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Geochemical-physical coupled study of the modern Arctic Ocean: GEOTRACES-ARCTIC

Periodic Reporting for period 2 - GCP-GEOTARCTIC (Geochemical-physical coupled study of the modern Arctic Ocean: GEOTRACES-ARCTIC)

Reporting period: 2017-07-01 to 2018-06-30

At present, our ability to evaluate the full impact of climate-driven alterations of the Arctic Ocean (sea ice cover, hydrography, circulation) and predict their future trajectory is limited by a poor understanding of the interacting chemical, physical and biological processes that shape the functional characteristics and resiliency of Arctic waters. To bridge this critical knowledge gap, a pan-Arctic field study (Arctic-GEOTRACES, has been coordinated between Canada, US, Germany and France to generate a quasi-synoptic database of biogeochemical tracers of circulation, ecosystem structure and productivity, and sea ice state. The Canadian program involves 28 investigators including biological, chemical and physical oceanographers, experimentalists and modellers.
Fully integrated in the Canadian Arctic program, the proposed research focused on trace elements (Rare Earth Elements; REEs) and isotopes (ɛNd, 230Th and 231Pa) that provide powerful tracers of ocean circulation and land/ocean chemical exchanges, both impacted by climate change in the Arctic. It was conducted on the Canadian section, itself connected to the US Arctic GEOTRACES section and the French GEOVIDE section by common cross-over intercalibration stations.
The proposed study consisted of a three-step approach:
i) Initial modelling study of Arctic Ocean dynamics, including off-line Lagrangian analyses to refine the sampling strategy for the tracers mentioned above;
ii) Measurements of these tracers on the Canadian section to specifically investigate land-ocean exchanges and circulation in the Arctic Ocean;
iii) Integration of these data into a fine resolution model coupling circulation, sea ice dynamics and biogeochemical processes to refine our understanding of circulation and to quantify land-ocean margin chemical exchanges of bioactive or water mass fingerprinting chemical elements.

Modelling and observational results notably allowed to evidence, in the sea-ice retreat enhancement context of the studied areas, important changes in the Arctic over the last decades, in terms of intermediate and deep circulation and of fluxes of biologic and lithogenic particles. Estimations of land-ocean flux budgets in the shallow Canadian Arctic Archipelago are ongoing and will allow to characterize the chemical contribution of this area to the crossing waters.
The first part of Dr Melanie Grenier's project was to prepare and participate to the GEOTRACES program of the Canadian Arctic cruise (July-August 2015). In this context, she first ran modeled Lagrangian analyses to refine the sampling location of a coastal station in Baffin Bay.
In parallel, Dr Grenier also prepared the sampling material to do the collection of the seawater samples and the on-board chemical extraction of the desired elements from seawater in appropriate conditions. She also followed a radionuclide safety course and familiarized herself with the methodology of the 230Th and 231Pa analyses in seawater.

Following their collection at sea, the data analysis of more than a hundred samples was performed in clean-labs and measured on advanced spectrometers, eventually leading to a set of ~320 data for the 3 geochemical isotopes. Among these data, some are the first to document some sampling areas. Prior to beginning the interpretation of the dataset, a procedure of data intercalibration and validation was followed by comparing profiles of stations crossed over by the US and French sections. A part of the validated dataset was integrated into a 3D model in the framework of a UBC Master Thesis (realized by Cindy Yu and supervised by Prof. Susan Allen) and contributed to the first modelled representation of 230Th and 231Pa in the Arctic. Dr Grenier was a member of Cindy Yu's supervisory committee.

Dr Grenier presented her results all along the project period, during the 2016 ArcticNet annual scientific meeting in Winnipeg (AB, CA), at the 2017 ASLO Aquatic Sciences Meeting in Honolulu (HI, USA), and at the 2018 Goldschmidt conference, in Boston (MA, USA). Several publications are in preparation. The submission of an article reviewing the evolution of dissolved 230Th, 231Pa and εNd in the Amerasian Basin over the last 4 decades is notably ongoing. It will be a companion paper to the modelling paper Cindy Yu is submitting to Journal of Geophysical Research, for which Dr Grenier is co-author. Dr Grenier plans to also write 2 other papers based on her 230Th, 231Pa and εNd results in the Labrador Sea and Baffin Bay. She is currently writing a letter on the CAA river REE results, and is co-author of a companion paper that Isabelle Baconnais (University of Saskatoon) is writing about the neodymium budget of the Canadian Arctic Archipelago.

Besides, Dr Grenier volunteered for several outreach activities during the incoming phase of her project (public raising awareness events, general audience publication, MSCA - If-GF and GEOTRACES program promotions).
The geochemical, analytical, computational and modelling skills acquired during the outgoing phase, coupled to management skill development, allowed the implementation of new knowledge in the incoming laboratory that will benefit to appropriately locally set-up similar tracer analysis and modelling methods. In addition, this project allowed to reinforce the collaboration between the two laboratories (LEGOS, Toulouse, France, and EOAS, Vancouver, Canada) and fed ideas to jointly continue the investigation of new resulting questions.

The management of the GCP-GEOTARCTIC project was a personal and professional challenge Dr Grenier took up, from which resulted multiple valuable consequences for her future career prospects. First, she learned how to organize and manage a multiple year project, scientifically, technically, and financially. In addition to the global geochemical database enrichment it provided, this project allowed Dr Grenier to assess the strengths and weaknesses of her geochemical and computational tools, and to highlight and mature remaining and rising questions she would like to focus on in priority in the future. Notably, her work demonstrated the presence of significant fluxes of particles, either biogenic or lithogenic, either lateral or vertical, impacting the continental margin areas of all the basins studied in this project, as well as the Canadian Arctic Archipelago. Temporal and spatial variability of these fluxes have been observed, and question our understanding of the implied processes. A part of the variability is likely related to climate change, and notably to the enhanced sea-ice retreat that increases the particle concentrations and fluxes, dynamically through enhanced continental weathering, biologically through enhanced productivity.

However, some of the observed variability might also be related to unknown behaviour or properties of the tracers used in the project, or to some natural process variability. Dr Grenier identified process studies that could be performed in natural laboratories to investigate the temporal and spatial variability of the Arctic. She would like to maintain or strengthen the collaborations set up through the GCP-GEOTARCTIC project, notably with the Canadian universities, to further investigate the natural and anthropic variability of the modern Canadian Pacific, Arctic and Atlantic areas. She is currently writing multi-year research proposals to submit to competitive examinations.
Oral presentation of the project results at the 2017 ASLO Aquatic Sciences Meeting (Hawaii, USA)
Polar bear on thin ice
View from the upper deck of the CCGS Amundsen
Project presentation to the Chief Science Advisor of Canada, Dr Mona Nemer (June 2018)
On-board clean lab setting up; distinct stages of Fe coprecipitation and settling
Total procedural blank and seawater samples after a first siphonning, settling again