Reconstructing the thermal structure of the upper ocean : insights from planktic foraminifera shell chemistry and alkenones in modern sediments of the tropical eastern Indian Ocean

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 26 (2011): PA3219, doi:10.1029/2011PA002132.Shell chemistry of planktic foraminifera and the alkenone unsaturation index in 69 surface sediment samples in the tropical eastern Indian Ocean off West and South Indonesia were studied. Results were compared to modern hydrographic data in order to assess how modern environmental conditions are preserved in sedimentary record, and to determine the best possible proxies to reconstruct seasonality, thermal gradient and upper water column characteristics in this part of the world ocean. Our results imply that alkenone-derived temperatures record annual mean temperatures in the study area. However, this finding might be an artifact due to the temperature limitation of this proxy above 28°C. Combined study of shell stable oxygen isotope and Mg/Ca ratio of planktic foraminifera suggests that Globigerinoides ruber sensu stricto (s.s.), G. ruber sensu lato (s.l.), and G. sacculifer calcify within the mixed-layer between 20 m and 50 m, whereas Globigerina bulloides records mixed-layer conditions at ∼50 m depth during boreal summer. Mean calcifications of Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, and Globorotalia tumida occur at the top of the thermocline during boreal summer, at ∼75 m, 75–100 m, and 100 m, respectively. Shell Mg/Ca ratios of all species show a significant correlation with temperature at their apparent calcification depths and validate the application of previously published temperature calibrations, except for G. tumida that requires a regional Mg/Ca-temperature calibration (Mg/Ca = 0.41 exp (0.068*T)). We show that the difference in Mg/Ca-temperatures of the mixed-layer species and the thermocline species, particularly between G. ruber s.s. (or s.l.) and P. obliquiloculata, can be applied to track changes in the upper water column stratification. Our results provide critical tools for reconstructing past changes in the hydrography of the study area and their relation to monsoon, El Niño-Southern Oscillation, and the Indian Ocean Dipole Mode.This project was funded by the German Ministry of Education and Research (BMBF project PABESIA) and the “Deutsche Forschungsgemeinschaft” (DFG project HE 3412/15–1)

Carbon and oxygen isotopic composition of planktonic foraminifera from laboratory culture, plankton tows and Recent sediment; implications for the reconstruction of paleoclimatic conditions and of the global carbon cycle

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Seasonal patterns of shell flux, delta O-18 and delta C-13 of small and large N. pachyderma (s) and G. bulloides in the subpolar North Atlantic

<p>Past water column stratification can be assessed through comparison of the delta O-18 of different planktonic foraminiferal species. The underlying assumption is that different species form their shells simultaneously, but at different depths in the water column. We evaluate this assumption using a sediment trap time-series of Neogloboquadrina pachyderma (s) and Globigerina bulloides from the NW North Atlantic. We determined fluxes, delta O-18 and delta C-13 of shells from two size fractions to assess size-related effects on shell chemistry and to better constrain the underlying causes of isotopic differences between foraminifera in deep-sea sediments. Our data indicate that in the subpolar North Atlantic differences in the seasonality of the shell flux, and not in depth habitat or test size, determine the interspecies Delta delta O-18. N. pachyderma (s) preferentially forms from early spring to late summer, whereas the flux of G. bulloides peaks later in the season and is sustained until autumn. Likewise, seasonality influences large and small specimens differently, with large shells settling earlier in the season. The similarity of the seasonal delta O-18 patterns between the two species indicates that they calcify in an overlapping depth zone close to the surface. However, their delta C-13 patterns are markedly different (> 1 parts per thousand). Both species have a seasonally variable offset from delta C-13(DIC) that appears to be governed primarily by temperature, with larger offsets associated with higher temperatures. The variable offset from delta C-13(DIC) implies that seasonality of the flux affects the fossil delta C-13 signal, which has implications for reconstruction of the past oceanic carbon cycle. Citation: Jonkers, L., S. van Heuven, R. Zahn, and F. J. C. Peeters (2013), Seasonal patterns of shell flux, delta O-18 and delta C-13 of small and large N. pachyderma (s) and G. bulloides in the subpolar North Atlantic, Paleoceanography, 28, 164-174, doi:10.1002/palo.20018.</p>