Mineralogy and microfabric of sediment from the Western Mediterranean Sea

Abstract

Analysis of bulk mineralogy and microfabric of sediments collected during Ocean Drilling Program Leg 161 indicates a notably uniform Neogene/Quaternary history of hemipelagic sedimentation in the Western Mediterranean Sea. Bulk X-ray diffraction data were collected for 857 samples from six drill sites in an east‐west transect from the Tyrrhenian Sea to the weste rn Alboran Sea. Semiquantitative relative abundances of clay minerals, calcite, quartz, and feldspar are reported. Age profiles of the major phases are used to describe downhole and east‐west variations in mineralogy of the dominant hemipelagic sediments. Although overall mineralogy is remarkably similar from site to site, some spatial variability was noted. Calcite content increases from an average of ~20% in the westernmost Alboran Sea to ~40% in the Tyrrhenian Basin. Calculated calcite burial fluxes correlate with sedimentation rates and define an opposite trend, with greatest accumulation rates in the western Alboran Sea. Quartz concentrations are slightly higher in hemipelagic sediments from the westernmost Alboran Sea sites than in the eastern Alboran, Balearic, and Tyrrhenian Basins. These compositional trends are thought to reflect the influence of detrital influx to the basins. Age profiles of calcite flux reveal peaks, many of which correlate between sites. Climate change, with associated changes in delivery of detrital material to the basins, and possibly in productivity, are likely responsible for the peaks in calcite accumulation. Backscattered electron images of 36 samples of hemipelagic sediment were collected and analyzed for areal percentages of grains, macropores, and matrix. Microfabric samples from a depth profile at Site 974, and across three time slices at all sites , show only minor variations in gross fabric related to age, depth of burial, or location. All samples are matrix dominated. Grai ns and macropores provide minor contributions to sediment fabric. Porosities calculated for the matrix material correlate well wit h total sample porosities.