Quantification of low molecular weight n-alkanes in lake sediment cores for paleoclimate studies

Organic biomarkers, such as n-alkanes, can be used as proxies in a range of paleoclimate and paleoenvironmental investigations. n-Alkanes accumulate in lacustrine sediments and can therefore be used to determine past terrestrial conditions by investigating chain length ratios and compound specific isotope signatures. Analytical methods for their quantification in sediments have been described previously; however, method validation has not been rigorously reported for the 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-C40n-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.