NITROGEN/CARBON ISOTOPE RATIOS FROM THE EARLY TO MIDDLE PROTEROZOIC SEDIMENTARY ROCKS, MCARTHUR BASIN, NORTHERN AUSTRALIA

Oxygenation of the Earth's surface is expected to be deeply linked to the evolution of life. Many sources of independent evidence suggest that the Earth’s atmospheric redox state has increased in two steps:around 2,400 Ma and 600 Ma (Holland, 2006). On the other hand, the ocean was mostly dominated by reducing conditions during the Archean, whereas the ocean-atmosphere system in the Phanerozoic was as well oxygenated as it is in the present. It has generally been assumed that the middle Proterozoic ocean was globally oxic at the surface and sulfidic (euxinic) in the deep ocean. Under these conditions, nitrogen limitation caused by deficiency of trace metals has been proposed as an explanation for the delay of eukaryotic diversification (Canfield, 1998; Anbar & Knoll, 2002). However, most evidence for the existence of euxinic ocean was from Fe speciation and sulfur isotopic data (e.g., Shen et al., 2002; 2003). To evaluate this hypothesis more clearly, we focused on the nitrogen cycle in the middle proterozoic ocean. We obtained sedimentary rocks from early to late middle Proterozoic sediments including carbonate rocks, alternation of mudstone and carbonate rocks, and shales collected from six drill cores in McArthur Basin, Northern Australia. We analyzed whole rock nitrogen and carbon isotope compositions of these rocks.

δ¹⁵N_TN values of the rocks in the Wollogorang and Barney Creek formations are relatively high, ranging from +2.6 to +7.2‰. δ¹⁵N_TN values gradually decrease from around +7 to +1‰ stratigraphically upward (the average δ¹⁵N_TN value is +3.5‰). A mechanism that can potentially explain the decrease in δ¹⁵N_TN value is increasing availability of nitrate. The high δ¹⁵N_TN values which observed in the Wollogorang and Barney Creek formations likely reflect the predominance of partial denitrification in the water-column. When the supply of nitrate exceeds the loss processes, such as denitrification and assimilation, oceanic nitrate become larger and it is not completely assimilated into organic nitrogen. There is also no obvious depth-dependent difference observed between lithologies and δ¹⁵N_TN values. An increasing nitrate reservoir may have been responsible for the decreasing δ¹⁵N_TN value, which implies an ocean oxygenation state that gradually increases from the early to late middle Proterozoic.