Abstract
Abstract. The oxygen isotopic composition of planktonic foraminifera tests is one of the widest used geochemical tools to reconstruct past changes of physical parameters of the upper ocean. It is common practice to analyze multiple individuals from a mono-specific population and assume that the outcome reflects a mean value of the environmental conditions during calcification of the analyzed individuals. Here we present the oxygen isotope composition of individual specimens of the surface-dwelling species Globigerinoides ruber and Globigerina bulloides from sediment cores in the Western Arabian Sea off Somalia, inferred as indicators of past seasonal ranges in temperature. Combining the δ18O measurements of individual specimens to obtain temperature ranges with Mg/Ca based mean calcification temperatures allows us to reconstruct temperature extrema. Our results indicate that over the past 20 kyr the seasonal temperature range has fluctuated from its present value of 16 °C to mean values of 13 °C and 11 °C for the Holocene and LGM, respectively. The data for the LGM suggest that the maximum temperature was lower, whilst minimum temperature remained approximately constant. The rather minor variability in lowest summer temperatures during the LGM suggests roughly constant summer monsoon intensity, while upwelling-induced productivity was lowered.
Highlights
Since Cesare Emiliani’s paper (1955) “Pleistocene Temperatures” indicated that the stable oxygen isotope ratio in the tests of foraminifera could be used to infer the temperature during calcification, the δ18O of planktonic foraminifera is one of the most applied methods for reconstructing past ocean temperature changes
The observed ranges in oxygen isotope data from the individual specimens are equivalent to temperature ranges of about 13 ◦C for G. bulloides and 11 ◦C for G. ruber (Fig. 2).The relatively small calcification temperature range of G. ruber indicates that this species does not calcify during the coldest conditions as its lowest temperature tolerance limit is reached
To reconstruct past environmental conditions focusing on seasonal change, we applied the single specimen approach on two planktonic foraminifera species (G. ruber and G. bulloides) from twelve, mostly -spaced samples from sediment core NIOP 905P (Fig. 3)
Summary
Since Cesare Emiliani’s paper (1955) “Pleistocene Temperatures” indicated that the stable oxygen isotope ratio in the tests of foraminifera could be used to infer the temperature during calcification, the δ18O of planktonic foraminifera is one of the most applied methods for reconstructing past ocean temperature changes. Oba (1990) interpreted the isotope differences in Globigerinoides sacculifer of up to 2.8 ‰ in sediment trap samples from northwest Pacific Ocean as a reflection of the temperatures of the vertical calcification range for this species (50–200 m) He further assigned the depth habitat of eleven species based on single specimen analyses (Oba, 1991). Highest seasonal temperature change in the tropics are presently found off Somalia: During late winter and spring, temperatures higher than 30oC prevail, yet during the upwelling season, sea-surface temperatures may drop down to 16 ◦C (Swallow and Bruce, 1966) This high seasonal amplitude in surface water temperatures of >14 ◦C, together with only minor variability in salinity (
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