[1] In order to assess the dissolution effect on foraminiferal Mg/Ca ratios, we analyzed Mg/Ca of seven planktonic foraminiferal species and four of their varieties from Caribbean core tops from $900-4700 m water depth. Depending on the foraminiferal species and variety, Mg/Ca start to decline linearly below ], similar to decreases of $0.5-0.8 mmol/mol per kilometer below $2500-3000 m water depth. Above these speciesspecific critical levels, Mg/Ca remains stable with higher intraspecific Mg/Ca variability than below. We developed routines to correct Mg/Ca from below these critical thresholds for dissolution effects, which reduce the overall intraspecific variability by $24-64%, and provide dissolution-corrected Mg/Ca appropriate to calculate Holocene paleotemperatures. When taking into account only dissolutionunaffected Mg/Ca from <2000 m, the systematic succession of foraminiferal species according to their Mg/ Ca reflects expected calcification depths.

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Mg/Ca ratios of planktonic foraminiferal tests are important tools for reconstructing past ocean temperatures at different levels of the upper water column. Yet numerous studies suggest a significant influence of calcite dissolution on Mg/Ca ratios lowering their initial signal recorded within a planktonic foraminiferal habitat. To determine the effect of dissolution, this study presents Mg/Ca ratios of eight planktonic foraminiferal species from the South China Sea sediment surface. Continuously decreasing with increasing water depth, the Mg/Ca ratios also decrease with calcite-saturation states close to and below saturation (bottom water Δ[CO 2− 3 ] <30 μmol kg −1 ) but are stable in well calcite-supersaturated bottom waters (>40 μmol kg −1 ). This preservation pattern compares well with examples of Mg/Ca dissolution from the tropical Atlantic Ocean and is independent of the foraminiferal species. Merging a global data set by separate normalization of 79 Mg/Ca data sets from the Pacific, Atlantic, and Indian Oceans, which removes thermal differences between the ocean regions and foraminiferal species, enabled us to quantify a global decrease in planktonic foraminiferal Mg/Ca ratios of 0.054 ±0.019 μmol mol −1 per μmol kg −1 below a critical threshold for dissolution of 21.3 ±6.6 μmol kg −1 . The absolute decline in Mg/Ca ratios, which is similar for all species, affects temperature estimates from (sub-)thermocline species more strongly than those from shallow dwellers. The water depth of this critical threshold in the global oceans shoals from >3.5 km in the North Atlantic to <0.5 km in the North Pacific based on calculations of the global calcite-saturation state from 6321 hydrographic stations. Above this critical threshold Mg/Ca ratios are well preserved, and paleotemperature estimates are broadly unaffected by dissolution.

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[1] We measured oxygen isotopes and Mg/Ca ratios in the surface-dwelling planktonic foraminifer Globigerinoides ruber (white s.s.) and the thermocline dweller Pulleniatina obliquiloculata to investigate upper ocean spatial variability in the Indo-Pacific Warm Pool (IPWP). We focused on three critical time intervals: the Last Glacial Maximum (LGM; 18-21.5 ka), the early Holocene (8-9 ka), and the late Holocene (0-2 ka). Our records from 24 stations in the South China Sea, Timor Sea, Indonesian seas, and western Pacific indicate overall dry and cool conditions in the IPWP during the LGM with a low thermal gradient between surface and thermocline waters. During the early Holocene, sea surface temperatures increased by ∼3°C over the entire region, indicating intensification of the IPWP. However, in the eastern Indian Ocean (Timor Sea), the thermocline gradually shoaled from the LGM to early Holocene, reflecting intensification of the subsurface Indonesian Throughflow (ITF). Increased surface salinity in the South China Sea during the Holocene appears related to northward displacement of the monsoonal rain belt over the Asian continent together with enhanced influx of saltier Pacific surface water through the Luzon Strait and freshwater export through the Java Sea. Opening of the freshwater portal through the Java Sea in the early Holocene led to a change in the vertical structure of the ITF from surface-to thermocline-dominated flow and to substantial freshening of Timor Sea thermocline waters.

[1] Different proxies for sea surface temperature (SST) often exhibit divergent trends for deglacial warming in tropical regions, hampering our understanding of the phase relationship between tropical SSTs and continental ice volume at glacial terminations. To reconcile divergent SST trends, we report reconstructions of two commonly used paleothermometers (the foraminifera G. ruber Mg/Ca and the alkenone unsaturation index) from a marine sediment core collected in the southwestern tropical Indian Ocean encompassing the last 37,000 years. Our results show that SSTs derived from the alkenone unsaturation index (U K′ 37 ) are consistently warmer than those derived from Mg/Ca by~2-3°C except for the Heinrich Event 1. In addition, the initial timing for the deglacial warming of alkenone SST started at~15.6 ka, which lags behind that of Mg/Ca temperatures by 2.5 kyr. We argue that the discrepancy between the two SST proxies reflects seasonal differences between summer and winter rather than postdepositional processes or sedimentary biases. The U K′ 37 SST record clearly mimics the deglacial SST trend recorded in the North Atlantic region for the earlier part of the termination, indicating that the early deglacial warming trend attributed to local summer temperatures was likely mediated by changes in the Atlantic Meridional Overturning Circulation at the onset of the deglaciation. In contrast, the glacial to interglacial SST pattern recorded by G. ruber Mg/Ca probably reflects cold season SSTs. This indicates that the cold season SSTs was likely mediated by climate changes in the southern hemisphere, as it closely tracks the Antarctic timing of deglaciation. Therefore, our study reveals that the tropical southwestern Indian Ocean seasonal SST was closely linked to climate changes occurring in both hemispheres. The austral summer and winter recorded by each proxy is further supported with seasonal SST trends modeled by Atmosphere-ocean General Circulation Models for our core site. Our interpretation that the alkenone and Mg/Ca SSTs are seasonally biased may also explain similar proxy mismatches observed in other tropical regions at the onset of the last termination.Citation: Wang, Y. V., G. Leduc, M. Regenberg, N. Andersen, T. Larsen, T. Blanz, and R. R. Schneider (2013), Northern and southern hemisphere controls on seasonal sea surface temperatures in the Indian Ocean during the last deglaciation, Paleoceanography, 28, 619-632,