Supply limited sediment transport in a high-discharge event of the tropical Burdekin River, North Queensland, Australia

Interactions between catchment variables and sediment transport processes in rivers are complex, and sediment transport behaviour during high‐flow events is not well documented. This paper presents an investigation into sediment transport processes in a short‐duration, high‐discharge event in the Burdekin River, a large sand‐ and gravel‐bed river in the monsoon‐ and cyclone‐influenced, semi‐arid tropics of north Queensland. The Burdekin's discharge is highly variable and strongly seasonal, with a recorded maximum of 40 400 m³ s^?1. Sediment was sampled systematically across an 800 m wide, 12 m deep and straight reach using Helley‐Smith bedload and US P‐61 suspended sediment samplers over 16 days of a 29‐day discharge event in February and March 2000 (peak 11 155 m³ s^?1). About 3·7 × 10⁶ tonnes of suspended sediment and 3 × 10⁵ tonnes of bedload are estimated to have been transported past the sample site during the flow event. The sediment load was predominantly supply limited. Wash load included clay, silt and very fine sand. The concentration of suspended bed material (including very coarse sand) varied with bedload transport rate, discharge and height above the bed. Bedload transport rate and changes in channel shape were greatest several days after peak discharge. Comparison between these data and sparse published data from other events on this river shows that the control on sediment load varies between supply limited and hydraulically limited transport, and that antecedent weather is an important control on suspended sediment concentration. Neither the empirical relationships widely used to estimate suspended sediment concentrations and bedload (e.g. Ackers & White, 1973) nor observations of sediment transport characteristics in ephemeral streams (e.g. Reid & Frostick, 1987) are directly applicable to this river.     

Pockmarks in the floor of Penobscot Bay,Maine

Hundreds of depressions (pockmarks) were found within a 40 square kilometer area of the sea floor near the head of Penobscot Bay, Maine. These roughly circular depressions range in diameter from 10 to 300 meters and extend as much as 30 meters below the surrounding sea floor. The pockmarks have formed in marine mud of Holocene age, which unconformably overlies glaciomarine deposits.The presence of shallow interstitial gas in the mud suggests that the pockmarks are related to the excipe of gas from the sediments, although other factors must be involved.     

The Creston,California, meteorite fall and the origin of L chondrites

It has been proposed that all L chondrites resulted from an ongoing collisional cascade of fragments that originated from the formation of the ~500 Ma old asteroid family Gefion, located near the 5:2 mean‐motion resonance with Jupiter in the middle Main Belt. If so, L chondrite pre‐atmospheric orbits should be distributed as expected for that source region. Here, we present contradictory results from the orbit and collisional history of the October 24, 2015, L6 ordinary chondrite fall at Creston, CA (here reclassified to L5/6). Creston's short 1.30 ± 0.02 AU semimajor axis orbit would imply a long dynamical evolution if it originated from the middle Main Belt. Indeed, Creston has a high cosmic ray exposure age of 40–50 Ma. However, Creston's small meteoroid size and low 4.23 ± 0.07° inclination indicate a short dynamical lifetime against collisions. This suggests, instead, that Creston originated most likely in the inner asteroid belt and was delivered via the ν₆ resonance. The U‐Pb systematics of Creston apatite reveals a Pb‐Pb age of 4,497.1 ± 3.7 Ma, and an upper intercept U‐Pb age of 4,496.7 ± 5.8 Ma (2σ), circa 70 Ma after formation of CAI, as found for other L chondrites. The K‐Ar (age ~4.3 Ga) and U,Th‐He (age ~1 Ga) chronometers were not reset at ~500 Ma, while the lower intercept U‐Pb age is poorly defined as 770 ± 320 Ma. So far, the three known L chondrites that impacted on orbits with semimajor axes a <2.0 AU all have high (>3 Ga) K‐Ar ages. This argues for a source of some of our L chondrites in the inner Main Belt. Not all L chondrites originate in a continuous population of Gefion family debris stretching across the 3:1 mean‐motion resonance.     

