Darren F. Mark

Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary

Impact Dating The large mass extinction of terrestrial and marine life—most notably, non-avian dinosaurs—occurred around 66 million years ago, at the boundary between the Cretaceous and Paleogene periods. But attributing the cause to a large asteroid impact depends on precisely dating material from the impact with indicators of ecological stress and environmental change in the rock record. Renne et al. (p. 684; see the Perspective by Pälike) acquired high-precision radiometric dates of stratigraphic layers surrounding the boundary, demonstrating that the impact occurred within 33,000 years of the mass extinction. The data also constrain the length of time in which the atmospheric carbon cycle was severely disrupted to less than 5000 years. Because the climate in the late Cretaceous was becoming unstable, the large-impact event appears to have triggered a state-shift in an already stressed global ecosystem. Radiometric dating establishes the mass extinction that killed the dinosaurs as synchronous with a large asteroid impact. [Also see Perspective by Pälike] Mass extinctions manifest in Earths geologic record were turning points in biotic evolution. We present 40Ar/39Ar data that establish synchrony between the Cretaceous-Paleogene boundary and associated mass extinctions with the Chicxulub bolide impact to within 32,000 years. Perturbation of the atmospheric carbon cycle at the boundary likely lasted less than 5000 years, exhibiting a recovery time scale two to three orders of magnitude shorter than that of the major ocean basins. Low-diversity mammalian fauna in the western Williston Basin persisted for as little as 20,000 years after the impact. The Chicxulub impact likely triggered a state shift of ecosystems already under near-critical stress.

Early Levallois technology and the Lower to Middle Paleolithic transition in the Southern Caucasus

An early assemblage of obsidian artifacts Levallois technology is the name for the stone knapping technique used to create tools thousands of years ago. The technique appeared in the archeological record across Eurasia 200 to 300 thousand years ago (ka) and appeared earlier in Africa. Adler et al. challenge the hypothesis that the techniques appearance in Eurasia was the result of the expansion of hominins from Africa. Levallois obsidian artifacts in the southern Caucasus, dated at 335 to 325 ka, are the oldest in Eurasia. This suggests that Levallois technology may have evolved independently in different hominin populations. Stone technology cannot thus be used as a reliable indicator of Paleolithic human population change and expansion. Science, this issue p. 1609 An assemblage of obsidian artifacts suggests independent origins of stone knapping in different hominin populations. The Lower to Middle Paleolithic transition (~400,000 to 200,000 years ago) is marked by technical, behavioral, and anatomical changes among hominin populations throughout Africa and Eurasia. The replacement of bifacial stone tools, such as handaxes, by tools made on flakes detached from Levallois cores documents the most important conceptual shift in stone tool production strategies since the advent of bifacial technology more than one million years earlier and has been argued to result from the expansion of archaic Homo sapiens out of Africa. Our data from Nor Geghi 1, Armenia, record the earliest synchronic use of bifacial and Levallois technology outside Africa and are consistent with the hypothesis that this transition occurred independently within geographically dispersed, technologically precocious hominin populations with a shared technological ancestry.

The ARGUS multicollector noble gas mass spectrometer: Performance for 40Ar/39Ar geochronology

We describe a new high-sensitivity multicollector noble gas mass spectrometer (ARGUS) that has been specifically designed for simultaneous collection of all Ar isotopes and hence is ideally suited for 40Ar/39Ar geochronology. The instrument uses a detector housing that holds five Faraday collectors. One collector is equipped with a 1011 ohm resistor (for 40Ar), and the remaining four are fitted with 1012 ohm resistors (for 36Ar to 39Ar). ARGUS has a mass resolution of 225–250 and a measured sensitivity of 7 × 10−14 mol/V at 200 μA trap current. During the course of a 1 month run cycle 145 air calibrations yielded a weighted average 40Ar/36Ar of 300.67 ± 0.07 (2σ, eight data points rejected, and mean square weighted deviation = 1.18). The ages of three mineral standards (Taylor Creek Rhyolite sanidine, Heidelberg biotite, and Alder Creek sanidine) are within error of the accepted ages, and uncertainties for two are improvements on previously published data. ARGUS decreases analytical time while allowing more measurements to be made at higher precision compared to standard peak-jumping single-collector mass spectrometers.

