Did major changes in the stable-isotope composition of Proterozoic seawater occur

Journal Article · Wed Feb 28 23:00:00 EST 1990 · Geology; (USA)

DOI:https://doi.org/10.1130/0091-7613(1990)018<0227:DMCITS>2.3.CO;2· OSTI ID:6140635

Burdett, J W; Arthur, M A ^[1]; Grotzinger, J P ^[2]

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1. Univ. of Rhode Island, Narragansett (USA)
2. Massachusetts Institute of Technology, Cambridge (USA)

Early Proterozoic marine carbonates of the Rocknest Formation (1.93-1.89 Ga) have very depleted {delta}{sup 18}O values (about 8{per thousand}) relative to younger, Late Proterozoic marine carbonates that formed in similar depositional environments. Two isotopic trends are superimposed on the data for open-marine components. The first involves stabilization of tidal-flat sediments during early, possibly reflux-type dolomitization by evaporative pore fluids enriched in {delta}{sup 18}O. The second trend toward isotopically light {delta}{sup 18}O values was established during dolomitization of open-marine facies in contact either with meteoric waters (mixing zone) or under conditions of higher temperatures during burial. This resulted in precipitation of blocky, pore-occluding cements. The isotopically most enriched ooids are the best preserved normal marine components and may suggest that the {delta}{sup 18}O of seawater was about {minus}9.75{per thousand} {plus minus}1.0{per thousand} (SMOW) at 1.9 Ga. This composition would require a major change in the balance of high-temperature oxygen isotopic exchange between seawater and basalt and low-temperature weathering in order to explain the 8{per thousand} positive shift in inferred seawater {delta}{sup 18}O between 1.9 and 1.0 Ga. Alternatively, the depleted {delta}{sup 18}O values represent an approximately 30-35 C higher temperature of surface waters at 1.9 Ga. The heaviest carbonate {delta}{sup 13}C values are +1.75{per thousand}, more enriched than previously reported for the Early Proterozoic on the basis of bulk-rock data.

• OSTI ID: 6140635
• Journal Information: Geology; (USA), Journal Name: Geology; (USA) Vol. 18:3; ISSN GLGYB; ISSN 0091-7613
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ALKALINE EARTH METAL COMPOUNDS
BASALT
CALCIUM CARBONATES
CALCIUM COMPOUNDS
CANADA
CARBON 13
CARBON COMPOUNDS
CARBON ISOTOPES
CARBONATE MINERALS
CARBONATE ROCKS
CARBONATES
CHEMISTRY
DOLOMITE
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
FLUIDS
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC FORMATIONS
GEOLOGIC MODELS
GROUND WATER
HYDROGEN COMPOUNDS
IGNEOUS ROCKS
ISOTOPE RATIO
ISOTOPES
LIGHT NUCLEI
MAGNESIUM CARBONATES
MAGNESIUM COMPOUNDS
METEORIC WATER
MINERALIZATION
MINERALS
NORTH AMERICA
NUCLEI
OXYGEN 18
OXYGEN COMPOUNDS
OXYGEN ISOTOPES
PALEOCLIMATOLOGY
PALEONTOLOGY
PRECAMBRIAN ERA
RESERVOIR FLUIDS
RESERVOIR TEMPERATURE
ROCKS
SEAWATER
SEDIMENTARY ROCKS
SEDIMENTS
STABLE ISOTOPES
VOLCANIC ROCKS
WATER
WATER CHEMISTRY
WEATHERING