A. Gilli

A sedimentary record of Holocene surface runoff events and earthquake activity from Lake Iseo (Southern Alps, Italy):

This study presents a record of Holocene surface runoff events and several large earthquakes, preserved in the sediments of pre-Alpine Lake Iseo, northern Italy. A combination of high-resolution seismic surveying, detailed sediment microfacies analysis, non-destructive core-scanning techniques and AMS 14C dating of terrestrial macrofossils was used to detect and date these events. Based on this approach, our data shed light on past seismic activity in the vicinity of Lake Iseo and the influence of climate variability and human impact on allochthonous detrital matter flux into the lake. The 19 m long investigated sediment sequence of faintly layered lake marl contains frequent centimetre- to decimetre-scale sandy-silty detrital layers. During the early to mid Holocene, these small-scale detrital layers, reflecting sediment supply by extreme surface runoff events, reveal a distinct centennial-scale recurrence pattern. This is in accordance with regional lake-level highstands and minima in solar activity and thus apparently mainly climate-controlled. After c. 4200 cal. yr BP, intervals of high detrital flux occasionally also correlate with periods of enhanced human settlement activity. In consequence, deposition of small-scale detrital layers during the late Holocene apparently reflects a rather complex interplay between climatic and anthropogenic influences on catchment erosion processes. Besides the small-scale detrital layers, five up to 2.40 m thick large-scale detrital event layers, composed of basal mass-wasting deposits overlain by large-scale turbidites, were identified, which are supposed to be triggered by strong earthquakes. The uppermost large-scale event layer can be correlated to a documented Mw=6.0 earthquake in ad 1222 in Brescia. The four other large-scale event layers are supposed to correspond to previously undocumented local earthquakes. These occurred around 350 bc, 570 bc, 2540 bc and 6210 bc and most probably also reached magnitudes in the order of Mw = 5.0–6.5.

Re-evaluation of Climate Change in Lowland Central America During the Last Glacial Maximum Using New Sediment Cores from Lake Petén Itzá, Guatemala

Glaciological data derived from moraines, and multiproxy data from lake sediment cores (e.g. fossil pollen, diatoms, and isotope data) indicate cooling in the Central American tropics during the last ice age. Contrary to prior inferences, however, new lake core data from Lake Peten Itza, lowland Guatemala, indicate that climate was not particularly dry on the Yucatan Peninsula during the last glacial maximum (LGM) chronozone, around 23,000–19,000 cal. yr BP. We present pollen and lithologic data from Lake Peten Itza and an improved chronology for climate changes in lowland Central America over the last 25,000 years. The driest period of the last glaciation was not the LGM, but rather the deglacial period (∼18,000–11,000 cal. yr BP). Causes of climate shifts during the last glaciation are ascribed to precessional changes in insolation, the position of the Inter-Tropical Convergence Zone, and southward penetration of polar air masses.

A 600-year sedimentary record of flood events from two sub-alpine lakes (Schwendiseen, Northeastern Switzerland)

Short sediment cores from two small interconnected sub-alpine lakes (Vorderer and Hinterer Schwendisee) in northeastern Switzerland contain a continuous record of environmental changes, which occurred during the last 600 years. Several light-colored layers within predominantly organic carbon-rich sediments display elevated density values and a high amount of detrital material. This composition points towards an allochthonous origin, which we interpret as material brought into the lake by unusually strong rainfall events. Based on this interpretation, four event horizons were identified in the sedimentary record. These horizons were dated combining a 137Cs and 210Pb-based chronology with meteorological and historical data. They occurred mostly within the 19th and at the beginning of the 20th century, whereas only one event occurred prior to 1800. The well-documented strong rainfall event that occurred on June 14, 1910 is observed in the sedimentary record as the thickest detrital horizon. These short-lived climatic events archived in the sediments of both lakes are superimposed on a longer-term environmental trend characterized by fluctuating conditions in organic productivity. The formation and preservation of authigenic minerals, such as siderite and vivianite, are most likely related to intervals of different primary productivity in the water column.

Lead (Pb) isotope baselines for studies of ancient human migration and trade in the maya region

We examined the potential use of lead (Pb) isotopes to source archaeological materials from the Maya region of Mesoamerica. The main objectives were to determine if: 1) geologic terrains throughout the Maya area exhibit distinct lead isotope ratios (206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb), and 2) a combination of lead and strontium ratios can enhance sourcing procedures in the Mesoamerica region. We analyzed 60 rock samples for lead isotope ratios and a representative subset of samples for lead, uranium, and thorium concentrations across the Maya region, including the Northern Lowlands of the Mexican Yucatan Peninsula, the Southern Lowlands of Guatemala and Belize, the Volcanic Highlands, the Belizean Maya Mountains, and the Metamorphic Province/Motagua Valley. Although there is some overlap within certain sub-regions, particularly the geologically diverse Metamorphic Province, lead isotopes can be used to distinguish between the Northern Lowlands, the Southern Lowlands, and the Volcanic Highlands. The distinct lead isotope ratios in the sub-regions are related to the geology of the Maya area, exhibiting a general trend in the lowlands of geologically younger rocks in the north to older rocks in the south, and Cenozoic volcanic rocks in the southern highlands. Combined with other sourcing techniques such as strontium (87Sr/86Sr) and oxygen (δ18O), a regional baseline for lead isotope ratios can contribute to the development of lead isoscapes in the Maya area, and may help to distinguish among geographic sub-regions at a finer scale than has been previously possible. These isotope baselines will provide archaeologists with an additional tool to track the origin and movement of ancient humans and artifacts across this important region.

Multidisciplinary distinction of mass-movements and flood induced deposits in lacustrine environments : implications for Holocene palaeohydrology and associated natural hazards (Lake Ledro, Southern Alps, Italy)

A. Simonneau, E. Chapron, B. Vannière, S. B. Wirth, A. Gilli, C. Di Giovanni, F. S. Anselmetti, M. Desmet, and M. Magny ISTO, UMR7327, CNRS; Univ. Orléans; BRGM, 1A rue de la Férollerie, 45071 Orléans Cedex 2, France Laboratoire de Chrono-Environnement, UMR6249, CNRS; UFR des Sciences et Techniques, 16 route de Gray, 25030 Besançon, France Geological Institute, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland Eawag, Department of Surface Waters, Überlandstrasse 133, 8600 Dübendorf, Switzerland 3205