EXPLORING THE RELATIONSHIP BETWEEN GLOBAL AND LOCAL REDOX PROXIES DURING THE GOBE: A CASE STUDY OF CARBONATES FROM THE LOWER FILLMORE FORMATION, UTAH

The Great Ordovician Biodiversification Event (GOBE) was one of the most important radiations in the history of life and is thought to have occurred primarily within continental shelf environments (e.g., Servais et al., 2008). However, recent geochemical studies suggest that the Ordovician deep ocean may have been euxinic (e.g., Thompson & Kah, 2012). The relationship between deep water euxinia and biodiversification in shelf environments is still unclear. Using local and global geochemical redox proxies, this paradox of deep ocean euxinia during the GOBE can be investigated. Abundances of total organic carbon (TOC) and total sulfur (TS) act as short term redox proxies for local depositional environments. On the other hand, sulfur isotopic compositions of carbonate associated sulfate (CAS) provide a global, long term redox proxy for the deep ocean.

Here we present TOC and TS abundances, as well as sulfur isotopic compositions of CAS (δ³⁴S_CAS) from Lower Ordovician (Tremadocian) carbonates of the Fillmore Formation of Utah (C Section of Hintze & Davis, 2003). Samples were collected at a resolution of 10m with the aid of a Jacob staff. TOC and TS abundances were measured using an ELTRA CS2000 Carbon and Sulfur Determinator. Sulfur isotopic analysis from CAS was conducted at the stable isotopes laboratory at UC Riverside.

δ³⁴S_CAS averages 34.5‰ VCDT, with heavier values of 40-50‰ VCDT at the bottom of the section and are similar to late Cambrian results (e.g., Kampschulte & Strauss, 2003). δ³⁴S_CAS values decrease upsection and are similar to mid-continent results from Gill et al. (2004). Our results are heavier by as much as 10-20‰ than the South American CAS results from Thompson & Kah (2012). TOC abundances average 0.2 wt.% and fall below 1 wt.% within maximum error. TS abundances are also low, with an average of 0.02 wt.% and with no values exceeding 0.03 wt.% with maximum error. The relatively heavy δ³⁴S_CAS values support the interpretation of euxinia in the deep ocean during the Early Ordovician. On the other hand, the TOC and TS abundance results suggest that the local depositional environment of the C Section was not anoxic. Our ongoing research will further explore the relationship between deep water euxinia and the GOBE (e.g., Thompson & Kah, 2012; Marenco et al., 2013).