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
de Jonge, C. (2015). 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. PhD Thesis. Universiteit Utrecht (UU): Utrecht. ISBN 978-94-6203-808-0. 254 pp. hdl.handle.net/1874/314974
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| Abstract |
Understanding and predicting climate variability is a major scientific challenge, especiallyas climate-induced environmental change will impact on human society. In order to constrainthe magnitude of this impact, models to predict future climates are increasingly complex, andpartly based on what is known of the climate in the past, both from the recent instrumental andhistorical records as well as from geological climate archives. In this thesis so-called proxiesare used to estimate past air temperatures, soil pH, and the amount of terrigenous organicmatter delivered to the marine system by rivers. To reconstruct these changes, the quantity anddistributions of two groups of lipids is studied; bacterial branched glycerol dialkyl glyceroltetraether lipids (brGDGTs) and bacteriohopanepolyols (BHPs).BrGDGTs are membrane-spanning lipids that are produced most likely by Acidobacteria.These ubiquitous bacteria occur in soil, peat bogs, and lake and coastal marine sedimentsand the water column. In soils, the number of methyl groups on the alkyl chains was shownto depend on the mean annual air temperature (MAT) and soil pH, while the number ofcyclopentane moieties correlates with soil pH. Based on this observation, the methylation indexof branched tetraethers (MBT) and the cyclisation index of branched tetraethers (CBT) wereintroduced to reconstruct MAT and soil pH. As brGDGTs are found in large quantities in soiland peat, it was assumed that their presence in freshwater and marine environments followedsoil erosion and transport through river systems. Hence, changes in the brGDGT compositionin river fan sediments, were assumed to reflect fluctuations in the past MAT and soil pH ofthe drainage basin of a river system. Furthermore, their presence allows reconstructing therelative contribution of soil-derived organic matter in the marine system, using the branchedand isoprenoid tetraether (BIT) index. A second group of lipids studied are BHPs, lipids thatare produced by a variety of (cyano)bacteria. The relative concentration of a set of soil-markerBHPs, expressed as the R’soil index, allows tracing the delivery of terrigenous organic matter tothe marine system. Furthermore, the presence of some BHPs can imply a more or less specificchemotaxonomic source.Previous studies of brGDGTs, described the abundance of only 9 brGDGT compounds, the5-methyl brGDGTs, where the penta- and hexamethylated brGDGTs have a methylation on the5 and/ or ?-5 position. In this thesis, the abundance of 15 brGDGT compounds is described,including 6 compounds that were previously partially or fully co-eluting with these 5-methylcompounds. The chemical structure of these novel compounds was determined, following theisolation of penta- and hexamethylated brGDGTs from a Siberian peat. They were describedto contain a methylation on the 6 and/ or ?-6 position, and are therefore referred to as 6-methylbrGDGTs. Using improved chromatography, these 6-methyl compounds were shown to beReferences, summary and acknowledgments245abundant in soils globally, where their relative concentration was strongly related to the soilpH. Their presence has an impact on the interpretation of currently used brGDGT palaeoclimateproxies, as their separate quantification results in a significantly improved soil pH proxy, and apH-independent temperature proxy.The Siberian mainland, a region that is particularly vulnerable to climate change, containsa vast amount of frozen permafrost. The current and accelerating rise in the concentration ofgreenhouse gasses in the atmosphere, that cause the warming of the globe, can lead to themobilization of this stock of fossil organic carbon. In a warmer climate, large changes in theexport of water and organic carbon to the Arctic Ocean are to be expected. The Yenisei Riveris the largest river of the Siberian mainland, draining the Mongolian steppe and highlandsand vast areas of Siberian temperate forests and taiga, before flowing into the Kara Sea, ashallow shelf sea of the Arctic Ocean. To perform informed palaeoclimate reconstructions ofthe Yenisei River watershed in particular, but also of Siberian watersheds in general, the source(i.e. soil-derived, riverine or marine) of the bacterial organic matter was determined in themodern Yenisei River and its outflow in the Kara Sea.Although the Yenisei River crosses several climatic zones, the brGDGTs suspended inthe water column of the Yenisei River show very little variation downstream the river. Thereconstructed temperature and pH using brGDGTs was distinct from the air temperature andsoil pH. The strong temperature gradient imposed on the watershed soils was thus not reflectedin the riverine brGDGT distribution. However, the reconstructed water temperature and pHapproached those of the Yenisei River water, indicating that the Yenisei River brGDGTs aredominated by riverine in-situ produced lipids.The prevalent assumption that the riverine brGDGT signal delivered to the ocean representsan average of the watershed was challenged by a study performed on Lake Baikal. Here, itis described how the contrasting brGDGT distribution of its main inflowing river, SelengaRiver, is strikingly different compared to that from the lake. A detailed study of this outflowsystem indicates that degradation or in-situ production of brGDGTs takes place. However, thebrGDGTs exported from the lake do not contribute to the brGDGT signal encountered in theYenisei River. Consequently, the watershed of Lake Baikal does not seem to contribute GDGTsto the marine system.Following up on this, the fate of riverine brGDGTs after their introduction in the marineenvironment was reconstructed. Moving downstream in the Yenisei River outflow, a strongshift is observed in the brGDGT distribution. It is postulated that this can be explained bydegradation, which will affect different brGDGT pools to a different extent. The performanceof the BIT index as a tracer for terrigenous and riverine bacterial organic matter was also tested.Although the contribution of the different brGDGT pools present (soil-derived, coastal cliffderivedand produced in-situ in the river) is unknown, the BIT-index performs well in the KaraSea system. These findings were compared with an independent set of bacteriohopanepolyolslipids. Six BHPs that are generally enriched in soils, i.e. the so-called soil-marker BHPs, areencountered both in the Yenisei River as its outflow. Both pools of bacterial OM show the samedecrease in concentration in the marine system.246Although in-situ production of brGDGTs only affects the modern sediments to a minorextent, it clearly influences downcore sediments, as was shown in the Kara Sea Trough. Here,sediments deposited since the deglaciation of this site (13 ka), illustrate that in-situ productionof marine brGDGTs can strongly influence their distribution. The shift from a system in frontof the river outflow to a system dominated by marine brGDGTs resulting from the substantialrise in sealevel since the last Glacial is also evident from the BIT index record. During this shiftsubstantial changes in the terrigenous sources of brGDGTs have occurred, resulting in changingbrGDGT distributions. Both processes strongly influence brGDGT-based palaeoclimatereconstructions.Overall, the work presented in this thesis has implications for brGDGT-based palaeoclimatereconstructions, not only in Siberian river systems, but also for brGDGTs encountered inriver fan sediments worldwide. Analytical developments allow performing palaeoclimatereconstructions with improved accuracy. However, the source of brGDGTs encountered inriver fan sediments was shown to be a complex mixture of riverine, marine and soil-derivedbrGDGTs. In modern sediments, their relative abundance can be influenced by degradation ofbacterial organic matter. On geological timescales, changes in sea level affect the importance ofmarine in-situ produced brGDGTs. The provenance of terrigenous brGDGTs delivered to andstored in the marine sediments can be modified by changes in the erosion of watershed soils, bythe presence of large lakes in the flowpath of the river, and by preferential degradation of morelabile brGDGT pools. BrGDGT-based palaeoclimate reconstructions thus have to be performedtaking these processes in mind. |
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