CONSTRAINING THE GLOBAL NATURE AND TIMING OF EDIACARAN C-ISOTOPIC REORDERING

Compound-specific carbon isotope analyses of Ediacaran biomarkers show a widespread change in isotopic patterns. We analyzed oils and/or rocks from Eastern Siberia, Oman and Australia and confirmed that, in sediments and oils older than 550 Ma, n-alkanes are enriched in ¹³C relative to the acyclic isoprenoids pristane and phytane. In younger sediments the n-alkanes are depleted compared to these isoprenoids with the possible exception of those deposited during a few Phanerozoic oceanic anoxic events.¹ We have found that this isotopic reordering appears to be global in nature and constrained its timing to ~550 Ma.

Pristane and phytane are considered to be derived from photosynthetic primary inputs and, based on established biosynthetic relationships of C3 organisms which dominate the modern ocean, should be ¹³C-enriched relative to n-alkanes from the same source. Therefore, the presence of n-alkanes with anomalously enriched isotopic compositions before 550 Ma may signify a high relative abundance of bacterial heterotrophs that extensively recycle organic matter (C_org) in the water column.²

The switch from anomalous isotopic ordering earlier in the Proterozoic to isoprenoid: n-alkyl biosynthetic relationships characteristic of the Phanerozoic is observed to take place in the Ediacaran. In Oman, this coincides with the termination of the Shuram Excursion when marine carbonates show very negative δ¹³C values with no corresponding shift in the isotopic composition of co-occurring C_org.³ This has been attributed to the oxidation of a large pool of C_org in the deep ocean³ with a corresponding fundamental change in C-cycle dynamics.⁴ Several hypotheses, ultimately linked to release of molecular oxygen via enhanced C_org burial, have been proposed to explain these phenomena. They include the evolution of: bilaterian animals with guts that rapidly export organic matter to the ocean floor as fecal pellets, reducing the amount of heterotrophy in the water column,² biomineralization, providing ballast for organic export,³ and algae with decay-resistant biopolymers.⁴ Alternatively, tectonism and the rifting of Rodinia may have indirectly increased the burial flux of C_org.

1. e.g. Grice et al. 2005 Science 307, 706. 2. Logan et al. 1995 Nature 376, 53. 3. Fike et al. 2006 Nature 444, 744. 4. Rothman et al. 2003 PNAS 100, 8124.