ChAOS

Description

The Arctic Ocean accounts for up to 14% of global atmospheric CO2 uptake. But only the carbon that is deposited at, and buried into, the Arctic seafloor removes it from the ocean-atmosphere system over long timescales. Hence, the Arctic seafloor, with its complex biological and biogeochemical processes, is inextricably involved in this crucial process of carbon sequestration. However, little is known about how the Arctic seafloor environment is responding to surface ocean changes. The ChAOS project will close this knowledge gap by studying an Arctic shelf seafloor system undergoing rapid environmental change using a multi-disciplinary approach.

The ChAOS project aims to better understand how changes in the Arctic Ocean sea ice cover and water mass distribution will affect biological and biogeochemical processes at the seabed. The seafloor is a highly dynamic environment that hosts a wide variety of biota, and plays a crucial role in carbon and nutrient cycling and burial. ChAOS will focus its research activities on the central and northern Barents Sea, a part of the Arctic shelf system that is strongly affected by modern climate change, and of interest to the hydrocarbon and fisheries industries.

Propagation of surface change to the seafloor

Rates of warming in the Arctic are amongst the highest globally. One of the most obvious manifestations is the dramatic reduction in summer sea ice extent and thickness over the past few decades. These changes in ice cover exert cascading effects on Arctic Ocean carbon and nutrient dynamics, causing important feedbacks on the local ecosystems, regional processes and the global climate system.

Carbon sequestration

The Arctic Ocean accounts for up to 14% of the global atmospheric CO2 uptake and is therefore of fundamental importance to the global carbon cycle. However, changes to certain key components of Arctic ecosystems, such as benthic faunal assemblages or the extent of carbon and nutrient burial, are often ignored in political and scientific discussions of a changing Arctic Ocean. The Arctic Ocean seafloor hosts a diverse and productive benthic ecosystem that is a crucial component of an intimately coupled benthic-pelagic system. The relative importance of benthic organisms in modulating sequestration, transformation and storage of bio-essential nutrients and carbon across the Arctic Ocean is still poorly constrained.

Seafloor processes

At the seafloor, a significant proportion of organic matter from marine, terrestrial, or sea ice sources is remineralised via microbially mediated processes that are coupled to the activity of benthic meio-, macro- and mega-fauna. These coupled biological and biogeochemical processes lead to a partition of the carbon and nutrient pools into a fraction that is recycled to drive a benthic-pelagic feedback loop, and a fraction that is buried in sediments.

The resulting feedbacks with water column processes (physical mixing, primary productivity) are more pronounced on the Arctic shelf than in the open ocean and play a crucial role for benthic-pelagic coupling and ecosystem productivity, as well as the long-term removal of carbon from the ocean-atmosphere system. Key uncertainties exist, however, in how changes in sea ice cover, with a trend to thinner and reduced ice cover that exhibits significant inter-annual variability, will alter existing biological community composition and structure, biogeochemical processes, and associated ecosystem functioning. Understanding these changes to the benthic environment is of critical importance to understanding the Arctic Ocean ecosystem as a whole.

Potential impacts of seafloor change

Recent research highlights that previously unrecognised biological and biogeochemical factors exert a significant control on the balance between organic matter degradation versus preservation in high latitude oceans. For example, the coupling of organic molecules to certain minerals or grain size fractions or the burial of large amounts of carbon in macro-benthic biomass. At the same time, longer and more extensive open water conditions, especially across ice-marginal Arctic shelves, could lead to prolonged growing seasons and enhanced CO2 sequestration into biomass. Eventually, this could result in a negative feedback on the CO2-induced greenhouse effect in the Arctic as more carbon gets sequestered into the sediment.

