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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 . 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  and diamond cell measurements for iron  which has up to now been lacking.
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.
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.
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.
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.
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.
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.
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.
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.
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.