Atmospheric dust variability from Arabia and China over the last 500,000 years

Atmospheric mineral dust aerosols affect Earth’s radiative balance and are an important climate forcing and feedback mechanism. Dust is argued to have played an important role in past natural climate changes through glacial cycles, yet temporal and spatial dust variability remain poorly constrained, with scientific understanding of uncertainties associated with radiative perturbations due to mineral dust classified as “very low”. To advance understanding of the dust cycle, we present a high-resolution dust record from the Red Sea, sourced principally from Arabia, with a precise chronology relative to global sea level/ice volume variability. Our record correlates well with a high-resolution Asian dust record from the Chinese Loess Plateau. Importing our age model from the Red Sea to the Chinese Loess Plateau provides a first detailed millennial-scale age model for the Chinese loess, which has been notoriously difficult to date at this resolution and provides a basis for inter-regional correlation of Chinese dust records. We observe a high baseline of dust emissions from Arabia and China, even through interglacials, with strong superimposed millennial-scale variability. Conversely, the distal EPICA Dome C Antarctic ice core record, which is widely used to calculate the radiative impact of dust variations, appears biased to sharply delineated glacial/interglacial contrasts. Calculations based on this Antarctic dust record will therefore overestimate the radiative contrast of atmospheric dust loadings on glacial/interglacial timescales. Additional differences between Arabian/Asian and circum-Saharan records reveal that climate models could be improved by avoiding ‘global mean’ dust considerations and instead including large-scale regions with different dust source variability.     

Uranium-series disequilibrium in tuffs from Yucca Mountain,Nevada, as evidence of pore-fluid flow over the last million years

Samples of tuff from boreholes drilled into fault zones in the Exploratory Studies Facility (ESF) and relatively unfractured rock of the Cross Drift tunnels, at Yucca Mountain, Nevada, have been analysed by U-series methods. This work is part of a project to verify the finding of fast flow-paths through the tuff to ESF level, indicated by the presence of ‘bomb’ ³⁶Cl in pore fluids. Secular radioactive equilibrium in the U decay series, (i.e. when the radioactivity ratios ²³⁴U/²³⁸U, ²³⁰Th /²³⁴U and ²²⁶Ra/²³⁰Th all equal 1.00) might be expected if the tuff samples have not experienced radionuclide loss due to rock-water interaction occurring within the last million years. However, most fractured and unfractured samples were found to have a small deficiency of ²³⁴U (weighted mean ²³⁴U/²³⁸U=0.95±0.01) and a small excess of ²³⁰Th (weighted mean ²³⁰Th/²³⁴U 1.10±0.02). The ²²⁶Ra/²³⁰Th ratios are close to secular equilibrium (weighted mean=0.94±0.07). These data indicate that ²³⁴U has been removed from the rock samples in the last ∼350 ka, probably by pore fluids. Within the precision of the measurement, it would appear that ²²⁶Ra has not been mobilized and removed from the tuff, although there may be some localised ²²⁶Ra redistribution as suggested by a few ratio values that are significantly different from 1.0. Because both fractured and unfractured tuffs show approximately the same deficiency of ²³⁴U, this indicates that pore fluids are moving equally through fractured and unfractured rock. More importantly, fractured rock appears not to be a dominant pathway for groundwater flow (otherwise the ratio would be more strongly affected and the Th and Ra isotopic ratios would likely also show disequilibrium). Application of a simple mass-balance model suggests that surface infiltration rate is over an order of magnitude greater than the rate indicated by other infiltration models and that residence time of pore fluids at ESF level is about 400 a. Processes of U sorption, precipitation and re-solution are believed to be occurring and would account for these anomalous results but have not been included in the model. Despite the difficulties, the U-series data suggest that fractured rock, specifically the Sundance and Drill Hole Wash faults, are not preferred flow paths for groundwater flowing through the Topopah Spring tuff and, by implication, rapid-flow, within 50 a, from the surface to the level of the ESF is improbable.     

