Amaelle Landais

One-to-one coupling of glacial climate variability in Greenland and Antarctica.

Precise knowledge of the phase relationship between climate changes in the two hemispheres is a key for understanding the Earth’s climate dynamics. For the last glacial period, ice core studies have revealed strong coupling of the largest millennial-scale warm events in Antarctica with the longest Dansgaard–Oeschger events in Greenland through the Atlantic meridional overturning circulation. It has been unclear, however, whether the shorter Dansgaard–Oeschger events have counterparts in the shorter and less prominent Antarctic temperature variations, and whether these events are linked by the same mechanism. Here we present a glacial climate record derived from an ice core from Dronning Maud Land, Antarctica, which represents South Atlantic climate at a resolution comparable with the Greenland ice core records. After methane synchronization with an ice core from North Greenland, the oxygen isotope record from the Dronning Maud Land ice core shows a one-to-one coupling between all Antarctic warm events and Greenland Dansgaard–Oeschger events by the bipolar seesaw6. The amplitude of the Antarctic warm events is found to be linearly dependent on the duration of the concurrent stadial in the North, suggesting that they all result from a similar reduction in the meridional overturning circulation.

A Review of Antarctic Surface Snow Isotopic Composition: Observations, Atmospheric Circulation, and Isotopic Modeling*

A database of surface Antarctic snow isotopic composition is constructed using available measurements, with an estimate of data quality and local variability. Although more than 1000 locations are documented, the spatial coverage remains uneven with a majority of sites located in specific areas of East Antarctica. The database is used to analyze the spatial variations in snow isotopic composition with respect to geographical characteristics (elevation, distance to the coast) and climatic features (temperature, accumulation) and with a focus on deuterium excess. The capacity of theoretical isotopic, regional, and general circulation atmospheric models (including “isotopic” models) to reproduce the observed features and assess the role of moisture advection in spatial deuterium excess fluctuations is analyzed.

Interglacials of the last 800,000 years

Interglacials, including the present (Holocene) period, are warm, low land ice extent (high sea level), end-members of glacial cycles. Based on a sea level definition, we identify eleven interglacials in the last 800,000 years, a result that is robust to alternative definitions. Data compilations suggest that despite spatial heterogeneity, Marine Isotope Stages (MIS) 5e (last interglacial) and 11c (~400 ka ago) were globally strong (warm), while MIS 13a (~500 ka ago) was cool at many locations. A step change in strength of interglacials at 450 ka is apparent only in atmospheric CO2 and in Antarctic and deep ocean temperature. The onset of an interglacial (glacial termination) seems to require a reducing precession parameter (increasing Northern Hemisphere summer insolation), but this condition alone is insufficient. Terminations involve rapid, nonlinear, reactions of ice volume, CO2, and temperature to external astronomical forcing. The precise timing of events may be modulated by millennial-scale climate change that can lead to a contrasting timing of maximum interglacial intensity in each hemisphere. A variety of temporal trends is observed, such that maxima in the main records are observed either early or late in different interglacials. The end of an interglacial (glacial inception) is a slower process involving a global sequence of changes. Interglacials have been typically 10–30 ka long. The combination of minimal reduction in northern summer insolation over the next few orbital cycles, owing to low eccentricity, and high atmospheric greenhouse gas concentrations implies that the next glacial inception is many tens of millennia in the future.

Abrupt change of Antarctic moisture origin at the end of Termination II

The deuterium excess of polar ice cores documents past changes in evaporation conditions and moisture origin. New data obtained from the European Project for Ice Coring in Antarctica Dome C East Antarctic ice core provide new insights on the sequence of events involved in Termination II, the transition between the penultimate glacial and interglacial periods. This termination is marked by a north–south seesaw behavior, with first a slow methane concentration rise associated with a strong Antarctic temperature warming and a slow deuterium excess rise. This first step is followed by an abrupt north Atlantic warming, an abrupt resumption of the East Asian summer monsoon, a sharp methane rise, and a CO2 overshoot, which coincide within dating uncertainties with the end of Antarctic optimum. Here, we show that this second phase is marked by a very sharp Dome C centennial deuterium excess rise, revealing abrupt reorganization of atmospheric circulation in the southern Indian Ocean sector.

Chronology reconstruction for the disturbed bottom section of the GISP2 and the GRIP ice cores: Implications for Termination II in Greenland

and d 18 Oatm in the trapped gases. Our reconstructed ages for basal ice samples are based on comparison of published measurements of CH4 and d 18 Oatm from the disturbed section of the GRIP and GISP2 cores with the same properties in the Vostok ice core. NGRIP d 18 Oice values are also used to constrain the chronology during the end of marine isotope stage 5e. For each sample, we assign an age that represents the unique or most probable time of gas trapping, given its gas composition. Of 157 samples with CH4 and d 18 Oatm data, 10 give unique ages. Twenty-five newly measured values of the triple isotope composition of O2 from the disturbed section of the GISP2 core add a third time-dependent gas property that agrees with our reconstruction. Our reconstruction supports earlier conclusions of Landais et al. (2003) that the disturbed section primarily includes ice from the last interglacial (MIS 5e) and the penultimate glacial period (MIS 6). The oldest ice in the basal layer of GISP2 and GRIP has an age � 237 ka. The climate history we derive suggests that the last interglacial at Summit, Greenland, around 127 ka was slightly warmer than the current interglacial period. Reduction of various ion concentrations in ice and thickening of the ice sheet during Termination II was similar to that in Termination I.

