Terrestrial influences on carbon burial at sea

Abstract

Over geologic time, a primary driver for change in atmospheric concentrations of gases such as methane, CO2, and O2 is burial of organic matter in marine sediments. After photosynthetic organisms produce organic matter and O2, these compounds are typically respired to CO2 in short order. However, burial of organic matter in sediments sequesters carbon for eons, allowing O2 to build up and playing a small role in reducing CO2 in the atmosphere. The exact mechanisms and rates of organic matter burial are therefore significant terms that need to be understood to fully comprehend earth processes. Organic matter preservation is linked to the burial of minerals, many of which are terrestrially derived. The favored hypothesis explaining organic–mineral associations is that the abundance of mineral surface area somehow protects organic matter from degradation. In this way, ocean sediments containing fine-grained minerals bury more organic matter than coarse-grained sediments. The mechanistic details of this remain poorly elucidated, but the relationship in the modern ocean is reasonably well described: ≈1 mg of C is buried along with each square meter of external mineral surface. This approach excludes the internal surfaces of expandable clay minerals, which have previously been thought to be relatively unimportant (1). The one notable exception to this generality occurs in suboxic and anoxic portions of the ocean, where organic matter is preserved in excess of 1 mg C per external square meter. It has generally been assumed that external mineral surface area controls “baseline” amounts of organic matter burial and that anoxia enhances burial. In PNAS, Kennedy and Wagner. (2) bring a unique perspective to this paradigm. They show that mineralogy, not just surface area, can play a large role in controlling organic matter burial. By measuring the internal surface area of expandable clays and relating that to changes in organic matter burial in Cretaceous black shales, they illustrate a process whereby both mineralogy and anoxia amplify carbon storage. The amplification effect seems to occur rapidly, on a temporal scale of decades rather than millenia.