Micrometeoritic Sulfur and the Worlds of Iron Sulfides and Thioesters

About 80% of the AMMs collected near the margin of the Antarctic ice sheets did show anomalous low sulfur contents (^0.1%) with regard to the value of ^3% measured in CM1 chondrites. So, sulfur could have been preferentially lost during either atmospheric entry (as SO2) or "cryogenic weathering" near the sea shore, involving the preferential leaching of sulfides in the fine-grained matrix of AMMs. The fact that their solar neon is not markedly lost favors this last possibility, because it is mostly retained in much more inert anhydrous silicates.

Duprat and Engrand (2003) recently recovered friable micrometeorites from snow samples deposited over the last few years in central Antarctica. These particles are essentially unweathered (Fig. 3.5), and their average sulfur content is approximately 5% (Fig. 3.6), a value even larger than that of the CMs (^3%). We make the simple maximizing assumption that all the sulfur that was released upon volatilization and/or melting into cosmic spherules gets initially oxidized in the early atmosphere (like organic carbon). This yields an enormous initial input rate of SO2, about 0.5 x 1016 g/year, that lasted about 100 Myr. This SO2 input was comparable to that of water and CO2. For a comparison, the amount of sulfur required to maintain the present day stratospheric sulfate aerosol layer on the Earth during quiet time period of volcanic activity is -1011 g/yr (Murphy, 2001).

What could be the contributions of this post-lunar SO2 input in prebi-otic chemistry? It was probably quickly transformed into stratospheric sulfate aerosols - i.e., mostly H2SO4 molecules with approximately 30% of water -that finally got deposited in early waters. A plausible reaction pathway to eliminate such an excess of sulfates was suggested by Andre Brack (Maurette et al., 2004b). Hydrothermal sources mostly formed at shallow depths in early water did probably functioned as "reactors" converting sulfates dissolved in water into both huge deposits of iron sulfides and exhalations of H2S (Fouquet

Micrometeorite Antarctic

Fig. 3.5. Scanning electron microscope observation of one of the smallest unmelted Antarctica micrometeorite recovered from fresh snow, at a depth of ~0.5 m, at Dome C. This new type of highly-friable-fine-grained micrometeorites is very rich in sulfur and nickel. This particle was partly masked to have a scale bar of 30 |J,m that corresponds to the size of a fairly large typical IDP collected in the stratosphere by NASA. Their discovery led to an increase to about 25% of the estimated proportion of the incoming flux of large (>100 ^m) micrometeorites that survive unmelted upon atmospheric entry. This high proportion cannot be predicted with previous models of atmospheric entry (Courtesy J. Duprat).

Fig. 3.5. Scanning electron microscope observation of one of the smallest unmelted Antarctica micrometeorite recovered from fresh snow, at a depth of ~0.5 m, at Dome C. This new type of highly-friable-fine-grained micrometeorites is very rich in sulfur and nickel. This particle was partly masked to have a scale bar of 30 |J,m that corresponds to the size of a fairly large typical IDP collected in the stratosphere by NASA. Their discovery led to an increase to about 25% of the estimated proportion of the incoming flux of large (>100 ^m) micrometeorites that survive unmelted upon atmospheric entry. This high proportion cannot be predicted with previous models of atmospheric entry (Courtesy J. Duprat).

et al., 1996). Surprisingly, micrometeoritic iron sulfides volatilized upon atmospheric entry would have been converted back into terrestrial iron sulfides, thus making the early oceans more suitable for swimming!

This amount of reprocessed sulfides is just enormous. The upper limit of their mass (about 7 x 1023 g) is equivalent to a global ~300-m-thick layer around the Earth. Thus, micrometeoritic sulfur could have intervened in pre-biotic chemistry in at least two different ways (Maurette et al., 2004). Sulfides are requested in the so-called iron sulfide "world" chemistry promoted by Wachstershauser (1988). Moreover, FeS and H2S can reduce CO2 to organic sulfides (thiols), as demonstrated in laboratory simulation of hydrothermal synthetic reactions (Heinen and Lauwers, 1996). Methyl- and ethyl-thiol were the principal thiols formed along with smaller amounts of others containing up to five carbon atoms. Thiols can lead to thioesters, which probably activated important organic prebiotic chemical reactions in the world of thiols proposed by de Duve (1998).

One can wonder about the final fate of this sulfur. In fact, it likely ended up being trapped in the upper mantle of the Earth like iridium. Indeed, by

Fig. 3.6. Energy dispersive X-ray spectrum of the bulk composition of one of the new highly friable Antarctic micrometeorites. The sulfur peak corresponds to a high sulfur content (~5%) mostly carried by iron sulfides. The weaker phosphorus peak corresponds to a concentration of approximately 0.5%. This element is stored in ferrihydrite, one of the two only known forms of magnetic iron hydroxide, which is probably quite abundant on the Martian surface (Courtesy C. Engrand).

Fig. 3.6. Energy dispersive X-ray spectrum of the bulk composition of one of the new highly friable Antarctic micrometeorites. The sulfur peak corresponds to a high sulfur content (~5%) mostly carried by iron sulfides. The weaker phosphorus peak corresponds to a concentration of approximately 0.5%. This element is stored in ferrihydrite, one of the two only known forms of magnetic iron hydroxide, which is probably quite abundant on the Martian surface (Courtesy C. Engrand).

diluting the total amount of micrometeoritic sulfur delivered to the Earth during the first 100 Myr of the post-lunar LHBomb in the upper mantle, one predicts that its content would be about 300 ppm. This value well fits the range of values (150-300 ppm) measured for the primitive upper mantle (Lorand, 1990), but also the highly siderophile character of S on the early Earth.

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