generally understood that the summer monsoon was stronger during interglacials than glacials (Prell et al., 1992 and references there in). Nevertheless, differential reaction of productivity proxies in different regions of the Arabian Sea leads to contradictory conclusions on the relationship between productivity and monsoonal strength (Clemens et al., 1991; Naidu and Shankar, 1999; Agnihotri et al., 2003; Ivanova et al., 2003). This paper discusses the relation between monsoonal driven biological productivity and the sedimentary record of calcium carbonate and organic carbon of the eastern Arabian Sea during the last 140 ka.

2. Oceanography

3. Material and methods In the eastern Arabian Sea, two gravity cores SL-1 (19159.90 N/651400 E) and SL-4 (151400 N/ 681350 E) were obtained during R.V. Meteor cruise 33/1, during September and October 1995, from 3167 m and 3820 m water depth respectively. Core length is 578 and 519 cm, respectively. Box Core SK 129-CR-05 (91210 N/711590 E), 552 cm core length, was collected during O.R.V. Sagar Kanya cruise 129, from 2300 m water depths (Fig. 1). Gravity cores SL-1 and SL-4 were sampled for biogeochemical analysis at 5–10 cm spacing (Tables 1 and 2; see Ivanova et al., 2003). Box core SK 129-CR-05 was subsampled at 4–10 cm intervals (Table 3; see Pattan et al., 2003). Oxygen isotopic ratios were measured on the planktic foraminiferal species Globigerinoides ruber (white) 4200 mm test size. Samples of SL-1 30°N

IRAN

PAKISTAN 25°

OMAN

INDIA

724

20°

SL-1 20 0m

SL-4

15°

3000 m

Circulation in the Arabian Sea is controlled by the seasonal reversal of the winds caused by the alternate heating and cooling of the Tibetian Plateau and the resulting pressure changes. During summer (June through September) strong southwesterly winds blow across the Arabian Sea, which cause offshore Ekman transport and intense upwelling along the Oman and Somalia margins and the southwest coast of India (Wyrtki, 1973). The upwelling process brings cold, nutrient-rich waters from a few hundred meters depth into the surface and fuels the biological productivity in the euphotic zone. Thus, the Arabian Sea is one of the most productive regions of the world oceans (Qasim, 1977). During winter, northeasterly winds suppress the upwelling and cause low biological productivity in the western Arabian Sea. In contrast, the northeasterly winds cause upwelling in the eastern Arabian Sea. Therefore, the seasonal reversal of the wind direction and associated circulation pattern has a direct bearing on the biological productivity in the Arabian Sea (Kobanova, 1968) and the lithogenic and biogenic flux to the sediment (Nair et al., 1989). In addition to the strong monsoonal character of the atmospheric and oceanic circulation, the Arabian Sea is unique

by the presence of a well developed oxygen minimum zone (OMZ) with one of the largest marine volumes of suboxic ocean water today (Morrison et al., 1998).

GC5

10°

SK 129-CR05

5°N 55°E

60°

65°

70°

75°

80°E

Fig. 1. Location of analyzed sediment cores along the eastern Arabian Sea (black dots), and additional sites under discussion (black circles). The 200 and 3000 m isobath are given for orientation.

ARTICLE IN PRESS M.V.S. Guptha et al. / Deep-Sea Research II 52 (2005) 1981–1993 Table 1 Core depth, age, stable oxygen isotope data, linear sedimentation rate (LSR), and calcite and organic carbon concentration of sediment core SL-1 Depth (cm) 1 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240

Age (ka BP) 0.49 1.70 3.20 4.80 6.30 7.80 9.70 11.60 13.50 15.40 17.30 17.90 18.50 19.00 19.70 20.30 20.90 21.50 22.00 22.60 23.20 23.80 24.40 25.00 25.60 26.20 26.80 26.94 27.08 28.08 29.07 30.07 31.06 32.06 33.05 34.05 35.04 36.04 37.03 38.03 39.02 40.02 41.02 42.01 43.01 44.00 45.00 45.99 46.99

d18O (%)

LSR (cm/ka)

2.09 1.89 1.46 1.04 1.27 0.73 0.47 0.06 0.15 0.37 0.20 0.04 0.03 0.01 0.28 0.02 0.11 0.02 0.08 0.09

3.33 3.13 3.33 3.33 2.63 2.63 2.63 2.63 2.63 8.33 8.33 10.00 7.14 8.33 8.33 8.33 10.00 8.33 8.33

0.02

8.33

0.12

8.33

0.09

8.33

0.09

35.48

0.11

5.02

0.34

5.02

0.45

5.02

0.24

5.02

0.40

5.02

0.75

5.02

0.69

5.02

0.26

5.02

0.49

5.02

0.24

5.02

CaCO3 (%)

Corg (%)

