Reconstructing sea surface temperature in the South Pacific using organic proxies

Abstract

Sea surface temperature (SST), at the interface between the atmosphere and the ocean, is an important element in the global climate system. Accurate estimates of past SSTs are indispensable for studying global climate and for validating the numerical models used for projections of future climate. SSTs prior to the instrumentation era could be reconstructed using climatically sensitive biomarkers such as alkenones and glycerol dialkyl glycerol tetraethers (GDGTs). Herein I use marine sediments, both core-tops and long piston cores, to further examine the applicability of organic SST proxies derived from the aforementioned biomarkers, with an emphasis on the relatively under-studied subantarctic Southeast Pacific. The appraisal of these proxies is based on the correlation of the core-top proxy index values with present-day climatological SST, and the comparison of the SST records inferred from these proxies with other regional SST records. The alkenone index (UK37 and UK 37) values in the South Pacific core-tops display linear relationships with climatological World Ocean Atlas 2009 (WOA09) SST at low temperatures (1 -12°C), with equally high r2 values (>0.93). These results suggest that both alkenone indices are highly correlated to SST even at high latitudes, rendering them appropriate for reconstructing SST in the subantarctic Pacific. However, these indices yield different Pleistocene SST patterns for study sites in the subantarctic sector of the Southern Ocean. Judging from the better structural fit of UK37 SST records with other subantarctic surface proxy records, including foraminiferal 18O and SST records inferred from diatom- and foraminiferal assemblages, it appears that the UK37 index results in more plausible paleo SST records in this region, as opposed to the commonly used UK 37 index. The GDGT index (TEX86 and TEX86L) values in the subpolar and polar core-tops (with overlying SSTs of -2 to 17°C) are not highly correlated (r2 values < 0.3) to the annual mean WOA09 SST. Plotting these indices against seasonal SSTs and water temperatures at various hydrographic boundaries does not improve the correlation. Nevertheless, when these data are combined with previously published core-top data (n = 630) spanning a SST range of -2 to 30°C, the r2 values of these relationships improve to ~ 0.8, with a better correlation for the TEX86 compared to TEX86L. The TEX86 calibration yields SST estimates that are in better agreement with the WOA09 SST for subpolar and polar regions, rendering it a better SST index in these regions. Meanwhile, applying the GDGT index on a Southeast Pacific sediment core yields SSTs that are colder than those inferred from the alkenones, and the GDGT-derived interglacial SSTs are ~3°C colder than annual mean WOA09 SST. Given the lack of seasonality in the archaeal abundance here, the cold-biased GDGT estimates probably reflect subsurface temperature instead of SST. The three SST records presented here allow for further understanding of the surface oceanographic variability in the subtropical and subantarctic Southeast Pacific over the past 700 kyr. The extent of glacial cooling increases with latitude, up to ~8°C in the subantarctic as opposed to the ~4°C in the subtropics. Intense cooling at high latitudes results in larger latitudinal SST gradients during glacials than interglacials. The alkenone-inferred SSTs along the latitudinal range of the Peru-Chile Current (PCC) imply massive equatorward migrations of the Southern Ocean frontal systems and increased equatorward transport of subantarctic waters owing to a stronger PCC during glacials. In addition, GDGT-derived temperatures suggest enhanced subsurface warming during MIS 11 and 13 in the subtropical Southeast Pacific, plausibly due to water column reorganization analogous to that occurring during modern-day El-Niño conditions.