Radiometric Ages Research Articles

A fully calibrated and updated mid-Holocene climate reconstruction for Eastern South America

Investigating the extent and climate implications of the mid-Holocene (MH - around 6000 years ago) is essential in better understanding the uncertainties associated with the ongoing and future projections of global warming. South America (SA) contains the biome with the largest biodiversity in the world, its population exceeds 400 million people, and its economy is mainly agricultural. Thus, studies involving South America's climate variability have become a subject of significant importance over the years. Paleoclimate archives provide unique indirect measurements, helping us to characterize past climates. However, the derived radiometric ages from several paleoarchives in SA published before the 2000s have not yet been calibrated, which represents a large source of uncertainty. In this article, we present a climate reconstruction for eastern SA during the MH with fully calibrated age models. We compiled 173 paleoclimate records, where 50 of them were calibrated in this study using Bayesian methods. Through this multiproxy approach and the elaboration of a new homogeneous quality index for proxies, our novel climate reconstruction sheds new light on the regional climate of SA climate during MH. All high quality (higher than 1) paleoarchives agree at regional scale. Compiled paleodata confirms reduced rainfall in Amazonia and a warmer and drier Southern SA, indicating an overall weakening of the South American Summer Monsoon during the MH. North East Brazil coast show higher than present moisture during the MH, while the border between Northeast Brazil and Southern SA shows divergent behavior, pointing to a local erratic rainfall regime with inputs of higher than present moisture conditions, most likely caused by changes in precipitation over the South Atlantic Convergence Zone, and a weaker than present South Atlantic Subtropical High during MH.

Martian surface dating model refinement based on Chang'E-5 updated lunar chronology function

Planetary surface ages are extremely important to geologic evolution studies. For extraterrestrial bodies, the crater size-frequency distribution dating method, including a crater production function and a chronology function, was established based on the radiometric ages of Apollo and Luna samples from the Moon. Recently, the radiometric age of a sample from Chang'e-5 mission added a data point where there was previously a gap in sample ages, and an updated lunar chronology function was proposed. This research aims to recalculate the Martian surface dating model from the newly updated lunar chronology function. The calculation takes account of the differences in impact rates, impact velocities, surface gravities, the threshold diameters of craters' transition from the strength to gravity regime and from simple to complex crater structures on the Moon and Mars. We use the Neukum production function, although other production functions could also be implemented with our method. A series of Martian isochrons is derived, and an approximated analytical form production function is provided for convenient applications in future studies. The Martian chronology function is finally obtained through fitting crater frequencies larger than 1.0 km in diameter with the corresponding ages of the isochrons. The resultant Martian PF and CF in this research are largely comparable with previous results. Based on the refined lunar chronology function, our model will enhance the reliability of the estimating Martian surface ages and we suggest it be used in future studies.

U-series and radiocarbon cross dating of speleothems from Nerja Cave (Spain): Evidence of open system behavior. Implication for the Spanish rock art chronology

