Within the framework of the QuakeRecNankai project, which focuses on the geological record of paleoearthquakes rupturing the Nankai-Suruga subduction zone, we acquired a grid of reflection-seismic profiles in the Fuji Five Lakes at the foot of Mount Fuji. The lake sedimentary architecture and the Holocene sedimentation were studied in detail, with a very high resolution GEOPULSE pinger system in October 2014. The Motosu, Sai, Kawaguchi and Yamanaka Lakes were covered by seismic grids with total lengths of 39 km, 24 km, 31 km and 28 km respectively. The seismic profiles provided a clear image of the entire (the upper 20-25 m on the reflection seismic profile) lacustrine infill in the deep Motosu and Sai Lakes. Regarding the shallow Yamanaka and Kawaguchi Lakes, gas blanking prevailed, but the upper 15 m of sedimentary infill was still visible locally. We establish a seismic stratigraphy for each lake over the Late Holocene period and integrate this into the wide range of published data on the Mount Fuji and Fuji Five Lakes. In particular we seek to tie the seismic stratigraphy to the geological record obtained in long drilling cores (Koshimizu et al., 2007) and to major eruption phases of the Mount Fuji. Our goal is to define a common time frame, which will enable integrated investigations of evidence for mass-wasting deposits, major eruptions and lake-level changes contained within the lakes.
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The period since the emergence of humans has displayed an ongoing biodiversity reduction named the Holocene extinction. Caused primarily by human impacts and climate change, this reduction is increasingly growing and has become a reason of global concern. In Europe, mountainous vegetation is prized as sources of high biodiversity and has been shown to be particularly sensitive to human action and climate changes. Otherwise palynological data represent the most widely available quantitative record of past environments and are an efficient tool for the reconstruction of vegetation and its responses to anthropogenic disturbances and/or abrupt climate changes. Mountainous pollen sequences therefore represent ideal archives for the study of the effects of past climate change and land-use on biodiversity over time scales that go beyond human life span. Within this framework, this work proposes to reconstruct the history of the landscape in the Champsaur Valley (French Alps) during the last 3400 years by means of a multidisciplinary approach. We also try to explain how and to which degree biodiversity have been affected by complex human/climate relationships. For that purpose, two high resolution pollen and NPPs records have been produced, one situated at the bottom of the valley and the other in the montane belt. They show in detail the local vegetation and human occupation history in two different types of topographic environments, covering the last 2000 and 3400 years respectively. Palynological richness and several diversity indexes have also been calculated for these sequences in order to reconstruct the evolution of biodiversity at the two different locations. On the other hand, modern pollen data and vegetation surveys from 49 sites, selected within different environmental and land-use contexts in the Champsaur Valley, were used to create a pollen-based transfer-function to quantify the pasture pressure, which has been the major human activity in the alpine valleys throughout the Late Holocene. Its application to the two well-dated pollen sequences allowed reconstructing the evolution of pasture-pressure through time. The pollen-based reconstructions were compared with changes in percentages of palynological and NPP pastoral indicators, but also with archaeological and historical evidences for the region. Comparisons show a good correlation but differences in the inferred intensity of the pastoral pressure. Pollen and NPPs assemblages, as well as biodiversity indexes and the pasture pressure estimated by our model, were also related to the Holocene climate variable, inferred from independent proxies such as
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