Diagenesis and convection reflected in pore water chemistry on the western flank of the East Pacific Rise, 20 degrees south

Abstract

NO 3 −, PO 4 3−, SiO 2, total alkalinity (TA), Ca 2+, Mg 2+ and O 2 were measured in seventeen in situ pore water profiles at 3 sites on the western flank of the East Pacific Rise at 20° S latitude. The top 1–2 m of the sediment was oxic at all sites. Stoichiometric modeling shows the CN of the decomposing organic matter to be about 106/6.6 while CP ranges from 106/0.7 to 106/0.25. The TA rise is greater than predicted by models assuming calcite saturation. suggesting that phases other than calcite are important in the alkalinity balance and control of CaCO 3 saturation. At Site III (crustal age 4.6 Myr), anomalous pore water NO 3 − and SiO 2 profiles were observed in areas of high heat flow (and inferred upwelling of pore waters through sediments) and low heat flow (and inferred downwelling). NO 3 − and SiO 2 profiles at the high heat flow site support the heat flow evidence for upwelling; the low heat flow NO 3 − and SiO 2 profiles give equivocal evidence for downwelling. No hydrothermal Ca 2+, Mg 2+, or F − anomalies were observed in the upwelling pore waters, suggesting the absence of significant water/rock reactions during convection at this site.