Geochemistry of Auckland Island Lake Sediments: Assessing Recent Sub-Antarctic Climate Change

Abstract

The Southern Hemisphere Westerly Winds (SHWW) are one of the primary controllers of air-ocean CO2 flux in the Southern Ocean. As the winds shift poleward and intensify with an increasingly positive Southern Annular Mode (SAM), upwelling of CO2-rich deep ocean water is enhanced, and the ocean's role in reducing the rate at which anthropogenic CO2 accumulates in the atmosphere is diminished. Furthermore, the strength and latitudinal position of the SHWW control storm tracks in the Southern Hemisphere and directly influence precipitation patterns in the South Island of New Zealand and throughout the Southern Hemisphere mid-latitudes. Despite their global significance, past variability of the SHWW is poorly understood. There are few terrestrial paleoclimate records of past SHWW variability, particularly at Sub-Antarctic latitudes where landmasses are scarce and the modern westerly maximum is located. Lake sediment cores from the Auckland Islands (50 deg S) provide an opportunity to study Holocene SHWW variability in this crucial gap. A high-resolution record of environmental change on the Auckland Islands over the last 550 years has been compiled from short sediment cores from three lakes using multiple physical and geochemical methods. The sediments collected are diatom- and plant macrofossil-rich and contain no carbonate. Down-core variations in the bulk sediment C/N ratio, magnetic susceptibility, and n-alkane distributions show an increase in terrestrial components of the sediment at about 300 years BP, while an overall decrease in the biogenic silica component of the sediment reveals a decline in lake productivity. These changes are indicative of an increase in precipitation causing additional influx of terrestrial material from the watershed and increased wind-driven mixing of the lake water column during a period of stronger westerly flow. Observed changes are broadly correlated with shifts in the SAM index as reconstructed by Abram et al. (2014). The dD of the C29 n-alkanes in modern lake sediment obtained from core tops appears to reflect local mean annual precipitation dD, and can potentially be applied as a proxy for the isotopic composition of precipitation, which likely reflects middle to high latitude temperature change and changes in the precipitation source region. When this record is compared to records from the South Island of New Zealand, wind strength appears to have an anti-phase relationship to that inferred in other studies, suggesting that the changes at the Auckland Islands may be due, in part, to latitudinal shifts in the westerly maximum, and not just changes in strength. Longer sediment cores from lakes on the Auckland Islands have the potential to produce high-resolution and continuous records of wind strength and temperature change throughout the Holocene, which will provide a useful comparison to records from the South Island and across the Southern Hemisphere for constructing new regional perspectives of Holocene shifts in the SHWW.