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Late Holocene history of the Fuji Five Lakes (Japan)
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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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Investigation of the Five Fuji Lakes and their potential of recording paleoearthquakes
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Within the framework of the QuakeRecNankai project, which focuses on the geological record of paleoearthquakes rupturing the Nankai-Suruga subduction zone, we sampled bottom sediments of the Fuji Five Lakes at the foot of Mount Fuji. In October 2014, we acquired 23 short gravity cores from Motosu, Sai, Kawaguchi and Yamanaka Lakes. We focus on investigating the sensitivity of each lake for recording natural hazards (storms, floods, volcanic eruptions, earthquakes). Recent natural hazards impacting the Fuji Five Lakes area are expected to be recorded as rapidly deposited layers within the background hemipelagic sediments. We seek to identify event deposits in the collected short cores based on visual identification combined with high resolution analyses, comprising geophysical and geochemical properties, grain size and X-ray images. We establish an age-depth model by combining radionuclide (14C,137Cs,210Pb) dating with the identification of historical tephra layers, in particular the one deposited during the Hoei eruption (AD 1707) of Mount Fuji. The sedimentary events in each lake are compared to a historical catalogue of natural hazards in the Fuji Five Lakes area, including historical records of megathrust earthquakes rupturing the Nankai subduction zone and other earthquakes occurring along the inland active faults that produced significant shaking (MKS intensity >7) in the Fuji Five Lakes area. The preliminary sedimentary study highlights the high sensitivity of Motosu Lake compared to the very shallow Yamanaka and Kawaguchi Lakes, which we attribute to its specific geomorphology. Motosu Lake is characterized by a deep basin, surrounded by very steep slopes. The generation of earthquake triggered mass transport deposits/turbidites and the preservation of the deposits are favored due to the architecture of the basin. The Fuji lake sensitivity to megathrust events will be the target of a future coring campaign to retrieve a longer geological record.
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New Geological Evidence of Past Earthquakes and Tsunami Along the Nankai Trough, Japan
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The east coast of Japan is prone to tsunamigenic megathrust earthquakes, as tragically demonstrated in 2011 by the Tōhoku earthquake (Mw 9.0) and tsunami. The Nankai Trough subduction zone, to the southwest of the area affected by the Tōhoku disaster and facing the densely populated and heavily industrialized southern coastline of central and west Japan, is expected to generate another megathrust earthquake and tsunami in the near future. This subduction zone is, however, segmented and appears to be characterized by a variable rupture mode, involving single- as well as multi-segment ruptures, which has immediate implications for their tsunamigenic potential, and also renders the collection of sufficiently long time records of past earthquakes and tsunami in this region fundamental for an adequate hazard and risk assessment. Over the past three decades, Japanese researchers have acquired a large amount of geological evidence of past earthquakes and tsunami, in many cases extending back in time for several thousands of years. This evidence includes uplifted marine terraces, turbidites, liquefaction features, subsided marshes and tsunami deposits in coastal lakes and lowlands. Despite these efforts, current understanding of the behaviour of the subduction zone still remains limited, due to site-specific evidence creation and preservation thresholds and issues over alternative hypotheses for proposed palaeoseismic evidence and insufficiently precise chronological control. Within the QuakeRecNankai project we are generating a long and coherent time series of megathrust earthquake and tsunami recurrences along the Nankai Trough subduction zone by integrating all existing evidence with new geological records of paleo-tsunami in the Lake Hamana region and of paleo-earthquakes from selected lakes in the Mount Fuji area. We combine extensive fieldwork in coastal plain areas and lakes, with advanced sedimentological and geochemical analyses and innovative dating techniques.
