Holocene relative sea-level variation and coastal changes in the Bay of Cádiz: New insights on the influence of local subsidence and glacial hydro-isostatic adjustments

Abstract

The Bay of Cádiz is located in South-western Spain (Andalusia region) and constitutes an example of a typical estuarine salt marsh environment. In this study we reconstruct its Holocene morpho-evolution and relative sea-level change history by assembling a geodatabase of geological sea-level markers derived from boreholes and bibliographic data, standardized to the most recent international guidelines for RSL studies. The identified high-precision sea-level index points were compared to a number of new site-specific glacio-hydro-isostatic adjustment (GIA) models in order to disentangle potential components which influenced the sea-level evolution and finally obtain the vertical displacement (VD) trends that affect this coastal area by using a Bayesian statistical approach including Monte Carlo simulations. In general, the whole area was affected by overall subsidence related to the local sediment compaction, which had an impact on the morpho-evolution of the different zones with a variable entity and completely outclassed the GIA-driven component. Between 6.7 and 3.0 ka BP, the northern sector of the bay was characterized by subsidence rates of about - 0.65 mm/yr while, during the last 3.0 ka, the general trend appears to be homogeneous for both the main sectors of the bay with an average subsidence rate of - 1.6 mm/yr. The Holocene RSL curve from the Gulf of Cádiz aligns with past reconstructions, revealing subtle differential trends in subsidence rates in the northern and southern sectors of the Bay of Cádiz due to varying substratum.