Sr isotopes and pore fluid chemistry in carbonate sediment of the Ontong Java Plateau; calcite recrystallization rates and evidence for a rapid rise in sea water Mg over the last 10 million years

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doi: 10.1016/j.gca.2006.06.009
Author(s): Fantle, Matthew S.; DePaolo, Donald J.
Author Affiliation(s): Primary:
University of California, Berkeley, Department of Earth and Planetary Sciences, Berkeley, CA, United States
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, 70(15), p.3883-3904. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. 65 refs.; illus., incl. 2 tables, sketch map
Summary: The 87Sr/86Sr ratios and Sr concentrations in sediment and pore fluids are used to evaluate the rates of calcite recrystallization at ODP Site 807A on the Ontong Java Plateau, an 800-meter thick section of carbonate ooze and chalk. A numerical model is used to evaluate the pore fluid chemistry and Sr isotopes in an accumulating section. The deduced calcite recrystallization rate is 2% per million years (%/Myr) near the top of the section and decreases systematically in older parts of the section such that the rate is close to 0.1/age (in years). The deduced recrystallization rates have important implications for the interpretation of Ca and Mg concentration profiles in the pore fluids. The effect of calcite recrystallization on pore fluid chemistry is described by the reaction length, L, which varies by element, and depends on the concentration in pore fluid and solid. When L is small compared to the thickness of the sedimentary section, the pore fluid concentration is controlled by equilibrium or steady-state exchange with the solid phase, except within a distance L of the sediment-water interface. When L is large relative to the thickness of sediment, the pore fluid concentration is mostly controlled by the boundary conditions and diffusion. The values of L for Ca, Sr, and Mg are of order 15, 150, and 1500 meters, respectively. LSr is derived from isotopic data and modeling, and allows us to infer the values of LCa and LMg. The small value for LCa indicates that pore fluid Ca concentrations, which gradually increase down section, must be equilibrium values that are maintained by solution-precipitation exchange with calcite and do not reflect Ca sources within or below the sediment column. The pore fluid Ca measurements and measured alkalinity allow us to calculate the in situ pH in the pore fluids, which decreases from 7.6 near the sediment-water interface to 7.1±0.1 at 400-800 mbsf. While the calculated pH values are in agreement with some of the values measured during ODP Leg 130, most of the measurements are artifacts. The large value for LMg indicates that the pore fluid Mg concentrations at 807A are not controlled by calcite-fluid equilibrium but instead are determined by the changing Mg concentration of seawater during deposition, modified by aqueous diffusion in the pore fluids. We use the pore fluid Mg concentration profile at Site 807A to retrieve a global record for seawater Mg over the past 35 Myr, which shows that seawater Mg has increased rapidly over the past 10 Myr, rather than gradually over the past 60 Myr. This observation suggests that the Cenozoic rise in seawater Mg is controlled by continental weathering inputs rather than by exchange with oceanic crust. The relationship determined between reaction rate and age in silicates and carbonates is strikingly similar, which suggests that reaction affinity is not the primary determinant of silicate dissolution rates in nature. Abstract Copyright (2006) Elsevier, B.V.
Year of Publication: 2006
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Algae; Alkaline earth metals; Boundary conditions; Calcite; Carbonate sediments; Carbonates; Cenozoic; Chemical composition; DSDP Site 590; Deep Sea Drilling Project; Depositional environment; Diffusion; Enrichment; Fluid phase; Foraminifera; Geochemistry; Hydrochemistry; IPOD; Invertebrata; Isotope ratios; Isotopes; Leg 130; Leg 90; Magnesium; Marine sediments; Mathematical methods; Metals; Microfossils; Miocene; Models; Nannofossils; Neogene; ODP Site 807; Ocean Drilling Program; Ontong Java Plateau; PH; Pacific Ocean; Paleo-oceanography; Paleoenvironment; Plantae; Pore water; Protista; Recrystallization; Sea water; Sediments; Sr-87/Sr-86; Stable isotopes; Strontium; Tertiary; West Pacific
Coordinates: N033622 N033626 E1563730 E1563728
S311002 S311001 E1632131 E1632130
Record ID: 2007110006
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands