Controls on pore fluid Mg isotopic composition in carbonate-rich sediments

Author(s): Chanda, P.; Fantle, M. S.
Author Affiliation(s): Primary:
Pennsylvania State University, University Park, PA, United States
Volume Title: AGU 2014 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2014; American Geophysical Union 2014 fall meeting, San Francisco, CA, Dec. 15-19, 2014. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: The Mg isotopic composition (δ26MgDSM3) 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 ppm) to -0.54±0.08 ppm (807A and 806B) and -0.39±0.04 ppm (762B). Below ∼110 mbsf at both 807A and 762B, pore fluid δ26Mg decreases noticeably by ∼0.1 ppm, while pore fluids at 806B exhibit a continual, gradual increase in δ26Mg to -0.17 ppm 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 ppm 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.
Year of Publication: 2014
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Alkaline earth metals; Cores; Equatorial Pacific; Exmouth Plateau; Indian Ocean; Integrated Ocean Drilling Program; Isotope ratios; Isotopes; Leg 122; Leg 130; Magnesium; Marine sediments; Metals; Mg-26/Mg-24; North Pacific; Northwest Pacific; ODP Site 762; ODP Site 807; Ocean Drilling Program; Ontong Java Plateau; Pacific Ocean; Sediments; Stable isotopes; West Pacific
Coordinates: S195315 S195314 E1121515 E1121514
N033622 N033626 E1563730 E1563728
Record ID: 2015092807
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