(Table 1) Geochemistry in interstitial water at DSDP Leg 90 holes

About 200 pore-water samples were recovered and analyzed from eight sites of DSDP Leg 90. At all sites on the carbonate- rich Lord Howe Rise, Ca2+ concentrations increase and Mg2+ concentrations decrease with increasing sub-bottom depth. The value of dCa2+/dMg2+ averages -0.45 mol/mol at these sites, an unusually small negative value in comparison with sites on basaltic crust. This supports the argument that the crust of the Lord Howe Rise is siliceous. Li+ concentrations increase and K+ concentrations decrease with depth. Both of these constituents are affected by reactions within the sediment column. Microbial sulfate reduction occurs to a small extent at all sites. In the upper 200 m of Site 594 on the south side of the Chatham Rise, sulfate reduction and alkalinity production are more pronounced.
Carbonate re-crystallization is indicated by large increases in Sr2+ concentrations with depth at all sites. The exact value of the Sr2+ maximum at each site is determined by equilibrium with respect to celestite (SrSO4). The greater the degree of microbial sulfate reduction, the higher is the pore water Sr2+ concentration. The diffusive flux of Sr2 out of the pore waters is consistent with an initial rate of carbonate re-crystallization of 10%/m.y. This re-crystallization must alter the isotopic and trace elemental compositions of some carbonate components.