And now….molybdenum isotopes! Ocean anoxia in the Proterozoic

“Everyone knows” that free atmospheric oxygen appeared about 2300 million years ago, thanks to the waste products of blue-green bacterial photosynthesis.  At least the land surface became an oxidising environment and a progressively redder place, as Fe-2 was oxidised to Fe-3 which forms insoluble oxides and hydroxides.  Paradoxically, the shallow sea floor of earlier times was redder than anything since, because of exactly the same oxygen-containing, ferric minerals.  It hosted the largest build-up of any metal concentration in Earth’s history; the banded iron formations (BIFs) that have for a century or more been the source of industrial iron.  A simple, and probably accurate explanation for BIFs is that iron dissolved in ocean water that lacked oxygen as Fe-2, and was supplied by sea-floor volcanism.  Once blue-green bacteria began pumping out oxygen, an oxidising reaction dumped both elements as slimy red sediment where the two met.  Dissolved iron consumed oxygen – just as well, because to most prokaryote life it is a poison – yet as oxygen productivity rose (and perhaps sea-floor spreading slowed) dissolved iron was increasingly removed by oxyidation from sea water.  The tipping point, when air contained oxygen and sea water became starved of iron (a vital micronutrient for phytoplankton) is difficult to address since the two chemical environments are so different and interact in complicated ways.  BIFs continued to form for about half a billion years after the first sign of atmospheric oxygen, then they disappear from the geological record at 1800 Ma ago.  There were minor reappearences in the Neoproterozoic, at the time of “Snowball Earth” events, and that is a fascinating topic in its own right.  Clearly, there was a long period of transition to what we can regard as a thoroughly modern world.  Studies that use sulphur isotopes suggest that in the Mesoproterozoic the upper ocean was oxygenated while bottom waters were perpetually akin to those of the Black Sea today.  Conditions in them may have been highly conducive to burial of dead organic matter – rapid drawdown of atmospheric CO­­­2, but allowing the massive production of methane by anaerobic bacteria.  Methane is a far more potent greenhouse gas than carbon dioxide, so controls over climate may have been very different from today’s.  Molybdenum offers an independent and potentially useful means of testing hypotheses about ocean chemistry.  It enters the sea in river water, which in post 2300 Ma times would have been oxygenated, allowing the formation of the soluble and very stable molybdate ion.  In anoxic ocean floor conditions, bacteria that generate hydrogen sulphide remove molybdenum as the sulphide, which is why modern Mo concentrations remain stable – it ends up in a very small percentage of ocean floor sediments.  The stable isotopes of molybdenum (97Mo and 95Mo) fractionate during precipitation of the element, the heavier one being preferentially removed during sulphide precipitation, to give high 97Mo/95Mo ratios in sediments.  The opposite seems to occur if precipitation is in the oxide form, as in sea-floor manganese nodules.  Geochemists from the Universities of Rochester and Missouri, USA have compared Mo isotopes from apparently anoxic Mesoproterozoic sediments with those in modern euxinic basins (Arnold, G.L. et al. 2004.  Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans.  Science v. 304, p. 87-90).  The Precambrian results are isotopically much lighter than modern ones, suggesting that 97Mo did not become enriched in seawater as a result of oxide precipitation in the equivalent of modern manganese nodules.  They estimate that 10 times more of the ocean floor was anoxic than today or since about 1300 Ma ago.  So far no comparable work has been done of the extremely abundant black shales and schists of the Neoproterozoic, that link with “Snowball Earth” events.  Whether or not “modern” redox conditions emerged 1300 Ma ago, with probably a big impact on climate controls, the oddest time climatically was between about 750 and 600 Ma ago.  Not only were there several dramatic coolings and warmings, but the main indicator of organic carbon burial, d13C, went haywire.  As did the BIFs, did ocean anoxic conditions once more get footholds.  Molybdenum isotope data seem likely to shed some light on  those strange times.

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