A multiproxy approach of the Holocene evolution of shelf–slope circulation on the NW Iberian Continental Shelf

Textural, mineralogical, geochemical and microfaunal data (benthic foraminifera) were studied along the OMEX core KSGX 40 recovered in the Galicia Mud Deposit, of the NW Iberian outer continental shelf, off the Ría de Vigo (North of Spain). This core included the records of the last ca. 4.8 ka cal BP and consists, from the base to the top, of a sedimentary sequence exhibiting gradual upward decrease in grain size. Sediments of this core are mainly siliciclastic, largely composed of quartz, K-feldspars, plagioclases, and phyllosilicates (mica/illite, kaolinite and chlorite) showing a great continental influence in this zone. Two periods of deposition of finer sediments are registered between ∼ 2.2–1.2 ka cal BP and ∼ 0.5–0 ka cal BP.Since the last ∼ 2.2 ka BP, but mainly during both muddy intervals, the Galicia Mud Deposit was nourished with a lower and finer supply of detrital minerals compensated by higher amounts of organic matter, as it is suggested by a Benthic Foraminifera High Productivity (BFHP) proxy. Processes involved in organic matter degradation by aerobic organisms led to depressed levels of oxygen in the sediments, as shown by a Benthic Foraminiferal Oxygen Index (BFOI). Peaks of redox-sensitive elements, like Mn, Fe, Cu, Ni, Cr, Co, Zn, Ni and Pb as well as the presence of diagenetic minerals, such as pyrite, suggest the development of anoxic conditions beneath the sedimentary surface and early diagenetic changes due to high organic matter flux, in both muddy intervals.Two different hydrodynamic regimes were inferred through the analysis of the different proxies (textural, mineralogical, geochemical and benthic foraminifera): (1) A strong hydrodynamic regime between ∼ 4.8 and 2.2 ka cal BP characterized by the prevalence of winter storms, which gave rise to a deep mixed layer on the shelf. (2) Weak hydrodynamic regime between ∼ 2.2–1.2 ka cal BP and ∼ 0.5–0 ka cal BP with a high predominance of upwelling and an increase in oceanic stratification.