ABOUT PROJECT

Permafrost thawing (PfT), Greenhouse Gases and Bio-essential and Toxic Trace Elements (BeT-TEs)

Warming of the Arctic and ensuing permafrost thawing (PfT) is enhancing fluxes of organic and inorganic matter (trace elements, nutrients and pollutants) to the Arctic Ocean. Increased organic matter (OM) decomposition in the water column and surface sediments may have important consequences for the ecosystem biogeochemistry (Reyes & Lougheed 2015). Trace elements (TEs) such as Fe, Mn, Co, Zn, Cd and Ni are essential for biological processes. Some of these elements could be toxic at elevated concentrations (i.e. Zn, Ni and Cd) (Turner at al. 2016) and some others, such as Hg and Pb, are always risk factors for serious toxicity in marine organisms and humans (Lamborg et al., 2014, Schuster et al. 2018). Warming accelerates degradation of terrestrial seabed PfT, releasing OM. The released OM following bacterial mineralization, increases pCO₂, induces de-oxygenation in the sediment and lower water column and increases nutrient concentrations in the shelf waters (Larsen et al., 2015). While the pCO2 increase drives a reduction of pH (ocean acidification) and outgassing of CO₂ from coastal waters, increases in primary productivity stimulated by the released nutrients and bio-essential trace elements (Fe, Mn, Zn and Co) will remove CO₂ from the atmosphere. Alternatively, primary productivity may be decreased due to increased high-turbidity river runoff, promoting stratification and restricting nutrient supply, and reduced water transparency due to increased OM content (from rivers and subsea permafrost). A large fraction of the Siberian Continental Shelf (SCS) is a source of atmospheric CO₂ because of eroded carbon oxidation and inputs from rivers (Semiletov et al., 2016). Sustained CH₄ release to the atmosphere from Arctic permafrost and dissociating hydrates are likely to be significant feedbacks to climate warming (Shakhova et al., 2015). Besides this, CH₄ release leads to a further acceleration of de- oxygenation and acidification. The net effect of these various processes on coastal biogeochemistry and their impacts on the transformation and mobility of bio-essential (e.g. Fe, Mn) and toxic elements (e.g. Hg, Pb) is currently unknown.

Transport of Bio-essential and Toxic Trace Elements by Trans Polar Drift (TPD)

Riverine and shelf-derived organic carbon, macro&micronutrients as well as toxic elements are rapidly transported from the Siberian continental shelf toward the Fram Strait by TPD on timescales of 1–3 years (Wheeling, 2020). That is the reason some of the mobilized BeT-TE (Fe, Co, Ni, Cu, Hg and Nd in the Siberian rivers, coastal waters and continental shelf are significantly elevated in the Arctic Ocean relative to other ocean basins (Charette et al. 2020). These elements may be carried by TDP beyond the central Arctic, may reach to the North Atlantic Ocean (Wheeling, 2020).

Geophysical Research: Oceans, 1Wheeling 2020).

Organic Matter and Bio-essential & Toxic Trace Elements interactions

Organic matter is a key controlling factor on BeT-TE biogeochemistry in the oceans, providing complexing ligands (Lohan et al, 2015; Waska et al., 2015) and scavenging/fractionation reagents (Lin et al., 2015; Chuang et al., 2015) and modifying redox conditions in the surface sediments. The pivotal role of OM on the methylation and de-methylation of mercury and its speciation in both water column and sediment is also a subject of ongoing research (Chakraborty et al., 2015). Organic matter, in both particulate and dissolved forms, has been shown to play an important role in the cycles of bio-essential metals such as Fe (Boyd et al. 2007; Dulailova et al. 2009; Ardelan et al. 2010) and toxic elements such as Hg (Hirose 2007; Schaefer and Morel, 2009; Mazrui et al., 2016) and may strongly affect bacterial and phytoplankton production in marine systems (Öztürk 2004). Complexation of BeT-TEs and OM constituents changes the solubilities of BeT-TEs in seawater, thus affecting residence times in the water column and transport. Organic matter that originate from terrestrial systems (allochthonous sources) and marine organisms (autochthonous sources) may have different complexation abilities depending on the element in consideration/ of interest. For example, while allochthonous OM showed strong binding ability to Pb compared to autochthonous OM, these variations were less pronounced for Zn and Cd (Chen et al., 2018). Increased OM inputs are expected in areas where the permafrost is thawing, and this may trigger major changes in the biogeochemistry of the SCS region. BeT-TEs-OM interactions are sensitive to temperature, photochemical destruction, particle density, salinity and pH, particle density and photochemical oxidation, especially in the changing polar coastal environments where OM is supplied by glacial melting and terrestrial and/or shelf processes (Boyd and Ellwood 2010; Hood 2015). Stockdale et al. (2016) reported that Pb speciation will be changed by ocean acidification and that this acidification can aggravate Pb toxicity for marine organisms (Han et al.2014). We also detected an increasing mobility in sedimentary Hg and Pb in our CO₂-seepage experiments (unpublished data).

