Skip to main content

The role of minerals in the oceanic carbon cycle

Periodic Reporting for period 3 - MINORG (The role of minerals in the oceanic carbon cycle)

Reporting period: 2020-06-01 to 2021-11-30

The carbon cycle is one of the most important elemental cycles on our planet because, via the cycling of carbon between the lithosphere, hydrosphere, atmosphere and biosphere, the carbon cycle controls climate and thus the habitability of our world. There are several aspects of the carbon cycle that must be better understood in order to shed light on how our planet became habitable and how our activities are shaping habitability into the future, but one in particular concerns the cycling of organic carbon in the oceans. The cycling of organic carbon in the oceanic environment is of fundamental importance to the Earth system on both the vastly long timescales that have created the planet we see today, and the much shorter timescales that we now rely on for our socioeconomic wellbeing. On multi-million year timescales the balance between the degradation and preservation of organic carbon in marine sediments has played a fundamentally important role in controlling atmospheric carbon dioxide and oxygen. On decadal to centennial timescales the balance between the degradation and preservation of organic carbon in seawater plays a pivotal role in controlling atmospheric carbon dioxide today, and thus in regulating modern climate and potentially mitigating against the effects of climate change.

Over these two different timescales the cycling of organic carbon in the oceans is of vital importance to the Earth system, but despite many years of research, we still do not fully understand how organic carbon escapes degradation and becomes preserved. All organic carbon is derived from living organisms, which are essentially quite easily broken down by microbes living in marine sediments and seawater, so the fact that any organic carbon is preserved is actually profoundly puzzling. To date several factors are known to be important for organic carbon preservation, including the amount of time that organic molecules are exposed to oxygen, but the correlations between these factors and organic carbon burial on a global scale are rather weak and it is clear that there must be one or more other processes responsible for preservation. In this regard recent work increasingly highlights the potential importance of sedimentary minerals for organic carbon protection and preservation. In particular sedimentary iron and manganese minerals have been shown to lock up organic carbon and protect it from degradation, with perhaps as much as 20% of all organic carbon in marine sediments associated with reactive iron phases, creating a so-called 'rusty sink' that might preserve organic carbon over thousands of years.

MinOrg is investigating the role of sedimentary minerals in the preservation of organic carbon in marine sediments in fine detail, through a combined experimental and modelling approach that uses cutting-edge molecular-level techniques to determine the exact mechanisms of organic carbon association with minerals and their preservation potential. Through careful experimentation and molecular to global modelling approaches the project addresses one overarching hypothesis, that minerals play a major role in the preservation of organic carbon in marine sediments and the production of long-lived organic carbon in seawater. There are four broad objectives to 1) determine the exact mechanisms of organic carbon association with sedimentary minerals; 2) determine whether and to what extent mineral associated organic carbon is protected from degradation; 3) quantify mineral promoted preservation of organic carbon in the oceanic carbon cycle; and 4) investigate the role of mineral promoted preservation over geological and anthropogenic timescales.

MinOrg will offer a new understanding of organic carbon preservation that can be used to better evaluate feedbacks between the oceanic carbon cycle and climate, over timescales relevant to both the evolution of our planet and the habitability of the Earth's surface over the coming centuries. This is of significant societal benefit as we strive to understand Earth history and the links between the biological and chemical components of our planet, and our role in a rapidly changing environment.
For O1 we have determined the mechanism of association of the most prevalent simple organic carbon molecules with iron minerals in marine sediments. We find that certain types of organic functional groups control carbon uptake, and that carbon molecules that are increasingly rich in these groups are more strongly attached to the mineral surfaces, and as a result, less readily degraded by chemical agents that mimic microbial attack. We have discovered a novel mechanism for the preservation of simple carbon molecules, catalysed by iron and manganese, that could account for a significant proportion of carbon preservation in the oceanic environment. These results have been written up and published as manuscripts in high impact journals, and several other manuscripts are in revision, submitted and in preparation.

For O2, we are investigating the extent to which our mineral associated carbon can be degraded by marine sediment microbes. The results have been written up and are being written up as manuscripts for high impact journals, and are either in revision or submitted, with several other manuscripts in preparation.

For O3 we are developing a new conceptual model for the cycling of carbon in marine sediments. A manuscript is in preparation for a peer-reviewed high impact journal.

For O4 we have identified an exciting avenue of research emerging as a result of our findings above, and a new team member has been hired to explore this. A manuscript is in preparation for a peer-reviewed high impact journal.
In O1 we designed an experimental campaign based on a series of simple carbon molecules, that build in chemical complexity, in which we can directly track carbon and mineral associations using new synchrotron spectroscopy tools. This represents a step change in how we investigate carbon and mineral interactions, and demonstrates that one type of organic functional group is especially important for the association of carbon with marine minerals. We are now probing the role of this group in the association of more complex forms of carbon with minerals.

In O2 have developed a new methodology for microbial incubation experiments, that goes beyond traditional approaches and now allows us to work with a much greater range of organic compounds than otherwise feasible. We are now deriving mineral associated carbon life times in marine sediment, and testing the microbial stability of the more complex forms of carbon in O1, where we can now explain observations of marine sediment biogeochemistry as a function of mineralogical and microbial processes.

In O3 and O4 we are developing a new conceptual model for the cycling of carbon in marine sediments, which represents a paradigm shift for how we think about degradation and preservation over anthropogenic and geological time. We now expect to quantify the role of mineral promoted carbon preservation in the oceanic carbon cycle, where initial results suggest that the catalytic role of iron and manganese is an important and previously overlooked route to carbon preservation and planetary homeostasis. We are now testing our theories about the Earth system role of iron and manganese using a new type of intermediate complexity biogeochemical model, facilitated by a new team member.
Schematic for the sorption and chemical transformation of organic carbon with marine minerals.