Abstract
The bulk organic matter composition (total organic carbon (TOC) content and δ13CTOC) and composition of isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGT) in surface sediments from 43 stations in the Berau River delta (east Kalimantan, Indonesia), including two coast-shelf transects and stations within the river mouth, were examined to reveal the spatial heterogeneity in these parameters in order to assess the impact of a tropical river loaded with suspended matter on the sedimentary organic matter in the shelf system. The high-resolution study showed that, despite the extensive transport of eroded soil material by the river to the sea, terrestrial organic matter and brGDGTs are only deposited on a relatively small part of the shelf. The concentrations of brGDGTs are highest (up to 120μgg−1 TOC) in sediments deposited in and close to the mouth of the Berau River and their distribution indicates that they represent a mixture of soil-derived and river in-situ produced brGDGTs. Crenarchaeol concentrations reach 700μgg−1 TOC in sediments deposited on the outer shelf due to Thaumarchaeotal production in shelf waters. This results in a strong gradient (0.93–0.03) in the BIT index, with high values in the river mouth and low values on the shelf. The decline in the BIT index is caused by both decreasing concentrations of the brGDGTs and increasing concentrations of crenarchaeol. The BIT index shows a highly significant but non-linear relationship with δ13CTOC. On the shelf, in the area not under the direct influence of the Berau River, cyclic brGDGTs become relatively dominant, most probably due to in-situ production in the alkaline pore waters of the surface sediments. The spatial heterogeneity of sources of brGDGTs on the Berau shelf complicates the use of brGDGTs as temperature proxies. Application of the global soil calibration to sedimentary mixtures of brGDGTs in the river-influenced area of the shelf results in a severe underestimation of mean annual air temperature (MAT) by 6°C. This is due to the mixed origin of the brGDGTs, which are not only derived from soil erosion but, likely, also from riverine production, as has been observed for other river systems.Comparison of the Berau shelf other shelf systems indicates that in-situ production of brGDGTs in shelf sediments is a widespread phenomenon that is especially pronounced at water depths of ca. 50–300m. It is hypothesized that this is so because benthic in-situ production of heterotrophic brGDGT-producing bacteria is fueled by the higher delivery of fresh organic matter to these sediments as the consequence of higher primary productivity in shelf waters and a decreased mineralization due to the relatively short settling times of particles on the shelf. For palaeoclimatic studies of marine shelf sediments the application of brGDGTs as proxies is severely complicated by the heterogeneity of sources of brGDGTs. Comparison of the brGDGT composition of soils with those of shelf sediments may assist in deciding if sedimentary brGDGTs are predominantly derived from soil erosion. Several methods to do so are discussed.
Highlights
Glycerol dialkyl glycerol tetraethers (GDGTs) are organic compounds occurring in membranes of archaea and bacteria and have recently raised substantial interest due to their potential as biomarkers and proxies
One of the interesting applications for palaeoclimatology is the use of branched GDGTs (brGDGTs) in shallow marine sediments to reconstruct continental climate (Weijers et al, 2007a) based on the premise that brGDGTs produced in soil are brought by rivers to coastal regions
The surface plots for TOC content and d13CTOC (Fig. 2A and B) reveal that in and close to the river mouth the TOC content of the surface sediments is high and d13CTOC values are most negative, while low TOC sediments with more positive stable carbon isotope values are found along the stations of the two transects further from the coast seawards and station 16A (Fig. 1)
Summary
Glycerol dialkyl glycerol tetraethers (GDGTs) are organic compounds occurring in membranes of archaea and bacteria and have recently raised substantial interest due to their potential as biomarkers and proxies (see Schouten et al, 2013b for a review). Since the distribution of the brGDGTs reflects the temperature and pH of the soil at the time of the production of the bacterial membranes, expressed in the MBT/CBT indices (Weijers et al, 2007b), the fossilized brGDGTs in coastal marine sediments may provide a record of past variations in the temperature of the catchment of the river system, enabling continental climate reconstructions. By reconstructing both continental and sea temperature using one marine core inferences about the contrast in temperature between sea and land may be made and its impact on the hydrology (e.g., Weijers et al, 2007a). Various complications with this approach, such as the provenance of soil-derived brGDGTs (Bendle et al, 2010) and potential in-situ production in the river (Zell et al, 2013a,b; De Jonge et al, 2014b), the oxic (Zell et al, 2014a,b; De Jonge et al, 2015) and anoxic (Liu et al, 2014; Xie et al, 2014) marine water column, and in marine sediments (e.g. Peterse et al, 2009a; Zhu et al, 2011), have been recognized
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