Iron (Fe) and other micronutrients. The Arctic marine system is still largely unexplored especially with regards to Fe and other micronutrients. The main sources of Fe and other bioactive trace metals are likely to be: i) the continental shelf, ii) sea ice melt, iii) allochthonous sources from PfT and decaying biota, iv) aeolian inputs. PfT may increase the mobility of usually ‘immobile’ inorganic components (Fe, Al and other elements), as observed in Siberian thermokarst lakes (Pokrovsky et al, 2011). A seasonal signature of trace metal concentrations in rivers has been reported to correlate with the depth of the thawed active layer in soil cores of Arctic watersheds (Barker et al, 2014). Surface ocean dissolved Fe concentrations in the Amundsen and Makarov Basins of the Arctic Ocean show a strong relationship with salinity, with Eurasian rivers as sources (Klunder et al., 2012). While large parts of the Arctic Ocean are replete in Fe and other bio-essential elements (Co, Mn), in some regions, like the surface waters of Barents and Kara seas, an observed Fe minima indicate depletion due to phytoplankton growth (Klunder et al., 2012). Seasonal inhibition of phytoplankton growth due to Fe limitation has been observed in the Fram Strait and is speculated to occur in the Labrador Sea as well (Bhatia et al., 2013).

Mercury (Hg). PfT is projected to increase the transfer of terrestrial Hg to Arctic coastal environments (Fisher et al. 2012; Rydberg et al. 2010; Schuster et al., 2018). This is likely to trigger cascading biogeochemical and ecological effects, which will lead to increased Hg species bioavailability, bioaccumulation, and biomagnification (Jonsson et al. 2017). Moreover, OM also strongly affects mercury’s speciation, transformation, transport, and toxicity in the aquatic environment (Schaefer and Morel, 2009; Zhong and Wang, 2009; Luengen 2012; Mazrui et al., 2016) together with other environmental and ecological drivers and stressors. The impact of OM on mercury speciation and its uptake are dependent on their biogeochemical characteristics (Graham et al., 2013; Jonsson et al., 2014). Most of the studies suggest higher Hg and MeHg uptake by cells in the presence of predominantly-marine OM compared to terrestrial OM (Zhong and Wang, 2009) but there are also reports that terrestrial OM, such as fresh humic OM leached from organic-rich soils, may also contribute significantly to methylation processes (Herrero Ortega et al., 2018). Since the methylation of inorganic divalent Hg to MeHg is primarily mediated by anaerobic micro- organisms (Gilmour et al., 2013), deoxygenation (e.g. due to the accumulation of labile OM) promotes methylation of Hg in the sediments. Deoxygenation can also create sulfidic conditions in the sediments, and it is well known that sulfide complexes of mercury together with OM can impact methylation processes (Ravichandran, 2004). Furthermore, combined effects of OM and Cl have been reported (Wang and Wang, 2010) in coastal water under heavy freshwater impact. Consequently, there is still much uncertainty regarding the cycling, transformation, transport, and toxicity of Hg, especially on local/regional scales. In a changing ocean the combined input of OM, Fe and Hg is likely to create competition between different phytoplankton and bacterial groups depending not only on their functional traits but also on their positions in the benthic-pelagic food web. Knowledge of the biogeochemistry of BeT-TEs and OM is essential for investigating the synergistic and antagonistic effects of PfT-derived emissions on Arctic microbial food webs, and how changes in BeT-TE-OM interactions due to climatic stressors may affect community composition of the phytoplankton bloom and bacterial activity in the Arctic waters.

