Neogene and Quaternary rollback evolution of the Tyrrhenian Sea, the Apennines, and the Sicilian Maghrebides

Reconstruction of the evolution of the Tyrrhenian Sea shows that the major stage of rifting associated with the opening of this basin began at similar to10 Ma. It involved two episodes of back arc extension, which were induced by the rollback of a west dipping subducting slab. The first period of extension (10-6 Ma) was prominent in the northern Tyrrhenian Sea and in the western part of the southern Tyrrhenian Sea. The second period of extension, mainly affected the southern Tyrrhenian Sea, began in the latest Messinian (6-5 Ma) and has been accompanied by subduction rollback at rates of 60-100 km Myr(-1). Slab reconstruction, combined with paleomagnetic and paleogeographic constraints, indicates that in the central Apennines, the latest Messinian (6-5 Ma) arrival of a carbonate platform at the subduction zone impeded subduction and initiated a slab tear and major strike-slip faults. These processes resulted in the formation of a narrow subducting slab beneath the Ionian Sea that has undergone faster subduction rollback and induced extreme rates of back arc extension.

5th International Symposium on Applied Isotope Geochemistry - Preface

Selected Papers from the 5th International Symposium on Applied Isotope Geochemistry, Heron Island, Great Barrier Reef, Australia, 26–30 May 2003

Subduction of the Nazca Ridge and the Inca Plateau: Insights into the formation of ore deposits in Peru

A large number of ore deposits that formed in the Peruvian Andes during the Miocene (15-5 Ma) are related to the subduction of the Nazea plate beneath the South American plate. Here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies, namely the Nazca Ridge in central Peru and the now-consumed Inca Plateau in northern Peru, at the subduction zone. Plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench. This is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction. It is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments. (c) 2005 Elsevier B.V All rights reserved.

Re-dating mid-Holocene betelnut (Areca catechu L.) and other plant use at Dongan, Papua New Guinea

Direct accelerator mass spectrometry (AMS) dating of anaerobically preserved plant remains from the Dongan site in New Guinea, combined with assessment of preservation condition, confirms earlier doubts about the antiquity of betel-nut (Areca catechu L.) found at the site. A possible sago leaf fragment is also identified as a modem contaminant. The mid-Holocene age of other fruit and nut remains is verified using these methods. The utility of AMS dating in combination with detailed archaeobotanical assessment is demonstrated, thus improving chronometric hygiene and with it knowledge of past plant use in Oceania.

Timing and duration of the last interglacial inferred from high resolution U-series chronology of stalagmite growth in Southern Hemisphere

High-precision Th-230-U-238 ages for a stalagmite from Newdegate Cave in southern Tasmania, Australia define a rare record of precipitation between 100 and 155 ka before the present. The fastest stalagmite growth occurred between 129.2 +/- 1.6 and 122.1 +/- 2.0 ka (similar to 61.5 mm/ka), coinciding with a time of prolific coral growth from Western Australia (128-122 ka). This is the first high-resolution continental record in the Southern Hemisphere that can be compared and correlated with the marine record. Such correlation shows that in southern Australia the onset of full interglacial sea level and the initiation of highest precipitation on land were synchronous. The stalagmite growth rate between 129.2 and 142.2 ka (similar to 5.9 mm/ka) was lower than that between 142.2 and 154.5 ka (similar to 18.7 mm/ka), implying drier conditions during the Penultimate Deglaciation, despite rising temperature and sea level. This asymmetrical precipitation pattern is caused by latitudinal movement of subtropical highs and an associated Westerly circulation, in response to a changing Equator-to-Pole temperature gradient. Both marine and continental records in Australia strongly suggest that the insolation maximum between 126 and 128 ka at 65 degreesN was directly responsible for the maintenance of full Last Interglacial conditions, although the triggers that initiated Penultimate Deglaciation (at similar to 142 ka) remain unsolved. (C) 2001 Elsevier Science B.V. All rights reserved.

