A paleolimnological reconstruction of mid and late holocene climate change in South Georgia

South Georgia is located at the barrier between Antarctica and the mid-latitudes which makes it a key location to determine the main drivers of past and present-day climate variability and to assess whether the climate in the South Atlantic was synchronous with Antarctica or South America. Here we performed a sedimentological, high resolution (ITRAX) geochemical, and fossil diatom and pigment analysis of a 5.41 m long, ca. 8000 cal yr BP, sediment core from Fan Lake, Annenkov Island, South Georgia (54°29’0’’S, 37°5’0’’W) in an attempt to separate the influence of Holocene palaeoclimatic variability from changes in catchment stability and glacier activity. While radiocarbon ages of events in the top 250 cm (c. 4 ka) of this core appear to be broadly in line with some other studies on South Georgia, the chronology of the lower half still poses several questions. The main lithological division in the profile is marked by the establishment of finely laminated sedimentation at c. 250 cm (4000 yrs BP) and is also picked out by the diatom and pigment analysis. This change is characterized by a reduction to low stable magnetic susceptibility values and a step-change increase in organic matter, and is most likely related to deglaciation of the lake catchment during the ‘Mid Holocene Hypsithermal’. Although the diatom composition is dominated by a single species (i.e., Cyclotella stelligera), relatively minor, but sometimes significant, fluctuations in other diatom species occur in the top 250 cm (mid-late Holocene). Interestingly, the most remarkable change in the diatom record occurs at c. 100 cm (1000 cal yr BP) and coincides with an increase in general lake productivity. We link these changes to increased catchment disturbance at c. 1000 cal yr BP, which is possibly associated with deglaciation following one of four relatively minor ‘post-cooling events’ during the late Holocene

Paleo-ecologisch onderzoek van een Holocene sequentie uit het Deurganckdok te Doel (Wase Scheldepolders, Noord-België)

De graafwerken van een getijdendok op de linkeroever van de Schelde nabij Doel (België) boden een uitstekende gelegenheid om de stratigrafie en de paleo-ecologie van Holocene afzettingen in de Beneden Scheldepolders te bestuderen. Een complete organische sequentie, gedateerd tussen 6000 en 1400 BP, werd op basis van pollen, sporen en andere microfossielen, macroresten en diatomeeën onderzocht. Naast een volledig goed ontwikkelde representatieve botanische evolutie vanaf het Atlanticum werd ook belangrijke informatie verzameld over twee specifieke gebeurtenissen. Enderzijds kon de uitbreiding en de paleo-ecologische omstandigheden van de late Calais inundatie, tussen 5800 en 5000 BP, worden gereconstrueerd. Anderzijds werd het mogelijk om een beter inzicht te verwerven over de tijdspanne tussen het einde van de veengroei rond de 6de-7de eeuw AD en de grote strategische overstromingen na 1580 AD. Een periode van non-depositie, gevolgd door lokale afzettingen van laatmiddeleeuwse strategische stormvloeden werd namelijk geattesteerd.The excavations of a huge tidal dock on the left bank of the Scheldt river near Doel (Belgium) provided an excellent opportunity for the study of the stratigraphy and palaeoecology of the Holocene deposits in the Lower Scheldt polders. A complete organic sequence from the period between 6000 and 1400 BP was radiocarbon dated and investigated by means of pollen, spores and other microfossils, macroremains and diatoms. Besides a good developed sequence of the botanical evolution since the Atlanticum, interesting information on two specific events was obtained. Firstly the extension and conditions of the late Calais inundation, between 5800 and 5000 BP could be reconstructed. Secondly it became possible to gain more insight in the timespan between the end of the peat growth around the 6th-7th century AD and the large strategic inundation after 1580 AD. A period of non deposition followed by local deposits of late medieval military storm surges could be recognised

Bacterial diversity assessment in Antarctic terrestrial and aquatic microbial mats : a comparison between bidirectional pyrosequencing and cultivation

The application of high-throughput sequencing of the 16S rRNA gene has increased the size of microbial diversity datasets by several orders of magnitude, providing improved access to the rare biosphere compared with cultivation-based approaches and more established cultivation-independent techniques. By contrast, cultivation-based approaches allow the retrieval of both common and uncommon bacteria that can grow in the conditions used and provide access to strains for biotechnological applications. We performed bidirectional pyrosequencing of the bacterial 16S rRNA gene diversity in two terrestrial and seven aquatic Antarctic microbial mat samples previously studied by heterotrophic cultivation. While, not unexpectedly, 77.5% of genera recovered by pyrosequencing were not among the isolates, 25.6% of the genera picked up by cultivation were not detected by pyrosequencing. To allow comparison between both techniques, we focused on the five phyla (Proteobacteria, Actinobacteria, Bacteroidetes, Firmicutes and Deinococcus-Thermus) recovered by heterotrophic cultivation. Four of these phyla were among the most abundantly recovered by pyrosequencing. Strikingly, there was relatively little overlap between cultivation and the forward and reverse pyrosequencing-based datasets at the genus (17.1–22.2%) and OTU (3.5–3.6%) level (defined on a 97% similarity cut-off level). Comparison of the V1–V2 and V3–V2 datasets of the 16S rRNA gene revealed remarkable differences in number of OTUs and genera recovered. The forward dataset missed 33% of the genera from the reverse dataset despite comprising 50% more OTUs, while the reverse dataset did not contain 40% of the genera of the forward dataset. Similar observations were evident when comparing the forward and reverse cultivation datasets. Our results indicate that the region under consideration can have a large impact on perceived diversity, and should be considered when comparing different datasets. Finally, a high number of OTUs could not be classified using the RDP reference database, suggesting the presence of a large amount of novel diversity

