Metamorphic turnover at 2 Ga related to two-stage assembly of Columbia.

Understanding the stabilization of cratons and how this is related to the onset of plate-tectonics is among the most important questions in geoscience. The assembly of Earth's first supercontinent Columbia represents the first lines of evidence for a global subduction network, when the oldest, deep subduction-related rocks have been reported. We combine the low-, intermediate- and high-T/P global metamorphic record with the two-stage assembly of the Nuna-Columbia supercontinent to address the significance of the oldest "cold" rocks (low-T/P) and the related emergence of bimodal metamorphic belts. For this purpose, we analyse two examples from Laurentia (including Greenland) and Australia between 2.0-1.8 Ga and 1.8-1.6 Ga. Two main observations are: (i) a first-stage (2.0-1.8 Ga) amalgamation of the megacontinent Nuna (precursor to Columbia) is characterized by bimodal metamorphism along major mobile belts suturing the megacontinent's center. In contrast, a second-stage (1.8-1.6 Ga) is dominated by the formation of soft collisional orogens during the final Columbia supercontinent assembly, recording intermediate- to high-T/P metamorphism; (ii) the metamorphic signature of the two assembly stages, featuring low- and intermediate-T/P rocks during Nuna assembly followed by their near absence during Columbia amalgamation, contrasts with the thermobaric ratios recorded by the Phanerozoic Gondwana-Pangea assembly, where intermediate and low-T/P rocks dominated the final stage of Pangea amalgamation. This discrepancy may signify substantial changes in intraplate metamorphism and minor rearrangements during Columbia assembly compared to major continent-continent collisions, such as the Appalachian-Variscan Orogen as well as production and fast exhumation of high- to ultra-high-pressure rocks during the assembly of the supercontinent Pangea. Furthermore, the variation of thermobaric ratios aligns with the concept of a two-stage mega-supercontinent formation, emphasizing differences between the potentially oldest and youngest supercontinent cycles.

Age constraints for the Trachilos footprints from Crete.

We present an updated time frame for the 30 m thick late Miocene sedimentary Trachilos section from the island of Crete that contains the potentially oldest hominin footprints. The section is characterized by normal magnetic polarity. New and published foraminifera biostratigraphy results suggest an age of the section within the Mediterranean biozone MMi13d, younger than ~ 6.4 Ma. Calcareous nannoplankton data from sediments exposed near Trachilos and belonging to the same sub-basin indicate deposition during calcareous nannofossil biozone CN9bB, between 6.023 and 6.727 Ma. By integrating the magneto- and biostratigraphic data we correlate the Trachilos section with normal polarity Chron C3An.1n, between 6.272 and 6.023 Ma. Using cyclostratigraphic data based on magnetic susceptibility, we constrain the Trachilos footprints age at ~ 6.05 Ma, roughly 0.35 Ma older than previously thought. Some uncertainty remains related to an inaccessible interval of ~ 8 m section and the possibility that the normal polarity might represent the slightly older Chron C3An.2n. Sediment accumulation rate and biostratigraphic arguments, however, stand against these points and favor a deposition during Chron C3An.1n.

Orbital forcing of ice sheets during snowball Earth.

The snowball Earth hypothesis-that a runaway ice-albedo feedback can cause global glaciation-seeks to explain low-latitude glacial deposits, as well as geological anomalies including the re-emergence of banded iron formation and "cap" carbonates. One of the most significant challenges to snowball Earth has been sedimentological cyclicity that has been taken to imply more climate dynamics than expected when the ocean is completely covered in ice. However, recent climate models suggest that as atmospheric CO2 accumulates, the snowball climate system becomes sensitive to orbital forcing. Here we show the presence of nearly all Milankovitch (orbital) cycles preserved in stratified banded iron formation deposited during the Sturtian snowball Earth. These results provide evidence for orbitally forced cyclicity of global ice sheets that resulted in periodic oxidation of ferrous iron. Orbital glacial advance and retreat cycles provide a simple mechanism to reconcile both the sedimentary dynamics and the enigmatic survival of multicellular life during snowball Earth.

Seismological evidence for the earliest global subduction network at 2 Ga ago.

