Glacial‐interglacial sea surface temperature changes across the subtropical front east of New Zealand based on alkenone unsaturation ratios and foraminiferal assemblages

We present sea surface temperature (SST) estimates based on the relative abundances of long‐chain C37 alkenones (U37K′) in four sediment cores from a transect spanning the subtropical to subantarctic waters across the subtropical front east of New Zealand. SST estimates from U37K′ are compared to those derived from foraminiferal assemblages (using the modern analog technique) in two of these cores. Reconstructions of SST in core tops and Holocene sediments agree well with modern average summer temperatures of ∼18°C in subtropical waters and ∼14°C in subpolar waters, with a 4°–5°C gradient across the front. Down core U37K′ SST estimates indicate that the regional summer SST was 4°–5°C cooler during the last glaciation with an SST of ∼10°C in subpolar waters and an SST of ∼14°C in subtropical waters. Temperature reconstructions from foraminiferal assemblages agree with those derived from alkenones for the Holocene. In subtropical waters, reconstructions also agree with a glacial cooling of 4° to ∼14°C. In contrast, reconstructions for subantarctic pre‐Holocene waters indicate a cooling of 8°C with glacial age warm season water temperatures of ∼6°C. Thus the alkenones suggest the glacial temperature gradient across the front was the same or reduced slightly to 3.5°–4°C, whereas foraminiferal reconstructions suggest it doubled to 8°C. Our results support previous work indicating that the STF remained fixed over the Chatham Rise during the Last Glacial Maximum. However, the differing results from the two techniques require additional explanation. A change in euphotic zone temperature profiles, seasonality of growth, or preferred growth depth must have affected the temperatures recorded by these biologically based proxies. Regardless of the specific reason, a differential response to the environmental changes between the two climate regimes by the organisms on which the estimates are based suggests increased upwelling associated with increased winds and/or a shallowing of the thermocline associated with increased stratification of the surface layer in the last glaciation.