47.23 49.34 58.99 58.62 59.70 53.35 48.57 50.71 41.97 39.31 39.76 41.17 44.95 62.02 44.18 46.33 46.64 44.98 47.50 43.51 41.95 41.80 36.32 38.12 42.06 41.18 41.08 41.07 39.47 37.06 39.23 40.69 36.35 40.36 41.35 40.36 39.41 44.53 42.33 39.83 39.20 35.18 32.33 32.11 25.96 30.85 33.34 32.38 38.10

0.44 0.61 0.47 0.35 0.10 0.43 0.12 0.12 0.33 0.22 0.42 0.06 0.38 0.31 0.47 0.56 0.37 0.40 0.49 0.73 0.37 0.32 0.51 0.28 0.54 0.47 0.29 0.28 0.70 0.36 0.24 0.21 0.60 0.34 0.34 0.23 0.33 0.32 0.24 0.56 0.75 0.61 0.85 0.77 0.50 0.38 0.49 0.73 0.56

1983

Table 1 (continued ) Depth (cm)

Age (ka BP)

245 250 255 260 265 270 275 280 285 290 295 300 305 310 315 320 325 330 335 340 345 350 355 360 365 370 375 380 385 390 395 400 405 410 415 420 425 430 435 440 445 450 455 460 465 470 475 480 485 490 495 500

47.98 48.98 49.97 50.97 51.96 52.96 53.95 54.95 55.94 56.94 57.93 58.93 59.96 60.98 62.01 63.03 64.06 65.09 66.11 67.14 68.16 69.19 70.45 71.71 72.96 74.22 75.48 76.74 77.99 79.25 82.08 84.90 87.50 90.10 91.10 92.10 93.05 94.00 95.00 96.00 96.95 97.90 98.90 99.90 100.85 101.80 102.80 103.80 105.23 106.65 108.08 109.50

d18O (%)

LSR (cm/ka)

0.50

5.02

0.41

5.02

1.04

5.02

0.69

5.02

0.84

5.02

0.39

5.02

0.03

4.87

0.57

4.87

0.36

4.87

0.25

4.87

0.76

4.87

0.50

3.98

0.77

3.98

1.13

3.98

1.39

3.98

0.79

1.77

0.49

1.92

0.64

5.00

1.27

5.26

1.34

5.00

1.57

5.26

1.37

5.00

1.42

5.26

1.90

5.00

1.62

3.51

CaCO3 (%)

Corg (%)

45.24 45.10 51.27 48.68 46.58 49.14 48.93 46.55 45.84 36.88 35.40 34.25 37.17 33.70 35.58 30.70 28.80 26.86 26.77 27.82 27.71 26.77 32.38 31.59 34.77 41.51 41.11 42.98 47.80 37.79 31.01 32.69 33.97 39.41 37.87 37.17 36.43 42.06 46.79 50.07 50.01 40.35 42.64 24.34 18.61 33.18 36.82 25.11 26.35 27.37 31.35 20.57

0.48 0.35 0.23 0.33 0.47 0.41 0.28 0.33 0.14 0.27 0.07 0.39 0.28 0.25 0.00 0.40 0.32 0.44 0.64 0.72 0.48 0.47 0.38 0.37 0.57 0.41 0.36 0.58 0.43 0.33 0.03 0.08 0.31 0.51 0.51 0.42 0.25 0.45 0.38 0.25 0.39 0.32 0.21 0.96 0.32 0.36 0.22 0.24 0.37 0.57 0.46 0.54

ARTICLE IN PRESS M.V.S. Guptha et al. / Deep-Sea Research II 52 (2005) 1981–1993

1984 Table 1 (continued ) Depth (cm)

Age (ka BP)

505 510 515 520 525 530 535 540 545 550 555 560 570

110.90 112.30 115.45 118.60 121.80 125.00 125.85 126.70 127.50 128.30 130.55 132.80 135.30

18

d O (%)

LSR (cm/ka)

1.33

3.57

1.60

1.59

2.03

1.56

1.17

5.88

1.36

6.25

0.36 0.00

2.22 4.00

CaCO3 (%)

Corg (%)

19.97 25.04 28.59 38.04 43.05 45.95 48.56 48.32 48.02 45.10 46.52 44.66

0.52 0.61 0.42 0.24 0.18 0.37 0.42 0.21 0.15 0.19 0.11 0.24

and SL-4 were analyzed using the Finnigan Mat 252 gas mass spectrometer linked to a CARBO Kiel carbonate preparation line at Leibniz Laboratory, Kiel University, Germany (long-term reproducibility of d18O data of 0.08%). SK129CR05 were analyzed at Shizuoka University, Japan, using Finnigan Mat 250 mass spectrometer (long-term reproducibility of d18O data of 0.05%). The values reported are relative to the PEEDEE Belemnite (PDB) scale. Age models of SL-1 and SL-4 are adopted from Ivanova et al. (2003), and of SK 129-CR05 from Pattan et al. (2003). Total carbon was measured with a Carlo Erba Carbon-Nitrogen Analyser 1500 (Milan, Italy). The precision of this method is 0.05% for carbon. Carbonate weight percentages were determined with a Wo¨sthoff Carmhograph 6 at Bochum, Germany. Organic carbon (Corg) was calculated as the difference between total carbon and carbonate carbon.

