MODELLING EARTH SYSTEM CHANGES THROUGH THE SHURUM-WONOKA ANOMALY

The Neoproterozoic recorded super-continent breakup, extensive glaciations and the first oxygenation of the deep ocean with a shift from sulfidic/ferruginous conditions to more oxic conditions, accompanied by the expansion of the first animals. Set within this tableau were enigmatic large-amplitude fluctuations in the isotopic composition of marine carbonate carbon (d¹³C_IC ) and oxygen (d¹⁸O) and more subdued changes in the isotope composition of marine organic carbon. Normally, carbon isotope changes are considered to reflect the burial history of inorganic and organic carbon into sediments, while the oxygen isotope record is considered to be more sensitive to post depositional diagenesis. The Neoproterozoic d¹³C_IC record, however, reveals prolonged excursions to less than mantle values that cannot be explained by our normal understanding of the carbon cycle.

We present a quantitative model of the Shurum-Wonoka anomaly; the largest, and most challenging to understand isotope event. Stratigraphic sections display the following common features: 1) d¹³C_IC minimums of ~-8 ‰, 2) a general correlation between d¹³C_IC and d¹⁸O within the anomaly and 3) lower carbon isotope fractionation between carbonate and organic carbon at lower values of d¹³C_IC, with a cross-plot slope of about 1. This unit slope seems to be unique to the Neoproterozoic in Earth history and not easily explained.

In our model, the carbon isotope excursions were driven by methane from sediment-hosted clathrate hydrate deposits. Being a powerful greenhouse gas, methane increased temperature and melted icecaps. These combined to produce a negative ¹⁸O anomaly, while the higher temperatures also accelerated the weathering of continental rocks, drawing down atmospheric CO₂. Lower CO₂, in turn, reduced the isotope fractionation between DIC and organic carbon during primary production. This cause and effect chain of events is qualitatively consistent with all observations. Earlier, others have suggested that methane release increased temperature and drew down CO₂. Our model goes much further in explaining all of the major features of the preserved carbon and oxygen isotope record.