Micro- to nano-scale characterization of martite from a banded iron formation in India and a lateritic soil in Brazil

The pseudomorphic transformation of magnetite into hematite (martitization) is widespread in geological environments, but the process and mechanism of this transformation is still not fully understood. Micro- and nano-scale techniques—scanning electron microscopy, focused ion bean transmission electron microscopy, and Raman spectroscopy—were used in combination with X-ray diffraction, Curie balance and magnetic hysteresis analyses, as well as Mössbauer spectroscopy on martite samples from a banded iron formation (2.9 Ga, Dharwar Craton, India), and from lateritic soils, which have developed on siliciclastic and volcanic rocks previously affected by metamorphic fluids (Minas Gerais, Brazil). Octahedral crystals from both samples are composed of hematite with minor patches of magnetite, but show different structures. The Indian crystals show trellis of subhedral magnetite hosting maghemite in sharp contact with interstitial hematite crystals, which suggests exsolution along parting planes. Grain boundary migrations within the hematite point to dynamic crystallization during deformation. Dislocations and fluid inclusions in hematite reflect its precipitation related to a hydrothermal event. In the Brazilian martite, dislocations are observed and maghemite occurs as Insel structures and nano-twin sets. The latter, typical for the hematite, are a transformation product from maghemite into hematite. For both samples, a deformation-induced hydrothermally driven transformation from magnetite via maghemite to hematite is proposed. The transformation from magnetite into maghemite comprises intermediate non-stoichiometric magnetite steps related to a redox process. This study shows that martite found in supergene environment may result from earlier hypogene processes.     

Expansion of the northern hemisphere subtropical high pressure belt: trends and linkages to precipitation and drought

A critical issue in research concerning long-term climate change is the relationship between circulation features and global temperature variations. We establish that the annual areal size of the northern hemisphere subtropical high pressure belt (SHPB), as defined by seven 500-hPa height isohypses, shares over 70% of the variability with global annual near-surface air temperature since 1948. The area enclosed by the 5850-m isohypse of the 500-hPa surface in the northern hemisphere has more than doubled since the 1950s, with greatest increases over northern Africa, the Middle East and India. A long-term historical run of a coupled global climate model shows rapidly increasing SHPB annual sizes, since the mid-1970s. Since the SHPB’s descending air produces increased aridity, SHPB expansion may transition humid regions to more arid lands. To examine this aspect, first, variations in recorded precipitation using a gridded database for the region experiencing expansion of the SHPB show a decrease in precipitation (though significant only at the 88% confidence level) over the last 60?years. Second, variations in a 0.5° spatial resolution monthly drought index, the Standardized Precipitation Evapotranspiration Index, are highly correlated (+0.78) with annual variations in the area enclosed by the 500-hPa height isohypses. These results support those of previous investigations that suggest further northward expansion of the northern hemisphere subtropical dry zones with continued global climate change.     

Exploring Paleogeographic Conditions at Two Paleolithic Sites in Navarino,Southwest Greece,Dated by Optically Stimulated Luminescence

In this paper, we employed optically stimulated luminescence (OSL) dating of sediments from two archaeological sites located in Navarino, Messenia, southwestern Greece, to deduce a chronology for the archaeological sites. Archaeological surveys identified two Paleolithic sites on fossilized coastal dunes. Chipped stone tool assemblages were identified eroding out of paleosols developed in the dunes. The assemblage from one site lacked distinct typological features and hence it was difficult to assign to a chronological period. The lithic assemblage from the other site contained artifacts that typologically can be assigned to the Levallois‐Mousterian. Previous efforts to date the artifact‐bearing sediments at these sites were unsuccessful. Using newer OSL dating methods (i.e., the Single‐Aliquot‐Regenerated Dose protocol and thermally transferred‐OSL[TT‐OSL]), we attempted to construct a chronological framework for Late Pleistocene human activity in the southwest Peloponnese. The revised OSL chronology for the first site is 28 ± 5 ka, while a luminescence age of 8 ± 1 ka for the second site only represents a later deflation event. Within the framework of Quaternary environmental change, the location of Paleolithic sites relative to the coast would have changed during the course of the Pleistocene. As a result, Paleolithic exploitation strategies would have been strongly influenced by the changing coastal geomorphology, encouraging hominids to adapt to new distributions of resources. OSL dating of the archaeological sites allowed us to connect traces of hominid activity with climatic stadials/interstadials of the later Pleistocene derived from existing relative sea‐level curves. Ultimately, these data permitted the reconstruction of regional Late Pleistocene paleogeography. © 2012 Wiley Periodicals, Inc.     