Two stalagmites from Nerja cave (Andalusia, Spain) were studied. The cave is well known because of its long human occupation from the Upper Palaeolithic to the Chalcolithic and its abundant parietal prehistoric Art. The aims of this study were twofold: i) to compare uranium/thorium (230Th/234U) and Carbon-14 (14C) ages obtained all along the growth axis of the stalagmites in order to understand the consequences of diagenetic processes on the validity of radiometric ages; ii) as one of the stalagmites contains black layers, attributed to combustion soot, to establish when these intense hearths were used and by which culture.230Th/234U and 14C ages were coupled with mineralogical studies using FTIR (Fourier-transform infrared spectroscopy) and thin section observations. The first stalagmite (GN16-9b) displays 230Th/234U ages in stratigraphic order, and compatible with 14C ages corrected for a few percent of dead carbon. Homogeneous composition of aragonitic crystals characterized by their needle-like texture is observed throughout this speleothem. For the second stalagmite (GN16-7), in contrast, 230Th/234U ages display large significant inversions and discordant results on the upper part and at the base of the stalagmite, suggesting a possible open system behavior for this chronometer. Interestingly, 14C ages are in stratigraphic order all along the stalagmite and are compatible with 230Th/234U ages only in its central part. Mineralogical studies display evidence of aragonite to calcite transformation at the top and a complex mineralogical assemblage with interlayered silicates (possibly clays) and calcitic mineralogy for the base of GN16-7. In these parts, discordant 230Th/234U ages were measured. In the middle part of the stalagmite, however, where the fibrous aragonite is well preserved, the 14C and 230Th/234U ages agree. Our data suggest that in the case of aragonite to calcite transformation as shown here, 230Th/234U ages are biased, but 14C ages seem to remain accurate, as already observed in aragonitic marine bio minerals. 14C ages obtained are used for the chronology of the soot layer, determined here between 7900 and 5500 years Cal BP, coherent with previous analysis of charcoals in the same sector of the cave. This study highlights the importance of working with at least two chronometers when stratigraphic age verification is not possible, as is the case of some parietal CaCO3 thin layers used for rock art dating. Recent 230Th/234U ages published for carbonate deposits on Spanish parietal Art are discussed in light of this demonstration.

Geochronological record of Cadomian exterior orogen reworking in bi-modal igneous protoliths of the Cabo Ortegal Allochthon (NW Iberia): the Cambrian onset of the Palaeozoic plate-tectonic cycle

ABSTRACT New geochemical data and radiometric ages of the protoliths of variably reworked metamafic (granulites and gabbros) and felsic rocks (orthogneisses) from the Cabo Ortegal allochthonous Complex are presented. Their igneous precursors were emplaced in ensialic environments and exhibit geochemical signatures transitional to tholeiitic character, probably originating in arc settings (with continental and oceanic components and minor crustal recycling). The new geochronological results (encompassing the Miaolingian and Furongian epochs of the Cambrian) link the aforementioned scenarios to a Cadomian magmatic arc. The 505Ma protoliths of calc-alkaline orthogneisses and mafic granulites would be the oldest in the sequence of intrusion events. The latter and the 498–486Ma protoliths of gabbros (emplaced in sedimentary host sequences later transformed into Variscan high-pressure and intermediate-pressure gneisses) can be considered as snapshots of the ongoing continental arc evolution. The 498–486Ma gabbro intrusives of these gneisses, together with the 505–503Ma orthogneisses, are ascribable to palaeo-geographically separate scenarios before the Variscan amalgamation of their country rocks. They would expand the initiation of rifting of the Gondwana continental margin to the Miaolingian Cambrian epoch. It is inferred that the Cadomian continental-arc (active at least since 750Ma) experienced in the 510–460Ma interval the consequences of a vanishing active subduction. Certain domains flanking the arc underwent forearc sedimentation and deformation, whereas others were affected by an increasing extension (rifting) and/or back-arc basin formation. The latter marked the initiation of the beginning of a Wilson Cycle, which ended in the late Palaeozoicwith the formation of the Variscan interior orogenic belt.

Late Cretaceous vertebrate faunal assemblages of the Manitoba escarpment, Canada: Implications for Western Interior Seaway provinciality and biodiversity

