A New Instrument and Method for Nitrogen-loss Studies in Oxygen Deficient Zones

• Main Author: Reed, Andrew
• Other Authors: McNeil, Craig
• Format: Thesis
• Language: English
• Published: 2018
• Subjects: Denitrification; Dissolved gases; Gas Tension Device; Henry's Law; Nitrogen; Oxygen Minimum Zones; Chemical oceanography; Physical oceanography; Ocean engineering; Oceanography; Antarctic; Ross Sea; Adelie Land; Antarc*
• Online Access: http://hdl.handle.net/1773/41831

Thesis (Ph.D.)--University of Washington, 2018 The ocean’s biogeochemical cycles are coming under increasing stress due to global climate change and anthropogenic emissions of carbon dioxide. Three different factors are stressing the oceans: rising temperatures, acidification, and deoxygenation [Gruber 2011]. Rising temperatures are predicted to increase stratification and slow-down global ocean circulation. We examine the change in the rates of formation and characteristics of Antarctic Bottom Water (AABW) in the Australia-Antarctic Basin from repeat hydrographic sections. Using changes in CFC-11 and CFC-12 concentrations, we find that AABW formation rates corrected for seasonality decrease by approximately 20%, from 0.38 ± 0.04 m2 s−1 in 1991 to 0.30 ± 0.02 m2 s−1 in 2008. Additionally, we find the sampled AABW warmed and increased in salinity, likely due to the increasing influence of bottom water formed in the Ross Sea over fresh, cold waters formed offshore of Adelie Land. Oceanic deoxygenation has the potential to significantly alter the global nitrogen cycle due to potential expansion of Oxygen Deficient Zones (ODZs), which are regions of the pelagic ocean where oxygen concentrations are nearly or functionally zero. In ODZs, microbes utilize biologically-available fixed nitrogen to either respire organic matter (denitrification) or fix new organic matter (anammox), converting the fixed nitrogen into N2-gas. An alternative method to the traditional method of measuring dissolved N2-gas by N2:Ar mass spectrometry is using in-situ measurements of total dissolved gas pressure (gas tension) using a gas tension device (GTD). We designed and characterized a new GTD which uses a custom designed small diameter (4 cm) thin (130 µm) incompressible composite Teflon-AF 2400 membrane. The new GTD eliminates issues of hydrostatic pressure-generated transients, changes to response times, and reverse osmosis, which plagued existing versions of GTDs using a compressible polydimethylsiloxane (PDMS) membrane. We demonstrate ...