Alkenones and hydrogen isotopic composition of n-alkanes as indicators of past temperature and hydrological variability using lacustrine sediments from Lake Toyoni (Japan).

Abstract:

A better understanding of decadal to centennial climate variability is vital to improve the accuracy of near future climate prediction. Hokkaido represents a region which has limited paleo-climate data and is sensitive to climate change. Instrumental data shows a good correlation between the temporal variability of temperature and rainfall with the Pacific Decadal Oscillation (PDO) and the East Asian Summer Monsoon (EASM) however instrumental data is limited to the past ~150 years. Therefore down-core reconstructions of temperature and precipitation prior to instrumental records are required to provide a better understanding of the long-term behaviour of the PDO and EASM systems in this region.

Lake Toyoni (Hokkaido, Japan) was investigated to provide high resolution lake temperature and hydrological reconstructions over the past 1000 years (1cm represents 3-8 years of sedimentation) using alkenones and the hydrogen isotopic composition of higher plant waxes (δD_(HPW)), respectively.

Lake Toyoni is the first lake from which alkenones are reported in Japan. C₃₇ alkenone concentrations in surface sediments are 18μg C₃₇ g⁻¹ of dry sediment and the dominant alkenone is C_37:4 . 18S DNA analysis revealed the presence of a single alkenone producer in Lake Toyoni and thus a single calibration for reconstructing lake temperature based on alkenone unsaturation patterns. Temperature reconstructions over the past 1000 years suggest lake water changes of 8-23°C which is in line with water temperature changes observed in Lake Toyoni. 

The molecular distributions and isotopic compositions of n-alkanes provide valuable paleo-environmental information. The dominant n-alkanes in Lake Toyoni are long chained (C₂₅-C₃₃) and are characterised by odd over even distribution. The source of long chained n-alkanes are from the surrounding terrestrial higher plants. Large fluctuations (~40‰) are documented in downcore δD_(HPW) representing hydrological changes in this region over the past 1000 years.

These results suggest that alkenones and the δD_(HPW)  preserved in Lake Toyoni record regional climate changes over decadal timescales. These fluctuations will be discussed in relation to globally recognised climate events, specifically the Medieval Warm Period and the Little Ice Age.