4. Results Oxygen isotope records obtained from G. ruber display distinct glacial/interglacial transitions in all three cores (Figs. 2–4) back to Marine Isotope Stage (MIS) 6, comprising a time span of about 140 ka. In the northeastern Arabian Sea core SL-1, calcium carbonate varies from 18.6% to 62.0%. The CaCO3 content decreases from MIS 6 to MIS

5. During interglacial MIS 3 and 5 both low and high CaCO3 values occur. Relatively low values occur during glacial MIS 4, intermediate values during MIS 2, and increasing values towards interglacial MIS 1. Organic carbon values range from 0.015% to 0.97%, with no systematic trends corresponding to glacials and interglacials (Fig. 2). In the mid-eastern Arabian Sea core SL-4, calcium carbonate content ranges from 4.4% to 77.6%, with relatively low values during MIS 6–4, and high percentages during MIS 3–1. The glacialto-interglacial transitions, MIS 6/5, MIS 4/3, and MIS 2/1, are marked by increasing CaCO3 values. Organic carbon concentrations vary from 0.54% to 1.3%, with no systematic variations corresponding to glacial and interglacial stages (Fig. 3). In the southeastern Arabian Sea core SK129CR05, calcium carbonate content varies from 30.3% to 66.1%, with higher values during interglacial than glacial stages. Organic carbon content varies from 0.3% to 1.8%, with highest values during interglacial MIS 5. Phases of enhanced organic carbon concentration during MIS 5 follow the warm intervals 5.5, 5.3, and 5.1 with a temporal lag. Organic carbon concentration during MIS 4 through 1 is relatively low, and does not show a systematic trend in relation to glacial and interglacial stages (Fig. 4).

5. Discussion 5.1. Calcium carbonate fluctuations Calcium carbonate and organic carbon records of the three cores SL-1, SL-4, and SK 129-CR05, from the northeastern to southeastern Arabian Sea, are largely different. Differences may be due to two mechanisms, (1) differential biological productivity in surface waters at the investigated sites, and/or (2) differential preservation/dilution of CaCO3 and organic carbon (see Ortiz et al., 2004). The two mechanisms are not entirely decoupled, and higher organic carbon content may cause enhanced CaCO3 dissolution (Emerson and Bender, 1981). Preservation of calcareous particles and organic matter follows different CaCO3 saturation and oxygen concentration of

ARTICLE IN PRESS M.V.S. Guptha et al. / Deep-Sea Research II 52 (2005) 1981–1993 Table 2 Core depth, age, stable oxygen isotope data, linear sedimentation rate (LSR), and calcite and organic carbon concentration of sediment core SL-4 Depth (cm) 0 1 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240

Age (ka BP)

d18O (%)

LSR (cm/ka)

CaCO3 (%)

Corg (%)

49.55 52.92 53.77 59.66 62.67 65.74 77.31 73.04 46.50 38.87 41.48 52.26 55.79 59.52 56.81 45.70 39.39 48.27 52.64 68.92 72.43 64.41 42.51 46.37 48.10 49.85 44.01 44.50 38.85 39.44 40.77 29.65 37.68 40.17 41.85 49.70 55.57 52.72 50.10 56.77 61.90 57.93 56.43 51.13 58.11 58.69 68.08 55.17 63.73

0.15 0.00 0.04 0.40 0.00 0.29 0.11 0.08 0.48 0.00 0.45 0.63 1.07 0.30 0.00 0.67 0.35 0.07 0.43 0.27 0.15 1.06 0.30 0.30 0.27 0.11 0.41 0.41 0.49 0.62 0.35 0.62 0.70 0.50 0.71 0.52 0.53 0.40 0.21 0.17 0.58 0.28 0.82 0.22 0.45 0.52 0.55 0.41 0.30

1.68 0.21 2.11 4.01 5.91 7.81 9.40 10.98 12.56 14.14 15.73 17.31 18.98 20.66 22.33 24.00 27.73 31.45 32.19 32.93 33.67 34.41 35.15 35.89 36.63 37.37 38.46 39.54 40.63 41.71 42.80 43.88 44.97 46.05 47.14 48.22 49.31 50.39 50.70 51.01 52.12 53.22 53.93 54.64 55.35 56.05 56.74 57.43 58.13 58.82

