Neoproterozoic sulfur-isotope variation in Australia and global implications

Seven sulfur-isotope stages are apparent in Australian Neoproterozoic secti ons. (1) From 840 Ma to the Sturtian glaciation (700 Ma) delta(34)S(sulfate ) varied little (+19 to +17.5 parts per thousand) and delta(34)S(sulfate) r anged from -20 to +23 parts per thousand. (2) The Sturtian glaciation was f ollowed by (3) a rise in delta(34)S(sulfate) to an average of + 30 parts pe r thousand (seen also in China, Namibia and Canada) and in delta(34)S(sulfa te) at least to +26 parts per thousand and possibly to +45 parts per thousa nd. The sharp rise in delta(34)S of all sulfur fractions (sulfate. sulfide, and organic) leads us to speculate that sulfide depleted in S-34 was depos ited on the abyssal plain and that residual sulfate enriched in S-34 remain ed beneath a stagnant, ice-covered ocean during the Sturtian glaciation and was brought to the shallow continental margin during the post-glacial tran sgression. (4) delta(34)S(sulfate) and delta(34)S(sulfate) fall from 640 to 607 Ma (delta(34)S(sulfate) down to + 17 parts per thousand) probably from oxygen excess (through massive organic carbon burial), which causes weathe ring of sulfide depleted in S-34 continental sulfide and previously deposit ed sulfide on the abyssal plain. (5) The oxic environment of the Marinoan ( 605-595 Ma) glaciation minimises sulfate-reduction (unlike the Sturtian) so that delta(34)S(sulfate) is unvarying. (6) The post-glacial environment ha s sulfide depleted in S-34 (average delta(34)S(sulfate) < -30 parts per tho usand). Burial of a substantial quantity of organic-carbon causes extensive bacterial reduction of sulfate and a rise in delta(34)S(sulfate) to a peak of at least +32 parts per thousand (7) around the Neoproterozoic-Cambrian boundary and a corresponding rise in delta(34)S(sulfate), explained by rele ase of methane gas from clathrate and a subsequent heatwave causing ocean s tagnation and oxygen depletion. (C) 2000 Elsevier Science B.V. All rights r eserved.