Geochemical signatures in contourites from the Gulf of Cadiz

Bottom currents are responsible for shaping the ocean floor and contributing to the architecture of deep-marine sediments. A prominent example of these deep-marine sediments is the contourite depositional system in the Gulf of Cadiz. This study considers the contourites recovered from IODP Expedition 339 and their relation to the Mediterranean Outflow Water (MOW). We have focussed, in particular, on sites U1389 (proximal to the Gibraltar Gateway), U1387 (in a medial position), and U1391 (distal).Paleorecords suggest that MOW strength and depth have varied over longer timescales (Hernandez-Molina et al., 2014; Stow et al., 2013). Particularly strong bottom currents can cause long periods of non-deposition and erosion and hence result in breaks in the sedimentary record (Stow et al, 2008). Such hiatuses, in some cases, can be connected to major changes in the past climate, which may be reflected in the lithology and geochemistry of rocks above and below these hiatuses. They may also be evident on high-resolution seismic profiles as distinct unconformities. Marked dolomitisation is recognised beneath the longer-duration hiatuses (> 1 My, Stow et al. 2013), especially at sites U1387 and U1391, and are clear evidence of such current action. Geochemical elemental distribution across these major hiatuses provides an indication of the current strength and variation. The absence of significant geochemical change across the more minor hiatuses is indicative of their lesser duration and/or the presence of a highly condensed section rather than true hiatus. We also present data on the geochemical signature of the bi-gradational contourite sequences, both shorter and longer duration. These data reflect the general uniformity of compositional and textural attributes throughout the Cadiz contourite succession, but also indicate compositional changes through the small-scale sequences. The lithological and diagenetic properties of the contourites are combined with their geochemical profiles to provide a better understanding of depositional processes, hiatuses and unconformities in deepwater settings.