Abstract
In this study, we investigated soils and river suspended particulate matter (SPM) collected in the Rhone and its tributary basins as well as marine surface sediments taken in the Rhone prodelta (Gulf of Lions, NW Mediterranean). Thereby, we traced the signal of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from the source to sink via the Rhone River and its tributaries and identified sources of brGDGTs in rivers and marine sediments. Soil pH rather than the mean annual air temperature (MAAT) explains most of the observed variances of the brGDGT distribution in our soil dataset. The observed changes in the distribution of brGDGTs in the river SPM indicate that brGDGTs brought by the river to the sea are primarily derived from the lower Rhone and its tributary soils, even though in situ production in the river itself cannot be excluded. In marine surface sediments, it appears that the input of riverine brGDGTs is the primary source of brGDGTs in the Rhone prodelta, although the brGDGT composition may be further modified by the in situ production in the marine environment. More work is required to assess fully whether brGDGTs can be used to reconstruct the terrestrial paleoenvironmental changes using marine sediment cores taken in the Rhone prodelta close to the river mouth.
Highlights
We have extended our previous studies to the methylation index of branched tetraethers (MBT)’/cyclization index of branched tetraethers (CBT) proxy, by analyzing soils and river suspended particulate matter (SPM) collected in Rhône and its tributary basins as well as marine surface sediments collected in the Rhône prodelta (Gulf of Lions, NW Mediterranean)
Our study conducted in the Rhône River system (NW Mediterranean) shows that the differences in soil pH are more important than those in mean annual air temperature (MAAT) in explaining the variance in the branched glycerol dialkyl glycerol tetraethers (brGDGTs) distribution in soils
The observed changes in the distribution of brGDGTs in the river SPM suggest that the provenance of brGDGTs brought by the river to the sea might be more restricted to certain areas in the lower Rhône River and its tributary basins
Summary
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) (see Schouten et al, 2013 for the review) are membrane-spanning lipids, most likely derived from anaerobic (Weijers et al, 2006a) and 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) as well as in lacustrine/marine sediments and water column (e.g., Blaga et al, 2009; Peterse et al, 2009a; Tierney and Russell, 2009; Pearson et al, 2011; Loomis et al, 2014; Weijers et al, 2014). The brGDGTs contain four to six methyl groups and up to two cyclopentane moieties formed by internal cyclization (Sinninghe Damsté et al, 2000; Weijers et al, 2006a, see Figure 1 for chemical structures). Four of these methyl groups are present in mid-chain positions of the two C28 linear chains of the tetraether structure, whilst the others are present at the C-5 and C-5 positions. Biological sources of other brGDGTs remain to be identified.
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