The earliest evidence for subduction, which could have been localized, does not signify when plate tectonics became a global phenomenon. To test the antiquity of global subduction, we investigated Paleoproterozoic time, for which seismic evidence is available from multiple continents. We used a new high-density seismic array in North China to image the crustal structure that exhibits a dipping Moho bearing close resemblance to that of the modern Himalaya. The relict collisional zone is Paleoproterozoic in age and implies subduction operating at least as early as ~2 billion years (Ga) ago. Seismic evidence of subduction from six continents at this age is interpreted as the oldest evidence of global plate tectonics. The sutures identified can be linked in a plate network that resulted in the assembly of Nuna, likely Earth's first supercontinent. Global subduction by ~2 Ga ago can explain why secular planetary cooling was not appreciable until Proterozoic time.

A new Miocene ape and locomotion in the ancestor of great apes and humans.

Many ideas have been proposed to explain the origin of bipedalism in hominins and suspension in great apes (hominids); however, fossil evidence has been lacking. It has been suggested that bipedalism in hominins evolved from an ancestor that was a palmigrade quadruped (which would have moved similarly to living monkeys), or from a more suspensory quadruped (most similar to extant chimpanzees)1. Here we describe the fossil ape Danuvius guggenmosi (from the Allgäu region of Bavaria) for which complete limb bones are preserved, which provides evidence of a newly identified form of positional behaviour-extended limb clambering. The 11.62-million-year-old Danuvius is a great ape that is dentally most similar to Dryopithecus and other European late Miocene apes. With a broad thorax, long lumbar spine and extended hips and knees, as in bipeds, and elongated and fully extended forelimbs, as in all apes (hominoids), Danuvius combines the adaptations of bipeds and suspensory apes, and provides a model for the common ancestor of great apes and humans.

First Precambrian palaeomagnetic data from the Mawson Craton (East Antarctica) and tectonic implications.

A pilot palaeomagnetic study was conducted on the recently dated with in situ SHRIMP U-Pb method at 1134 ± 9 Ma (U-Pb, zircon and baddeleyite) Bunger Hills dykes of the Mawson Craton (East Antarctica). Of the six dykes sampled, three revealed meaningful results providing the first well-dated Mesoproterozoic palaeopole at 40.5°S, 150.1°E (A95 = 20°) for the Mawson Craton. Discordance between this new pole and two roughly coeval poles from Dronning Maud Land and Coats Land (East Antarctica) demonstrates that these two terranes were not rigidly connected to the Mawson Craton ca. 1134 Ma. Comparison between the new pole and that of the broadly coeval Lakeview dolerite from the North Australian Craton supports the putative ~40° late Neoproterozoic relative rotation between the North Australian Craton and the combined South and West Australian cratons. A mean ca. 1134 Ma pole for the Proto-Australia Craton is calculated by combining our new pole and that of the Lakeview dolerite after restoring the 40° intracontinental rotation. A comparison of this mean pole with the roughly coeval Abitibi dykes pole from Laurentia confirms that the SWEAT reconstruction of Australia and Laurentia was not viable for ca. 1134 Ma.

Messinian age and savannah environment of the possible hominin Graecopithecus from Europe.

Dating fossil hominids and reconstructing their environments is critically important for understanding human evolution. Here we date the potentially oldest hominin, Graecopithecus freybergi from Europe and constrain the environmental conditions under which it thrived. For the Graecopithecus-bearing Pikermi Formation of Attica/Greece, a saline aeolian dust deposit of North African (Sahara) provenance, we obtain an age of 7.37-7.11 Ma, which is coeval with a dramatic cooling in the Mediterranean region at the Tortonian-Messinian transition. Palaeobotanic proxies demonstrate C4-grass dominated wooded grassland-to-woodland habitats of a savannah biome for the Pikermi Formation. Faunal turnover at the Tortonian-Messinian transition led to the spread of new mammalian taxa along with Graecopithecus into Europe. The type mandible of G. freybergi from Pyrgos (7.175 Ma) and the single tooth (7.24 Ma) from Azmaka (Bulgaria) represent the first hominids of Messinian age from continental Europe. Our results suggest that major splits in the hominid family occurred outside Africa.