The role of antecedent morphology and changing sediment sources in the postglacial palaeogeographical evolution of an incised valley: The sedimentary record of the Ría de Arousa (NW Iberia)

Incised valleys represent dynamic coastal and shelf systems in continuous change in response to different temporal and spatial scales. Their sedimentary record offers valuable data to unveil the effect of different drivers, such as relative sea-level rise, on the evolution of coastal systems and provide invaluable records of palaeoenvironmental changes. In this work, the palaeogeographical evolution of the Ría de Arousa (NW Spain) from the Last Glacial Maximum to the present is reconstructed by identifying the main elements of sedimentary architecture. These elements are characterised using a multidisciplinary approach combining seismic, sedimentary, geochemical and palynological data. The general succession of the sedimentary infilling of the incised valley records an overall transgression in response to postglacial sea-level rise. A large fluvial valley evolves into a tide-dominated estuary and later to a wave-dominated estuary as it is progressively flooded. The complex interaction between variable rates of relative sea-level rise and the antecedent morphology exerts a strong control on hydrodynamics within the valley controlling the sediment dynamics and facies distribution, displaying some similarities with conceptual facies models for tide- and wave-dominated estuaries. The analysis of sediment sources also denotes significant changes in response to varying hydrodynamic regimes and climate, shifting from a predominant fluvial source in the earliest phases of the transgression to an increase in local sources (cliffs and local coastal deposits), including a likely reworking of older deposits (sediment recycling) during the Early Holocene. These changing sediment sources may have a clear impact on the palynological signal recorded within the incised valley, hypothetically masking and overprinting the climatic signal during phases of intense reworking of older deposits. Sediment recycling seems extensive within the incised valley and represents a key process that needs to be evaluated to understand the source-to-sink sediment flux to the deeper ocean. Its impact on the sedimentary records may have major implications for palaeoenvironmental studies in coastal settings, stressing the importance of applying multi-proxy approaches.