Holocene subsurface transport between subpolar and subtropical gyre in the North Atlantic

Abstract:

The transport of warm waters from the subtropics into the subpolar North Atlantic (NA) strongly affects the climate variability of Europe and governs the strength of NA deepwater convection and the resulting AMOC strength. Modern oceanographic studies and climate models propose that this transport is mainly driven by the interaction between the subtropical (STG) and the subpolar (STG) gyre via subsurface pathways. Paleoceanographic studies postulate that subsurface transport of warm saline water from the STG may counterbalance freshening of the SPG and stabilize the Holocene AMOC mode. We tested this hypothesis, with new stable isotope and Mg/Ca analyses from surface and subsurface dwelling foraminifera obtained from a core site situated at the northern boundary of the STG.

Our new dataset indicates stable sea surface conditions and stronger variability in the subsurface temperature and salinities. During early Holocene similar subsurface temperatures and salinities in STG and SPG indicate that both regions were directly influenced by the same water mass that probably originated from NH meltwater discharges (e.g. Thornalley et al., 2009). This period is followed by a strong Mid-Holocene divergence between subpolar and STG salinity and temperature records, while the latter two properties in the STG converge towards the records from the subtropical source region (Bahr et al., 2013). We infer that this convergence was driven by a change in the source of subsurface waters due to the expansion of subtropical water masses in the Mid-Holocene. The Mid- to late Holocene (after 6 ka BP) is characterized again by in phase variations of subsurface subpolar and STG records although both temperatures and salinities differ in absolute values. This resemblance indicates a strong connection via subsurface transport between the STG and the subpolar region that probably stabilized the late Holocene NA gyre circulation.