Glacial disparities in Intermediate Mode Water advection in the South Pacific Gyre

Abstract:

The Intermediate Mode Waters formed in the Southern Ocean are critical for the lower thermocline ventilation process in the Southern Hemisphere Gyres. They also might have served as the most relevant pathways transporting climatic signals from high to low latitudes via the “oceanic tunneling” on glacial/interglacial time scales. Despite the importance of the Southern Ocean Intermediate Waters (SOIWs), our understanding on the long–term evolution, exact advection paths, and impact on the South Pacific Gyre’s thermocline is still fragmentary.

Here, we present a 200 kyr record of paired Mg/Ca ratios and stable oxygen isotope from surface dweller and deep dwelling planktonic foraminifera, from the South Pacific Gyre (SPG). On average, the Mg/Ca–derived sea Surface Temperatures (Globigerina bulloides) show similar conditions during the LGM and Marine Isotope Stage (MIS) 6 (9.4 °C versus 9.9 °C). In contrast, our Mg/Ca–derived subsurface temperatures (Globorotalia inflata and Globorotalia truncatulinoides) suggest LGM from ~3 to ~2 °C colder than MIS 6. The reconstructed subsurface ice volume corrected stable oxygen isotope ratio of seawater (δ¹⁸O_sw-ivc, proxy for local salinity changes) suggests opposing glacial subsurface conditions, i.e., slightly saltier–than–Holocene during MIS 6 to fresher–than–Holocene during MIS 2. Considering that subsurface hydrography at the core site is plausibly driven by the formation and/or advection of SOIWs from the South East Pacific, our results provide further support on the relevance of subsurface processes in the Southern Ocean transferring climatic signals (temperature and salinity) to the SPG. Furthermore, the contrasting subsurface glacial scenarios at the SPG’s thermocline imply that the advection of SOIWs during glacial stages could be highly variable during different glacial stages.