Northern and southern hemisphere controls on seasonal sea surface temperatures in the Indian Ocean during the last deglaciation

Abstract

[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,

Variability of Indonesian Throughflow and Borneo Runoff During the Last 14 kyr

“…This cool air mass would explain the low U K′ 37 ‐derived SST (Figure b). In contrast, Mg/Ca‐derived SST (Figure b), which closely follows Southern Hemisphere δ 18 O ice core records (Figure c), may have been mainly influenced by southeasterly trade winds during austral winter, as proposed for records from offshore East Africa (Wang et al, ). Changes in local insolation may have additionally contributed to these seasonally biased temperature records.…”

“…Today, tropical U K 0 37 temperatures are linked to the seasonal productivity of coccolithophorids, which thrive in periods of intensified upwelling and upper ocean mixing during the boreal winter monsoon and develop Paleoceanography 10.1002/2016PA003030 intense blooms in early spring (Chen et al, 2007;Harada et al, 2001). In contrast, modern tropical Mg/Caderived SST generally reflects boreal summer temperatures (Timmermann et al, 2014;Wang et al, 2013). A strong seasonality in YD temperature reconstructions may be related to an intensified cross equatorial atmospheric flow.…”

Hydroclimate variability in the Nile River Basin during the past 28,000 years

“…Reproducibility (the standard deviations of multiple analyses) 133 varied between 0‰ and 0.5‰ (average 0.1‰) for δ 13 C, and 0‰ and 4‰ (average 1‰) for 134 δD. Leaf wax δD values were corrected for global ice volume changes using the method 135 described by Wang et al [2013]. 136 7 Glycerol dialkyl glycerol tetraethers (GDGTs) were analyzed at NIOZ Royal 137…”

Reply to comment by Rashid et al. on “Asynchronous variation in the East Asian winter monsoon during the Holocene”

“…On point (c), along with the remark that the Mg-/Ca-derived SSTs from core SK237-GC04 [Saraswat et al, 2013] does not share the above mentioned trends, we emphasize that (1) the MAT records are more and more questioned in the paleocommunity since, in the MARGO effort, it has been identified to be responsible for a much stronger level of spatial heterogeneity in the tropics than has been simulated by coupled model runs (see, e.g., discussion in Kageyama et al [2013] and Lea et al [2014]), and (2) by mixing the alkenone-derived and Mg/Ca-derived SST, Rashid et al [2016] seem to ignore the increasing number of tropical to subtropical SST records suggesting that mixing SST proxies may lead to misinterpret SST signals because they do not record SST over the same seasons [Koutavas and Sachs, 2008;Leduc et al, 2010;Schneider et al, 2010;Lohmann et al, 2013;Wang et al, 2013;Hessler et al, 2014;Timmermann et al, 2014;Leduc et al, 2014]. Among above mentioned articles, those that deal with SST estimates from the northern tropics all interpret alkenone-derived SSTs as being reflective of winter-skewed SST records.…”