Climate variability in central China over the last 1270 years revealed by high-resolution stalagmite records

Oxygen and carbon isotopic variations in the upper section of a stalagmite (SF-1) from Buddha Cave (33°40′N 109°05′E) dated by ²³⁰Th/²³⁴U ²¹⁰Pb and lamination counting to a time resolution as fine as 1–3 years have recorded climate changes in central China for the last 1270 years. The changes include those corresponding to the Medieval Warm Period Little Ice Age and 20th-century warming lending support to the global extent of these events. The isotopic records also show cycles of 33, 22, 11, 9.6, and 7.2 years. The 33-year cycle could well represent the ∼35-year periodicity of climate fluctuations previously recognized in China and Europe. Cycles of 22, 11, and 9.6 years have often been associated with the Sunspot or lunar-orbit variations. The 7.2-year cycle was recently identified also in tree-ring records from an area close to Buddha Cave. These cycles suggest that external forcing (e.g. solar irradiance) may affect the summer monsoon over eastern China. The general consistency between the climate characteristics inferred from the stable isotope records of SF-1 and those from other proxy records underscores the value of stalagmites as recorders of paleoclimate.     

Shaping the Australian crust over the last 300 million years: Insights from fission track thermotectonic imaging and denudation studies of key terranes

Apatite fission track thermochronology is a well‐established tool for reconstructing the low‐temperature thermal and tectonic evolution of continental crust. The variation of fission track ages and distribution of fission track lengths are primarily controlled by cooling, which may be initiated by earth movements and consequent denudation at the Earth's surface and/or by changes in the thermal regime. Using numerical forward‐modelling procedures these parameters can be matched with time‐temperature paths that enable thermal and tectonic processes to be mapped out in considerable detail. This study describes extensive Australian regional fission track datasets that have been modelled sequentially and inverted into time‐temperature solutions for visualisation as a series of time‐slice images depicting the cooling history of present‐day surface rocks during their passage through the upper crust. The data have also been combined with other datasets, including digital elevation and heat flow, to image the denudation history and the evolution of palaeotopography. These images provide an important new perspective on crustal processes and landscape evolution and show how important tectonic and denudation events over the last 300 million years can be visualised in time and space. The application of spatially integrated denudation‐rate chronology is also demonstrated for some key Australian terranes including the Lachlan and southern New England Orogens of southeastern Australia, Tasmania, the Gawler Craton, the Mt Isa Inlier, southwestern Australian crystalline terranes (including the Yilgarn Craton) and the Kimberley Block. This approach provides a readily accessible framework for quantifying the otherwise undetectable, timing and magnitude of long‐term crustal denudation in these terranes, for a part of the geological record previously largely unconstrained. Discrete episodes of enhanced denudation occurred principally in response to changes in drainage, base‐level changes and/or uplift/denudation related to far‐field effects resulting from intraplate stress or tectonism at plate margins. The tectonism was mainly associated with the history of continental breakup of the Gondwana Supercontinent from Late Palaeozoic time, although effects related to compression are also recorded in eastern Australia. The results also suggest that the magnitude of denudation of cratonic blocks has been significantly underestimated in previous studies, and that burial and exhumation are significant factors in the preservation of apparent ‘ancient’ features in the Australian landscape.     

Aspects of human physical and behavioural evolution during the last 1 million years

This paper reviews some of the main advances in our understanding of human evolution over the last 1 million years, presenting a holistic overview of a field defined by interdisciplinary approaches to studying the origins of our species. We begin by briefly summarizing the climatic context across the Old World for the last 1 million years before directly addressing the fossil and archaeological records. The main themes in this work explore (i) recent discoveries in the fossil record over the last 15 years, such as Homo naledi and Homo floresiensis; (ii) the implications of palaeogenetics for understanding the evolutionary history of, and relationships between, Neanderthals, Denisovans and Homo sapiens; (iii) the interplay between physiology and metabolic demand, landscape use, and behavioural adaptations in the evolution of morphological and behavioural innovation; and (iv) recent advances in archaeological understanding for the behavioural record, in particular that of the Neanderthals. This paper seeks to provide a broad‐scale, holistic perspective of our current understanding of human evolution for the last 1 Ma, providing a reference point for researchers that can be built upon as new discoveries continue to develop the landscapes of human evolution. © 2019 The Authors. Journal of Quaternary Science Published by Wiley Periodicals, Inc.     

