Provenance of tetraether membrane lipids in a large temperate lake (Loch Lomond, UK): implications for glycerol dialkyl glycerol tetraether (GDGT)-based palaeothermometry

[摘要] The application of glycerol dialkyl glycerol tetraether (GDGT)-basedpalaeoenvironmental proxies, such as the branched vs. isoprenoidaltetratether (BIT) index, TEX₈₆ and the MBT–CBT palaeothermometer, haslately been expanded to lacustrine sediments. Given recent researchidentifying the production of branched, bacterial GDGTs (brGDGTs) withinlakes, it is necessary to ascertain the effect of this lacustrine productionon GDGT-based proxies. This study profiles a temperate, monomictic lake (LochLomond, UK), analysing labile intact polar GDGT lipids (IPLs) and resilientcore GDGT lipids (CLs) in catchment soils, small tributary rivers, lake waterand lake sediments.

Loch Lomond consists of two basins bisected by the Highland Boundary Fault,resulting in a mesotrophic to oligotrophic gradient from south to north. Thenorth basin is fjord-like, while the south basin is shallow with a lowlandcatchment. Besides abundant influxes of allochthonous soil- and peat-derived(CL) brGDGTs, brGDGTs are produced in a variety of settings in Loch Lomond.Rather than integrating a scattered soil signal, there is some evidence thatsmall rivers may contribute to the brGDGT pool through addition of brGDGTsproduced in situ in these streams. Three hundred days of settling particles and watercolumn profiles of suspended particulate matter (SPM; March and September2011) reveal brGDGT production throughout the water column, with (IPL andCL) brGDGT distributions varying by basin. In lake sediments, in situ brGDGTproduction affects the distributions of sedimentary brGDGTs despite highsoil- and peat-derived organic matter influxes from the catchment.

MBT–CBT-derived mean annual air temperature (MAAT) estimates from soil,river and lake sediments vary widely. A strong bias towards higher MAATs inthe south and lower MAATs in the north basin further complicates theapplication of the proxy. These results emphasise that caution must beexercised when applying the MBT–CBT palaeothermometer to individual lakes inwhich the use of the proxy has not been validated and therefore the factorsaffecting its application are not well understood. Despite elevated BITindices, (partly) due to in situ brGDGT production, reliable TEX₈₆ lakesurface temperature (LST) estimates were obtained from SPM with BIT indicesup to 0.9. Lower north basin sediments yielded accurate LST estimates butrequire further evaluation to properly constrain the application of theTEX₈₆ proxy.

[学科分类] 地球化学与岩石