CF201812480
The 8.2 ka event in western Europe: an analogue for the impacts of possible future meltwater discharge to the North Atlantic
J-3
Doctorat Programme Doctoral
Terre & Univers
Auvergne-Rhône-Alpes
Disciplines
Géophysique
Laboratoire
ENVIRONNEMENTS, DYNAMIQUES ET TERRITOIRES DE LA MONTAGNE (EDYTEM)
Institution d'accueil
UNIVERSITE GRENOBLE ALPES
Ecole doctorale
SISEO : Sciences et Ingenierie des Systèmes, de l'Environnement et des Organisations - ED 489

Description

The consequences of the accelerated melting of the Greenland ice-sheet on North Atlantic circulation and its influence on western Europe and Mediterranean climate is still unclear, yet it is of crucial importance for these densely populated areas. More palaeoclimate data are needed to compare with climate model outputs and improve their parameters. 8.2 thousand years ago, a large meltwater discharge occurred in the North Atlantic whilst the Earth was already in an interglacial state similar to the present day. Accordingly, the event represents an excellent analogue to document the regional impacts of significant meltwater incursions. The PhD project objective is to propose a spatial reconstruction of climate change triggered by the 8.2 ka event across western Europe, from SW France to Macedonia, with a particular focus on the northern margin of the Mediterranean, whose climate is particularly sensitive to even modest hydrological changes. For this purpose, the candidate will study speleothem archives (stable isotopes, trace elements geochemistry, U-Th dating), which can provide high-resolution palaeoclimate proxy time series, anchored in time by absolute and precise chronologies, allowing far-field comparisons. Trends in rainfall, temperature and vegetation response obtained from the speleothem proxy-series will then be augmented by lake- and marine-sediment records to complement the pattern of regional climate changes. Finally, the palaeodata will be compared to the simulations obtained with an isotope-enabled Earth system model: site by site data-model comparisons will be undertaken in order to refine the various forcing scenarios. The PhD project will be conducted in collaboration with the University of Pisa and AWI, and a shared-supervision (‘cotutelle’) will be established with the University of Melbourne.

Context and Motivation

Global warming is causing, amongst other things, accelerated melting of the Greenland ice sheet. The consequences for climate over the coming century remain uncertain. Meltwaters have the potential to slow down ocean circulation patterns in the North Atlantic, which transports tropical-sourced heat to NW Europe. Such a slow down would cause regional cooling, leading to changes in rainfall as far afield as the southern tropics, with significant impacts on societies. Although the 5th IPCC Assessment Report (AR5) concludes that an abrupt interruption to North Atlantic circulation has a low probability of occurrence this century (Summary 2014), it has not been discounted entirely. A recent comprehensive modelling study suggests that meltwater-driven changes in deep-water convection in the Labrador Sea (south of Greenland) could lead to abrupt climate change over the coming decades, even under conservative CO2-emission scenarios (Sgubin et al. 2017 Nature Comm.). However, the pressing problem is the lack of agreement amongst models on the likely tipping point required to trigger abrupt climate change, a fact also highlighted in the AR5. Accordingly, there is a need to improve models to reach a broad consensus regarding thresholds and impact, and one way to achieve this is to test their outputs of past abrupt events against palaeoclimate data.
The 8.2 ka event was a centennial cooling episode that affected the North Atlantic and western Europe during the early Holocene. Caused by a large meltwater release from proglacial lakes on the margin of the remnant Laurentide ice sheet, it offers an ‘interglacial analogue’ in the study of the effects of high-latitude meltwater incursions on ocean circulation and climate. However, determining the extent of this cooling, as well as the feedbacks on atmospheric circulation, has proved difficult, largely due to the inability of many palaeoclimate archives to capture with accuracy and reliability the impacts of the event.


Scientific Objectives

The aim of this project is to unravel the climate responses to the 8.2 ka event over western Europe using an assemblage of speleothem records. Palaeoclimate proxy time series from these speleothems will provide a picture of how the region responded to the 8.2 ka meltwater incursion, with a particular emphasis on temperature, precipitation and vegetation changes. The proxy data will be compared with palaeoclimate model simulations to evaluate how accurately the latter match the inferred climate changes. Such data-model comparisons provide a means by which to refine and improve model skill. This will have flow-on effects to model projections: determining the extent and intensity of the climate responses to this event over western Europe will give a strong basis to develop a consensus about potential abrupt climate change in the coming century in response to Greenland ice-sheet melting and its consequences for North Atlantic circulation.
The PhD objectives and deliverables will include a spatial reconstruction of climate change triggered by the 8.2 ka event across western Europe, with a particular focus on the well-populated Mediterranean margin of western and central Europe, from the Atlantic to Macedonia. Speleothem archives can provide accurately dated and highly resolved palaeoclimate records, enabling far-field comparisons with palaeoclimate archives regionally and worldwide. This will allow unravelling the atmospheric teleconnections associated with the event. The climate modelling, drawing on state-of-the-art knowledge about the 8.2 ka event, will provide an opportunity to calibrate the required meltwater input in the North Atlantic to provoke the climatic changes observed in the speleothem and other records.

