Abstract
Abstract. Branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are thought to be transported from soil to marine sediment by rivers, have been used to reconstruct the mean annual air temperature (MAAT) and soil pH of the drainage basin using the methylation index of branched tetraethers (MBT, recently refined as MBT') and cyclization index of branched tetraethers (CBT) from coastal marine sediment records. In this study, we trace the brGDGTs from source to sink in the Tagus River basin, the longest river system on the Iberian Peninsula, by determining their concentration and distribution in soils, river suspended particulate matter (SPM), riverbank sediments, marine SPM, and marine surface sediments. The concentrations of brGDGTs in river SPM were substantially higher and their distributions were different compared to those of the drainage basin soils. This indicates that brGDGTs are mainly produced in the river itself. In the marine environment, the brGDGT concentrations rapidly decreased with increasing distance from the Tagus estuary. At the same time, the brGDGT distributions in marine sediments also changed, indicating that marine in situ production also takes place. These results show that there are various problems that complicate the use of the MBT'/CBT for paleoreconstructions using coastal marine sediments in the vicinity of a river. However, if the majority of brGDGTs are produced in the river, it might be possible to reconstruct the environmental (temperature and pH) conditions of the river water using appropriate aquatic calibrations, provided that marine core locations are chosen in such a way that the brGDGTs in their sediments are predominantly derived from riverine in situ production.
Highlights
Branched glycerol dialkyl glycerol tetraethers are membrane-spanning lipids, most likely derived from heterotrophic (Pancost and Sinninghe Damsté, 2003; Oppermann et al, 2010; Weijers et al, 2010) bacteria that occur ubiquitously in peat (e.g., Weijers et al, 2006a) and soil (e.g., Weijers et al, 2007a)
Our study shows that in the case of the Tagus River system, there are several problems concerning the use of the MBT’/cyclization index of branched tetraethers (CBT) to reconstruct river basin mean annual air temperature (MAAT) and soil pH
BrGDGTs in the river suspended particulate matter (SPM) are mainly produced in situ
Summary
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane-spanning lipids, most likely derived from heterotrophic (Pancost and Sinninghe Damsté, 2003; Oppermann et al, 2010; Weijers et al, 2010) bacteria that occur ubiquitously in peat (e.g., Weijers et al, 2006a) and soil (e.g., Weijers et al, 2007a). The major brGDGTs contain a variety of methyl groups (4–6) and may contain up to two cyclopentane moieties formed by internal cyclization (Sinninghe Damsté et al, 2000; Weijers et al, 2006a). Four of these methyl groups are present in mid-chain positions of the two C28 linear chains of the tetraether structure (Fig. 1), whilst the others are present at the C-5 and C-5’ positions. De Jonge et al (2013, 2014) identified four new isomers of the previously described pentamethylated and hexamethylated brGDGTs in a Siberian peat and suspended matter of the Yenisei River These isomers are characterized by the presence of methyl groups at the C-6 and C-6’ instead of the C-5 and C-5’ positions. It remains unclear whether other bacteria are able to produce brGDGTs
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