K-T transition in Deccan traps of central India marks major marine seaway across India.

Deccan intertrappean sediments in central India are generally considered as terrestrial deposits of Maastrichtian age, but the Cretaceous–Tertiary (K–T) position is still unknown. Here we report the discovery of the K–T transition, a marine incursion and environmental changes preserved within the intertrappean sediments at Jhilmili, Chhindwara District, Madhya Pradesh. Integrative biostratigraphic, sedimentologic, mineralogic and chemostratigraphic analyses reveal the basal Danian in the intertrappean sediments between lower and upper trap basalts that regionally correspond to C29r and the C29R/C29N transition, respectively. Intertrappean deposition occurred in predominantly terrestrial semi-humid to arid environments. But a short aquatic interval of fresh water ponds and lakes followed by shallow coastal marine conditions with brackish marine ostracods and early Danian zone P1a planktic foraminifera mark this interval very close to the K–T boundary. This marine incursion marks the existence of a nearby seaway, probably extending inland from the west through the Narmada and Tapti rift valleys. The Jhilmili results thus identify the K–T boundary near the end of the main phase of Deccan eruptions and indicate that a major seaway extended at least 800 km across India.

Measuring large topographic change with InSAR: lava thicknesses, extrusion rate and subsidence rate at Santiaguito volcano, Guatemala

Lava flows can produce changes in topography on the order of 10s-100s of metres. A knowledge of the resulting volume change provides evidence about the dynamics of an eruption. We present a method to measure topographic changes from the differential InSAR phase delays caused by the height differences between the current topography and a Digital Elevation Model (DEM). This does not require a pre-event SAR image, so it does not rely on interferometric phase remaining coherent during eruption and emplacement. Synthetic tests predicts that we can estimate lava thickness of as little as �9 m, given a minimum of 5 interferograms with suitably large orbital baseine separations. In the case of continuous motion, such as lava flow subsidence, we invert interferometric phase simultaneously for topographic change and displacement. We demonstrate the method using data from Santiaguito volcano, Guatemala, and measure increases in lava thickness of up to 140 m between 2000 and 2009, largely associated with activity between 2000 and 2005. We find a mean extrusion rate of 0.43 +/- 0.06 m3/s, which lies within the error bounds of the longer term extrusion rate between 1922-2000. The thickest and youngest parts of the flow deposit were shown to be subsiding at an average rate of �-6 cm/yr. This is the first time that flow thickness and subsidence have been measured simultaneously. We expect this method to be suitable for measurment of landslides and other mass flow deposits as well as lava flows.

Towards radiocarbon calibration beyond 28ka using speleothems from the Bahamas

We present a new speleothem record of atmospheric Δ14C between 28 and 44 ka that offers considerable promise for resolving some of the uncertainty associated with existing radiocarbon calibration curves for this time period. The record is based on a comprehensive suite of AMS 14C ages, using new low-blank protocols, and U–Th ages using high precision MC-ICPMS procedures. Atmospheric Δ14C was calculated by correcting 14C ages with a constant dead carbon fraction (DCF) of 22.7 ± 5.9%, based on a comparison of stalagmite 14C ages with the IntCal04 (Reimer et al., 2004) calibration curve between 15 and 11 ka. The new Δ14C speleothem record shows similar structure and amplitude to that derived from Cariaco Basin foraminifera (Hughen et al., 2004, 2006), and the match is further improved if the latter is tied to the most recent Greenland ice core chronology (Svensson et al., 2008). These data are however in conflict with a previously published 14C data set for a stalagmite record from the Bahamas — GB-89-24-1 (Beck et al., 2001), which likely suffered from 14C analytical blank subtraction issues in the older part of the record. The new Bahamas speleothem ∆14C data do not show the extreme shifts between 44 and 40 ka reported in the previous study (Beck et al., 2001). Causes for the observed structure in derived atmospheric Δ14C variation based on the new speleothem data are investigated with a suite of simulations using an earth system model of intermediate complexity. Data-model comparison indicates that major fluctuations in atmospheric ∆14C during marine isotope stage 3 is primarily a function of changes in geomagnetic field intensity, although ocean–atmosphere system reorganisation also played a supporting role.

Biology as an agent of chemical and mineralogical change in soil

Earthworms have a significant impact on the functioning of soils and the processes that occur within them. Here we review our work on the impact of earthworms on soil mineralogy and chemistry, in particular focusing on the contribution of earthworms to mineral weathering and calcium carbonate in soils and the impact that earthworms have on metal mobility at contaminated sites.

