Description: Fragilariopsis kerguelensis (O’ Meara) Hust. is a ubiquitous diatom of the Southern Ocean. Its thick frustules are the numerically dominant component of the siliceous sediment layer covering large parts of the seafloor beneath. Morphometric variability of frustules of this diatom has been of interest for paleoenvironmental reconstructions. Recently, two morphotypes differentiated by the morphometric descriptor rectangularity were described from a Southern Ocean sediment core, the relative abundance of which correlated with reconstructed paleotemperatures. In the present study, we use semi-automated microscopic and image analysis methods to answer whether these morphotypes also appear in recent assemblages, and if yes, do their distributions reflect geographic location or environmental factors. Three transects from the water column, sampled along the Greenwich meridian with hand nets, and one sediment surface transect from the South Pacific, were analyzed. In each of these transects, both morphotypes were detected, and annual mean sea surface temperatures (SST) were found to be a good predictor of their relative abundances. The transition between dominance of one or the other morphotype appeared roughly between the Antarctic Polar Front and the Southern Boundary of the Antarctic Circumpolar Current. Although more extensive circumpolar sampling will be needed to confirm the generality of our conclusions, the observed morphometric cline is a novel aspect of the biology of this species and can in the future potentially be used for further developing paleoproxies especially for highly F. kerguelensis-dominated sediment in the Southern Ocean.

Description: Organic geochemical and micropaleontological analyses of surface sediments collected in the southern Drake Passage and the Bransfield Strait, Western Antarctic Peninsula, enable a proxy-based reconstruction of recent sea ice conditions in this climate-sensitive area. We study the distribution of the sea ice biomarker IPSO25, and biomarkers of open marine environments such as more unsaturated highly branched isoprenoid alkenes and phytosterols. Comparison of the sedimentary distribution of these biomarker lipids with sea ice data obtained from satellite observations and diatom-based sea ice estimates provide for an evaluation of the suitability of these biomarkers to reflect recent sea surface conditions. The distribution of IPSO25 supports earlier suggestions that the source diatom seems to be common in near-coastal environments characterized by annually recurring sea ice cover, while the distribution of the other biomarkers is highly variable. Offsets between sea ice estimates deduced from the abundance of biomarkers and satellite-based sea ice data are attributed to the different time intervals recorded within the sediments and the instrumental records from the study area, which experienced rapid environmental changes during the past 100 years. To distinguish areas characterized by permanently ice-free conditions, seasonal sea ice cover and extended sea ice cover, we apply the concept of the PIP25 index from the Arctic Ocean to our data and introduce the term PIPSO25 as a potential sea ice proxy. While the trends in PIPSO25 are generally consistent with satellite sea ice data and winter sea ice concentrations in the study area estimated by diatom transfer functions, more studies on the environmental significance of IPSO25 as a Southern Ocean sea ice proxy are needed before this biomarker can be applied for semi-quantitative sea ice reconstructions.