Recent sedimentation-erosion processes along the Nalón estuary (Asturias, N Spain)

Coastal marshes from the northern Atlantic Iberian margin are unique sedimentary environments filtering and storing chemical pollutants, nutrients and terrestrial carbon,proxies that may allow deciphering their environmental transformation. The Nalón estuary marshes (Asturias, N Spain) have been severely impacted by human activities such as construction of dikes, land reclamation and coal and mercury mining effluents. We analyzed foraminiferal assemblages from 18 surface samples collected from salt marsh and tidal flat settings along the axis of the estuary and a 50-cm-long core recovered from the lower estuary San Juan de la Arena salt marsh in order to understand recent coastal sedimentation and erosion processes. Salt marsh surface samples are characterized by Trochammina inflata, Entzia macrescens, Miliamminafusca andHaplophragmoideswilbertitaxa in the living and dead assemblages. Tidal flat samples from the middle estuary are dominated bythe living speciesHaynesina germanica, Ammonia tepida, Cribroeplhidium excavatumand M. fusca. In turn, deadassemblages reveal an elevated proportion (max. 95%) of salt marsh taxa, much higher than those determined in other tidal flats from this coastal region, that could derive from the ongoing erosion of adjacent salt marsh areas.The stratigraphy of the sedimentary core is distinguished by two intervals with characteristic microfossil assemblages,whose depositional environment has been inferred from those analogues represented by the surface samples. The lowermost unit (50-28 cm), with a mixture of H. germanica, M.fusca, A. tepida and C. excavatum,has been interpreted as an intertidal flat environment, deposited before the development of the San Juan de la Arena salt marsh, namely previous to the 1950s, as observed in aerial photographs. The second unit (28-0 cm) is characterized mainly byE. macrescensand T. inflata, followed by M. fusca and H. wilberti, representing a salt marsh environment. The salt marshwas developed after the 1950s probably in response to an important accumulation of sediment in the lower estuary. However, as observed in aerial photographs, this salt marsh is no longer active owing to very active erosional processes, possibly associated to the construction of a dock in front of this area in ~2004-2006. On the whole, the foraminiferal analysis of the Nalón estuary sedimentary record hasrevealedsignificant changes in the sedimentation and erosion processesalong the middle and lower estuary during the 20^th and 21^st centuries.

Acknowledgements: Research supported by the Spanish MINECO RTI2018-095678-B-C21(MCIU/AEI/FEDER, UE) project.