GEOCHEMICAL CONSTRAINTS on THE ORGIN OF C, O AND S ISOTOPIC VARIATIONS IN NEOPROTEROZOIC CARBONATES

The Shuram-Wonoka negative δ¹³C anomaly in sedimentary carbonates is recognized in multiple sections globally but it’s origin remains unclear. Fluid-rock reaction is almost certainly involved, as sediments of this age have seen one to many episodes of fluid interaction. A common feature is the strong correlation between the negative δ¹³C_carb values and low δ¹⁸O_carb which places important constraints on the origin of the anomaly. First, the very low δ¹³C carbonates must have (neo)formed in the presence of a fluid decidedly different from seawater, with depleted δ¹⁸O. Second, the sub-linear correlations imply both mixing and reaction between isotopically depleted and enriched components. The details depend in part on the temperature of formation of the carbonates, which in most cases is poorly known. The source of the isotopically depleted fluid could be high pCO₂ basinal fluids generated at higher T, although the temperature of final reaction to produce depleted carbonates need not be especially high. Alternatively, the fluid could be a meteoric fluid with a large component of low ¹³C DIC derived from organic C, although it may be hard to achieve sufficiently high DIC levels. CH₄ is a possible source of light DIC but any model requires that the low ¹³C and ¹⁸O were introduced together. However subsequent reaction with a low-C fluid can obscure the correlation with δ¹⁸O. Further, any model must include a viable mechanism to introduce sufficient depleted C (and in some cases Mg ) - redox and mass balance are hard constraints. The fluid-rock ratios (F/R) necessary have implications for carbonate-associated sulfate (CAS). Available data indicate that K_SO4 for SW-aragonite is near unity. Such a low value implies that δ³⁴S_CAS is sensitive to low F/R, and also suggest that models of CAS alteration patterns can help estimate paleoseawater properties. Calculations with available data from the Quanji group (Shen et al. 2010) suggest [SO₄⁼]_sw ≥ 10 mmol/kg. Finally there is the possibility that low TOC sediments have a substantial contribution to their δ¹³C_org signature from transported carbon in petroleum-bearing fluids, a ubiquitous process in sedimentary basins. Resolution of these questions requires improved data on basin thermal and fluid history as well as improved modeling approaches.