CONICET | Buscador de Institutos y Recursos Humanos

Salt marsh soils commonly store large amounts of organic carbon because of a high plant biomass production and slow decay rates due to anoxic conditions. Under a rising sea level, salt marshes with adequate sediment input can vertically accrete to match the rise in water level, and the organic carbon content in soils, especially in deeper layers, is a measure of long-term carbon storage. In this work we describe a recent change in carbon storage at the Bahía Blanca estuary, a highly dynamic coastal environment in the South Western Atlantic. In the study area, the relative sea level reached around 6 m above present about 6000 years ago, and the late Holocene falling trend has resulted in wide low-lying coastal terraces, inherited from the former estuarine dynamics. Through the quantification of recent changes in land cover, surface elevation profiles, and soil surveys, we estimated that 21.5 km2 of Sarcocornia perennis marshes converted to mudflats between 1967 and 2005, and 3.4 kg C m-2 were lost from the soil organic carbon pool. The observed marsh loss and the erosion of soft sediments are in agreement with the present rising trends in relative sea level. The coastal platform presently occupied by Sarcocornia perennis marshes derives from marine deposits that formed during the late Holocene, under a slightly higher relative sea level. Under the current conditions, these low lying coastal landforms became relict and their erosion may be accelerated by the recent rise in sea level. A Holocene relative sea level higher than present characterized most coastlines of the southern hemisphere. Although our work is geographically restricted, we provide a description of a transgressive coastal system where salt marsh soils act as a net source of organic carbon. Given the role that wetlands soils play in the global carbon cycle, further work is needed to evaluate the potential to extrapolate the observations to a broader scale.