Tracking Arctic/Subarctic Export Production Variability Using Compound-Specific Isotope Analysis of Amino Acids

Abstract

The Arctic and subarctic seas have been experiencing dramatic reductions in sea ice extent as a result of climate change. Changes in sea ice extent and surface primary production are expected to impact the timing, composition, and quality of export production that supports deep-water benthic food webs, having cascading effects on energy transfer, carbon (C) cycling, and food web functioning in Arctic and subarctic ecosystems. Enhanced understanding of primary production, export production, and carbon sequestration in sea-ice influenced regions is key to predict future productivity and community responses to the changing climate. The traditional approach to interpret stable isotope values of bulk organic materials has been widely used to study organic matter sources and cycling in different ecosystems. However, bulk stable isotopes can be difficult to interpret due to variations in source signatures and effects of metabolic processes, trophic transfer, and microbial alteration. Stable isotope values of carbon and nitrogen (δ13C and δ15N) in amino acids (AAs) have been proven to provide specific information about C and N sources, shifts in nutrient sources, and physiological and heterotrophic processing of the organisms. Compound-specific isotope analysis (CSIA) of AAs has emerged as a powerful tool for tracing organic C and N in marine food webs, sinking particles, and ancient sediments, allowing for more accurate interpretation of stable isotope data. With the novel CSIA-AA techniques, this thesis investigates different components of the biological pump in the Canadian Arctic and subarctic seas: sea ice algae and pelagic algae, exported sinking particles, and archived marine sediments. Chapter 2 explores differences in δ13C-AA and δ15N-AA signatures between sea ice and pelagic algae collected in Canadian Arctic and subarctic seas. For the first time, distinct δ13C-EAA fingerprints and different degrees of heterotrophic reworking are revealed between sea ice and pelagic algae. These results highlight the potential of CSIA-AA to trace C sources from sea ice and pelagic origins and to evaluate the efficiency of the biological pump in polar marine environments. Building on these findings, Chapter 3 applies CSIA-AA proxies on a two-year time-series of exported sinking particles collected in sediment traps in the northwestern Labrador Sea to explore the sources and composition of organic C and N in sinking OM. I reveal that sea ice algae and exported zooplankton fecal pellets can be a critical source of C and N for benthic fauna. Chapter 4 reports the first coupled AA δ13C and δ15N sediment core records spanning most of the Holocene period in northeastern Baffin Bay. I demonstrate robust long-term preservation of CSIA-AA proxies despite noticeable alteration in AA concentrations. These results expand limited existing CISA-AA data on multi-millennial sediments from temperate seas to polar regions and highlight the potential of AAs as reliable tracers to provide novel information for longer-term paleo-reconstruction. Together, this thesis paves the way for the use of a newly developed biomarker for sea ice algae and other existing CSIA-AA proxies that are proven to be reliable OM tracers across different sedimentary regimes and time scales in ice-covered Arctic and subarctic seas, which is essential for predicting future responses of Arctic/subarctic ecosystems to long-term climate change.