Attenuation of diffusive noble-gas transport in laminated sediments of the Stockholm Archipelago

A sediment core has been sampled at a spatial resolution of about two centimeters for noble-gas analysis of the sediment pore water. This high-resolution sampling allowed an excess of dissolved atmospheric gases to be identified in the pore water at a sediment depth of 40 cm. Sedimentological evidence indicates that the observed gas excess is related to a past mass movement that trapped and conserved the gas surplus for more than a century. The measured noble-gas concentrations support the hypothesis that, depending on the sediment texture or particular structures in the sediment, the transport of noble gases in the pore space can be significantly attenuated compared to molecular diffusion in bulk water. The atmospheric noble-gas abundance in the pore water allows the deposition of sediments originally located near the air/water interface to be identified. Although the nature of the physical processes that limit the fluid transport in the pore space could not be fully assessed, our observations indicate that noble-gas concentration profiles can be archived for unexpectedly long times in specific sedimentary sequences. This finding represents a significant step forward in the interpretation of noble-gas concentrations in sediment pore waters in terms of past environmental conditions.