Showing papers by "Jérôme Chappellaz published in 2020"

Abrupt CO2 release to the atmosphere under glacial and early interglacial climate conditions.

Abstract: Pulse-like carbon dioxide release to the atmosphere on centennial time scales has only been identified for the most recent glacial and deglacial periods and is thought to be absent during warmer climate conditions. Here, we present a high-resolution carbon dioxide record from 330,000 to 450,000 years before present, revealing pronounced carbon dioxide jumps (CDJ) under cold and warm climate conditions. CDJ come in two varieties that we attribute to invigoration or weakening of the Atlantic meridional overturning circulation (AMOC) and associated northward and southward shifts of the intertropical convergence zone, respectively. We find that CDJ are pervasive features of the carbon cycle that can occur during interglacial climate conditions if land ice masses are sufficiently extended to be able to disturb the AMOC by freshwater input.

Millennial-scale atmospheric CO 2 variations during the Marine Isotope Stage 6 period (190–135 ka)

Abstract: . Using new and previously published CO2 data from the EPICA Dome C ice core (EDC), we reconstruct a new high-resolution record of atmospheric CO2 during Marine Isotope Stage (MIS) 6 (190 to 135 ka) the penultimate glacial period. Similar to the last glacial cycle, where high-resolution data already exists, our record shows that during longer North Atlantic (NA) stadials, millennial CO2 variations during MIS 6 are clearly coincident with the bipolar seesaw signal in the Antarctic temperature record. However, during one short stadial in the NA, atmospheric CO2 variation is small ( ∼5 ppm) and the relationship between temperature variations in EDC and atmospheric CO2 is unclear. The magnitude of CO2 increase during Carbon Dioxide Maxima (CDM) is closely related to the NA stadial duration in both MIS 6 and MIS 3 (60–27 ka). This observation implies that during the last two glacials the overall bipolar seesaw coupling of climate and atmospheric CO2 operated similarly. In addition, similar to the last glacial period, CDM during the earliest MIS 6 show different lags with respect to the corresponding abrupt CH4 rises, the latter reflecting rapid warming in the Northern Hemisphere (NH). During MIS 6i at around 181.5± 0.3 ka, CDM 6i lags the abrupt warming in the NH by only 240± 320 years. However, during CDM 6iv ( 171.1± 0.2 ka) and CDM 6iii ( 175.4± 0.4 ka) the lag is much longer: 1290± 540 years on average. We speculate that the size of this lag may be related to a larger expansion of carbon-rich, southern-sourced waters into the Northern Hemisphere in MIS 6, providing a larger carbon reservoir that requires more time to be depleted.

Continuous In Situ Measurement of Dissolved Methane in Lake Kivu Using a Membrane Inlet Laser Spectrometer

Abstract: . We report the first high-resolution continuous profile of dissolved methane in the shallow water of Lake Kivu, Rwanda. The measurements were performed using an in situ dissolved gas sensor, called Sub-Ocean, based on a patented membrane-based extraction technique coupled with a highly sensitive optical spectrometer. The sensor was originally designed for ocean settings, but both the spectrometer and the extraction system were modified to extend the dynamical range up to 6 orders of magnitude with respect to the original prototype (from nmol L −1 to mmol L −1 detection) to fit the range of concentrations at Lake Kivu. The accuracy of the instrument was estimated to ±22 % ( 2σ ) from the standard deviation of eight profiles at 80 m depth, corresponding to ±0.112 mbar of CH4 in water or ±160 nmol L −1 at 25 ∘ C and 1 atm. The instrument was able to continuously profile the top 150 m of the water column within only 25 min. The maximum observed mixing ratio of CH4 in the gas phase concentration was 77 %, which at 150 m depth and under thermal conditions of the lake corresponds to 3.5 mmol L −1 . Deeper down, dissolved CH4 concentrations were too large for the methane absorption spectrum to be correctly retrieved. Results are in good agreement with discrete in situ measurements conducted with the commercial HydroC® sensor. This fast-profiling feature is highly useful for studying the transport, production and consumption of CH4 and other dissolved gases in aquatic systems. While the sensor is well adapted for investigating most environments with a concentration of CH4 up to a few millimoles per liter, in the future the spectrometer could be replaced with a less sensitive analytical technique possibly including simultaneous detection of dissolved CO2 and total dissolved gas pressure, for exploring settings with very high concentrations of CH4 such as the bottom waters of Lake Kivu.

