Abstract
Abstract. The application of glycerol dialkyl glycerol tetraether (GDGT)-based palaeoenvironmental proxies, such as the branched vs. isoprenoidal tetratether (BIT) index, TEX86 and the MBT–CBT palaeothermometer, has lately been expanded to lacustrine sediments. Given recent research identifying the production of branched, bacterial GDGTs (brGDGTs) within lakes, it is necessary to ascertain the effect of this lacustrine production on GDGT-based proxies. This study profiles a temperate, monomictic lake (Loch Lomond, UK), analysing labile intact polar GDGT lipids (IPLs) and resilient core GDGT lipids (CLs) in catchment soils, small tributary rivers, lake water and 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. The north basin is fjord-like, while the south basin is shallow with a lowland catchment. 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 that small rivers may contribute to the brGDGT pool through addition of brGDGTs produced in situ in these streams. Three hundred days of settling particles and water column profiles of suspended particulate matter (SPM; March and September 2011) reveal brGDGT production throughout the water column, with (IPL and CL) brGDGT distributions varying by basin. In lake sediments, in situ brGDGT production affects the distributions of sedimentary brGDGTs despite high soil- 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 in the south and lower MAATs in the north basin further complicates the application of the proxy. These results emphasise that caution must be exercised when applying the MBT–CBT palaeothermometer to individual lakes in which the use of the proxy has not been validated and therefore the factors affecting its application are not well understood. Despite elevated BIT indices, (partly) due to in situ brGDGT production, reliable TEX86 lake surface temperature (LST) estimates were obtained from SPM with BIT indices up to 0.9. Lower north basin sediments yielded accurate LST estimates but require further evaluation to properly constrain the application of the TEX86 proxy.
Highlights
Palaeoclimate reconstructions are essential for our understanding of climate dynamics and the factors that affect them and are important in order to project future climate change with greater reliability
BrGDGT distributions are similar in intact polar GDGT lipids (IPLs) and core GDGT lipids (CLs) bacterial GDGTs (brGDGTs) (Fig. 5a, b)
In Loch Lomond the lacustrine-produced brGDGTs form a substantial proportion of the total brGDGT pool in sediments, despite the soil CL brGDGT signature originating from a large, soil-organic-matter-rich catchment area
Summary
Palaeoclimate reconstructions are essential for our understanding of climate dynamics and the factors that affect them and are important in order to project future climate change with greater reliability. The relative amount of cyclopentane moieties and methyl branches contained in these membrane lipids, expressed in the cyclisation and methylation indices of branched tetraethers (CBT and MBT, respectively), are related to soil pH and mean annual air temperature (MAAT) (Weijers et al, 2007a; Peterse et al, 2012). Upon soil erosion these compounds are transported to the marine realm, and the MBT–CBT proxy has been successfully applied to near-coastal marine sediments containing considerable terrestrial organic matter to reconstruct river basin integrated continental MAAT (e.g. Weijers et al, 2007b, c; Schouten et al, 2008). In order to extend the continental coverage of temperature reconstructions, application of the MBT–CBT proxy to potentially high-resolution lacustrine sediment records is a logical step
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