Origin and evolution of tidal depressions in a tidal flat and their role in carbon sequestration in the Bahía Blanca Estuary (Argentina)

Abstract

Tidal flats play an essential role in the global carbon cycle. Intertidal environments are very dynamic and have several erosive landforms such as tidal depressions (TD), although TD genesis is still poorly understood. The tidal flats of the Bahía Blanca Estuary (BBE) are ideal for TD’s study since they are hardly altered by human activity. In this site is possible to distinguish a gradation from a non- bioturbed zone to one dominated by erosive landforms. This preliminary study aims to determine the origin and evolution of TD in tidal flats and evaluate their role as carbon sinks based on an integrated sedimentological and geomorphological study. We found that the origin of TD in siliciclastic tidal flat is associated with the activity of the Neohelice granulata crabs. N. granulata selects finer particles and organic matter to build burrows; also alter the physical properties of the sediment. The loss of sediment structure by the increase in the density of excavations added to the tidal dynamics favour the burrows's collapse giving rise to the tidal depression. Tidal dynamics affect sediment stability by exerting a hydrostatic pressure on the burrow when flooding and emptying of burrows. The effect of N. granulata in the sediment record is clearly identifies by an increase water content with a decrease in the sediment density. Furthermore, it is possible to identify changes in the grain size distribution curves of the typical tidal flat facies as a second mode at 10 μm. These features can be recognised and used as a tool to identify TD in the geological record. TD with occluded crab burrows can be important organic carbon sinks due to the organic matter fixation under suitable conditions that delay its decomposition. Due to the large number of TD (of the order of 35 million), we estimated that the C sequestration by these landforms might be of the order 1.8 × 10–3 Gtn without considering that may be stored directly on the tidal flats themselves.