Origin, abundance and storage of organic carbon and sulphur in the Holocene Humber Estuary: emphasizing human impact on storage changes

Andrews, Julian and Samways, Gregory and Dennis, Paul and Maher, Barbara (2000) Origin, abundance and storage of organic carbon and sulphur in the Holocene Humber Estuary: emphasizing human impact on storage changes. Geological Society Special Publications, 166. pp. 145-170. ISSN 0305-8719

Abstract

An organic carbon (Corg) and sulphur (S) storage inventory for Holocene sediments in the Humber Estuary is established; sources of organic matter and their variation over time are identified, and with chronological control, the importance of estuarine sediments as Corg and S stores is demonstrated. Humber Holocene sediments are grouped into seven widespread environmental facies with statistically significant geochemical data sets: (1) oak-hazel fenwood (OHF); (2) alder carr (AC), appearing as peats in core; (3) river channel muds or sands (Rcm/s); (4) high saltmarsh (HSM); (5) low saltmarsh (LSM); (6) intertidal mudflat (ITMF); and (7) a sandy facies (S). Carbon, nitrogen and sulphur (CNS) abundances show that these facies have diagnostic geochemical signatures and δ13C values for bulk organic matter exhibit a range of average values: −28‰ (terrestrial peats), −27‰ (HSM), and −24.5‰ (ITMF) reflecting the up core transition from terrestrial peats through saltmarshes to more open marine mudflat environments as regional sea-level rose. Chronology and average sedimentation rates are partly constrained by radiocarbon dates; palaeomagnetic techniques helped define discrete sediment packages and discontinuities (time gaps). Although the Humber Holocene sediment record is not continuous, long-term sedimentation rates (about 1 mm a−1) show that sediment accretion kept pace with regional sea-level rise between 6 and 2 cal. ka BP. This sedimentation rate, combined with core evidence to allow a geographic reconstruction of the palaeo-Humber (3–2cal. ka BP), is used to calculate storage values for Corg and S in the various environments of the palaeo-Humber. Comparison of the Corg and S sedimentation and storage terms for the palaeo-Humber with modern values highlights the impacts of reclamation and commercial/urban development in the estuary in the last 300 years. OHF and AC peats, which were the largest Corg and S stores in the palaeo-estuary, are now absent (reclaimed), while saltmarshes are no longer widespread. Conservative calculations show a net decrease in Corg deposition from about 3.2 × 105 tonne in the palaeo-estuary to no more than 2.5 × 103 tonne today, a >99% reduction in potential Corg storage capacity. The total modern yearly S deposition is approximately 2% of its value 2ka ago. Removal of saltmarsh and associated brackish-freshwater wetland suggests that suspended sediment and associated Corg and S are currently bypassing former (Holocene) storage areas and may be impacting North Sea biogeochemical cycling.