A one‐million‐year history of a north‐south segment of the Subtropical Front, east of New Zealand

Abstract

Planktic foraminiferal assemblages from Ocean Drilling Program (ODP) site 1119 and Deep Sea Drilling Project (DSDP) site 594 are used to estimate sea surface temperatures (SSTs) over the past ∼900 kyr (marine isotope stages (MISs) 23–1) in the southwest Pacific Ocean, east of New Zealand. Comparison of these long SST records allows us to document the cyclic migrational history of water masses and fronts, especially of the N‐S oriented segment of the Subtropical Front (STF), known as the Southland Front. During most interglacials, summer SSTs were 10–12°C in both sites, indicating that both were overlain by Subantarctic Water (SAW) and that the Southland Front was located landward of them. During the peaks of MISs 11 and 5.5, summer SST estimates were considerably warmer than modern, suggesting that the southern edge of the E‐W trending STF expanded southward over the Bounty Trough, and the Southland Current contained a greater proportion of Subtropical Water (STW) than it does today. During most glacials, summer SSTs in inshore ODP site 1119 (7–10°C) were ∼4°C warmer than in DSDP site 594. This implies that the Southland Front migrated offshore with lowered sea levels to lie between the two sites. During MIS 20, SSTs in ODP site 1119 dropped to those of DSDP site 594, indicating that there was no Southland Front in existence and no detectable STW entrained in the Southland Current flowing around the south end of New Zealand. The low glacial SST estimates in DSDP site 594 (∼4°C) might be a result of persistent eddies of Circumpolar Subantarctic Water spinning off the Subantarctic Front into the Bounty Trough during glacials.