Revisiting Branched Glycerol Monoalkyl Glycerol Tetraether Paleothermometry in Tropical Lacustrine Sediment Archives: Insight from Lake Malawi

Branched glycerol monoalkyl glycerol tetraethers (brGMGTs) are a class of archaeal membrane-spanning lipids produced in the water column and anoxic sediment of lacustrine and marine settings. Like related glycerol dialkyl glycerol tetraethers (GDGTs), these compounds have been investigated for their potential application to paleoclimate reconstructions, particularly in tropical lakes. The mechanisms governing their production and structural distribution, however, have remained challenging to disentangle. Here we analyze brGMGTs from a section of the Lake Malawi Drill Core (MAL05-1) spanning several glacial-interglacial cycles to evaluate the relationship between these compounds and their changing environmental context. We find that many major brGMGTs exhibit notable sensitivity to changes in temperature and lake depth at Malawi, and we apply this insight to capture variability of brGMGTs produced predominantly in the anoxic water column through a new index (termed the ALX), which exhibits a moderately strong correlation with paleotemperatures and lake level at Malawi. We additionally revisit brGMGTs analyzed from modern East African lake sediments and find that the ALX strongly correlates with mean annual air temperature at these sites. These findings corroborate foundational insights from previous studies investigating brGMGTs as potential paleothermometers, while addressing some of the complications known to compromise previously-developed brGMGT temperature indices.