Abstract
Abstract. The study of past sedimentary records from coastal margins allows us to reconstruct variations in terrestrial input into the marine realm and to gain insight into continental climatic variability. There are numerous organic proxies for tracing terrestrial input into marine environments but none that strictly reflect the input of river-produced organic matter. Here, we test the fractional abundance of the C32 alkane 1,15-diol relative to all 1,13- and 1,15-long-chain diols (FC32 1, 15) as a tracer of input of river-produced organic matter in the marine realm in surface and Quaternary (0–45 ka) sediments on the shelf off the Zambezi and nearby smaller rivers in the Mozambique Channel (western Indian Ocean). A Quaternary (0–22 ka) sediment record off the Nile River mouth in the eastern Mediterranean was also studied for long-chain diols. For the Mozambique Channel, surface sediments of sites most proximal to Mozambique rivers showed the highest F1, 15 − C32 (up to 10 %). The sedimentary record shows high (15–35 %) pre-Holocene F1, 15 − C32 and low (< 10 %) Holocene F1, 15 − C32 values, with a major decrease between 18 and 12 ka. F1, 15 − C32 is significantly correlated (r2 = 0.83, p < 0.001) with the branched and isoprenoid tetraether (BIT) index, a proxy for the input of soil and river-produced organic matter in the marine environment, which declines from 0.25 to 0.60 for the pre-Holocene to < 0.10 for the Holocene. This decrease in both FC32 1, 15 and the BIT is interpreted to be mainly due to rising sea level, which caused the Zambezi River mouth to become more distal to our study site, thereby decreasing riverine input at the core location. Some small discrepancies are observed between the records of the BIT index and FC32 1, 15 for Heinrich Event 1 (H1) and the Younger Dryas (YD), which may be explained by a change in soil sources in the catchment area rather than a change in river influx. Like for the Mozambique Channel, a significant correlation between FC32 1, 15 and the BIT index (r2 = 0.38, p < 0.001) is observed for the eastern Mediterranean Nile record. Here also, the BIT index and FC32 1, 15 are lower in the Holocene than in the pre-Holocene, which is likely due to the sea level rise. In general, the differences between the BIT index and FC32 1, 15 eastern Mediterranean Nile records can be explained by the fact that the BIT index is not only affected by riverine runoff but also by vegetation cover with increasing cover leading to lower soil erosion. Our results confirm that FC32 1, 15 is a complementary proxy for tracing riverine input of organic matter into marine shelf settings, and, in comparison with other proxies, it seems not to be affected by soil and vegetation changes in the catchment area.
Highlights
Freshwater discharge from river basins into the ocean has an important influence on the dynamics of many coastal regions
Following Sinninghe Damsté (2016), we calculated the #ringstetra to determine if the branched glycerol dialkyl glycerol tetraethers (brGDGTs) are produced in situ in the surface sediments or derived from the continent
The low #ringstetra indicate that there is likely limited in situ sedimentary production of brGDGTs in the sediments of the Mozambique coastal shelf area except for the samples closest to the Madagascar coast where high #ringstetra values and low branched and isoprenoid tetraether (BIT) values indicate in situ production of brGDGTs
Summary
Freshwater discharge from river basins into the ocean has an important influence on the dynamics of many coastal regions. Terrestrial OM can be differentiated from marine OM using carbon to nitrogen (C / N) ratios and the bulk carbon isotopic composition (13C) of sedimentary OM (e.g., Meyers, 1994). The abundance of N-free macromolecules such as lignin or cellulose result in organic carbon-rich plant tissues that lead to an overall higher C / N ratio for terrestrial OM compared to aquatic organisms (Hedges et al, 1986). This ratio may be biased when plant tissues gain nitrogen during bacterial degradation and when planktonic OM preferentially loses nitrogen over carbon during decay (Hedges and Oades, 1997). C4 plants have δ13C values of around −12 ‰ (Fry and Sherr, 1984; Collister et al, 1994; Rommerskirchen et al, 2006), and a substantial C4 plant contribution can make it difficult to estimate the proportion of terrestrial to marine OM in certain settings (Goñi et al, 1997)
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