2.13

2.63

2.03

2.63

1.66

3.15

1.27

3.16

0.20

3.15

0.55

2.99

0.43

2.99

0.30

1.34

0.52

6.76

0.36

6.76

0.29

6.76

0.64

6.76

0.48

4.61

0.74

4.61

0.67

4.61

0.69

4.61

0.86

4.61

1.09

4.61

0.90

16.13

0.85

4.52

0.77

7.04

0.91

7.09

0.80

7.25

0.61

7.19

1985

Table 2 (continued ) Depth (cm)

Age (ka BP)

245 250 255 260 265 270 275 280 285 290 295 300 305 310 315 320 325 330 335 340 345 350 355 360 365 370 375 380 385 390 395 400 405 410 415 420 425 430 435 440 445 450 455 460 465 470 475 480 485 490 495 500 519

59.51 60.20 60.90 61.59 62.28 62.97 65.70 70.61 72.52 74.43 75.30 76.17 77.05 77.92 78.79 79.66 81.33 83.00 84.68 86.35 88.02 89.69 91.36 93.04 94.71 96.38 97.62 98.85 99.84 100.83 102.32 103.80 106.48 109.15 111.83 114.50 117.17 119.84 122.52 125.19 125.79 126.39 126.99 127.59 128.19 128.84 129.51 130.17 130.83 131.50 132.16 132.82 135.34

d18O (%)

LSR (cm/ka)

0.19

7.25

0.34

7.19 7.25

0.43

0.79

0.51

2.62

0.90

5.74

0.41

5.74

1.31

5.74

0.86

2.99

0.81

2.99

1.00

2.99

1.51

2.99

1.26

4.05

1.01

5.05

1.27

3.37

1.22

1.87

1.40

1.87

1.46

1.87

1.52

1.87

1.16

8.33

0.81

7.55

0.78

7.52

0.56 0.46

7.55 7.54

CaCO3 (%)

Corg (%)

67.49 32.64 23.22 30.53 46.15 41.69 39.43 18.34 16.43 17.13 20.75 25.95 34.20 15.94 23.62 24.05 31.19 37.78 45.38 25.54 27.39 22.69 25.42 23.48 18.93 27.92 29.63 31.14 31.42 34.43 36.97 32.59 33.51 25.76 32.85 40.15 41.59 32.42 35.65 31.51 23.41 25.79 53.20 39.58 29.37 30.45 32.92 22.36 25.06 25.08 32.16 26.45

0.52 0.09 0.16 0.46 1.03 0.47 0.41 0.03 0.97 0.71 0.35 0.80 0.54 0.44 0.59 0.67 0.58 1.28 0.47 1.26 0.73 0.82 0.78 0.30 0.33 0.31 0.32 0.26 0.44 0.73 0.58 0.37 0.47 1.06 0.31 0.29 0.49 0.44 0.21 0.22 0.32 0.52 0.32 0.20 0.28 0.09 0.33 0.24 0.36 0.30 0.38 0.70

ARTICLE IN PRESS M.V.S. Guptha et al. / Deep-Sea Research II 52 (2005) 1981–1993

1986

Table 3 Core depth, age, stable oxygen isotope data, linear sedimentation rate (LSR), and calcite and organic carbon concentration of sediment core SK 129-CR05 Depth (cm) 0 4 6 8 10 12 14 16 18 20 22 24 26 28 30 34 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 104

Age (ka BP)

0.44 0.92 1.39 1.86 2.33 2.80 3.27 3.75 4.22 4.69 5.16 5.63 6.10 6.58 7.52 8.46 8.93 9.41 9.88 10.35 10.82 11.29 11.76 12.25 12.77 13.30 13.82 14.34 14.86 15.38 15.91 16.43 16.95 17.47 17.99 18.52 19.04 19.56 20.08 20.61 21.13 21.65 22.17 22.69 23.22 23.74 24.28 25.39

d18O (%)

LSR (cm/ka)

2.50 2.76

4.24

2.38

4.24

2.78

4.24

2.52

4.24

2.27

4.24

2.48

4.24

2.24

4.24

CaCO3 (%)

Corg (%)

63.66

0.44

63.50

0.40

64.61

0.40

63.86 66.29

1.89

4.24

1.56

4.24

1.46

4.24

1.40

4.24

0.96

4.24

0.42

3.83

0.49

3.83

0.53

3.83

0.74

3.83

0.46

3.83

0.45

3.83

0.47

3.83

0.36

61.88

0.38

60.85 58.79 54.38

0.38 0.33 0.30

54.37 45.31 46.19 41.82 44.32 45.33

3.83

0.60

3.83

0.55

3.83

0.52

3.83

1.31

3.60

0.45

64.67

54.87

0.62

0.39

0.30 0.39 0.36 0.41 0.43 0.66 0.67

43.91

0.71

42.79

0.73

43.66

0.70

44.05

0.72

42.91

0.70

41.81

0.70

41.29

0.52

38.91 39.25 37.58

0.62 0.62 0.55

Table 3 (continued ) Depth (cm) 108 112 116 120 124 126 128 130 132 134 136 138 142 146 150 152 154 156 158 160 162 164 166 168 170 172 174 176 178 180 182 184 186 188 190 192 196 200 204 208 210 212 216 218 220 224 228 230 232 236 240 242