Lack of data and modelling uncertainties

However, modelling the response of the Arctic Ocean carbon and nutrient cycles to reduced sea ice and its associated, and partly counteracting, effects (deeper light penetration, longer growth seasons, increased water column stratification, ocean acidification, warming) is difficult due to the lack of data and an incomplete mechanistic understanding of the changing Arctic Ocean seafloor. It is currently unclear which fraction of carbon, phosphorus, nitrogen, silica or iron will be metabolised and transformed at the seafloor, which interactions between microbial and macro-benthic activity dominate these transformations, and what the effects are on ecosystem structure and functioning, under changing sea ice conditions.

Although sophisticated, multi-component diagenetic models have emerged over the past two decades, most regional to global scale biogeochemical models and Earth system models do not resolve the complexity of the benthic environment. The primary constraint here is the high computation cost of simulating all essential redox and equilibrium reactions within marine sediments that control carbon burial and benthic recycling fluxes, a problem that is exacerbated if a variety of benthic environments are to be spatially resolved. Most models neglect or simplify biogeochemical processes using a limited number of parameters in the sediment and, in so doing, misrepresent the complexities of organism-sediment interactions and benthic-pelagic coupling.

ChAOS Aims

ChAOS will quantify the effect of changing sea ice cover on organic matter quality, benthic biodiversity, biological transformations of carbon and nutrient pools, and resulting ecosystem function at the Arctic Ocean seafloor. We will achieve this by determining the amount, source, and bioavailability of organic matter and associated nutrients exported to the Arctic seafloor; its consumption, transformation, and cycling through the benthic food chain; and its eventual burial or recycling back into the water column.

We will study these coupled biological and biogeochemical processes by combining

• a detailed study of representative Arctic shelf sea habitats that intersect the ice edge, with
• broad-scale in situ validation studies and shipboard experiments,
• manipulative laboratory experiments that will identify causal relationships and mechanisms,
• analyses of highly spatially and temporally resolved data obtained by the Canadian, Norwegian and German Arctic programmes to establish generality, and
• we will integrate new understanding of controls and effects on biodiversity, biogeochemical pathways and nutrient cycles into modelling approaches to explore how changes in Arctic sea ice alter ecosystems at regional scales.

We will focus our research on the Barents Sea, a part of the Arctic Ocean where drastic changes in sea ice cover and water mass distribution are the main environmental control. Common fieldwork campaigns will form the core of our research activity. Our focal region is a N-S transect along 30 degree East in the Barents Sea where ice expansion and retreat are well known and safely accessible. We will also use additional cruises to locations that share similar sediment and water conditions, retrieving key species for extended laboratory experiments that consider future environmental forcing. Importantly, the design of our campaign is not site specific, allowing our approach to be applied in other areas that share similar regional characteristics. This flexibility maximizes the scope for coordinated activities between all CAO projects (pelagic or benthic), and with scientists from other nations (e.g., Norway).

In support of our field campaign, and informed by the analysis of field samples and data obtained by our international partners (in Norway, Canada, USA, Italy, Poland and Germany), we will conduct a range of well-constrained laboratory experiments, exposing incubated natural sediment to environmental conditions that are most likely to vary in response to the changing sea ice cover, and analysing the response of biology and biogeochemistry to these induced changes in present versus future environments (e.g., ocean acidification, warming). We will use existing complementary data sets provided by international project partners to achieve a wider spatial and temporal coverage of different parts of the Arctic Ocean. The unique combination of expertise (microbiologists, geochemists, ecologists, modellers) and facilities across eight leading UK research institutions will allow us to make new links between the quantity and quality of exported OM as a food source for benthic ecosystems, the response of the biodiversity and ecosystem functioning across the full spectrum of benthic organisms, and the effects on the partitioning of carbon and nutrients between recycled and buried pools. To link the benthic sub-system to the Arctic Ocean as a whole, we will establish close links with complementary projects studying biogeochemical processes in the water column, benthic environment (and their interactions) and across the land-ocean transition. This will provide the combined data sets and process understanding, as well as novel, numerically efficient upscaling tools, required to develop predictive models (e.g., MEDUSA) that allow for a quantitative inclusion seafloor into environmental predictions of the changing Arctic Ocean.