Community zonation of the Late Cretaceous Western Interior Seaway (WIS) has been suggested for bivalves, cephalopods, foraminifera, gastropods, and tetrapods. Most proposed WIS community zones consist of a northern and southern subprovince with a gradational boundary across central or south-central North America. Since it has been over three decades since the WIS community zonation hypothesis has been investigated for vertebrates, recent radiometric age determinations, taxonomic revisions, additional specimen discoveries, and recently available online museum specimen catalogues allow for an updated description of Manitoba (MB) escarpment faunal assemblages and testing of the community zonation hypothesis. Nine time bins were used to represent nine Upper Cretaceous lithostratigraphic units of the MB escarpment to test the zonation hypothesis consistency for nearly the entire Late Cretaceous (~71–95 Ma). Relatively high genus-level community similarity values (25–50%) of south-central WIS localities and low values (<20%) of localities furthest north and south support the existence of a central subprovince during late Cenomanian to early Turonian and late Coniacian to early Campanian times, when the gradational subprovincial boundary would have been furthest south between Kansas and Texas localities. Comparatively low genus-level community similarity values (<25%) of all localities south of MB during mid-Cenomanian and early to mid-Campanian times indicate the southern subprovincial boundary was farthest north between MB and South Dakota localities during these time intervals and had migrated throughout the Late Cretaceous. This work highlights significant fluctuations in vertebrate community zonation throughout the WIS through time and space and offers insight into the magnitude of compositional and palaeoecological changes that can occur in shallow marine vertebrate communities over an approximately 25 million year interval.

Metal-rich organic matter and hot continental passive margin: drivers for Devonian copper-cobalt-germanium mineralization in dolomitized reef-bearing carbonate platform

The abundance and types of reef-bearing carbonate platforms reflect the evolution of Devonian climate, with conspicuous microbial-algal reefs in the warm Early and Late Devonian and sponge-coral reefs in the cooler Middle Devonian. A dolomitized Wenlock-Lower Devonian microbial-algal reef-bearing carbonate platform hosts epigenetic copper-cobalt-germanium (Cu-Co-Ge) sulfide mineralization at Ruby Creek-Bornite in the Brooks Range, Alaska. Here, we present rhenium-osmium (Re-Os) radiometric ages and molybdenum and sulfur (δ98/95Mo = +2.04 to +5.48‰ and δ34S = −28.5 to −1.8‰) isotope variations for individual Cu-Co-Fe sulfide phases along the paragenetic sequence carrollite-bornite-pyrite. In the context of a hot, extensional passive margin, greenhouse conditions in the Early Devonian favored restriction of platform-top seawater circulation and episodic reflux of oxidized brines during growth of the carbonaceous carbonate platform. Molybdenum and sulfur isotope data signal the stepwise reduction of hot brines carrying Cu during latent reflux and geothermal circulation for at least ca. 15 million years from the Early Devonian until Cu-Co sulfide mineralization ca. 379–378 million years ago (Ma) in the Frasnian, Late Devonian (weighted mean of Re-Os model ages of carrollite at 379 ± 15 Ma [n = 4]; Re-Os isochron age of bornite at 378 ± 15 Ma [n = 6]). On the basis of petrographic relationships between sulfides and solid bitumen, and the Mo and S isotope data for sulfides, we imply that the endowment in critical metals (e.g., Co, Ge, Re) in the Ruby Creek-Bornite deposit is linked to the activity of primary producers that removed trace metals from the warm Early Devonian seawater and concentrated Co, Ge, and Re in algal-bacterial organic matter in carbonate sediments.

Paleoclimate-induced stress on polar forested ecosystems prior to the Permian–Triassic mass extinction

The end-Permian extinction (EPE) has been considered to be contemporaneous on land and in the oceans. However, re-examined floristic records and new radiometric ages from Gondwana indicate a nuanced terrestrial ecosystem response to EPE global change. Paleosol geochemistry and climate simulations indicate paleoclimate change likely caused the demise of the widespread glossopterid ecosystems in Gondwana. Here, we evaluate the climate response of plants to the EPE via dendrochronology snapshots to produce annual-resolution records of tree-ring growth for a succession of late Permian and early Middle Triassic fossil forests from Antarctica. Paleosol geochemistry indicates a shift in paleoclimate towards more humid conditions in the Early and early Middle Triassic relative to the late Permian. Paleosol morphology, however, supports inferences of a lack of forested ecosystems in the Early Triassic. The plant responses to this paleoclimate change were accompanied by enhanced stress during the latest Permian as determined by high-resolution paleoclimate analysis of wood growth intervals. These results suggest that paleoclimate change during the late Permian exerted significant stress on high-latitude forests, consistent with the hypothesis that climate change was likely the primary driver of the extinction of the glossopterid ecosystems.

