DOI: 10.1594/pangaea.723370
title: Mg/Ca and Sr/Ca in calcites and estimated distribution coefficients in marine sediments (Table 2)

description: Recrystallization processes in marine sediments can alter the extent to which biogenic calcite composition serves as a proxy of oceanic chemical and isotopic history. Models of calcite recrystallization developed to date have resulted in significant insights into these processes, but are not completely adequate to describe the conditions of recrystallization. Marine sediments frequently have concentration gradients in interstitial dissolved calcium, magnesium, and strontium which have probably evolved during sediment accumulation. Realistic, albeit simplified, models of the temporal evolution of interstitial water profiles of Ca, Mg, and Sr were used with several patterns of recrystallization rate variation to predict the composition of recrystallized inorganic calcite. Comparison of predictions with measured Mg/Ca and Sr/Ca ratios in severely altered calcite samples from several Deep Sea Drilling Project sites demonstrates that models incorporating temporal variation in interstitial water composition more successfully predict observed calcite compositions than do models which rely solely on present-day interstitial water chemistry. Temporal changes in interstitial composition are particularly important in interpreting Mg/Ca ratios in conjunction with Sr/Ca ratios. Estimates of Mg distribution coefficients from previous observations in marine sediments, much lower than those in laboratory studies of inorganic calcite, are confirmed by these results. Evaluation of the effects of diagenetic alteration of biogenic calcium carbonate sediment must be a site-specific process, taking into account accumulation history, present interstitial chemistry and its variation in the past, and sample depths and ages.
description: Distribution coefficients are calculated from calcite compositions and solution compositions. Interstitial water results include those based on present-day interstitial water composition ("present") and three based on time-averaged interstitial water compositions (“uniform”, “age-dependent”, and “Sr-maximum”). Model parameters for each site were defined as follows (with gradients in mM/m and Sr concentrations in mM): Site 288, accumulation = 1000 m at 10 m/m.y., Ca gradient = 0.023, Mg gradient = -0.025, Sr maximum of 0.7 at 340 m; Site 289, accumulation = 1300 m at 10 m/m.y., Ca = 0.028, Mg = -0.033, Sr maximum of 0.9 at 200 m; Site 366, accumulation = 1000 m at 10 m/m.y., Ca = 0.056, Mg = -0.051, Sr maximum of 1.2 at 500 m; Site 369, accumulation = 600 m at 5 m/m.y., Ca = 0.037, Mg = -0.029, Sr maximum of 1.1 at 460 m. For these sites, model runs used accumulations of 300 and 500 m before Ca and Mg gradients developed; Sr concentrations were constant below the Sr maximum. Site 526, accumulation = 500 m at 5 m/m.y., no Ca or Mg gradient, Sr maximum of 0.24 at 100 m, Sr decreases to 0.08 at depth; results in parentheses for Site 526 use a Ca gradient = 0.050. Distribution coefficients are also calculated based on present-day seawater composition (“seawater”). Depth, sediment for average calculation is given as the deepest depth (mbsf)
description: Supplement to: Delaney, Margaret Lois (1989): Temporal changes in interstitial water chemistry and calcite recrystallization in marine sediments. Earth and Planetary Science Letters, 95(1-2), 23-37