Earth and Planetary Sciences Letters

Published by Elsevier
Online ISSN: 0012-821X
Publications
Article
A suite of morphologically distinctive silicon carbide (SiC) grains from the Orgueil and Murchison carbonaceous chondrite meteorites contains Si and C of highly anomalous isotopic composition. All of the SiC grains in this suite are characterized by a distinctive platy morphology and roughly developed hexagonal crystal forms that allow them to be distinguished from other types of SiC found in the host meteorites. The δ^(29)Si and δ^(30)Si values of individual SiC crystals deviate from those of normal solar material by more than 100‰, while the δ^(13)C values range from 150 to 5200‰. Isotopically normal C and Si are not found in any of these SiC crystals. The SiC grains belonging to this morphological suite are isotopically distinct from fine-grained SiC aggregates and other morphological types of SiC in unequilibrated meteorites. The ^(29)Si/^(28)Si and ^(30)Si/^(28)Si ratios of these platy grains are well correlated and define a linear array that does not pass through the composition of normal, solar Si. This behavior contrasts sharply with the diverse and poorly correlated Si isotopic compositions shown by the total SiC population. We suggest that the distinctive morphological characteristics and comparatively simple Si isotope systematics identify the platy SiC crystals as a genetically related family, formed around a single, isotopically heterogeneous presolar star or an association of related stars. The enrichments in ^(13)C and the Si isotope systematics of the platy SiC are broadly consistent with theoretical models of nucleosynthesis in low-mass, carbon stars on the asymptotic giant branch. The Si isotope array most plausibly reflects mixing between ^(28)Si-rich material, inherited from a previous generation of stars, and material enriched in ^(29)Si and ^(30)Si, produced in intershell regions by neutron capture during He-burning. ^(13)C is also produced in intershell regions by proton reactions on ^(12)C seed nuclei and is carried withs-process nuclei to the stellar envelope by convection which penetrates down to the He shell. The absence of a correlation between the Si and C isotopic compositions of the SiC suggests either episodic condensation of SiC, extending over several thermal pulses, in the atmosphere of a single star, or derivation of the SiC from several stars characterized by different rates of ^(13)C production. In the multiple star scenario, the linear correlation of the ^(29)Si/^(28)Si and ^(30)Si/^(28)Si ratios among the platy SiC indicates that these stars evolved from a common Si seed composition under similar conditions of neutron-capture nucleosynthesis. The ^(29)Si/^(30)Si ratio of the SiC, inferred by us to be produced by neutron capture in the stellar interior, is distinct from values calculated from models of nucleosynthesis in AGB stars.
 
Article
We present a feasibility study for using (236)U as an oceanic circulation tracer based on depth profiles of (236)U and (137)Cs in the Japan/East Sea. The concentration of the predominantly anthropogenic (236)U, measured with Accelerator Mass Spectrometry (AMS), decreased from (13±3)×10(6) atom/kg in surface water to (1.6±0.3)×10(6) atom/kg close to the sea floor (2800 m). The profile has a smooth trend with depth and concentration values are generally proportional to that of (137)Cs for the same water samples, but with a slightly lower ratio of (137)Cs/(236)U below 2000 m. The cumulative inventory of dissolved (236)U in the water column was estimated to be (13.7±0.9)×10(12) atom/m(2), which is similar to the global-fallout level (17.8×10(12) atom/m(2)) in Japan. Additional analyses of suspended solids (SS) and bottom sediments yielded negligible amounts of (236)U. Our results suggest that (236)U behaves as a conservative nuclide in seawater, with potential advantages over other tracers of oceanic circulation.
 
The coral core HMF-1 under X-ray: a well-defined stratigraphy going back 100 years. The prominent dense double band in the piece in the upper left corner marks the years 1998 and 1999, when the colony was stressed by both exceptionally high sea surface temperatures (due to the 1998 El Nino) and Hurricane Mitch (Carilli et al., 2009a).
Global stratospheric 236U fall-out in kg/month. Northern (solid) and southern (dashed) hemispheric fall-out from testing of thermonuclear devices are shown separately. This is a new version of the model presented in the UNSCEAR (2000) report, adapted to gauge 236U production from these tests. The initial output of the model is actually in terms kt TNT equivalent fission yield, which has been converted to kg of 236U using our conversion factor (7.4kg per Mt TNT equivalent) derived from the 236U/238U ratios in the coral core record. The fall-out model indicates that by the end of 1964 more than 80% of all bomb-produced 236U had already been deposited on the earth's surface.
Sketch of the model depicting the flux of 236U between the boxes. The fall-out is separated into a Caribbean (“northern”) and a south Atlantic (“southern”) fall-out which accumulates in the mixed layer until the water arrives at the location of our coral core. During transit from the south Atlantic to the coral core some 236U is lost to the ocean interior due to diffusion.
The 236U/238U record from the HMF-1 coral core compared to our simple model: the 236U/238U data is shown with associated error bars for the isotope ratio (1σ) and the timing uncertainty for the coral slices (total range estimate); the model is shown as a black line interpolated between the calculated points.
Surface currents of the South Atlantic and the location of coral core HMF-1. Shown are the approximate locations of the South Equatorial Current (SEC) with the southern (sSEC), central (cSEC), and northern (nSEC) branch; the South Equatorial Counter Current (SECC); the Equatorial Undercurrent (EUC); the North Equatorial Counter Current (NECC); the north equatorial current (NEC) and the North Brazil Current (NBC).(Stramma and Schott, 1999) The cross-hair marks the location of our coral core. The solid line marks the shortest plausible trajectory from the central South Atlantic to the Caribbean Sea.
Article
Anthropogenic (236)U (t ½=23.4 My) is an emerging isotopic ocean tracer with interesting oceanographic properties, but only with recent advances in accelerator mass spectrometry techniques is it now possible to detect the levels from global fall-out of nuclear weapons testing across the water column. To make full use of this tracer, an assessment of its input into the ocean over the past decades is required. We captured the bomb-pulse of (236)U in an annually resolved coral core record from the Caribbean Sea. We thereby establish a concept which gives (236)U great advantage - the presence of reliable, well-resolved chronological archives. This allows studies of not only the present distribution pattern, but gives access to the temporal evolution of (236)U in ocean waters over the past decades.
 
Article
We have measured the helium abundance and isotopic composition of a suite of Lower Ordovician marine limestones and associated fossil meteorites from Kinnekulle, Sweden. Limestone 3He/4He ratios as high as 11.5 times the atmospheric value in fused samples and up to 23 times atmospheric in a single step-heat fraction indicate the presence of extraterrestrial helium, and demonstrate that at least a fraction of the extraterrestrial 3He carried by interplanetary dust particles must be retained against diffusive and diagenetic losses for up to 480 Ma. The carrier phase has not been identified but is not magnetic. Extrapolation of high-temperature 3He diffusivities in these sediments is consistent with strong retention of extraterrestrial 3He under ambient Earth-surface conditions. Combination of the observed helium concentrations with sedimentation rates estimated from conodont biostratigraphy suggest that the flux of extraterrestrial 3He in the Early Ordovician was about 0.5 x 10(-12) cm3 STP cm-2 ka-1, ignoring potential post-deposition helium loss. This value is indistinguishable from the average 3He flux estimated for the Cenozoic Era. In contrast, previous studies of fossil meteorites, Ir abundances, and Os isotopic ratios in the limestone suggest that the total accretion rate of extraterrestrial material during the studied interval was at least an order of magnitude higher than the Cenozoic average. This disparity may reflect significant post-depositional loss of 3He from IDPs within these old limestones; if so, the match between the Ordovician flux and the Cenozoic average would be fortuitous. Alternatively, the size distribution of infalling objects during the Early Ordovician may have been enriched only in extraterrestrial material too large to retain 3He during atmospheric entry heating (> approximately 30 micrometers). The fossil meteorites themselves also preserve extraterrestrial helium. Meteorite 3He concentrations of 2 to 9 x 10(-12) cm3 STP g-1 are several orders of magnitude lower than found in most modern meteorites, suggesting very substantial helium loss (probably >99.9%) from these chemically altered objects. The Meteorites carry 3He concentrations only a factor of a few higher than the host limestones. The meteorites themselves cannot be the source of the extraterrestrial 3He observed in the limestones.
 
Article
Large systematic discrepancies currently exist among the carbon data reported for oceanic basalts by different laboratories. These discrepancies are likely attributable both to non-uniform criteria for the removal of carbon contamination from the samples, and to systematic errors in analytical procedures. In order to solve these problems, investigators must agree upon effective criteria for the removal of carbon contamination, and they must create an accurately calibrated set of balsaltic glass carbon standards.
 
