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Project Progress

Bjerrum and Canfield (2002) first suggested a “phosphorus crisis” during the Archean in which they suggested phosphorus (a limiting nutrient for microorganisms) would have been highly depleted from seawater due to its high absorption affinity to iron oxides. Konhauser et al. (2007) refuted this idea by suggesting that high Si concentrations in Archean seawater would have limited to ability of phosphorus to absorb to iron oxides, and thus a “phosphorus crisis” was unlikely. These debates notwithstanding, it is recognized that continental weathering is the most likely source for phosphorus in seawater. Traditional assessments of continental weathering have been done using Sr isotope compositions of marine carbonate (e.g., Shields and Veizer 2002); however, Archean carbonate is rare and in many instances altered to the point where the primary ocean signal has been lost. The low solubility and high Sr contents (>2000 ppm) of marine barite makes them perfect targets to assess the Sr isotope composition of the fluids from which they precipitated. There are little reliable data from carbonate at this time, thus the barite from the 3.23 Ga Fig Tree Group are crucial for testing the possible influences that continental weathering may have had on ocean chemistry. Strontium and oxygen isotope results are shown in Figure 1. The positive trend suggests that barite formed as a mixture of hydrothermal fluid and seawater, thus at a minimum, seawater must have had a Sr isotope composition of ~0.7015, much higher than previously reported (~0.7007) by Shields and Veizer (2002).

In Figure 2, we plot our new results for 3.2 Ga barite with other Sr isotope data. Also plotted are data from McCulloch (1994) for hydrothermal barite of 3.4 Ga age, which McCulloch noted likely reflected a mantle value and hence a lower limit to the seawater Sr isotope composition. Our new results suggest that the assumed alignment of the seawater and mantle Sr isotope curve in the Archean (Figure 2) is likely incorrect, and requires a significantly greater continental input via continental weathering. This conclusion is counter to the arguments that the continents were submerged throughout most of the Archean (Flament et al., 2013). This is an important result because it implies that microbial organisms may not have been nutrient limited, supporting the discoveries that a diverse microbial ecology existed during the Archean.

References Cited

Bjerrum CJ, Canfield DE (2002) Ocean productivity before about 1.9 Gyr limited by phosphorus adsorption onto iron oxides. Nature. 417: 159-162.
Flament N, Coltice N, Rey PF (2013) The evolution of the ⁸⁷Sr/⁸⁶Sr of marine carbonate does not constrain continental growth. Precambrian Research. 229: 177-188.
Konhauser KO, Lalonde SV, Amskold L, Holland HD (2007) Was There Really an Archean
Phosphate Crisis? Science. 315: 1234.
Shields G, Veizer J (2002) Precambrian marine carbonate isotope database: Version 1.1. Geochemistry, Geophysics, Geosystems. 3: 1-12.