The Gulf of Cádiz Contourite Laboratory: Sediment variability from Recent to 300kya

Abstract

Integrated Ocean Drilling Program Expedition 339 drilled contourite deposits in the Gulf of Cádiz and West Iberian margin. These recovered sedimentary successions provide key data for examining the development of the Mediterranean Outflow Water and the subsequent evolution of a complex contourite depositional system. In this study, the major terrigenous mineral phases at Sites U1387 and U1389 were identified to define sediment composition and interpret its provenance. Temporal changes in sediment composition are analyzed to ~300 kya to identify changes in provenance, continental weathering regimes, and/or dispersal patterns. The shipboard age model was used to assign sediment ages using a constant sedimentation rate of 25 cm/ky for Site U1387 and 40 cm/ky for Site U1389. The samples investigated range from Recent to 300 kya, with a sampling interval of ~4 ky. Randomly mounted pressed powder samples were examined using x-ray diffractometry to identify bulk mineralogy. Semiquantitative mineral abundances in each sample are estimated by comparing the ratio of a mineral’s selected peak area to that of the 4.26Å quartz peak. Preliminary results show that the primary mineral phases include: quartz, calcite, dolomite, aragonite, feldspar, and a variety of clay minerals. This composition is consistent with onshore lithologies, as well as shipboard XRD results. At Site U1389, calcite and 7Å intensity ratios are higher in interglacial periods while dolomite decreases. Site U1387 samples show higher dolomite/quartz values in the glacial period measured (MIS 8). This illustrates preferential deposition of calcite and the 7Å clays during times of weakened MOW flow at a site, and preferential deposition of dolomite and quartz during times of strong MOW flow at a site. This difference in behavior may be controlled by the relatively smaller grain size of calcite and the 7Å clays compared to quartz and dolomite.