Speleothems as sensitive recorders of volcanic eruptions - the Bronze Age Minoan eruption recorded in a stalagmite from Turkey

Mounting evidence exists that variations in sulphur content in stalagmites are closely linked to changes in volcanic or anthropogenic atmospheric sulphur. The strong dependency of sulphur on soil pH and ecosystem storage, however, can result in a delay of several years to decades in the registration of volcanic eruptions and anthropogenic emissions by stalagmites. Here we present synchrotron-radiation based trace element analysis performed on a precisely-dated section of a stalagmite from Sofular Cave in Northern Turkey. As this section covers the time interval of the intensively studied Minoan volcanic eruption between 1600 and 1650 BC, we can test whether this vigorous eruption can be traced in a stalagmite. Of all measured trace elements, only bromine shows a clear short-lived peak at 1621±251621±25 BC, whereas sulphur and molybdenum show peaks later at 1617±251617±25 and 1589±251589±25 respectively. We suggest that all trace element peaks are related to the Minoan eruption, whereas the observed phasing of bromine, molybdenum and sulphur is related to differences in their retention rates in the soil above Sofular Cave. For the first time, we can show that bromine appears to be an ideal volcanic tracer in stalagmites, as it is a prominent volatile component in volcanic eruptions, can be easily leached in soils and rapidly transferred from the atmosphere through the soil and bedrock into the cave and stalagmite respectively. Highly resolved oxygen and carbon isotope profiles indicate that the Minoan eruption had no detectable climatic and environmental impact in Northern Turkey.

Late Quaternary environmental change in the interior South American tropics: new insight from leaf wax stable isotopes

Stable isotope analysis of leaf waxes in a sediment core from Laguna La Gaiba, a shallow lake located at the Bolivian margin of the Pantanal wetlands, provides new perspective on vegetation and climate change in the lowland interior tropics of South America over the past 40,000 years. The carbon isotopic compositions (δ13C) of long-chain n-alkanes reveal large shifts between C3-and C4-dominated vegetation communities since the last glacial period, consistent with landscape reconstructions generated with pollen data from the same sediment core. Leaf wax δ13C values during the last glacial period reflect an open landscape composed of C4grasses and C3herbs from 41–20ka. A peak in C4abundance during the Last Glacial Maximum (LGM, ∼21ka) suggests drier or more seasonal conditions relative to the earlier glacial period, while the development of a C3-dominated forest community after 20 ka points to increased humidity during the last deglaciation. Within the Holocene, large changes in the abundance of C4 vegetation indicate a transition from drier or more seasonal conditions during the early/mid-Holocene to wetter conditions in the late Holocene coincident with increasing austral summer insolation. Strong negative correlations between leaf wax δ13C and δD values over the entire record indicate that the majority of variability in leaf wax δD at this site can be explained by variability in the magnitude of biosynthetic fractionation by different vegetation types rather than changes in meteoric water δD signatures. However, positive δD deviations from the observed δ13C–δD trends are consistent with more enriched source water and drier or more seasonal conditions during the early/mid-Holocene and LGM. Overall, our record adds to evidence of varying influence of glacial boundary conditions and orbital forcing on South American Summer Monsoon precipitation in different regions of the South American tropics. Moreover, the relationships between leaf wax stable isotopes and pollen data observed at this site underscore the complementary nature of pollen and leaf wax δ13C data for reconstructing past vegetation changes and the potentially large effects of such changes on leaf wax δD signatures.

Magnetotelluric deep soundings, gravity and geoid in the south Sao Francisco craton: Geophysical indicators of cratonic lithosphere rejuvenation and crustal underplating