Early-Mid Holocene climatic variations in Tasmania, Australia: multi-proxy records in a stalagmite from Lynds Cave

Mass spectrometric uranium-series dating and C-O isotopic analysis of a stalagmite from Lynds Cave, northern Tasmania, Australia provide a high-resolution record of regional climate change between 5100 and 9200 yr before present (BP). Combined delta(18)O, delta(13)C, growth rate, initial U-234/U-238 and physical property (color, transparency and porosity) records allow recognition of seven climatic stages: Stage I ( > 9080 yr BP) - a relatively dry period at the beginning of stalagmite growth evidenced by elevated U-234/U-238; Stage II (9080-8600 yr BP) - a period of unstable climate characterized by high-frequency variability in temperature and bio-productivity; Stage 111 (8600-8000 yr BP) - a period of stable and moderate precipitation and stable and high bio-productivity, with a continuously rising temperature; Stage IV (8000-7400 yr BP) - the warmest period with high evaporation and low effective precipitation (rainfall less evaporation); Stage V (7400-7000 yr BP) - the wettest period with highest stalagmite growth and enhanced but unstable bio-productivity; Stage VI (7000-6600 yr BP) - a period with a significantly reduced precipitation and bio-productivity without noticeable change in temperature; Stage VII (6600-5100 yr BP) - a period of lowest temperature and precipitation marking a significant climatic deterioration. Overall, the records suggest that the warmest climate occurred between 8000 and 7400 yr BP, followed by a wettest period between 7400 and 7000 yr BP. These are broadly correlated with the so-called 'Mid Holocene optimum' previously proposed using pollen and lake level records. However, the timing and resolution of the speleothem. record from Lynds Cave are significantly higher than in both the pollen and lake level records. This allows us to correlate the abrupt change in physical property, delta(18)O, delta(13)C, growth rate, and initial U-234/U-238 of the stalagmite at similar to8000 yr BP with a global climatic event at Early-Mid Holocene transition. (C) 2001 Elsevier Science B.V. All rights reserved.

The geochemistry of late Archaean microbial carbonate: Implications for ocean chemistry and continental erosion history

Trace element concentrations and combined Sr- and Nd-isotope compositions were determined on stromatolitic carbonates (microbialites) from the 2.52 Ga Campbellrand carbonate platform (South Africa). Shale-normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive La and Y anomalies and in being depleted in light rare earth elements. In contrast to modem seawater (and microbialite proxies), the 2.52 Ga samples lack a negative Ce anomaly but possess a positive Eu anomaly. These latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where, unlike today, hydrothermal Fe input was not oxidised, and scavenged and rare earth elements were not coprecipitated with Fe-oxyhydroxides. The persistence of a positive Eu anomaly in relatively shallow Campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated (Archaean) to continental erosion-dominated (Phanerozoic) rare earth element flux ratio. The dominant hydrothermal input is also expressed in the initial Sr- and Nd-isotope ratios. There is collinear variation in Sr-Nd systematics, which range from primitive values (Sr-87/Sr-86 of 0.702386 and epsilon (Nd) of +2.1) to more evolved crustal ratios. Mixing calculations show that the range in trace element ratios (e.g., Y/Ho) and initial isotope ratios is not a result of contamination by trapped sediment, but that the chemical band isotopic variation reflects carbonate deposition in an environment where different water masses mixed. Calculated Nd flux ratios yield a hydrothermal input into the 2.52 Ga oceans one order of magnitude larger than continental input. Such a change in flux ratio most likely required substantially reduced continental inputs, which could, in turn, reflect a plate tectonic causation (e.g., reduced topography or expansion of epicontinental seas). Copyright (C) 2001 Elsevier Science Ltd.

Complex natural resonances of conducting planar objects buried in a dielectric half-space

A scheme is presented to incorporate a mixed potential integral equation (MPIE) using Michalski's formulation C with the method of moments (MoM) for analyzing the scattering of a plane wave from conducting planar objects buried in a dielectric half-space. The robust complex image method with a two-level approximation is used for the calculation of the Green's functions for the half-space. To further speed up the computation, an interpolation technique for filling the matrix is employed. While the induced current distributions on the object's surface are obtained in the frequency domain, the corresponding time domain responses are calculated via the inverse fast Fourier transform (FFT), The complex natural resonances of targets are then extracted from the late time response using the generalized pencil-of-function (GPOF) method. We investigate the pole trajectories as we vary the distance between strips and the depth and orientation of single, buried strips, The variation from the pole position of a single strip in a homogeneous dielectric medium was only a few percent for most of these parameter variations.