Rainer Petschick

Clay mineral distribution in surface sediments of the South Atlantic: sources, transport, and relation to oceanography

Abstract Surface samples, mostly from abyssal sediments of the South Atlantic, from parts of the equatorial Atlantic, and of the Antarctic Ocean, were investigated for clay content and clay mineral composition. Maps of relative clay mineral content were compiled, which improve previous maps by showing more details, especially at high latitudes. Large-scale relations regarding the origin and transport paths of detrital clay are revealed. High smectite concentrations are observed in abyssal regions, primarily derived from southernmost South America and from minor sources in Southwest Africa. Near submarine volcanoes of the Antarctic Ocean (South Sandwich, Bouvet Island) smectite contents exhibit distinct maxima, which is ascribed to the weathering of altered basalts and volcanic glasses. The illite distribution can be subdivided into five major zones including two maxima revealing both South African and Antarctic sources. A particularly high amount of Mg- and Fe-rich illites are observed close to East Antarctica. They are derived from biotite-bearing crystalline rocks and transported to the west by the East Antarctic Coastal Current. Chiorite and well-crystallized dioctaedral illite are typical minerals enriched within the Subantarctic and Polarfrontal-Zone but of minor importance off East Antarctica. Kaolinite dominates the clay mineral assemblage at low latitudes, where the continental source rocks (West Africa, Brazil) are mainly affected by intensive chemical weathering. Surprisingly, a slight increase of kaolinite is observed in the Enderby Basin and near the Filchner-Ronne Ice shelf. The investigated area can be subdivided into ten, large-scale clay facies zones with characteristic possible source regions and transport paths. Clay mineral assemblages of the largest part of the South Atlantic, especially of the western basins are dominated by chlorite and illite derived from the Antarctic Peninsula and southernmost South America and supported by advection within the Circumantarctic Deep Water flow. In contrast, the East Antarctic provinces are relatively small. Assemblages of the eastern basins north of 30°S are strongly influenced by African sources, controlled by weathering regimes on land and by a complex interaction of wind, river and deep ocean transport. The strong gradient in clay mineral composition at the Brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins.

Terrigenous Sediment Supply in the Polar to Temperate South Atlantic: Land-Ocean Links of Environmental Changes during the Late Quaternary

Terrigenous sediment parameters in modern 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. Mass-accumulation 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 of terrigenous 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 100-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.

Kaolinite and Chlorite as Tracers of Modern and Late Quaternary Deep Water Circulation in the South Atlantic and the Adjoining Southern Ocean

In the South Atlantic and adjoining Southern Ocean the kaolinite/chlorite-ratio in Late Quaternary sediments are an alternative deep water proxy to benthic foraminiferal proxies and carbonate preservation indices that is even suitable in regions with poor carbonate preservation. This paper shows the relationship between modern abyssal circulation and the kaolinite/chlorite-ratio and presents reconstructions of deep and bottom water advection based on the kaolinite/ chlorite proxy. We also discuss the limitations and future perspectives of the kaolinite/chlorite proxy. Latitudinal and water depth-related patterns of the kaolinite/chlorite-ratio in surface sediments correspond to the modern deep and bottom water mass distribution. Kaolinite originates from low-latitudes and traces North Atlantic Deep Water (northern-source deep water) advection to the south. Chlorite from the southern high-latitudes is exported via northward advecting Antarctic Bottom Water and Circumpolar Deep Water (southern-source deep and bottom water). Deep-sea sedimentation in regions underlying the Antarctic Circumpolar Current was current-dominated throughout the Late Quaternary. Temporal variations of the kaolinite/chlorite-ratio in response to glacial-interglacial cycles reflect changing deep water mass configurations, suggesting a shallowing and northward retreat of northern-source deep water and accordingly wider expansion of southern-source deep and bottom water masses during glacial times relative to interglacial times. Submarine topography influenced the spatial and temporal patterns of deep water mass distribution.