Acceleration of Late Pleistocene activity of a Central European fault driven by ice loading

We studied the southern part of the NW-SE trending Sudetic Marginal fault (SMF), situated at the northeastern limit of the Bohemian Massif in central Europe, to assess its Quaternary activity. Eighteen trenches and thirty-four electric resistivity profiles were performed at Bílá Voda to study the fault zone and 3-dimensional distribution of a beheaded alluvial fan on the NE side of the fault. We interpret a small drainage, located about 29–45 m to the SE of the fan apex, as the only plausible source channel implying a similar amount of left-lateral offset. The alluvial fan deposits' radiometric ages range between about 24 and 63 ka, but postglacial deposits younger than 11 ka are not displaced, indicating that all motion occurred in the late Pleistocene. The site lies ∼150 km south of the late Pleistocene Weichselian maximum (∼20 ka) ice sheet front. We model the effects of the ice load on lithospheric flexure and resolved fault stresses, and show that slip on the SMF was promoted by the presence of the ice sheet, resulting in a late Pleistocene slip rate of ∼1.1+2.3/−0.6 mm/yr. As the most favorable time for glacial loading-induced slip would be during the glacial maximum between about 24 and 12 ka, it is doubtful that the slip rate remained constant during the entire period of activity, and if most slip occurred during this period, the short-term rate may have been even higher. Considering that the modern maximum principal stress (σ1) is oriented nearly parallel to the Sudetic Marginal fault (NNW-SSE) and is thus unfavorable for fault motion, our observations suggest that the likelihood of continued motion and earthquake production is much lower in the absence of an ice sheet.

Monazite as a control on Th/U in magmatic zircon

Zircon is one of the most widely studied minerals for geochronology, and it has been successfully used to determine the age of young eruptions (< 300 ka) through U-Th dating. However, the processes leading to Th/U variations between cogenetic zircon crystals that are often much larger than expected from partitioning considerations are still not fully understood, despite the critical importance of zircon Th/U variability for isochron dating, for the disequilibrium correction of radiometric ages, and for identifying different zircon generations. Here, we present 238U-230Th disequilibrium ages and trace element abundances of zircon from the Quaternary Pudahuel Ignimbrite, a caldera-forming eruption associated to the Maipo volcanic complex in central Chile. Zircon chemical signatures show a decrease of Th/U along with decreasing Ti-in-zircon temperatures that can be related to the presence of accessory minerals which incorporate Th over U into its structure. These variations can also be identified along individual depth profiles, suggesting that monazite co-crystallization is the key cause for a sharp rimward decrease in zircon Th/U. Apatite fractionation is expected to have little impact on Th/U in the remaining melt, and in consequence U-Th activity ratios in zircon because it mostly saturates at temperatures above zircon saturation, and partitioning differences between Th and U are minor. By contrast, monazite fractionation strongly controls Th/U in Pudahuel rhyolite melts, especially at low temperatures. The isochron age calculated from zircon with low Th/U (238U/232Th activity >5.2) crystallized at low temperature below monazite saturation (167 ± 8 ka; 1σ error) is younger than for zircon with high Th/U (238U/232Th < 5.2). This presents the most precise age constraint available for Pudahuel Ignimbrite, albeit as a maximum eruption age. These results are particularly important in the study of young eruptions, where accurate ages are important to reliably interpret eruptive recurrence and volcanic chronostratigraphy, which in turn can affect the hazard awareness of volcanic systems.

