Western Arabian Sea SST during the penultimate interglacial: A comparison of U₃₇^K′ and Mg/Ca paleothermometry

Abstract

[1] Millennial-scale records of planktonic foraminiferal Mg/Ca, bulk sediment U 37 K 0 , and planktonic foraminiferal d 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 0 -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 37and Mg/Ca temperatures are 1.5°C warmer than during the Holocene, while the U 37 K 0 -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 0 -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 0 -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 0 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 d 18 O profile, and Mg/Ca-based temperature returns to lower values before d 18 O has reached minimum interglacial values. This indicates a substantial lead in Arabian Sea warming relative to global ice melting.

Sensitivity of Red Sea circulation to sea level and insolation forcing during the last interglacial

“…However, the membrane lipid distribution in the present day northern Red Sea correlates neither with seasonal or deep water temperatures nor with salinity, nutrient concentrations, or diagenetic effects . Additionally, our findings are consistent with estimates of SST increases during termination II of 2-3 • C in the Arabian Sea using different proxies (Emeis et al, 1995;Rostek et al, 1997;Saher et al, 2009) and are similar to the magnitude of SST changes in tropical oceans across termination II (McCulloch and Esat, 2000;Kukla et al, 2002). The SST increase during the termination I in the northern Red Sea was reported as about 1 • C (TEX 86 reconstruction, Trommer et al, 2010) or 2 • C (U K 37 reconstruction, Arz et al, 2007).…”

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

“…However, measurement uncertainties of 0.7°C for U K′ 37 or 1.2°C for Mg/Ca of the G. ruber cannot explain the up to 4°C SST offset between alkenone and Mg/Ca paleothermometers. Furthermore, large SST offsets (up to 3°C) between the two proxies are also independently observed in core‐top sediments [ Fallet et al ., ] as well as at glacial/interglacial timescales in the Mozambique Channel [ Caley et al ., ; Saher et al ., ]. These observations suggest that warmer U K′ 37 SSTs and cooler Mg/Ca SSTs are robust features in the western tropical Indian Ocean.…”

“…Our records corroborate SST mismatches recorded by different temperature proxies during the penultimate interglacial period in the Arabian Sea [ Saher et al ., ], suggesting the seasonal effects on SSTs are a pervasive feature in the western tropical Indian Ocean during terminations. In addition, our hypothesis also explains discrepancies in SST records for deglacial timing and absolute temperature values that have been recorded in the tropical Indian Ocean [ Bard et al ., ; Caley et al ., ; Kiefer et al ., ; Naidu and Govil , ; Saraswat et al ., ; this study].…”

Alkenone Paleotemperature Determinations

“…In particular, how do we interpret disagreements larger than the ostensible absolute temperature errors of each method (e.g., Saher et al, 2009)? To be clear, this is a case of looking at the glass as half empty, for many comparisons show quite good agreement between proxies (Dekens et al, 2008).…”