Carbon stable isotope (δ¹³C) and elemental (TOC, TN) geochemistry in saltmarsh surface sediments (Western Brittany, France): a useful tool for reconstructing Holocene relative sea‐level

Abstract

Providing a dense network of Holocene relative sea‐level (RSL) records still faces some challenges, such as the paucity of microfossil assemblages within some Holocene sedimentary sequences. Recently, the isotopic (δ13C) and elemental geochemistry [total organic carbon (TOC), total nitrogen (TN) and C/N ratios] of saltmarsh sediments has showed potential for RSL reconstruction. This study presents the results of new δ13C, TOC and TN measurements obtained on surface sediments of two marshes of the Brittany region (France). Overall, the distribution of δ13C, TOC, TN and C/N values shows clear dependencies with elevation. Sediments from above and below the high tide level show contrasting TN contents. Clustering analyses identified three elevation‐dependent groups, characterized by specific values of δ13C, TOC and TN. The use of this geochemical reference for RSL reconstruction is tested on a core retrieved from one of the marshes. The sea‐level index points obtained plot in good accordance with the microfauna‐based RSL positions previously obtained in the region. Some diagenetic losses in TN are observed between modern and Holocene sediments. This does not appear to be harmful to the accuracy of the RSL reconstructions but suggests that C/N ratios may be unreliable palaeoenvironmental proxies in this specific context.

Testing the Utility of Geochemical Proxies to Reconstruct Holocene Coastal Environments and Relative Sea Level: A Case Study from Hungry Bay, Bermuda

“…Measurements made on bulk sediment include total organic carbon (TOC), stable carbon isotopes (δ 13 C), the ratio of carbon to nitrogen (C:N), and a suite of values from Rock-Eval pyrolysis. Analysis of TOC, δ 13 C, and C:N (usually measured simultaneously) is an established technique in paleoenvironmental research including reconstructing Holocene RSL (e.g., Goslin et al, 2017;Lamb et al, 2006). Rock-Eval pyrolysis is an established technique for evaluating organic material in source rocks (and their modern analogs) for hydrocarbon characterization and exploration (e.g., Girkin et al, 2018;Lafargue, Marquis & Pillot, 1998), but is increasingly being used to generate paleoenvironmental reconstructions from recent sediments (e.g., Ariztegui et al, 2001;Lacey et al, 2018;Newell et al, 2016;Stein, 2008;section 3).…”

“…The use of these proxies is underpinned by reasoning through analogy, in which the observable and measurable characteristics of modern sedimentary environments are used to recognize and interpret their equivalents preserved in the sedimentary record (Jackson & Williams, 2004). A first step in establishing the utility of a proxy is to test if it can reliably distinguish the environment in which modern samples accumulated, which is often achieved by objectively grouping samples (e.g., Goslin et al, 2017;Wilson, 2017). In studies seeking to reconstruct RSL, a key goal is to determine if a proxy (or multiple proxies used together) can distinguish sediment that accumulated above the influence of salt-water tides in a freshwater marsh from inter-tidal sediment that accumulated in a saline mangrove or salt marsh.…”

Ecological risk assessment of heavy metals in the coastal sediment in the South-western Bay of Bengal

“…A high SOC was noticed at MH-10 in the wet period (Table 1). Generally, the concentrations of SOC were higher in the silt and clay in both near-shore and offshore sediments, indicating that SOC is derived from marine and terrestrial inputs (Goslin et al, 2017).…”