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Palaeoenvironments and climate during the Late Glacial and Early Holocene in Sandy Flanders (NW Belgium): the high-resolution multiproxy record of the Moervaart palaeolake
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During the last three decades intense archaeological prospection has taken place in the region of Sandy Flanders (Belgium), which is now one of the most intensively surveyed areas of NW Europe. This has led to the production of archaeological distribution maps, which show a distinct pattern regarding the temporal and spatial distribution of these archaeological sites. Some areas with a presumed high ecological value seem to have been attractive settlement locations in Prehistory. Habitations however seem to have ‘moved’ in time, and the same areas became completely ‘empty’ during Protohistory and even the Roman Period. During the Late Glacial and Holocene the landscape in the Belgian area of Sandy Flanders was subjected to major changes due to climatic fluctuations, and besides human factors, environmental conditions may have influenced settlement conditions throughout time and played a role in this change in site location and the occupational history of the region. In this light an inter-disciplinary project ’Prehistoric settlement and land-use systems in Sandy Flanders (NW Belgium): a diachronic and geoarchaeological approach’ (GOA project, UGent), involving archaeology, geography, palaeoecology, sedimentology and geophysical survey, has been undertaken. The study of both ‘empty’ and densely inhabited areas is ongoing and aims at analyzing the settlement dynamics of the area of Sandy Flanders in terms of environmental potentials. Likewise, we seek to investigate the reasons why some areas, which were inhabited in previous periods, were apparently not attractive anymore from the Metal Ages onwards. Indeed, to determine the suitability of a certain land type for a certain activity, it is necessary to understand the different types of land use, the soil characteristics and the environment at different time intervals. We present here the first results of the palaeoecological (mainly palynology and NPPs, but also plant macroremains, charcoal, diatoms, ostracods, molluscs, beetles and Chironomideae) and sedimentological (water content, LOI, magnetic susceptibility, gamma-density) approaches, which have been undertaken on the site of the Moervaart Depressie, a shallow but large palaeolake first densely inhabited and then deserted. They provide new insights in the palaeolandscape evolution of this area during the Late Glacial and the early Holocene, and may allow us to evaluate in detail how and to which degree this evolution determined the pre- and protohistoric occupation and exploitation within Sandy Flanders. Furthermore, significant emphasis is placed on the impact of prehistoric populations on both local and regional landscapes.
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Reconstruction of environmental and climatic changes during the Late Glacial at Moerbeke (Flemish Valley, Belgium) using multiproxy lake sediment analyses
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The Flemish Valley (NW Belgium) is a relatively flat and low-lying area situated at the southern limit of the lowland cover sand region of the NW European plain. During the Late Pleniglacial and the Late Glacial, numerous shallow lakes were formed. These depressions are important and sensitive ecosystems providing excellent continental archives to investigate past environmental changes since the last glaciation, making this area of particular interest. However, although previous pollen analyses have punctually been carried out in the Flemish Valley, studies employed in combination with other proxy records are rare, and temporal resolutions stay low and not suited to catch abrupt and short changes as climatic crises may be. In order to better understand the natural processes which occurred in these particular ecosystems, an integrated research program based on a multiproxy approach has recently been undertaken on the Late Glacial. The fundamental aim is to produce a detailed and quantified reconstruction of past environments at high temporal resolution in relation with climate variability by means of independent proxies. For that purpose a 70m long trench was dug at Moerbeke through the deepest part of the Moervaart Depression, one of the largest palaeolake of Europe. Several sequences revealing a contrasted stratigraphy (lake marl, gyttja, peaty deposits and sandy layers alternating) are investigated using biological indicators (pollen, NPPs, plant macrofossils, charcoal, diatoms, ostracods, mollusks, insects), sedimentological (LOI, magnetic susceptibility, granulometry, gamma-density), chronological (AMS 14C and OSL dating, tephra layers) and geochemical proxies (isotopes). Using multiproxy comparisons, we will try to assess if the vegetation and other environmental indicators have changed concomitantly and simultaneously and to decipher between the most sensitive palaeoenvironmental indicators to regional climatic conditions.
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Understanding prehistoric settlement and land-use systems in Sandy Flanders (NW Belgium) since the last 15000 years: the high-resolution palaeoenvironmental multiproxy records of the Moervaart area
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After three decades of intense archaeological prospection in Sandy Flanders, detailed archaeological distribution maps have been produced, showing a distinct pattern regarding the temporal and spatial distribution of the archaeological sites. In order to explain the observed changes in site location an inter-disciplinary project 'Prehistoric settlement and land-use systems in Sandy Flanders (NW Belgium): a diachronic and geoarchaeological approach' has been undertaken. The starting point is that, besides human factors, environmental conditions may have influenced settlement conditions through time, and our research aims thus at analyzing the occupational history of the area in terms of environmental potentials. Indeed, since the Late Pleniglacial the landscape in this area was subjected to major changes due to abrupt climatic fluctuations, and numerous, generally small but elongated sand dunes and shallow lakes were formed, which may have played a role in the settlement dynamics. Palaeoecological (pollen and NPPs, plant macroremains, charcoal, diatoms, ostracods, molluscs, beetles, chironomids) and sedimentological (water content, LOI, magnetic susceptibility, gamma-density, granulometry) analyses are ongoing to reconstruct local and regional past environments and climate change at high resolution during the Late Glacial and Early Holocene. This contribution presents the first palaeoenvironmental results obtained for three sedimentary sequences collected in the Moervaart area, first densely inhabited and then deserted, where a trench and 15 mechanical corings have been made at five different locations along a N-S transect: from a deksandridge to a palaeochannel crossing a shallow but large palaeolake, through the deepest part of the depression. They provide new insights in the palaeolandscape evolution of this area, and may allow us to evaluate in detail how and to which degree this evolution determined the prehistoric occupation and exploitation within Sandy Flanders.
