The earliest diagenetic changes that may be observable in a Recent marine sediment are those due to chemical reactions between the solid minerals and trapped sea water. The nature and extent of the reactions may be interpreted from the chemical composition of the interstitial water in combination with the mineralogy of the sediment. The water composition may be a more sensitive clue to some reactions than the mineralogy of the solid.

Interstitial waters have been extracted from four different sets of long cores of fine-grained recent marine sediment of the Atlantic Ocean using a modified filter press. Four cores come from an area off Cape Cod, Massachusetts; two cores are from the western side of the Atlantic off the coast of Brazil; three cores are from the Romanche Deep in the equatorial Atlantic; seven samples came from the preliminary test for the Mohole, off Guadalupe Island in the Pacific. More than 150 samples of waters, similar in composition to sea water, have been analyzed for Na, K, Ca, Mg, Cl, and SiO₂. It appears that in the sediment waters, in comparison with the overlying sea water, there is some tendency for sodium and chloride to increase slightly, calcium to decrease, magnesium to rema n constant, and silica to increase greatly. Wide variations in water composition are correlative with sediment lithology. Changes in water composition may be due to post-burial reactions between sea water and clay and carbonate minerals as well as to salt-filtering effects of the compacting clay membranes. Calcium concentration may be reduced by precipitation of secondary CaCO₃. Sodium and chloride increase as a result of salt-filtering, although some paleosalinity effects may be possible in some nearshore areas where hydrography was strongly affected by Pleistocene events. Silica increases as a consequence of the dissolution of amorphous siliceous organisms and is probably poised at concentrations in equilibrium with montmorillonite. Shipboard determinations of pH on fresh co es compared with pH of

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aerated squeezed waters indicate that CO₂ pressures in the sediment may be an order of magnitude higher than that in equilibrium with the atmosphere, an effect probably related to the bacterial decomposition of organic matter.

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