Millennial-scale variability in Southern Hemisphere westerly wind activity over the last 5000 years in SW Patagonia

Here we report results from a high-resolution palynological record and stratigraphic/geochronologic data related to a Neoglacial event in Torres del Paine National Park, southern Chile (51°S, 71°W), to investigate climatic variations in Southwest Patagonia during the last 5000 years. The record reveals a stepwise expansion of Nothofagus-dominated woodlands and forests with discrete pulses at 4400, 2900, 1300, and 570 cal yr BP. Superimposed upon this trend we identify a relative opening of the woodlands between 4100–2900 and 2300–1300 cal yr BP. Closed-canopy forests dominated the landscape between 570–60 cal yr BP, followed by a rapid decline at the end of the 19th century that coincided with intense fire activity and the appearance of Rumex cf. acetocella, an exotic species introduced by European settlers. We interpret these changes as variations in the intensity and/or position of the southern margin of the westerly winds, which culminated with a net eastward shift of the forest–steppe ecotone during the Little Ice Age. We propose that millennial-scale changes in either the latitudinal position and/or the overall strength of the southern westerlies may be responsible for vegetation changes, fire occurrence, and the dynamic behavior of Patagonian glaciers during the last 5000 years. Because the modern maximum in near-surface wind velocities and precipitation is located between 48° and 50°S, we suggest that the core of the southern westerlies may have achieved this modern position ～570 years ago.     

The last 1.35 million years at Tenaghi Philippon: revised chronostratigraphy and long-term vegetation trends

In addition to being of interest to ancient Greek and Roman historians, the site of Philippi, NE Greece, has long been noted in Quaternary circles for providing the longest continuous European pollen record, spanning the last one million years. Here the original age model is re-evaluated and a new marine-terrestrial correlation is proposed. An astronomical calibration procedure, based on a correspondence between changes in certain vegetation elements and March and June perihelion configurations, suggests that the base of the sequence extends back to 1.35 million years ago. The revised chronological framework for the Tenaghi Philippon sequence provides an opportunity to examine the long-term behaviour of individual taxa and vegetation trends within the context of global climate changes. Comparisons reveal a close correspondence between the terrestrial and marine records, in terms of orbital and suborbital variability. However, joint time-frequency analysis of the arboreal pollen record shows that the obliquity and eccentricity/precession signals persist into the ‘100-kyr’ and ‘41-kyr’ worlds, respectively, suggesting the operation of additional climate mechanisms that are independent of high-latitude glacial–interglacial effects. Unlike ice core and marine sequences, no change in the magnitude of interglacial tree population expansions is observed after the Mid-Brunhes Event. Instead, the Tenaghi Philippon record suggests a major shift in the vegetational composition of interglacials after MIS 16, with the establishment of forests of reduced diversity and a ‘modern’ appearance.     

Discussion and Reply: Shaping the Australian crust over the last 300 million years: Insights from fission track thermotectonic imaging and denudation studies of key terranes

Ellis Fjord is a small, fjord‐like marine embayment in the Vestfold Hills, eastern Antarctica. Modern sediment input is dominated by a biogenic diatom rain, although aeolian, fluvial, ice‐rafted, slumped and tidal sediments also make a minor contribution. In areas where bioturbation is significant relict glaciogenic sediments are reworked into the fine‐grained diatomaceous sediments to produce poorly sorted fine sands and silts. Where the bottom waters are anoxic, sediments remain unbioturbated and have a high biogenic silica component. Three depositional and non‐depositional facies can be recognised in the fjord: an area of non‐deposition around the shoreline; a relict morainal facies in areas of low sedimentation and high bioturbation; and a basinal facies in the deeper areas of the fjord.     

