Distribution Of brGDGTs Research Articles

Evaluating branched tetraether lipid-based palaeotemperature proxies in an urban, hyper-eutrophic polluted lake in South Africa

We evaluate the application of the branched glycerol dialkyl glycerol tetraether (br GDGT) based palaeotemperature and palaeoenvironmental proxy to a hyper-eutrophic, polluted and shallow oxic lake. Lake Zeekoevlei is the largest freshwater lake in South Africa, located close to Cape Town. We use published lake-based and soil-based calibration equations, and compare the reconstructed mean annual air temperature (MAT) with regional (South African) and local (Cape Town) instrumental temperature records. The distribution of br GDGTs in the lake sediments is influenced by air temperature. The lake-based calibration equation, which uses the methylation index of branched tetraethers/cyclisation ratio of branched tetraethers (MBT/CBT), formulated for African lakes (Tierney et al., 2010), fits well with the instrumental temperature records. Moreover, the CBT-derived pH likely reflects historic socioeconomic changes in catchment. Our results suggest that a polluted/hyper-eutrophic status and shallow water urban setting do not preclude application of the MBT/CBT-MAT proxy. However, further research is necessary to understand the behaviour of br GDGT–producing bacteria in polluted and highly productive lakes.

Distribution of branched and isoprenoid tetraether lipids in an oligotrophic and a eutrophic Swiss lake: Insights into sources and GDGT-based proxies

Distributions of isoprenoid (isoGDGT) and branched glycerol dialkyl glycerol tetraethers (brGDGTs) were measured in the water column and sediments of the eutrophic Lake Lugano and the oligotrophic Lake Brienz, Switzerland. Absolute concentrations of isoprenoid, i.e. archaeal GDGTs, were highest in the euphotic zone of both lakes, as well as in sediments deposited at times when lake eutrophication occurred. This indicates that GDGT concentrations may be used as indicators for primary productivity. Both lakes, including the anoxic bottom water of Lake Lugano, are characterised by GDGT distributions typical for group I Crenarchaeota with GDGT-0/crenarchaeol ratios of around 1. Comparison of the distribution of brGDGTs with isoGDGTs and other terrestrial biomarkers throughout the Lake Lugano water column, together with CBT/MBT-derived temperatures that resemble that of the lake, suggest significant in situ production. BIT index values for Lake Brienz sediments (ca. 0.4) were significantly higher than water column values (ca. 0.1), most probably because terrestrial run off events were not captured during water sampling. TEX 86 – derived temperatures reflect surface water conditions to within a few degrees, while lower values obtained from deeper water layers suggest a contribution of in situ produced isoGDGTs. For both lake sediments, TEX 86-derived temperatures could be matched reasonably with mean annual lake surface water temperature variation, albeit with a larger offset for Lake Lugano. This suggests that absolute temperatures can only be reconstructed from lake sediments for which a local calibration is known.