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2008 Annual Science Report

University of Wisconsin Reporting  |  JUL 2007 – JUN 2008

Biogenic Formation of High-Magnesium Calcite in Sulfide-Rich Systems

Project Summary

Calcite minerals that contain high proportions of magnsium are well-known biogneic minerals in modern marine environments, reflecting metastable incorporation of Mg in the calcite lattice. In the modern oceans, calcite may form in the presence of sulfide in hydrothermal systems, or, in the ancient Earth, high-magnesium calcite may have formed in the presence of high ambient sulfide (produced by bacterial sulfate reduction or hydrothermal systems), and this project is aimed at understanding the mechamisms involved in producing high-magnesium calcite in the presence of high dissolved sulfide.

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

We have investigated the effect of sulfide (both aqueous species and solid mineral pyrite) on incorporation of magnesium into calcite and the formation of high magnesian calcite at room temperature. High magnesian calcite (up to 25 ~ 30 mole% of MgCO3) can precipitate from solutions with Mg/Ca ratios of ~5 (similar to the ratio of seawater) in the presence of pyrite powders and / or aqueous HS-. Sulfides serve as catalysts that lower the dehydration energy of the Mg-water complex and enhance incorporation of Mg into the calcite. Both pyrite and aqueous sulfide can be products of sulfate-reducing bacteria in anaerobic environments through the reactions of:

CH4 + SO2-4 => HCO3 + HS + H2O


2CH2O + SO2-4 => 2HCO3 + HS + H+

The sulfide will serve as catalyst for formation of magnesian calcite via oxidation of organics:

HCO-3 + (Ca2+ + Mg2+) => (Ca, Mg)CO3 + H+

High-magnesian calcite formed through this process will be depleted in heavy carbon isotopes (have low 13C/12C ratios). The goal is that integrated features from crystal morphology, texture, chemical and isotope compositions can be used as collective evidence for biosignature. We have started investigation of natural high magnesian calcite from modern sea sediments that are closely associated with pyrite micro-crystals, methane hydrates, and proto-dolomite micro-crystals.

    Huifang Xu Huifang Xu
    Project Investigator
    Eric Roden Eric Roden
    Fangfu Zhang
    Doctoral Student

    Objective 4.1
    Earth's early biosphere

    Objective 6.1
    Environmental changes and the cycling of elements by the biota, communities, and ecosystems

    Objective 7.1
    Biosignatures to be sought in Solar System materials