Biogeochemical processes affecting the distribution and vertical transport of hydrocarbon residues in the coastal Mediterranean

Sediments, sediment trap material, dominant surface plankton and collected fecal material were sampled concurrently with surface seawater in a coastal Mediterranean ecosystem and analyzed for non-volatile hydrocarbons and chlorinated hydrocarbons. Results showed consistent partitioning of hydrocarbon classes between dissolved and particulate phases of surface waters which appeared to be related to component solubility and particle availability. Analysis of biological materials showed the biota were important not only in packaging residues into large, fast sinking particles, but also in modifying the composition of components through metabolism and selective incorporation into body tissues and feces. Apparent sedimentation rate was calculated by analysis of ²¹⁰Pb in sediment core samples and used to estimate average deposition rates of organics to the sea/sediment interface. The flux of particles through 100 m, as measured in the trap material in this sampling interval, was sufficient to balance most of the petroleum input to the sediments but accounted for only 17% of the average flux of PCBs to the sediments, and virtually none of the more soluble chlorinated hydrocarbon flux. Vertical transport via large fecal material compared to average background particles was seasonally low corresponding to a seasonal minimum in plankton biomass in late summer. Results show that hydrocarbon residues transported long distances away from input sources are highly modified, pointing to the geochemical significance of physical-chemical partitioning between seawater phases, incorporation into organisms and fecal material and biological/chemical degradation.     

Sensitivity-guided reduction of parametric dimensionality for multi-objective calibration of watershed models

Problem complexity for watershed model calibration is heavily dependent on the number of parameters that can be identified during model calibration. This study investigates the use of global sensitivity analysis as a screening tool to reduce the parametric dimensionality of multi-objective hydrological model calibration problems while maximizing the information extracted from hydrological response data. This study shows that by expanding calibration problem formulations beyond traditional, statistical error metrics to also include metrics that capture indices or signatures of hydrological function, it is possible to reduce the complexity of calibration while maintaining high quality model predictions. The sensitivity-guided calibration is demonstrated using the Sacramento Soil Moisture Accounting (SAC-SMA) conceptual rainfall–runoff model of moderate complexity (i.e., up to 14 freely varying parameters). Using both statistical and hydrological metrics, optimization results demonstrate that parameters controlling at least 20% of the model output variance (through individual effects and interactions) should be included in the calibration process. This threshold generally yields 30–40% reductions in the number of SAC-SMA parameters requiring calibration – setting the others to a priori values – while maintaining high quality predictions. Two parameters are recommended to be calibrated in all cases (percent impervious area and lower zone tension water storage), three parameters are needed in drier watersheds (additional impervious area, riparian zone vegetation, and percent of percolation going to tension storage), and the lower zone parameters are crucial unless the watershed is very dry. Overall, this study demonstrates that a coupled, multi-objective sensitivity and calibration analysis better captures differences between watersheds during model calibration and serves to maximize the value of available watershed response time series. These contributions are particularly important given the ongoing development of more complex integrated models, which will require new tools to address the growing discrepancy between the information content of hydrological data and the number of model parameters that have to be estimated.     

Expanding the Cenozoic paleoceanographic record in the Central Arctic Ocean: IODP Expedition 302 Synthesis

The Arctic Coring Expedition (ACEX) proved to be one of the most transformational missions in almost 40 year of scientific ocean drilling. ACEX recovered the first Cenozoic sedimentary sequence from the Arctic Ocean and extended earlier piston core records from ≈1.5 Ma back to ≈56 Ma. The results have had a major impact in paleoceanography even though the recovered sediments represents only 29% of Cenozoic time. The missing time intervals were primarily the result of two unexpected hiatuses. This important Cenozoic paleoceanographic record was reconstructed from a total of 339 m sediments. The wide range of analyses conducted on the recovered material, along with studies that integrated regional tectonics and geophysical data, produced surprising results including high Arctic Ocean surface water temperatures and a hydrologically active climate during the Paleocene Eocene Thermal Maximum (PETM), the occurrence of a fresher water Arctic in the Eocene, ice-rafted debris as old as middle Eocene, a middle Eocene environment rife with organic carbon, and ventilation of the Arctic Ocean to the North Atlantic through the Fram Strait near the early-middle Miocene boundary. Taken together, these results have transformed our view of the Cenozoic Arctic Ocean and its role in the Earth climate system.     

