LATE QUATERNARY CLIMATE HISTORY OF COASTAL ANTARCTIC ENVIRONMENTS: A MULTI-PROXY APPROACH (LAQUAN) (EV/01)

anglais. Novel biological proxies and inference models were developed to reconstruct past environmental changes in Antarctic ice-free regions. Reference datasets of cyanobacterial sequences, diatoms and pigments were constructed in order to study the present diversity and distribution of biota in benthic microbial mats from Antarctic lakes. These datasets were subsequently used for comparison between living and fossil floras or to develop inference models to quantitatively reconstruct past environmental changes in East Antarctica. Paleolimnological analyses and application of the models revealed the history of late Quaternary variation in climate, ultraviolet (UV) radiation, and relative sea-level in the Larsemann Hills. The study of modern cyanobacterial diversity showed that each lake is quite unique in terms of diversity. Every single lake studied resulted in the discovery of new Operational Taxonomic Units (OTUs), which suggests that there is a lot more diversity to discover. The majority of the genotypes are restricted to Antarctica and sometimes, even present only in one sample, which hints to the existence of endemic cyanobacteria. A taxonomic inventory of the diatom flora from the Larsemann Hills similarly revealed that Antarctic endemics account for about 40 % of all freshwater and brackish taxa. Fossil cyanobacterial sequences were found in layers of up to 9000 years old. The validation of fossil sequences of Progress and Heart Lake cores by both laboratories allowed us to assess that a majority of cyanobacterial sequences found in sedimentary core layers were really from fossil organisms. Comparison between the modern and fossil diversity revealed that most fossil sequences were also present in modern samples. The main problems encountered were related to the presence of good-quality bacterial DNA that act as competitor of fossil DNA during PCR, downcore degradation of fossil DNA, and the selective, group-specific resistance of cyanobacterial DNA to degradation. The main paleoenvironment-related results can be summarized as follows. During the Last Glacial Maximum one of the main peninsulas in the Larsemann Hills was only partly glaciated, as evidenced by uniquely long lake sedimentary records extending into the previous interglacial period (Eemian). Diatom-based inference models revealed that this interglacial was probably warmer and wetter than the Holocene, which was further supported by the presence of currently sub-Antarctic endemics in the Eemian diatom flora. The last glacial period was characterized by dry and cold conditions prevailing over the Larsemann Hills. The levels of the cyanobacterial UV-screening compound scytonemin in fossil microbial communities from this period were three times higher than the present-day values. Shortly after 13,500 yr BP, deglaciation of the Larsemann Hills and the continental shelf in Prydz Bay intensified. The collapse of this part of the East Antarctic Ice Sheet (EAIS) thuscoincided and may have contributed to melting water pulse 1A, which was one of the most rapid sea-level rises since the end of the last glacial period. During the Holocene, several warm periods were detected, coinciding with more productive coastal waters. Marine sediments in isolation basins from these periods are characterized by open water taxa and high chlorophyll a concentrations. Based on a relative sea level curve, we inferred that during the last warm period (the Hypsithermal) increased moisture supply to this part of the EAIS might have contributed to the global sea level fall between 4000 and 2500 yr BP. The high sediment accumulation rate in the isolation lakes further enabled us to identify several dry episodes and periods of higher UV radiation penetration during the past 2000 years. Together, our results highlight the potential of coastal Antarctic lakes for the reconstruction of past environmental changes and underscore the need for continued studies of lacustrine sediment sequences from this climate sensitive region.

Projet EV/01 – “Late quaternary climate history of coastal antarctic environments: a multi-proxy approach (LAQUAN)”