Millennial‐scale records of planktonic foraminiferal Mg/Ca, bulk sediment U 37 K′ , and planktonic foraminiferal δ 18 O are presented across the last two deglaciations in sediment core NIOP929 from the Arabian Sea. Mg/Ca‐derived temperature variability during the penultimate and last deglacial periods falls within the range of modern day Arabian Sea temperatures, which are influenced by monsoon‐driven upwelling. The U 37 K′ ‐derived temperatures in MIS 5e are similar to modern intermonsoon values and are on average 3.5°C higher than the Mg/Ca temperatures in the same period. MIS 5e U 37 K′ and Mg/Ca temperatures are 1.5°C warmer than during the Holocene, while the U 37 K′ ‐Mg/Ca temperature difference was about twice as large during MIS 5e. This is surprising as, nowadays, both proxy carriers have a very similar seasonal and depth distribution. Partial explanations for the MIS 5e U 37 K′ ‐Mg/Ca temperature offset include carbonate dissolution, the change in dominant alkenone‐producing species, and possibly lateral advection of alkenone‐bearing material and a change in seasonal or depth distribution of proxy carriers. Our findings suggest that (1) Mg/Ca of G. ruber documents seawater temperature in the same way during both studied deglaciations as in the present, with respect to, e.g., season and depth, and (2) U 37 K′ ‐based temperatures from MIS 5 (or older) represent neither upwelling SST nor annual average SST (as it does in the present and the Holocene) but a higher temperature, despite alkenone production mainly occurring in the upwelling season. Further we report that at the onset of the deglacial warming, the Mg/Ca record leads the U 37 K′ record by 4 ka, of which a maximum of 2 ka may be explained by postdepositional processes. Deglacial warming in both temperature records leads the deglacial decrease in the δ 18 O profile, and Mg/Ca‐based temperature returns to lower values before δ 18 O has reached minimum interglacial values. This indicates a substantial lead in Arabian Sea warming relative to global ice melting.
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.
Abstract. Mg/Ca ratios measured in benthic foraminifera have been explored as a potential palaeothermometry proxy for bottom water temperatures (BWT). Mg/Ca-BWT calibrations from the Indian Ocean are rare and comprise conflicting results. Inconsistencies between studies suggest that calibrations may need to be region specific. The aim of this study was to develop benthic foraminifera (Uvigerina peregrina, Cibicidoides wuellerstorfi and Cibicidoides mundulus) based Mg/Ca – BWT calibrations in the tropical western Indian Ocean. Testing variations of existing analytical protocols, aimed at optimising cleaning of the foraminifera while avoiding sample loss in the process, entailed that a previously established protocol by Barker et al. (2003) was the most suitable for our study. The majority of samples of Cibicidoides mundulus and Uvigerina peregrina, however, remained contaminated, rendering those data unusable for Mg/Ca core-top calibrations. Only Mg/Ca ratios in Cibicidoides wuellerstorfi allowed a tentative Mg/Ca - BWT calibration with the relationship being: Mg/Ca = 0.19 ± 0.02 ∗ BWT + 1.07 ± 0.03, 𝑟2 = 0.87. While this result differs to some degree from previous studies it principally suggests that existing core-top calibrations from the wider Indian Ocean can be applied to core-tops in the western Indian Ocean. The agreement of Mg/Ca ratios at lower temperatures in Cibicidoides wuellerstorfi, Cibicidoides mundulus and Uvigerina peregrina with Mg/Ca ratios reported for these species at low temperatures in other studies supports this conclusion. Many uncertainties surrounding the Mg/Ca proxy exist and more calibration studies are required to improve this method.
Abstract. The Asian monsoon system is a crucial part of the global climate system affecting a significant proportion of the world population. Understanding the controls for changes in the monsoon system is crucial for meaningful assessments of future climate change. The Arabian Sea is part of the wider Asian monsoon system and has been studied regarding controls of monsoon variability through time. In this study we present sea surface temperature data from 37–67 ka BP from sediment core NIOP 929 from the western Arabian Sea assessing the importance of northern/southern hemispheric climate change driving monsoon circulation in the Arabian Sea. Earlier work implies a straightforward link between monsoon variation in the Arabian Sea and northern hemisphere millennial scale climate change during glacial periods, as depicted in Greenland ice cores. We present a new millennial-scale Mg/Ca based sea surface temperature reconstruction based on the planktic foraminifera species G. bulloides and G. ruber. We use these data to calculate seasonal sea surface temperatures. The SST data are variable with a maximum short-term change of 8–9 °C. The variations in our SST records appear not related to change in either hemisphere in a straightforward fashion by not showing a phase-locked relation to millennial scale change in Greenland or Antarctic ice core records. We discuss these changes in the context of the Arabian Sea potentially being a “melting pot” with both the northern and the southern hemisphere exerting influence on a seasonal scale.
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