In the south Sao Francisco craton a circular and 8-m amplitude geoid anomaly coincides with the outcropping terrain of an Archean-Paleoproterozoic basement. Broadband magnetotelluric (MT) data inversions of two radial profiles within the positive geoid and Bouguer gravity anomaly yield geo-electrical crustal sections, whereby the lower crust is locally more conductive (10 to 100 Omega m) in spatial coincidence with a denser lower crust modeled by the gravity data. This anomalous lower crust may have resulted from magmatic underplating, associated with Mesoarchean and Proterozoic episodes of tholeiitic dike intrusion. Long-period MT soundings reveal a low electrical resistivity mantle (20 to 200 Omega m) from depths beyond 120 km. Forward geoid modeling, using the scope of the low electrical resistivity region within the mantle as a constraint, entails a density increase (40 to 50 kg/m(3)) possibly due to Fe enrichment of mantle minerals. However, this factor alone does not explain the observed resistivity. A supplemented presence of small amounts of percolated carbonatite melting (similar to 0.005 vol.%), dissolved water and enhanced oxygen fugacity within the peridotitic mantle are viable agents that could explain the less resistive upper mantle. We propose that metasomatic processes confined in the sub-continental lithospheric mantle foster the conditions for a low degree melting with variable CO(2), H(2)O and Fe content. Even though the precise age of this metasomatism is unknown it might be older than the Early Cretaceous based on the evidence that a high-degree of melting in a lithospheric mantle impregnated with carbonatites originated the tholeiitic dike intrusions dispersed from the southeastern border of the Sao Francisco craton, during the onset of the lithosphere extension and break-up of the western Gondwana. The proxies are the NE Parana and Espinhaco (130 Ma, Ar/Ar ages) tholeiitic dikes, which contain (similar to 3%) carbonatites in their composition. The occurrence of a positive geoid anomaly (+ 10 m) and pre-tholeiites (age > 138 Ma), carbonatites and kimberlites along the west African continental margin (Angola and Namibia) reinforces the presumed age of the Sao Francisco-Congo craton rejuvenation to be prior to its fragmentation in the Lower Cretaceous. (C) 2010 Elsevier B.V. All rights reserved.

The onset of flood basalt volcanism, Northern Parana Basin, Brazil: A precise U-Pb baddeleyite/zircon age for a Chapeco-type dacite

We report the first U-Pb baddeleyite/zircon date for a felsic volcanic rock from the Parana Large Igneous Province in south Brazil. The new date of 134.3 +/- 0.8 Ma for a hypocrystalline Chapeco-type dacite from Ourinhos (northern Parana basin) is an important regional time marker for the onset of flood basalt volcanism in the northern and western portion of the province. The dated dacite was erupted onto basement rocks and is overlain by a high-Ti basalt sequence, interpreted to be correlative with Pitanga basalts elsewhere. This new U-Pb date for the Ourinhos dacite is consistent with the local stratigraphy being slightly older than the few reliable step-heating (40)Ar/(39)Ar dates currently available for overlying high-Ti basalts (133.6-131.5 Ma). This indicates an similar to 3 Ma time span for the building of the voluminous high-Ti lava sequence of the Parana basin. On the other hand, it overlaps the (40)Ar/(39)Ar dates (134.8-134.1 Ma) available for the stratigraphically older low-Ti basalt (Gramado + Esmeralda types) and dacite-rhyolite (Palmas type) sequences from South Brazil, which is consistent with the short-lived character of this volcanism and its rapid succession by the high-Ti sequence. (C) 2010 Elsevier B.V. All rights reserved.

Terrane transfer during the Grenville orogeny: tracing the Amazonian ancestry of southern Appalachian basement through Pb and Nd isotopes

Whole rock Pb isotope data can be used to determine the provenance of different blocks within the Rodinia supercontinent, providing a test for paleogeographic reconstructions. Calculated isotopic values for the source region of the Grenville-deformed SW Amazon craton (Rondonia, Brazil), anchored by published U-Pb zircon ages, are compared to those from the Grenville belt of North America and Grenvillian basement inliers in the southern Appalachians. Both the SW Amazon craton and the allochthonous Blue Ridge/Mars Hill terrane are defined by a similar Pb isotopic signature, indicating derivation from an ancient source region with an elevated U/Pb ratio. In contrast, the Grenville Province of Laurentia (extending from Labrador to the Llano Uplift of Texas) is characterized by a source region with a distinctly lower, time-integrated U/Pb ratio. Published U-Pb zircon ages (ca. 1.8 Ga) and Nd model ages (1.4-2.2 Ga) for the Blue Ridge/Mars Hill terrane also suggest an ancient provenance very different from the rest of the adjacent Grenville belt, which is dominated by juvenile 1.3-1.5 Ga rocks. The presence of mature continental material in rocks older than 1.15 Ga in the Blue Ridge/ Mars Hill terrane is consistent with characteristics of basement rocks from the SW Amazon craton. High-grade metamorphism of the Blue Ridge/Mars Hill basement resulted in purging of U, consistent with observations of the rest of the North American Grenville province. In contrast, the Grenvillian metamorphic history of the Amazon appears to have been much more heterogeneous, with both U enrichment and U depletion recorded locally. We propose that the Blue Ridge/ Mars Hill portion of the Appalachian basement is of Amazonian provenance and was transferred to Laurentia during Grenvillian orogenesis after similar to1.15 Ga. The presence of these Amazonian rocks in southeastern Laurentia records the northward passage of the Amazon craton along the Laurentian margin, following the original collision with southernmost Laurentia at ca. 1.2 Ga. (C) 2004 Elsevier B.V. All rights reserved.

