Branched GDGT signals in fluvial sediments of the Danube River basin: Method comparison and longitudinal evolution

Abstract

Abundances and distributional changes of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in fluvially influenced sediments are used in various paleoclimate studies to reconstruct variations in soil export, continental air temperature and soil pH in corresponding river basins. For accurate interpretation of these records, it is important to understand the provenance and the evolution of biomarker signals as they move through the river system. Here we investigate the brGDGT composition of modern river sediments of the Danube River, the second largest river in Europe. BrGDGT-based mean annual air temperature and soil pH parallel the actual values of air temperature and soil pH from the upper to the lower basin, showing that signals predominantly reflect local as opposed to basin-wide environmental conditions. Furthermore, data generated using the recently developed method with improved chromatography, separating the 6-methyl-isomers from the 5methyl-isomers, was compared with that resulting from the conventional method. We show that the temperatures and pH values reconstructed using the data obtained by improved chromatography best resemble the local environmental conditions throughout the Danube river basin. Our results highlight the importance of in-depth studies within river systems to better understand the provenance of biomarker signals in fluvially derived sedimentary archives.

Evolution of biomolecular loadings along a major river system

“…brGDGTs: The preparation of a brGDGT fraction from the neutral phase and its subsequent measurement is described in Freymond et al (2017). The brGDGT concentration data reported here were acquired with the 'classic' method, i.e.…”

“…The brGDGT concentration data reported here were acquired with the 'classic' method, i.e. without the separation of 5-methyl and 6methyl brGDGT isomers, using an HPLC-APCI-MS equipped with a Grace Prevail cyano column (150 mm  2.1 mm, 3 mm) (Schouten et al, 2007;Freymond et al, 2017). This form of data is presented so that all available Danube catchment data, including samples that were not measured with the 'improved chromatography' method, could be used.…”

Branched GDGTs as Proxies in Surface Sediments From the South-Eastern Brazilian Continental Margin

“…The brGDGTs in the marine sediments were first assumed to represent an integrated signal of the entire river basin from where the soil brGDGTs derive (Weijers et al, 2007a). However, it has been shown that the reconstructed MAT and soil pH can be biased toward a certain area and/or period of increased input of brGDGTs to the river (Freymond et al, 2017). In addition, the hydrodynamics and microbial activity during fluvial transport might influence the distribution of brGDGTs along the river (Zell et al, 2013a(Zell et al, ,b, 2014aDe Jonge et al, 2014b, 2016, and the marine in situ production of brGDGTs may affect the original signal of the soil-derived material (Peterse et al, 2009;Zhu et al, 2011;Hu et al, 2012;Liu et al, 2014;Zell et al, 2014a,b;Sinninghe Damsté, 2016).…”

Long-chain diols in rivers: distribution and potential biological sources

“…There were 46 riverbed sediments (main river and tributaries, Fig. 2a) collected as described by Freymond et al (2018) in spring 2013 and 2014. The riverbed sediments were wet sieved with Milli-Q water over a 63 µm sieve on a (Usman et al, 2018) with enhanced view of the delta region, (b) concentrations of LCDs in SPM (collected during wet and dry seasons), and (c) concentrations of LCDs in sediments (collected during wet and dry seasons).…”

“…The extracts were dried under N 2 and an 80 % aliquot was further processed for analysis. The samples from the Danube were extracted as described by Freymond et al (2018) using microwave extraction (MARS) with 9 : 1 DCM : MeOH (v/v, 25 min at 100 • C). The Meteor-Poseidon Black Sea surface sediments were extracted three times ultrasonically with a 9 : 1 DCM : MeOH (v/v) solvent mixture after addition of 1.96 µg of C 22 7,16-diol as the internal standard.…”