Article
The degree of impact-induced devolatilization of nonporous serpentine, porous serpentine, and deuterium-enriched serpentine was investigated using two independent experimental methods, the gas recovery method and the solid recovery method, yielding consistent results. The gas recovery method enables determination of the chemical and hydrogen isotopic composition of the recovered gases. Experiments on deuterium-enriched serpentine unambiguously identify the samples as the source of the recovered gases, as opposed to other possible contaminants. For shock pressures near incipient devolatilization (Pinitial = 5.0 GPa), the hydrogen isotopic composition of the evolved gas is similar to that of the starting material. For higher shock pressures the bulk evolved gas is significantly lower in deuterium than the starting material. There is also significant reduction of H2O to H2 in gases recovered at higher shock pressures, probably caused by reaction of evolved H2O with the metal gas recovery fixture. The hydrogen isotopic fractionation between the evolved gas and the residual solid indicates nonequilibrium, kinetic control of gas-solid isotopic ratios. In contrast, gaseous H2O-H2 isotopic fractionation suggests high temperature (800-1300 K) isotopic equilibrium between the gaseous species, indicating initiation of devolatilization at sites of greater than average energy deposition (i.e., shear bands). Impact-induced hydrogen isotopic fractionation of hydrous silicates during accretion can affect the distribution of hydrogen isotopes of planetary bodies during accretion, leaving the interiors enriched in deuterium. The significance of this process for planetary development depends on the models used for extrapolation of the observed isotopic fractionation to devolatilizations greater than those investigated experimentally and assumptions about timing and rates of protoatmosphere loss, frequency of multiple impacts, and rates of gas-solid or gas-melt isotopic re-equilibration. A simple model indicates that substantial planetary interior enrichments of D/H relative to that of the incident material can result from impact-induced hydrogen fractionation during accretion.
 
Article
We report precise 207Pb/206Pb single zircon evaporation ages for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, and from granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group yield ages between 3445 +/- 3 and 3416 +/- 5 Ma and contain older crustal components represented by a 3504 +/- 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 +/- 5 and 3225 +/- 3 Ma in age which we interpret to reflect the time of crystallization. A surprisingly complex xenocryst population in one sample documents ages from 3323 +/- 4 to 3522 +/- 4 Ma. We suspect that these xenocrysts were inherited, during the passage of the felsic melts to the surface, from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite-trondhjemite plutons along the southern and western margins of the BGB, and units predating any of the exposed greenstone or intrusive rocks. Several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 Ma. coeval with or slightly older than Onverwacht felsic volcanism, while the Kaap Valley pluton along the northwestern margin of the belt is coeval with Fig Tree dacitic volcanism. These results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites. Some of the volcanic plutonic units contain zircon xenocrysts older than any exposed rocks. These indicate the existence of still older units, possibly stratigraphically lower and older portions of the greenstone sequence itself, older granitoid intrusive rocks, or bodies of older, unrelated crustal material. Our data show that the Onverwacht and Fig Tree felsic units have distinctly different ages and therefore do not represent a single, tectonically repeated unit as proposed by others. Unlike the late Archaean Abitibi greenstone belt in Canada, which formed over about 30 Ma. exposed rocks in the BGB formed over a period of at least 220 Ma. The complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones. A surprising similarity in rock types, tectonic evolution, and ages of the BGB in the Kaapvaal craton of southern Africa and greenstones in the Pilbara Block of Western Australia suggests that these two terrains may have been part of a larger crustal unit in early Archaean times.
 
Article
The Chicxulub impact crater in Mexico is the site of the impact purported to have caused mass extinctions at the Cretaceous/Tertiary (K/T) boundary. 2-D hydrocode modeling of the impact, coupled with studies of the impact site geology, indicate that between 0.4 and 7.0 x 10(17) g of sulfur were vaporized by the impact into anhydrite target rocks. A small portion of the sulfur was released as SO3 or SO4, which converted rapidly into H2SO4 aerosol and fell as acid rain. A radiative transfer model, coupled with a model of coagulation indicates that the aerosol prolonged the initial blackout period caused by impact dust only if the aerosol contained impurities. A larger portion of sulfur was released as SO2, which converted to aerosol slowly, due to the rate-limiting oxidation of SO2. Our radiative transfer calculations, combined with rates of acid production, coagulation, and diffusion indicate that solar transmission was reduced to 10-20% of normal for a period of 8-13 yr. This reduction produced a climate forcing (cooling) of -300 Wm-2, which far exceeded the +8 Wm-2 greenhouse warming, caused by the CO2 released through the vaporization of carbonates, and therefore produced a decade of freezing and near-freezing temperatures. Several decades of moderate warming followed the decade of severe cooling due to the long residence time of CO2. The prolonged impact winter may have been a major cause of the K/T extinctions.
 
Article
We present a study of the textural signature of terrestrial weathering and related biological activity in the Tatahouine meteorite. Scanning and transmission electron microscopy images obtained on the weathered samples of the Tatahouine meteorite and surrounding soil show two types of bacteria-like forms lying on mineral surfaces: (1) rod-shaped forms (RSF) about 70-80 nm wide and ranging from 100 nm to 600 nm in length; (2) ovoid forms (OVF) with diameters between 70 and 300 nm. They look like single cells surrounded by a cell wall. Only Na, K, C, O and N with traces of P and S are observed in the bulk of these objects. The chemical analyses and electron diffraction patterns confirm that the RSF and OVF cannot be magnetite or other iron oxides, iron hydroxides, silicates or carbonates. The sizes of the RSF and OVF are below those commonly observed for bacteria but are very similar to some bacteria-like forms described in the Martian meteorite ALH84001. All the previous observations strongly suggest that they are bacteria or their remnants. This conclusion is further supported by microbiological experiments in which pleomorphic bacteria with morphology similar to the OVF and RSF objects are obtained from biological culture of the soil surrounding the meteorite pieces. The present results show that bacteriomorphs of diameter less than 100 nm may in fact represent real bacteria or their remnants.
 
Article
An important but poorly understood factor that affects diffusion rates is the role of speciation during diffusion of a multi-species component. The diffusion of such a component is complicated by the different diffusion coefficient of each species and the interconversion reactions among the species. These complexities can be treated by a diffusion equation that incorporates the diffusive fluxes of all species contributing to the concentration of the component. The effects of speciation on the diffusion of the component can be investigated experimentally in some simple cases by measuring concentration profiles of all species developed during diffusion experiments or by studying some of their other consequences. Experimental data on water diffusion in rhyolitic glasses indicate that although dissolved water is present as two species. H_2O molecules and OH groups, molecular H_2O is the dominant diffusing species at very low to high water concentrations. This explains the apparently complex behavior of water diffusion. Experimental data on oxygen diffusion in some silicates using ^(18)O tracers in the form of H_(2)^(18)O are consistent with the idea that ^(18)O transport is dominated by diffusion of H_2O molecules even at lower water contents (ppm or less). This explains why oxygen transport depends on the presence of water and generally depends on water fugacity linearly. For this mode of oxygen transport, there is a simple theoretical relationship between the effective total oxygen diffusion coefficient and the total water diffusion coefficient that is a function of only the water concentration of the silicate at low water content. This relationship appears to describe quantitatively the existing data over a wide range in water contents and diffusion coefficients in several phases.
 
Article
Ages of craters in the record of impacts on earth may be uniformly period, totally random, or a mixture of the two. These alternatives are studied through numerical simulation wherein time-series analysis is performed on real and simulated sequences to which random noise has been added to represent age-dating uncertainty. We conclude that the real record is most likely to have been generated by a mixture of random and periodic impacts, with the random events constituting the majority.
 
Article
The volume of Earth's oceans may be determined by a dynamic mechanism involving exchange of water between the crust and the mantle. Fast-spreading mid-ocean ridges are currently submerged to a depth at which the pressure is close to the critical pressure for seawater. This ensures optimal convective heat transport and, hence, maximal penetration of hydrothermal circulation along the ridge axes. The oceanic crust is hydrated to a depth of a kilometer or more and can therefore carry a substantial flux of water to the upper mantle when it is subducted. The current ingassing rate of water by this process is probably at least sufficient to balance the outgassing rate. If the oceans were shallower, as they may have been in the distant past, convective heat transport would be reduced and the depth of hydrothermal penetration and crustal hydration would decrease. Outgassing would exceed ingassing and ocean volume would increase. The system is self-stabilizing as long as the depth of the oceans does not exceed its present value. This mechanism could explain why continental freeboard has remained approximately constant since the Archean despite probable increases in continental area.
 
