Investigating the source and fate of organic nitrogen in Arctic marine sediments using a novel multi-proxy approach

Abstract

Nitrogen plays a critical role in controlling marine primary productivity and what is subsequently buried in the sediments, and is becoming increasingly important across the Arctic Ocean as sea ice melts and light availability is no longer the dominant control. Changes in the nitrogen source to primary producers alter the isoscape of a region and subsequently what is buried in marine sediments. It is crucial we understand these alterations of the isoscape and how these might change in the future as the Arctic continues to change in response to climate warming. Here, as part of the ARISE project we focus on two unique areas of the Arctic, the East Siberian Arctic Shelf (ESAS) and the Barents Sea and Fram Strait region. Marine surface sediments of both regions were analysed using a multi-proxy approach involving elemental composition (carbon and nitrogen), bulk stable isotopes (13C and 15N), macromolecular analyses (Py-GCMS; pyrolysis gas chromatography mass spectrometry), amino acid composition (using GC-FID; gas chromatography flame ionisation detection) and compound specific isotope (15N) analysis (GC-IRMS; gas chromatography isotope ratio mass spectrometry). Analysis of the ESAS sediments indicate a dominance of terrestrially derived OM in the nearshore areas and distinct terrestrial to marine transects in an offshore direction across all proxies used. The 15N at both a bulk and compound specific (amino acid) level differed between the different nearshore regions of the ESAS indicating a qualitative difference in the terrestrial nitrogen delivered to these areas. This correlation persisted offshore suggesting the nearshore patterns impact offshore sediments. In addition, the link between bulk and amino acid analyses confirms that inorganic nitrogen is not the primary cause of the near shore differences observed at bulk level. In the Barents Sea and Fram Strait regions the major controls on sedimentary organic matter composition were the relative position of the sample to the polar front, marginal ice zone and/or terrestrial inputs. Unlike the ESAS region, inorganic nitrogen produced a de-coupling between bulk and amino acid 15N, highlighting the limitations of using bulk analyses in this region. The baseline 15N varied across the region depending on the major controls above and in some areas whether nitrate was limited in the water column. Collectively, this study shows that baseline values in the isoscape differ across the Arctic as well as within regions, highlighting the need for each region to be studied as individual areas. In addition, as these regions change in response to climate change it will be vital to have a well constrained baseline for accurate assessments of sedimentary organic matter compositions and the quality of organic matter delivered from various sources. This project forms a vital contribution to the ARISE projects first effort in producing these baseline values and trying to understand what underpins these changes.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stephen Boult (Supervisor) & Bart Van Dongen (Supervisor)