How well do integrated assessment models simulate climate change?

Integrated assessment models (IAMs) are regularly used to evaluate different policies of future emissions reductions. Since the global costs associated with these policies are immense, it is vital that the uncertainties in IAMs are quantified and understood. We first demonstrate the significant spread in the climate system and carbon cycle components of several contemporary IAMs. We then examine these components in more detail to understand the causes of differences, comparing the results with more complex climate models and earth system models (ESMs), where available. Our results show that in most cases the outcomes of IAMs are within the range of the outcomes of complex models, but differences are large enough to matter for policy advice. There are areas where IAMs would benefit from improvements (e.g. climate sensitivity, inertia in climate response, carbon cycle feedbacks). In some cases, additional climate model experiments are needed to be able to tune some of these improvements. This will require better communication between the IAM and ESM development communities.     

Spatial variation in fluid flow and geochemical fluxes across the sediment-seawater interface at the Carlos Ribeiro mud volcano (Gulf of Cadiz)

Submarine mud volcanism is an important pathway for transfer of deep-sourced fluids enriched in hydrocarbons and other elements into the ocean. Numerous mud volcanoes (MVs) have been discovered along oceanic plate margins, and integrated elemental fluxes are potentially significant for oceanic chemical budgets. Here, we present the first detailed study of the spatial variation in fluid and chemical fluxes at the Carlos Ribeiro MV in the Gulf of Cadiz. To this end, we combine analyses of the chemical composition of pore fluids with a 1-D transport-reaction model to quantify fluid fluxes, and fluxes of boron, lithium and methane, across the sediment-seawater interface. The pore fluids are significantly depleted in chloride, but enriched in lithium, boron and hydrocarbons, relative to seawater. Pore water profiles of sulphate, hydrogen sulphide and total alkalinity indicate that anaerobic oxidation of methane occurs at 34-180 cm depth below seafloor. Clay mineral dehydration, and in particular the transformation of smectite to illite, produces pore fluids that are depleted in chloride and potassium. Profiles of boron, lithium and potassium are closely related, which suggests that lithium and boron are released from the sediments during this transformation. Pore fluids are expelled into the water column by advection; fluid flow velocities are 4 cm yr⁻¹ at the apex of the MV but they rapidly decrease to 0.4 cm yr⁻¹ at the periphery. The associated fluxes of boron, lithium and methane vary between 7-301, 0.5-6 and 0-806 mmol m⁻² yr⁻¹, respectively. We demonstrate that fluxes of Li and B due to mud volcanism may be important on a global scale, however, release of methane into the overlying water column is suppressed by microbial methanotrophy.     

Combining advanced NMR techniques with ultrahigh resolution mass spectrometry: A new strategy for molecular scale characterization of macromolecular components of soil and sedimentary organic matter

Molecular level characterization of complex biopolymers in nature is a key element to understanding the composition of natural organic matter (NOM) and fossil organic matter formation, such as kerogen and coal. Characterization of such geopolymers is difficult due to their heterogeneity and insolubility in common aqueous or organic solvents. Here, a strategy for extracting a representative sample is presented using the combined techniques of nuclear magnetic resonance (NMR) and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) for chemical analysis and characterization of NOM. A variety of NOM samples (wood, kerogen, bitumen, whole sediments) are shown as examples for implementing the strategy, which include solvent extractions using pyridine. For most samples, the extracts are confirmed to be chemically representative of the insoluble solid, by comparing the liquids NMR spectrum of the extract to that of the whole, unfractionated NOM utilizing high resolution magic angle spinning (HRMAS) NMR. To assist in unambiguous peak assignment, a technique for post acquisition spectral denoising, using wavelet transformation (WT), is also employed on the 1D and 2D NMR spectra. The findings from the NMR data lay a foundation for the subsequent electrospray ionization FTICR-MS analysis of the pyridine extracts, since this instrument has the limitation of only analyzing liquid samples. The FTICR-MS analyses can, therefore, be used to molecularly represent the structural components of the NOM. Various compositional insights have been obtained on these samples, indicating the efficacy of the analytical techniques used.     

Modelling tidal current turbine wakes using a coupled RANS-BEMT approach as a tool for analysing power capture of arrays of turbines

An improved method is developed to couple an inner domain solution of the blade element momentum theory with an outer domain solution of the Reynolds averaged Navier Stokes equations for evaluating performance of tidal current turbines. A mesh sensitivity study shows that a mesh of at least 6 M cells with at least 40% of these within the turbine wake is required to ensure satisfactory convergence of the velocity deficit. In addition to the usually applied axial momentum source terms, angular momentum and turbulence intensity source terms are shown to be required to model the near wake evolution. Three different lateral turbine spacing of 2, 4 and 6 turbine diameters are used to demonstrate the influence of the effective channel blockage on the velocity distribution in the turbine bypass region, the rate of spread of the wake and the recovery of velocity distribution. A final study shows that for a fixed number of turbines minimising the lateral spacing within each row, with a small number of staggered rows spaced as longitudinally as far apart as practical, is the most effective strategy for energy capture.     

Late Devonian OIB alkaline gabbro in the Yarlung Zangbo Suture Zone: Remnants of the Paleo-Tethys?

