Climatologist Dr. Judith Curry rips UN IPCC’s ‘expert judgement’ that humans are ‘extremely likely’ responsible for more than half of warming since 1951
Nature Geoscience. doi:10.1038/ngeo2238
Author: John Kessler
The release of large quantities of methane from ocean sediments might affect global climate change. The discovery of expansive methane seeps along the US Atlantic margin provides an ideal test bed for such a marine methane–climate connection.
Nature Geoscience. doi:10.1038/ngeo2233
Authors: William Nardin & Douglas A. Edmonds
River deltas support a disproportionate percentage of the world’s population and some are drowning as sea level rises. Resilient deltas theoretically balance relative sea-level rise with vertical growth from surface sedimentation. Vegetation generally enhances inorganic sedimentation and resiliency in some settings, such as tidal saltwater marshes, but the effect of vegetation on freshwater marshes in river deltas is less clear. Here we use a hydrodynamic numerical model to simulate deposition in a river delta with varying vegetation characteristics and water discharge and show that vegetation does not always enhance sedimentation on a freshwater marsh. For a given flood, we find that intermediate vegetation height and density are optimal for enhancing both sand and mud deposition, whereas tall or dense vegetation causes sand to remain in the river channel, reducing marsh sedimentation. A multivariate regression analysis of remote-sensing data from Wax Lake Delta, Louisiana, USA shows that the delta exhibits a hydrodynamic response to vegetation in agreement with model predictions. Because most sediment is delivered to freshwater deltaic marshes by infrequent storm and flood events, we further suggest that the timing of such events relative to seasonal vegetation growth determines the integrated effect of vegetation on delta resiliency.
Record of the ancient martian hydrosphere and atmosphere preserved in zircon from a martian meteorite
Nature Geoscience. doi:10.1038/ngeo2231
Authors: A. A. Nemchin, M. Humayun, M. J. Whitehouse, R. H. Hewins, J-P. Lorand, A. Kennedy, M. Grange, B. Zanda, C. Fieni & D. Deldicque
Mars exhibits ample evidence for an ancient surface hydrosphere. The oxygen isotope compositions of carbonate minerals and alteration products in martian meteorites suggest that this ancient hydrosphere was not in isotopic equilibrium with the martian lithosphere. Martian meteorite NWA 7533 is composed of regolith breccia from the heavily cratered terrains of ancient Mars and contains zircon grains for which U–Pb ages have been reported. Here we report variations between the oxygen isotopic compositions of four zircon grains from NWA 7533. We propose that these variations can be explained if the mantle melts from which the zircon crystallized approximately 4.43 Gyr ago had assimiliated 17O-enriched regolith materials, and that some of the zircon grains, while in a metamict state, were later altered by low-temperature fluids near the surface less than 1.7 Gyr ago. Enrichment of the martian regolith in 17O before the zircon crystallized, presumably through exchange with the 17O-enriched atmosphere or hydrosphere during surface alteration, suggests that the thick primary atmosphere of Mars was lost within the first 120 Myr after accretion. We conclude that the observed variation of 17O anomalies in zircon from NWA 7533 points to prolonged interaction between the martian regolith, atmosphere and hydrosphere.
Nature Geoscience. doi:10.1038/ngeo2227
Authors: Elizabeth C. Sibert, Pincelli M. Hull & Richard D. Norris
Open-ocean ecosystems experienced profound disruptions to biodiversity and ecological structure during the Cretaceous/Palaeogene mass extinction about 66 million years ago. It has been suggested that during this mass extinction, a collapse of phytoplankton production rippled up the food chain, causing the wholesale loss of consumers and top predators. Pelagic fish represent a key trophic link between primary producers and top predators, and changes in their abundance provide a means to examine trophic relationships during extinctions. Here we analyse accumulation rates of microscopic fish teeth and shark dermal scales (ichthyoliths) in sediments from the Pacific Ocean and Tethys Sea across the Cretaceous/Palaeogene extinction to reconstruct fish abundance. We find geographic differences in post-disaster ecosystems. In the Tethys Sea, fish abundance fell abruptly at the Cretaceous/Palaeogene boundary and remained depressed for at least 3 million years. In contrast, fish abundance in the Pacific Ocean remained at or above pre-boundary levels for at least four million years following the mass extinction, despite marked extinctions in primary producers and other zooplankton consumers in this region. We suggest that the mass extinction did not produce a uniformly dead ocean or microbially dominated system. Instead, primary production, at least regionally, supported ecosystems with mid-trophic-level abundances similar to or above those of the Late Cretaceous.
Nature Geoscience. doi:10.1038/ngeo2232
Authors: A. Skarke, C. Ruppel, M. Kodis, D. Brothers & E. Lobecker
Methane emissions from the sea floor affect methane inputs into the atmosphere, ocean acidification and de-oxygenation, the distribution of chemosynthetic communities and energy resources. Global methane flux from seabed cold seeps has only been estimated for continental shelves, at 8 to 65 Tg CH4 yr−1, yet other parts of marine continental margins are also emitting methane. The US Atlantic margin has not been considered an area of widespread seepage, with only three methane seeps recognized seaward of the shelf break. However, massive upper-slope seepage related to gas hydrate degradation has been predicted for the southern part of this margin, even though this process has previously only been recognized in the Arctic. Here we use multibeam water-column backscatter data that cover 94,000 km2 of sea floor to identify about 570 gas plumes at water depths between 50 and 1,700 m between Cape Hatteras and Georges Bank on the northern US Atlantic passive margin. About 440 seeps originate at water depths that bracket the updip limit for methane hydrate stability. Contemporary upper-slope seepage there may be triggered by ongoing warming of intermediate waters, but authigenic carbonates observed imply that emissions have continued for more than 1,000 years at some seeps. Extrapolating the upper-slope seep density on this margin to the global passive margin system, we suggest that tens of thousands of seeps could be discoverable.