Uncovering the Ediacaran phosphorus cycle.

Phosphorus is a limiting nutrient that is thought to control oceanic oxygen levels to a large extent1-3. A possible increase in marine phosphorus concentrations during the Ediacaran Period (about 635-539 million years ago) has been proposed as a driver for increasing oxygen levels4-6. However, little is known about the nature and evolution of phosphorus cycling during this time4. Here we use carbonate-associated phosphate (CAP) from six globally distributed sections to reconstruct oceanic phosphorus concentrations during a large negative carbon-isotope excursion-the Shuram excursion (SE)-which co-occurred with global oceanic oxygenation7-9. Our data suggest pulsed increases in oceanic phosphorus concentrations during the falling and rising limbs of the SE. Using a quantitative biogeochemical model, we propose that this observation could be explained by carbon dioxide and phosphorus release from marine organic-matter oxidation primarily by sulfate, with further phosphorus release from carbon-dioxide-driven weathering on land. Collectively, this may have resulted in elevated organic-pyrite burial and ocean oxygenation. Our CAP data also seem to suggest equivalent oceanic phosphorus concentrations under maximum and minimum extents of ocean anoxia across the SE. This observation may reflect decoupled phosphorus and ocean anoxia cycles, as opposed to their coupled nature in the modern ocean. Our findings point to external stimuli such as sulfate weathering rather than internal oceanic phosphorus-oxygen cycling alone as a possible control on oceanic oxygenation in the Ediacaran. In turn, this may help explain the prolonged rise of atmospheric oxygen levels.

Seawater-buffered diagenesis, destruction of carbon isotope excursions, and the composition of DIC in Neoproterozoic oceans.

Carbonate sediments of nonglacial Cryogenian (659 to 649 Ma) and early Ediacaran (635 to 590 Ma) age exhibit large positive and negative δ13Ccarb excursions in a shallow-water marine platform in northern Namibia. The same excursions are recorded in fringing deep-sea fans and in carbonate platforms on other paleocontinents. However, coeval carbonates in the upper foreslope of the Namibian platform, and to a lesser extent in the outermost platform, have relatively uniform δ13Ccarb compositions compatible with dissolved inorganic carbon (DIC) in the modern ocean. We attribute the uniform values to fluid-buffered diagenesis that occurred where seawater invaded the sediment in response to geothermal porewater convection. This attribution, which is testable with paired Ca and Mg isotopes, implies that large δ13Ccarb excursions observed in Neoproterozoic platforms, while sedimentary in origin, do not reflect the composition of ancient open-ocean DIC.

Earthquake lubrication and healing explained by amorphous nanosilica.

During earthquake propagation, geologic faults lose their strength, then strengthen as slip slows and stops. Many slip-weakening mechanisms are active in the upper-mid crust, but healing is not always well-explained. Here we show that the distinct structure and rate-dependent properties of amorphous nanopowder (not silica gel) formed by grinding of quartz can cause extreme strength loss at high slip rates. We propose a weakening and related strengthening mechanism that may act throughout the quartz-bearing continental crust. The action of two slip rate-dependent mechanisms offers a plausible explanation for the observed weakening: thermally-enhanced plasticity, and particulate flow aided by hydrodynamic lubrication. Rapid cooling of the particles causes rapid strengthening, and inter-particle bonds form at longer timescales. The timescales of these two processes correspond to the timescales of post-seismic healing observed in earthquakes. In natural faults, this nanopowder crystallizes to quartz over 10s-100s years, leaving veins which may be indistinguishable from common quartz veins.