Age (ka BP)

d18O (%)

LSR (cm/ka)

CaCO3 (%)

Corg (%)

26.50 27.61 28.72 29.83 30.94 31.50 32.06 32.61 33.17 33.72 34.28 34.83 35.94 37.06 38.17 38.72 39.28 39.83 40.39 40.94 41.50 42.06 42.61 43.17 43.72 44.28 44.83 45.39 45.94 46.50 47.06 47.61 48.17 48.72 49.28 49.83 50.94 52.06 53.17 54.28 54.83 55.39 56.50 57.06 57.61 58.72 59.58 59.96 60.35 61.12 61.88 62.27

0.77 0.98 0.95 0.93 1.21

3.60 3.60 3.60 3.60 3.60

42.46 43.05 43.75 42.65 41.71 43.67

0.50 0.58 0.54 0.50 0.52 0.56

1.38

3.60 42.76

0.50

1.38

3.60 45.85

0.58

1.42

3.60

1.42 1.16 1.52

3.60 3.60 3.60

46.69 47.01 48.55 50.56 50.94

0.52 0.45 0.51 0.41 0.43

1.45

3.60 52.61

0.50

1.31

3.60 48.77

0.40

1.27

3.60 47.64

0.40

1.33

3.60 49.51

0.51

1.19

3.60 48.34

0.49

1.28

3.60 50.54

0.61

1.36

3.60 49.49

0.64

1.25

3.60 49.32

0.63

1.43

3.60 47.08

0.62

1.44

3.60 48.87 44.98 45.55 45.50 48.00

0.54 0.53 0.54 0.61 0.65

51.41

0.60 0.52

45.80 47.08 44.03

0.78 0.56 0.49

42.71 40.39 50.14

0.63 0.57 0.64

2.06

1.94

1.41

1.58

3.60

3.60

5.20

5.20

ARTICLE IN PRESS M.V.S. Guptha et al. / Deep-Sea Research II 52 (2005) 1981–1993 Table 3 (continued ) Depth (cm) 244 248 252 254 256 260 264 266 268 272 276 278 280 284 288 292 296 300 302 304 308 312 316 320 322 324 328 330 332 336 340 344 348 350 352 356 360 364 368 370 372 376 380 384 388 390 392 396 400 404 408 410 412

Age (ka BP) 62.65 63.42 64.19 64.58 64.96 65.73 66.50 66.88 67.27 68.04 68.81 69.19 69.58 70.35 71.12 71.88 72.65 73.42 73.81 74.11 74.68 75.25 75.82 76.39 76.68 76.96 77.54 77.82 78.11 78.68 79.25 80.18 81.11 81.58 82.05 82.99 83.92 84.86 85.80 86.27 86.73 87.67 88.61 89.54 90.48 90.95 91.36 92.21 93.05 93.89 94.74 95.16 95.58

1987

Table 3 (continued ) d18O (%)

1.14

1.87

1.40

1.54

1.95

2.03

1.78

1.86

2.38

1.74

2.03

1.98

2.01

1.86

LSR (cm/ka)

CaCO3 (%)

Corg (%)

Depth (cm)

Age (ka BP)

42.44 39.20 37.57

0.39 0.31 0.38

34.43 34.27 34.30

0.40 0.46 0.52

32.43 30.40 30.09

0.56 0.58 0.60

29.82 31.33 33.84 34.09 36.05 35.28

0.64 0.68 0.73 0.81 0.81 0.76

34.05 31.30 36.20 41.14 44.35

0.64 0.54 0.56 0.62 0.67

46.64 42.91

0.61 0.48

51.53 51.64 48.39 49.03 50.55

0.73 0.88 1.33 0.64 0.44

41.79 43.05 49.19 51.61 50.31

0.29 0.35 0.53 0.82 1.11

52.46 52.46 52.71 51.49 51.03

1.44 1.36 1.49 1.55 1.72

416 420 424 428 430 432 436 440 444 448 450 452 456 460 464 468 470 472 476 480 484 488 490 492 496 500 504 508 510 512 516 520 524 528 530 532 536 540 544 548 550

96.43 97.27 98.11 98.96 99.38 99.96 101.10 102.24 103.38 104.52 105.09 105.66 106.80 107.94 109.08 110.22 110.79 113.56 118.69 123.82 124.63 125.46 125.88 126.29 127.13 127.96 128.79 129.63 130.04 130.46 131.29 132.13 132.96 133.79 134.21 134.63 135.46 136.29 137.13 137.96 138.38

