SULFIDE STRESS ON ANIMALS IN NEOPROTEROZOIC OCEANS

Inferences from molecular clocks and organic geochemistry suggest that animals, as well as several other eukaryotic lineages, arose ~800 Ma. Oxygen levels at this time are believed to have been ~1-10% of modern levels. The ecology of modern low-oxygen zones and simple modeling suggest the earliest animals could have coped with these levels. In modern low-O₂ settings, however, sulfide is a synergistic stressor that binds to cytochrome oxidase and consequently inhibits aerobic respiration. To evaluate the impact of sulfide as an additional stress on benthic life in Neoproterozoic oceans, we analyzed a dataset of >4500 iron geochemical measurements from mudstones deposited throughout Earth history; of these >2400 are inferred to have been deposited under oxic water columns and are hence relevant to aerobic life. The pyrite iron contents of Neoproterozoic samples from oxic environments are significantly lower than those of Mesoproterozoic or Paleozoic samples, and on average are more than five times lower than modern oxic samples. As reactive iron is an effective sulfide sink, sulfide will not build up within pore waters or reach the sediment-water interface until essentially all reactive iron has been sulfidized. High ratios of pyritic iron to reactive iron in ancient oxic sediments can therefore be used to fingerprint paleoenvironments with appreciable pore water sulfide. In contrast to other time intervals, less than one percent of samples (12 of 1243) from Neoproterozoic successions show evidence of this phenomenon. Thus, early animals and other eukaryotes on Tonian and Cryogenian seafloors experienced little benthic sulfide stress. The data further limit the scope for interpreting early multicellular eukaryotes in terms of symbioses with sulfide oxidizing bacteria. Modern reducing environments such as seagrass and mangrove beds, large organic falls and very productive upwelling zones where such symbioses are common are likely to have Phanerozoic origins.