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Interactions entre Climat et Pastoralisme dans le Champsaur (Alpes Occidentales Françaises): leurs effets sur l'évolution de la biodiversité au cours des 4 derniers millénaires
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Ce travail retrace l'histoire du paysage dans deux sites localisés dans le Champsaur (Alpes Occidentales Françaises), le Lac de Faudon (1577 m) et le Laus des Combettes (1175 m) en soulignant les relations entre l'évolution de la diversité et les changements du climat et des pratiques humaines des derniers 4000 ans. L'analyse pollinique de 49 échantillons de mousses, prélevés dans différents contextes environnementaux et anthropiques dans la Vallée du Champsaur, a servi de base pour l'élaboration d'une fonction de transfert vers la pression de pâturage connue pour chaque parcelle échantillonnée. Son application aux données polliniques provenant des deux séquences fossiles a permis de quantifier la pression pastorale au cours des derniers millénaires. Les résultats se montrent cohérents avec d'autres indicateurs de pastoralisme, tels que les spores de champignons coprophiles. Les données historiques et archéologiques concordent avec les données paléoécologiques dans la mise en évidence d'une présence humaine continue sur les deux sites à partir de l'Antiquité. La diversité pollinique atteint des seuils sans précédents lors de l'intensification du pastoralisme observée au cours de l'Optimum Climatique Médiéval (850-550 cal BP) correspondant à une augmentation de la démographie et à une présence humaine stable. Une brusque diminution de la diversité est enregistrée au début du Petit Age Glaciaire (650-380 cal BP) et correspond à une baisse des activités pastorales ainsi qu'à des phases de guerres et de tueries entre fermiers voisins pour l'exploitation du bois et des pâtures. La pénurie de bois et les famines reportées semblent liées à l'effet conjoint de la dégradation climatique et de la surexploitation des ressources opérées par l'Homme au cours de l'optimum médiéval. Les données suggèrent une transformation importante dans la composition de la flore, résultant en un écosystème fortement dépendant de l'Homme et hautement fragilisé face aux péjorations climatiques.
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Paléoenvironments et climat au cours du Tardiglaciaire en Vallée Flamande (Belgique): l’enregistrement multi-proxy haute résolution du paléolac du Moervaart
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La région de la Vallée Flamande (Nord-Ouest de la Belgique), est une zone relativement plane de basse altitude située à la limite Sud de la couverture sableuse de la grande plaine du Nord-Ouest de l’Europe. A la fin du dernier Pléniglaciaire et au cours du Tardiglaciaire, de nombreuses dépressions lacustres et zones humides s’y sont formées. Ces milieux sont d’importants écosystèmes pourvoyeurs d’excellentes archives pour l’étude des changements paléoenvironnementaux depuis la dernière glaciation, ce qui fait de ce secteur des Flandres une zone d’intérêt particulier. Pourtant, bien que des analyses polliniques y aient été ponctuellement réalisées, très rares sont celles menées en conjonction avec d’autres indicateurs, et leurs résolutions temporelles très faibles restent inadaptées pour mettre en évidence des changements brefs et abrupts comme peuvent l’être certaines crises climatiques. Afin de mieux comprendre les processus naturels intervenus au sein de ces écosystèmes sensibles de la Vallée Flamande, une étude basée sur une approche multi-proxy a été récemment entreprise sur la période du Tardiglaciaire. L’objectif fondamental est d’aboutir à une reconstitution détaillée et quantitative des environnements passés à haute résolution temporelle en relation avec la variabilité climatique au moyen d’indicateurs indépendants. Pour ce faire, une tranchée de 70m de long a été creusée dans la partie la plus profonde de la dépression du Moervaart, un des plus grands paléolacs d’Europe. Plusieurs séquences, montrant une stratigraphie contrastée (marnes lacustres, gyttja, dépôts tourbeux et passées sableuses en alternance), sont en cours d’analyses par le biais d’indicateurs biologiques (pollen, NPP, macrorestes végétaux, charbons, diatomées, ostracodes, mollusques, insectes), sédimentologiques (LOI, susceptibilité magnétique, granulométrie, densité), chronologiques (datations radiocarbone et OSL, téphras) et géochimiques (isotopes). Cette contribution se propose de comparer les résultats livrés par la palynologie à ceux issus des autres champs disciplinaires. Il s’agit ici d’évaluer si la végétation et les autres indicateurs ont évolué de manière concomitante et simultanée et d’identifier les indicateurs paléoenvironnementaux les plus sensibles aux conditions climatiques régionales.
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Amniotic eggshells from the Haţeg Basin (Upper Cretaceous, Romania).
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RBINS Staff Publications
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Vertebrate remains in the 2003 Bernissart drill: histological assessment.
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RBINS Staff Publications
