Foraminiferal production and monsoonal upwelling in the Arabian Sea: evidence from sediment traps

Abstract

Planktonic foraminifera collected in sediment traps in the Arabian Sea during 1986 and 1987 responded to the southern Asian monsoon with changes in productivity, relative abundance of species and isotopic shell chemistry. Most species of foraminifera increased in flux shortly after the advent of the southwest monsoon. G. bulloides increased its production rate by three orders of magnitude. The isotopic chemistry of G. ruber recorded the increase in monsoon upwelling by increasing its δ 18 O values by about 1‰, accurately reflecting the average 4°C sea surface temperature decrease associated with the upwelling. The mean value of δ 18 O for G. ruber was greater in the western Arabian Sea than in the central or eastern basins because upwelling in that region cools surface water. The carbon isotopic composition of G. ruber does not have a clear temporal or geographical relationship to upwelling. While its δ 13 C values decreased in the western Arabian Sea during the upwelling event, the mean δ 13 C values remained higher in the western than in the eastern and central Arabian Sea. This longitudinal gradient is opposite to that expected from the geographical gradient of upwelling: the region with the most intense upwelling should have lower δ 13 C values in surface waters because of the upwelling of low-δ 13 C water to the surface.

Tropical climate variability from the last glacial maximum to the present

“…At most sites, Mg/Ca SSTs are either at or slightly below the observed annual average SST, in accordance with the observation that G. ruber lives year round in the Arabian Sea, but experiences blooms during both monsoon seasons (Curry et al, 1992;Conan and Brummer, 2000;Peeters et al, 2002). The reconstructed west-to-east SST gradient of 2 -4C is consistent with the observed annually averaged and summer SST gradients of 2°C and 4°C, respectively.…”

“…G. bulloides is only present at this site during the summer upwelling season (e.g. Curry et al, 1992). While G. ruber is present year round, its abundance increases during both the summer and winter monsoon seasons (e.g.…”

“…While the percentage of G. bulloides is not strictly a proxy for SST, it has been historically used in this region as an indicator of the strength of summer monsoon upwelling, as the foraminifer is only present during the upwelling season today (e.g. Curry et al, 1992). The second record is an alkenone SST reconstruction published by Higginson et al (2004).…”

“…While G. ruber is present year round, its abundance increases during both the summer and winter monsoon seasons (e.g. Curry et al, 1992).…”

Miocene–Pliocene surface-water hydrography of the eastern equatorial Atlantic

“…Studies of sediment traps and core tops have also found that foraminifer δ 13 C is lowest in samples collected during upwelling times or under upwelling centers in many instances (Curry et al, 1992;Kroon and Ganssen, 1989;Sautter Reynolds and Thunell, 1991;Wefer et al, 1983) However, other studies have found a weak relationship between upwelling and foraminifer δ 13 C. For example, Curry et al (1983) found no obvious relationship between δ 13 C and upwelling in near-surface growing species (G. sacculifer and G. ruber) collected in sediment traps from the Panama Basin. Likewise, Curry et al (1992) found that although there was a distinct decrease in foraminifer δ 13 C during peak upwelling in the Arabian Sea, there was not a significant change in foraminifer δ 13 C along a transect from upwelling centers to non-upwelling regions. Finally, Ravelo and Fairbanks (1995) found no clear relationship between foraminifer δ 13 C and depth habitat inferred from shell δ 18 O, despite the large vertical changes in water δ 13 C from the surface mixed layer to thermocline waters.…”