Late Holocene intensification of the westerly winds at the subantarctic Auckland Islands (51° S), New Zealand

摘要

The Southern Hemisphere westerly winds (SHWWs) play a major role in controlling wind-driven upwelling of Circumpolar Deep Water (CDW) and out-gassing of CO2 in the Southern Ocean, on interannual to glacial-interglacial timescales. Despite their significance in the global carbon cycle, our understanding of millennial- and centennial-scale changes in the strength and latitudinal position of the westerlies during the Holocene (especially since 5000 yr BP) is limited by a scarcity of palaeoclimate records from comparable latitudes. Here, we reconstruct middle to late Holocene SHWW variability using a fjord sediment core collected from the subantarctic Auckland Islands (51 degrees S, 166 degrees E), located in the modern centre of the westerly wind belt. Changes in drainage basin response to variability in the strength of the SHWW at this latitude are interpreted from downcore variations in magnetic susceptibility (MS) and bulk organic delta C-13 and atomic C = N, which monitor influxes of lithogenous and terrestrial vs. marine organic matter, respectively. The fjord water column response to SHWW variability is evaluated using benthic foraminifer delta O-18 and delta C-13, both of which are influenced by the isotopic composition of shelf water masses entering the fjord. Using these data, we provide marine and terrestrial-based evidence for increased wind strength from similar to 1600 to 900 yr BP at subantarctic latitudes that is broadly consistent with previous studies of climate-driven vegetation change at the Auckland Islands. Comparison with a SHWW reconstruction using similar proxies from Fiordland suggests a northward migration of the SHWW over New Zealand during the first half of the last millennium. Comparison with palaeoclimate and palaeoceanographic records from southern South America and West Antarctica indicates a late Holocene strengthening of the SHWW after similar to 1600 yr BP that appears to be broadly symmetrical across the Pacific Basin. Contemporaneous increases in SHWW at localities on either side of the Pacific in the late Holocene are likely controlled atmospheric teleconnections between the low and high latitudes, and by variability in the Southern Annular Mode and El Nino-Southern Oscillation.