Abstract850 surface samples mostly from abyssal sediments of the South Atlantic and the Antarctic Ocean were investigated for clay content and composition. Maps of relative clay mineral content were compiled, which improve previous maps by showing more details, especially at high latitudes. Large-scaled relations regarding the origin and transport paths of detrital clay are revealed. Near
One of the five largest mass extinctions of the past 600 million years occurred at the boundary of the Triassic and Jurassic periods, 201.6 million years ago. The loss of marine biodiversity at the time has been linked to extreme greenhouse warming, triggered by the release of carbon dioxide from flood basalt volcanism in the central Atlantic Ocean. In contrast, the biotic turnover in terrestrial ecosystems is not well understood, and cannot be readily reconciled with the effects of massive volcanism. Here we present pollen, spore and geochemical analyses across the Triassic/Jurassic boundary from three drill cores from Germany and Sweden. We show that gymnosperm forests in northwest Europe were transiently replaced by fern and fern-associated vegetation, a pioneer assemblage commonly found in disturbed ecosystems. The Triassic/Jurassic boundary is also marked by an enrichment of polycyclic aromatic hydrocarbons, which, in the absence of charcoal peaks, we interpret as an indication of incomplete combustion of organic matter by ascending flood basalt lava. We conclude that the terrestrial vegetation shift is so severe and wide ranging that it is unlikely to have been triggered by greenhouse warming alone. Instead, we suggest that the release of pollutants such as sulphur dioxide and toxic compounds such as the polycyclic aromatic hydrocarbons may have contributed to the extinction.
The continental settings of Central Asia witnessed increased desertification during the Cenozoic as a result of mountain uplift and the Paratethys retreat. The interaction of these tectonic-scale processes with orbitally forced climate change and their influence on Asia's atmospheric moisture distribution are poorly constrained. A Miocene succession of continental mudflat deposits, exposed in the Aktau Mountains (Ili Basin, south-east Kazakhstan), has great potential as a terrestrial palaeoclimate archive. About 90 m of the 1700 m thick succession comprise alluvial mudflat deposits and appear as cyclic alternation of coarse sheet floods, mudflat fines and semi-arid hydromorphic soils. In this study, bulk-sediment mineralogy and geochemistry, magnetic susceptibility, sediment colour and palynology are used to reconstruct environmental conditions by determining changes and forcing mechanisms in the intensity of sediment discharge, weathering and pedogenesis. The results presented here indicate four major periods of arid soil formation and one palustrine interval characterized by higher evaporation rates under highly alkaline/saline conditions. A positive correlation between weathering indices and the Mg/Al ratio suggest that these horizons correspond to maximum rates of evapotranspiration and aridity. The formation of mudflat fines is, instead, interpreted as representing higher detrital sediment production by more intense alluvial fan activity during times of higher precipitation. Time series analysis of weathering indices, colour and magnetic susceptibility data yields cycle-to-frequency ratios with the potential to represent Milankovitch cyclicity with short and long eccentricity as dominant periodicities. Periods of pronounced aridity, paced by long eccentricity forcing, reflect changes in moisture availability. On longer tectonic timescales, the persistent appearance of gypsum indicates a shift towards more arid conditions. This trend in climate is considered to result from the closure of the eastern gateway of the Mediterranean to the Indian Ocean that restricted circulation and enhanced salinity within the Eastern Paratethys.
Terrigenous sediment parameters in modem sea-bottom samples and sediment cores of the South Atlantic are used to infer variations in detrital sources and modes of terrigenous sediment supply in response to environmental changes through the late Quaternary climate cycles. Massaccumulation rates of terrigenous sediment and fluxes of ice-rafted detritus are discussed in terms of temporal variations in detrital sediment input from land to sea. Grain-size parameters ofterrigenous mud document the intensity of bottom-water circulation, whereas clay-mineral assemblages constrain the sources and marine transport routes of suspended fine-grained particulates, controlled by the modes of sediment input and patterns of ocean circulation. The results suggest low-frequency East Antarctic ice dynamics with dominant lOO-kyr cycles and high rates of Antarctic Bottom Water formation and iceberg discharge during interglacial times. In contrast, the more subpolar ice masses of the Antarctic Peninsula also respond to short-term climate variability with maximum iceberg discharges during glacial terminations related to the rapid disintegration of advanced ice masses. In the northern Scotia Sea, increased sediment supply from southern South America points to extended ice masses in Patagonia during glacial times. In the southeastern South Atlantic, changes in regional ocean circulation are linked to global thermohaline ocean circulation and are in phase with northern-hemispheric processes of ice build-up and associated formation of North Atlantic Deep Water, which decreased during glacial times and permitted a wider extension of southern-source water masses in the study area.
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