Article
We examined the cause of the fluctuation in recurrence intervals of an M4.8±0.1 ‘characteristic earthquake’ sequence off-Kamaishi, NE Japan by comparing the recurrence intervals with creep (quasi-static slip) rates around the asperity estimated from small repeating earthquake data. The eight recurrence intervals (5.52 yrs on average) of the sequence are almost constant but have small fluctuations (standard deviation of 0.68 yrs) characterized by a longer recurrence interval following a shorter one. From the analysis of small repeating earthquakes from 1990 to 2003, we found a significant acceleration of creep on the plate boundary during and after an earthquake swarm in 1992 to the east of the M4.8±0.1 event. The acceleration seems to have migrated from east (shallower part of the plate boundary) to west (deeper part) over about 80 km in 2 yrs to reach the asperity of the M4.8±0.1 events. The 1995 event that occurred with the shortest recurrence interval was probably advanced by this acceleration of creep in around 1994. We also found similar earthquake swarms prior to the 1962 and 1973 ‘characteristic’ events that occurred after the second and third shortest recurrence intervals respectively. Therefore it is possible that the 1962 and 1973 events were also advanced by creep accelerations. The three pairs of longer and shorter recurrence intervals we observed suggest the existence of creep deceleration following the acceleration. We conclude that the recurrence interval fluctuations of the M4.8±0.1 events are mainly controlled by temporal changes of creep around its asperity.
 
Article
We have measured compressional wave velocity (VP) in talc as a function of temperature at 0.5 GPa. VP falls from 5.27±0.23 km s−1 at 25°C to 4.35±0.15 km s−1 at 800°C. Combining these results with a previously published bulk modulus (K) and δK/δP, gives a Poisson’s ratio (ν) of 0.268 and a shear modulus (G) of 22.6 GPa under atmospheric conditions. Assuming that ν is independent of temperature gives a δK/δT value of −19.3±0.64 MPa K−1 at 0.5 GPa. We used the experimental data, in combination with published data, to calculate bounds on the elastic moduli of a peridotite hydrated with talc just above cold subducting lithosphere. We find that an assemblage hydrated by only 0.9 wt% H2O has elastic wave velocities (calculated from the Voigt–Reuss–Hill average) lower than the anhydrous rock by an average of 7.6% for VP, and 9.7% for shear waves (Vs) at 700°C. These results are consistent with observations of converted phase behavior in the vicinity of the surface of a cold subducting slab, and may place the low-velocity layer observed in the lowermost mantle wedge, not the uppermost slab as has previously been suggested.
 
Article
A theoretical model of grain size variation of domain transitions in titanomagnetite (x = 0.6) as a function of oxidation (z) is presented. The superparamagnetic (SP) to single-domain (SD) transition ds, the SD to two-domain (TD) transition d0, the TD to three-domain (3D) transition and the pseudo-single domain (PSD) to multi-domain (MD) transition are calculated as a function of z. It is shown that all the transition grain sizes increase with z, except for the PSD-MD transition for z > 0.6. The calculations predict that ds increases from 0.044 to 0.197 μm, d0 increases from 0.54 to 13 μm, the TD-3D transition increases from 1.6 to 49 μm as z varies from 0 to 0.8. The PSD-MD transition increases from 42 μm at z = 0 to 150 μm at z = 0.6, whereas between z = 0.6 to z = 0.8, the PSD-MD transition decreases to 49 μm. Qualitatively, the model explains some of the trends in magnetic properties of submarine basalts with low-temperature oxidation. Quantitatively, the model does give reasonable estimates of the PSD-MD boundary and d0, which are close to the experimental values for x = 0.6 and z = 0. Furthermore, the model predicts that psarks or two-domain grains could be the major contributors to the remanence of oxidized submarine pillow basalts.
 
Article
The melting curve of aluminum was measured in a laser-heated diamond cell up to a pressure of 0.8 Mbar in order to test the agreement between this technique and shock wave measurements, which has been lacking in the case for iron. At this pressure, which is over an order of magnitude higher than in previous experiments [1, 2], the melting temperature is 3800 K, comparable to that measured for iron at 2 Mbar [3]. The present results for aluminum extrapolate smoothly to the previous melting measurements in a multi-anvil apparatus to 60 kbar and to the calculated shock melting point of 4750 K at 1.25 Mbar. They are also in excellent agreement with theoretical calculations. A review of the shock data reported for Al, Ta and Mo, close-packed metals, in which a break in the sound velocity-pressure curve is used to determine the melting pressure, shows that the change in velocity at melting is about 10% for all three metals. In the case of iron, the sound velocity data have been used to infer two transitions: a solid-solid transition at 2.0 Mbar and melting at 2.4 Mbar, each of these transitions having about a 5% change in sound velocity. It is unlikely that a phase transition between close-packed cubic structures will have a 5% velocity change, the same as is found in the melting transition. We therefore suggest that for iron there exists only a single transition, starting at 2.0 Mbar, a region of incomplete shock melting between 2 and 2.4 Mbar, and a total change in sound velocity of about 10%, which is closer to the value of the other metals studied. This interpretation introduces a very good agreement between the shock melting results of Brown and McQueen [4] and diamond cell measurements for iron [3] which has up to now been lacking.
 
Article
We present results from the investigation of the primary productivity record over the last 250 kyr in the North Canary Basin (30°N) off Northwest Africa. Two distinct productive systems interfere in this area: the oligotrophic open ocean and the upwelling filament off Cape Ghir, that occasionally carries offshore cool nutrient-rich waters. The following geochemical and micropaleontological paleoproductivity proxies have been used in our study: calcium carbonate, barium excess (Baexcess), total organic carbon (TOC) and diatoms. Time series analysis of these proxies indicates that paleoproductivity in the North Canary Basin underwent important changes following precession and eccentricity cycles. While the precessional signal appears to be mainly related to trade wind strength, superimposed peaks in Baexcess, TOC and diatom records point to large productivity events at Terminations I, II and III. Lowering of the North Atlantic sea surface temperatures by melt water discharges which in turn strengthened the Azores high-pressure center and increased trade wind velocities is postulated as the mechanism to explain the enhancement of the coastal upwelling and associated filaments at terminations. Additionally, the Canary Current may play a role in transmitting cold melt waters and nutrients from higher latitudes to the North Canary Basin.
 
Article
A paleoenvironmental reconstruction of terrestrial environments in Southwestern France between 33 and 15 cal kyr BP is provided using δ13C and δ15N variations in collagen of three herbivorous mammals. Altogether 161 analyses have been carried out on collagen extracted from skeletal fragments of reindeer, horse and Bos/Bison from four successive chronological phases covering the end of MOIS 3 and MOIS 2. The δ13C values of ungulate collagen are clearly separated between the studied species. They are interpreted as reflecting a stable dietary adaptation in a changing environment. The variations of δ15N values of ungulate collagen are significant, especially between specimens from MOIS 3 and specimens from MOIS 2, with a minimum during the Last Glacial Maximum. This phenomenon seems to reflect changes in the activity of nitrogen cycling processes associated with permafrost development. Carbon and nitrogen isotopic composition of fossil herbivore collagen are worth investigating as paleoecological and paleoenvironmental tracers in Upper Pleistocene periglacial continental contexts.
 
Article
The production of 10Be in the Earth’s atmosphere depends on the galactic cosmic ray influx that, in turn, is affected by the solar surface magnetic activity and the geomagnetic dipole strength. Using the estimated changes in 10Be production rate and the geomagnetic field intensity, variations in solar activity are calculated for the last 200 ka. Large variations in the solar activity are evident with the Sun experiencing periods of normal, enhanced and suppressed activity. The marine δ18O record and solar modulation are strongly correlated at the 100 ka timescale. It is proposed that variations in solar activity control the 100 ka glacial–interglacial cycles. However, the 10Be production rate variations may have been under-estimated during the interval between 115 ka and 125 ka and may have biased the results. Future tests of the hypothesis are discussed.
 
Article
Observational evidence for sea-level change along the French Mediterranean coast has been examined and compared with glacio-hydro-isostatic models to predict the spatial and temporal patterns of change for about the past 30 000 14C years. These predictions are pertinent to discussions of changing ocean volumes during this interval, the tectonic stability or otherwise of the coastal areas, mantle rheology, and the timing of possible human occupation of the now submerged coastal plain and caves, such as Cosquer Cave near Marseille. The principal results from the analysis are: (i) sea levels along this section of the coast have risen continually since the time of the last glacial maximum (LGM) and at no time during the Holocene has the mean sea level been higher than that of today. (ii) The coast has been tectonically stable between Marseille and Nice as well as further to the west in Roussillon. Western Corsica may have experienced a slow tectonic uplift of between 0.15–0.3 mm/year for the past 3000 years but northernmost Corsica appears to have been stable during this same interval. (iii) During the LGM, sea levels along the coast and immediate off-shore areas stood at between 105–115 m below present level, the range reflecting the importance of the isostatic contributions. During oxygen isotope stage 3, sea levels do not appear to have risen locally above about −60 m. (iv) The rebound parameters (describing the mantle rheology and ice sheets) required to match the limited observational evidence are consistent with the results of similar analyses carried out for other parts of Europe. Because of its distance from the former northern ice sheets, the isostatic factors are particularly sensitive to the value of the lower-mantle viscosity. (v) The model predictions for sea-level change at the Cosquer Cave site and for its immediate environments indicate that the cave was last readily accessible before about 10 700±500 14C years (about 12 500±500 cal. years) BP and that the cave entrance was completely flooded by 9000±200 radiocarbon years BP (between about 9800 and 10 300 calibrated years BP). The cave was above sea level throughout the oxygen isotope stage 3.
 
