FACTORS INFLUENCING THE PRESERVATION OF ORGANIC CARBON IN THE NEOPROTEROZOIC DOUSHANTUO FORMATION

Stable C-isotope records, S-isotope records and molecular organic fossils indicate that oxygen concentrations in the atmosphere and ocean increased in the late Precambrian to levels that supported complex multicellular life. The exact timing and duration of this event is unclear, but the driver behind this oxygenation event is thought to be increased rates of organic carbon (OC) burial. It has been proposed that enhanced burial of OC in the latest Precambrian was caused by: 1) increases in sediment accumulation rates in response to breakup of a supercontinent, or, 2) an end to nutrient scavenging by anoxic/sulfidic deep ocean waters that up until that point limited primary productivity. These mechanisms invoke unsustainable increases in the supply of OC or sediment to produce a sustained increase in OC burial and O2 to Phanerozoic levels. Given the long held view of the importance of anoxic conditions to the efficiency of OC preservation in sediments, and that oceanic redox conditions are inferred to have changed during the Neoproterozoic, we have investigated a third possible explanation for the late Precambrian O2 rise – that the efficiency of OC preservation changed during the Neoproterozoic. To investigate this possibility requires condensed, organic-rich sections spanning the period of oxygenation. Slow sediment accumulation rates minimize the impact of varying dilutional effects of detrital material on total organic carbon (TOC) content of sediments and ensure long exposure times at the sediment-water interface. In this setting OC content (TOC) would have been particularly sensitive to factors slowing OC break down, allowing a comparison of the effect of 1) mineral sheltering, 2) changes in redox, 3) chemical transformation of OC. The late Neoproterozoic Doushantuo Formation, in southern China, is a condensed section containing organic-rich (~ 5% TOC) black shales. It was deposited between the end of Marinoan-age glaciation and the first appearance of bilaterians, and is inferred to span the oxygenation event. Here we show preliminary results of a comparative investigation of the controls of TOC content of the Doushantuo. Application of this method to other sections could help determine controls of OC preservation and therefore clarify reasons for enhanced rates of OC burial in the late Precambrian.