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

Some late Precambrian limestones and dolostones exhibit exceptionally low δ¹³C_PDB 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 δ¹⁸O_carb is an unrelated diagenetic overprint fortuitously correlated with δ¹³C_carb. 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) δ¹⁸O_PDB 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) δ¹⁸O_carb positively correlate with δ¹⁸O of the fluid (δ¹⁸O_SMOW = -5 to +9‰) from which carbonate grew/interacted. (3) δ¹³C_carb correlates with T and δ¹⁸O_fluid; that is, the lowest δ¹³C_carb ~ -9‰ is associated with the lowest T and δ¹⁸O_fluid whereas δ¹³C approaching typical marine carbonates show higher T (~110¢ªC) and δ¹⁸O_fluid 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