Diagenetic Reactions in Deeply Buried Sediments of the Japan Sea: A Synthesis of Interstitial Water Chemistry Results from Leg 127 and Leg 128

Abstract

Shipboard and shore-based analyses of -230 interstitial-water samples gathered at six sites during ODP Legs 127 and 128 indicate that diagenetic reactions within sediment columns throughout the Japan Sea record the influence of bacterial degradation of organic matter, transformation of amorphous biogenic opaline silica to opal-CT, and formation and alteration of carbonate phases, as well as alteration of crystalline basement and volcanic ash.Organic matter oxidation not only affects the distribution of oxidants (SO 4 , Mn, and CO 2 ) and dissolved metabolites (alkalinity, NH 4 , and PO 4 ), but also controls the distribution of many authigenic solid phases. Site 794, located deep in the Yamato Basin, is the only site where sulfate is not completely consumed, whereas Sites 798 and 799 record the total consumption of sulfate at shallow depths and concomitant extreme generation of alkalinity, NH 4 , and PO 4 . Authigenic carbonates, phosphates, and barite also record the importance of bacterial activity. The diagenetic opal-A/opal-CT transition is a notable chemical, diagenetic, and stratigraphic horizon that occurs more shallowly in the basinal sites because of their greater geothermal gradient. Due to its effect on the porosity and permeability of the involved lithologies, the siliceous phase transformation effectively restricts diffusion between reactions occurring in the uppermost sediment column from those involved in deep basement alteration. Processes that influence dissolved Ca, Mg, Sr, and alkalinity tend to be preferentially dominant at either basinal (Site 794, 795, and 797) or ridge (all other) sites, due to the distinctly different depths and sedimentation rates of the two depositional regimes. At basinal sites, where sediment accumulation is relatively slow and occurs below the CCD, basement alteration is the dominant influence on dissolved Ca and Mg profiles. At ridge sites, where accumulation is relatively fast and occurs above the CCD, diagenetic reactions such as calcite dissolution, inorganic carbonate precipitation, dolomitization, and siderite/ankerite formation control Ca and Mg behavior. Basement alteration reactions are responsible for large overall decreases in Mg, K, Rb, Li, B, δ l8 0, δD, 87 Sr/ 86 Sr, and Na with depth, as well as for increases in Ca and Sr. Alteration of ash in the sediment column also influences these downhole profiles. Site 795, in the Japan Basin, preserves the greatest diagenetic signal of basement alteration. Gradients resulting from basement alteration document the importance of diffusive chemical exchange between the endogenic and exogenic reservoirs.

Site U1427

“…The dissolved H 4 SiO 4 profile at Site U1427 (Table T12; Fig. F41) resembles those at other sites drilled in the marginal basin (Murray et al, 1992), with one main exception. The mudline sample has a concentration of 145 µM.…”

Site U1423

“…The other main process affecting the Ca, Mg, and Sr profiles at Site U1423 is basalt alteration, as was seen in many of the ODP Leg 127/128 interstitial water profiles (Murray et al, 1992). This general reaction consumes Mg and produces both Ca and Sr (Gieskes and Lawrence, 1981).…”

“…An increase in K concentrations immediately below the seafloor was also found at Site U1422 and may reflect exchange during authigenic mineral formation. The decrease in K concentrations with depth perhaps results from further reactions with ash and basalt (Murray et al, 1992), although formation of glauconite would also remove K + from interstitial water .…”

Methods

“…Fourth, previous work, both on piston cores and on sediment recovered from drilling during Legs 127/ 128, has emphasized that deep-sea sediment across the Marginal sea spans a particularly wide range of chemical environments derived from microbially mediated organic carbon degradation (Masuzawa and Kitano, 1983;Murray et al, 1992). For example, some locations (Site 794) have a relatively modest maximum alkalinity (<12 mM) and little to no gas in interstitial water at depth; other locations (ODP Site 798) have very high alkalinity (>75 mM) and abundant methane within 100 m of the seafloor Ingle, Suyehiro, von Breymann, et al, 1990).…”