Marine refractory dissolved organic carbon and transgressive black shales

Abstract

<p indent="0mm">The marine refractory dissolved organic carbon (RDOC) pool, derived from the microbial carbon pump (MCP), holds a tremendous amount of carbon, as much as that in today’s atmosphere; hence, an understanding of this pool is crucial for investigating carbon cycling and climate change at the global scale. The size of the RDOC pool dynamically evolves over time. In the geological past, particularly during the Neoproterozoic to Cambrian transition, the marine RDOC pool might have reached a hundred to a thousand times its present size. However, the patterns of the reduction in the RDOC pool through time and the means by which RDOC was transferred into the geological reservoir have been poorly known to science. Black shales, which are the source rocks of oil and gas reservoirs and are enriched in many rare metals, are economically important sedimentary rocks. Black shale is characterized by the enrichment of clay sediments and organic matter (total organic carbon (TOC) &gt;1%). At least a dozen depositional models have been proposed to interpret various types of black shales based on the depositional environments and three major controls on organic content, i.e., the production and supply of organic matter, the demand and consumption of oxygen, and the rate of sedimentation. Organo-clay interactions and potential organic sources of the bulk RDOC in ancient oceans have not been fully considered. Clay minerals, e.g., smectite and illite, are well known for their capability of adsorption and preservation of dissolved organic matter in significant quantities. Organic compounds adsorbed by clays are present dominantly within interlayer sites and thus are difficult to be decomposed or used by microbes. Accordingly, here, I propose a hypothetical mechanism of RDOC deposition and preservation, which also functions as an alternative model for the origin of transgressive black shales that always abruptly contact the underlying rock unit. The proposed mechanism is outlined as follows: During global-scale sea level rise driven by crustal subsidence or climate change, upwelling currents transport RDOC-rich water bodies from deep oceans to clay-rich shallow sea depositional environments, where clay sediments mix well with the RDOC; the mixture triggers the interplay between clay minerals and organic matter, and thus leads to large amounts of RDOC being absorbed by clay sediments; after deposition, the clay sediments are able to absorb organic matter from pore water; finally, the organic-rich transgressive clay sediments form black shales through diagenesis. Experiments indicate clays are capable of absorbing as much as 12% or more dissolved organic matter, far more than the minimal 1% TOC in black shales. Therefore, it is reasonable to assume that the RDOC is an important organic source in transgressive black shales. The new model provides a potential link between ancient marine RDOC and black shales; hence, it might offer a mechanism for transferring marine RDOC into the geological carbon reservoir and thereby present an additional path of marine carbon cycling. Unlike the previous model that invoked primary production as the major organic source, the present model emphasizes the contribution of RDOC to the organic matter of black shales. However, it does not refute the contributions of any other organic source or any other model for transgressive black shales. To test this hypothesis, future work may include enhanced experimental studies of the interplay between clay minerals and organic compounds under different settings of temperature, pressure, and redox state, establish a set of geological criteria to recognize RDOC from organic-rich rocks, e.g., black shales; and quantitatively model the evolution of RDOC over geological time. Each of these tests would provide critical information for understanding the global carbon cycles of the past, present, and future.