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

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.