New Evidence of Holocene Mass Wasting Events in Recent Volcanic Lakes from the French Massif Central (Lakes Pavin, Montcineyre and Chauvet) and Implications for Natural Hazards

International audienceHigh-resolution seismic profiling (12 kHz) surveys combined with sediment cores, radiocarbon dating, tephrochronology and multibeam bathymetry (when available) allow documentation of a range of Holocene mass wasting events in nearby contrasting lakes of volcanic origin in the French Massif Central (45°N, 2°E): two deep maar lakes (Pavin and Chauvet) and a shallow lake (Montcineyre) dammed by the growth of a volcano. In these lacustrine environments dominated by authigenic sedimentation, recent slide scars, acoustically transparent to chaotic lens-shaped bodies, slump deposits or reworked regional tephra layers suggest that subaqueous mass wasting processes may have been favoured by gas content in the sediments and lake level changes. While these events may have had a limited impact in both lakes Chauvet and Montcineyre, they apparently favoured the development of lacustrine meromicticity in maar Lake Pavin along with possible subaerial debris flows resulting from crater outburst events

Comparison of modeled and reconstructed changes in forest cover through the past 8000 years: Eurasian perspective

Reproducing the tree cover changes throughout the Holocene is a challenge for land surface-atmosphere models. Here, results of a transient Holocene simulation of the coupled climate-carbon cycle model, CLIMBER2-LPJ, driven by changes in orbital forcing, are compared with pollen data and pollen-based reconstructions for several regions of Eurasia in terms of changes in tree fraction. The decline in tree fraction in the high latitudes suggested by data and model simulations is driven by a decrease in summer temperature over the Holocene. The cooler and drier trend at the eastern side of the Eurasian continent, in Mongolia and China, also led to a decrease in tree cover in both model and data. In contrast, the Holocene trend towards a cooler climate in the continental interior (Kazakhstan) is accompanied by an increase in woody cover. There a relatively small reduction in precipitation was likely compensated by lower evapotranspiration in comparison to the monsoon-affected regions. In general the model-data comparison demonstrates that climate-driven changes during the Holocene result in a non-homogeneous pattern of tree cover change across the Eurasian continent. For the Eifel region in Germany, the model suggests a relatively moist and cool climate and dense tree cover. The Holzmaar pollen record agrees with the model for the intervals 8-3 ka and 1.7-1.3 ka BP, but suggests great reduction of the tree cover 3-2 ka and after 1.3 ka BP, when highly developed settlements and agriculture spread in the region. © The Author(s) 2011

Comments on Anti-phase oscillation of Asian monsoons during the Younger Dryas period: Evidence from peat cellulose delta(13)C of Hani, Northeast China

In their recent paper, Hong et al. (2010; Anti-phase oscillation of Asian monsoons during the Younger Dryas period: Evidence from peat cellulose delta C-13 of Hani, Northeast China, Palaeogeography, Palaeoclimatology, Palaeoecology 297, 214-222) discuss bulk peat sample cellulose delta C-13 data from a fen in northeast China as a proxy for East Asian summer monsoon intensity during the Late Glacial period. Based on their own results, cited papers, and an extensive re-interpretation of sedimentological and palynological data from nearby Lake Sihailongwan, 'Hong et al. (2010) construct a hypothesis of contrasting moisture conditions in northern and southern China, with wet conditions in the north during the Younger Dryas period and an anti-phase behaviour of Indian- and East Asian summer monsoon intensity. However, we do not approve of the reinterpretation of our Lake Sihailongwan data by 'Hong et al. (2010) and must strongly reject it. We show here that neither the Hong et al. (2010) fen data, nor the Lake Sihailongwan data or any other cited data allow for the sound assumption of an intensified East Asian summer monsoon in northeastern China during the Younger Dryas. The Late Glacial variability of the fen data found by 'Hong et al. (2010) can be easily explained by changes in the plant assemblage down core and thus by the composition of the peat. Furthermore, the use of bulk peat cellulose delta C-13 data as a precipitation proxy remains unproven for that area. Hence, there is no basis for a model contrasting Indian and East Asian summer monsoons during that period. (C) 2011 Elsevier B.V. All rights reserved

