Sources and sinks of branched tetraether lipids and bacteriohopanepolyols in a major river system (Yenisei River – Kara Sea) : Implications for their application as geochemical tracers

Abstract

Understanding and predicting climate variability is a major scientific challenge, especially as climate-induced environmental change will impact on human society. In order to constrain the magnitude of this impact, models to predict future climates are increasingly complex, and partly based on what is known of the climate in the past, both from the recent instrumental and historical records as well as from geological climate archives. The Siberian mainland is a region that is particularly vulnerable to climate change, and it contains a vast amount of frozen permafrost. The current and accelerating rise in the concentration of greenhouse gasses in the atmosphere, that cause the warming of the globe, can lead to the mobilization of this stock of fossil organic carbon. In a warmer climate, large changes in the export of water and organic carbon to the Arctic Ocean are to be expected. The Yenisei River is the largest river of the Siberian mainland, draining the Mongolian steppe and highlands and vast areas of Siberian temperate forests and taiga, before flowing into the Kara Sea, a shallow shelf sea of the Arctic Ocean.

To estimate past air temperatures and soil pH of the Siberian mainland and the amount of terrigenous organic matter delivered to the Kara Sea by the Yenisei River, the quantity and distributions of bacterial branched glycerol dialkyl glycerol tetraether lipids (brGDGTs) was studied. They have been used previously for palaeoclimate reconstructions in palaeosoils, lake and marine sediments. We made analytical developments that allowed quantifying six new brGDGT compounds with a methylation on the 6 and/ or ω-6 position, and are therefore referred to as 6-methyl brGDGTs. Their separate quantification allowed performing brGDGT-based palaeoclimate reconstructions with improved accuracy. However, the source of brGDGTs encountered in river fan sediments was shown to be a complex mixture of riverine, marine and soil-derived brGDGTs. In modern sediments, their relative abundance can be influenced by degradation of bacterial organic matter. On geological timescales, changes in sea level affect the importance of marine in-situ produced brGDGTs. The provenance of terrigenous brGDGTs delivered to and stored in the marine sediments can be modified by changes in the erosion of watershed soils, by the presence of large lakes in the flowpath of the river, and by preferential degradation of more labile brGDGT pools. BrGDGT-based palaeoclimate reconstructions thus have to be performed taking these processes in mind. Overall, the work presented in this thesis has implications for brGDGT-based palaeoclimate reconstructions, not only in Siberian river systems, but also for brGDGTs encountered in river fan sediments worldwide.