Net community production (NCP) and gross primary production (GPP) are two key processes in the marine carbon cycle which can be quantified in situ using triple oxygen isotopes (TOI) and continuous measurements of O2/Ar. Unfortunately, two key issues involved with measuring and interpreting TOI and O2/Ar data remain unresolved, namely, measurement uncertainty and spatial variability. A scientist from Woods Hole Oceanographic Institution plans to carry out an intercalibration effort between eight laboratories worldwide to address whether differences in TOI signatures observed between sample sites are results of real differences in the oceanic isotopic signature of oxygen or whether they are measurement artifacts. As part of the intercalibration, a suite of gas, homogenized water, equilibrated water, and seawater samples will be distributed to the laboratories to assess each step of the measurement process. In addition, the researcher will participate in a cruise of opportunity to examine the spatial variability in TOI and O2/Ar ratios and to determine the causes for this variability. To accomplish this goal, several grid surveys of continuous surface ocean O2/Ar measurements from an equilibrator inlet mass spectrometer (EIMS) and high resolution TOI sampling will be conducted. A video plankton recorder (VPR-II) will be towed on the cruise to continuously measure profiles of O2, plankton and seston abundance, temperature, salinity, fluorescence, photosynthetically available radiation, and turbidity in the upper 150 m of the water column. Various statistical estimators (co-spectra, wavelets, probability density functions) will be used to investigate linkages between hydrographic properties, biogeochemical properties, O2/Ar ratios, and TOI. This work, which will be the first to combine an EIMS and the VPR-II, will quantify spatial variability on scales from two to one thousand kilometers.
As regards broader Impacts, as a result of this intercalibration effort, tried and tested biogeochemical tracers for determining in situ gross primary productivity and net carbon productivity would be available to the marine sciences community. One graduate student would be supported and trained as part of this project.
Normal 0 false false false EN-US X-NONE X-NONE The ocean plays a key role in the global carbon cycle. About one-third of the CO2 emitted by fossil fuel burning is taken up by the ocean. Just as plants on land photosynthesizeand take up CO2 (hence the campaign to plant trees to offset CO2 emissions), photosynthetic algae in the ocean called phytoplankton take up CO2. Thus there is a lot of scientific interest in how much CO2 the phytoplankton take up, what factors this uptake depends on, how variable it is, and how might it change in the future. Many different tools are used to assess the amount of photosynthesis and the net amount of CO2 uptake by the phytoplankton. One techniques used is called gas tracers and consists of measurements of gases produced or consumed by photosynthesis and respiration (the reverse reaction to photosynthesis). In this work, I examined some key uncertainties in the application of gas tracers to studying photosynthesis and respiration. I also investigated small-scale spatial variability in rates of photosynthesis and respiration as determined by rates produced from gas tracers.Rates of photosynthesis and respiration are not uniform throughout the ocean. Rather there are patches of productivity separated by patches of lifeless waters. In this study, I examined the statistics of the patchiness of the net amount of photosynthesis and respiration and found that the patchiness in that term is similar to that of temperature. This suggests that physical processes may dominate variability in rates of photosynthesis and respiration at small spatial scales (<10 km).The patches of productivity can be seen in the surface water through applications of gas tracers determined from mass spectrometers onboard ships. They also can be seen in deeper waters (up to 140 m depth) by profiles of oxygen collected from an undulating instrument that is towed from the back of the research vessel. This instrument, called the video plankton recorder II (VPR-II) was outfitted with oxygen sensors as part of this research. The resulting high resolution oxygen profiles revealed exciting "hotspots" in biological productivity – small spots (several kilometer in horizontal direction, ~ten meters in vertical direction) with much lower oxygen and much higher fluorescence (a measure of productivity) than the surrounding water. I have developed a conceptual model of generation of these hotspots.Triple oxygen isotopes are a gas tracer system used for measuring rates of photosynthesis. Isotopes are atoms that have the same number of electrons but different atomic masses and thus have different rates of reaction. In the case of oxygen, there are three stable isotopes: O-16, O-17, and O-18. Processes that occur on the surface of the earth such as photosynthesis, produce oxygen with always the same ratio of O-16 to O-17 to O-18. In contrast, processes in the top layer of the atmosphere result in changes to this ratio. Some of this air with the anomalous ratio is mixed into the ocean. Thus the measurements of the ratio of O-16 to O-17 to O-18 in the ocean serve as a "made-in" tag describing if the oxygen in the surface ocean is produced from photosynthesis or if it is mixed in from the atmosphere. This "tag" can then be converted to a quantitative rate of photosynthesis.The triple oxygen technique is powerful for quantifying the rates of photosynthesis in the ocean. However, the measurements are technically challenging since the variations in the ratio of the isotopes is so small. Thus as part of this research, I led an international intercalibration study to ensure that all groups in the world measuring these isotopes were doing it in a correct and reproducible manner. The intercalibration study involved several parts so that we could determine what caused different labs to disagree. Thus we measured samples of water and gas prepared in the laboratory as well as samples from the open ocean. We found that generally the labs agreed very well but that one of the "short-cuts" that two of the groups was using lead to erroneous results. We also reached consensus on an important value – the value of the triple oxygen isotopic ratio in water that has been in contact with air but had no photosynthesis. In the past, there were disagreements in the community about that value.This study had a significant training component, providing research experience for a high school student, two undergraduates, and a graduate student. The high school student had her first lengthy experience in the lab and field as part of this work – she measured the triple oxygen isotopes in a local salt marsh, as that was a more accessible environment than the open ocean. One of the undergraduates and I developed a new application for triple oxygen isotopes - using them in inverted "bell-jars" in shallow environments to quantify how much productivity is happening on the seafloor.
