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Large-scale liquid immiscibility at the top of the Bushveld Complex, South Africa
J.A. VanTongeren Yale University, Kline Geological Laboratory, New Haven, Connecticut 06520, USA and E.A. Mathez. Posted online 19 April 2012; doi: 10.1130/G32980.1.
The Bushveld Complex layered intrusion is one of only a handful of exhumed fossil mafic magma chambers on Earth, and it is vital for understanding of magma evolution and differentiation prior to eruption. Scientists have long argued over whether these magma chambers evolve to produce Fe-rich or Si-rich magmas near the end of their crystallization and solidification, and the answer has important implications for the production of the variety of magmas we see today. In this study, J.A. VanTongeren and colleagues look at the mineral apatite, which constitutes only a small fraction of the rock but concentrates a large proportion of the rare earth elements. They found that the apatite near the top of the magma chamber has two very distinct populations, one with low concentrations of rare earth elements in the lower layer and the other with high concentrations in the upper layer. These and other observations in this interval are consistent with large-scale liquid immiscibility in which the original magma splits into a dense iron (Fe)-rich magma that sinks to form the lower layer, and a silicon (Si)-rich magma that buoyantly rises to form the upper layer. These results suggest that some mafic magmas can evolve to produce both Fe-rich and Si-rich compositions.
Substantial biologically mediated phosphorus depletion from the surface of a Middle Cambrian paleosol
L.B. Horodyskyj et al., ASU Online, Arizona State University, Tempe, Arizona 85287, USA. Posted online 19 April; doi: 10.1130/G32761.1.
Recent work on a 503-million-year-old soil shows evidence of the earliest chemical footprints of mycorrhizal fungi, a symbiotic fungus that provides a majority of today's land plants with essential nutrients. Early land plants formed partnerships with fungus capable of extracting the limited nutrient phosphorus out of rock minerals. This symbiotic partnership enabled successful colonization of the terrestrial landscape. The soil investigated by L.B. Horodyskyj and colleagues shows substantial loss of the phosphorus- and calcium-rich mineral apatite from the surface of the soil. This loss could not be explained by non-biological causes alone. Mycorrhizal fungi are the suspected culprit because in modern soils, when dissolving apatite, they preferentially use phosphorus and leave calcium behind. The additional calcium in the soil can result in the formation of distinctive clay flow structures. The complete loss of apatite from the surface and the high concentration of clay flow structures in this zone of loss point to mycorrhizal fungi as the likely culprit. If true, this pushes back the earliest evidence for these organisms by 43 million years and casts a revealing light on the players involved in ecosystem reorganization during an understudied period of time.
Nonbiomineralized carapaces in Cambrian seafloor landscapes (Sirius Passet, Greenland): Opening a new window into early Phanerozoic benthic ecology
M. Gabriela Mángano et al., Dept. of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon SK S7N 5E2, Canada. Posted online 19 April; doi: 10.1130/G32853.1.
Sirius Passet in northern Greenland is the most remote and least well known of the sites containing exceptionally preserved Cambrian Explosion biotas. This study by scientists from the University of Saskatchewan, the Natural History Museum of Denmark, Durham University, the Oxford University Museum of Natural History, and the University of Texas sheds new light on the bottom ecology of Burgess Shale-type communities. The paper documents a complex assemblage of trace fossils (including simple trails, branching burrow systems, dendritic and annulated structures) directly associated with non-mineralized arthropod carapaces. Biogenic structures are found both above and below the carapaces, but complex, three-dimensional structures seem to be restricted to the lower interface. While some of these trace fossils can be related to scavenging and/or deposit feeding, others suggest grazing on and farming of bacteria. The organic surface of the carapace at the sediment water interface created a chemical gradient promoting the growth of sulfur-bacteria that served as primary food for small invertebrates. Authors M. Gabriela Mángano and colleagues suggest that these arthropod carcasses deposited on the Early Cambrian sea floor may have acted as attractors, supporting a community of small invertebrates, in an analogous way to whale carcasses in modern sea floors.
Survival of a submarine canyon during long-term outbuilding of a continental margin
David Amblas et al., GRC Geociències Marines, Departament d'Estratigrafi a, Paleontologia i Geociències Marines, Universitat de Barcelona, E-08028 Barcelona, Catalonia, Spain. Posted online 19 April; doi: 10.1130/G33178.1.
