(U,Th)-He dating of Pleistocene carbonates: Analytical methods and age calculations.

The uranium-thorium-helium ((U,Th)-He) dating method applied to calcium carbonate speleothems holds much promise for constraining the timeline of hominin evolution, as well as for palaeoclimatology research beyond the range of U/Th disequilibrium dating. Technical problems are posed by often low U concentrations and the requirement that samples need to be individually removed from the ultrahigh vacuum (UHV) system after helium is extracted from them, to be then analyzed for U, Th content and U series disequilibrium. We describe a low-cost furnace with this capability, constructed from standard UHV components, as well as the methods for subsequent U/Th disequilibrium analysis using multicollector ICP mass spectrometry. We present analytical and numerical solutions to determine (U,Th)-He ages for four conditions that have been encountered: (1) for an age range in which the residual activity ratio (234 U/238 U) can still be resolved but not that of (230 Th/234 U) (up to about 3000 ka), (2) for an age range where neither can be resolved (unlimited), (3) for ages up to 1000 ka where both activity ratios may be resolvable, and (4) for cases where (234 U/238 U) and (230 Th/234 U) indicate ages < 200 ka but (U,Th)-He systematics point to much older ages.•Helium extraction is carried out using an in-house built vacuum furnace that allows for large sample sizes (20 to 100 mg) of powdered carbonate material wrapped in Cu foil.•U and Th are separated together using Eichrom UTEVAⓇ resin and their isotope abundances are measured together in a single 2-cycle dynamic run using the multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS).•(U,Th)-He ages are calculated using four methods that take into account the U/Th disequilibria.

The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa.

New ages for flowstone, sediments and fossil bones from the Dinaledi Chamber are presented. We combined optically stimulated luminescence dating of sediments with U-Th and palaeomagnetic analyses of flowstones to establish that all sediments containing Homo naledi fossils can be allocated to a single stratigraphic entity (sub-unit 3b), interpreted to be deposited between 236 ka and 414 ka. This result has been confirmed independently by dating three H. naledi teeth with combined U-series and electron spin resonance (US-ESR) dating. Two dating scenarios for the fossils were tested by varying the assumed levels of 222Rn loss in the encasing sediments: a maximum age scenario provides an average age for the two least altered fossil teeth of 253 +82/-70 ka, whilst a minimum age scenario yields an average age of 200 +70/-61 ka. We consider the maximum age scenario to more closely reflect conditions in the cave, and therefore, the true age of the fossils. By combining the US-ESR maximum age estimate obtained from the teeth, with the U-Th age for the oldest flowstone overlying Homo naledi fossils, we have constrained the depositional age of Homo naledi to a period between 236 ka and 335 ka. These age results demonstrate that a morphologically primitive hominin, Homo naledi, survived into the later parts of the Pleistocene in Africa, and indicate a much younger age for the Homo naledi fossils than have previously been hypothesized based on their morphology.

Geological and taphonomic context for the new hominin species Homo naledi from the Dinaledi Chamber, South Africa.

We describe the physical context of the Dinaledi Chamber within the Rising Star cave, South Africa, which contains the fossils of Homo naledi. Approximately 1550 specimens of hominin remains have been recovered from at least 15 individuals, representing a small portion of the total fossil content. Macro-vertebrate fossils are exclusively H. naledi, and occur within clay-rich sediments derived from in situ weathering, and exogenous clay and silt, which entered the chamber through fractures that prevented passage of coarser-grained material. The chamber was always in the dark zone, and not accessible to non-hominins. Bone taphonomy indicates that hominin individuals reached the chamber complete, with disarticulation occurring during/after deposition. Hominins accumulated over time as older laminated mudstone units and sediment along the cave floor were eroded. Preliminary evidence is consistent with deliberate body disposal in a single location, by a hominin species other than Homo sapiens, at an as-yet unknown date.

Re-appraisal of the stratigraphy and determination of new U-Pb dates for the Sterkfontein hominin site, South Africa.

