Contourites and Climate: New Evidence from the Gulf of Cadiz, IODP Expedition 339

Citation

Smillie Z & Stow D (2017) Contourites and Climate: New Evidence from the Gulf of Cadiz, IODP Expedition 339. 7th Annual Science Meeting of the Marine Alliance for Science and Technology for Scotland (MASTS), Glasgow, 04.10.2017-06.10.2017.

Abstract
The role of bottom (contour) currents in deposition, erosion and shaping the deep seafloor is now widely recognised, and their influence on margin sedimentation is much greater than previously understood. Drift geometry and evolution, as well as erosional elements and widespread depositional hiatuses can be clearly recognised by their distinctive seismic characteristics.The Gulf of Cadiz Contourite Depositional System (CDS) holds an unmistakable signal of Mediterranean Outflow Water following its exit through the Strait of Gibraltar and its flow along the mid-slope (500-1500 m water depth). It also represents a key area for understanding the interaction of controls on contourite sedimentation along the margin, including tectonics, sea-level, sediment supply and climate. Following the Integrated Ocean Drilling Program (IODP) Expedition 339, extensive work has been carried out on the evolution of the MOW, the timing and effects of tectonic activity, and the nature and architecture of sedimentation along the Cadiz margin. In this study, we focus in particular on the climate signal evident within contourites.The Gulf of Cadiz is the world{\textquoteright}s premier contourite laboratory and thus presents an ideal testing ground for the contourite paradigm. Following examination of over 4.5 km of contourite cores, the existing models for contourite deposition are validated. Contourites typically display very uniform compositional and textural attributes, a noted absence of primary sedimentary structures and an intense continuous bioturbation throughout. They are characterised by bi-gradational sequences, from inverse to normal grading, as well as by a range of partial sequences. Such sequences reflect cyclic changes in bottom current velocity, sediment supply and primary productivity in surface waters. Extensive hiatuses are evident across much of the Cadiz CDS, with particularly long duration discontinuities noted during the Late Pliocene and Early Quaternary. Where the duration of hiatuses exceeds 0.5-1.0 My, they are marked by dolomite cementation of the sediment, which we interpret to reflect seafloor hardground development. Cumulative evidence shows that variation in contourite characteristics is the norm at all scales of observation, from the sequence stratigraphic context (100s of metres), to seismic cycles (10s of metres), to sediment facies cycles depicted in the standard facies model (metres), and to the less regular alternation of coarser-grained lenses and layers with fine muddy contourites (centimetres). Controls on such cyclic variation include sea-level, climate, ocean resonance and current instabilities. However, recent drilling results show that an orbital cyclic signal is commonly very strong and that contourite drifts provide ideal sedimentary successions for paleoceanographic studies.Preliminary work has shown a remarkable record of orbital-scale variation in bulk sediment properties of contourites at several of the drift sites and a good correlation between all sites. The climate control on contourite sedimentation is clearly significant at this scale. The organic carbon content of Cadiz contourites (up to 2% TOC) is found to be relatively high for normal deep marine sedimentation, but no clear climate signal has yet been resolved. Work in progress, including isotope-ratio mass spectrometry (CF-IRMS) of the organic carbon-13, inorganic carbon-13 and oxygen-18 of the contourites will determine the nature of controls on the millennial scale sediment cycles. They will also provide further evidence on the nature and effects of the dolomite layers associated with depositional hiatuses.