Different resistance mechanisms to desiccation of a pair of Nostoc sp. strains from aquatic and terrestrial habitats of Antarctica

Cyanobacteria are photosynthetic autotrophic micro-organisms that can survive in a variety of extreme environments around the world, such as polar regions. They usually live in association with other organisms and form biological soil crusts in terrestrial environments or thick benthic mats in aquatic environments, where they can develop particular structures such as benthic pinnacle mats. Cyanobacteria have developed different resistance traits to cope with extreme and contrasted environments, such as the production of pigments to absorb UV radiation, or a polysaccharidic matrix to withstand desiccation. However, a better understanding of their ecophysiological mechanisms is necessary to clarify how these processes work. In the present study, the ecophysiological performance of two Nostoc sp. strains was investigated under dehydration and subsequent re-hydration stress. The chosen pair of strains shares 100 % of 16S rRNA gene similarity but have different ecologies. Strain ULC180 and ULC008 from the BCCM culture collection were isolated from a granitic outcrop in the Sør Rondane Mountains (East Antarctica) and from an Antarctic lake (Larsemann Hills, East Antarctica), respectively. To determine their capacity to withstand desiccation, both strains were cultured in BG110 medium, then left dehydrating onto filters placed into a desiccation chamber until complete desiccation (Fv/Fm around 0) and re-hydrated for up to 72 h. Samples for pigments, exopolysaccharide (EPS) and RNA extractions were taken when the desiccation was complete and after 10 min, 2 h, 24 h and 72 h of rewetting whilst Fv/Fm was measured before sampling. Our preliminary EPS and pigments results suggest a higher resistance of the terrestrial strain to desiccation. However further analyses of the gene expression by RNA-seq are in progress for a better understanding of the different mechanisms developed by these two strains

Development of a regional glycerol dialkyl glycerol tetraether (GDGT)-temperature calibration for Antarctic and sub-Antarctic lakes

A regional network of quantitative reconstructions of past climate variability is required to test climate models. In recent studies, temperature calibration models based on the relative abundances of sedimentary glycerol dialkyl glycerol tetraethers (GDGTs) have enabled past temperature reconstructions in both marine and terrestrial environments. Nevertheless, to date these methods have not been widely applied in high latitude environments due to poor performance of the GDGT–temperature calibrations at lower temperatures. To address this we studied 32 lakes from Antarctica, the sub-Antarctic Islands and Southern Chile to: 1) quantify their GDGT composition and investigate the environmental controls on GDGT composition; and 2) develop a GDGT–temperature calibration model for inferring past temperatures from Antarctic and sub-Antarctic lakes. GDGTs were found in all 32 lakes studied and in 31 lakes branched GDGTs (brGDGTs) were the dominant compounds. Statistical analyses of brGDGT composition in relation to temperature, pH, conductivity and water depth showed that the composition of brGDGTs is strongly correlated with mean summer air temperature (MSAT). This enabled the development of the first regional brGDGT–temperature calibration for use in Antarctic and sub-Antarctic lakes using four brGDGT compounds (GDGT-Ib, GDGT-II, GDGT-III and GDGT-IIIb). A key discovery was that GDGT-IIIb is of particular importance in cold lacustrine environments. The addition of this compound significantly improved the model's performance from r2=0.67r2=0.67, RMSEP-LOO (leave-one-out) = 2.23 °C, RMSEP-H (h-block) = 2.37 °C when applying the re-calibrated global GDGT–temperature calibration to our Antarctic dataset to r2=0.83r2=0.83, RMSEP-LOO = 1.68 °C, RMSEP-H = 1.65 °C for our new Antarctic calibration. This shows that Antarctic and sub-Antarctic, and possibly other high latitude, palaeotemperature reconstructions should be based on a regional GDGT–temperature calibration where specific compounds can be identified and included to improve model performance. Finally, downcore temperature reconstructions using the new Antarctic brGDGT–temperature calibration were tested in sub-Antarctic Fan Lake from South Georgia providing a proof of concept for the new calibration model in the Southern Hemisphere