Terrestrial influences on carbon burial at sea

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.

Why is this intriguing? The findings demonstrate a previously unrecognized link between terrestrial and marine processes that act in unison to sequester carbon. It is not just the delivery of mineral to the ocean that matters, but the timing and type of mineral delivered. Increases in expandable clay mineral content under anoxic conditions dramatically increases carbon burial, enhancing carbon storage by as much as a factor of 10.

The fact that mineralogy plays a role in controlling carbon burial in the sea is a bit of a surprise. This is because the various

Mineralogy, not just surface area, can play a large role in controlling organic matter burial.

metals, oxides, and organic materials in sediment pore waters interact with and largely nullify any mineral-specific surface properties (3). Nonetheless, there have been glimpses that the internal (expandable) surface area of clay minerals could be important (4, 5). What Kennedy et al. show is that under anoxic conditions the amount of organic matter stored on expandable clays is dramatically higher than observed under more typical settings. The effect of mineralogy enhances the effect of anoxia, and vice versa. This strongly suggests that the type and timing of sediment delivery from land can influence the quantities (and perhaps types?) of organic matter preserved in the ocean.

Ocean Anoxic Events

Organic matter-rich ocean sediments are often indicators of oceanic anoxic events (OAEs), a link documented more than 30 y ago (6). The significance of OAEs to the global carbon cycle and their use as paleoceanographic indicators of climate change are fertile areas of research because they contain clues to both the past and to a changing world in the Anthropocene (7). OAEs form when increased continental runoff delivers nutrients that stimulate algal growth. This causes an increase in organic matter sinking to the ocean's interior, thus increasing oxygen demand (think “global eutrophication”). Simultaneously, surface water dilution increases stratification and discourages mixing or ventilation, isolating deeper waters from the atmosphere. A common record of these OAEs is enhanced organic matter burial in sediments. Previously it has been believed that this is solely the result of the onset of anoxia, but Kennedy et al. eloquently illustrate the role that terrestrial mineralogy can play, a role that in the present example acts as a negative feedback against the expansion of the anoxic waters and against global warming.

Looking Back to See the Future

It has been proposed that OAEs contain a positive feedback loop enhancing their stability via efficient recycling of P and N nutrients, which leads to more primary productivity, more sinking organic matter, etc. (8, 9). This is perhaps reinforced by ocean acidification, which reduces biomineral formation and thus causes organic matter to sink slower (10), and ocean warming, which increases stratification (11). This has led scientists to suggest that the ocean is trending toward larger suboxic and anoxic zones. The role of soil erosion on this remains controversial (12, 13), but Kennedy et al. provide a unique angle on this debate and a unique avenue for investigation. If anoxic zones expand into regions where expandable clays are eroding, will enhanced organic matter burial act to mute the expansion of ocean dead zones? Enhanced preservation of organic matter in the presence of expandable clays should act as a negative feedback on the growth of OAEs. Along with burial of the organic carbon and the lack of utilization of O₂, organic N and P should be buried as well, preventing them from being remineralized and released back to the water. The effect of expandable clay delivery therefore should enhance carbon storage and diminish the intensity of the stressor. Much work remains to be done to determine how broadly representative this research may be. Understanding the role of mineralogy on carbon burial in marine sediments will require the effective integration of modern and paleo research, as well as integration of terrestrial and marine research approaches. Minerals travel from mountains to abyssal plains, telling a story the entire route. Kennedy and Wagner help us listen.