From diagenesis to anchimetamorphism, upper Austroalpine sedimentary cover in Bavaria and Tyrol

AbstractVitrinite reflectance and illite crystallinity of a great number of samples of fine-grained Mesozoic sediments from the Northern Calcareous Alpes between Allgau and the Karwendel Mountains have been investigated. Coalification varies from 0,4 % Rr to 6,2 % Rmax and allows a significant description of the low diagenetic temperature distribution in the greatest part of this area. For most stratigraphiC units, reliable correlations between vitrinite reflectance and illite crystallinity can be established. By means of both methods the fields of diagenesis and anchimetamorphism are subdivided into five stages.A comprehensive coal rank map, based on outcrop specimen is published. It represents the amount and the distribution of the weak alpidic temperature effects within the uppermost Austroalpine nappes. From the northern to the southern rim of the Northern Calcareous Alps and in each tectonic unit, a general temperature increase is charateristic. Nevertheless, rank disconformities and anomalies are wi...

Element mobility and volumetric strain in brittle and brittle–viscous shear zones of the superdeep well KTB (Germany)

Abstract Mass-balance studies of brittle and brittle–viscous shear zones of the superdeep well KTB (Germany) show that element mobility and associated volumetric strain is markedly different in metabasites and paragneisses. Shear zones in metabasites show thickening and volume increase due to mineralization of prehnite (±epidote±calcite) within open fractures and pore space. Gains in Al 2 O 3 , CaO and SiO 2 are compatible with these observations. Shear zones in paragneisses show either constant volume or volume loss. Volume loss of the paragneiss shear zones can be explained by pressure solution of quartz and by retrograde mica-forming reactions associated with significant gains in potassium-group elements (K, Rb, Ba) and losses in SiO 2 . The differences in volumetric strain between paragneiss and metabasite shear zones can be explained by the different deformation mechanisms and rheology of paragneiss and metabasite at T =ca. 250–350°C. The rigid metabasites supported long-lived open fractures and pores where new minerals could precipitate from a fluid phase. The paragneisses, on the other hand, were weak because of increasing amounts of crystal plasticity and pressure solution of quartz which is typical for the brittle–viscous transition of quartz-bearing rocks. Open fractures and pores were rapidly closed hampering the deposition of new minerals. Graphite enrichment has been found in both paragneiss and metabasite shear zones. There is clear evidence that graphite enrichment results from fluid phases which carried C into the shear zones from external sources suggesting that C was largely mobile during the shearing processes.

Terrigenous Flux in the Rio Grande Rise Area during the Past 1500 ka: Evidence of Deepwater Advection or Rapid Response to Continental Rainfall Patterns?

Surface sediment samples and three gravity cores from the eastern terrace of the Vema Channel, the western flank of the Rio Grande Rise, and the Brazilian continental slope were investigated for physical properties, grain size, and clay mineral composition. Discharge of the Rio Doce is responsible for kaolinite enrichments on the slope south of 20° and at intermediate depths of the Rio Grande Rise. The long-distance advection of kaolinite with North Atlantic Deep Water from lower latitudes is of minor importance as evidenced by low kaolinite/chlorite ratios on the Mid-Atlantic Ridge. Cyclic variations of kaolinite/chlorite ratios in all our cores, with maxima in interglacials, are attributed to low- and high-latitude forcing of paleoclimate on the Brazilian mainland and the related discharge of the Rio Doce. A long-term trend toward more arid and “glacial” conditions from 1500 ka to present is superimposed on the glacial-interglacial cyclicity.

Testing 3D computer simulation of carbonate platform growth with REPRO: the Miocene Llucmajor carbonate platform (Mallorca)

Abstract REPRO, a 3D computer-based simulation program for reef and carbonate platform growth, was tested using published data from the late Miocene Llucmajor carbonate platform in Mallorca. The device central to the program is a depth-dependent logistic growth function for carbonate growth combined with linear diffusion, so accounting for the lateral spreading of reefs. Evolution of the carbonate platform was simulated using a composite sea level curve consisting of superposed sine functions with different frequencies and amplitudes. Overall carbonate platform shape and internal geometries as produced by the program match fairly well with the outcrop data. Thus we demonstrate that REPRO has the potential to model lateral facies changes and lateral variations in the depositional geometries of carbonate platforms in three dimensions.

