INCREASING OXYGENATION AND THE ORDOVICIAN RADIATION: AN INVESTIGATION USING TH/U IN THE LOWER-MIDDLE ORDOVICIAN OF UTAH, USA

The Ordovician Radiation (OR) was one of the most important diversification events in the history of life. A number of factors contributed to the OR, including climate cooling, continental configuration, and paleoecological factors. Studies of Cambrian and Early Ordovician strata have concluded that the Early Paleozoic was a time of recurrent, widespread anoxia in the oceans. It has been suggested that a change to more oxidizing conditions during the Early-Middle Ordovician contributed to the OR. In order to investigate seawater oxygenation associated with the OR, we measured Th/U from carbonates of the mixed carbonate-siliciclastic Lower-Middle Ordovician succession near Ibex, Utah, USA.

The Th/U composition of carbonates reflects changes in the global availability of dissolved U in seawater. The U inventory of the oceans is affected by anoxic deposition in the deep ocean as U becomes insoluble under reducing conditions and is sequestered in black shales. Highly insoluble Th has only one redox state and thus is not affected by changes in global redox. Therefore, an increase in anoxic sedimentation in the oceans results in the drawdown of U and an increase in Th/U of shallow water carbonates. Because Th reflects detrital input, studies of long-term changes to Th/U are ideally performed in clean carbonates with consistent detrital input through time. Although mixed-carbonate siliciclastic settings do not fit this ideal, they can provide useful Th/U information if detrital input can be well-constrained. To better understand Th/U results we used solution ICP-MS to constrain indicators of diagenesis and terrigenous input. We found that Th/U does not vary with indicators of diagenesis. However, in intervals with high terrigenous input, Th abundance is much higher than that of U and Th/U does not reflect global seawater.

The highest Th/U values occur at the base of the studied succession, coincident with the last positive carbon isotopic excursion of the Early Ordovician, which has been interpreted to reflect enhanced organic carbon burial under reducing conditions. Th/U decreases and remains low for the rest of the succession except during an interval of increased terrigenous input. These results support the hypothesis that the OR was facilitated by an increase in seawater oxygen during the Early-Middle Ordovician.