Fourteen-Year Fluvial Sediment Record Shows Non-Conservativeness of Organic Tracers: Recommendations for Sediment Fingerprinting

Abstract

While tracing the sources of fluvial sediment using carbon and nitrogen stable isotopic ratios (δ¹³C and δ¹⁵N) has progressed significantly over the last two decades, the conservativeness of these tracers remains questionable. Recent work indicates that δ¹³C and δ¹⁵N alterations in streambed deposition zones likely represent the largest source of uncertainty impacting usefulness of the isotopic ratios as tracers. Here we report a 14-year dataset of δ¹³C and δ¹⁵N of fluvial sediment from a streambed-dominated basin in Kentucky, USA, and employ empirical model decomposition (EMD) to identify dominant temporal trends that may impact conservativeness. Results from EMD show significant seasonality of δ¹³C and δ¹⁵N for sediment as well as underlying multi-year variation. The seasonal and multi-year variance account for 72% and 50% of the total data variation for δ¹³C and δ¹⁵N, respectively. The prominent seasonality for δ¹³C and δ¹⁵N show a mean intra-annual change of 0.6‰ and 1.1‰, respectively, and the seasonal change is attributed to algal accrual and organic matter turnover in the streambed sediment deposits. Mixing model simulations show that the mean streambed isotopic ratios should be separated from other sediment sources by 3.0‰ and 3.6‰ for δ¹³C and δ¹⁵N, respectively, to achieve 90% accuracy in source apportionment when the isotopic ratios are used independently; and the mean streambed value of both isotopic ratios should be separated from other sediment sources by 3.0‰ when δ¹³C and δ¹⁵N are used in combination. Our results lead to the recommendation that isotope ratios of sources be separated by at least 3‰ when the streambed is expected to be a prominent sediment source, which far exceeds the prior recommendation of 1‰ mean separation of sources.