Evaluating pore fluid Mg isotopic and elemental constraints on seawater Mg chemistry in the Cenozoic

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http://minmag.geoscienceworld.org/content/76/6/1692.full.pdf+html
Author(s): Fantle, Matthew S.; Teng, Fang-Zhen
Author Affiliation(s): Primary:
Pennsylvania State University, Department of Geosciences, University Park, PA, United States
Other:
University of Arkansas, United States
Volume Title: Goldschmidt 2012 abstract volume
Source: Mineralogical Magazine, 76(6), p.1694; Goldschmidt 2012, Montreal, QC, Canada, June 24-29, 2012. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0026-461X CODEN: MNLMBB
Note: In English. 2 refs.
Summary: Interpreting Cenozoic climate and environment accurately using marine geochemical proxies requires high fidelity proxies that are resistant to diagenetic alteration over tens of millions of years. For mineral-based proxies that do not passively sample seawater, such as Mg/Ca and stable isotopic proxies, it is also necessary to place limits on the chemistry of seawater (i.e, the solution from which the minerals precipitate). The latter is predominantly true for Cenozoic proxies over time scales longer than elemental residence times (τ) in the ocean (τMg∼10 Ma; τCa∼1 Ma). This study utilizes depositional reactive transport models of carbonate diagenesis to simulate measured pore fluid and solid Mg isotopic and elemental data from ODP Site 807A (ave. CaCO3 ∼ 92 wt%). The ultimate goal of the iterative modeling approach taken is to (1) evaluate the extent to which pore fluid chemistry can be used to elucidate the Mg chemistry of the Cenozoic ocean and (2) estimate the degree to which diagenesis alters the Mg isotope and Mg/Ca proxies. Previously published recrystallization rates, based on Ca and Sr isotopic and elemental data, constrain the models. The Mg isotopic composition (δ26MgDSM3) of 807A pore fluids varies systematically from -0.79 to -0.25ppm between the shallowest (13.4 mbsf) and deepest (738 mbsf) fluids. There is notable structure in pore fluid δ26Mg as a function of depth that is not well explained by diffusion, either of an initial seawater signal or between boundaries that reflect modern seawater (upper) and basement basaltic Mg (lower). Accordingly, reaction must be considered. In the current study, endmember model scenarios are used to evaluate the applicability (and uncertainty) of pore fluid chemical data for constraining past seawater chemistry. The modeling assumes that carbonate recrystallization is the dominant reaction controlling the alteration of Mg pore fluid chemistry in the sedimentary column. The first endmember scenario generates seawater Mg concentration and δ26Mg curves for a range of partition coefficients (KMg) assuming a constant value for the diagenetic fractionation factor. A second set of simulations assumes seawater Mg concentrations consistent with evaporite fluid inclusion data and constrains KMg and the Mg isotopic evolution of the ocean accordingly. The model results are subsequently used to assess the potential for diagenetic alteration of bulk carbonate. Given low Mg concentrations, reactions rates on the order of <2%/Ma, and an assumed equilibrium fractionation factor and partition coefficient that are different at depth than in the surface ocean, there is considerable leverage to change both the Mg isotopic composition and Mg/Ca of bulk carbonates over tens of millions of years.
Year of Publication: 2012
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Alkaline earth metals; Ca-44/Ca-40; Calcium; Cenozoic; Chemical composition; Equatorial Pacific; Integrated Ocean Drilling Program; Isotope ratios; Isotopes; Leg 130; Magnesium; Metals; Mg-26/Mg-24; North Pacific; Northwest Pacific; ODP Site 807; Ocean Drilling Program; Ontong Java Plateau; Pacific Ocean; Paleo-oceanography; Paleoclimatology; Sea water; Stable isotopes; West Pacific
Coordinates: N033622 N033626 E1563730 E1563728
Record ID: 2014024927
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