Re-Os isotope and highly siderophile element systematics of the Parana continental flood basalts (Brazil)

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

New historical archeointensity data from Brazil: Evidence for a large regional non-dipole field contribution over the past few centuries

We report new archeointensity data obtained from the analyses of baked clay elements (architectural and kiln brick fragments) sampled in Southeast Brazil and historically and/or archeologically dated between the end of the XVIth century and the beginning of the XXth century AD. The results were determined using the classical Thellier and Thellier protocol as modified by Coe, including partial thermoremanent magnetization (pTRM) and pTRM-tail checks, and the Triaxe protocol, which involves continuous high-temperature magnetization measurements. In both protocols, TRM anisotropy and cooling rate TRM dependence effects were taken into account for intensity determinations which were successfully performed for 150 specimens from 43 fragments, with a good agreement between intensity results obtained from the two procedures. Nine site-mean intensity values were derived from three to eight fragments and defined with standard deviations of less than 8%. The site-mean values vary from similar to 25 mu T to similar to 42 mu T and describe in Southeast Brazil a continuous decreasing trend by similar to 5 mu T per century between similar to 1600 AD and similar to 1900 AD. Their comparison with recent archeointensity results obtained from Northeast Brazil and reduced at a same latitude shows that: (1) the geocentric axial dipole approximation is not valid between these southeastern and northeastern regions of Brazil, whose latitudes differ by similar to 10 degrees, and (2) the available global geomagnetic field models (gufm1 models, their recalibrated versions and the CALSK3 models) are not sufficiently precise to reliably reproduce the non-dipole field effects which prevailed in Brazil for at least the 1600-1750 period. The large non-dipole contribution thus highlighted is most probably linked to the evolution of the South Atlantic Magnetic Anomaly (SAMA) during that period. Furthermore, although our dataset is limited, the Brazilian archeointensity data appear to support the view of a rather oscillatory behavior of the axial dipole moment during the past three centuries that would have been marked in particular by a moderate increase between the end of the XVIIIth century and the middle of the XIXth century followed by the well-known decrease from 1840 AD attested by direct measurements. (C) 2011 Elsevier B.V. All rights reserved.

Archeointensity in Northeast Brazil over the past five centuries

This study presents the first archeointensity results from Northeast Brazil obtained from 14 groups of architectural brick fragments sampled in the city of Salvador, Bahia State (13 degrees S, 38.5 degrees W) and dated between the middle of the XVIth century and the beginning of the XIXth century. The dating is ascertained by historical documents complemented by archeological constraints, yielding in all cases age uncertainties of less than 50 years. Analyses were carried out using two experimental protocols: 1 the ""zero field-in field"" version of the classical Thellier and Thellier method as proposed by Coe (TT-ZI), including partial thermoremanent magnetization (pTRM) and pTRM-tail checks, and 2 the Triaxe procedure involving continuous high temperature magnetization measurements. Both TRM anisotropy and cooling rate effects were taken into account for the intensity determinations. The cooling rate effect was further explored for the TT-ZI protocol using three increasing slow cooling times (5 h, 10 h and 25 h) between 450 C and room temperature. Following archeological constraints, the slowest cooling time was retained in our study, yielding decreases of the raw intensity values by 4% to 14%. For each fragment, a mean intensity was computed and retained only when the data obtained from all specimens (between 2 and 6) satisfied a coherence test at similar to 5%. A total of 57 fragments (183 specimens) was considered for the computations of site-mean intensity values, with derived standard deviations of less than 8% of the corresponding means. When separately computed using the two experimental techniques, the site-mean intensity values always agree to within 5%. A good consistency is observed between intensity values of similar or close ages, which strengthen their reliability. Our data principally show a significant and continuous decrease in geomagnetic field intensity in Northeast Brazil between the first half of the XVIIth century and the XXth century. One result dated to the second half of the XVIth century further suggests that the geomagnetic field intensity reached a maximum around 1600 AD. This evolution is in good agreement with that expected in the city of Salvador from the available global geomagnetic field models. However, the accuracy of these models appears less well constrained between similar to 1550 AD and similar to 1650 AD. (C) 2010 Elsevier B.V. All rights reserved.