Multimethod palaeointensity results from a rapidly emitted upper Miocene lava flow sequence in São Vicente (Cape Verde): new data for the African record

SUMMARY We present new radiometric ages and full-vector palaeomagnetic data from São Vicente Island, Cape Verde. Samples were obtained from a sequence of 11 flows. A K-Ar age determination was performed on the lowermost flow of the sequence, yielding an upper Miocene age of 6.07 ± 0.16 (2σ) Ma. Considering the normal polarity observed in all flows, the age of the sequence could be narrowed down to an interval between 6.03 and 6.23 Ma. The mean palaeomagnetic direction of the studied sequence was compared with the expected direction, calculated from the African Apparent Polar Wander Path (5 Ma). While inclinations approximately agree, the palaeodeclination appears westwardly deviated by 26.5° ± 2.9°. Moreover, nearly no scatter of virtual geomagnetic poles around their mean value is observed and its comparison with the scatter observed in palaeomagnetic directions obtained from geomagnetic field model SHA.DIF.14k suggests that the sequence was very rapidly emplaced in only a few centuries. Thus, the obtained anomalous palaeomagnetic direction seems to reflect the effect of non-averaged secular variation, although a local vertical-axis counter-clockwise rotation of the sequence, cannot be entirely ruled out. Thellier–Coe palaeointensity experiments on 66 specimens yielded 22 successful determinations and multispecimen (MS) palaeointensity experiments on 11 samples yielded 8 successful determinations. In the upper part of the sequence, Thellier–Coe determinations fail due to the presence of multidomain (MD) grains, but the MS method yields successful results in several of these flows, demonstrating that this method can provide reliable palaeointensity determinations on samples with a higher MD grain fraction. A mean palaeointensity BSV = (18.5 ± 1.9) µT is obtained from the joint analysis of the results from both methods. This value lies clearly below the present-day value BP = 33.1 μT. The virtual axial dipole moment (VADM = 4.3 × 1022 Am2), however, cannot be considered anomalous if compared to available upper Miocene–Pliocene data and represents a new high-reliability contribution to the very limited African palaeointensity database.

Deformation along the Deseado Massif (Patagonia, Argentina) during the Jurassic Period and its relationship with the Gondwana breakup: paleomagnetic and geochronological constraints

This work presents the analysis of paleomagnetic results obtained from four sampling areas of the Jurassic Bahía Laura Complex in the Deseado Massif and their implications on the regional deformation history during the breakup of Gondwana. Paleomagnetic data show cessation of tectonic block rotations about vertical axes around 160 Ma and a change from a transtensional to a mainly extensional tectonic regime. Two biotite samples yielded 40Ar–39Ar radiometric ages: one from the eastern outcrops of the Chon Aike Formation (plateau age of 184.66 ± 0.55 Ma) and the other from the La Matilde Formation (plateau age of 157.40 ± 0.65 Ma), located at the central part of the Deseado Massif. Finally, an apparent polar wander path (APWP) was calculated for the Jurassic of South America (200 to 140 Ma). This APWP indicates that South America experienced a northward drift between 200 Ma and 170 Ma, a clockwise rotation (~10°) between 170 Ma and 160 Ma, and a westward drift between 160 Ma and 140 Ma. The 170–160 Ma rotation could has been triggered by the combined effects of the uncoupling of the Antarctic Peninsula, the high rates of subduction of the Phoenix plate beneath Patagonia, the opening of the Rocas Verdes basin and the Weddell Sea, and the cessation of the Tethys slab-pull. Furthermore, the calculated APWP for South America does not support a Jurassic massive true polar wander event.