Article
Giant piston core MD99-2269 recovered 25 m of sediment in Hunáfloáall, a deep trough on the North Iceland margin fronting the Iceland Sea, and the site of a shelf sediment drift. The rate of sediment accumulation is 2 m/kyr (5 yr/cm); the core terminated in the Vedde tephra (∼12 cal ka). The sediment was sampled at between 5 and 50 yr/sample, including rock magnetic, grain-size, and sediment properties. Data reduction was carried out using principal component analysis. Two PC axes for the 5-yr/sample magnetic data are strongly correlated with measures of coercivity (ARM20 mT/ARM) and magnetic concentrations (kARM). In turn ARM20 mT/ARM is highly correlated (negatively) with grain-size and the mean size of the sortable silt fraction. Analyses of the two PC axes with MTM spectral methods indicate a series of significant (>99%) periodicities at millennial to multidecadal scales, including those at ∼200, 125, and 88 yr which are associated with solar variability. We also document a strong correlation between the sediment magnetic properties and the ∂18O on benthic foraminifera on the North Iceland inner shelf. We hypothesize that the links between variations in grain-size, magnetic concentrations, and solar forcing are controlled by atmospheric and oceanographic changes linked to changes in the relative advection of Atlantic and polar waters along the North Iceland margin. Today these changes are associated with variations in the deep convection in the Greenland and Iceland Seas. The precise linkages are, however, presently elusive although a combination of coarser sediments and low ∂18O values define a Holocene thermal maximum between ∼8 and 6 cal ka.
 
Article
Extensive ice cover on the Tibetan Plateau would significantly influence Earth’s climate in general and the Asian monsoon system in particular, but extent and timing of Quaternary glaciations in Tibet remain highly controversial. We dated erratics on top of moraines in the climatic key areas of Central and East Tibet using cosmogenic 10Be, 26Al, and 21Ne. Consistent exposure ages obtained by various nuclides indicate a continuous period of exposure since the deposition of the samples. Our data imply that glacial advances were restricted to a few 10 km during the last 170 kyr in Central Tibet and during the peak of the last glaciation (∼24–13 kyr ago) in Eastern Tibet. Advances of Tibetan glaciers were much less prominent than elsewhere in the northern hemisphere most likely due to very arid conditions and high sublimation rates. A proposed ice-dome covering the entire Plateau can be excluded. Thus, albedo increase of Tibet most likely did trigger neither northern hemisphere ice ages nor paleomonsoon changes during the last two glacial cycles. The glacial advance during Marine Isotope Stage 2 in East Tibet and the absence of significant glacial events during the Holocene suggest a relation of snowline lowering in East Tibet to North Atlantic cooling events rather than to periods of high precipitation by an intensified monsoon.
 
Article
Cosmogenic 10Be in river-borne quartz sand records a time-integrated erosion rate representative of an entire drainage basin. When sequestered in a terrace of known age, paleo-erosion rates may be recovered from the nuclide content of the terrace material. Paleo-erosion rates between 30 and 80 mm/kyr are determined from terrace sediments 200 to 30 000 yr in age of the Allier and Dore Rivers, France, and the Meuse (Maas) River, the Netherlands. Erosion rates determined from cosmogenic nuclides on terraces from the Allier River are consistent with rates derived from the sedimentary fill of a lake in the Allier catchment. A strong decrease in cosmogenic nuclide-derived erosion rates from terraces of the Meuse River with Late Pleistocene to Holocene age is observed. The paleo-erosion signal from cosmogenic nuclides records projection of the elevated Late Pleistocene erosion rate into the time-integrated rates derived from Middle European rivers.
 
Article
Time series of diatom accumulation rates (DAR) and assemblage composition for the last 200 000 yr from the Atlantic major coastal upwelling loci of the eastern boundary currents and eastern Equator are compared together with independent proxies of export production, to upper ocean productivity and to nutrient concentration and utilization. Fluctuations in DAR are concomitant at all sites and in good correspondence to the other independent productivity proxies indicating glacial periods as the most productive times of the past 200 000 yr in both hemispheres of the eastern Atlantic. Maximum productivity, about one order of magnitude higher than at present, occurred at the last glacial maximum (LGM). Despite the ‘Atlantic’ similarities in both coastal and equatorial upwelling types, important regional and site specific variations emerge. In the southern Hemisphere site, a contradiction of proxies, diatoms and Corg, is noted at the base of stage 6 (185 000 yr). The good correlation found between the Corg and the benthic foraminifera fluxes is interpreted as a reflection of a shift in the phytoplankton community structure related to either a change in the N:P:Si ratio, or similar nutrient conditions associated with increased water column stratification.
 
Article
A new direct pollen–orbital tuning procedure, based on a correspondence between changes in certain vegetation elements and March and June perihelion configurations, is applied to the Tenaghi Philippon record, northeast Greece. The development of a refined chronological scheme allows comparisons to be made with records of climate variability from the North Atlantic as well as of global sea level/ice volume and atmospheric CO2 content. On orbital frequencies, the comparison reveals a close correspondence between relative ice volume extent and tree population size during glacial intervals. During interglacial and interstadial periods the degree of forest development is more closely associated with high-latitude insolation and related climate regimes rather than extent of residual ice volume. On suborbital frequencies, the Greek record shows similar repeat times in peaks of steppe vegetation with North Atlantic ice-rafting events, but the amplitude of this variability in the two records is not always proportional. Overall, what emerges is that the major shifts in the relative abundance of forest v. open vegetation communities at Tenaghi Philippon on orbital and suborbital frequencies over the last 450 000 years are coherent with high-latitude changes affecting atmospheric and oceanic circulation. Glacial decreases in atmospheric CO2 content contributed to the elimination of tree populations by exacerbating water stress conditions during arid intervals, but do not appear to be the primary driver of the observed vegetation changes at this latitude. Variations in CO2 levels (in the order of 40 ppmv) between different interglacial/interstadials periods with adequate moisture availability appear to have had a limited effect on the nature and size of tree populations.
 
Article
Abyssal peridotites are fragments of the oceanic upper mantle. Previous studies have indicated that their Os isotopic compositions span a wide range, including values more radiogenic than most estimates of the average bulk Earth 187Os/188Os ratio. This is difficult to reconcile with their derivation as residues of MORB partial melting. We present results that suggest that some of this variation, particularly the higher values, may be of secondary origin. Comparison of altered rims and unaltered cores of two mylonitic peridotites reveal that submarine weathering can increase 187Os/188Os ratios by more than 5%. Leaching experiments demonstrate the presence of a radiogenic, easily leachable phase that is most easily explained as a seawater-derived component. Thus, Os isotopic results from abyssal peridotites must be interpreted with some caution unless it can be shown that the effects of seawater alteration were minor. We develop criteria based on Sr and Nd isotopic measurements, as well as major element compositional changes, that allow strongly altered samples to be identified. Applying these criteria, we find 10 samples, including literature data as well as new analyses, that have apparently suffered only minimal alteration. These samples range in 187Os/188Os from 0.1221 to 0.1270, with a mean of 0.1246 and a standard deviation of 0.0014. Thus, we see no evidence for depleted mantle rocks more enriched in 187Os/188Os than the preferred value of ∼ 0.127. Finally, if Os is lost from peridotites during weathering, it may provide a significant source of mantle Os to the world's oceans.
 
Article
A depth profile of14C in lunar rock 12 002 has been measured using six specimens with well-defined depth below the exposed surface of the rock. The results obtained show an excess surface activity attributed to solar particle flux.
 
Article
Shergotty–Nakhla–Chassigny (SNC) meteorites provide the only available samples of Martian material. The stable permanent magnetization of SNC meteorites has been traditionally attributed to magnetite (Fe3O4) or pyrrhotite (Fe7S8). On the basis of rock magnetic, microscopic, and electron microprobe analyses on rock chips and mineral separates, we suggest that a new material (Fe–Cr–Ti spinel) is responsible for the stable paleomagnetic record of Martian meteorite SaU 008. It is possible that SaU 008 acquired a primary remanence of thermal origin from the Martian crustal field. However, this proposition requires further testing because the effect of shock events on Fe–Cr–Ti spinel is unknown.
 
