How to be a Prehistoric Weatherman: Using n-alkanes as a Proxy for Holocene Climate and Hydrology, Southwest South Island, New Zealand

Abstract

The latitudinal position and strength of the Southern Hemisphere westerly winds (SHWW) plays a critical role in global CO2 air-sea flux and the distribution of rainfall in the southern mid-latitudes. Strengthening and southward shifting westerlies are thought to be reducing the efficiency of the Southern Ocean carbon sink, which has direct implications for modern atmospheric CO2 concentrations. Southern New Zealand intersects the northern margin of the SHWW belt, where a direct relationship exists between annual precipitation and zonal wind speeds. Reconstructing past hydrological variations from the southwest of the South Island (SWSI) can provide a regional record of climatic response to changing SHWW. A 5.4 m sediment core spanning the last 10.8 ka was recovered from South Mavora Lake, Southland. Magnetic susceptibility, bulk organic C and N isotopes and concentrations, and lipid biomarker (C21-C33 n-alkanes) concentrations, distributions, and hydrogen isotope values (δD) serve as proxies for change in lacustrine productivity, relative proportions of terrestrial and aquatic input, and hydrology. Modern SWSI meteoric water isotope values (δ18O, δD) collected over a 12-month period, and meteorlogical station data, show orographic rainout and air temperature are the primary drivers of hydrological isotope composition in SWSI. Downcore interpretation of data suggests a period of increased precipitation, rapid warming, and greater terrestrial input from 10.8-9.0 ka, likely corresponding to weaker westerly influence over SHWW. From 9.0-7.0 ka, decreasing δDn-alkanes shows gradual cooling, δ13C and ACL suggest increased aquatic productivity, and stratigraphy shows an increase in storm strength. From 7.0-5.1 ka δDn-alkanes and δ13C are characteristic of a relatively stable temperate climate, Paq and C/N ratios suggest a relatively humid environment, and stratigraphy showed an increase of storm events. From 5.1-3.6 ka δDn-alkanes showed a large cold excursion followed by gradual warming, Paq and stratigraphy reflected a significant increase in storm event frequency and strength, and an increase in ACL reflected the expansion of cool-moist Nothofagus menziesii into the region. From 3.6 ka to present δDn −alkanes showed a cooling trend to present day, likely related to strengthening of the SHWW, and low amplitude and frequency variation in Paq and decreased storm events signaled a gradual decrease in precipitation to modern day conditions.