Sheldon and Retallack (2002) have suggested that the presence of berthierine in high-latitude soils is an indication of soil oxygen consumption by an influx of atmospheric methane to form carbon dioxide, which in turn warmed the earliest Triassic, giving rise to a postapocalyptic greenhouse. We do not dispute the possibility of a postapocalyptic greenhouse, nor that this could explain the existence at high latitudes of a mineral normally associated with low-latitude soils. However, there are a number of issues that we wish to discuss which collectively weaken the case for direct involvement of atmospheric methane in the genesis of earliest Triassic soils.
1. Berthierine is known from present-day, high-latitude settings, including high-latitude freshwater lakes (Shterenberg et al., 1968) and arctic desert soils (Kodama and Foscolos, 1981). Neither of these occurrences can reasonably be related to a postapocalyptic greenhouse.
2. Sheldon and Retallack have not demonstrated unequivocally that the Fe in the 7 Å (X-ray diffraction peak) green clay is ferrous. Until recently (Huggett and Gale, 2002) ferric iron in 7 Å green clay was only known to occur in odinite, which in its pure form is known only from the present. However, we now have unpublished electron energy loss spectrometry evidence that clay in Eocene shallow marine sediments, formerly thought to be berthierine, has 100% ferric iron. This is an important point because the 7 Å green clay is being used as evidence of reducing conditions at the time it formed.
3. Even if the clays described by Sheldon and Retallack were shown to be ferrous, the possibility remains that reduction occurred through burial-related processes. Berthierine commonly occurs as (or replaces) ooids, which are known to form in high-energy oxic marine environments, or as a result of pedogenic processes, specifically laterite formation (e.g., Amouric et al., 1986; Taylor, 1990). In true berthierine the iron is almost 100% ferrous (Brindley, 1982), yet berthierine is typically found in the deposits of paleoenvironments thought to be oxic. This apparent paradox is easily resolved if the berthierine formed after burial. We agree with Sheldon and Retallack (2002) that berthierine may form either by replacement of kaolinite and iron (though their equation 1 is oversimplified—it does not include the Mg component of berthierine) or by replacement of other clays, especially smectite. However, it is not clear that this is necessarily a one-step reaction, and we point out that berthierine may also form by replacement of odinite. Indeed, it is possible that odinite is the most frequent immediate precursor to berthierine. Diagenetic alteration of odinite involves loss of Mg and reduction of Fe3+ to Fe2+. Because sediment typically enters a reducing environment within ~1 m of burial, this may explain why odinite is rare or possibly absent from ancient sediments. It is notable that Sheldon and Retallack attribute the presence of chlorite in the Graphite Peak and Allan Hills paleosols to low-temperature metamorphism. The same possibility—Fe-reduction during diagenesis—should be considered for the origin of berthierine. If the berthierine described by Sheldon and Retallack formed after burial there is no necessity to link its origin to high soil methane concentrations.