Testing the utility of an organic geochemical approach to Holocene sea-level change in the Shannon estuary

Abstract

This project tests a recently developed, geochemically-based technique for relative sea- level (RSL) reconstruction to assess its applicability in the Shannon region. The technique is underpinned by the identification of altitudinal variations within intertidal sediment of δ13C and C/N resulting from differential deposition of the two main sources of organic matter (OM) to saltmarshes: plant detritus and particulate organic matter (POM). Previous application in the UK has demonstrated the occurrence of these geochemical gradients across contemporary marshes and their preservation within the sedimentary record. To assess the utility of this approach within Ireland, this study is broken into four main investigations into: 1/ the effects of acid pre-treatment methods to remove inorganic carbon; 2/ the occurrence of contemporary geochemical gradients of organic matter across intertidal sites; 3/ the alteration to geochemical values within recently buried intertidal sediment; and 4/ the occurrence of geochemical gradients in older Holocene sediment from the Shannon estuary. For the first investigation, two acid pre-treatment methods were applied to replicate sediment from three saltmarsh sites prior to isotope ratio mass spectrometry (IRMS) analysis to allow inter- and intra-method comparison. It was found that while acid pre-treatment does cause variation in δ13C, δ15N and C/N ratios, the magnitude of these differences is small when compared to the range of values encountered across the inter-tidal to supra-tidal environments of the study sites: C/N ranges are from 4-9; δ13C from 4- 11 % and δ15N from 6-10%, compared to mean deviations due to acid pre-treatment method of up to 0.8 for C/N, 0.3% for δ13C and 0.5% for δ15N. Consequently, any bias associated with the choice of acid pre-treatment is not sufficient to obscure source provenance in a contemporary environment.