The Miocene/Pliocene Boundary In The North East Java Basin, Indonesia

As a part of the LEMIGAS Research Programme on Neogene Epoch Boundaries in the Indonesian Tertiary Basins, in 1981 fiscal year is selected North East Java Basin as an object for research. In this paper we will stress our work on the Miocene/ Pliocene Boundary of the basin based on planktonic Foraminifera. More recently a lot of paleontologists have been working on the Miocene/Pliocene Boundary problem. Bandy (1963) used the first appearance of Sphaeroidinella dihescens as a boundary in the Phillipine, whereas boundary based on the initial apppearance of Globoratalia margaritae. carried out an examination of samples from the Trubi Marl of the Zanclean Sicily and came to the conclusion approximately with middle N18 of this zonal scheme. Bolli ( 1966, revision in 19/0 ) put this stage In 1969, Blow Berggren (1973) proposed the limit of this stage boundary based on the last occurrence of Globoquadrina dehiscens. Kaneps (1974) based on this observation on planktonic Foraminifera of the DSDP cores from the Eastern Equatorial Pacific Ocean came to the conclusion that the Miocene/Pliocene Boundary lies at the base of Globoratalia tumida zone. The base of this zone was placed at the horizon of the initial appearance of Globoratalia tumida while the upper boundary was placed at the horizon of reversal in coiling direction of Pulleniatina primalis from left below to right above. Pringgoprawiro and Baharuddin (1979) on the other hand proposed the initial appearance of Sphaeroidinella immatura as a boundary between these two periods. The purpose of this paper is to study the ranges of some selected important species, and to detect how far these species can be used as a tool for Miocene/Pliocene Boundary in the North East Java Basin. No radiometric age dating has been done in the area.

Sulfide-associated hydrothermal dolomite and calcite reveal a shallow burial depth for Alpine-type Zn-(Pb) deposits

Abstract Difficulties in dating Mississippi Valley–type (MVT) mineral deposits and the often closely associated dolomitization have led to controversy regarding their origin. We report the first radiometric ages for the Gorno mining district in northern Italy, an example of the Alpine subclass of MVT deposits. U-Pb ages of hydrothermal carbonates pre- and postdating the ore-forming event show that base-metal mineralization occurred shortly after the deposition of the Carnian host rocks. This implies that the Gorno ore deposits formed at shallow burial depth prior to the Early Jurassic western Tethys rifting phase. Contemporaneous Triassic magmatism and extensional tectonics likely contributed to the high geothermal heat fluxes required to drive the mineralizing system. Our study reinforces the need for reliable geochronological data for metallogenic models and warns against a general application of classic North American MVT models to similar deposits worldwide.

Recognition of Detrital Carbonaceous Material in the Ryoke Metamorphic Belt by Using Raman Thermometry: Implications for Thermal Structure and Detrital Origin

Abstract Raman spectroscopy of carbonaceous material (RSCM) thermometry is an effective method to determine the thermal structure of metasedimentary domains with high spatial resolution. The mean value of numerous measurements in a single sample is used for temperature estimation. A potential complicating factor is the presence of detrital carbonaceous material (CM) that can introduce bias in the results. Detrital CM generally shows a high degree of crystallinity making it difficult to discriminate detrital and metamorphic CM in high-grade metamorphic domains. Application of the RSCM thermometry to 100 metamudstone samples from the Hongusan area of the Ryoke metamorphic belt reveals multimodal degrees of crystallinity of CM in many samples indicating the common presence of detrital CM grains. Application of a new procedure to filter out the effects of detrital CM reveals a thermal structure that can be explained as the result of the overprinting of two distinct thermal regimes related to regional and subsequent contact metamorphism. The ability to recognize the presence of detrital CM and measure its crystallinity can be used to infer the metamorphic grade present in the hinterland from which the detrital material is derived. A combination of this information on metamorphic temperature with geochemical and radiometric age data reported in the literature indicates that the detrital CM was derived from basement rocks of the Korean Peninsula.

Astronomically forced cycles in Middle Permian fluvial sediments from Karoo Basin (South Africa)

