Liliane Merlivat

Deuterium excess in marine water vapor: Dependency on relative humidity and surface wind speed during evaporation

We provide the first continuous measurements of isotopic composition (δD and δ18O) of water vapor over the subtropical Eastern North Atlantic Ocean from mid-August to mid-September 2012. The ship was located mostly around 26°N, 35°W where evaporation exceeded by far precipitation and water vapor at 20 m largely originated from surface evaporation. The only large deviations from that occurred during a 2 day period in the vicinity of a weak low-pressure system. The continuous measurements were used to investigate deuterium excess (d-excess) relation to evaporation. During 25 days d-excess was negatively correlated with relative humidity (r2 = 0.89). Moreover, d-excess estimated in an evaporative model with a closure assumption reproduced most of the observed variability. From these observations, the d-excess parameter seems to be a good indicator of evaporative conditions. We also conclude that in this region, d-excess into the marine boundary layer is less affected by mixing with the free troposphere than the isotopic composition. From our data, the transition from smooth to rough regime at the ocean surface is associated with a d-excess decrease of 5‰, which suggests the importance of the ocean surface roughness in controlling d-excess in this region.

The warm oceanic surface layer: Implications for CO2 fluxes and surface gas measurements

The ocean and air-sea interface are important for the exchange of heat, momentum, water vapour and carbon-dioxide. The details of the exchange mechanisms, which are often coupled and complex, have to be understood in order to assess the future role of the global oceans in climate change. Recently, much attention has focused on the thermal skin effect, cooling or warming of the uppermost millimeters of the sea surface, and its global implications for enhancing CO 2 uptake [Robertson and Watson, 1992 ; Van Scoy et al., 1995]. Routinely, air-sea flux estimates derived from oceanic pCO 2 measurements are corrected for the usually cooler thermal skin by applying a correction factor [Sarmiento and Sundquist, 1992 ; Wong et al., 1995]. Here we describe how near surface warming of the upper few meters of the ocean by solar radiation, routinely observed but not fully appreciated in this context, can significantly affect the net daily exchange of CO 2 over and above the skin effect, and can even lead to a reversal of the direction of the air-sea flux calculated from pCO 2 measurements and wind speed alone. The warming and cooling cycle produces a net asymmetry between CO 2 invasion and evasion, having the effect of decreasing CO 2 invasion and increasing CO 2 evasion.

The influence of short-term wind variability on air-sea CO2 exchange

Quantifying the regional and global exchange of CO2 between the ocean and atmosphere requires knowledge of the factors that affect CO2 gas transfer (e.g., wind speed) and the air-sea difference in partial pressure of CO2 (pCO2). A major uncertainty is the effect of short-term variability on air-sea CO2 flux. Using high sampling frequency wind speed and pCO2 data collected during deployments of the autonomous CARbon Interface OCean Atmosphere (CARIOCA) buoy, we compare CO2 fluxes at different sampling frequency of wind speed (i.e., hourly versus daily averaged). Air-sea CO2 flux was up to three times greater if high frequency wind data was used rather than daily average values. This difference arises from the non-linear relationship between wind speed and CO2 gas transfer coefficient, and a better representation of wind distribution at a higher frequency (i.e., hourly) of sampling. This finding has significant implications for determining regional and global air-sea CO2 fluxes, and understanding of the global carbon cycle.

A new optical sensor for PCO2 measurements in seawater

Abstract A knowledge of the distribution of the partial pressure of CO2 at the surface of the ocean is needed to quantify the absorption of atmospheric CO2 by gas exchange. Owing to the large space and time variability of this quantity at the surface of the ocean, an experimental approach aiming to make time-series measurements from unattended platforms must be envisaged. In this context we have developed a PCO2 sensor which meets the requirements for being installed on a buoy for approximately 1 year. The principle of the sensor is based on a colorimetric method. The selected dye is thymol blue. Its spectra and variation of its pK vs. temperature have been determined. A dedicated optical spectrophotometer has been built making absorbance measurements at three wavelengths. The sensor includes a sensitive volume enclosed with two silicone membranes and filled with thymol blue in an ionic medium. It is related to the spectrophotometer with silica fiber optics. Two calibration experiments were made over a fugacity of CO2 (fCO2) range extending from 340 μatm to 590 μatm. They lasted 3 and 4 days, respectively and showed that in the range 340–600 μatm a variation of ± 1 μatm is seen by the sensor and that the standard error of the fit of the two calibration curves is, respectively, ± 3 μatm and ±5 μatm.

