Non-Equilibrium Fractionation Factors for D/H and 18O/16O During Oceanic Evaporation in the North-West Atlantic Region

Ocean isotopic evaporation models, such as the Craig-Gordon model, rely on the description of nonequilibrium fractionation factors that are, in general, poorly constrained. To date, only a few gradient-diffusion type measurements have been performed in ocean settings to test the validity of the commonly used parametrization of nonequilibrium isotopic fractionation during ocean evaporation. In this work, we present 6 months of water vapor isotopic observations collected from a meteorological tower located in the northwest Atlantic Ocean (Bermuda) with the objective of estimating nonequilibrium fractionation factors (k, ‰) for ocean evaporation and their wind speed dependency. The Keeling Plot method and Craig-Gordon model combination were sensitive enough to resolve nonequilibrium fractionation factors during evaporation resulting into mean values of k18 = 5.2 ± 0.6‰ and k2 = 4.3 ± 3.4‰. Furthermore, we evaluate the relationship between k and 10-m wind speed over the ocean. Such a relationship is expected from current evaporation theory and from laboratory experiments made in the 1970s, but observational evidence is lacking. We show that (a) in the observed wind speed range [0–10 m s−1], the sensitivity of k to wind speed is small, in the order of −0.2‰ m−1 s for k18, and (b) there is no empirical evidence for the presence of a discontinuity between smooth and rough wind speed regime during isotopic fractionation, as proposed in earlier studies. The water vapor d-excess variability predicted under the closure assumption using the k values estimated in this study is in agreement with observations over the Atlantic Ocean.publishedVersio

Inter-comparison of salt effect correction for δ 18 O and δ 2 H measurements in seawater by CRDS and IRMS using the gas-H 2 O equilibration method

The isotope composition of seawater is an efficient method for detecting mixing between water masses. To measure long term or large scale hydrological processes at the ocean surface, it is necessary to be able to precisely compare datasets produced by different laboratories. The oxygen and hydrogen isotope (δ18O and δ2H) composition of marine waters can be measured using isotope ratio mass spectrometry (IRMS) and near-infrared laser absorption spectroscopy (LS) techniques. The IRMS and equilibration method is thought to provide results on the activity scale, while LS provides results on the concentration scale. However, the effect of dissolved seawater salts on the measurement is not sufficiently assessed and seems sometimes contradictory in the literature. For this purpose, we made artificial seawater and a pure NaCl solution from a freshwater of known isotope composition. The solutions were measured by four different laboratories allowing us to compare the two techniques. We show that minor corrections are necessary to correct seawater measurements for the salt effect and report them on the concentration scale. Interestingly, seawater measurements using LS (type Picarro) coupled to a liner are not on the concentration scale and require a correction of ~ 0.09‰ for δ18O, while the correction is relatively less significant for δ2H (~ 0.13‰). Moreover, we found for IRMS measurements that the salt effect can differ between different laboratories but seems reproducible for a given laboratory. A natural sea water sample was then analyzed by the different laboratories participating in the study. We found that applying the corrections increases the reproducibility of the isotope measurement significantly, with inter-laboratory standard deviation decreasing from 0.06 to 0.02‰ and 0.55 to 0.23‰ for δ18O and δ2H, respectively. Thus, comparing sea water datasets produced in different laboratories requires that each laboratory carries out its own calibration with artificial seawater and presents measurements on the concentration scale

Non-Equilibrium Fractionation Factors for D/H and 18O/16O During Oceanic Evaporation in the North-West Atlantic Region

Ocean isotopic evaporation models, such as the Craig-Gordon model, rely on the description of nonequilibrium fractionation factors that are, in general, poorly constrained. To date, only a few gradient-diffusion type measurements have been performed in ocean settings to test the validity of the commonly used parametrization of nonequilibrium isotopic fractionation during ocean evaporation. In this work, we present 6 months of water vapor isotopic observations collected from a meteorological tower located in the northwest Atlantic Ocean (Bermuda) with the objective of estimating nonequilibrium fractionation factors (k, ‰) for ocean evaporation and their wind speed dependency. The Keeling Plot method and Craig-Gordon model combination were sensitive enough to resolve nonequilibrium fractionation factors during evaporation resulting into mean values of k18 = 5.2 ± 0.6‰ and k2 = 4.3 ± 3.4‰. Furthermore, we evaluate the relationship between k and 10-m wind speed over the ocean. Such a relationship is expected from current evaporation theory and from laboratory experiments made in the 1970s, but observational evidence is lacking. We show that (a) in the observed wind speed range [0–10 m s−1], the sensitivity of k to wind speed is small, in the order of −0.2‰ m−1 s for k18, and (b) there is no empirical evidence for the presence of a discontinuity between smooth and rough wind speed regime during isotopic fractionation, as proposed in earlier studies. The water vapor d-excess variability predicted under the closure assumption using the k values estimated in this study is in agreement with observations over the Atlantic Ocean

High-resolution record of Northern Hemisphere climate extending into the last interglacial period

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