54.04 51.61 49.29 53.51 51.27

1.42 1.48 1.77 1.11 1.27

51.32

0.84

5.20

5.20

5.20

5.20

5.20

7.00

7.00

7.00

7.00

4.27

4.27

4.27

4.27

4.27

2.15

4.74

2.14

4.74

d18O (%)

LSR (cm/ka)

2.44

4.74

2.60

4.74

1.99

3.51

2.26

3.51

2.10

3.51

2.02

3.51

2.86

0.78

2.65

4.80

2.46

4.80

1.09

4.80

0.82

4.80

0.94

4.80

0.98

4.80

0.93

4.80

CaCO3 (%)

Corg (%)

51.35 48.88 51.36 53.57

0.67 1.23 0.65 0.51

46.54 44.91 48.90 49.79 51.38

0.28 0.39 0.54 0.72 0.94

53.79 52.86 51.99 50.99 50.21

0.72 0.71 0.90 1.29 1.40

49.62 54.17 51.56 55.06

1.47 0.98 1.06 0.88 0.34

52.04 50.23 50.99 48.82 47.42

0.57 0.76 0.49 0.86 0.70

48.34 48.42 52.67 47.98 44.68

0.47 0.62 0.97 0.59 0.50

46.55 43.15 46.52 41.18 42.69 43.70

0.57 0.36 0.34 0.34 0.30 0.34

subsurface to bottom water masses, which follow the glacial/interglacial cycle (Ivanova et al., 2003). Carbonate dissolution is supposed to be minor in the analyzed cores, because water depth of SL-1 and SK-129-CR05 cores is well above the modern calcite and foraminiferal lysocline depth in this part of the Arabian Sea (3500 m; Cullen and Prell, 1984). Ivanova et al. (2003) suggested that

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1988

1

2

LSR (cm / ka)

3

4

5

6

35.5

10 8 6 4 2

Corg (%)

0 1.0

0.5

CaCO3 (%)

0.0 70 50 30

δ18O (‰)

10 -2.5 -1.5 -0.5 0.5 0

20

40

60

80

100

120

140

Age (ka BP) Fig. 2. Oxygen isotope stratigraphy, down core variation of calcium carbonate (CaCO3) and organic carbon (Corg), and LSR of the core SL-1. LSR ¼ linear sedimentation rate.

dissolution did not alter the planktic foraminiferal assemblage significantly, and the CaCO3 record of SL-1 and SL-4 is rather unaltered. Turbidites are rare at SL-4 (Singh et al., 2002), and may have also been captured by the records given for SL-1 and SK 129-CR05. The SK-129-CR05 carbonate record shows a systematic trend in relation with glacial and interglacial stages, and is interpreted to display changing climate and productivity. Eastern Arabian Sea calcium carbonate changes do not correspond uniformally either to typical Pacific Pattern of high and low values in glacial and interglacials, respectively (Farrell and Prell, 1989), or an Atlantic Pattern of low and high values in glacial and interglacials, respectively (Damuth, 1975). In SK129-CR05 at the southeastern Arabian Sea, terrigenous matter content is higher during glacials than interglacials (Pattan

et al., 2003), showing negative correlation with carbonate record. Therefore, variation in calcium carbonate content may primarily be due to dilution by terrigenous material rather than productivity changes. The input of terrigenious material to the Arabian Sea is related to monsoon intensity and varies on a regional scale across the Arabian Sea (Sirocko, 1989). High linear sedimentation rate (LSR) during late MIS 6, 5, and 4, at SL-1 and SK-129-CR05, correlates with low concentration of CaCO3 and Corg and is possibly caused by enhanced terrigenious supply (Figs. 3 and 4). The lower sea level (120 m) during glacials may have favored the erosion of exposed continental shelf and shortened the pathway of suspended river load in to the deep sea at all these core locations. Major sedimentary cycles in the eastern Arabian

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1

2

3

5

6

16.1

10

LSR (cm / ka)

4

1989

8 6 4 2

Corg (%)

0 1.5 1.0

SL-4

0.5

CaCO3 (%)

0.0 90 70 50 30

δ18O (‰)

10 -2.5 -1.5 -0.5 0.5 0

20

40

60

80

100

120

140

Age (ka BP) Fig. 3. Oxygen isotope stratigraphy, down core variation of calcium carbonate (CaCO3) and organic carbon (Corg), and LSR of the core SL-4. LSR ¼ linear sedimentation rate.

