Investigating the Possible Role of Chitin Deposition in the Shuram-Wonoka δ13C Excursion

Abstract:

Preceding the rapid rise in the number of recorded animal genera in the Cambrian Period by a few tens of Myr, the Ediacaran Shuram-Wonoka δ¹³C excursion is the largest negative carbon-13 anomaly recorded in Earth’s history, and its cause remains disputed. This global event, with δ¹³C_PDB recorded as low as -12‰, roughly coincides with the timing of the widespread appearance of trace fossils. In contrast to the soft body forms of Ediacaran metazoa, fossils of Cambrian metazoa suggest generally shelly structures, bearing resemblance to modern chitinous animals. Direct laboratory evidence for the use of chitin as a major structural component by animals has been found in 505 Ma Cambrian sponge fossils (Ehrlich et al. 2013), while fossils of other Cambrian animals show widespread evidence of structures for which modern analogues are chitinous, such as molluscan radulae (Caron et al., 2006). Motivated by the widespread appearance of apparently chitinous metazoa roughly subsequent to the Shuram-Wonoka excursion, and noting that chitin in modern marine animals typically exhibits significant carbon-13 depletion as great as δ¹³C_PDB ~ -20‰ (Schimmelmann 2011), I will discuss the hypothesis that the Shuram-Wonoka excursion records the development of chitin-based biological structures and associated chitin deposition. While chitin deposited in sediments usually degrades to other molecules (including inorganic constituents) relatively rapidly, typically decreasing in preserved abundance by orders of magnitude over tens of millions of years, I will discuss the results of preliminary field and laboratory work seeking to test this hypothesis using a variety of techniques, including polysaccharide-specific fluorescent staining to search for possible remnant detrital chitin in the late Neoproterozoic Johnnie Formation of the southwestern United States.