Indian monsoon variability in the Mahanadi Basin over the last two glacial cycles and its implications on the Indonesian throughflow

The orbital-scale variability of the Indian summer monsoon (ISM) has been influenced by multiple factors, such as atmospheric CO₂ concentration, global ice volume, and insolation. Proxies for weathering activity and paleo-productivity provide potential insights into the driving forces of its variability. We documented multi-proxy data at IODP Site U1445, located in the Mahanadi Basin of the northwestern Bay of Bengal, to find out ISM variability over the last 200 ka. The proxy records, such as Nd/Sr isotopes of detrital particles, clay mineral compositions of the fine-grained sediments, biogenic opal and CaCO₃, organic carbon contents, and carbon isotopes of organic matter, represent sediment sources, weathering patterns, and paleo-productivity related to the ISM variability. Detrital Nd/Sr isotope data and clay mineral compositions suggest that the sediments at Site U1445 originated mainly from the Ganges, Brahmaputra, and Meghna rivers without dramatic provenance change between the glacial and interglacial periods. The weathering activity inferred from clay mineral compositions and the paleo-productivity shift reconstructed by biogenic opal and CaCO₃ contents suggest that the land-sea interactions were closely linked to the ISM precipitation between the glacial and interglacial periods. High precipitation by the strong ISM resulted in intense chemical weathering and dominant biogenic opal deposition during the interglacial periods. In contrast, low precipitation by the weak ISM led to reduced chemical weathering and predominant CaCO₃ deposition during the glacial periods. Further, the ISM variability driving the land-sea interactions in the Mahanadi Basin was modulated by the Indonesian throughflow (ITF). Our study emphasizes the role of low-latitude forcing of climatic changes in the strong relationship between the ISM and ITF over orbital periods, providing a base for future investigations.     

Relative sea-level rise and climate change over the last 1500 years

We constructed a detailed relative sea-level rise curve for the last 1500 years using a novel approach, i.e. charting the rate of relative sea-level rise using microfaunal and geochemical data from a coastal salt marsh sequence (Clinton, CT, USA). The composition of benthic foraminiferal assemblages and the iron abundance in peats were used to describe shifts in marsh environment through time quantitatively. The resulting sea-level rise curve, with age control from ¹⁴C dating and the onset of anthropogenic metal pollution, shows strong increases in the rate of relative sea-level rise during modern global warming (since the late nineteenth century), but not during the Little Climate Optimum (ad 1000–1300). There was virtually no rise in sea-level during the Little Ice Age (ad 1400–1700). Most of the relative sea-level rise over the last 1200 years in Clinton appears to have occurred during two warm episodes that jointly lasted less than 600 years. Changes from slow to fast rates of relative sea-level rise apparently occurred over periods of only a few decades. We suggest that changes in ocean circulation could contribute to the sudden increases in the rate of relative sea-level rise along the northeastern USA seaboard. Relative sea-level rise in that area is currently faster than the worldwide average, which may result partially from an ocean surface effect caused by hydrodynamics. Our data show no unequivocal correlation between warm periods (on a decaal to centennial time-scale) and accelerated sea-level rise. One period of acclerated sea-level rise may have occurred between about ad 1200 and 1450, which was the transition for the Little Climate Optimum to the Little Ice Age, i.e. a period of cooling (at least in northwestern Europe). Local changes in tidal range might also have contributed to this apparent increase in the rate of relative sea-level, however. The second period of accelerated sea-level rise occurred during the period of modern global warming that started at the end of the last century.     

The climate of Scotland over the last 5000 years inferred from multiproxy peatland records: inter‐site correlations and regional variability

The mid to late‐Holocene climates of most of Scotland have been reconstructed from seven peat bogs located across north–south and east–west geographical and climatological gradients. The main techniques used for palaeoclimatic reconstruction were plant macrofossil, colorimetric humification, and testate amoebae analyses, which were supported by a radiocarbon‐based chronology, aided by markers such as tephra isochrons and recent rises in pine pollen and in spheroidal carbonaceous particles (SCPs). Field stratigraphy was undertaken at each site in order to show that the changes detected within the peat profiles were replicable. Proxy climate records were reconstructed using detrended correspondence analysis (DCA) of the plant macrofossil data and a mean water table depth transfer function on the testate amoebae data. These reconstructions, coupled with the humification data, were standardised for each site and used to produce a composite record of bog surface wetness (BSW) from each site. The results show coherent wet and dry phases over the last 5000 years and suggest regional differences in climate across Scotland, specifically between northern and southern Scotland. Distinct climatic cycles are identified, all of which record a millennial‐scale periodicity which can be correlated with previously identified marine and ice core Holocene cycles. The key role of the macrofossil remains of Sphagnum imbricatum, a taxon now extinct on many sites, is discussed in relation to the identified climatic shifts. Copyright © 2005 John Wiley & Sons, Ltd.     

