T113 Understanding High-Amplitude Stable Isotope Variations in Proterozoic Strata (Posters)

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

INSIGHTS on CARBON AND OXYGEN ISOTOPES COMPOSITIONS OF NEOPROTEROZOIC CARBONATES FROM CLUMPED ISOTOPE THERMOMETRY

Paper number: T113-7
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

Some late Precambrian limestones and dolostones exhibit exceptionally low δ13CPDB values (down to ~ -12 ‰) and positive correlations between δ13C and δ18O. This might reflect marine deposition from an ocean globally low in δ13C due to unusual disturbances in the global C cycle [1]. In this case, such C excursions are either linked to decreases in seawater δ18O, or low δ18Ocarb is an unrelated diagenetic overprint fortuitously correlated with δ13Ccarb. Alternatively, these isotope systematics might reflect early diagenesis in the presence of meteoric waters (rich in low-δ13C organic C) [2], or later high temperature fluid-rock interactions [3].

The carbonate clumped isotope thermometer provides new constraints on this debate since it can be used to reconstruct the T of carbonate growth or diagenesis and the δ18O of fluids from which analyzed carbonates grew [4].

We report initial results of clumped, C and O isotope compositions for Ediacaran age carbonates showing a large range in δ13C values correlating to δ18O, as well as samples deviating from this correlation towards lower δ18O values. We find: (1) δ18OPDB of carbonate (from -9 to -2‰) is uncorrelated with T (from 40 to 110ºC). Thus, previous suggestions that δ18O is a good indicator of high-T diagenesis are not supported. However, (2) δ18Ocarb positively correlate with δ18O of the fluid (δ18OSMOW = -5 to +9‰) from which carbonate grew/interacted. (3) δ13Ccarb correlates with T and δ18Ofluid; that is, the lowest δ13Ccarb ~ -9‰ is associated with the lowest T and δ18Ofluid whereas δ13C approaching typical marine carbonates show higher T (~110¢ªC) and δ18Ofluid similar to formation waters (≥ +4‰).

We conclude: all studied samples have undergone some degree of post-depositional diagenesis; they must be considered suspect as records of marine T, δ18O and may be δ13C; the lowest δ13C values reflect relatively shallow early diagenesis dominated by low-δ13C, low-δ18O (presumably meteoric) water; and higher δ13C and δ18O carbonate values reflect deeper recrystallization in rock-buffered systems containing pore waters far higher in δ18O and δ13C. It remains difficult to understand why exceptionally low δ13C values appear to be typical of Neoproterozoic carbonates.

[1] Fike et al (2006) Nature; [2] Knauth and Kennedy (2009) Nature; [3] Derry (2010) EPSL; [4] Ghosh et al. (2006) GCA