The formation of such sedimentary rocks could be caused by a large influx of positively charged ions, as would be produced by rapid weathering during the extreme greenhouse following a snowball Earth event. The isotopic signature of the cap carbonates is near −5 ‰, consistent with the value of the mantle—such a low value is usually/could be taken to signify an absence of life, since photosynthesis usually acts to raise the value; alternatively the release of methane deposits could have lowered it from a higher value, and counterbalance the effects of photosynthesis.