A paleoclimate record with tephrochronological age control for the last glacial-interglacial cycle from Lake Ohrid, Albania and Macedonia

Lake Ohrid is probably of Pliocene age, and the oldest extant lake in Europe. In this study climatic and environmental changes during the last glacial-interglacial cycle are reconstructed using lithological, sedimentological, geochemical and physical proxy analysis of a 15-m-long sediment succession from Lake Ohrid. A chronological framework is derived from tephrochronology and radiocarbon dating, which yields a basal age of ca. 136 ka. The succession is not continuous, however, with a hiatus between ca. 97.6 and 81.7 ka. Sediment accumulation in course of the last climatic cycle is controlled by the complex interaction of a variety of climate-controlled parameters and their impact on catchment dynamics, limnology, and hydrology of the lake. Warm interglacial and cold glacial climate conditions can be clearly distinguished from organic matter, calcite, clastic detritus and lithostratigraphic data. During interglacial periods, short-term fluctuations are recorded by abrupt variations in organic matter and calcite content, indicating climatically-induced changes in lake productivity and hydrology. During glacial periods, high variability in the contents of coarse silt to fine sand sized clastic matter is probably a function of climatically-induced changes in catchment dynamics and wind activity. In some instances tephra layers provide potential stratigraphic markers for short-lived climate perturbations. Given their widespread distribution in sites across the region, tephra analysis has the potential to provide insight into variation in the impact of climate and environmental change across the Mediterranean.     

Pedoarchaeology of Cactus Hill,a sandy Paleoindian site in southeastern Virginia,U.S.A.

Cactus Hill is located in the Virginia Coastal Plain on a terrace above the Nottoway River. The site has a record of occupation that spans the Holocene and also offers evidence of humans late in the Pleistocene before Clovis time. Soil investigations identified several deposit types, and demonstrated that multisequal eolian sands forming the site's primary core are arrayed in spatially and temporally discrete horizons. Resting atop an ancient paleosol, the earliest sand stratum (19,540 ± 70 ¹⁴C yr B.P.) is marked by a conspicuous but culturally sterile buried surface horizon. Eolian sand above this surface supports another sequum in which Clovis and underlying “Blade” artifacts are associated with a fainter surface horizon and pronounced subsoil lamellae. Early Archaic and successively younger artifacts occur above the Clovis level in a more weakly developed uppermost sequum. This soil and cultural stratigraphy, together with considerations of regional topography, demonstrate that the landscape has evolved incrementally since about the last glacial maximum. © 2004 Wiley Periodicals, Inc.     

The Extended Delaunay Method Applied to First Order Resonance

This paper is concerned with the extended Delaunay method as well as the method of integration of the equations, applied to first order resonance. The equations of the transformation of the extended Delaunay method are analyzed in the (p + 1)/p type resonance in order to build formal, analytical solutions for the resonant problem with more than one degree of freedom. With this it is possible to gain a better insight into the method, opening the possibility for more generalized applications. A first order resonance in the first approximation is carried out, giving a better comprehension of the method, including showing how to eliminate the ‘Poincaré singularity’ in the higher orders. This revised version was published online in July 2006 with corrections to the Cover Date.     

河北承德黑山铁矿床热液成矿特征及流体包裹体研究

黑山大型钒钛磁铁矿矿床产于大庙斜长岩杂岩体中,是承德地区最重要的"大庙式"岩浆型铁矿床。笔者在矿区野外地质观察过程中发现,穿插于斜长岩中的铁磷矿脉、磁铁矿硫化物矿脉有热液成矿作用的显示,表明黑山铁矿床成因除传统认为的岩浆期结晶、熔离、矿浆贯入成矿作用外,还有热液期的成矿作用发生。本文对热液成矿期铁磷矿石中磷灰石和矿化蚀变石英中的流体包裹体进行了显微测温和激光拉曼光谱分析。结果表明,磷灰石中原生包裹体可以分为气液两相包裹体、含子矿物包裹体、含液态CO2三相包裹体、单液相、单气相包裹体5类,均一温度主要集中于180～420℃,盐度主要集中于6.2%～38.9%NaCleq,流体包裹体气相成分主要为CO2、N2和CH4,液相成分主要为H2O,固相成分主要为方解石、石盐、白云石及铁氧化物子矿物。石英中流体包裹体类型和成分与磷灰石中的类似,但固相成分未发现石盐和不透明金属子矿物,均一温度变化于149～422℃,盐度变化于5.7%～22.9%NaCleq。成矿流体为CaCl2-NaCl-H2O-CO2体系,均一温度和盐度呈现正相关连续渐变的特征。铁磷矿石的磷灰石中原生包裹体为流体包裹体,盐度高,子矿物种类复杂,组成中富含CO2和CH4等,这些特征显示成矿流体以岩浆热液为主;成矿机理可能与大气降水对岩浆热液的稀释有关。