A numerical model of the evolution of ocean sulfate and sedimentary sulfur during the last 800 million years

A reservoir model describing the time evolution of the sedimentary cycle of sulfur over the past 800 my has been developed. As a first approximation, the ocean sulfate concentration is assumed to be time-independent. With this assumption, the model is integrated backward in time and a new initialization procedure is derived in order to calculate the present state of the system which must be compatible with both observational data and model equations. The effects of a variation of the present state of the cycle on its past evolution are investigated. It is found that, when the present gypsum reservoir content is too low or when the weathering rate constants are too high, no acceptable solution can be obtained for the evolution of the cycle, since one reservoir is forced to depletion. The sensitivity of the model to the mean isotopic composition of the sedimentary system and to the fractionation factor during pyrite formation is also studied.Moreover, a model with time-dependent ocean sulfate concentration was developed. The existence of an acceptable solution appears to be linked to the steady state hypothesis for ocean sulfate, since a model with no acceptable steady state solution may be integrated until t = −800 my without any problem of reservoir depletion when the time-dependent equations are used.A tentative evolution of the ocean sulfate concentration is calculated. It is shown that this concentration is negatively correlated to the δ³⁴S of seawater sulfate. The carbon cycle is modelled in order to compare the calculated δ¹³C of carbonate deposits to the observational data.     

The vegetation and climate of northwest Iberia over the last 14,000 years

A new pollen record from an upland lake in north-west Spain, Laguna de la Roya, spans the last ca 14,500 yrs and includes clear evidence of a Weichselian Lateglacial event correlative with the Younger Dryas. Pollen-climate response surfaces have been used to make quantitative reconstructions of palaeoclimate conditions at this and two other sites in the region. These reconstructions indicate that the climate was dry and cool during both the Late Weichselian and the Younger Dryas; in contrast, conditions during the Lateglacial Interstadial were relatively moist. During the early Holocene the climate was more continental in character than it has been for the last three millenia. Human activity has had a substantial impact upon the upland vegetation around Laguna de la Roya only during the last two millennia.     

Impact of climate variability in the western Mediterranean during the last 20,000 years: oceanic and atmospheric responses

Eolian input fluctuations and paleoceanographic conditions in the western Mediterranean have been reconstructed by using a multi-proxy approach to infer climate variability for the last 20,000 yr. A sediment core from the East Alboran Sea basin provides high-resolution geochemical, mineralogical and grain size distribution records from the Last Glacial Maximum onward. The accurate chronology obtained, closely related with the North Greenland Ice core Project, has provided a detailed paleoclimate reconstruction at the centennial–millennial time scale. Mineral and chemical compositions as well as sediment grain size distribution have been used to establish both fluvial and eolian input variations, deep-water current intensity, and ventilation and productivity conditions at the time of deposition. During the cold period Greenland Stadial-2a, the analyzed proxies evidence significant climate oscillations. Redox proxies, showing marked Mn/Al and Fe/Al peaks, as well as granulometric parameters, support major changes in thermohaline circulation, with active circulation triggered by the Western Mediterranean Deep Water formation. Ba/Al ratios and the presence of authigenic barite point to high productivity during the Last Heinrich event. At this time, Zr/Al, Si/Al and Ti/Al ratios as well as quartz and palygorskite contents also corroborate dryer conditions in the Alboran borderlands, with a major reorganization of atmospheric circulation at the beginning of the deglaciation and during the Last Heinrich event. In contrast, a decreasing trend in the Zr/Al ratio and increasing Mg/Al, K/Al and illite + chlorite/kaolinite ratios would indicate enhanced river runoff and supporting lower eolian dust input and wetter periods during the Bölling–Alleröd. Short-term and abrupt climate oscillations such as the Intra-Alleröd Cold Period are likewise reflected by minor yet conspicuous changes in eolian proxies. During the Younger Dryas, we distinguish an initial cold and dry phase, with major westerlies intensity, thermohaline reactivation, and high productivity levels, and a second wetter phase characterized by sporadic rainfalls over poorly vegetated surrounding continental areas. Then, during the Holocene, alternating phases of major humidity and aridity were recorded by major fluctuations in detrital input. These are also correlated with recurrent forest cover declines reported from pollen records, with a noteworthy early decline in the African Humid Period. During the Holocene, an initial demise of the Last Organic-Rich Layer is also evidenced, tentatively associated with aspiration forces promoted by intense thermohaline circulation through the Gibraltar Strait.     

