TH/U IN CARBONATES AS A TOOL FOR INTERPRETING CARBON ISOTOPIC EXCURSIONS: INSIGHTS FROM THE EDIACARAN-CAMBRIAN TRANSITION IN MONGOLIA

The Ediacaran-Cambrian (E-C) transition is of interest to geoscientists because of its importance to the study of early animal life. The E-C boundary is defined by the first occurrence of the trace fossil Treptichnus pedum at the GSSP at Mistaken Point, Newfoundland. In mixed carbonate-siliciclastic successions around the world, the first occurrence of T. pedum coincides with a negative carbon isotopic anomaly in carbonates. Most research on the E-C excursion has focused on its utility for correlation, leaving some ambiguity as to the causes of the anomaly.

Th/U in clean carbonates can be used to understand changes in global carbon burial, and can thus be used to interpret isotopic excursions. Uranium is soluble under oxic conditions and is incorporated into carbonates in proportion to its availability in seawater. Under anoxic conditions, U becomes insoluble and is deposited with organic matter in black shales. Thus, prolonged intervals of anoxic deposition in the deep ocean removes U from seawater and leads to lower U abundances in carbonates. Thorium is largely insoluble and insensitive to redox and is used to constrain sedimentation rates when interpreting the U content of carbonates. Thus, increasing Th/U in carbonates can reflect increased rates of organic carbon burial in the deep ocean. Carbon isotopic excursions can be influenced by multiple factors including inputs of anomalous carbon to the ocean (e.g., volcanism and terrestrial weathering) and changes in the rate of organic carbon burial. Because of the link between U availability in seawater and organic carbon burial, Th/U can be useful for understanding isotopic excursions.

Here we report preliminary Th/U results from the E-C transition at Bayan Gol, Mongolia. Late Ediacaran strata exhibit low Th/U values resulting from high U contents which decrease upsection toward the negative isotopic excursion. This decrease in U content likely reflects enhanced burial of organic carbon during the late Ediacaran. U contents exhibit a well-defined increase during the negative excursion, suggesting that the isotopic excursion is largely due to decreased organic carbon burial. U contents remain relatively stable above the E-C excursion with minor decreases. Thus, the E-C negative excursion possibly reflects a short-lived oxygenation event at the end of the Ediacaran.