Assessing δ13C and C/N ratios from organic material in archived cores as Holocene sea level and palaeoenvironmental indicators in the Humber Estuary, UK

Abstract

Organic δ 13C and C/N analyses of estuarine deposits provide proxies for changes in the source of organic matter, which can be driven by fluctuations in relative sea level, river discharge, and catchment disturbance. Here we present the results of a comprehensive vegetation and sediment δ 13C and C/N survey of Welwich Marsh (outer Humber Estuary, UK), together with high-resolution δ 13C and C/N analyses of Holocene cores collected nine years previously from the Humber Estuary and the Lincolnshire Marshes, Eastern England, UK. The contemporary intertidal δ 13C and C/N dataset shows a gradual increase in surface sediment δ 13C with decreasing marsh height and suggests that δ 13C is controlled by the degree of tidal inundation and thus reflects organic matter source. However, sediment C/N ratios are less sensitive to tidal changes and the recent introduction of C 4 salt-marsh species complicates the contemporary analogue. The Holocene δ 13C and C/N records are in general agreement with existing microfossil data and provide additional palaeoenvironmental information. This includes support for an estuary-wide expansion of marine conditions from c. 3.3 ka cal. yr BP, followed by a contraction of marine conditions after c. 2.7 ka cal. yr BP, and evidence for an increase in delivery of terrigenous organic matter to the inner estuary in the late Holocene. Bulk organic δ 13C and C/N analysis is shown to be a reliable and independent indicator of coastal environmental change and is therefore a complementary technique to the more commonly used microfossil approach. However, this study also shows that in some circumstances the technique may be compromised when applied to sediments from cores that have been stored for a period of time.