Quantifying the effect of diagenetic recrystallization on the Mg isotopic composition of marine carbonates

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doi: 10.1016/j.gca.2017.01.010
Author(s): Chanda, Piyali; Fantle, Matthew S.
Author Affiliation(s): Primary:
Pennsylvania State University, Department of Geosciences, University Park, PA, United States
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, Vol.204, p.219-239. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. Includes appendices. 103 refs.; illus., incl. 5 tables, sketch map
Summary: The Mg and Sr isotopic compositions (δ26Mg and 87Sr/86Sr) of pore fluids and bulk carbonates from Ocean Drilling Project Site 1171 (South Tasman Rise; 2148.2 m water depth) are reported, in order to evaluate the potential of diagenesis to alter carbonate-based geochemical proxies in an open marine system. Given the trace amounts of Mg in marine carbonates relative to coexisting pore fluids, diagenesis can alter carbonate δ26Mg, a promising proxy for seawater δ26Mg that may help elucidate long-term changes in the global Mg cycle. Constraints on the effect of diagenetic recrystallization on carbonate δ26Mg are therefore critical for accurate proxy interpretations. This study provides context for assessing the fidelity of geochemical proxy-reconstructions using the primary components (i.e., foraminiferal tests and nannofossils) of bulk carbonate sediments. We find that pore fluid δ26Mg values (on the DSM3 scale) at Site 1171 increase systematically with depth (from -0.72 ppm to -0.39 ppm in the upper ∼260 m), while the δ26Mg of bulk carbonates decrease systematically with depth (from -2.23 ppm to -5.00 ppm in the upper ∼260 m). This variability is ascribed primarily to carbonate recrystallization, with a small proportion of the variability due to down-hole changes in nannofossil and foraminiferal species composition. The inferred effect of diagenesis on bulk carbonate δ26Mg correlates with down-core changes in Mg/Ca, Sr/Ca, Na/Ca, and 87Sr/86Sr. A depositional reactive-transport model is employed to validate the hypothesis that calcite recrystallization in this system can generate sizeable shifts in carbonate δ26Mg. Model fits to the data suggest a fractionation factor and a partition coefficient that are consistent with previous work, assuming calcite recrystallization rates of ≤7%/Ma constrained by Sr geochemistry. In addition, either partial dissolution or a distinctly different previous diagenetic regime must be invoked in order to explain aspects of the elemental chemistry and 87Sr/86Sr of relatively deep sediments from Holes A and C. This study indicates that the dynamics of a given sedimentary system can significantly alter bulk carbonate geochemistry, and presents a framework for considering the potential impact of such alteration on picked archives such as foraminiferal tests and nannofossils. Ultimately, this study contributes to the development of δ26Mg as a proxy for seawater δ26Mg by quantifying the susceptibility of carbonate δ26Mg to diagenetic alteration, particularly in sediments in open marine systems. This study suggests that because of the sensitivity of carbonate δ26Mg to diagenetic recrystallization, it can, in certain systems, be used to quantify the impact of diagenesis on carbonate-based geochemical proxies.
Year of Publication: 2017
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 06 Petrology, Sedimentary; Alkaline earth metals; Carbonate sediments; Carbonates; Chromatograms; Crystal chemistry; Diagenesis; Equatorial Pacific; ICP mass spectra; Integrated Ocean Drilling Program; Ion chromatograms; Isotope ratios; Isotopes; Leg 130; Leg 189; Magnesium; Marine sediments; Mass spectra; Mathematical methods; Metals; Mg-26/Mg-24; North Pacific; Northwest Pacific; ODP Site 1171; ODP Site 807; Ocean Drilling Program; Ontong Java Plateau; Pacific Ocean; Recrystallization; Sea water; Sediments; South Pacific; South Tasman Rise; Southwest Pacific; Spectra; Sr-87/Sr-86; Stable isotopes; Strontium; Tasman Sea; West Pacific
Coordinates: S483000 S482900 E1490700 E1490600
Record ID: 2017078119
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands