Deep diagenesis in tephra-rich sediments from the Lesser Antilles Volcanic Arc

Abstract:

We measured dithionite extractable iron and manganese along with a variety of bulk sedimentary solid and dissolved phases to constrain the diagenetic reactions occurring within the sediment package of the Grenada Basin, which resides within the southern Lesser Antilles volcanic arc region. Core material was obtained during IODP Expedition 340, and for this presentation we focus primarily on two sites (U1399 and U1400) located off the island of Martinique. Sediments in this region include tephra-rich volcanic sands, hemipelagic mud sequences, and carbonate-rich sequences, and vary widely in their proportions over short time and distance intervals. Regardless of the main sediment type, organic carbon contents are uniformly low with average values of 0.28 ± 0.08% for U1399 and 0.23 ± 0.15% for U1400. Carbonate contents are more variable ranging between 1 and 40% in both cores and these variations likely reflect variable dilution with volcanogenic sediment. Pore fluids in U1399 exhibit a decrease in Ca, Mg, and Sr with depth, indicative of carbonate precipitation, and a decrease in total dissolved S to values less than 10 mM by 160 meters. A reaction zone is present at ~70 meters in core U1400, with a slight increase in ammonium and alkalinity but a small decrease in total dissolved S. Ca, Mg, and Sr show little to no change with depth. Pore fluid Sr isotope data show a surprisingly narrow range of values suggesting minimal reaction of pore fluid with volcanic material. Reactive Fe ranges from 0.15 to 1.5% in both cores whereas reactive Mn ranges from 0 to 0.3%. For this presentation we assess the relationships among reactive iron, manganese, organic carbon and other constituents with an emphasis on how the presence of volcanic material might influence the cycling of these components during diagenesis.