Accretion of extraterrestrial matter during the last 80 million years and its effect on the marine osmium isotope record

The flux of particulate extraterrestrial (ET) matter to the deep-sea has been calculated using a four-component mixing model based on osmium concentrations and isotope ratios in slowly accumulating pelagic sediments from the Pacific Ocean. Nineteen published bulk-leach osmium isotope data pairs that cover the last 80 million years have been used for the calculation. The calculated annual particulate flux, averaged over several 100 ka by slow accumulation and bioturbation, ranges from ∼18,000 to ∼67,000 tons, with a mean value of ∼37,000 ± 13,000 tons. The data indicate no significant variability in the flux with time, except at the K-T boundary and are thus compatible with results based on Ir accumulation in deep-sea sediments. The inferred constancy of the ET matter flux is in contrast to recent results based on the ET ³He flux that integrates only a small size fraction of the entire ET matter flux. ³He data indicate variability by a factor of 6 over the past 70 Ma and a fivefold increase in the ET flux between 3 Ma and 1 Ma that is not seen in the bulk ET matter flux based on osmium isotope data.The apparent constancy of the cosmic matter flux over the past 80 Ma stands in marked contrast to the dynamic nature of the marine osmium isotope record and indicates that dissolution of cosmic matter does not drive changes in this record, except at the K-T boundary. A comparison between osmium isotope and Ir data from two pelagic sediment cores from the Pacific (DSDP Site 596 and LL44-GPC3) indicates that the seawater-soluble fraction of ET Os cannot exceed ∼36 kg/a and most likely is significantly smaller. The maximum value of 36 kg/a can account for approximately one half of the seawater-soluble ET matter flux necessary to balance the radiogenic continental runoff of Os without any additional unradiogenic source. Simple mass balance calculations indicate that an additional unradiogenic source of Os to the oceans, most likely alteration of oceanic crust, is required to balance the present-day seawater osmium-isotopic composition. This source is probably more important in balancing radiogenic continental runoff than is dissolution of cosmic matter in seawater.     

Glacier and climatic fluctuations inferred from tree-growth variations over the last 250 years, central southern Norway

A Pinus sylvestris (Scots pine) tree-growth series from near the tree-line in upper Gud- brandsdalen, southern Norway (Slistad 1957) is analysed, and possible relationships to glacier and climatic fluctuations are explored. The series is smoothed by harmonic analysis, and calibrated using a variety of independently derived glaciological, geomorphological, and climatological data from Storbreen (Jotunheimen), the Storbreen gktschervorfeld, and Dombås meteorological station. The curves are then used to make inferences about the number, date, magnitude, and duration of glacier fluctuations and summer temperature fluctuations from 1700 A.D. to 1950 A.D. At least 10 major oscillations in the tree-growth data, reflecting fluctuations in summer temperatures of amplitude 1.0^°C to 3.0°C, are believed to be indicative of glacier fluctuations. Periods of reduced tree-growth are a response to cool phases and indicate glacier advances after a lag of about 4 years. These short-term oscillations are superimposed on a general trend of glacier retreat and a long-term warming of 1.0°C indicated by a glacier equilibrium-line displacement of 140 m to 145 m since 1750 A.D. Problems and prospects for development of dendroglaciological and dendro-climatological techniques are discussed.     

A Quaternary Timeline: a teaching tool for interpreting palaeoenvironmental, palaeoclimatic and archaeological change over the last 500,000 years

This short paper presents a case study for teaching elements of Quaternary Science to a non-specialist audience. A guided hands-on practical exercise in sedimentology and stratigraphy, palaeoenvironmental analysis and Palaeolithic archaeology was supported by the development of a ‘Quaternary Timeline’ chart, which introduces the concepts of changing climates and environments and characterises the broad expression of these changes in the terrestrial and fossil records. Using information from the practical exercises, the figure allows a step-by-step interpretation to be built up of past landscapes, including an understanding of the nature of fluvial responses to Quaternary climate change, a reconstruction of the contemporary flora and mammalian fauna and an analysis of the activities of early human populations.     

