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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - ICY-LAB (Isotope CYcling in the LABrador Sea)

Teaser

Summary

The high-latitude regions are experiencing some of the most rapid changes observed in recent decades: polar temperatures are rising twice as fast as the global mean and there are concerns about the impact of sea-ice and glacier retreat on global oceans and climate. The high-latitude North Atlantic is also a key region for ecologically and economically important natural resources such as fisheries. How these resources will change in the future depends strongly on the response of marine biogeochemical cycling of essential nutrients to increasing anthropogenic stress.
Diatoms are photosynthetic algae that are responsible for nearly half of the export of carbon from the sea surface to the seafloor, and they are a sensitive indication of the state of nutrient cycling. Diatoms are one of many organisms that precipitate biogenic opal, an amorphous glass made of silica, to form protective skeletons, and one of the essential nutrients is therefore dissolved silicon. The response of the silicon cycle to changing environmental conditions is critical for both carbon and nutrient cycling and it can now be addressed through high precision silicon isotopes, which is the focus of ICY-LAB.
The approach will be to capture the whole silicon cycle system in areas of marked environmental change using careful field sampling strategies - with research expeditions to coastal Greenland and the open ocean Labrador Sea - coupled with cutting-edge analytical methods. The results will lead to an unprecedented and cross-disciplinary view of nutrient cycling, biomineralisation, and the taxonomy and biogeography of siliceous organisms in an ecologically important region of the North Atlantic.
The overarching theme of ICY-LAB is to understand nutrient and isotope cycling in the climatically critical but understudied regions of the Labrador Sea and Greenland fjords, in two major Work Packages (WP) investigating: (WP1) the impact of glaciers and fjord processes, and the role of ocean circulation and marine biology, on nutrient cycling in the region, using systematically collected samples that will fully capture the Si cycle; (WP2) the biogeographical controls on sponge and diatom distribution in the Labrador Sea and Greenland fjords.

The specific objectives of WP1 are to:
1) Collect water samples in order to constrain the relative inputs of Si from glacial meltwater and different water masses in the study area;
2) Collect biological and particulate samples in order to constrain the role of particulate silica precipitation and dissolution on the silicon (Si) cycle in the study area;
3) Link the silicon cycle to high-latitude North Atlantic biogeochemistry using water and sedimentary column modelling.

The specific objectives of WP2 are to:
1) Collect and identify benthic organisms (focussing primarily on sponges) in order to carry out biogeographical analyses, and isolate suitable specimens for biomineralisation and natural product research;
2) Collect and identify planktonic organisms in order to carry out biogeographical and geochemical analyses.

Work performed

All our sampling objectives are now met, with additional samples collected in summer 2018 through collaboration with the Greenland Institute of Natural Resources to complement the samples collected during a major oceanic research expedition in summer 2017 on board the RRS Discovery [DY081, July 5th-August 8th 2017]. These samples included high resolution physical oceanographic sensor data, geochemical data from waters and sediments (WP1), biological specimens from the water column and sea-floor communities (WP2) and additional habitat data.

Sample analysis for WP1 is underway, with silicon isotope measurements complete for glacial input and oceanic sediment cycling studies, with one paper accepted for publication and another in preparation. All ancillary data (e.g. seawater nutrients, oxygen isotopes, carbonate chemistry) are completed, and a synthesis paper has been submitted and is currently under review. The new data have also been disseminated at several national and international meetings and conferences. Geochemical modelling for subglacial weathering complete, and underway for porewater analyses.

Sample analysis is also underway for WP2, with higher level sponge taxonomy complete. Detailed taxonomic work and isotopic analysis has been carried out on some of the sponge groups (with one paper submitted and currently under review, also presented at conferences), and is underway for the rest of the collection. Pigment and microbial analyses (for identification purposes) from the seawater samples are complete.

Final results

The new data from expedition DY081 reveal the impact of glacial meltwater on marine biogeochemical cycling of macronutrients and trace metals, and the impacts on sensitive marine ecosystems, from multiple different approaches (Hendry et al., under revision). Further work will elucidate the nature of these impacts further, including additional analysis of sensor data, further isotopic studies, and biogeographical research.

Specifically in terms of WP1, our findings show that subglacial weathering releases significant quantities of both dissolved silicon and reactive particulate silica, both of which have the potential to release nutrients for diatom growth (Hawkings et al., 2017). The isotopic composition of this silicon reveals, for the first time, information about the subglacial processes that are responsible for its release (Hawkings et al., 2018; Hatton et al., accepted). We also find that the Greenland marine shelf sediments release a significant amount of dissolved silicon, more than previously expected, and can use isotope geochemistry to understand the chemical changes active in the sediments (Ng et al., in preparation). Further analyses will investigate biogeochemical cycling within glacial fjords, the conduits between the glaciers themselves and the marine system, likely highlighting the important role played by these transition zones.

Addressing WP2, our results so far have highlighted new benthic species and new ranges of benthic organisms. Further taxonomic, genomics and biogeographical analyses will help to reveal the impact of meltwater, biogeochemistry and physics, on the distribution of seafloor-dwelling organisms. Our results also highlight the impact of meltwaters on the distribution and phytophysiology of planktonic organisms, including unexpected and previously undocumented adaptations of diatoms to these high-latitude environments (Hendry et al., under review).