We report evidence for Milankovitch cycles discovered in Middle Permian strata of the fluvial Abrahamskraal formation, lower Beaufort Group, in Karoo Basin of the Northern Cape Province, South Africa. Statistical analyses of ranked lithologies and of major element oxides have been used to obtain clusters of elements that capture lithological variations and reflect changes of the sedimentary environment through time. Spectral analysis of these elemental statistical groups reveal significant meter-scale sedimentary cycles of 67 m, 17.5 m, 5.9 m and 3.5–2.8 m, which can be interpreted as the sedimentary expression of astronomical forcing, based on the available estimate of sedimentation rate. The identified periods of short-eccentricity, precession and obliquity show a good match with those predicted for Middle Permian times, providing a data-based validation of the astronomical theory. Cycle counting integrated with available UPb dating, provides a cyclochronological calibration for Wordian normal magnetozones and, combined with radiometric ages, indicates an age of 266.5 ± 0.26 Ma for the end of Kiaman superchron. Recognition of the orbitally driven sedimentation in Gondwana supercontinent suggest a global extension of astronomical influence of Permian climate and confirms empirical knowledge of Earth's astronomical parameters before 260 million years ago. The new data demonstrate a rare case of astronomically paced cyclicity in fluvial deposits and a unique cyclostratigraphic record of the Middle Permian Gondwana supercontinent whose sedimentation reflects orbitally-paced precipitation changes.

Late Cenozoic channel migration of the proto-Yangtze River in the delta region: Insights from cosmogenic nuclide burial dating of onshore boreholes

A thorough magnetostratigraphic chronology of the Yangtze River Delta is problematic due to coarse-grained (i.e., poorly magnetized) deposits and sedimentary discontinuity. This study presents cosmogenic nuclide data which facilitate 26Al/10Be burial dating to determine the absolute age of horizons from four previously reported long onshore boreholes in the Yangtze Delta region that record sediment provenance change to the Yangtze River. Dates obtained close to the provenance change horizons in boreholes RGK15 and ZKJ39 from the northern delta were 4.99 (+1.26/−0.81) Ma and 4.74 (+0.94/−0.70) Ma, the oldest radiometric ages of borehole sediments yet to date. The provenance change horizon in boreholes LQ19 and LQ11 in the southern delta were dated to 0.94 (+0.17/−0.16) Ma and 0.57 ± 0.17 Ma, respectively. These new ages combined with previous lithofacies and provenance studies imply that the proto-Yangtze River first entered the northern delta prior to ~5 Ma, then migrated gradually southward to the present estuary location after 0.94 Ma and moved even further south at 0.57 Ma. The southward migration history of the proto-Yangtze River channel sheds lights not only on the tectonic subsidence widely occurring on the east China coast during the Quaternary, but also on the somewhat controversial connection history of the Yangtze River to the East China Sea.

Carbonatite, aillikite and olivine melilitite from Zandkopsdrift, Namaqualand, South Africa: Constraints on the origin of an unusual lamprophyre-dominated carbonatite complex and the nature of its mantle source

We present a chemical and stable and radiogenic isotope data set for carbonatite, aillikite (ultramafic lamprophyre) and olivine melilitite from the Zandkopsdrift carbonatite complex in Namaqualand, South Africa. This carbonatite complex is associated with the ≈500-km-long, NE-SW oriented, age-progressive (40–80 Ma) Namaqualand-Bushmanland-Warmbad ultramafic alkaline magmatic lineation that consists mainly of olivine melilitite and kimberlite. Zandkopsdrift has a radiometric age of 55 ± 4 Ma and is unusual among southern African carbonatites because the dominant rock type exposed is aillikite (a variety of carbonated ultramafic lamprophyre), with carbonatite occurring mainly as a network of subvertical dykes in the southwest of the complex and olivine melilitite occurring as meter-sized blocks of float derived from the subsurface. Calcite and ferroan dolomite carbonatites are present but Mg-rich dolomitic carbonatite varieties are lacking. All rock types appear to have experienced at least moderate differentiation and all show elevated δ18OSMOW values for carbonate minerals, whereas phlogopite and clinopyroxene from these rock types fall near mantle values (with ranges of +13.3 to +21.8‰ and + 4.9 to +5.4‰ for carbonates and silicates, respectively), indicative of interaction with hydrous fluids at low-temperature. Sr-Nd-Pb isotope compositions indicate moderate to strong HIMU signatures (e.g., 206Pb/204Pbi = 19.4 to 21.0) with limited SrNd isotope variation and subvertical NdHf isotope arrays extending well below the ocean island basalt field and terrestrial array. Uncertainty regarding the differentiation history of the three rock types complicates estimation of primary magma compositions but strong chemical similarities between the carbonatites and aillikites, plus their close physical intermingling, strongly suggests that they are related by fractional crystallization to a common parental magma, whereas their relationship to the melilitites is less straightforward. The aillikites and melilitites have compositions of the REE and other elements similar to near-primary versions of these rock types from other alkaline igneous provinces globally, which have been explained as the products of low degrees of partial melting at 4–7 GPa (for aillikites) and slightly higher degrees of melting at 3–3.5 GPa (for melilitites). Lower pressures of generation and distinct trace element characteristics of the Zandkopsdrift melilitites could be explained by these originating as melts of a garnet wehrlite halo around the magmatic conduit formed by metasomatic reaction between carbonated melts and lithospheric peridotite. Most Zandkopsdrift carbonatites have high Lu/Hf ratios which, combined with their relatively low 176Hf/177Hf values, suggests that their mantle sources could not have formed more than 30–50 Myr prior to emplacement of the complex. This implies that a mantle plume had a role in their formation, either as a direct source of melts or as a source of heat with which to melt pre-existing metasomatic heterogeneities.

