Surface water dynamics of the last 20 kyr documented by coccolithophores in the Gulf of Cadiz

A centennial scale study has been performed on the coccolith assemblage of the Integrated Ocean Drilling Program Site U1390 in the Gulf of Cadiz in order to identify changes in the surface water dynamics during the last 20 kyr. The dataset is integrated with selected planktonic foraminifer taxa and with available sedimentary and paleoceanographic proxies. The reconstructed surface water conditions controlling coccolithophores shifted from a prevailing influence of Portugal Current during the Last Glacial Maximum, to enhanced advection of subtropical water during the Holocene. Millennial to centennial-scale fluctuations are also well recorded based on abundance fluctuations of key coccolithophore taxa. During Heinrich Stadial 1 and Younger Dryas, wind/ frontal-induced upwelling favored nutrient availability and productivity in surface waters, although colder temperatures during Heinrich Stadial 1 inhibited coccolithophore proliferation. Combining calcareous plankton and oxygen isotope data it was possible to highlight that Heinrich Stadial 1 has an internal variability, marked by three distinct phases. Cold and productive surface water conditions occurring during HS1a and HS1c were interrupted during HS1b by enhanced influence of the warm Azores Current at the core location, which acted in phase with increased Mediterranean Outflow Water strength. The sharpest abundance of the polar-subpolar/ melt-water related taxa, observed during HS1c, suggests this was the most extreme cold climate phase in the Gulf of Cadiz during the last 20 kyr. During the Bølling-Allerød interstadial, the northward extension of the subtropical gyre, coupled with enhanced fluvial discharge, determined the increase of warm water taxa and Emiliania huxleyi large morphotype, which was able to adapt to less saline waters. During the Holocene, mainly during the last 8 kyr, over the main warming trend, coccolith assemblages display short-term fluctuations. The N ratio index together with the pattern of Gephyrocapsa oceanica, compared with a palaeo record of NAO, indicates that centennial-scale events of enhanced mixing are in phase with more arid conditions on land. This pattern is related with periods of (+) North Atlantic Oscillation leading to a prevalence of strong northerly winds and enhanced mixing during the winter season.