Beschreibung: The aim of this project was to investigate nearshore depositional processes in the Arctic within the last century. For this study sediment cores were retrieved from lagoon and marine sites at Kanivaliuraq – Ptarmigan Bay (lagoon) and Herschel Basin (marine). The lagoon (YC18-PB-SC01, 64 cm length) and two marine (YC18-HB-GC01, 40 cm length; and PG2303-1, 20 cm length) sediment cores, were collected at 3, 18 and 43 m water depth, respectively. Sampling missions took place in April 2016 and August 2018 in the nearshore areas of the southern Beaufort Sea along the Yukon North Slope in the western Canadian Arctic. Lagoon core YC18-PB-SC01 was taken with a percussion corer and marine cores with an UWITEC gravity corer. The cores were kept cool and dark until further analysis in the laboratories. Mineralogy composition was obtained by X-ray diffraction (XRD) of bulk sediment (well ground and mixed) through a Siemens D500 X-Ray diffractometer and the spectra were resolved with EVA Bruker-AXS software. The grain size analysis results are presented in three groups, particles between 2 mm and 63 µm, 63 µm and 2 µm and smaller than 2 µm. All samples were treated with peroxide (30%) before laser diffraction to remove organic matter particles. Grain size analysis was performed using a Microtrac MRB's Bluewave and a Partica HORIBA laser diffraction device. The 210Pb activities were obtained by assuming a secular equilibrium with 210Po and using a spike of 209Po. Samples were digested by acid solutions (HNO3, HCl, HF and HNO3+H2O2). Both polonium radionuclides were deposited on a silver disk and measured by alpha spectrometry using ORTEC detectors coupled to MaestroTM data acquisition software. 226Ra and 137Cs activity were assessed by high-resolution gamma spectrometry through a germanium detector (ORTEC). The ages, mass and sediment accumulation rates (MAR and SAR, respectively) were inferred by a Constant Flux model also known as Constant Rate of Supply model. All uncertainties were calculated by Monte Carlo method.

Beschreibung: Microfaunal assemblages of benthic foraminifera, ostracods, and tintinnids from two marine sediment cores retrieved from the Herschel Basin of the Canadian Beaufort Sea shelf document relationships with environmental parameters such as salinity, sea-ice cover, and turbulence. Cores YC18-HB-GC01 and PG2303-1 were collected at 18 and 32 m water depth, respectively. At these sites, sediment accumulation rates range between 0.6 and 1.7 cm yr–1 allowing a near-annual temporal resolution over the last 50 years. Multivariate analyses indicate that benthic foraminiferal assemblages respond primarily to food supply. Dissimilarities between the microfaunal assemblages of the two cores are mainly the result of bottom water salinity levels linked to water depth. High abundance of the benthic foraminiferal species Elphidium clavatum and occurrences of Elphidium bartletti point to varying, but relatively low, salinities at the shallow core site YC18-HB-GC01, which may be affected by variations in the summer halocline depth. Higher species diversity and more abundant Cassidulina reniforme and Stainforthia feylingi characterize the deeper core PG2303-1, which might reflect more stable conditions and higher bottom-water salinities throughout the studied time interval. The most important microfaunal shift of the last 50 years, observed in the shallower longer core YC18-HB-GC01, occurred at the turn of the 21st century. Prior to ∼2000 CE, the presence of Islandiella norcrossi indicates more stable and saline conditions. Since ∼2000 CE, increased abundances of Haynesina nivea and of the ciliate Tintinnopsis fimbriata suggest decreased salinity and increased turbidity. An increased abundance of Eoeponidella pulchella after ∼2000 CE suggests a concurrent increase in productivity in the last two decades. This shift is nearly synchronous with a decrease in mean summer sea-ice concentration, which can play an important role in bottom water stability on the shelf. Easterly winds can induce a reduction in the sea-ice cover, but also foster a westward spreading of the Mackenzie River plume and the upwelling of nutrient-rich Pacific waters onto the shelf. Both factors would explain the increased freshening and productivity of the Herschel Basin. The last two decades were also marked by a decrease in ostracod abundance that may relate to higher water turbidity. This study shows that combining information from benthic foraminifera, ostracods, and tintinnids provides a comprehensive insight into recent hydrographic/climatic changes in nearshore Arctic habitats, where productivity is critical for the food security of local communities.