A set of sedimentary cores, high resolution swath bathymetry and subbottom profiler data provides new insights on sedimentary processes in meromictic maar Lake Pavin, France.Three sedimentary environments (i.e., littoral, plateau and basin) have been identified in the lake from sediment composition using bulk organic geochemistry and the analysis of hydroacoustic images. Various forms of rapidly deposited layers (RDLs) have been identified and radiocarbon dated. An up to date stratigraphy of sedimentary events matching coeval RDLs across the lake is presented and illustrates a wide range of natural hazards linked to Lake Pavin during the last 2,000 years. In AD 600, a sudden lake outburst triggered a slump deposit along with a 9 m lake-level drop that drove shifts in sedimentary organic matter composition. Outside the lake, outburst flood deposits have been described downstream and provide sedimentary evidence for this event. The lake-level drop also favored the generation of gravity reworking phenomena, as shown by (1) a regional earthquake-triggered large slope failure on the plateau connected to a mass-wasting deposit in the basin dated to AD 1300, and (2) a succession of turbidites in AD 1825 and AD 1860 contemporaneous to two historic earthquakes, suggesting that this lake is sensitive to earthquakes with a minimum epicentral intensity of V. Finally, past observations of lake water color changes in AD 1783 and AD 1936, similar to reports in other meromictic lakes, match iron-rich deposits identified in maar lake sediments and suggest that Lake Pavin could have undergone limnic eruptions.
International audienceThis paper presents the potential of 5β-stanols and bile acids to act as fecal source biomarkers in order to identify and characterize past agropastoral activities in archaeological and natural archives. First of all, a molecular inventory of 5β-stanols and bile acids was made on fresh fecal human and domestic animal samples, using the same methodology to define the specificity of these molecular biomarkers. The selected species were cow, horse, pig and sheep as they are the major domestic species present in European archaeological sites. To our knowledge, our work constitutes the first report on fecal biomarkers in sheep feces. Bile acids can distinguish diet regime and species having the same diet with greater specificity than 5β-stanols. Fresh human fecal material and historical/archaeological fecal material were analyzed to assess their stability through time by calculating the coprostanol/epi-coprostanol (Cp/epi-Cp) and deoxycholic acid/cholic acid (DOC/C) ratios. Results show that bile acids are more resistant to diagenesis than 5β-stanols, at least on a 700-year time scale. Human and domestic animal fecal molecular imprints were then compared to the molecular content of 65 samples retrieved from archaeological sites, soils and lacustrine sediments to test their ability to trace past land-use dynamics. This study is the first to combine bile acids and 5β-stanols to identify a source of fecal material in lacustrine sediments. The combination of sterols and bile acids can be used in a variety of natural archives and archaeological contexts to define the origin of fecal material, to specify land-use, and to reconstruct past pastoral practices at various spatio-temporal scales
International audienceIn this chapter we present an up-to-date database of sedimentary sequences retrieved from Lake Pavin during the last 50 years in both oxic and anoxic waters. The detailed history of this mid Holocene crater lake can be reconstructed from the correlation of radiocarbon dated sedimentary sequences retrieved from the deep central basin, a subaquatic plateau and littoral environments. High-resolution measurements of sediment composition (diffuse spectral refl ectance, XRF core scanning) combined with the analysis of organic matter composition and preliminary pollen and diatom assemblages investigations on selected sediment cores are used to reconstruct (i) the evolution since ca. 7000 cal BP of Lake Pavin limnology together with its radiocarbon reservoir effect and (ii) the impact of a wide range of subaquatic slope failure events. Such a multidisciplinary approach of Lake Pavin basin fi ll revealed contrasted sedimentation patterns just after the volcanic eruption and following the development of a dense vegetation cover along the slopes of the crater. Pavin sedimenta-tion is rapidly and largely dominated by organic rich and fi nely laminated diatomite formation , but several short periods of enhanced mineral inputs might refl ect the infl uence of wetter periods, such as the Little Ice Age. Over the last millennium two large subaquatic mass wasting events are also identifi ed and may have signifi cantly impacted its limnology
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.
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