Dispersant Use and a Bioremediation Strategy as Alternate Means of Reducing Impacts of Large Oil Spills on Mangroves: The Gladstone Field Trials

Over a three-year period (1995–1998), we studied short-term effects of dispersant use and a bioremediation strategy in two consecutive field trials in sub-tropical Australian mangroves. In each case, weathered oil was applied, and a large spill simulated, in mature Rhizophora stylosa trees around 4–9 m tall. In the first trial, we used Gippsland light crude oil with or without dispersant, Corexit 9527. In the second, a bioremediation strategy followed application of Gippsland oil or Bunker C fuel oil. Bioremediation involved forced aeration with supplemental application of nutrients. Dispersant use had an overall positive benefit shown as reduced tree mortality. By contrast, there was no apparent reduction in mortality of trees with bioremediation. However, one year after oiling, leaf densities of surviving trees were greater in bioremediation plots than in controls, and less in oil-only plots. These and other results have been incorporated into spill response management strategies in Australia.     

Refining Hernando de Soto's Route Using Electric Circuit Theory and CircuitScape

A least-cost path (LCP) analysis and a circuit theory analysis were used to estimate the path followed by Hernando de Soto as he crossed the Appalachian Mountains between Tennessee and North Carolina in 1540. The analyses were performed on the slope of the terrain and on a function of the slope that estimates hiking speed. The analyses were performed on data sets with 90-m and 180-m spatial resolutions. Three potential routes were found and compared. The most novel element of the work was the use of CircuitScape software, which returned the likelihood that each cell in the raster data was a part of de Soto's route. This clearly illuminated areas where the estimated routes were more constrained and areas where de Soto would have been free to take alternate paths without increasing travel time. The two analysis methods, LCP and circuit theory, corroborate one another and provide insight into de Soto's journey.     

Mineralogical and strontium isotopic record of hydrothermal processes in the lower ocean crust at and near the East Pacific Rise

Tectonic exposures of upper plutonics (>800 m) that are part of a contiguous section of young East Pacific Rise (EPR) crust at the Hess Deep Rift provide the first regional-scale constraints on hydrothermal processes in the upper plutonic crust formed at a fast-spreading ridge. Submersible-collected samples recovered over a 4-km-wide region show that the sheeted dike complex is largely underlain by a 150- to 200-m-thick gabbro unit, followed by a more primitive gabbronorite unit. Gabbroic samples are variably altered by pervasive fluid flow along fracture networks to amphibole-dominated assemblages. The gabbroic rocks are significantly less altered (average 11% hydrous phases) than the overlying sheeted dike complex (average 24%), and the percentage of hydrous alteration diminishes with depth. Incipient, pervasive fluid flow occurred at amphibolite facies conditions (average 720°C), with slightly higher temperatures in the lower 500 m of the section. The extent of subsequent lower-temperature alteration is generally low and regionally variable. The gabbroic samples are slightly elevated in ⁸⁷Sr/⁸⁶Sr relative to fresh rock values (0.7024) and less enriched than the overlying sheeted dike complex. ⁸⁷Sr/⁸⁶Sr for the pervasively altered gabbroic samples ranges from 0.70244 to 0.70273 (mean 0.70257), tonalites is 0.7038, and pyroxene hornfels ranges from 0.70259 to 0.70271. ⁸⁷Sr/⁸⁶Sr does not vary with depth, and there is a strong positive correlation with the percentage of hydrous phases. Strontium contents of igneous and hydrothermal minerals, combined with bulk rock ⁸⁷Sr/⁸⁶Sr, indicate that Sr-isotopic exchange is largely controlled by the uptake of fluid ⁸⁷Sr/⁸⁶Sr in hydrous minerals and does not require Sr gain or loss. The minimum, time-integrated fluid–rock ratio for the sheeted dike complex and upper plutonics is 0.55–0.66, and the fluid flux calculated by mass balance is ~2.1 to 2.5 × 10⁶ kg m⁻², 30–60% higher than fluid fluxes calculated in the same manner for sheeted dike complexes on their own at Hess and Pito Deeps, and Ocean Drilling Program Hole 504B. Alteration patterns within the upper plutonics evolved in response to axial magma chamber (AMC) dynamics at the EPR, such that magma replenishment led to assimilation and thermal metamorphism of the country rock, and the position of the hydrothermal root-zone tracked the vertical migration of the AMC. The freshness of the lowermost gabbroic rocks suggests that pervasive fluid flow does not lead to significant fluid and heat fluxes at and near fast-spreading ridges.