Knowledge Bank

The Gulf of Cádiz and West Iberian margin are the location of a complex contourite depositional system. Expedition 339 of the Integrated Ocean Drilling Program collected sediment cores from multiple sites in this area. In order to understand the history of the Mediterranean Outflow Water and how it has changed through glacial and interglacial periods, primary mineral phases in samples from two sites were identified. X-Ray diffraction (XRD) analyses of randomly oriented pressed powders were used to determine bulk mineralogy. Diffraction peak intensity ratios of major mineral phases, relative to quartz, provide a semiquantitative analysis. Ages were calculated using the shipboard age/depth model based on paleomagnetic and biostratigraphic data, and range from 300 ka to 1 Ma, covering multiple glacial cycles. Fifty-four samples were used from Site U1387, ranging from a depth of 90.72 meters composite depth (mcd) to 275.17 mcd. Sixty-nine samples were used from Site U1389, ranging from a depth of 156.14 mcd to 369.17 mcd. Site U1387 had a constant sedimentation rate of 27.5 cm/ky. Site U1389 had a sedimentation rate of 52.2 cm/ky above a widespread hiatus at 217.44 mbsf and 23.9 cm/ky below that hiatus. Primary mineral phases include quartz, calcite, dolomite, aragonite, plagioclase, illite, a 7 Å clay, and various interstratified clays. Mineralogical data are consistent with the Expedition 339 shipboard XRD results, and onshore lithologies support an interpretation of local provenance. Compositional changes at the positions of the upper Mediterranean Outflow Water and the lower Mediterranean Outflow Water were considered during glacial and interglacial episodes, and variations were explained in terms of changes in weathering and in sediment transport paths. Overall, there are no consistent variations in mineral abundances between glacial versus interglacial periods. This indicates that the Gulf of Cádiz contourite depositional system is complex, influenced by multiple controls.