Impact of the Mediterranean-Atlantic connectivity and the late Miocene carbon shift on deep-sea communities in the Western Alboran Basin

Abstract

Integration of foraminiferal and geochemical data (stable isotope and elemental composition) from West Alboran Basin (WAB) ODP Site 976 allowed evaluation of the effects of the initial Mediterranean – Atlantic restriction event preceding the Messinian Salinity Crisis (MSC) in a context of late Miocene cooling and diminishing water – mass exchange close to Gibraltar Strait. At 7.17 Ma a prominent shift in benthic foraminifer abundances from dominantly oxic taxa to species tolerating oxygen deficiency, paired with a drop in δ13C values, suggest that the restriction of the Mediterranean-Atlantic gateways profoundly affected the WAB deep waters. From 7.17 Ma onward, deep-water stagnation increased the bottom water residence time and led to oxygen depletion. Similar changes, already identified in other Mediterranean basins imply that the first signs of Mediterranean-Atlantic restriction significantly predated the onset of the MSC also in the WAB, an area sometimes considered more under the influence of the Atlantic. Simultaneously, a marked amplitude increase of several element-log ratios reveals a clear cyclical pattern related with precession. Together with new δ18O data, the identification of cyclical patterns allowed improving the age model of Site 976 and consequently enabled an accurate correlation with other Mediterranean, mostly land-based sections. Comparing the records, we were able to correlate the event at a basinal scale and to refine thermohaline circulation models of the Mediterranean after 7.17 Ma. Because this Mediterranean-scale change was contemporaneous with the global Late Miocene Carbon Isotope Shift (LMCIS) it was important to discern between global and local effects. Given the synchronicity of the global and local Mediterranean change in the δ13C record, a global effect certainly affected the Mediterranean Basin. However, opposite phase relations of the global and local δ13C signals with orbital parameters, paired with a higher magnitude change identified in our WAB isotope record suggests that the local imprint overruled the global one.