Holocene oceanographic variability linked to the Guinea Dome development recorded in a deep-sea sediment core off Cabo Verde

The Guinea Dome, located in the eastern tropical North Atlantic, is produced by cyclonic circulation associated with the eastward North Equatorial Countercurrent, the northward Mauritanian Current and the westward North Equatorial Current, which causes the uplift of the isotherms in the Guinea Dome. This oceanographic feature is important for the regional atmosphere-ocean dynamics, and its variability was suggested to be linked with precipitation changes in North Africa, at least at a decadal scale. Characterizing the development of the dome through the Holocene will contribute to understand the prominent environmental changes that occurred regionally during this period, as evidenced by the green-to-desert Sahara transition at the end of the African Humid Period (ca. 6,000 years ago).

A 35cm sediment multicore, extracted southwest off Cabo Verde during the iMirabilis2 scientific cruise, at a water depth of 4,394 m, is being investigated. We present planktonic foraminifera counts and X-ray fluorescence (XRF) scanning data to reconstruct palaeoceanographic and sediment input changes during the Holocene. An age-depth model for the sediment core was established with three samples dated by the radiocarbon method, indicating that the sediment was deposited from 11,180 to 1,257 calibrated years before present (cal. BP).

Planktonic foraminifera results show a gradual but important change in the assemblages throughout the core, where the abundance of species preferring warmer waters increase by 44% towards the top of the core. These results are interpreted as warming of the surface water masses during the Holocene, as a result of reduced influence of the Guinea Dome due to its change of location to a more southern position and/or as a consequence of a weakening of the dome. X-ray fluorescence scan variations along the core show that the faunal shift is encompassed by differences in the terrigenous sediment supply, indicating changes in the inland climate regime. For instance, changes in the Ti/Fe, Ti/Al and Al/Ca ratios are proxies for the fluvial/aeolian sedimentary input and the hinterland climate variability. An increase of the high river discharge indicators is displayed between 10 and 6 kyr BP, probably as a consequence of the increased precipitation that took place during the African Humid Period.

Further ongoing geochemical analyses of foraminifera shells will provide information regarding the temperature, salinity and productivity of both, the mixed layer and the sub-thermocline, which will improve the characterization of the variability that the Guinea Dome experienced during the Holocene.