Article
Chronological studies of brecciated lunar meteorites have proved difficult, because they are mixtures of materials from various sources and the radiometric “clocks” are sometimes affected by the subsequent impact events on the Moon. Here, we report the in-situ U–Pb dating of phosphates in lunar meteorite, Meteorite Hills (MET) 01210, which is a regolith breccia consisting of low-Ti mare basalt clasts and mineral fragments with a minor anorthositic component. In-situ analyses of four merrillite and four apatite grains in MET 01210, which are resistant to secondary events, resulted in a 207Pb/206Pb–204Pb/206Pb isochron age of 3904 ± 85 Ma (95% confidence limit). This phosphate formation age, when considered as the crystallisation age of this low-Ti basalt, is similar to crystallization ages of 3.8–3.9 Ga for unbrecciated low-Ti basalt meteorites, Asuka 881757 and Yamato 793169. This result reinforces the hypothesis that all these three meteorites originated from the same area on the Moon and were launched by a single impact event, consistent with the similarity of launch ages, mineralogical and geochemical signatures.
 
Article
Mineralogy, melt inclusions in crystals and reverse zoning in minerals of oceanic basalts can be used to get an insight into the functioning of a magma chamber beneath a spreading zone. Dredged basalts from the Central Indian Ridge (CIR), 21 km from the Rodriguez triple junction (RTJ) exhibit two groups of olivine and plagioclase of different composition.The first group is composed of forsteritic olivine (Fo90) and very anorthitic plagioclase (An89–90) resorbed megacrysts, containing numerous melt inclusions. Those observed in the core of the plagioclase megacrysts are the most primitive. Their compositions have been considered as the parental magma composition (Mg-number= 0.77; low TiO2: 0.31 wt.%; and low Na2O: 1.2–1.4 wt.%). These magmas have a density about 2.70 g cm−3 and viscosity close to 100 poises at 1250°C (liquidus temperature). In the diagramFeO*/MgO vs. (TiO2MgO) × 100, the zigzag evolution of the melt inclusions from core to rim of these plagioclase megacrysts has been interpreted as the result of three cycles of injection of parental liquid, mixing and olivine crystallization.The second group of minerals is essentially composed of more evolved olivine (Fo86–88) and plagioclase (An67–72) euhedral mesocrysts. Some olivines are reverse-zoned (3 steps). They crystallized from hybrid liquids resulting from three successive mixings between batches rising from the base of the magma chamber and an evolved magma at the top of the reservoir. Time between the mixings can be calculated from the diffusion profiles of Ni in the reverse-zoned olivines. These three mixings have been achieved within 4 days.A two-layer model for the magmatic reservoir is proposed to account for the mineralogy, the melt inclusions and glassy matrix compositions of those basalts. An estimation of the total volume of the chamber has been made, knowing the volume of extruded magmas at zero age. The chamber would have a radius of 400–500 m. Relative volume of lower layer where the first group of minerals crystallized and the upper layer where the second group of minerals formed is around 1:3.
 
Article
We have completed a systematic survey for hydrothermal activity along the Reykjanes Ridge from 57°45′N to 63°09′N. Shipboard determinations of total dissolvable Mn (TDMn) and dissolved Si, CH4 and H2 have revealed evidence for only one site of hydrothermal activity along this section of ridge crest, the Steinahóll vent-field at 63°06′N [1]. Our measurements confirm this site to be a stable hydrothermal convection cell of at least two years' duration. The site is situated in just 250–350 m of seawater and, unlike deeper-sited vent-fields (e.g. TAG, 3650 m, 26°N MAR), is notable for the formation of bubble-rich plumes which have been imaged using a high-frequency (38 kHz) echo-sounder. High dissolved gas concentrations of up to 18 nmol/l CH4 and 30 nmol/l H2 coincide with these bubble-rich plumes. The Steinahóll plume is also characterised by high total dissolvable Mn (TDMn) anomalies (up to 60 nmol/l) and dissolved Si anomalies (∼ 2 μmol/l) yielding molar TDMn/CH4 and TDMn/Si ratios of ∼ 3.5 and 0.03, respectively, directly comparable to the TAG hydrothermal plume.The absence of any indication of hydrothermal activity along the remainder of the Reykjanes Ridge is intriguing. Between 11°N and 40°N on the Mid-Atlantic Ridge, previous studies have revealed evidence for hydrothermally active sites approximately every 150 km. For the 300 km of ridge crest studied in detail here (750 km total) only one site has been found. Thus, despite its proximity to the Iceland hot-spot, the incidence of hydrothermal activity along this section of ridge crest appears to be only 20–50% of that measured elsewhere along the slow-spreading Mid-Atlantic Ridge.
 
Article
Rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere. Strength versus depth profiles, calculated for conditions appropriate for slow-spreading ridges, indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30%. Results from flexural fault models indicate that if serpentinization is isolated to fault zones, strain localization should be enhanced, providing an explanation for the variations in the style of normal faulting along slow-spreading ridge segments. At segment centers, where serpentinites are scarce, deformation is accommodated on closely spaced faults with small throws. At the segment ends, where serpentinites are most abundant, faults are widely spaced and have large throws.
 
EPR plate boundary configuration in the ARGO survey area. On left, blow-up of Sea MARC II sonar record between approximately 9030 ' and 42'N showing paired black acoustic reflectors and white shadows characteristic of a well-developed ASC. In middle, plate boundary determined from Sea MARC II (SM II) sonar data, 9054 '-I5'N, from Macdonald et al. [29]. The presence of the ASC is denoted by a solid line; the dashed line shows where the axial zone appears as a discontinuous, "'fuzzy" trace in the Sea MARC II records. On right, ASC configuration determined from ARGO sonar records. Width of the ASC is measured across the top of this feature between the outermost bounding walls. Agreement between Sea MARC II and ARGO delineation of the ASC is excellent considering the difference in resolution and areal coverage for the two systems.
A. ARGO 100 kHz sonar image recorded on Line 19. White areas are strong acoustic reflectors and black is acoustic shadow (opposite of Sea MARC II record in Fig. 4). Total width of record is -300 m. B. Interpretative sketch of sonar image shown in A with the ASC outlined. Heavy line denotes limit of the sonar image. The eastern ASC wall is well-defined by the start of the acoustic shadow. An area along the western wall has been traced to show the prominent shadows cast by the eastern wall on the highly reflective terrain of the western wall. Within the ASC floor, especially along its eastern margin, a linear array of reflectors (shown by stippled areas) is interpreted either as uncollapsed remnants of the volcanic roof which once existed over the area now occupied by the ASC or as a linear volcanic construct formed by fissure eruptions. The jagged shadows cast by the eastern ASC wall are formed by overhanging, large uncollapsed volcanic crusts that were often visually observed when ARGO was towed along the upper rim of the ASC. The scalloped (in plan-view) character of the ASC walls, the uncollapsed roof remnants, and the often-observed volcanic pillars, collapse pits, and collapse debris within the floor of the ASC all suggest that the ASC is largely a collapse feature associated with magma drainback or drainout.
A. Top view of an active black smoker edifice along the base of the west wall of the ASC in Segment B2 near 9°45'N. Electronic Still Camera (ESC) image, depth 2540 m, altitude = 7.0 m, heading 350 ° (towards top of image). Horizontal width of image is -4 m. Note "smoke" plume which is wafting from the chimney spires. Edifice is -7 m high and -2 m wide near the top; note that several smaller chimneys have coalesced to form the top of the edifice. B. Top view of inactive sulfide edifice near the east wall of the ASC in Segment C near 9°41'N. Electronic Still Camera (ESC) image, depth 2560 m, altitude = 8.0 m, heading 350°(towards top of image). Horizontal width of image is -4.8 m, image area = 15.6 m 2. Edifice is -6 m high and -2.2 m wide near the top; note numerous small chimneys which extend upward from the top of edifice. The edifice appears to be a complex structure created by coalescence of the smaller chimneys.
Article
Using the near-bottom ARGO imaging system, we visually and acoustically surveyed the narrow ( < 200 m wide) axial zone of the fast-spreading East Pacific Rise (EPR) along 83 km of its length (9°09′–54′N), discovered the Venture Hydrothermal Fields, and systematically mapped the distribution of hundreds of hydrothermal features relative to other fine-scale volcanic and tectonic features of the ridge crest. The survey encompasses most of a 2nd order ridge segment and includes at least ten 4th order (5–15 km) segments defined by bends or small lateral offsets of the ridge crest or axis (Devals). 4th order segmentation of the ridge crest is clearly expressed in the high-resolution ARGO data by the fine-scale behavior of the ridge axis and by changes in the characteristics of the axial zone (axial lava age, extent of fissuring, axial morphology and structure, etc.) across segment boundaries. The distribution and along-strike variability of hydrothermal features corresponds closely to the morphotectonic/structural segmentation of the ridge. On the 2nd order scale, we find that high T hydrothermal activity correlates with: (1) shallowing of the axial magma chamber (AMC) reflector to depths < 1.7 km beneath the ridge axis; and, (2) with the presence of a well-developed axial summit caldera (ASC). Previous work refers to this feature as an axial summit graben (ASG); however, the extent of volcanic collapse along the ASG revealed by the ARGO survey adds to evidence that on fast-spreading ridges it is an elongate volcanic caldera rather than a tectonic graben, and supports the introduction of “axial summit caldera” as a more accurate descriptor. All but 1 of the 45 active high T vent features identified with ARGO are located within 20 m of the margins of the ASC. Despite the significant extent of our track coverage outside the ASC, no important signs of venting were seen beyond the axial zone. On the 4th order scale, the abundance and distribution of hydrothermal features changes across 4th order segment boundaries. We find that high T vents are most abundant where: (1) the ASC is very narrow (40–70 m), (2) the AMC reflector is most shallow ( < 1.55 km beneath the axial zone), and (3) the axial lavas are youngest and least fissured. To explain the observed distribution of vent activity, a two-layer model of ridge crest hydrothermal flow is proposed in which 3-D circulation at lower T in the volcanic section is superimposed on top of axis-parallel high T circulation through the sheeted dike complex. In the model, circulation parallel to the ridge axis is segmented at the 4th order scale by variations in thermal structure and crustal permeability which are directly associated with the spacing of recent dike intrusions along strike and with cracking down into the sheeted dikes, especially along the margins of the ASC. Based on ratios between numbers of active high T vents and inactive sulfide deposits along particular 4th order segments, and on corresponding volcanic and tectonic characteristics of these segments, we suggest that the individual 4th order segments are in different phases of a volcanic-hydrothermal-tectonic cycle that begins with fissure eruptions, soon followed by magma drainback/drainage and accompanying gravitational collapse, possible development of an ASC, and onset of hydrothermal activity. The hydrothermal activity may wax and continue for up to several hundred years where an ASC is present. The latest phase in the cycle is extensive tectonic fissuring, widening of the ASC by mass wasting along its margins, and waning of hydrothermal activity. In the ARGO area, where full spreading rates are 11 cm/yr, the entire cycle takes less than ∼ 1000 years, and the tectonic phase does not develop where the time interval between eruptions is significantly less than 1000 years.
 
