Radiocarbon dating of marine samples from Gulf of Cadiz: The reservoir effect

The 14C content of marine shells can be used as a proxy for past coastal upwelling intensity. The Iberian coast of the Gulf of Cadiz can be divided in two regions with different oceanographic conditions: the southern Portuguese coast with an active upwelling, influenced by the Northeastern Atlantic circulation, and the southwestern Andalusian coast where, due to its configuration, a wind-driven coastal upwelling is absent. Because these oceanographic conditions ΔR, a parameter that quantifies the marine reservoir effect, has positive values with a weighted mean of 65 ± 20 14C yr (southern Portuguese coast) or negative values with a weighted mean of −135 ± 20 14C yr (Andalusian coast). Nevertheless, between 4400 BP and 4000 BP ΔR values are strongly positive, suggesting a very strong upwelling existed in the entire region of the Gulf of Cadiz. This does not reflect wind-driven processes, but probably an eastward extension of the Azores Front, which is characterized by locally intense upwelling, along the Azores Current that penetrated into the Gulf of Cadiz. The study highlights the importance of using appropriate calibration methods when developing 14C chronologies from the Gulf of Cadiz, as well as the potential of ΔR for reconstructing palaeoceanographic conditions.