First Partial Skeleton of a 1.34-Million-Year-Old Paranthropus boisei from Bed II, Olduvai Gorge, Tanzania

Recent excavations in Level 4 at BK (Bed II, Olduvai Gorge, Tanzania) have yielded nine hominin teeth, a distal humerus fragment, a proximal radius with much of its shaft, a femur shaft, and a tibia shaft fragment (cataloged collectively as OH 80). Those elements identified more specifically than to simply Hominidae gen. et sp. indet are attributed to Paranthropus boisei. Before this study, incontrovertible P. boisei partial skeletons, for which postcranial remains occurred in association with taxonomically diagnostic craniodental remains, were unknown. Thus, OH 80 stands as the first unambiguous, dentally associated Paranthropus partial skeleton from East Africa. The morphology and size of its constituent parts suggest that the fossils derived from an extremely robust individual who, at 1.338±0.024 Ma (1 sigma), represents one of the most recent occurrences of Paranthropus before its extinction in East Africa.

Early oxygenation of the terrestrial environment during the Mesoproterozoic

Geochemical data from ancient sedimentary successions provide evidence for the progressive evolution of Earth’s atmosphere and oceans. Key stages in increasing oxygenation are postulated for the Palaeoproterozoic era (∼2.3 billion years ago, Gyr ago) and the late Proterozoic eon (about 0.8 Gyr ago), with the latter implicated in the subsequent metazoan evolutionary expansion. In support of this rise in oxygen concentrations, a large database shows a marked change in the bacterially mediated fractionation of seawater sulphate to sulphide of Δ34S < 25‰ before 1 Gyr to ≥50‰ after 0.64 Gyr. This change in Δ34S has been interpreted to represent the evolution from single-step bacterial sulphate reduction to a combination of bacterial sulphate reduction and sulphide oxidation, largely bacterially mediated. This evolution is seen as marking the rise in atmospheric oxygen concentrations and the evolution of non-photosynthetic sulphide-oxidizing bacteria. Here we report Δ34S values exceeding 50‰ from a terrestrial Mesoproterozoic (1.18 Gyr old) succession in Scotland, a time period that is at present poorly characterized. This level of fractionation implies disproportionation in the sulphur cycle, probably involving sulphide-oxidizing bacteria, that is not evident from Δ34S data in the marine record. Disproportionation in both red beds and lacustrine black shales at our study site suggests that the Mesoproterozoic terrestrial environment was sufficiently oxygenated to support a biota that was adapted to an oxygen-rich atmosphere, but had also penetrated into subsurface sediment.

The Origin of The Acheulean: The 1.7 Million-Year-Old Site of FLK West, Olduvai Gorge (Tanzania).

The appearance of the Acheulean is one of the hallmarks of human evolution. It represents the emergence of a complex behavior, expressed in the recurrent manufacture of large-sized tools, with standardized forms, implying more advance forethought and planning by hominins than those required by the precedent Oldowan technology. The earliest known evidence of this technology dates back to c. 1.7 Ma. and is limited to two sites (Kokiselei [Kenya] and Konso [Ethiopia]), both of which lack functionally-associated fauna. The functionality of these earliest Acheulean assemblages remains unknown. Here we present the discovery of another early Acheulean site also dating to c. 1.7 Ma from Olduvai Gorge. This site provides evidence of the earliest steps in developing the Acheulean technology and is the oldest Acheulean site in which stone tools occur spatially and functionally associated with the exploitation of fauna. Simple and elaborate large-cutting tools (LCT) and bifacial handaxes co-exist at FLK West, showing that complex cognition was present from the earliest stages of the Acheulean. Here we provide a detailed technological study and evidence of the use of these tools on the butchery and consumption of fauna, probably by early Homo erectus sensu lato.

Evidence for methane in Martian meteorites

The putative occurrence of methane in the Martian atmosphere has had a major influence on the exploration of Mars, especially by the implication of active biology. The occurrence has not been borne out by measurements of atmosphere by the MSL rover Curiosity but, as on Earth, methane on Mars is most likely in the subsurface of the crust. Serpentinization of olivine-bearing rocks, to yield hydrogen that may further react with carbon-bearing species, has been widely invoked as a source of methane on Mars, but this possibility has not hitherto been tested. Here we show that some Martian meteorites, representing basic igneous rocks, liberate a methane-rich volatile component on crushing. The occurrence of methane in Martian rock samples adds strong weight to models whereby any life on Mars is/was likely to be resident in a subsurface habitat, where methane could be a source of energy and carbon for microbial activity.

