Controls on Pore Fluid Mg Isotopic Composition in Carbonate-rich Sediments

Abstract:

The Mg isotopic composition (δ²⁶Mg_DSM3) of pore fluids and bulk carbonates from ODP Sites 762B, 807A, and 806B are presented. The objective of this study is to elucidate the major controls on marine pore fluid δ²⁶Mg, specifically the effect of authigenic clay precipitation in carbonate-rich sediments. Such studies are important for quantifying the leverage that exists in carbonate section to drive diagenetic alteration, and also for identifying geochemical reactions occurring in deep marine sediments that may impact the global geochemical cycles of elements such as Ca and Mg.

 The general pore fluid δ²⁶Mg trend at each of the three sites is a systematic increase with depth. In the upper ~108 m sections of all three sites, the pore fluid δ²⁶Mg gradually increases from the modern seawater value (-0.80 ± 0.04 ‰) to -0.54 ± 0.08 ‰ (807A and 806B) and -0.39± 0.04 ‰ (762B). Below ~110 mbsf at both 807A and 762B, pore fluid δ²⁶Mg decreases noticeably by ~ 0.1‰, while pore fluids at 806B exhibit a continual, gradual increase in δ²⁶Mg to -0.17‰ at 679.9 mbsf.

 Simple reactive transport modeling suggests that the general increase in pore fluid δ²⁶Mg with depth is likely explained by diffusion and calcite recrystallization. However, diffusive communication with an isotopically distinct lower boundary alone cannot explain the observed shifts of -0.1‰ in pore fluid δ²⁶Mg at depth, unless there is an increase in fractionation factor (from 0.9960 to 0.9995) associated with reaction between the sediment and pore fluid. Mineralogical evidence from Site 762B indicates that the Mg isotopic shift in the pore fluids occurs at a depth interval (between 106.8 mbsf and 135.3 mbsf) where clay (mainly illite and smectite) content increases from 3% to 11%, CaCO₃ content decreases from 92% to 86%, and porosity decreases by ~20%. The isotopic and mineralogical data are consistent with the formation of secondary clays that preferentially sequesters isotopically heavy Mg into the solid phase, implying that clay precipitation is relevant to the geochemical evolution of pore fluids even in carbonate-rich sedimentary columns. More detailed reactive transport modeling that includes reactions between pore fluids and both calcite and clay minerals is employed to constrain clay reaction rates and an empirical fractionation factor associated with clay precipitation.