The resemblance between subaerial and submarine canyons has led to the long-standing view of submarine canyons as purely erosive landforms. Yet submarine canyons are observed both on and beneath the seafloor along continental slopes that are growing from long-term accumulation of sediment. This study by David Amblas and colleagues documents the Pleistocene coevolution of a submarine canyon and adjacent slope along the Ebro Margin (NW Mediterranean) using a 3-D seismic image of the seafloor and subsurface. Seismic reflectors beneath the present-day canyon and adjacent slope show that net accumulation has occurred in both areas over the last 500,000 years. Seismic mapping reveals a mid-Pleistocene canyon beneath the modern canyon that is morphologically similar in planform but exhibits a different long profile shape. An explanation for the change in long-profile shape is proposed in terms of the dominant sedimentation processes in the canyon. This study aims to broaden thinking about canyon evolution and the processes that govern it during outbuilding of a continental margin.
An earthquake slip zone is a magnetic recorder
Yu-Min Chou et al., Dept. of Geosciences, National Taiwan University, 1, Roosevelt Road Section 4, Taipei 10617, Taiwan. Posted online 19 April; doi: 10.1130/G32864.1.
This study shows for the first time that during an earthquake, the cm-to-mm-thick slip zone is a magnetic recorder. Generally, people emphasize the possible imprint of co-seismic current magnetic overprint. Here, Yu-Min Chou and colleagues show that the magnetic record is due to the dual action of temperature drop and chemical processes. This means that the magnetic record is robust at the scale of geological time. Their study further shows that (1) a rock magnetic investigation, fully automated, allows the identification of the mm-thick Chi-Chi slip zone (Mw 7.6, 1999, Taiwan); (2) the 16-cm-thick layer of intensively deformed rocks is remagnetized during earthquake due to the presence of fluids; and (3) they provide a fault gouge's magnetic record cycle in the event of a large earthquake. This study constitutes a new, fast, and non-destructive way to find the most recent principal slip zone of a fault.
Detrital zircon provenance from the Neuquén Basin (south-central Andes): Cretaceous geodynamic evolution and sedimentary response in a retroarc-foreland basin
A. Di Giulio et al., Dipartimento di Scienze della Terra, Università di Pavia, Corso Strada Nuova 65, 27100 Pavia, Italy. Posted online 19 April; doi: 10.1130/G33052.1.
A. Di Giulio and colleagues reconstruct the surface response, in terms of drainage pattern changes, to the Cretaceous geodynamic reorganization of the Andean subduction zone between 36 degrees and 41 degrees south by studying the geochronology-based provenance of alluvial detrital zircons. The age spectra obtained by 500 spot uranium-lead (U-Pb) ages record an eastward provenance of detritus coming from the foreland during the Early Cretaceous backarc extensional stage, followed by westward-sourced clastics coming from the Cordillera during the Cenomanian. This drainage pattern reversal fits the regional unconformity in the sedimentary record that is linked to the geodynamic reorganization of the continental margin from an extensional to a compressional regime, forcing the Neuquén Basin to evolve from a retroarc to a foreland stage. After this inversion, the clastic systems progressively returned to be mainly fed by the foreland, due to the uplift of the peripheral bulge as a consequence of the Late Cretaceous thrust front migration. This tectonic evolution of the Neuquén Basin and the related response of the drainage pattern are thought to be the surface expression of the dip decrease of the Benioff subduction zone.
Growth of subtropical forests in Miocene Europe: The roles of carbon dioxide and Antarctic ice volume
N. Hamon et al., Institut de Paléoprimatologie, Paléontologie Humaine: Evolution et Paléoenvironnements, CNRS-INEE, Université de Poitiers, 86022 Poitiers, France; and Laboratoire des Sciences du Climat et de l'environnement, CNRS-CEA-UVSQ, 91191 Gif-sur-Yvette, France. Posted online 19 April; doi: 10.1130/G32990.1.
The middle Miocene (about 13-17 million years ago) is a crucial period for the evolution of hominoids (apes), and it corresponds to their appearance in Europe. Because hominoids can only live in (sub-)tropical forests, their dispersion from Afro-Arabia to Europe is closely linked to the vegetation changes in this region during the middle Miocene. Data-based reconstructions by N Hamon and colleagues indicate the development of subtropical forests in Europe between 15 and 17 million years ago, which corresponds to the middle Miocene climatic optimum. However, the context in which the middle Miocene climatic optimum occurred still lacks constraints, particularly in terms of atmospheric pCO2 and ice-sheet volume and extent. Using a coupled atmosphere-ocean general circulation model and a dynamic vegetation model, Hamon and colleagues performed experiments with varying pCO2 levels and Antarctic ice-sheet configurations for the middle Miocene climatic optimum. Their results suggest that a small East Antarctic Ice Sheet (25% of present-day ice volume), together with higher than present pCO2 values, are in better agreement with available European middle Miocene data and the dispersion of apes from Afro-Arabia to Europe during the middle Miocene.
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