Estimation of gas record alteration in very low accumulation ice cores

Abstract: . We measured the methane mixing ratios of enclosed air in five ice core sections drilled on the East Antarctic Plateau. Our work aims to study two effects that alter the recorded gas concentrations in ice cores: layered gas trapping artifacts and firn smoothing. Layered gas trapping artifacts are due to the heterogeneous nature of polar firn, where some strata might close early and trap abnormally old gases that appear as spurious values during measurements. The smoothing is due to the combined effects of diffusive mixing in the firn and the progressive closure of bubbles at the bottom of the firn. Consequently, the gases trapped in a given ice layer span a distribution of ages. This means that the gas concentration in an ice layer is the average value over a certain period of time, which removes the fast variability from the record. Here, we focus on the study of East Antarctic Plateau ice cores, as these low-accumulation ice cores are particularly affected by both layering and smoothing. We use high-resolution methane data to test a simple trapping model reproducing the layered gas trapping artifacts for different accumulation conditions typical of the East Antarctic Plateau. We also use the high-resolution methane measurements to estimate the gas age distributions of the enclosed air in the five newly measured ice core sections. It appears that for accumulations below 2 cm ice equivalent yr−1 the gas records experience nearly the same degree of smoothing. We therefore propose to use a single gas age distribution to represent the firn smoothing observed in the glacial ice cores of the East Antarctic Plateau. Finally, we used the layered gas trapping model and the estimation of glacial firn smoothing to quantify their potential impacts on a hypothetical 1.5-million-year-old ice core from the East Antarctic Plateau. Our results indicate that layering artifacts are no longer individually resolved in the case of very thinned ice near the bedrock. They nonetheless contribute to slight biases of the measured signal (less than 10 ppbv and 0.5 ppmv in the case of methane using our currently established continuous CH4 analysis and carbon dioxide, respectively). However, these biases are small compared to the dampening experienced by the record due to firn smoothing.

Le Méthane et le destin de la Terre : Les Hydrates de méthane : rêve ou cauchemar?

Abstract: Les hydrates de methane representent une phase solide constituee de glace et de methane: elles constituent sur terre plusieurs dizaines de milliers de milliards de tonnes de gaz, ce qui represente un tresor energetique inoui, ... et devoile un danger potentiel pour l'humanite, aussi bien sur le plan climatologique que geologique... Cet ouvrage scientifique, ecrit par quatre experts, fait le point sur ce phenomene qui interesse les plus hautes autorites mondiales. L'ouvrage presente ainsi: -definition et proprietes des clathrates -les HM en milieu oceanique -les HM du Permafrost -Rappels sur l'effet de serre -Le cycle du methane -les cycles glaciaires-interglaciaires -le role du Methane dans l'histoire de la Terre -Les HD, source potentielle d'energie.

Antarctic air bubbles and the long-term ice core record of CO2 and other greenhouse gases

Abstract: Nature has been continuously sampling the atmosphere at the surface of Antarctica throughout the ages. Atmospheric gases trapped in Antarctic ice provide the most direct record of changes in greenhouse gas levels during the past 800,000 years. The best-documented and reliable trace-gas records are for CO2 and CH4, and Antarctic ice is the key player in recording past atmospheric CO2. They are archives of the past and a window to the present and future of the interplay between greenhouse gases and climate. We discuss the pioneering work of glaciologists measuring CO2 in the air extracted from Antarctic ice, which confirmed Arrhenius’ prediction about the role of atmospheric carbon dioxide in ice age climate. We detail here how the ice core record has been progressively extended to four and then to eight glacial-interglacial cycles (i.e., over the last 800,000 years). The Antarctic ice record highlights the tight coupling of atmospheric CO2 and Antarctic climate on the timescales of glacial-interglacial cycles for the entire 800,000-year interval. This close linkage suggests that glacial-interglacial variations of CO2 explain a large fraction of glacial-interglacial climate changes observed in the Antarctic ice record, which is consistent with modeling results. We present more recent works showing a near synchronous phasing between Antarctic temperature and CO2 during the last deglaciation and pinpointing the important role of oceanic circulation in both heat transport and CO2 outgassing. We also briefly explore the prospect for investigating Antarctic ice older than 1 million years to document what is often called the enigma of the Mid-Pleistocene Transition around 1 million years ago.