Award: OCE-1434417

In light of the rapid changes in the Arctic ecosystem, it is now imperative to characterize its biogeochemistry and establish a baseline from which to monitor future changes. Decreasing sea ice cover and longer ice-free seasons are giving rise to improved conditions for marine plant growth. With more light penetrating the sea surface, availability of the nutrient nitrogen will exert increasing influence on the fertility of the Arctic Ocean and ultimately on carbon dioxide in the atmosphere. However negative effects could occur as increased plant production could also give rise to increased losses of nitrogen nutrients from continental shelves and slopes as a consequence of a process called denitrification which produces biologically unavailable N2 gas.  Greater organic matter input to sediments is known to favor this process.  Increased denitrification could, paradoxically, lead to a consequent reduction in overall plant production due to nitrogen scarcity.

The relative scarceness of studies addressing Arctic N biogeochemistry may be partly due to difficulty of access. The inherent spatial and temporal variability of biologically mediated fluxes compounds this problem with respect to understanding Arctic N-cycling; Shipboard ‘direct’ rate measurements, which are time-consuming and labor-intensive, typically under-sample and cannot accurately estimate integrated fluxes. To remedy this, this project pursued geochemical approaches utilizing dissolved gas ratios, which integrate over the spatial and temporal variability of biological N transformations, as they bear the imprint of denitrification.The primary goal of this project was to develop a first order understanding of denitrification’s importance to the regional nitrogen budget, examine the importance of the Chukchi shelf/slope in this respect, and investigate the water mass connections using geochemical signals to Canada and Makarov Basins of the Western Arctic Ocean.

During the US Arctic GeoTraces cruise, samples were collected for laboratory based dissolved gas analyses however a significant number of samples appear to have been compromised.  We attribute this problem in part to harsh conditions during filling of sample bottles as any accidental inclusion of even very small gas bottles adulterates the results. To remedy this, we took advantage of another sampling opportunity in later summer of 2017 and an experienced gas sampler from our group participated in a cruise of the RV Healy to the Bering and Chukchi Seas.  Physical inspection of these samples upon their return to our lab was promising with respect to their integrity.  

Specific objectives has been to produce a detailed sections of biologically produced N2 gas dissolved in seawater between the Bering Strait and the North Pole.  These data are then compared to another measure of denitrification (N deficit) determined from seawater nitrate and phosphate concentrations.  Large N deficits are known for the Pacific Inflow across the Bering Strait into the Arctic ocean and has been hypothesized to be a geochemical signature of high denitrification in sediments of the Chukchi shelf and slope.  The new samples collected on the RV Healy were from locations that were well suited to meet these objectives with respect to the highest N deficit waters found at intermediate depth above the Chukchi slope.

We found relatively large concentrations of biologically produced N2 gas at about 200 m over the Chukchi Slope, the same depth as a maximum N deficit and a minimum in O2.   We conclude that the observed N deficit in the Western Arctic Ocean is indeed produced by denitrification. As water column O2 gas concentration is not low enough to support denitrification, it is sensible to assume that Chukchi shelf and slope sediments are responsible.  We do have direct evidence from the minima in O2 gas concentration co-occurring with maxima in biologically produced N2 and N deficit.  The variations in oxygen isotope ratio support that most of the respiration that produced this O2 minima must have occurred in sediment.   These data further support biogeochemical impacts in the deep Arctic Ocean mostly associated with prior contact with shelf and slope sediments.

 

 

Last Modified: 09/26/2019
Modified by: Mark A Altabet