Molecular and bulk geochemical proxies in sediments from the Conwy Estuary, UK

Raquel A Lopes Dos Santos,Christopher H Vane

doi:10.1016/j.orggeochem.2020.104119

Raquel A Lopes Dos Santos, Christopher H Vane

Open Access

https://doi.org/10.1016/j.orggeochem.2020.104119

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Journal: Organic Geochemistry	Publication Date: Sep 8, 2020
Citations: 4	License type: cc-by

Affiliation: British Geological Survey

Abstract

Owing to their restricted riverine flow and greater numbers, small estuaries (annual discharge < 2000 m3 s−1) are important sites of organic carbon (OC) storage in sediments. The Conwy Estuary is an exemplar small rural-agricultural catchment which delivers a diverse pool of terrestrial OC to the Irish Sea. Thirty-nine surface sediments, covering a distance of 10.5 km, were taken transversal as well as longitudinal to the estuary to examine local and distant catchment inputs. Biomarkers (branched Glycerol Dialkyl Glycerol Tetraethers - brGDGTs, n-alkanes) and molecular and bulk proxies (BIT, Paq, TAR, δ13C, C/N), showed sediments were dominated by terrestrial inputs, with only sediments of the three most seaward sites showing stronger physical and geochemical characteristics of a marine influence (e.g., sandier sediments and lower BIT). Biomarker concentrations in the Conwy were comparable to the more urbanised Thames and the Clyde estuaries. The three estuaries showed similar fractional abundance of the brGDGTs, recording an inverse trend between the hexamethylated and the tetramethylated brGDGTs, and showing the pentamethylated brGDGTs as the dominant group. A systematic decrease in terrestrial biomarkers from bank to the main channel confirmed either a strong localised erosion/runoff process or a decrease in water velocity leading to a higher deposition.

Highlights

Rivers and estuaries transport vast amounts of carbon from land to oceans and play an important role in the global carbon cycle through their high rates of carbon respiration and sequestration (Galy et al, 2015)
The results of the molecular and bulk geochemical analyses show that most of the sedimentary organic carbon (OC) accumulating in the Conwy estuary originates from terrestrial sources
The physical characteristics of the sediments and principal component analysis (PCA) of the entire data-set suggests that grain size and type of OC were possible factors controlling the molecular values and bulk proxies

Summary

Introduction

Rivers and estuaries transport vast amounts of carbon from land to oceans and play an important role in the global carbon cycle through their high rates of carbon respiration and sequestration (Galy et al, 2015). The amount, origin and fate of carbon transported globally through rivers and estuaries has been the subject of numerous studies (e.g., Schlesinger and Melack, 1981; Galy et al, 2015). The majority of these investigations have focused on large river-estuaries such as the Amazon (Kim et al, 2012). If the sedimentary transfer of carbon from land to open ocean is to ever be fully understood, there is a need to track carbon flows and quantify carbon stored in smaller riverestuaries.

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