Hydrogen isotope abundances in the solar system. Part I: Unequilibrated chondrites☆

Abstract Concentrations and isotopic compositions of H extracted by pyrolysis under a He flow were determined in three unequilibrated type 3 ordinary chondrites: Chainpur, Bishunpur and Ngawi. In Chainpur, a D-depleted component ( δD = −500‰ ) is removed at low temperature (200 to 400°C), whereas at temperatures ranging between 450°C and 900°C, a D-rich component is evolved with a bimodal release pattern. The isotopic composition patterns reported as a function of temperature are qualitatively similar in the three meteorites, with minimum and maximum values at 300–400°C and 700–900°C, respectively. Consequently, the bulk isotopic composition of each meteorite is interpreted as resulting from the mixing of two components, the low-δD fraction constituting 80 to 90% of the total H. The assumption that a carbonaceous chondrite-like polymer is the carrier of the D-rich H is supported by two observations: i) the D-rich H is released with bimodal patterns, and ii) oxidation of whole-rock samples with H2O2 removes this D-rich hydrogen (as well as a significant fraction of the D-depleted com ponent). For each meteorite the peak in H concentration associated with the marked increase in the δD pattern permits us to calculate an isotopic composition for the D-rich H. The calculated δD values never exceed ca. + 5500%. and no experimental indication of the presence of any D-richer H was found. Uncertainties on these determinations do not permit us to establish the presence of a unique D-rich component, common to these three meteorites. In Chainpur the low-δD hydrogen is concentrated in the amorphous matrix surrounding the chondrules, while the D-rich component is located inside the chondrules or at their surfaces.

Intercomparison of shipboard and moored CARIOCA buoy seawater fCO2 measurements in the Sargasso Sea

The ocean is an important sink for carbon and heat, yet high-resolution measurements of biogeochemical properties relevant to global climate change are being made only sporadically in the ocean at present. There is a growing need for automated, real-time, long-term measurements of CO in the ocean using a network of sensors, strategically placed on ships, 2 moorings, free-drifting buoys and autonomous remotely operated vehicles. The ground-truthing of new sensor technologies is a vital component of present and future efforts to monitor changes in the ocean carbon cycle and air-sea exchange of CO . 2 . A comparison of a moored Carbon Interface Ocean Atmosphere CARIOCA buoy and shipboard fugacity of CO 2

Variability of the net air-sea CO2 flux inferred from shipboard and satellite measurements in the Southern Ocean south of Tasmania and New Zealand

processes. For Chl > 0.37 mg m � 3 , pCO2 is negatively correlated with Chl, indicating that pCO2 variability is mostly controlled by carbon fixation by biological activity. We deduce fields of pCO2 and of air–sea CO2 fluxes from satellite parameters using pCO2-SST, pCO2-chlorophyll relationships and air–sea gas exchange coefficient, K, from satellite wind speed. We estimate an oceanic CO2 sink from December 1997 to December 1998 of � 0.08 GtC yr � 1 with an error of 0.03 GtC yr � 1 . This sink is approximately 38% smaller than that computed from the Takahashi et al. (2002) climatological distribution of DpCO2 for the 1995 year but with the same K (� 0.13 GtC yr � 1 ). When we correct ocean pCO2 for the interannual variability between 1995 and 1998, the difference is even larger, and we cannot reconcile both estimates in February–March and from June to November. This strengthens the need of new in situ measurements for validating extrapolation methods and for improving knowledge of interannual pCO2 variability.

