Southwest Pacific subtropics responded to last deglacial warming with changes in shallow water sources

Abstract

This study examined sources of mixed layer and shallow subsurface waters in the subtropical Bay of Plenty, New Zealand, across the last deglaciation (~30–5 ka). δ18O and δ13C from planktonic foraminifera Globgerinoides bulloides and Globorotalia inflata in four sediment cores were used to reconstruct surface mixed layer thickness, δ18O of seawater (δ18OSW) and differentiate between high- and low-latitude water provenance. During the last glaciation, depleted planktonic δ18OSW and enriched δ13C (−0.4–0.1‰) indicate surface waters had Southern Ocean sources. A rapid δ13C depletion of ~1‰ in G. bulloides between 20 and 19 ka indicates an early, permanent shift in source to a more distal tropical component, likely with an equatorial Pacific contribution that persisted into the Holocene. At 18 ka, a smaller but similar shift in G. inflata δ13C depletion of ~0.3‰ suggests that deeper subsurface waters had a delayed reaction to changing conditions during the deglaciation. This contrasts with the isotopic records from nearby Hawke Bay, to the east of the North Island of New Zealand, which exhibited several changes in thermocline depth indicating switches between distal subtropical and proximal subantarctic influences during the early deglaciation ending only after the Antarctic Cold Reversal. Our results identify the midlatitude subtropics, such as the area around the North Island of New Zealand, as a key region to decipher high- versus low-latitude influences in Southern Hemisphere shallow water masses.