Sterkfontein Caves is the single richest early hominin site in the world with deposits yielding one or more species of Australopithecus and possible early Homo, as well as an extensive faunal collection. The inability to date the southern African cave sites accurately or precisely has hindered attempts to integrate the hominin fossil evidence into pan-African scenarios about human evolutionary history, and especially hominin biogeography. We have used U-Pb and U-Th techniques to date sheets of calcium carbonate flowstone inter-bedded between the fossiliferous sediments. For the first time, absolute age ranges can be assigned to the fossil-bearing deposits: Member 2 is between 2.8 +/- 0.28 and 2.6 +/- 0.30 Ma and Member 4 between 2.65 +/- 0.30 and 2.01 +/- 0.05 Ma. The age of 2.01 +/- 0.05 Ma for the top of Member 4 constrains the last appearance of Australopithecus africanus to 2 Ma. In the Silberberg Grotto we have reproduced the U-Pb age of approximately 2.2 Ma of for the flowstones associated with StW573. We believe that these deposits, including the fossil and the flowstones, accumulated rapidly around 2.2 Ma. The stratigraphy of the site is complex as sediments are exposed both in the underground chambers and at surface. We present a new interpretation of the stratigraphy based on surface mapping, boreholes logs and U-Pb ages. Every effort was made to retain the Member system, however, only Members 2 and 4 are recognized in the boreholes. We propose that the deposits formally known as Member 3 are in fact the distal equivalents of Member 4. The sediments of Members 2 and 4 consisted of cone-like deposits and probably never filled up the cave. The U-Th ages show that there are substantial deposits younger than 400 ka in the underground cave, underlying the older deposits, highlighting again that these cave fills are not simple layer-cakes.

Geological setting and age of Australopithecus sediba from southern Africa.

We describe the geological, geochronological, geomorphological, and faunal context of the Malapa site and the fossils of Australopithecus sediba. The hominins occur with a macrofauna assemblage that existed in Africa between 2.36 and 1.50 million years ago (Ma). The fossils are encased in water-laid, clastic sediments that were deposited along the lower parts of what is now a deeply eroded cave system, immediately above a flowstone layer with a U-Pb date of 2.026 +/- 0.021 Ma. The flowstone has a reversed paleomagnetic signature and the overlying hominin-bearing sediments are of normal polarity, indicating deposition during the 1.95- to 1.78-Ma Olduvai Subchron. The two hominin specimens were buried together in a single debris flow that lithified soon after deposition in a phreatic environment inaccessible to scavengers.

New Australopithecus robustus fossils and associated U-Pb dates from Cooper's Cave (Gauteng, South Africa).

Australopithecus robustus is one of the best represented hominin taxa in Africa, with hundreds of specimens recovered from six fossil localities in the Bloubank Valley area of Gauteng Province, South Africa. However, precise geochronological ages are presently lacking for these fossil cave infills. In this paper, we provide a detailed geological background to a series of hominin fossils retrieved from the newly investigated deposit of Cooper's D (located partway between Sterkfontein and Kromdraai in the Bloubank Valley), including uranium-lead (U-Pb) ages for speleothem material associated with A. robustus. U-Pb dating of a basal speleothem underlying the entire deposit results in a maximum age of 1.526 (+/-0.088) Ma for Cooper's D. A second U-Pb date of ca. 1.4 Ma is produced from a flowstone layer above this basal speleothem; since this upper flowstone is not a capping flowstone, and fossiliferous sediments are preserved above this layer, some of the hominins might be slightly younger than the calculated age. As a result, we can broadly constrain the age of the hominins from Cooper's D to between 1.5 and approximately 1.4 Ma. Extinct fauna recorded in this comparatively young deposit raise the possibility that the Bloubank Valley region of South Africa represented a more stable environmental refugium for taxa relative to tectonically more active East Africa. The sediments of the deposit likely infilled rapidly during periods when arid conditions prevailed in the paleoenvironment, although it is unclear whether sediment deposition and bone deposition were necessarily contemporaneous occurrences. We reconstruct the paleoenvironment of Cooper's D as predominantly grassland, with nearby woodlands and a permanent water source. The hominin teeth recovered from Cooper's D are all from juveniles and can be confidently assigned to A. robustus. In addition, two juvenile mandibular fragments and an adult thoracic vertebra are tentatively attributed to A. robustus.

Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary.

