A multi-proxy look at deglacial southwest Pacific Ocean sea surface temperature and thermocline source water changes

Glacial-interglacial changes in global and regional temperature have been linked to changes in insolation, winds, and ocean circulation. This study reconstructs sea surface temperature (SST) and source water provenance across the last deglaciation (~30-5 kyr BP) in five sediment cores in the Bay of Plenty, New Zealand and places these in a regional context by comparing these results to previously published work. SST reconstructions from Mg/Ca ratios in the planktonic foraminifera Globogerina bulloides and alkenones (Ú) track different seasons’ SST spring and summer, respectively. During the last glacial maximum (LGM, 26-22 kyr BP), summer SSTs average 16.4°C while spring SSTs were 13.6; about 4°C cooler than modern. The seasons track well with each other and maintain a constant offset of 3.3-2.8°C as temperatures increase into the Holocene, peaking at 21.7°C for summer and 18.4°C for spring. Comparison to model reconstructions of local insolation yielded correlation to winter insolation from the LGM (~26 kyr BP) to the Antarctic Cold Reversal ACR (~14.1 kyr BP) after which SSTs correlate well to their respective seasonal insolation. Comparison of this study’s temperatures to published SSTs indicate that deglacial warming of subtropical waters differ from subantarctic waters that warmed later and by 2°C more than subtropical waters. δ18O and δ13C from planktonic foraminifera G. bulloides and Globorotalia inflate were used to reconstruct δ18O of seawater (δ18Osw) and track source water provenance. In the LGM, depleted δ18OSW averaging 0.2‰, and enriched δ13C ranging between -0.4-0.1‰ indicate shallow water masses had a strong local Southern Ocean component. A step change occurs at 20.1 kyr BP where δ13C depletes to -1.3‰ that suggests a deglacial shift in shallow subsurface water mass source location to a distal subtropical component likely sourcing through the equatorial Pacific that persists into the Holocene. A regional comparison indicates numerous switches between distal-subtropical and proximal- subantarctic influences during the early deglaciation. This ends at the ACR, which figures as a tipping point for stabilization and onset of modern circulation.M.S.Includes bibliographical referencesby Benedetto Schiraldi Jr