Insights into the morphology of the Serra da Cangalha impact structure from geophysical modeling

Forward modeling is commonly applied to gravity field data of impact structures to determine the main gravity anomaly sources. In this context, we have developed 2.5-D gravity models of the Serra da Cangalha impact structure for the purpose of investigating geological bodies/structures underneath the crater. Interpretation of the models was supported by ground magnetic data acquired along profiles, as well as by high resolution aeromagnetic data. Ground magnetic data reveal the presence of short-wavelength anomalies probably related to shallow magnetic sources that could have been emplaced during the cratering process. Aeromagnetic data show that the basement underneath the crater occurs at an average depth of about 1.9 km, whereas in the region beneath the central uplift it is raised to 0.51 km below the current surface. These depths are also supported by 2.5-D gravity models showing a gentle relief for the basement beneath the central uplift area. Geophysical data were used to provide further constraints for numeral modeling of crater formation that provided important information on the structural modification that affected the rocks underneath the crater, as well as on shock-induced modifications of target rocks. The results showed that the morphology is consistent with the current observations of the crater and that Serra da Cangalha was formed by a meteorite of approximately 1.4 km diameter striking at 12 km s-1.

Giant magnetofossils and hyperthermal events

Magnetotactic bacteria biomineralize magnetic minerals with precisely controlled size, morphology, and stoichiometry. These cosmopolitan bacteria are widely observed in aquatic environments. If preserved after burial, the inorganic remains of magnetotactic bacteria act as magnetofossils that record ancient geomagnetic field variations. They also have potential to provide paleoenvironmental information. In contrast to conventional magnetofossils, giant magnetofossils (most likely produced by eukaryotic organisms) have only been reported once before from Paleocene-Eocene Thermal Maximum (PETM; 55.8 Ma) sediments on the New Jersey coastal plain. Here, using transmission electron microscopic observations, we present evidence for abundant giant magnetofossils, including previously reported elongated prisms and spindles, and new giant bullet-shaped magnetite crystals, in the Southern Ocean near Antarctica, not only during the PETM, but also shortly before and after the PETM. Moreover, we have discovered giant bullet-shaped magnetite crystals from the equatorial Indian Ocean during the Mid-Eocene Climatic Optimum (similar to 40 Ma). Our results indicate a more widespread geographic, environmental, and temporal distribution of giant magnetofossils in the geological record with a link to "hyperthermal" events. Enhanced global weathering during hyperthermals, and expanded suboxic diagenetic environments, probably provided more bioavailable iron that enabled biomineralization of giant magnetofossils. Our micromagnetic modelling indicates the presence of magnetic multi-domain (i.e., not ideal for navigation) and single domain (i.e., ideal for navigation) structures in the giant magnetite particles depending on their size, morphology and spatial arrangement. Different giant magnetite crystal morphologies appear to have had different biological functions, including magnetotaxis and other non-navigational purposes. Our observations suggest that hyperthermals provided ideal conditions for giant magnetofossils, and that these organisms were globally distributed. Much more work is needed to understand the interplay between magnetofossil morphology, climate, nutrient availability, and environmental variability.

Re-Os isotope and highly siderophile element systematics of the Parana continental flood basalts (Brazil)

Basalts of the Parana continental flood basalt (PCFB) province erupted through dominantly Proterozoic continental crust during the Cretaceous. In order to examine the mantle source(s) of this major flood basalt province, we studied Os, Sr, Nd, and Pb isotope systematics, and highly siderophile element (HSE) abundances in tholeiitic basalts that were carefully chosen to show the minimal effects of crustal contamination. These basalts define a precise Re-Os isochron with an age of 131.6 +/- 2.3 Ma and an initial Os-187/Os-188 of 0.1295 +/- 0.0018 (gamma Os-187 = +2.7 +/- 1.4). This initial Os isotopic composition is considerably more radiogenic than estimates of the contemporary Depleted Mantle (DM). The fact that the Re-Os data define a well constrained isochron with an age similar to Ar-40/Ar-39 age determinations, despite generally low Os concentrations, is consistent with closed-system behavior for the HSE. Neodymium, Sr, and Pb isotopic data suggest that the mantle source of the basalts had been variably hybridized by melts derived from enriched mantle components. To account for the combined Os, Nd, Sr, and Pb isotopic characteristics of these rocks, we propose that the primary melts formed from metasomatized asthenospheric mantle (represented by arc-mantle peridotite) that underwent mixing with two enriched components, EM-I and EM-II. The different enriched components are reflected in minor isotopic differences between basalts from southern and northern portions of the province. The Tristan da Cunha hotspot has been previously suggested to be the cause of the Parana continental flood basalt magmatism. However, present-day Tristan da Cunha lavas have much higher Os-187/Os-188 isotopic compositions than the source of the PCFB. These data, together with other isotopic and elemental data, preclude making a definitive linkage between the Tristan plume and the PCFB. (C) 2012 Elsevier B.V. All rights reserved.