Animal-like multicellular fossils appeared towards the end of the Precambrian, followed by a rapid increase in the abundance and diversity of fossils during the Early Cambrian period, an event also known as the 'Cambrian explosion'. Changes in the environmental conditions at the Precambrian/Cambrian transition (about 542 Myr ago) have been suggested as a possible explanation for this event, but are still a matter of debate. Here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 Myr. We find a transient molybdenum isotope signal immediately after the Precambrian/Cambrian transition. Using a box model of the oceanic molybdenum cycle, we find that intense upwelling of hydrogen sulphide-rich deep ocean water best explains the observed Early Cambrian molybdenum isotope signal. Our findings suggest that the Early Cambrian animal radiation may have been triggered by a major change in ocean circulation, terminating a long period during which the Proterozoic ocean was stratified, with sulphidic deep water.

Geochemistry: how well can Pb isotopes date core formation?

Timescale and the physics of planetary core formation are essential constraints for models of Earth's accretion and early differentiation. Wood and Halliday use the apparent mismatch in core-formation dates determined from tungsten (W) and lead (Pb) chrono-meters to argue for a two-stage core formation, involving an early phase of metal segregation followed by a protracted episode of sulphide melt addition. However, we show here that crust-;mantle Pb isotope systematics do not require diachronous core formation. Our observations indicate that very early (< or = 35 Myr) core formation and planet accretion remain the most plausible scenario.

Comparative study of the temporal evolution of atmospheric lead deposition in Scotland and eastern Canada using blanket peat bogs.

The temporal evolution of atmospheric lead deposition and its possible sources were assessed in eastern Canada and in western Scotland, using blanket peat bogs as geochemical archives. Short cores were taken from two remote sites located close to the sea. Significant lead enrichments in the upper layers at both sites reflect the increasing emission of lead into the atmosphere due to anthropogenic activities during the last century. At the Scottish site, a region under aeolian influence from Europe, anthropogenic derived lead could be recognized by the distinctive unradiogenic composition (206Pb/207Pb ratios down to approximately 1.115), being clearly different from the pre-industrial values (206Pb/207Pb approximately 1.166). In contrast, the lead pollution in eastern Canada (influenced by North American sources) is identified by a more radiogenic lead isotope composition (206Pb/207Pb ratios up to approximately 1.199) compared to preindustrial values (206Pb/207Pb approximately 1.161). Emission inventories and isotope characteristics suggest that industrial (coal burning, mining) and traffic (leaded gasoline) outputs are the most likely sources during the first and the second half of the 20th century, respectively, in both, western Scotland and eastern Canada alike. The Scottish record is in line with previous studies of past atmospheric lead deposition. However, the Canadian deposit suggests that the wind derived, pre-industrial lead, is less radiogenic as previously implied using sediment archives. These results are thus the first to report pre-industrial lead isotope ratios and concentrations of atmospheric derived aerosols in North America.

Combined chemical separation of Lu, Hf, Sm, Nd, and Rees from a single rock digest: precise and accurate isotope determinations of Lu-Hf and Sm-Nd using multicollector-ICPMS.

A combined procedure for separating Lu, Hf, Sm, Nd, and rare earth elements (REEs) from a single sample digest is presented. The procedure consists of the following five steps: (1) sample dissolution via sodium peroxide sintering; (2) separation of the high field strength elements from the REEs and other matrix elements by a HF-free anion-exchange column procedure; (3) purification of Hf on a cation-exchange resin; (4) separation of REEs from other matrix elements by cation exchange; (5) Lu, Sm, and Nd separation from the other REEs by reversed-phase ion chromatography. Analytical reproducibilities of Sm-Nd and Lu-Hf isotope systematics are demonstrated for standard solutions and international rock reference materials. Results show overall good reproducibilities for Sm-Nd systematics independent of the rock type analyzed. For the Lu-Hf systematics, the reproducibility of the parent/daughter ratio is much better for JB-1 (basalt) than for two analyzed felsic crustal rocks (DR-N and an Archaean granitoid). It is demonstrated that this poorer reproducibility of the Lu/Hf ratio is truly caused by sample heterogeneity; thus, results are geologically reasonable.