Oxygen Isotopic Composition of Multiple Species of Planktonic Foraminifera: Recorders of the Modern Photic Zone Temperature Gradient

Abstract

Abundances of 12 species of planktonic foraminifera collected in two plankton tows from the east tropical Atlantic are compared to the chlorophyll content and the temperature of the sea water from which they were collected. As expected from previous work in the tropics, all dominant tropical species occur in greatest abundance within the photic zone. Many species occur in greatest abundances in the seasonal thermocline in association with the maximum chlorophyll concentration, while a few algal symbiont-bearing species occur in greatest abundance in the mixed layer. The •80 measurements of planktonic foraminifera shells from core top sediment samples confirm the vertical stratification within the photic zone that is suggested by the relationship between hydrography and abundances found in the plankton tows and found in the statistical study by Ravelo et al. [ 1990]. Comparison between the measured •80 values of planktonic foraminifera with the predicted •80 profiles of the overlying water column at three core locations indicate that species abundances in the sediment record the seasonally integrated conditions of the photic zone and suggests that the abundance of a species in the sediment depends on whether the preferred ecological conditions of that species may be 1Now at Marine Sciences, Paper number 92PA02092 0883-8305/92/92PA-02092510.00 found within the photic zone of the overlying water column sometime during the year. Species which calcify below the photic zone have only trace relative abundances. Finally, it appears that the total range of •18 0 values of the dominant species approximates the predicted annual •80 of calcite range in the upper 80 m of the water column.

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

“…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. Hence, the utility of foraminifer δ 13 C for deducing vertical hydrographic structure and upwelling remains an open problem.…”

“…Monsoonal upwelling produces a very shallow and intense thermocline in the western Arabian Sea, which may account for the <0.5‰ δ 18 O difference between mixedlayer taxa and thermocline species (Kroon and Darling, 1995;Kroon and Ganssen, 1989). Similarly, Ravelo and Fairbanks (1992) report a δ 18 O difference of ~0.6‰−0.7‰ between G. sacculifer and N. dutertrei in a core from the northwest African upwelling center. Sediment traps in the Arabian Sea accumulate virtually all their particle and foraminiferal flux during upwelling periods (Curry et al, 1992).…”

“…Several studies have shown that G. sacculifer calcifies largely in the surface mixed layer or very shallow thermocline while N. dutertrei grows in the thermocline (Curry et al, 1983;Fairbanks et al, 1982;Ravelo and Fairbanks, 1992;Ravelo and Shackleton, 1995;Sautter Reynolds and Thunell, 1991). Foraminifers caught in vertically stratified plankton tows yield δ 18 O consistent with the water depths and temperatures from which they were collected (Fairbanks et al, 1982).…”

“…Much the same is true for the eastern equatorial Atlantic, where the surface ocean has a strong, shallow thermocline throughout the year. There, the δ 18 O gradient between G. sacculifer and N. dutertrei is about 1.5‰ (Ravelo and Fairbanks, 1992).…”

Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in &delta;¹⁵N-signals of suspended matter and surface sediments

“…Around longitude 13.75 E, however, there is a deep chlorophyll maximum (DCM) at the depth of the thermocline ( Figs 3C, F, H). DCM are a common feature in many regions of the tropical Atlantic (Ravelo & Fairbanks 1992). The phenomenon was observed during both METEOR cruise M57-3 and the AHAB 05-expedition and vanishes south of 23 S. (Fig.…”