Sequestration of Martian CO2 by mineral carbonation

Carbonation is the water-mediated replacement of silicate minerals, such as olivine, by carbonate, and is commonplace in the Earth’s crust. This reaction can remove significant quantities of CO2 from the atmosphere and store it over geological timescales. Here we present the first direct evidence for CO2 sequestration and storage on Mars by mineral carbonation. Electron beam imaging and analysis show that olivine and a plagioclase feldspar-rich mesostasis in the Lafayette meteorite have been replaced by carbonate. The susceptibility of olivine to replacement was enhanced by the presence of smectite veins along which CO2-rich fluids gained access to grain interiors. Lafayette was partially carbonated during the Amazonian, when liquid water was available intermittently and atmospheric CO2 concentrations were close to their present-day values. Earlier in Mars’ history, when the planet had a much thicker atmosphere and an active hydrosphere, carbonation is likely to have been an effective mechanism for sequestration of CO2.

Discussion on ‘A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications’Journal, Vol. 168, 863–872

We welcome the contribution of Parry et al . (2011) concerning the age of the globally important Rhynie hot-spring system, Aberdeenshire, Scotland. However, although we accept that they have provided a valuable and robust U–Pb age constraint for the Milton of Noth Andesite and the Rhynie outlier stratigraphic succession, we do not accept that they have constrained the timing of hot-spring activity, as suggested. The geophysical trace of the unexposed Milton of Noth Andesite indicates that it lies immediately adjacent to the Rhynie Fault Zone and runs parallel to it for some 2 km. It may even lie within the Rhynie Fault Zone (see discussion by Rice & Ashcroft 2004). Its stratigraphic position is therefore uncertain. Parry et al . (2011) stated that the exact stratigraphic position is not important because their preferred stratigraphic locations lie within the Tillybrachty Sandstone and Dryden Flags Formations, both of which have consistently yielded spores belonging to the polygonalis–emsiensis Spore Assemblage Biozone. This assertion is not valid. The polygonalis–emsiensis Spore Assemblage Biozone may have lasted several million years and thus, potentially, there could be a significant hiatus between eruption of the andesite and the onset of hot-spring activity. They further assume that the andesite was linked to an andesitic magma chamber at depth, which powered the hydrothermal system. This remains speculation. Accepting the uncertainty in the stratigraphic position and by asserting a maximum thickness of sediment between the andesite and the Rhynie Cherts and a conservative deposition rate, Parry et al . (2011) attempted to temporally link the andesite to the cherts by calculating the time interval represented by the intervening sediment. The estimation of time depends on the absence of unconformities, disconformities and faulting, with such assumptions and uncertainties casting doubt on the thickness of the intervening sediments …

Geochemistry of Compositionally Distinct Late Cretaceous Back-Arc Basin Lavas: Implications for the Tectonomagmatic Evolution of the Caribbean Plate

The Cretaceous Blue Mountain Inlier of eastern Jamaica contains the Bellevue lavas, which represent a Mid- to Late Campanian back-arc basin succession of tholeiitic volcanic rocks. The lavas are composed of basic/intermediate and intermediate/acidic subgroups that can be related by intraformation fractional crystallization. Trace element and Hf radiogenic isotope data reveal that the mantle component of the Bellevue magmas is consistent with derivation from a mantle plume (oceanic plateau) source region. Modeling indicates that the magmas formed by 10%–20% partial melting of an oceanic plateau mantle source comprising spinel peridotite that had previously undergone approximately 5%–7.5% prior melt extraction in the garnet stability field. Trace element and radiogenic isotope systematics suggest that the Bellevue mantle source region was contaminated with a slab-derived component from both the altered basaltic slab and its pelagic sedimentary veneer. The data from the Bellevue lavas support the plateau reversal model of Caribbean tectonic evolution, whereby subduction on the Great Arc of the Caribbean was to the northeast until the Caribbean oceanic plateau collided with the southern portion of the Great Arc in the Santonian (85.8–83.5 Ma), resulting in subduction polarity reversal and thus southwest-dipping subduction. This polarity reversal allowed oceanic plateau source regions to be melted beneath a new back-arc basin to the southwest of the Great Arc.