Glacial sea surface and subsurface temperature reconstructions off northern Chile (27.5°S) from 970 ka to present

Subantarctic surface and intermediate waters flow equatorwards along the Chilean continental margin within the Peru-Chile Current system while Subtropical Surface and Equatorial Subsurface Waters flow poleward. Pacific Deep water also flows poleward here. Thus, there is an interplay of high versus low latitude sourced waters throughout the water column. The degree of north/southward advection of upper column subantarctic/subtropical waters varies on glacial-interglacial time scales, global cooling favours northwards advection of subantarctic waters, global warming can promote southwards advection of subtropical waters.

We investigate the nature of this interaction for the past 970 kyr using a splice of gravity core GeoB3375-1 and MeBo Site GeoB15016 from off northern Chile (27.5°S, 70°W). We measured stable isotopes and Mg/Ca in Globigerina bulloides (surface dweller) and Globorotalia inflata (sub-surface dweller). The number of foraminifera available for Mg/Ca analyses varied significantly and we used the annotated number of individuals per sample to generate pseudo-abundance records of both species. During interglacial periods, they are both scarce indicating conditions non-favorable for their proliferation, potentially due to stronger advection of nutrient-poor subtropical waters. Exceptions are Marine Isotope Stages (MIS) 23 and 25 during the Mid-Pleistocene Transition, which show, similarly to glacial periods, high abundances. Because of the scarcity in foraminifera, our Mg/Ca-derived surface and subsurface temperature (SST, subSST) records are discontinuous and represent mostly glacial periods (see Caniupán´s et al. contribution for alkenone-based interglacial SST estimations). Through the past 970 kyr, glacial Mg/Ca-derived SST and subSST (upper thermocline) became progressively colder towards present. This is in line with equatorial Pacific records pointing to a low latitude Pacific-wide signal. The difference between surface and subsurface temperatures does not show a clearly defined pattern. The largest difference occurs from MIS 4 to 1 (6-8°C) suggesting strong water column stratification, while during glacial MIS 8, 10, and 16 both records are nearly similar, which we interpret as well mixed conditions in the upper water column.