(Table 2) 87Sr/86Sr ratios of DSDP Site 38-336 samples

Studies of the chemical composition of the bulk sediments and their <1µm fraction indicate the presence of large amounts of volcanic ash, especially in sites close to Iceland. The presence of authigenic clay minerals (illite/smectite) suggests that alteration of this volcanic ash is an important process.

A study of Site 348 reveals that dissolved Ca gradients are caused mainly by transport from the underlying basalts, but that decreases in Mg and d18O (H2O) are largely due to reactions involving alteration of volcanic ash in the sediments. Studies of 87Sr/86Sr distributions of dissolved Sr confirm this process in those sites with high contents of volcanic matter. On the other hand, in sites where the chemical composition of dissolved constituents has been little influenced by alteration reactions. 87Sr/86Sr ratios reveal complete diffusive exchange with present-day seawater.