Mg isotopes in pleagic carbonates and associated pore-fluids; insights into diagenesis of low-Mg biogenic carbonates

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Author(s): Higgins, J. A.; Schrag, D. P.
Author Affiliation(s): Primary:
Princeton University, Department of Geosciences, Princeton, NJ, United States
Harvard University, United States
Volume Title: AGU 2011 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2011; American Geophysical Union 2011 fall meeting, San Francisco, CA, Dec. 5-9, 2011. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Paleoceanographic proxies based on the concentration and/or isotopic composition of magnesium in biogenic carbonates may be aliased by post-depositional re-crystallization. Observed changes in the chemistry (Sr, Sr isotopes, Ca isotopes) of pore-fluids in pelagic carbonate sediments indicate that re-crystallization is capable of altering a significant fraction of the bulk sediment. To better understand the implications of diagenetic re-crystallization for paleoceanographic proxies based on Mg, we measured the Mg isotopic composition of pelagic carbonates and associated pore-fluids from ODP sites in the Atlantic (site 1265) and Pacific (site 807) ocean basins. The Mg isotopic composition of the carbonate sediments is very similar at both sites and shows systematic variability over the Cenozoic, ranging from -3.88 to -5.12ppm. In general samples from different size fractions of the sediment (<65 µm and >250 µm) have indistinguishable Mg isotopes, with a few notable exceptions in young (0 to 2 Ma) samples. The Mg isotopic composition of the pore-fluid at both sites systematically increases with depth by ∼0.4 and 0.8ppm, at sites 1265 and 807, respectively. We explore these data sets using a numerical model of sediment diagenesis which includes Mg, Ca, and Sr isotopes. We discuss implications for paleoceanographic proxies which depend on Mg in pelagic carbonates, in particular changes in the Mg isotopic composition of seawater over the Cenozoic.
Year of Publication: 2011
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Alkaline earth metals; Atlantic Ocean; Biogenic processes; Carbonates; Concentration; Diagenesis; Equatorial Pacific; Integrated Ocean Drilling Program; Isotopes; Leg 130; Leg 208; Magnesium; Metals; North Pacific; Northwest Pacific; ODP Site 1265; ODP Site 807; Ocean Drilling Program; Ontong Java Plateau; Pacific Ocean; Paleo-oceanography; Pore water; Sea water; Sediments; South Atlantic; Walvis Ridge; West Pacific
Record ID: 2018028918
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by, and/or abstract, Copyright, American Geophysical Union, Washington, DC, United States

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