CEMENT STRATIGRAPHY SUGGESTS CHEMICALLY DISTINCT WATER MASSES IN THE MESOPROTEROZOIC OCEAN

The presence (or absence) of distinct water masses in the Proterozoic has proven difficult to establish due to a number of factors, including diagenetic alteration, lack of well-constrained basin geometry, and absence of significant water depth changes in the typically epeiric settings of most Proterozoic carbonate platforms. Furthermore, when distinct geochemical differences are preserved as stratigraphic or facies variation, it is difficult to determine whether geochemical variation is primary and, if so, whether it represents water masses that were arrayed laterally or vertically in the ancient ocean. The shallow-water nature of epeiric sea environments, in particular, leaves little opportunity to probe the vertical structure of the water column directly.

The occurrence of distinctive, multilayered voids in the stromatolitic reef facies of the Mesoproterozoic Atar Group, Mauritania, permits evaluation of a range of fluids that interacted with Atar Group strata and suggests a means of discerning the presence of ancient water masses. Within individual voids, different phases possess distinct geochemistry, and multiple generations of the same phase commonly show a tight range of isotopic composition, suggesting that superimposed phases within voids record primary water mass composition, with only limited post-depositional alteration. Isotopic and elemental compositions of these phases define trajectories that can be interpreted to reflect three distinct fluid compositions: (1) ¹⁸O-enriched and highly reducing; (2) ¹⁸O-depleted and highly reducing; and (3) ¹⁸O-depleted and mildly reducing. Geochemical evidence for dysoxic or anoxic waters suggests the presence of a redox gradient in the Atar Basin, which might also be reflected in the presence of herringbone carbonate as a first-generation of void-filling cement associated with marine transgression. In low-oxygen, low-sulfate conditions, deepwater anoxia may also be associated with ferruginous oceanic conditions and/or high dissolved organic carbon. Mixing of such deeper water masses with surface waters would be expected to result in a temporary increase in carbonate saturation, suggesting a mechanism for cement precipitation in the voids of the Atar Group stromatolite reefs.