Structural and gold mineralizing evolution of the world-class orogenic Mana district,Burkina Faso: Multiple mineralizing events over 150 million years

The Mana district, located in the northern part of the Birimian Houndé greenstone belt in western Burkina Faso, is a world-class Paleoproterozoic orogenic gold district (∼8 Moz) including five gold deposits (Fofina, Nyafé, Siou, Wona-Kona and Yaho). These deposits are located in specific lithostratigraphic domains, and gold is controlled by various structural features. Deposit- and regional-scale mapping, intrusion age and geochemistry, as well as airborne aeromagnetic and electrical resistivity geophysical data, were used to decipher the tectonic evolution of each gold deposit and the district. Five deformational and four gold mineralizing events were recognized.The first deformation event (D1_MD: E-W oriented shortening) affected the metamorphosed volcanic and sedimentary rocks of the Lower Birimian group. This early deformation episode was correlated with the formation of gently N-plunging folds (F_1MD) and N-S-striking thrusts faults coeval with emplacement of the pre- to synkinematic Wona-Kona and Siou plutons dated at ∼2172 Ma, under greenschist facies metamorphism. The quartz-carbonate veins (V_1MD) at Fofina and Siou formed during D1_MD at Eoeburnean time, manifesting the first gold event at approximately ∼2172 Ma.The following deformation event (D2_MD: E-W oriented extension) is associated with the deposition of the Upper Birimian group (Mana basin) overlying the Lower Birimian group. The geometry of the Mana basin is controlled by the Mana and Maoula shear zones. The Tarkwaian-type rock formation overlying the Upper Birimian group, controlled by the Wona-Kona and Siou shear zones, is constrained at the end of D2_MD or at the beginning of the D3_MD event with a maximum deposition age at ∼2113 Ma.The third deformation event (D3_MD: E-W to WNW-ESE transpression) affected the entire supracrustal rock. Such event is correlated with the formation of map-scale F_3MD folds and dextral shear zones during the Eburnean orogeny (∼2113–2090 Ma). A second gold mineralizing event occurred during D3_MD and is manifested by quartz-carbonate veins (V_3MD) and disseminated sulfides at the Yaho, Fofina and Nyafé and possibly Wona-Kona deposits.The fourth deformation event (D4_MD: NNW-SSE transpression) is correlated with sinistral shearing along the major transcurrent faults and the development of asymmetric NNE-striking folds (F_4MD) associated with vertical fold axes. Syn-D4_MD mineralization is characterized mainly by a strong silicification (Si_4MD) with disseminated pyrite and arsenopyrite along the Wona-Kona shear zone and by tiny quartz-carbonate veinlets (V_4MD). This event is considered the main gold-bearing event in the western margin of the Mana district.The fifth and last deformation event (D5_MD) is brittle in character and was responsible for the formation of E-W subvertical crenulation cleavages and reverse faults under overall N-S shortening. This late deformation event is tentatively associated with a last gold event recorded as free gold associated with muscovite in brittle fractures developed in competent orebodies at the Wona-Kona and Siou deposits. This event could be as young as ∼2022 Ma, the age obtained from Ar-Ar datation of muscovite-schists at the Wona-Kona deposit.Our main contribution is that we decipher multiple gold mineralizing events at the district scale based on deposit- and regional-scale mapping. It is interpreted that gold was introduced as early as ∼2172 Ma and possibly as late as ∼2022 Ma during at least 3 or even 4 shortening tectonic events in a timeframe not yet recognized at the district scale for all the Birimian belts.     

