Seasonal variability in concentrations and fluxes of glycerol dialkyl glycerol tetraethers in Huguangyan Maar Lake, SE China: Implications for the applicability of the MBT–CBT paleotemperature proxy in lacustrine settings

Abstract

Glycerol dialkyl glycerol tetraethers (GDGTs) are increasingly used for paleoclimate studies of marine and lacustrine environments. Although GDGT-based proxies have been applied to various lake environments globally, little is known about the distribution of GDGTs in lakes in subtropical South China, which are mainly controlled by the East Asian monsoonal system. Therefore, we investigated the distribution of GDGTs in Huguangyan Maar Lake (HML) in order to examine their characteristics as paleoclimate proxies in subtropical South China. Suspended particulate matter (SPM) in the water column was sampled monthly over a one year period at three different depths. Sediment traps were deployed simultaneously at the same locations and settling particles were collected monthly over a one year period too. In order to identify the sources of GDGTs in HML, lake bottom sediments and soil samples from the catchment were collected. The distribution and abundance of GDGTs in all samples were determined and GDGT-based temperatures were calculated.The concentrations and fluxes of GDGTs of settling particles in HML were dominated by branched GDGTs, with a minor contribution of isoprenoid homologues. The maximum concentrations and fluxes of branched GDGTs occurred during the winter months, when the water column was well mixed due to the strong East Asian winter monsoon. Also, maximal concentrations and fluxes of branched GDGTs were observed at the deeper water mass. These observations suggest that in situ production of branched GDGTs takes place mainly in the deeper part of the water column and/or at the sediment–water interface, but may be influenced by seasonal variability in the primary production in the shallower water mass of HML. The temperatures calculated from the methylation of branched tetraethers (MBT) and the cyclization ratio of branched tetraethers (CBT) of SPM and settling particles were lower than local air temperature (AT) during spring and summer when the water column was in thermal stratification, and were higher than AT during fall and winter when the water column was isothermal. However, the MBT–CBT based temperatures were in good agreement with AT on an annual time scale. Therefore they can be used as a proxy of local air temperature. Furthermore, the variations in concentration and flux of GDGTs are synchronous with the East Asian monsoon annual cycle. The overall amplitude of the estimated monthly temperature is however smaller than that of the measured air temperature. The mean MBT–CBT based temperature estimates (23.4±0.6°C) of lake bottom sediments for that one year period agree with the local average mean air temperature (23.1±0.5°C) for the preceding 45years, providing further support to the reliability of this proxy and the interpretation of the results. Both long term stratification and mixing events in water column might present a challenge to the application of branched GDGT-based paleoenvironmental proxies in lakes. The present study reveals that GDGTs in lakes within the area influenced by the East Asian monsoon have a considerable potential for paleoclimatic studies but it is important to determine the location and nature of the sources of GDGTs sampled in lacustrine environments prior to their interpretation as paleoclimatic indicators. More generally, this study identifies factors which have to be considered in order to obtain reliable palaeotemperature data from GDGTs in lacustrine settings.