Figure F15. Plot of dissolved Ca²⁺, Mg²⁺, and SiO₂ at northern margin (Woodlark Rise) sites. The shapes of the Ca²⁺ and Mg²⁺ profiles reflect the effects of carbonate diagenesis and a variable extent of alteration of volcanic matter in the upper portion of the sedimentary column of each site. Deeper downhole, alteration of volcanics exerts a more dominant control on pore-water profiles. Large increases in dissolved Ca²⁺ result primarily from alteration of plagioclase whereas depletion of Mg²⁺ reflects its uptake during authigenic smectite formation. The dissolved SiO₂ profiles depict a downhole progression including dissolution of volcanic ash in the upper sedimentary column, and silicification reactions deeper downhole. Particularly elevated SiO₂ concentrations deep in Site 1118 are attributable to an abundance of fresh to slightly altered volcanic matter in higher porosity sediments.