The Saxothuringian-Rhenohercynian boundary underneath the Vogelsberg volcanic field: evidence from basement xenoliths and U-Pb zircon data of trachyte

The present study is focusing on Variscan basement xenoliths, which are present inside pyroclastic rocks of the Miocene Vogelsberg volcanic field (VVF). The investigated samples have been collected from quarries situated at the SW margin of the Vogelsberg (Nickel quarry, Nidda Ober-Widdersheim) and at the NE margin of the Vogelsberg (Rauher Berg quarry, Alsfeld-Brauerschwend). Moreover, we investigated different types of trachyte, which are present (1) as xenoliths inside an alkali basaltic tuff breccia of the Nickel quarry, (2) as trachytic lava from an outcrop at the Hauserhof (Nidda OberWiddersheim), and (3) as part of a trachytic/phonolitic lava dome drilled in the central VVF near Schotten-Sichenhausen. Apart from one garnet-bearing sample, the investigated basement xenoliths consist of phyllite and basic to felsic metavolcanics. Both the metamorphic index minerals and the quartz deformation microfabrics suggest that these rocks underwent greenschist facies metamorphism, and thus can be attributed to the Northern Phyllite Zone of the Variscan chain. These findings allow tracing the Northern Phyllite Zone from the Rhenish Massif via the VVF to the Harz Mountains. Variscan basement underneath the VVF is also documented by inherited U-Pb zircon ages obtained from the Hauserhof trachyte (366 ± 8.5 Ma and 294 ± 3 Ma) and from the trachyte of the Sichenhausen-Eschwald drilling (323 ± 12 Ma to 344 ± 6 Ma and ages ranging from the Middle Proterozoic to Cambrian/Ordovician boundary). The Permian and Carboniferous ages correlate with widespread late-Variscan volcanic activity in Central Europe at ~290 Ma, and with granitoid intrusions along the Rheic suture during the main phase of the collision of the Rhenohercynian and Saxothuringian domains at ~340 Ma, respectively. For the Latest Devonian ages (366 ± 8.5 Ma) several scenarios are possible. Middle Proterozoic to Cambrian/Ordovician ages are regarded as reworking of pre-Variscan sedimentary and magmatic material. The new ages and the garnetbearing xenolith, the metamorphic grade of which does not fit with the greenschist facies rocks of the Northern Phyllite Zone, suggest that rocks of both the Northern Phyllite Zone and the Mid-German Crystalline Zone are present underneath the volcanic rocks of the VVF. The U-Pb zircon ages of trachytic xenoliths of the Nickel quarry suggest the emplacement of a trachytic lava dome at 66.7 ± 0.4 Ma. Similar ages have been obtained from camptonitic dykes and trachyte intrusions within Permian and Triassic sediments exposed at the southwestern margin of the VVF. The large amount of trachyte together with a significant positive magnetic anomaly in the southwestern VVF might indicate a magma chamber that developed at shallow crustal levels below the southwestern part of the present Vogelsberg during the Upper Cretaceous. The new results have been used to develop a model for the early evolution of the southwestern Vogelsberg. Kurzfassung: Die vorliegende Arbeit befasst sich mit variszischen Grundgebirgsxenolithen, die sich in vulkanischen Gesteinen des tertiaren Vogelsberg-Vulkangebiets (VVG) befinden. Die untersuchten Proben stammen aus Steinbruchen am SW-Rand des Vogelsberges (Steinbruch Nickel, Nidda Ober-Widdersheim) sowie am NE-Rand des Vogelsberges (Steinbruch Rauher Berg, Alsfeld-Brauerschwend). Auserdem untersuchten wir verschiedene Arten von Trachyt, der (1) als xenolithische Einschlusse in einer alkalibasaltischen Tuffbrekzie des Steinbruchs Nickel, (2) als trachytische Lava an einem Aufschluss nahe des Hauserhofs (Nidda Ober-Widdersheim) und (3) als Teil eines trachytisch-phonolitischen Lavadoms in einer Bohrung nahe Schotten-Sichenhausen vorkommt.

Climatically forced moisture supply, sediment flux and pedogenesis in Miocene mudflat deposits of south-east Kazakhstan, Central Asia

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 Asias 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.

Erratum to “Testing 3D computer simulation of carbonate platform growth with REPRO: the Miocene Llucmajor carbonate platform (Mallorca)” [Palaeogeogr. Palaeoclimatol. Palaeoecol. 175 (2001) 239–247]

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