Sea were possibly controlled by variation in the supply of terrigenous matter and the monsoonal driven productivity and depositional flux of CaCO3. 5.2. Organic carbon production In the two northern cores SL-1 and SL-4, systematic variation in organic carbon content across glacial and interglacial MIS are not obvious (Fig. 2 and 3). Similarly, at ODP Site 724 in the northwestern Arabian Sea (Fig. 1) organic carbon content do not vary systematically corresponding to glacial and interglacials (Zahn and Pedersen, 1991). In the southern core SK129-CR05, highest organic carbon content occurs during MIS 5, with some lag to the warmest intervals 5.5 and 5.3 (Fig. 4). During MIS 3 and 1, however, organic carbon content is low.

Variation in organic carbon content may be attributed to changes in either productivity or preservation or both (Emerson, 1985; Emerson and Hedges, 1988; Hedges and Keil, 1995). Paropakari et al. (1993) argued that preservation of organic carbon is determined by the level of oxidation, respectively, the presence of an oxygen minimum zone in the Arabian Sea. Reichart et al. (1998) conclude that variations in organic carbon content in the northern Arabian Sea are not caused by fluctuations in the OMZ, and instead are primarily controlled by changes in surface water productivity. Various studies have mainly focused on the use of organic carbon in deep-sea sediments as a paleoproductivity index (Sarnthein et al., 1988; Lyle et al., 1988). Both production and oxygen availability can, however, not be independent of each other (Sarma, 2004). Another cause of changing organic carbon ratios could be

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1990

1

2

3

4

5

6

LSR (cm / ka)

8 6 4 2

Corg (%)

1.5 1.0 0.5 0.0 80

CaCO3 (%)

SK 129-CR05

0 2.0

60 40

δ18O (‰)

20 -3.0 -2.0 -1.0 0.0 0

20

40

60

80

100

120

140

Age (ka BP) Fig. 4. Oxygen isotope stratigraphy, down core variation of calcium carbonate (CaCO3) and organic carbon (Corg), and LSR of the core SK129-CR05 (after Pattan et al. 2003). LSR ¼ linear sedimentation rate.

dilution by inorganic components (Calvert et al., 1995). Increased sedimentation rate enhances organic carbon preservation in sediments (Heath et al., 1977; Sarnthein et al., 1988). Surface production and sedimentation rate are often positively correlated. However, increased sedimentation rates in the eastern Arabian Sea are not synchronous to increasing monsoon strength (Zahn and Pedersen, 1991). Phase-relationship between proxies of monsoonal forced bioproductivity and organic carbon accumulation rates along the western Arabian Sea imply that either bulk sedimentation rates impose a major influence on the preservation of organic carbon or that oceanic carbon production is not directly linked to monsoon-driven upwelling (Murray and Prell, 1992). A reliable organic carbon-based index of productivity must

therefore be corrected for non-biogenic components. During MIS 5, high organic carbon rates (1.8%) and C/N ratios 428, in core SK129CR05 (Pattan et al., 2003), suggest high terrigenous organic carbon input. Consequently, high organic carbon ratios during MIS 5 at SK129CR05 do not result from enhanced marine productivity alone but are affected by continentally derived organic matter. 5.3. Relationship between monsoon strength, calcium carbonate and organic carbon fluctuations Numerous paleoclimatic studies have shown that summer monsoon is stronger during interglacials than glacials in the western Arabian Sea (Prell and Kutzbach, 1987; Clemens et al., 1991;

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Anderson and Prell, 1993; Ivanova et al., 2003, and references therein) as well as in the eastern Arabian Sea (Rostek et al., 1993; Naidu and Shankar, 1999). Flux of lithogenic matter, bulk carbonate, organic carbon, and foraminiferal tests is tightly coupled to monsoon intensity in the western, central, and eastern Arabian Sea, as shown by sediment trap studies (Haake et al., 1993). A distinct bi-modal distribution is observed representing high fluxes during summer and winter monsoons and minimum flux during inter-monsoon (Nair et al., 1989; Curry et al., 1992; Rixen et al., 1996). This implies that the biological productivity in the modern Arabian Sea is higher during monsoons and lowest during inter-monsoons. It is suggested that there is a strong link between the precessional component of the Earth’s orbital parameters, productivity in the Arabian Sea (Prell, 1984; Rostek et al., 1997; Budziak et al., 2000) and intensity of summer monsoon winds over the Arabian Sea (Clemens et al., 1991). Further, insolation changes associated with the precessional cycle play a dominant role in forcing the SW monsoon strength rather than the glacial and interglacial boundary conditions (Clemens et al., 1991). However, coherency of productivity in the eastern Arabian Sea and boreal summer insolation exhibits a time lag of 6 to 9 ka (Clemens et al., 1991; Altabet et al., 1995; Ivanova et al., 2003). A similar time lag might obscure also the CaCO3 and Corg record at the three sites analyzed here. Based on the phase relationships between percent total organic carbon (TOC), TOC flux and summer insolation maxima in the precessional frequency band, it is suggested that the TOC signal reflects either preservation changes due to enhanced sedimentation rates or to the production of organic carbon and is not directly linked to monsoonal upwelling (Murray and Prell, 1992). In contrast, variation in paleoproductivity in the western Arabian Sea is probably linked to deeper mixing of surface water caused by stronger NE monsoonal winds and a longer NE monsoonal season, which is supplemented by the strength of SW monsoon winds in the Arabian Sea (Emeis et al., 1995). In the eastern Arabian Sea the TOC and alkenone maxima are in phase with maximum ice volume indicating an additional influence of