Methodology and Planning

Most of the speleothems for this study have already been sampled; additional samples are available via our collaborators. Field sites give spatial coverage across a west-east transect from SW France to Macedonia, via Ardèche, Marseille, Tuscany, Sardinia, and Croatia. In each speleothem, the time interval capturing the 8.2 ka event (~8.5 to 8.0 ka) will be targeted for high-resolution analyses. O and C stable isotope-time series (proxies for temperature and rainfall, and vegetation-soil activity, respectively) will be compiled at the highest possible resolution (sub-decadal) using a NewWave micromill and conventional dual-inlet mass spectrometry in Melbourne. Trace-element analyses (laser-ablation ICP-MS, in Melbourne) will provide additional information on palaeohydrology, and will potentially resolve annual geochemical cycles that can be used to refine the time-series chronologies. Uranium-thorium dating (Melbourne) will be conducted on each speleothem proxy-series to produce age models with uncertainties better than a few decades. This is essential for precise comparisons across a large spatial domain. Clumped-isotope analysis (Melbourne) will also be applied to the most suitable samples to ascertain the extent of the temperature depression during the 8.2 ka event.
Trends in rainfall, temperature and vegetation response through the region during the 8.2 ka event arising from the speleothems will be augmented by lake- (e.g. Annecy, Ammersee, Ledro, Ohrid; published or ongoing work) and marine-sediment records (e.g. ODP975, MD90-917) to present a more complete picture of regional climate changes. The palaeodata will be compared to climate model simulations (AWI). The model will be forced by various meltwater scenarios (rate/volume of hosing, meltwater entry points) to evaluate how well the outputs agree with the palaeodata. Time series of climate parameters (e.g. seasonal and annual rainfall amounts, rainfall isotopes, seasonal and mean annual temperature, etc.) representing the grid cells over the cave sites (adjusted for local characteristics, e.g. altitude) will enable detailed site-by-site data-model comparisons.
Schedule:
09/18 to 09/19: Speleothem sampling, cave monitoring, collation of weather and rainfall isotope data; literature review; collation of other high-resolution palaeoenvironmental data; sampling and preliminary O and C analyses and U-Th dating.
09/19 to 09/20: High-resolution microsampling for stable isotopes on the 8.0-8.5 ka period, additional U-Th dating to refine the age models, high-resolution trace element analyses by LA-ICPMS and clumped-isotope analyses on suitable samples in Melbourne; comparison of speleothem results with other palaeoclimate data; integration with palaeoclimate modelling at AWI.
09/20 to 09/21: finalizing proxy-time series and models; communications at international conferences; preparation of papers for high-ranked journals; writing of the thesis; data archiving in international climate databases.

Joint PhD Advisor : Russell Drysdale / The University of Melbourne (Australia)

Compétences requises

The successful candidate will hold a Master’s degree in earth science, environmental science or a related discipline, with a superior ranking. Experience in one or more of the following is highly desirable: stable isotope geochemistry; trace element geochemistry; uranium-thorium dating; data handling (time series); Quaternary field geology; karst hydrology; climate modelling; palaeoclimatology. The candidate will be highly motivated, well organised, and be prepared to: undertake field and laboratory work, present research at national and international conferences, travel to foreign laboratories to conduct analyses, and write research papers for publication. The candidate will be required to work as part of a multinational research team and should be fluent in English. We encourage applications from suitably qualified women as well as from persons from disadvantaged backgrounds.

Your profile is eligible to apply for the PhD/Doctorate program "Make Our Planet Great Again" if:

- You have a Master's degree or you will pass a Master's degree before August 31, 2018
- You have lived in France for less than 90 days since April 1, 2016
- You are exclusively a foreign national

How to APPLY?


Please send your CV and a letter of motivation to the following contact : isabelle.couchoud@univ-smb.fr

Mots clés

Abrupt climate change Holocene Modelling North Atlantic circulation Speleothem Western Europe

Offre financée

Type de financement
Contrat Doctoral
Montant du financement
1350 € Net / mois

Dates

Date limite de candidature 29/04/18

Durée36 mois

Date de démarrage01/09/18

Date de création06/04/18

Langues

Niveau de français requisAucun

Niveau d'anglais requisB2 (intermédiaire)

Possibilité de faire sa thèse en anglais

Divers

Frais de scolarité annuels400 € / an

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