Milankovitch and beyond - Glacial climate variability and vegetation changes over various timescales reconstructed from annually laminated sediments (Shihailongwan Maar, North-Eastern China)

Orbitally forced cyclicity culminating in the alternation of glacials and interglacials is the most outstanding feature of the Quaternary. At sub-orbital scale, recurrent Dansgaard–Oeschger (D/O) events are the most prominent variations of the last glacial period. First recognised in the Greenland ice core records, their global occurrence has been proven. However, unravelling effects of such rapid climate change on vegetation, including potential lead/lag relationships, remains a challenge. The results presented here from the annually laminated sedimentary sequence of Sihailongwan Lake provide the first comprehensive palaeoecological record covering the period from about ~ 65,000 to 14,000 cal yrs. BP (MIS 2-4) from monsoonal North-Eastern China. A reliable chronology has been established by both varve counting and 40 calibrated AMS 14C age determinations and a pollen sampling resolution of ~65 years allows peak-to-peak correlations between the pollen signals and 18O data from the Greenland ice cores and Chinese speleothem records. The pollen data indicate mosaic-like occurrence of woodland and steppe biotopes close to the study site during the last glacial period. Tree communities in the lake’s catchment area primarily include Betula, Larix, Alnus fruticosa, Picea and Salix - all of them constituents of taiga ecosystems and cold-deciduous forests. Beyond it, the regular occurrences of Ulmus and Fraxinus pollen grains during the pleniglacial point to the existence of favorable micro-habitats for (cool-) temperate trees not far away from the study site. Spectral analyses of pollen data derived from various taxa provide evidence of Milankovitch-, millennial- and centennial-scale climate variability during the study period. Corresponding to the precession cycle, the vegetation experienced repeated low-frequency changes characterized by the alternative growth and decline of taiga-forest and steppe biotopes. Shorter periods with ameliorated climate coincide within error margins with known D/O cycles from Greenland ice core records. An additional ~200 year periodicity of the pollen record is near the solar deVriess/Suess cycle. The new data from Sihailongwan Lake attest to changes of the East Asian monsoon system during the last Glacial over various time scales and illuminate immediate vegetations responses. The evident synchronicity of climate changes in the North Atlantic region and East Asia supports the theory of strong atmospheric coupling between both regions. Our high resolution data allows examination of response of vegetation in the context of different forcing mechanisms in more detail than has previously been possible

Influence of bottom water anoxia on nitrogen isotopic ratios and amino acid contributions of recent sediments from small eutrophic Lonar Lake, central India

Lonar Lake is a eutrophic, saline soda lake with permanently anoxic deep water. The high pH and deoxygenation result in very elevated δ15N of suspended particulate matter (SPM) and sediments due to denitrification and pH-related loss of gaseous ammonium. SPM and sinking particles are predominantly aquatic in origin, whereas surface sediments are of mixed terrestrial plant and planktonic source. An indicator of degradation intensity was derived from a principal component analysis of the spectral distribution of amino acids and named Lonar degradation index (LI). A ratio of individual amino acids (Ox : Anox ratio) was additionally used to determine the relative degree of aerobic vs. anaerobic degradation. These two biogeochemical indicators can be used to detect changes in degradation intensity and redox conditions in the geological history, and thus the paleoclimatic interpretation of Lonar sediments. Surface sediments can be divided into three zones: (1) a nearshore, oxic zone of predominantly aquatic organic matter, in which oxidation leads to a strong diagenetic increase of δ15N; (2) an alluvial zone with a predominance of isotopically depleted land plant and soil organic matter degraded under oxic conditions; and (3) an anoxic, deep zone, which receives aquatic organic matter and land plant–derived material transported near the bottom and in which organic matter is well preserved due to anoxic diagenetic condition