Article
Weekly average atmospheric 210Pb flux data collected between March 1989 and February 2001 are used to construct a record of Australian dust incursions onto the west coast, South Island, New Zealand. Dusts collected from New Zealand glaciers were found to contain a mixture of local New Zealand and long range Australian material, based on novel binary and tertiary mixing models of their ultra trace element chemistries. Trace element characteristics further allowed determination of the provenance of the long range dust component within Australia to better than 200 km resolution. 210Pb analyses of these chemically characterised dust samples show that activity is related to the percentage of Australian dust in a linear fashion. However, 210Pb activity of Australian dusts collected in New Zealand is orders of magnitude greater than that measured in the alluvial sediments of the provenance areas. Australian dusts collected in New Zealand are also highly enriched in 210Pb compared to dusts collected in Australia. This shows that dust scavenges atmospheric 210Pb which can therefore be used as an effective tracer of long range dust transport. Previously obtained average atmospheric 210Pb flux data can thus be converted into the first record of long range Australian dust flux in New Zealand.
 
Article
Pleistocene stable carbon isotope (δ13C) records from surface and deep dwelling foraminifera in all major ocean basins show two distinct long-term carbon isotope fluctuations since 1.00 Ma. The first started around 1.00 Ma and was characterised by a 0.35‰ decrease in δ13C values until 0.90 Ma, followed by an increase of 0.60‰ lasting until 0.50 Ma. The subsequent fluctuation started with a 0.40‰ decrease between 0.50 and 0.25 Ma, followed by an increase of 0.30‰ between 0.25 and 0.10 Ma. Here, we evaluate existing evidence and various hypotheses for these global Pleistocene δ13C fluctuations and present an interpretation, where the fluctuations most likely resulted from concomitant changes in the burial fluxes of organic and inorganic carbon due to ventilation changes and/or changes in the production and export ratio. Our model indicates that to satisfy the long-term ‘stability’ of the Pleistocene lysocline, the ratio between the amounts of change in the organic and inorganic carbon burial fluxes would have to be close to a 1:1 ratio, as deviations from this ratio would lead to sizable variations in the depth of the lysocline. It is then apparent that the mid-Pleistocene climate transition, which, apart from the glacial cycles, represents the most fundamental change in the Pleistocene climate, was likely not associated with a fundamental change in atmospheric pCO2. While recognising that high frequency glacial/interglacial cycles are associated with relatively large (100 ppmv) changes in pCO2, our model scenario (with burial changes close to a 1:1 ratio) produces a maximum long-term variability of only 20 ppmv over the fluctuation between 1.00 and 0.50 Ma.
 
Article
New K/Ar dating and geochemical analyses have been carried out on the WNW–ESE elongated oceanic island of S. Jorge to reconstruct the volcanic evolution of a linear ridge developed close to the Azores triple junction. We show that S. Jorge sub-aerial construction encompasses the last 1.3 Myr, a time interval far much longer than previously reported. The early development of the ridge involved a sub-aerial building phase exposed in the southeast end of the island and now constrained between 1.32 ± 0.02 and 1.21 ± 0.02 Ma. Basic lavas from this older stage are alkaline and enriched in incompatible elements, reflecting partial melting of an enriched mantle source. At least three differentiation cycles from alkaline basalts to mugearites are documented within this stage. The successive episodes of magma rising, storage and evolution suggest an intermittent re-opening of the magma feeding system, possibly due to recurrent tensional or trans-tensional tectonic events. Present data show a gap in sub-aerial volcanism before a second main ongoing building phase starting at about 750 ka. Sub-aerial construction of the S. Jorge ridge migrated progressively towards the west, but involved several overlapping volcanic episodes constrained along the main WNW–ESE structural axis of the island. Mafic magmas erupted during the second phase have been also generated by partial melting of an enriched mantle source. Trace element data suggest, however, variable and lower degrees of partial melting of a shallower mantle domain, which is interpreted as an increasing control of lithospheric deformation on the genesis and extraction of primitive melts during the last 750 kyr. The multi-stage development of the S. Jorge volcanic ridge over the last 1.3 Myr has most likely been greatly influenced by regional tectonics, controlled by deformation along the diffuse boundary between the Nubian and the Eurasian plates, and the increasing effect of sea-floor spreading at the Mid-Atlantic Ridge.
 
Article
High-resolution records of glacial–interglacial variations in biogenic carbonate, opal, and detritus (derived from non-destructive core log measurements of density, P-wave velocity and color; r≥0.9) from 15 sediment sites in the eastern equatorial (sampling resolution is ∼1 kyr) clear response to eccentricity and precession forcing. For the Peru Basin, we generate a high-resolution (21 kyr increment) orbitally-based chronology for the last 1.3 Ma. Spectral analysis indicates that the 100 kyr cycle became dominant at roughly 1.2 Ma, 200–300 kyr earlier than reported for other paleoclimatic records. The response to orbital forcing is weaker since the Mid-Brunhes Dissolution Event (at 400 ka).A west–east reconstruction of biogenic sedimentation in the Peru Basin (four cores; 91–85°W) distinguishes equatorial and coastal upwelling systems in the western and eastern sites, respectively. A north–south reconstruction perpendicular to the equatorial upwelling system (11 cores, 11°N–8°S) shows high carbonate contents (≥50%) between 6°N and 4°S and highly variable opal contents between 2°N and 4°S. Carbonate cycles B-6, B-8, B-10, B-12, B-14, M-2, and M-6 are well developed with B-10 (430 ka) as the most prominent cycle. Carbonate highs during glacials and glacial-interglacial transitions extended up to 400 km north and south compared to interglacial or interglacial–glacial carbonate lows. Our reconstruction thus favors glacial–interglacial expansion and contraction of the equatorial upwelling system rather than shifting north or south. Elevated accumulation rates are documented near the equator from 6°N to 4°S and from 2°N to 4°S for carbonate and opal, respectively. Accumulation rates are higher during glacials and glacial–interglacial transitions in all cores, whereas increased dissolution is concentrated on Peru Basin sediments close to the carbonate compensation depth and occurred during interglacials or interglacial–glacial transitions.
 