The distribution of helium 3 in the deep western and southern Indian Ocean

Almost a decade after the Geochemical Ocean Sections Study Indian Expedition, the new deep 3He data from the INDIGO program give a further insight into the distribution of this tracer in the Indian Ocean. This distribution exhibits some major features related on one hand to a hydrothermal 3He input in the Gulf of Aden and on the Mid-Indian Ocean Ridge, and on the other to the origin of the water masses and to the characteristics of the deep circulation. The main pattern is a significant north-south 3He gradient, with deep waters of the southern ocean showing δ3He values around 8–9% due to the influence of the Atlantic deep waters poor in 3He and relatively high values in the northern and central regions (15% to 18% between 2000 m and 3000 m depth) originating from the hydrothermal activity. In the easternmost part of the basin, the 3He values exhibit a significant increase at shallower depths (around 1000 m) probably due to the Pacific water flow through the Indonesian sills, whereas the data in the Indian sector of the Antarctic ocean show a maximum of the order of 10%, south of the Polar Front, interpreted as showing the presence of the Pacific deep waters in the Antarctic Circumpolar Current. These different aspects are summarized by mapping the horizontal distribution of the δ3He maxima all over the Indian basin. This map points out some characteristics of the deep circulation but also stresses the need for further measurements in order to clarify the description of this tracer in several key areas.

The relevant time scales in estimating the air–sea CO2 exchange in a mid-latitude region

A 1D biogeochemical model simulating the nitrogen and carbon cycles is validated, using continuous observations obtained with the Carioca buoy (sea surface temperature (SST), pCO2; fluorescence) at the DYFAMED station (NW Mediterranean Sea) in 1995–1997, as well as other in situ data. Although the average surface pCO2 is generally slightly over-saturated (878matm), the NW Mediterranean Sea is a weak sink for atmospheric CO2 (0.1570.07 mol C m � 2 yr � 1 ), because the highest fluxes occur in winter and spring during the period of undersaturation when the winds are strong. The seasonal cycle is modulated by synoptic events (Mistral bursts), which can have a strong impact on the behaviour of the system and on the fluxes in winter and spring. While the atmospheric forcing constrains the annual balances and fluxes of tracers and CO2, the winter pre-conditioning of the bloom plays a major role in driving the interannual variability. Sensitivity analyses indicate that atmospheric forcing in this highly variable region should be averaged over periods of no longer than one week to simulate the biological and air–sea CO2 fluxes correctly. Also, because the model used is rather simple, this time scale is an upper limit. The sampling period must be no greater than a few days in order to estimate the CO2 fluxes with an error smaller than 20%. In the NW Mediterranean Sea, it is difficult to use ‘‘satellite proxies’’ (SST; sea colour) to estimate annual air–sea CO2 fluxes because SST does not vary much in winter and during the beginning of the bloom, while chlorophyll can either remain low (winter) or change rapidly (beginning of the bloom). At least several direct observations of pCO2 are needed during winter to define ‘‘initial conditions’’. r 2002 Published by Elsevier Science Ltd.

Tritium in the western Mediterranean Sea during 1981 Phycemed cruise

Abstract We report on simultaneous hydrological and tritium data taken in the western Mediterranean Sea during April 1981 and which implement our knowledge of the spatial and temporal variability of the convection process occurring in the Northern Basin (Gulf of Lion, Ligurian Sea). The renewal time of the deep waters in the Medoc area is calculated to be 11 ± 2 years using a box-model assymption. An important local phenomenon of “cascading” off the Ebro River near the Spanish coast is, noticeable by the use of tritium data. In the Sardinia Straits area tritium data indicate very active mixing between 100 and 500 m depth. The tritium subsurface maxima in Sardinia Straits suggests the influence of not only the Levantine Intermediate Water (LIW) but also an important shallower component. In waters deeper than 500m, an active mixing occurs between the deep water and the LIW via an intermediate water mass from the Tyrrhenian Sea by “salt-fingering”. Assuming a two end-member mixing. We determine the deep tritium content in the Sardinia Channel to be 1.8 TU. For comparison, the deep tritium content of the Northern Basin is equal to 1.3 TU. Tritium data relative to the Alboran Sea show that a layer of high tritium content persists all along its path from Sardifia to Gibraltar on a density surface shallower than the intermediate water. The homogeneity of the deep tritium concentrations between 1200 m depth and the bottom corroborate the upward “pumping” and westward circulation of deep waters along the continental slope of the North African Shelf. From the data measured in the Sardinia Straits and in the Alboran Sea, and upper limit of the deep advection rate of the order of 0.5 cm s−1 is estimated.