2000 Years of Sub-Antarctic Climate: The Use of n-alkanes and Compound-Specific Isotopes for the Paleoclimate Reconstruction of Auckland Island

The Southern Ocean’s air-sea CO2 flux is heavily influenced by the strength and position of the Southern Hemisphere westerly winds (SHWWs) over the southern mid-latitudes. An increasingly positive southern annular mode (SAM) causes these winds to intensify and move southward. This results in the upwelling of CO2 rich waters, and therefore a reduction in the Southern Ocean’s anthropogenic carbon sink capabilities. Southern Hemisphere storm tracks are also influenced by the strength and positioning of the SHWWs, directly affecting precipitation patterns over the South Island and associated sub-Antarctic islands of New Zealand, which lie in the northern margin of the wind system. The scarcity of sub-Antarctic land masses means that few terrestrial paleoclimate studies exist in the heart of this important climate system. Auckland Island (50°S) is located within the modern westerly maximum, and contains several lakes from which sediment cores can be collected and used to fill gaps in the sub-Antarctic Holocene era. Physical and chemical biomarkers within sediments can hold vital information on past climate systems. n-Alkanes accumulate in lacustrine sediments and can be used to determine past terrestrial conditions by investigating chain length ratios and compound specific isotope analysis. Analytical methods for their quantification in sediments have been described previously; however, method validation has not been rigorously reported for low molecular weight (LMW) n-alkanes, which have recently been gaining attention as useful proxies. Using spike and recovery experiments with a simple sand matrix, we found that LMW n-alkanes are prone to low recovery due to volatilization when following common protocols for quantification. This is alarming because low recovery can lead to misinterpretation of n-alkane results. On the other hand, we found that isotope fractionation was fortunately not observed even when volatilization loss was high. We addressed the problem of LMW n-alkane volatilization by developing an alternative extraction and sample preparation method. The optimized method, which employs pressurized liquid extraction, was tested by using it to extract n-alkanes from sediments from three Southern New Zealand lakes (Hinemoa, Ohau and Thomas) with different properties. Recovery of the C15-C40 n-alkanes ranged from 32–66% for Lake Hinemoa, 54–85% for Lake Thomas and 61–89% for Lake Ohau. Variation in n-alkane recovery from the three lakes was attributed to differences in sediment properties, notably organic matter and particle size, and an approach for calculating ‘adjusted concentration’ with a sediment-specific adjustment factor for each n-alkane is suggested. This n-alkane extraction and quantification method was then applied to a 6.3 m sediment core from Auckland Island. Compound-specific isotope ratios and bulk sediment C/N ratios were used to create a high resolution 2000 year paleoclimate record. Down-core changes in n-alkane δD reveals a trend toward warmer, more northerly sourced storms over the last 2000 years, with a short, yet sustained decrease between 800 and 200 years ago. Variations in n-alkane ratios, as well as C/N, are in agreeance with δD over the last 900 years, showing the effect of increasing precipitation washing greater amounts of terrestrial organic matter into the lake. When this record is compared to Australasian and Antarctic temperature reconstructions (the PAGES 2k), it is apparent that the temperature trends of Auckland Island follow closely to that of the former, and that climate on Auckland Island is indeed influenced by shifts in the SHWWs