Article
A set of time integrated cosmogenic 3He production rates in olivines for the last 1.35 Ma are presented. We investigated three basaltic lava flow tops from Lanzarote, Canary Islands, Spain. The 40Ar/39Ar ages determined for those basalt flows by incremental laser heating of leached groundmass samples are 152±26 ka, 281±19 ka and 1.35±0.01 Ma (±2σ). Three or four different olivine phenocryst samples have been analyzed from each flow for their cosmogenic 3He abundances. The resulting 3He production rates in olivine at sea level at 29° latitude are 82±14 and 82±8 atoms g−1 a−1, as obtained from the 152 ka and 281 ka old flows, respectively. Considering effects of erosion on the 1.35 Ma old flow we find that the production rate of 82±8 atoms g−1 a−1 is consistent with the cosmogenic 3He production during the last 1.35 Ma. There appears to be a 14% discrepancy between previously published production rates derived at higher latitudes and altitudes if the scaling factors of Lal are used to compare results. This discrepancy is greatly reduced, however, if the revised scaling factors of Dunai (this issue) are applied. Using the new scaling factors we derive a production rate for cosmogenic 3He in olivine at sea level and high latitudes (>60°) of 118±11 atoms g−1 a−1 (±2σ). The correspondingly revised value of Cerling and Craig, and Ackert et al.) is 123±6 atoms g−1 a−1 (±2σ). The mean value of these two calibrations is 121±6 atoms g−1 a−1 (±2σ). We suggest that the production rate of 121±6 atoms g−1 a−1 at sea level and high latitudes may be applied to the complete time range where paleomagnetic data indicate that there was no long-term averaged intensity variation in the Earth’s magnetic field, i.e. over the last 10 Ma.
 
Article
Diabase sills are widespread within the Mesoproterozoic marine clastic and carbonate platformal sedimentary rocks in the northern North China Craton (NCC). Our new LA-ICP-MS zircon and baddeleyite dating on a diabase sill sample emplaced into the Mesoproterozoic Wumishan Formation yielded weighed mean 207Pb/206Pb ages of 1345 ± 12 Ma (95% confidence, MSWD = 1.8, N = 18) and 1353 ± 14 Ma (95% confidence, MSWD = 0.67, N = 15), respectively, indicating emplacement of the sills at the Mid-Mesoproterozoic (Ectasian) instead of the Late Paleozoic or Mesozoic as previously regarded. Existence of large volumes of 1.35 Ga diabase sills in the northern NCC indicates that the northern NCC underwent strong extension and mafic magmatism during Mid-Mesoproterozoic (Ectasian) time, probably related to the breakup of the Columbia (Nuna) supercontinent. Compared with the 1.4–1.3 Ga mafic dyke swarms (sills) in other cratons, we suggested that the NCC was a member of Nena including the Laurentia (North America and Greenland), Siberia and Baltica cratons in the supercontinent. The NCC was likely connected to the Siberia and North America (Laurentia) cratons during late Paleoproterozoic to Mid-Mesoproterozoic times, and breakup of the NCC away from the Columbia supercontinent occurred at around 1.35 Ga, which are consistent with the paleomagnetic results. The new zircon and baddeleyite geochronological results on the diabase sills provide not only magmatic evidence for Precambrian supercontinent reconstruction but also important constraint on ages of the Meso-Neoproterozoic strata in the NCC.
 
Article
We have determined the solidus temperatures and liquid compositions for batch melts of fertile and depleted lherzolite in the 0–15% melting interval at 1.5 GPa. Because establishment of equilibrium is difficult at very low melt fractions we have used an iterative technique in which the liquids from sandwich-type experiments are synthesized and tested for multiple saturation at the same pressure, temperature conditions as those of the sandwich experiment. Only when all 4 mantle phases are stable and of correct composition is the result accepted as being at equilibrium. This technique permits accurate determination of solidus temperatures to ±10°C and provides the compositions of low-degree mantle melts. We find that at degrees of batch melting above 3%, liquids produced from a fertile peridotite (MORB-Pyrolite) contain ∼49% SiO2, have <4% Na2O and have Fe–Mg partitioning relationships with the solid phases which are typical of basalt. As the degree of melting drops to 0% in fertile peridotite (MORB-Pyrolite), SiO2 content of the melt increases to 53% and the Na2O content to 8%. The melts remain strongly nepheline-normative and Fe favours the melt relative to olivine even more strongly than normal. We ascribe this latter observation to Na–Fe3+ coupling in the melt at high Na2O content. These high-alkali and high-silica melts are not observed at the solidus of depleted peridotite (Tinaquillo Lherzolite). These experimental results provide the first direct tests of fractional melting models which invoke the successive extraction of low-degree partial melts. No melting model currently available provides a good description of the compositions of low-degree melts from a fertile peridotite at 1.5 GPa.
 
Article
Coupled relative geomagnetic paleointensity (RPI) and oxygen isotope records are used to construct RPI and oxygen isotope stacks for the last 1.5 Myr. The coupled correlations are accomplished using the Match algorithm (Lisiecki, L.E., and Lisiecki, P.A., 2002. Application of dynamic programming to the correlation of paleoclimate records. Paleoceanography, 17: 1049, doi:10.1029/2001PA000733) to simultaneously correlate isotope and RPI records. The simultaneous match reduces the degree of freedom associated with correlations using RPI or oxygen isotope records alone. The overall compatibility of RPI and oxygen isotopes indicates a dominant global (but independent) component in both signals. The local wavelet power spectrum (LWPS) for the RPI stack indicates little significant orbital power, although the accompanying oxygen isotope stack has all the LWPS characteristics expected of a high-resolution oxygen isotope record containing orbital frequencies. The PISO-1500 stack represents a new stratigraphic template that can be used to correlate among marine sediment records and link them to polar ice cores via variations in cosmogenic nuclide production. Scaling the stack to values for virtual axial dipole moment (VADM) indicate maxima at ~ 15 × 1022 Am2 and minima that imply a threshold of ~ 2.5 × 1022 Am2 below which values are associated with either polarity reversals or magnetic excursions.
 
Article
The origin of 1.9-1.7 Ga crust in Europe, Greenland and North America can be evaluated using Nd and other isotopic information. Terrains stabilized 1.9-1.7 Ga ago make up 74% of the pre-1.6 Ga continental area, but interpretation of Nd data shows that only ca. 50% of this is new mantle-derived crust, while the other half is recycled or reactivated Archean material. These terrains can be divided into (1) those with > 90% reactivated Archean crust, concentrated in northern Greenland and Canada, whose origin is enigmatic, and (2) those with > 80% newly-differentiated material, containing only a limited Archean contribution, probably in the form of recycled sediments. The new terrains occur principally in a wide zone from Arizona through Colorado, Michigan, South Greenland, Sweden, Finland to the western U.S.S.R., and they border the present southern margin of the Wyoming, Superior, North Atlantic and Kola Archean blocks. The new terrains contain a high proportion of volcanic and plutonic rocks resembling those of present-day island arcs and continental margins, and they aomost certainly represent a major subduction-related mantle-to-crust differentiation during 1.9-1.7 Ga. Igneous activity was complex in pattern from area to area, with sequential accretion of volcano-plutonic belts outwards from Archean continents. The overall crust production rate from the northern continents in the period 1.9-1.7 Ga was ∼ 1.2 km3/a, which is very slightly greater than the total Phanerozoic island-arc accretion rate. Thus Phanerozoic or only slightly higher rates of subduction can explain the 1.9-1.7 Ga new terrains only if most subduction-related igneous activity in the 1.9-1.7 Ga world was concentrated in the Arizona-Finland zone. Interpretation of limited geological information available from other continents suggests that this was not the case, and that a crust production rate around double the present one prevailed 1.9-1.7 Ga ago. The results document major crustal growth during Proterozoic time.
 
Article
Paleointensities from Precambrian rocks are rare and might be biased by remagnetization processes. Here we present new analyses of samples from a 1.786 Ga gabbro near Hoting, Central Sweden. Rock magnetic and mineralogical analyses indicate that one of the sites (site 5) may be pristine, whereas the others exhibit evidence of alteration.Characteristic remanent magnetization was determined using principal component analysis for each sample and was compared with results obtained in a previous study of Elming et al. (2009). Intensity measurements from site 5 show higher values compared to those of the other sites, suggesting that alteration processes may lead to underestimation of the field intensity.After cooling rate and anisotropy correction, the field moment at 1.786 Ga was estimated to be 25.6 ± 3.3 ZAm2 and 15.2 ± 6.1 ZAm2 from site 5 only and from all sites respectively. We consider the result from site 5 to be more accurate owing to the lack of evidence for alteration; our estimates agree well with the Proterozoic VDM values suggested by Biggin et al. (2009).Highlights► Measured paleointensity of a 1.786 Ga old Gabbro from Sweden. ► Magnetic mineralogy and its alteration analysed with rock magnetics and microscopy. ► Pristine samples yield higher paleointensities than altered ones. ► One of seven sites is pristine and yields the most reliable results. ► These high quality measurements indicate a low intensity during Precambrian.
 