Re-Os geochronology for the Cambrian SPICE event: Insights into euxinia and enhanced continental weathering from radiogenic isotopes

AbstractThe late Cambrian Steptoean positive carbon isotope excursion (SPICE) represents a major perturbation to the global carbon cycle and was associated with trilobite extinctions and expansion of anoxic and/or euxinic water masses during episodes of eustatic sea-level change. We present a new Re-Os age together with Os and Nd isotope stratigraphy and major- and trace-element data from the Alum Shale Formation (Scania, Sweden). The Re-Os age of 494.6 ± 2.9 Ma is from the interval of peak δ13Corg values, providing the first radiometric age constraint for this Cambrian carbon isotope excursion, interpreted as a possible pre-Mesozoic ocean anoxia event, and the timing of biomere-level extinctions. The Os isotope chemostratigraphic profile can be explained by an increase in terrigenous weathering prior to the SPICE, potentially driven by sea-level fall, and in agreement with enhanced nutrient supply, primary productivity, and organic matter burial as the driver of the SPICE event. Post-SPICE, the Os isotopes become increasingly unradiogenic; however, invariant εNd(t) values argue against a change in provenance and instead support a decrease in the continental weathering flux, possibly related to eustatic sea-level rise.

Author Correction: Updated lunar cratering chronology model with the radiometric age of Chang’e-5 samples

Author Correction: Updated lunar cratering chronology model with the radiometric age of Chang’e-5 samples

Calibrating the Russøya excursion in Svalbard, Norway, and implications for Neoproterozoic chronology

Abstract The Tonian–Ediacaran Hecla Hoek succession of Svalbard, Norway, represents one of the most complete and well-preserved Neoproterozoic sedimentary successions worldwide. With diverse fossil assemblages, an extensive carbonate δ13C record, and sedimentary evidence for two distinct Cryogenian glaciations, this succession will continue to yield insights into the Neoproterozoic Earth system; however, at present there are no direct radiometric age constraints for these strata. We present two new Re-Os ages and initial Os isotope data that constrain the timing of Neoproterozoic glaciation in Svalbard, providing further support for two globally synchronous Cryogenian glaciations and insight into pre- and post-snowball global weathering conditions. An age from the Russøya Member (Elbobreen Formation) facilitates correlation of the negative carbon isotope excursion recorded therein with the pre-glacial “Islay” excursion of the Callison Lake Formation of northwestern Canada and the Didikama and Matheos Formations of Ethiopia. We propose that this globally synchronous ca. 735 Ma carbon isotope excursion be referred to as the Russøya excursion with northeastern Svalbard as the type locality. This new age provides an opportunity to construct a time-calibrated geological framework in Svalbard to assess connections between biogeochemical cycling, evolutionary innovations within the eukaryotes, and the most extreme climatic changes in Earth history.