2.1 Potential for academic impact of the research project

BEST-Siberian will address important present scientific challenges by filling critical knowledge gaps with respect to fluxes and transformations of BeT-TEs and potential organism/ecosystem responses in the Arctic Ocean under PfT (Objectives 1-3). This knowledge could transform the current scientific conception of Arctic sensitivity to climate change and cascade effects of PfT (see Figure 1). The project will also address holistic/system-level responses of Arctic marine ecosystems to PfT through the development of state-of-the- art models and model parameterizations that will allow new insights into the role of the Arctic Ocean in global biogeochemical cycles and the investigation of novel and potentially important feedbacks to climate change within the next generation of ESMs (Objectives 4, 5, Figure 1). With regard to societal challenges, BEST-Siberian will take a novel approach to develop and explore new paradigms for knowledge co- production through stakeholder involvement (Objective 7), aiming to guide the research towards areas of maximal relevance to societal issues while also involving and educating stakeholders whose activities may ultimately be impacted directly by new project knowledge or by management decisions taken in light of the project research.

The outcomes of BEST-Siberian will help us to evaluate the impact of Siberian continental shelf to TPD biogeochemistry. In the near future the TPD will become even more important to biogeochemistry and ecosystems in the Arctic Ocean and beyond. Understanding the elemental transport of the TPD could help researchers track flows and uptake of BeT-TEs into The North Atlantic waters.

Finally, BEST-Siberian will contribute to several UN Sustainable Development goals, specifically: climate action (13) and life below water (14), since we will advance knowledge of climate change impacts on and feedbacks from marine ecosystems and biogeochemical processes; good health and well being (3) and clean water and sanitation (6), since we will investigate potential toxin release and broader PfT impacts on water quality; and no poverty (1), zero hunger (2), and decent work and economic growth (8), since we will assess impacts on ocean productivity and potential impacts on fisheries and food for indigenous people, and provide knowledge for improved management and exploitation of marine resources.

2.2 Potential for societal impact of the research project (optional)

BEST-Siberian will improve knowledge of marine processes necessary to safeguarding the marine environment. By focusing on the coastal system, Outcomes of BEST-Siberian will help to evaluate ecosystem goods and services critical to first coastal and subsequently to all human communities. BEST-Siberian focus also supports the concern for food security in the region and beyond.

The project will help find better solutions to cope with the impacts of climate change by determining the key feedback mechanisms underlying climate change, including societal response to policies, will be determined together with the sensitivities and uncertainties of the system.

BEST-Siberian incorporates the social sciences into its work, and its results will be used to assess the vulnerability of coastal communities that are strongly and directly dependent on marine ecosystem goods and services. Outcomes of the project will also be used to inform coastal communities by focusing on economic sectors directly and most immediately impacted by marine conditions. In addition, BEST-Siberian will draw upon the insights produced by stakeholder workshops, and the close collaboration with ecosystem modelers, products from these activities can be used to predict the strategic choices of stakeholders in given situations. Stakeholder involvement and an emphasis on dissemination will help produce management and policy options that are flexible, legitimate, and sustainable, enabling easier implementation of political decisions and regulations.

Overarching Project Goal

To deliver improved-targeted environmental and socio-ecological response understanding for the Arctic Ocean under the impact of permafrost thawing in a changing climate

Project Objectives

1. Characterize the fluxes of bio-essential/toxic elements (BeT-TEs) from major terrestrial and seabed sources and the distribution of BeT-TEs throughout the water column and surface sediments.

2. Quantify changes in organic matter (OM) supply under permafrost thawing (PfT) and impacts on ecology and BeT-TE transformation/mobility on the Siberian Continental Shelf (SCS).

3. Empirically derive organism and ecosystem responses to changing Arctic Ocean drivers (Fe, light, ocean acidification)

4. Develop and improve local and basin-scale biogeochemical models for the SCS and wider Arctic Ocean by integrating data from new field expeditions and experiments.

5. Determine the sensitivity of Arctic Ocean ecosystem functioning and potential climate feedbacks due to PfT-derived perturbations to ocean biogeochemistry.

6. Develop, test, and refine parameterizations of key processes in the Arctic marine system response to PfT for use into Earth System Models (ESMs).

7. Co-produce and populate a stakeholder-scientific socioecological framework to optimally deliver new project knowledge for improved understanding of ecosystem service sensitivity.