The Yarlung Zangbo Suture Zone (YZSZ) is believed to be composed of material largely derived from the destruction of the Neo-Tethys that occurred from early Mesozoic to early Cenozoic. We report here geochronological and petrological data obtained for newly discovered alkaline gabbro blocks embedded in a mélange zone of the western YZSZ. Single zircon U–Pb analyses from one representative gabbro sample by SIMS (Secondary Ion Mass Spectrometry) yielded a combined crystallization age of about 363.7 ± 1.7 Ma (1σ). In situ Hf isotopic analyses yielded ε_Hf(t) values of + 2.6 to + 5.5, suggesting an enriched mantle source. All of the gabbro samples show typical Ocean Island Basalt (OIB) affinity with little or no continental crust contamination. They also display strong geochemical similarities with the Hawaii basalts and the Xigaze seamount basalts suggestive of their intra-oceanic setting. These observations, in combination with the Early Carboniferous layered gabbros reported at Luobusa, indicate that these rocks could represent remnants of the Paleo-Tethys. We propose that a branch ocean separating the Western Qiangtang terrane and the Lhasa terrane from the Gondwana continent might have been present during the Late Devonian and the Early Carboniferous, providing new constrains on the configuration of Paleo-Tethys in Tibetan Plateau during early Late Paleozoic.     

The chemical changes of DOM from black waters to coastal marine waters by HPLC combined with ultrahigh resolution mass spectrometry

How dissolved organic matter (DOM) undergoes chemical changes during its transit from river to ocean remains a challenge due to its complex structure. In this study, DOM along a river transect from black waters to marine waters is characterized using an offline combination of reversed-phase high performance liquid chromatography (RP-HPLC) coupled to electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS), as well as tandem ESI-FTICR-MS. In addition, a water extract from degraded wood that mainly consists of lignins is used for comparison to the DOM from this transect. The HPLC chromatograms of all DOM samples and the wood extract show two major well-separated components; one is hydrophilic and the other is hydrophobic, based on their elution order from the C₁₈ column. From the FTICR-MS analysis of the HPLC fractions, the hydrophilic components mainly contain low molecular weight compounds (less than 400 Da), while the hydrophobic fractions contain the vast majority of compounds of the bulk C₁₈ extracted DOM. The wood extract and the DOM samples from the transect of black waters to coastal marine waters show strikingly similar HPLC chromatograms, and the FTICR-MS analysis further indicates that a large fraction of molecular formulas from these samples are the same, existing as lignin-like compounds. Tandem mass spectrometry experiments show that several representative molecules from the lignin-like compounds have similar functional group losses and fragmentation patterns, consistent with modified lignin structural entities in the wood extract and these DOM samples. Taken together, these data suggest that lignin-derived compounds may survive the transit from the river to the coastal ocean and can accumulate there because of their refractory nature.     

Interannual variability of tropical cyclone activity along the Pacific coast of North America

The interannual variability of near-coastal eastern North Pacific tropical cyclones is described using a data set of cyclone tracks constructed from U.S. and Mexican oceanic and atmospheric reports for the period 1951-2006. Near-coastal cyclone counts are enumerated monthly, allowing us to distinguish interannual variability during different phases of the May-November tropical cyclone season. In these data more tropical cyclones affect the Pacific coast in May-July, the early months of the tropical cyclone season, during La Niña years, when equatorial Pacific sea surface temperatures are anomalously cool, than during El Niño years. The difference in early season cyclone counts between La Niña and El Niño years was particularly pronounced during the mid-twentieth century epoch when cool equatorial temperatures were enhanced as described by an index of the Pacific Decadal Oscillation. Composite maps from years with high and low near-coastal cyclone counts show that the atmospheric circulation anomalies associated with cool sea surface temperatures in the eastern equatorial Pacific are consistent with preferential steering of tropical cyclones northeastward toward the west coast of Mexico.     

Syngenetic gold in western Victoria: occurrence,age and dimensions

In western Victoria, a widespread stratiform style of gold enrichment in Palaeozoic black mudstone and chert—clearly different from the classic mesothermal quartz vein deposits of the Victorian goldfields—has been confirmed by whole-rock geochemistry and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). This enrichment pre-dates compaction, deformation and low-grade metamorphism of the sedimentary host-rocks, and therefore possibly developed diagenetically during slow deposition of the thin carbonaceous black mudstone beds and the thinner layers of chert. These paired strata have been documented at many locations in three regional outcrop areas of chevron-folded Cambrian and Lower Ordovician metasediments in the Stawell and Bendigo Zones, where they are interbedded with quartz-rich turbidites. The layers were named ‘indicators’ by the early miners, who found locally rich nuggety gold deposits at intersections between these layers and mesothermal quartz veins. Gold is present in euhedral pyrite crystals in both black shale and chert. LA-ICPMS analysis of individual pyrite crystals in the indicator beds shows that pyrite is enriched in Au, As, Sb, Se, Te and Bi. The Au content of pyrite varies from 0.03 to 2.69 ppm with a mean of 0.58 ppm and shows a positive correlation with As, which varies from 1000 to 6000 ppm. Many pyrite crystals show enrichment of gold in their cores and depletion in their rims, confirming the likely syngenetic or syndiagenetic accumulation of gold during pyrite formation in the sediments. Prior to regional metamorphism, folding and faulting, the many indicator strata in the outcrop areas were parts of an extensive marine sequence of Late Cambrian and Early Ordovician age. The former primary source of this mineralisation is considered to have been one or more contemporaneously exhalative submarine hydrothermal systems. Thus, the older Palaeozoic sediments of the western Lachlan Fold Belt were significantly enriched in syngenetic gold in the Early Palaeozoic, at least 40 million years before emplacement of the quartz – gold vein deposits of the goldfields.