The C32 alkane-1,15-diol as a proxy of late Quaternary riverine input in coastal margins
Julie Lattaud1,2, Denise Dorhout1,2, Hartmut Schulz3, Isla S. Castañeda1,2,a, Jaap S. Sinninghe Damsté1,2,4, and Stefan Schouten1,2,41NIOZ Royal Netherlands Institute for Sea Research, Department of Marine Microbiology and Biogeochemistry, The Netherlands 2Utrecht University, The Netherlands 3University of Tübingen, Department of Geosciences, Hölderlinstraasse 12, 72074 Tübingen, Germany 4Utrecht University, Department of Earth Sciences, Faculty of Geosciences, Budapestlaan 4, 3584 CD Utrecht, The Netherlands apresent address: University of Massachusetts, Department of Geological sciences, 244 Morrill Science Center, Amherst, USA
Received: 09 Mar 2017 – Accepted for review: 13 Mar 2017 – Discussion started: 14 Mar 2017
Abstract. The study of past sedimentary records from coastal margins allows us to reconstruct variations of 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 riverine organic matter input. Here, we test the fractional abundance of the C32 alkane 1,15-diol relative to all 1,13- and 1,15-diols (F1,15-C32) as a tracer of riverine input 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 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 BIT index, a proxy for soil and riverine input, which declines from 0.25–0.60 for the pre-Holocene to < 0.10 for the Holocene. This decrease of both F1,15-C32 and the BIT is interpreted to be mainly due to an increasing 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 F1,15-C32 for Heinrich Event 1 (H1) and Younger Dryas (YD), which can 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 F1,15-C32 and the BIT index (r2 = 0.38, p < 0.001) is observed for Eastern Mediterranean Nile record. Here also, the BIT index and F1,15-C32 are lower in the Holocene than in the pre-Holocene, which is likely due to the sea level rise. In general, the differences between BIT index and F1,15-C32 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 F1,15-C32 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.
Lattaud, J., Dorhout, D., Schulz, H., Castañeda, I. S., Sinninghe Damsté, J. S., and Schouten, S.: The C32 alkane-1,15-diol as a proxy of late Quaternary riverine input in coastal margins, Clim. Past Discuss., doi:10.5194/cp-2017-43, in review, 2017.