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Indian Ocean high-productivity event (10–8 Ma): Linked to global cooling or to the initiation of the Indian monsoons?
USA, and Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, Connecticut 06520-8109, USA
Anil K. Gupta*
Raj K. Singh Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721 302, W.B., India Sudheer Joseph
Ellen Thomas Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459-0139,
Uplift of the Himalayas and Tibetan Plateau (ca. 10–8 Ma)
has been said to be the main cause of the origin or intensification of the Indian monsoon system, because mountains modulate the land-sea thermal contrast. The intensification of the monsoons, in turn, is seen as the cause of major changes in fauna and flora on land (as a result of changing precipitation patterns) as well as in the Indian Ocean, where the monsoons drive increased upwelling and thus increased productivity. We argue that the interactions between the elevation of the Himalayas and Tibetan Plateau, the onset of the monsoons, and their effects on the Indian Ocean biota remain uncertain. The timing of these events (uplift, monsoons, and biotic change) is not well constrained. Neogene deep-sea benthic foraminiferal faunal and isotope records of the Ninetyeast Ridge combined with published data show that a major increase in bio- genic productivity occurred at 10–8 Ma throughout the Indian Ocean, the equatorial Pacific, and southern Atlantic. We suggest that this Indian Ocean high-productivity event was not simply the result of monsoon-induced upwelling or nutrient delivery from the weathering of newly uplifted mountains, but may have been caused by strengthened wind regimes resulting from global cooling and the increase in volume of the Antarctic ice sheets.
Keywords: Indian monsoons, Indian Ocean productivity, benthic fo- raminifera, stable isotopes, Neogene, Ocean Drilling Program, Antarc- tic ice sheets.
The Indian (or Asian) summer and winter monsoons, marked by seasonally reversing winds, influence precipitation and runoff as well as vegetation in South Asia and the biota in the Indian Ocean (Fig. 1). It has been suggested that uplift of the Himalayas and Tibetan Plateau heightened the land-sea thermal contrast (Hahn and Manabe, 1975), thus causing the origin or strong intensification of the Indian mon- soons, with their heavy summer rainfall (summer monsoon) over the Indian subcontinent (e.g., Ruddiman and Kutzbach, 1989; Hastenrath, 1991), and their cold dry winter monsoons. According to this hypoth- esis, changes in the elevation of the Himalayan-Tibetan region have modulated the development of the South Asian monsoons since the middle to late Miocene (Molnar et al., 1993; Clift et al., 2002). Various proxy records have been interpreted as indicating that the monsoons started or strongly intensified between ca. 10 and 8 Ma, as a response to Himalayan-Tibetan uplift to at least about half of its present eleva- tion (e.g., Prell and Kutzbach, 1992; Rea, 1992). These proxies indicate that the intensification of monsoonal winds led to increased upwelling in the Arabian Sea (Kroon et al., 1991) and eastern Indian Ocean (Singh and Gupta, 2004), a shift from C3- to C4-type vegetation on land (Quade et al., 1989), and increased terrigenous flux to the Indian Ocean as a result of increased weathering and erosion in the uplifted
mountainous region (Prell and Kutzbach, 1992). The high weathering rates increased nutrient flux (including P) to the oceans, increasing oceanic productivity even more (Filipelli, 1997).
Changes in the flora and fauna of the Indian subcontinent and the Indian Ocean thus have been linked primarily to the development of the Indian monsoons, which in turn has been linked to the uplift of the Himalayas and Tibetan Plateau. However, the timing of the three- component sets of events necessary to this model (1—uplift of the Himalayas and Tibetan Plateau, 2—changes in climate, and 3—chang- es in biota) is not well constrained (Table 1). This model thus is far from proven, although it or parts of it may be valid (e.g., see Hay et al., 2002).
In contrast, we propose that the biotic changes ca. 10–8 Ma in the Indian Ocean were not simply related to the initiation or intensi- fication of the monsoons, because they occurred over a much larger region and extended into the Pacific and the Atlantic Oceans, as a part of the so-called ‘‘biogenic bloom’’ (Dickens and Owen, 1999; Her- moyian and Owen, 2001). The strengthening of the Indian monsoon system, if it occurred at this time, may have been only one of the several responses to global climate change. We present benthic fora- miniferal faunal and isotope data from Ocean Drilling Program (ODP) Sites 752, 757, and 758 (Leg 121) and Deep Sea Drilling Project (DSDP) Sites 214 and 216 (Leg 22) in the eastern and southeastern Indian Ocean (Fig. 1); we then compare these to published carbonate data (Peterson et al., 1992).
LOCATION AND METHODS
ODP Hole 752A (lat 30 53.48 S, long 93 34.65 E; water depth 1086 m) is on the Broken Ridge, whereas Holes 757B (lat 17 01.46 S, long 88 10.90 E; water depth 1652 m) and 758A (lat 5 23.05 N, long 90 21.67 E; water depth 2924 m) are located on the Ninetyeast Ridge (Fig. 1), as are DSDP Holes 214 (lat 11 20.21 S, long 88 43.08 E; water depth 1671 m) and 216A (lat 01 27.73 N; long 90 12.48 E; water
Figure 1. Location of Ocean Drilling Program (circles) and Deep Sea Dril- ling Project (triangles) sites used, with directions of summer (continuous ar- rows) and winter monsoon (broken arrows) winds.
2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or email@example.com.
Geology; September 2004; v. 32; no. 9; p. 753–756; doi: 10.1130/G20662.1; 3 figures; 1 table; Data Repository item 2004131. 753
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