7 may 2024
Defense of PhD thesis of Joanna Charton
Title
Paleoclimatic reconstruction for the past 40,000 years in the
Kerguelen Archipelago from glacial chronologies constrained by the
cosmogenic nuclide chlorine-36
Jury
- Tibor DUNAI (PR), Institute of Geology and Mineralogy, Univ. Cologne - Rapporteur
- Yann ROLLAND (MCF), EDYTEM, Univ. Savoie Mont Blanc - Rapporteur
- Olga SOLOMINA (DR), Institute of Geography, Russian Academy of Sciences - Examinatrice
- Denis MERCIER (PR), Sorbonne Université, LGP – Examinateur & président du jury
- Vincent JOMELLI (DR), CNRS CEREGE, AMU - Directeur de thèse
- Irene SCHIMMELPFENNIG (CR), CNRS CEREGE, AMU - Co-directrice de thèse
- Vincent FAVIER (Physicien adjoint), IGE, Université Grenoble Alpes - Membre invité
- Deborah VERFAILLIE (CR), CNRS CEREGE, AMU – Membre invitée
Abstract
The Kerguelen Archipelago (49°S, 69°E) is the one of the few places in the sub-Antarctic
sector of the Indian Ocean that host numerous glacial landforms, making it an excellent
study site for investigating glacier fluctuations and their associated paleocli mates.
However, little is known about glacier fluctuations from Marine Isotopic Stage 3 (MIS 3;
60-26.5 ka) in the sub-Antarctic. The objectives of this dissertation are to constrain the
glacier chronology of the Kerguelen Archipelago with the aim to compare it with other
southern mid-latitude regions and to investigate possible climate drivers. To do so, I used
the cosmic-ray exposure (CRE) dating method (principally 36Cl, and complemented by a
few 10Be and 26Al measurements) on samples of moraine boulders, erratics and glacially
polished bedrocks collected at ten different sites. The oldest evidence of glacier
culmination occurred during MIS 3, as indicated by direct moraine dating at 42.2 ± 4.9 ka
at the Port-aux-Français site. Subsequent culminations occurred during the global Last
Glacial Maximum (gLGM; 26.5 – 19 ka) at 21.5 ± 3.2 ka at the Port Jeanne D’Arc site and at
21.4 ± 3.7 ka and 19.4 ± 2.6 at Baie Larose site. The presence of a MIS 3 moraine that has
not been obliterated by a gLGM advance suggests that the ⁓42.2 ka glacier extent was at
least as large as the gLGM glacier maxima on the archipelago. 36Cl ages of glacially
polished bedrock surfaces sampled in different locations of the archipelago range from
⁓39 ka to ⁓19 ka. We interpret these results as reflecting periods of deglaciation that
occurred in between the two glacier culminations and right after the gLGM on Kerguelen.
These glacier maxima on Kerguelen may have been in phase with regional cold Sea
Surface Temperature (SST) records. However, the climate drivers responsible for the
larger MIS 3 glacier maximum remain unclear, although changes in precipitation may
have superimposed on temperature changes. During the Late Glacial (19 – 11.7 ka), glacial
advances occurred at 16.0 ± 1.9 ka and at 12.9 ± 1.7 ka in Val Travers, at 13.6 ± 1.8 ka in
the Arago Glacier valley and at 14.3 ± 2.3 ka in the Gentil Glacier forefield, probably
associated with the large-scale climatic signal of the Heinrich Stadial 1 and/or Antarctic
Cold Reversal events. So far, no Early nor Mid-Holocene moraines have been found on
Kerguelen, indicating that glaciers had probably receded significantly during these
periods. This is in agreement with the formation of peat bogs previously evidenced
suggesting extensive deglaciation during the Early and Mid-Holocene periods. 36Cl ages
of polished bedrock surfaces (ranging from ~4.4 ka to ~14 ka) at Ampere Glacier site also
suggest that this valley was ice free for several millennia during the Holocene. Finally,
glaciers seem to have re-advanced only during the Late Holocene at 2.6 ± 1 ka on Gentil
Glacier site, then within the last millennium, at ~1 ka, ~430 yr and ~300 yr at Ampere
Glacier site. The Holocene variations of Kerguelen glaciers were probably controlled by
the combined effects of SSTs related to the variations of the Antarctic Polar Front and
fluctuations of precipitation related to long-term variations of the Southern Annular
Mode. Our results are generally consistent with observations from elsewhere in the
Southern Hemisphere during the Late Pleistocene. However, the multi-millennial
evolution of glaciers on Kerguelen during the Holocene is original in contrast to the
evolution of glaciers located in New-Zealand and Patagonia. On Kerguelen glaciers were
similar in the Early Holocene to modern extent and did not significantly re-advance until
the last millennium. Finally, glaciological modeling and observations confirm the recent
decrease of the largest ice body hosted by the archipelago, the Cook Ice Cap, and future
simulations suggest its complete disappearance by the end of the century.