1991

stronger and prolonged NE monsoon winds associated with cold climates as proposed by Rostek et al. (1997). Therefore, in the eastern Arabian Sea paleoproductivity is predominantly linked to variations in the NE monsoon winds, probably related to precessional cycles and changes in global ice volume (Budziak et al., 2000). Increased productivity in the eastern Arabian Sea during MIS 5 is either due to SW or NE monsoonal-driven upwelling (Ivanova et al., 2003). Enhanced paleoproductivity in the eastern Arabian Sea during glacials is, however, unlikely (Madhupratap et al., 1996) Based on organic carbon accumulation rates of sediment core GC5 off SW India (Fig. 1), it was shown that SW monsoons were strong during late Holocene, and NE monsoons were more intense during last glacial maximum (LGM) (Thamban et al., 2001). High organic carbon accumulation rates during late Holocene and LGM and low accumulation rates during early Holocene in the core GC5 are strongly influenced by changing sedimentation rates. Therefore, organic carbon accumulation rates do not unequivocally warrant conclusion either on monsoon strength or biological productivity.

6. Conclusions Calcium carbonate and organic carbon concentration in the northeastern (SL-1 and SL-4) and southeastern Arabian Sea cores (SK 129-CR05) are different, and may display differential relation to monsoonal intensity. Calcium carbonate changes in the eastern Arabian Sea do not show a consistent pattern similar to Atlantic or Pacific carbonate cycle pattern, and are rather controlled by regionally varying conditions of productivity and sedimentation than by global climate change. Short-term and high-amplitude changes of calcium carbonate and organic carbon concentration obscure a glacial/interglacial signal at the northern sites SL-1 and SL-4. At the southern site SK 129-CR05, down-core changes in CaCO3 concentration could be attributed to glacial/interglacial changes. High organic carbon concentration during MIS 5 follows warm intervals, 5.5, 5.3,

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and 5.1, with a temporal lag. Organic carbon concentration during MIS 4 through MIS 1 is rather uniform and low.

Acknowledgements We thank Dr. A.S. Naidu and Dr. R. Ramesh for their critical comments and suggestions, which improved the readability and interpretations, the Captain, crew and other participants of the R. V. Meteor cruise 33/1 and O. R. V. Sagar Kanya cruise 129 for help and cooperation in collecting the cores. We acknowledge the Department of Ocean Development (DOD, New Delhi) for providing the ship time and Department of Science and Technology, New Delhi for financial support. This is National Institute of Oceanography Contribution 3995. References Agnihotri, R., Sarin, M.M., Somayajulu, B.L.K., Bull, A.J.T., Burr, G.S., 2003. Late-Quaternary biogenic productivity and organic carbon deposition in the eastern Arabian Sea. Paleogeography, Paleoclimatology, Paleoecology 197, 43–60. Altabet, M.A., Francois, R., Murray, D.W., Prell, W.L., 1995. Climate-related variations in denitrification in the Arabian Sea from sediment 15N/14N ratios. Nature 373, 506–509. Anderson, D.M., Prell, W.L., 1993. A 300 kyr record of upwelling off Oman during the Late Quaternary: evidence of the Asian southwest monsoon. Paleoceanography 8, 193–208. Bhusan, R., Dutta, K., Somayajulu, B.L.K., 2001. Concentrations and burial fluxes of organic and inorganic carbon on the eastern margins of the Arabian Sea. Marine Geology 178, 95–113. Budziak, D., Schneider, R.R., Rostek, F., Mu¨ller, P.J., Bard, E., Wefer, G., 2000. Late Quaternary insolation forcing on total organic carbon and C37 alkenone variations in the Arabian Sea. Paleoceanography 15, 307–321. Calvert, S.E., Pedersen, T.F., Naidu, P.D., von Stackelberg, U., 1995. On the organic carbon maximum on the continental slope of the eastern Arabian Sea. Journal of Marine Research 53, 269–296. Clemens, S.C., Prell, W.L., Murray, D.W., Shimmield, G.B., Weedon, G.P., 1991. Forcing mechanisms of the Indian monsoon. Nature 353, 720–725. Cullen, J.L., Prell, W.L., 1984. Planktonic foraminifera of the northern Indian Ocean: distribution and preservation in surface sediments. Marine Micropaleontology 9, 1–52.

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