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Hamme, RC, Emerson SR, Severinghaus JP, Long MC, Yashayaev I.  2017.  Using noble gas measurements to derive air-sea process information and predict physical gas saturations. Geophysical Research Letters. 44:9901-9909.   10.1002/2017gl075123   AbstractWebsite

Dissolved gas distributions are important because they influence oceanic habitats and Earth's climate, yet competing controls by biology and physics make gas distributions challenging to predict. Bubble-mediated gas exchange, temperature change, and varying atmospheric pressure all push gases away from equilibrium. Here we use new noble gas measurements from the Labrador Sea to demonstrate a technique to quantify physical processes. Our analysis shows that water-mass formation can be represented by a quasi steady state in which bubble fluxes and cooling push gases away from equilibrium balanced by diffusive gas exchange forcing gases toward equilibrium. We quantify the rates of these physical processes from our measurements, allowing direct comparison to gas exchange parameterizations, and predict the physically driven saturation of other gases. This technique produces predictions that reasonably match N-2/Ar observations and demonstrates that physical processes should force SF6 to be approximate to 6% more supersaturated than CFC-11 and CFC-12, impacting ventilation age calculations. Plain Language Summary Gases dissolved in the ocean are important because they influence oceanic habitats and Earth's climate. Physics and biology combine to control the amounts of gases like carbon dioxide, oxygen, and nitrogen in the ocean. Our research seeks to disentangle and quantify the competing effects of physics and biology on dissolved gases. We present very precise measurements of dissolved noble gas concentrations (neon, argon, and krypton) in the Labrador Sea, one of the few places on Earth where the surface and deep ocean communicate with each other. Because noble gases have no biological function, responding only to physical processes in the ocean, we use these measurements to discover the amounts of physical processes that affect gases during the winter at this site, like rapid cooling of the water or bubbles injected by breaking waves. From these amounts of physical processes, we calculate the concentrations of nitrogen and chlorofluorocarbons if only physical processes affected these gases. Our work will allow oceanographers to better estimate the rate that bioavailable nutrients are being removed from the ocean (a process that biologically creates nitrogen gas) and to better determine how the ocean moves from observations of changing chlorofluorocarbons in the ocean.

Hamme, RC, Severinghaus JP.  2007.  Trace gas disequilibria during deep-water formation. Deep-Sea Research Part I-Oceanographic Research Papers. 54:939-950.   10.1016/j.dsr.2007.03.008   AbstractWebsite

We present high-precision measurements by a new isotope dilution technique of a suite of inert gases in the North Pacific. Remarkably smooth gradients in Ar, Kr and Xe from near equilibrium in intermediate waters to several percent undersaturated in deep waters were observed. The general pattern in the deepest waters was that Ar, Kr and Xe were undersaturated (Ar least and Xe most), while N-2 was close to equilibrium, and Ne was supersaturated. We propose that this pattern was produced by the interaction between the different physical properties of the gases (solubility and the temperature dependence of solubility) with the rapid cooling and high wind speeds that characterize deep-water formation regions. In a simple model of deep-water formation by convection, the saturations of the more temperature-sensitive gases were quickly driven down by rapid cooling and could not reequilibrate with the atmosphere before the end of the winter. In contrast, the gas exchange rate of the more bubble-sensitive gases (Ne and N-2) was able to meet or exceed the drawdown by cooling. Our simple convection model demonstrates that the heavier noble gases (Ar, Kr and Xe) are sensitive on seasonal timescales to the competing effects of cooling and air-sea gas exchange that are also important to setting the concentration Of CO2 in newly formed waters. (c) 2007 Elsevier Ltd. All rights reserved.

Headly, MA, Severinghaus JP.  2007.  A method to measure Kr/N-2 ratios in air bubbles trapped in ice cores and its application in reconstructing past mean ocean temperature. Journal of Geophysical Research-Atmospheres. 112   10.1029/2006jd008317   AbstractWebsite

We describe a new method for precise measurement of Kr/N-2 ratios in air bubbles trapped in ice cores and the first reconstruction of atmospheric Kr/N-2 during the last glacial maximum (LGM) similar to 20,000 years ago. After gravitational correction, the Kr/N-2 record in ice cores should represent the atmospheric ratio, which in turn should reflect past ocean temperature change due to the dependence of gas solubility on temperature. The increase in krypton inventory in the glacial ocean due to higher gas solubility in colder water causes a decrease in the atmospheric inventory of krypton. Assuming Kr and N-2 inventories in the ocean-atmosphere system are conserved, we use a mass balance model to estimate a mean ocean temperature change between the LGM and today. We measured Kr/N-2 in air bubbles in Greenland (GISP2) ice from the late Holocene and LGM, using the present atmosphere as a standard. The late Holocene delta Kr/N-2 means from two sets of measurements are not different from zero (+0.07 +/- 0.30 parts per thousand and -0.14 +/- 0.93 parts per thousand), as expected from the relatively constant climate of the last millennium. The mean delta Kr/N-2 in air bubbles from the LGM is -1.34 +/- 0.37 parts per thousand. Using the mass balance model, we estimate that the mean temperature change between the LGM ocean and today's ocean was 2.7 +/- 0.6 degrees C. Although this error is large compared to the observed change, this finding is consistent with most previous estimates of LGM deep ocean temperature based on foraminiferal delta O-18 and sediment pore water delta O-18 and chlorinity.

Huber, C, Beyerle U, Leuenberger M, Schwander J, Kipfer R, Spahni R, Severinghaus JP, Weiler K.  2006.  Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements. Earth and Planetary Science Letters. 243:61-73.   10.1016/j.epsl.2005.12.036   AbstractWebsite

We present elemental and isotopic measurements of noble gases (He, Ne, Ar, Kr, and Xe), oxygen and nitrogen of firn air from two sites. The first set of samples was taken in 1998 at the summit of the Devon Ice Cap in the eastern part of Devon Island. The second set was taken in 2001 at NGRIP location (North Greenland). He and Ne are heavily enriched relative to Ar with respect to the atmosphere in the air near the close-off depth at around 50-70 in. The enrichment increases with depth and reaches the maximum value in the deepest samples just above the zone of impermeable ice where no free air could be extracted anymore. Similarly, elemental ratios of O(2)/N(2), O(2)/Ar and Ar/N(2) are increasing with depth. In contrast but in line with expectations, isotopic ratios of (15)N/(14)N, (18)O/(16)O, and (36)Ar/(40)Ar show no significant enrichment near the close-off depth. The observed isotopic ratios in the firn air column can be explained within the uncertainty ranges by the well-known processes of gravitational enrichment and thermal diffusion. To explain the elemental ratios, however, an additional fractionation process during bubble inclusion has to be considered. We implemented this additional process into our firn air model. The fractionation factors were found by fitting model profiles to the data. We found a very similar close-off fractionation behavior for the different molecules at both sites. For smaller gas species (mainly He and Ne) the fractionation factors are linearly correlated to the molecule size, whereas for diameters greater than about 3.6 A the fractionation seems to be significantly smaller or even negligible. An explanation for this size dependent fractionation process could be gas diffusion through the ice lattice. At Devon Island the enrichment at the bottom of the firn air column is about four times higher compared to NGRIP. We explain this by lower firn diffusivity at Devon Island, most probably due to melt layers, resulting in significantly reduced back diffusion of the excess gas near the close-off depth. The results of this study considerably increase the understanding of the processes occurring during air bubble inclusion near the close-off depth in firn and can help to improve the interpretation of direct firn air measurements, as well as air bubble measurements in ice cores, which are used in numerous studies as paleo proxies. (c) 2006 Elsevier B.V. All rights reserved.