Article
Ten sites of basaltic lava flows from the Koolau series (island of Oahu, Hawaii) were selected to improve the determination of the Matuyama time-averaged geomagnetic field between 1.8 and 2.6 Ma. Identical primary directions with reverse polarity carried by magnetite were isolated by alternating-field and thermal demagnetization in 13–16 specimens from each site. The final mean direction points south while the inclination is 7° lower than the inclination of the geocentric axial dipole (GAD). This offset is reduced after correcting the site latitude for the Pacific plate motion, which has been neglected so far in most studies. The mean corrected direction is almost perfectly antipodal to the Brunhes mean direction derived from the Honolulu series in the same island. Despite this correction, the inclination remains 3.6° larger than the inclination of the axial dipole and cannot be accounted for by a small axial quadrupole which would reverse with the dipole. The mean inclinations of other Hawaiian records (mostly younger than 1 Ma) involving long sequences of lava flows exhibit similar deviations from the GAD which are too large to be accounted for by a small axial quadropole. Tilting of all sections is difficult to defend for so many localities and persistant secondary components have not been detected. Thus these inclination anomalies would reflect the presence of a long-term standing component under Hawaii but crustal magnetization linked to the volcanic edifice cannot be completely ruled out.
 
Article
The Kapuskasing Structural Zone was the site of local crustal thrusting and uplift within the Superior craton, presumably in response to stresses applied at the margin of the craton during the Paleoproterozoic ca. 1.9 Ga. We use the present-day heat flow data in the Superior Province to calculate the strength of the lithosphere and to show that the uplift took place in the weakest part of the craton. After eliminating heat flow data that were affected by the uplift and later events (Keweenawan rifting), we calculate the heat flow and the temperature field in the lithosphere at 1.9 Ga. We show that the Kapuskasing region was in the center of a “thermal anomaly” at this time. The strength of the lithosphere in this region may have been up to 2–3 orders of magnitude lower than in surrounding parts of the craton, depending on crustal heat source distribution. Uplift and the resulting erosion of the upper crust, as evidenced by geothermobarometry data from around Kapuskasing, led to a reduced crustal heat production compared to the average crust of the southern Superior Province. This reduced heat production resulted in low crustal temperatures and a strong lithosphere permitting the long term preservation of a crustal root.
 
Article
We report volatile (H2O, CO2, F, S, Cl) and trace element data for submarine alkalic basalt glasses from the three youngest Samoan volcanoes, Ta'u, Malumalu and Vailulu'u. Most samples are visibly sulfide saturated, so have likely lost some S during fractionation. Cl / K ratios (0.04–0.15) extend to higher values than pristine MORBs, but are suspected to be partly due to source differences since Cl / K roughly varies as a function of 87Sr / 86Sr. There are no resolvable differences in the relative enrichment of F among sources, and compatibility of F during mantle melting is established to be nearly identical to Nd. Shallow degassing has affected CO2 in all samples, and H2O only in the most shallowly erupted samples from Vailulu'u. Absolute water contents are high for Samoa (0.63–1.50 wt.%), but relative enrichment of water compared to equally incompatible trace elements (Ce, La) is low and falls entirely below normal MORB values. H2O / Ce (58–157) and H2O / La (120–350) correlate inversely with 87Sr / 86Sr compositions (0.7045–0.7089). This leads us to believe that, because of very fast diffusion of hydrogen in mantle minerals, recycled lithospheric material with high initial water and trace element content will lose water to the drier ambient mantle during storage within the inner Earth. The net result is the counter-intuitive appearance of greater dehydration with greater mantle enrichment. We expect that subducted slabs will experience a two-stage dehydration history, first within subduction zones and then in the ambient mantle during long-term convective mixing.
 
Article
Recent research has shown that ombrotrophic mires can yield a proxy climate signal based on changes in the degree of peat humification [1,2], and that oceanic margin sites show the most sensitive record [3]. We compare humification records for the last 1000 yr from two radiocarbon-dated blanket peat profiles from western Ireland, and show several corresponding periods of climatic fluctuations. Periods favouring reduced peat decomposition, suggestive of wetter and/or cooler climatic conditions, seem to coincide with periods of reduced sunspot activity and atmospheric14C anomalies.Whilst climatologists have continued to debate a link between solar variability on a century timescale and climate change, the exact nature of that link remains elusive [4–7]. The results from Ireland demonstrate that data derived from mires could be relevant to the debate as to the extent of solar forcing in natural climatic variability, and the curves shown provide a continuous record to add to previous evidence for the so-called ‘Medieval Optimum’ and ‘Little Ice Age’ [8]. The record may imply that, during the past millennium, climatic change at the oceanic margin of the northeast Atlantic largely corresponded to inferred variations in solar output.
 
Article
An archeomagnetic study was carried out on 24 pottery fragments from Grand Banks, southern Ontario. 14C dating on maize found in the same pit as the potsherds gives an age in the range A.D. 990–1160 (modal age A.D. 1030). Hysteresis measurements indicate that the natural remanent magnetization is carried by pseudosingle-domain magnetite grains. Paleointensity experiments were done on 63 samples using a Thellier-type double-heating method. Half of the samples were heated in helium and the other half in air. Hysteresis measurements on samples after heating to various temperatures reveal only slight mineralogical changes during heating steps. Thirty reliable results give a paleofield intensity of 42.0±7.4 μT, and a virtual axial dipole moment of 7.0±1.3×1022 Am2. This result is in agreement with the low virtual axial dipole moment values given by studies on material from Ontario compared to the values from the southwestern USA in the interval A.D. 900–1400.
 
Article
Sample 10071, 33 is a thin section of Apollo 11 ferrobasalt showing an unusual dual texture. The major portion of the sample is very similar to other fine grained Apollo 11 basalts, but the thin section also includes material with a distinct variolitic texture. The two areas are separated by a sharp boundary and the mineralogy and composition of the two textural types are quite distinct. The mineralogy and chemistry of the variolitic portion show it to be the product of rapid cooling of a liquid, intermediate between the typical Apollo 11 ferrobasalt and the associated Si and K-rich mesostasis. This liquid is the result of fractional crystallization of a magma of composition closely corresponding to the major portion of the 10071 system, followed by crystal-liquid separation. The sample provides strong and direct evidence for igneous differentiation on the lunar surface.
 
Article
The geochemical and geodynamical parameters that may influence the composition of island-arc basalts (IAB's) are evaluated. Systematic correlations amongst high-field strength (HFS) elemental ratios (Zr/Nb, Sm/Nb and TiO2/Zr) relative to Nb abundances, indicate that HFS element systematics are not controlled by the presence of residual Nb-bearing phases in the slab. This provides confirmation of models whereby high-field strength (HFS) and HREE elements remain immobile during slab-fluxing processes and are thus derived from the mantle wedge without additional enrichments from the slab. In contrast enrichment of large-ion-lithophile elements (LIL) such as Rb, Cs, Ba, Sr, Pb, U and LREE (i.e., La, Ce) in IAB's is consistent with slab involvement, with their relative enrichment, being due to a combination of both their high rock-melt incompatibility and slab-“fluid” mobility. As a consequence, the low abundances of HFS elements such as Nb, Ti, Zr, and Hf in IAB's reflect a depleted (relative to MORB source) mantle wedge overlying the subduction slab. Depletion of the arc mantle wedge in HFS elements is attributed to previous melting events in the mantle wedge, and to geodynamic conditions associated with the formation and evolution of coupled island arcs and back-arc basins. These processes ensure a budgetary deficit in the HFS elements relative to those elements derived from the subducted slab (predominantly LILE and LREE). Thus, although in MORB's and OIB's, Nb has a similar incompatibility to U, in subduction zones the main factor controlling its abundance is its highly immobile character, particularly relative to elements like U which are mobile during prograde dehydration reactions in the slab. Based on these observations, a quantitative model has been developed for IAB petrogenesis with the transfer of trace elements from the slab to the mantle wedge being modelled with empirical slab-“fluid” partition coefficients whilst the mantle-wedge to arc-crust transfer is constrained by melt-solid partitioning. The empirically derived slab-“fluid” partition coefficients indicate that the enrichment factors characteristic of slab fluxing processes have a distinctive pattern particularly for the elements Nb, U, Th, and Sr.
 
Top-cited authors
Albrecht W. Hofmann
  • Max Planck Institute for Chemistry
G. J. Wasserburg
  • California Institute of Technology
Jan D Kramers
  • University of Johannesburg
Janne Blichert-Toft
  • Ecole normale supérieure de Lyon
Francis Albarède
  • Ecole normale supérieure de Lyon