Sr isotopes and pore fluid chemistry in carbonate sediment of the Ontong Java Plateau: Calcite recrystallization rates and evidence for a rapid rise in seawater Mg over the last 10 million years

The ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations in sediment and pore fluids are used to evaluate the rates of calcite recrystallization at ODP Site 807A on the Ontong Java Plateau, an 800-meter thick section of carbonate ooze and chalk. A numerical model is used to evaluate the pore fluid chemistry and Sr isotopes in an accumulating section. The deduced calcite recrystallization rate is 2% per million years (%/Myr) near the top of the section and decreases systematically in older parts of the section such that the rate is close to 0.1/age (in years). The deduced recrystallization rates have important implications for the interpretation of Ca and Mg concentration profiles in the pore fluids. The effect of calcite recrystallization on pore fluid chemistry is described by the reaction length, L, which varies by element, and depends on the concentration in pore fluid and solid. When L is small compared to the thickness of the sedimentary section, the pore fluid concentration is controlled by equilibrium or steady-state exchange with the solid phase, except within a distance L of the sediment-water interface. When L is large relative to the thickness of sediment, the pore fluid concentration is mostly controlled by the boundary conditions and diffusion. The values of L for Ca, Sr, and Mg are of order 15, 150, and 1500 meters, respectively. L_Sr is derived from isotopic data and modeling, and allows us to infer the values of L_Ca and L_Mg. The small value for L_Ca indicates that pore fluid Ca concentrations, which gradually increase down section, must be equilibrium values that are maintained by solution-precipitation exchange with calcite and do not reflect Ca sources within or below the sediment column. The pore fluid Ca measurements and measured alkalinity allow us to calculate the in situ pH in the pore fluids, which decreases from 7.6 near the sediment-water interface to 7.1 ± 0.1 at 400-800 mbsf. While the calculated pH values are in agreement with some of the values measured during ODP Leg 130, most of the measurements are artifacts. The large value for L_Mg indicates that the pore fluid Mg concentrations at 807A are not controlled by calcite-fluid equilibrium but instead are determined by the changing Mg concentration of seawater during deposition, modified by aqueous diffusion in the pore fluids. We use the pore fluid Mg concentration profile at Site 807A to retrieve a global record for seawater Mg over the past 35 Myr, which shows that seawater Mg has increased rapidly over the past 10 Myr, rather than gradually over the past 60 Myr. This observation suggests that the Cenozoic rise in seawater Mg is controlled by continental weathering inputs rather than by exchange with oceanic crust. The relationship determined between reaction rate and age in silicates and carbonates is strikingly similar, which suggests that reaction affinity is not the primary determinant of silicate dissolution rates in nature.     

Global,regional and local scale factors determining glaciation extent in Eastern Siberia over the last 140,000 years

The aim of this work is to evaluate the effect of variations of greenhouse gas concentrations, orbital parameters, sea-surface conditions, vegetation and dust deposition on the extent of East Siberian mountain glaciations during the late Quaternary. An atmosphere-only general circulation model is used in a series of 16 sensitivity tests at high spatial resolution over the region of interest to systematically evaluate the relative importance of these different forcing parameters. No attempt was made to reproduce in detail the history of late Quaternary mountain glaciations in East Siberia, because, given (a) the temporal and spatial scarcity of available evidence of mountain glaciations in this region and (b) the large uncertainties concerning the boundary conditions to be prescribed in this model, such an exercise must necessarily remain incomplete and partially inconclusive. The results of this study suggest that moisture delivery from the Atlantic is an important factor determining mountain glacier mass balance in Eastern Siberia and is very sensitive to the geometry of the West Eurasian ice sheet. This means that variable moisture blocking by the West Eurasian ice sheet during the Weichselian is the most important single factor explaining the opposite history of glacier and ice sheet extent in West and East Eurasia during the Weichselian. This work confirms earlier results showing that the large 140 kyr BP West Eurasian ice sheet caused regional-scale cooling extending towards Eastern Eurasia. Nevertheless, the simulated response of the regional summer temperature (and thus glacier extent because of the strong dependency of glacier mass balance of summer melt rates) is to a very large extent directly determined by insolation. For the Early Weichselian, this leads to a clear maximum of local glacier extent at 70 kyr BP, which is in line with the variations of top-of-the-atmosphere insolation on orbital time scales, but to some degree at odds with geological evidence which suggests larger glacier extent at 115 and 90 kyr BP than at 70 kyr BP. Through snow feedbacks, the effects of changes in the prescribed vegetation distribution and dust deposition rate are also substantial. In summary, it appears that the broad features of late Quaternary glaciation history in Eastern Eurasia can be understood in terms of known forcings.