The first results of water vapour monitoring at the coastal station Dumont d’Urville will help to link the isotopes records from ice cores to the ice-ocean-atmosphere interactions.
During the austral summer 2016-17, an infrared spectrometer monitoring water stable isotopes in the vapour has been deployed by Camille Bréant from LSCE (Laboratoire des Sciences du Climat et de l’Environnement, Paris, France) at the coastal station of Dumont d’Urville in East Antarctica (see Fig. 1). This campaign has provided continuous measurements of isotopic composition of the water vapour for 40 days, showing important diurnal cycles (up to 5‰ in δ18O) which differs from all other coastal records from Polar Regions (see Fig. 2). Important precipitation events are also imprinted in the records.
Abstract: Orbital tuning is central for ice core chronologies beyond annual layer counting, available back to 60 ka (i.e. thousands of years before 1950) for Greenland ice cores. While several complementary orbital tuning tools have recently been developed using δ18Oatm, δO2⁄N2 and air content with different orbital targets, quantifying their uncertainties remains a challenge. Indeed, the exact processes linking variations of these parameters, measured in the air trapped in ice, to their orbital targets are not yet fully understood.
Abstract: Improving the representation of the hydrological cycle in atmospheric general circulation models (AGCMs) is one of the main challenges in modeling the Earth’s climate system. One way to evaluate model performance is to simulate the transport of water isotopes. Among those available, tritium is an extremely valuable tracer, because its content in the different reservoirs involved in the water cycle (stratosphere, troposphere, and ocean) varies by order of magnitude. Previous work incorporated natural tritium into Laboratoire de Météorologie Dynamique Zoom (LMDZ)-iso, a version of the LMDZ general circulation model enhanced by water isotope diagnostics.
Abstract: The isotopic compositions of oxygen and hydrogen in ice cores are invaluable tools for the reconstruction of past climate variations. Used alone, they give insights into the variations of the local temperature, whereas taken together they can provide information on the climatic conditions at the point of origin of the moisture.
The last interglacial period (LIG, ~129-116 thousand years ago) provides the most recent case study for multi-millennial polar warming above pre-industrial level and a respective response of the Greenland and Antarctic ice sheets to this warming, as well as a test bed for climate and ice sheet models. Past changes in Greenland ice sheet thickness and surface temperature during this period were recently derived from the NEEM ice core records, North-West Greenland. The NEEM paradox has emerged from an estimated large local warming above pre-industrial level (7.5 ± 1.8°C at the deposition site 126 ka ago without correction for any overall ice sheet altitude changes between the LIG and pre-industrial) based on water isotopes, together with limited local ice thinning, suggesting more resilience of the real Greenland ice sheet than shown in some ice sheet models.
Climate reconstruction from ice cores rely on good knowledge of equilibrium and kinetic isotopic fractionation at each step of the water cycle. One of the strongest limitations when interpreting water isotopes in remote Antarctic ice cores is the formulation of the isotopic fractionation at solid condensation (vapour to ice). The uncertainties associated with the coefficients for equilibrium fractionation and water vapour diffusion in air make the formulation of isotopic fractionation at solid condensation only empirical.
The PAGES PALeo constraints on SEA level rise 2 (PALSEA2) working group organized their third workshop in July 22-24, 2015 (Atmosphere and Ocean Research Institute, University of Tokyo). This workshop aimed at integrating data and models to better understand and constrain past changes in sea level, the cryosphere and climate during past warm periods when ice extent was similar to or less than that at present (e.g. Mid-Holocene, last interglacial, mid-Pliocene) with a key goal being to place improved constraints on the amplitudes and rates of sea level changes during such periods.
Outcomes from previous workshops of the PALSEA2 working group have been published recently in Science this summer. The study led by Assistant Professor Andrea Dutton (University of Florida) outlines advances and challenges involved in constraining ice sheet sensitivity to climate change with the use of paleo-sea level records.
The Last Interglacial (LIG) represents an invaluable case study to investigate the response of components of the Earth system to global warming. However, the scarcity of absolute age constraints in most archives leads to extensive use of various stratigraphic alignments to different reference chronologies. This feature sets limitations to the accuracy of the stratigraphic assignment of the climatic sequence of events across the globe during the LIG. Here, we review the strengths and limitations of the methods that are commonly used to date or develop chronologies in various climatic archives for the time span (~140-100 ka) encompassing the penultimate deglaciation, the LIG and the glacial inception. Climatic hypotheses underlying record alignment strategies and the interpretation of tracers are explicitly described. Quantitative estimates of the associated absolute and relative age uncertainties are provided.
The implementation of tritium in a GCM, and subsequent model-data comparison with a large available dataset, is a useful complement to the existing approach of using stable water isotope tracers. The method promises to provide better constraints on: (1) the intrusions and transport of air masses from the stratosphere and (2) the dynamics of the water cycle in the model. The next step will be to implement the bomb-tritium in LMDZ-iso. The atmospheric thermonuclear tests in the 1950s and early 1960s were indeed responsible for the massive injection of tritium in the atmosphere. The detailed information on these nuclear tests (dates, locations, altitudes, yields, etc) has now been released by governments and will be used to reconstruct a realistic atmospheric bomb-tritium input function. Simulations of bomb-tritium will be a very useful tool to evaluate the dynamics of the hydrological cycle in the LMDz-iso model in response to the bomb-tritium transient, and to revisit the issue of the stratospheric injections in relation to the modelled vertical advection.
Cauquoin, A., Jean-Baptiste, P., Risi, C., Fourré, E., Stenni, B., and Landais, A. The global distribution of natural tritium in precipitation simulated with an Atmospheric General Circulation Model and comparison with observations. Earth Planet. Sci. Lett., accepted.
 TU : Tritium Unit, où 1 TU correspond à un rapport tritium/hydrogène de 10-18
Cette étude est financée par l’ERC COMBINISO (306045)
The oldest ice core records are obtained on the East Antarctic plateau. The composition in stable isotopes of water (δ18O, δD, δ17O) permits to reconstruct the past climatic conditions over the ice sheet and also at the evaporation source. Paleothermometer accuracy relies on good knowledge of processes affecting the isotopic composition of surface snow in Polar Regions. Both simple models such as Rayleigh distillation and global atmospheric models with isotopes provide good prediction of precipitation isotopic composition in East Antarctica but post deposition processes can alter isotopic composition on site, in particular exchanges with local vapour. To quantitatively interpret the isotopic composition of water archived in ice cores, it is thus essential to study the continuum water vapour – precipitation – surface snow – buried snow.
In contrast to the East Asian and African monsoons the Indian monsoon is still poorly documented throughout the last climatic cycle (last 135,000 years). Pollen analysis from two marine sediment cores (NGHP-01-16A and NGHP-01-19B) collected from the offshore Godavari and Mahanadi basins, both located in the Core Monsoon Zone (CMZ) reveals changes in Indian summer monsoon variability and intensity during three contrasting climatic periods: the Holocene, the Heinrich Stadial (HS) 2 and the Marine Isotopic Stage (MIS) 5/4 during the ice sheet growth transition. During the first part of the Holocene between 11,300 and 4200 cal years BP, characterized by high insolation (minimum precession, maximum obliquity), the maximum extension of the coastal forest and mangrove reflects high monsoon rainfall.