The modern record of “concretionary” carbonate: Reassessing a discrepancy between modern sediments and the geologic record

• Authigenic carbonate common in marine sediments under productive water columns
• Geochemical similarities between pore waters and carbonates indicate likelihood of modern authigenesis
• Identification of concretionary carbonate precluded due to coring limitations
• Suggests that modern concretion analogs exist

Carbonate concretions have long been recognized as common constituents of sedimentary rocks exposed in outcrop. Although formation mechanisms vary, organic matter degradation often plays a role in carbonate concretion authigenesis, providing a source of alkalinity for mineral production. Perhaps surprisingly, modern examples of concretionary carbonates are rarely identified in cores despite ubiquitous organic matter degradation in marine sediments. This has prompted the general notion that the modern record of concretionary carbonate does not mirror that preserved in the geologic record. Here, we present data from continental margin sediments that receive enhanced delivery of organic matter and demonstrate that concretionary carbonates are indeed recovered from many of these sites. In addition, sites where concretionary carbonates have not been recovered exhibit chemical profiles that suggest local authigenesis. The lack of concretion recovery from some locations likely reflects the poor potential for modern coring techniques to adequately capture concretions or provide context for their identification once recovered. Concretionary carbonate recovered from the Peru Margin, Cascadia, the Gulf of California, the Cariaco Basin and the western margin of southern Africa exhibit evidence for modern or very recent formation via methanogenesis or the anaerobic oxidation of methane. Demonstrably modern organotrophic sulfate reduction concretions have not been found.