Abstract
The Southern Hemisphere westerly winds (SHWW) play a major role in controlling wind-induced upwelling of Circumpolar Deep Water and outgassing of CO₂ in the Southern Ocean, on interannual to glacial-interglacial timescales. Despite their importance, our understanding of millennial-scale changes in the strength and latitudinal position of the SHWW during the Holocene is limited by few paleoclimate records from sensitive regions, that are often not in agreement, especially after 5,000 yr BP. Paleoclimate records from the Southern Hemisphere mid-latitudes utilise the strong positive correlation between precipitation and wind speed to track changes in the SHWW field over the Holocene. Sediment cores recovered from fjords located in the modern core of the westerly wind belt, along the eastern margin of the subantarctic Auckland Islands (51°S, 166°E) are ideally located to reconstruct changes in westerly wind-derived precipitation. Here, drainage basin and fjord responses to variability in the strength and position of the SHWW are reconstructed from bulk organic δ¹⁵N, and the δ¹⁸O and paleoecology of benthic foraminifera, which monitor the influx of terrestrial organic matter (OMterr) and changes in salinity, respectively. The response of these proxies to strengthened winds is informed by modern fjord process studies, which reveal that increased delivery of OMterr and lowered salinity (as a result of wind-induced mixing of fjord waters) occurs during high winds in Hanfield Inlet.
Holocene benthic assemblages from Hanfield Inlet are dominated by Nonionellina flemingi, with subdominant Cassidulina carinata and Quinqueloculina seminula. Species-specific vital offsets from equilibrium for stable carbon and oxygen isotopic composition (Δδ¹³C and Δδ¹⁸O) of foraminiferal calcite are reported here for five species. The first record for New Zealand endemic N. flemingi is provided, where Δδ¹³C is -2.63 ±0.59‰ and Δδ¹⁸O is +0.73 ±0.32‰. Epifaunal Cibicides spp. consistently precipitates both carbon and oxygen isotopes in near equilibrium with ambient bottom water, and is considered the most reliable recorder of fjord water δ¹³CDIC and δ¹⁸O. Because infaunal N. flemingi shows consistent downcore offsets of δ¹⁸O relative to Cibicides spp., it is used in paleoceanographic reconstructions of fjord water salinity downcore.
Downcore multiproxy reconstructions reveal evidence for millennial-scale westerly wind variability, a general strengthening of westerlies over the past 5,000 years, and consistently strong winds from 2,000 to 500 yr BP at the Auckland Islands. Comparison with contemporaneous paleoclimate records from southern South America suggests that SHWW were zonally symmetric across the Pacific south of 50 degrees S, with multi-millennial scale strengthening of the westerlies since 2,000 yr BP, and a poleward shift from 800 to 500 yr BP. Concomitant increases in benthic foraminiferal δ¹³C of epifaunal Cibicides spp. and the bulk sediment δ¹³C since 2,000 yr BP may be associated with increased δ¹³CDIC of oceanic water external to the fjord. This may reflect increased rates of air-sea CO₂ exchange, related to increased westerly wind speeds over the Southern Ocean.