THE ANTINOMY OF ORGANIC AND INORGANIC CARBON ISOTOPES AT CA. 551 MA: A RECORD OF BIOLOGICAL RESPONSES TO OCEAN REDOX CHANGE?

An unusually negative inorganic carbon isotope excursion with d¹³C_carbonate values as low as -12‰ has been documented from Ediacaran successions in Oman, Australia, northern India, south China and western US. The duration and global synchrony of this d¹³C anomaly in individual successions remain largely unknown, but in south China, the top of the excursion is constrained by the U-Pb zircon age of 550.5±0.8 Ma. The large magnitude (up to 12‰ shift) and potentially long duration (millions of years) of this excursion challenge most of the existing paleoceanographic models related to Neoproterozoic carbon cycles. A theoretically permitted but not adequately tested model is the remineralization of a large dissolved organic carbon (DOC) reservoir stored in the ocean (Rothman et al., 2003). This model would require significant ocean anoxia and oxygen fluctuations associated with the negative d¹³C excursions that have not been documented so far. The occurrence of an up to 10-m-thick organic-rich (TOC up to 10%) black shale unit at the top of the d¹³C excursion in south China and northern India supports in general an ocean anoxic event at the end of the excursion. The intriguing phenomenon is the large organic carbon isotope variability. Data from India, Oman, and western US show no significant d¹³C_org variations associated with a ≥ 12‰ shift in d¹³C_carbonate. In south China, a large (up to 12‰) negative d¹³C_org shift is found in proximal platform sections, but a much smaller (< 6‰) d¹³C_org shift in the marginal section, plus an up to 7‰ difference in absolute d¹³C_org values. Such variations may record different biological responses to depositional environments. Isotope studies of modern algae species indicate that d¹³C values lower than –30‰ are mostly produced by red algae that relies on diffusive CO₂ entry. The presence of red algae species and their low d¹³C_org values identical to that of kerogen from ca. 551 Ma black shales suggest that primary production by such species may have dominated the total organic production in restricted shelf-lagoonal environments, while their contribution in marginal open-marine settings may have been much less. An integrated sedimentological, paleobiological, and geochemical study is needed to test the hypothesis.