Reply to comment by Martin F. Miller on “Record of δ18O and 17O-excess in ice from Vostok Antarctica during the last 150,000 years”

that this excess was lower in glacial than in interglacial times. We further suggested that the change in 17 O-excess indicates smaller effect of kinetics signifying higher normalized humidity in the oceanic source region during glacials. In his comment, Miller [2008] questions the validity of these conclusions on three grounds. First, he emphasizes that the measurements are reported with respect to the VSMOW standard and not with respect to ocean water and therefore 17 O-excess with respect to the ocean may not exist. We note, however, that this point was raised by us [Landais et al., 2008, p. 3] and we emphasized the need for precise calibration with respect to seawater. Recently, we [Luz and Barkan, 2008] have made precise calibration and reported that the triple oxygen-isotope composition of VSMOW is similar to seawater. Therefore, we conclude that Antarctic precipitation unambiguously contains 17 O-excess with respect to the ocean.

The Triple Isotopic Composition of Oxygen in Leaf Water and Its Implications for Quantifying Biosphere Productivity

Publisher Summary Among the main processes that affect global climate changes, are the interactions between the atmosphere and biosphere. Climatic conditions control the biosphere productivity and, in turn, vegetation strongly influences climate through the emission and consumption of greenhouse gases and through the terrestrial albedo. The only effective way to understand these interactions is based on examination of the past climate changes and the associated biosphere evolution. This chapter describes some of the basic principles underlying the mass-dependent and mass-independent fractionation in the oxygen cycle. It then details how the change in the global oxygen-based biosphere productivity can be inferred from the past changes in the triple isotopic composition of O2. The necessity to know precisely the relationship between δ17O and δ18O during leaf transpiration, is emphasized. The results of the experimental studies on transpiration isotope effects that include— variations along a leaf, daily variations, the influence of plant species, and the effects of environmental and climatic conditions— are reported. Finally, these results are used to perform a global budget of the three isotopes of oxygen in the atmosphere and show the implications for the estimate of the ratio between the last glacial maximum (LGM) and the present-day oxygen biosphere productivities.

Direct, Precise Measurements of Isotopologue Abundance Ratios in CO2 Using Molecular Absorption Spectroscopy: Application to Δ17O

We present an ultrasensitive absorption spectrometer based on a 30 Hz/s stability, sub-kHz line width laser source coupled to a high-stability cavity-ring-down-spectroscopy setup. It provides direct and precise measurements of the isotopic ratios δ17O and δ18O in CO2. We demonstrate the first optical absorption measurements of 17O anomalies in CO2 with a precision better than 10 ppm, matching the requirements for paleo-environmental applications. This illustrates how optical absorption methods have become a competitive alternative to state-of-the-art isotopic ratio mass spectrometry techniques.

Asuma : un raid scientifique pour documenter la zone côtière de l'Antarctique

Le bilan de masse de surface des grandes calottes, c’est-à-dire le bilan comptable entre apports (précipitation, dépôt de neige par le vent, givre) et pertes (fonte, sublimation, érosion de la neige par le vent) de masse d’eau en surface des calottes, réagit en permanence aux variations du climat. Selon les estimations actuelles, l’augmentation de l’accumulation de neige en surface de l’Antarctique prévue pour la f in du XXIe siècle (15 % environ) représentera une compensation de l’élévation du niveau des mers d’environ 5 cm (voire 15 cm d’ici à 2200). Cette évolution prend en compte les conséquences de l’augmentation de l’humidité atmosphérique en réponse au réchauffement climatique, mais prend mal en considération les changements potentiels de circulation atmosphérique au-dessus de l’océan Austral et le long des côtes qui bordent l’Antarctique. Pourtant, en raison des variations attendues du gradient de pression entre moyennes et hautes latitudes, des déplacements du rail des dépressions sont prévus dans l’hémisphère Sud au cours du prochain siècle. C’est d’ailleurs déjà le cas, et des effets devraient déjà se faire sentir sur le bilan de masse de surface de l’Antarctique de l’Est. Certes, le bilan de masse de surface de cette partie du continent ne semble pas avoir connu de tendance notable au cours des dernières décennies, mais cette conclusion est facilement remise en doute en raison du manque de données de terrain de long terme, tout particulièrement dans la zone côtière de cette partie du continent. Le 1er décembre 2016, le raid scientif ique Asuma (improving the Accuracy of the Surface Mass balance of Antarctica) quittait la base de Cap Prud’homme en Antarctique, à quelques kilomètres de la base française de Dumont-d’Urville, en direction du centre du continent (figure 1). À bord de quatre tracteurs à chenilles et d’une dameuse, cinq scientifiques de l’Institut des géosciences de l’environnement (IGE) étaient épaulés par trois mécaniciens de l’Institut polaire PaulÉmile-Victor (Ipev) et un médecin. Pendant un peu plus d’un mois, ils ont parcouru 1 371 km sur la calotte polaire pour contribuer à améliorer la connaissance du continent antarctique et mieux évaluer les variations spatiotemporelles de son bilan de masse de surface et relier ces variations à d’éventuels changements de circulation dans la région.