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Diffusion-dominated profiles with their coupled changes for and have been successfully modelled using diffusion coefficients that vary with depth ( McDuff, 1978 McDuff and Gieskes, 1976). In this case, the excess upward diffusion of Ca 2+ is compensated by a downward diffusional flux of seawater-supplied Na + that is reflected by a distinct Na + gradient. Where hydrolysis reaction rates are low, Mg uptake may exceed Ca release. Negative linear correlation between Ca 2+ and δ 18O in diffusion-dominated pore-water profiles of the DSDP (redrawn and modified from Gieskes, 1983 data from Lawrence and Gieskes, 1981). Upward diffusion of Ca is driven by the low calcium concentration of seawater (11 mM), which is about 1/5 of that of magnesium (55 mM).įigure 9.4. Excess Mg supplied by downward diffusion is also removed with hydroxyl ions, whereas the Ca that is set free, for example by the breakdown of feldspars, diffuses away.
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The Mg and part of the OH-ions are removed by the precipitation of Mg-bearing secondary minerals such as brucite and saponite. The negative linear correlation between the concentration changes Δ and Δ is due to hydrolysis reactions of aluminosilicates in the basaltic crust that release Mg, Ca, hydroxyl-ions and silicic acid as main dissolved species. Diffusion-dominated sites are characterized by negative linear correlations between the concentration changes for calcium (Δ) and magnesium (Δ) and between calcium concentration and oxygen-isotope ratios ( Figs. The sites with gradients are either diffusion-dominated or reaction-dominated. For a thickness of more than 500 m, sites without gradients do not exist ( Fig. Where the sediment thickness exceeds 200 m, the number of drill sites displaying pore-water gradients increases significantly.