Closed‐Form Approximations for Two‐Dimensional Groundwater Age Patterns in a Fresh Water Lens

Simple closed‐form approximations are presented for calculating the steady‐state groundwater age distribution in two‐dimensional vertical cross sections of idealized fresh water lenses overlying salt water, for aquifers that are vertically semi‐infinite and of finite thickness. The approximations are developed on the basis of existing one‐dimensional analytical solutions for travel‐time calculation in fresh water lenses and approximate streamline formulations. The two‐dimensional age distributions based on the closed‐form solutions match convincingly with numerical simulations. As expected, notable deviations from the numerical solution are encountered at the groundwater flow divide and when submarine groundwater discharge occurs. Ratios of recharge over hydraulic conductivities are varied to explore how the magnitude of the deviations changes, and it is found that the approximate closed‐form solutions perform well over a range of conditions found in natural systems.     

A relative sea-level history for Arviat,Nunavut, and implications for Laurentide Ice Sheet thickness west of Hudson Bay

Thirty-six new and previously published radiocarbon dates constrain the relative sea-level history of Arviat on the west coast of Hudson Bay. As a result of glacial isostatic adjustment (GIA) following deglaciation, sea level fell rapidly from a high-stand of nearly 170 m elevation just after 8000 cal yr BP to 60 m elevation by the mid Holocene (~ 5200 cal yr BP). The rate of sea-level fall decreased in the mid and late Holocene, with sea level falling 30 m since 3000 cal yr BP. Several late Holocene sea-level measurements are interpreted to originate from the upper end of the tidal range and place tight constraints on sea level. A preliminary measurement of present-day vertical land motion obtained by repeat Global Positioning System (GPS) occupations indicates ongoing crustal uplift at Arviat of 9.3 ± 1.5 mm/yr, in close agreement with the crustal uplift rate inferred from the inferred sea-level curve. Predictions of numerical GIA models indicate that the new sea-level curve is best fit by a Laurentide Ice Sheet reconstruction with a last glacial maximum peak thickness of ~ 3.4 km. This is a 30–35% thickness reduction of the ICE-5G ice-sheet history west of Hudson Bay.     

All adrift: aviation,shipping, and climate change policy

All sectors face decarbonization for a 2 °C temperature increase to be avoided. Nevertheless, meaningful policy measures that address rising CO₂ from international aviation and shipping remain woefully inadequate. Treated with a similar approach within the United Nations Framework Convention on Climate Change (UNFCCC), they are often debated as if facing comparable challenges, and even influence each others’ mitigation policies. Yet their strengths and weaknesses have important distinctions. This article sheds light on these differences so that they can be built upon to improve the quality of debate and ensuing policy development. The article quantifies ‘2 °C’ pathways for these sectors, highlighting the need for mitigation measures to be urgently accelerated. It reviews recent developments, drawing attention to one example where a change in aviation mitigation policy had a direct impact on measures to cut CO₂ from shipping. Finally, the article contrasts opportunities and barriers towards mitigation. The article concludes that there is a portfolio of opportunities for short- to medium-term decarbonization for shipping, but its complexity is its greatest barrier to change. In contrast, the more simply structured aviation sector is pinning too much hope on emissions trading to deliver CO₂ cuts in line with 2 °C. Instead, the solution remains controversial and unpopular – avoiding 2 °C requires demand management.

Policy relevance

The governance arrangements around the CO₂ produced by international aviation and shipping are different from other sectors because their emissions are released in international airspace and waters. Instead, through the Kyoto Protocol, the International Civil Aviation Authority (ICAO) and the International Maritime Organization (IMO) were charged with developing policies towards mitigating their emissions. Slow progress to date, coupled with strong connections with rapidly growing economies, has led to the CO₂ from international transport growing at a higher rate than the average rate from all other sectors. This article considers this rapid growth, and the potential for future CO₂ growth in the context of avoiding a 2 °C temperature rise above pre-industrial levels. It explores similarities and differences between these two sectors, highlighting that a reliance on global market-based measures to deliver required CO₂ cuts will likely leave both at odds with the overarching climate goal.     

Experimental simulation of oxygen isotopic exchange in olivine and implication for the formation of metamorphosed carbonaceous chondrites

We have conducted hydration–dehydration experiments on terrestrial olivine to investigate the behavior of oxygen isotopic fractionation to test the hypothesis that multiple cycles of aqueous and thermal processing on a parent asteroid comprise a genetic relationship between CM2s and metamorphosed carbonaceous chondrites (MCCs). Two experiments were undertaken. In the first experiment, serpentine was obtained by hydrating terrestrial olivine (Fo_90.9) in the laboratory. During this experiment, olivine was reacted with isotopically heavy water (δ¹⁸O 21.5‰) at T = 300 °C,  = 300 bar, for 100 days. The oxygen isotopic composition of the experimental serpentine was enriched in ¹⁸O (by 10 ‰ in δ¹⁸O) due to exchange of oxygen isotopes between olivine and the ¹⁸O‐rich water. Dehydrated serpentine was then produced during laboratory heating experiment in vacuum, at T = 930 °C, for 1 h. The oxygen isotopic composition of the dehydrated serpentine was enriched in ¹⁸O by a further 7 ‰. The net result of the hydration–dehydration process was an enrichment of ¹⁸O in the final material by approximately 17‰. The new experimental results suggest that the oxygen isotopic compositions of MCCs of the Belgica‐like group, including Dhofar 225 and Dhofar 725, could be derived from those of typical CM2 chondrites via several cycles of hydration–dehydration caused by aqueous alteration and subsequent thermal metamorphism within their parent asteroids.     

WAVERYS: a CMEMS global wave reanalysis during the altimetry period

Ocean Dynamics - As part of the Copernicus Marine Service, WAVERYS is the multi-year wave reanalysis that provides global wave data with a fine grid resolution of 1/5°. This wave reanalysis...     

When is star formation episodic? A delay differential equation 'negative feedback' model

We introduce a differential equation for star formation in galaxies that incorporates negative feedback with a delay. When the feedback is instantaneous, solutions approach a self-limiting equilibrium state. When there is a delay, even though the feedback is negative, the solutions can exhibit cyclic and episodic solutions. We find that periodic or episodic star formation only occurs when two conditions are satisfied. First the delay time-scale must exceed a cloud consumption time-scale. Secondly, the feedback must be strong. This statement is quantitatively equivalent to requiring that the time-scale to approach equilibrium be greater than approximately twice the cloud consumption time-scale. The period of oscillations predicted is approximately four times the delay time-scale. The amplitude of the oscillations increases with both feedback strength and delay time.
We discuss applications of the delay differential equation (DDE) model to star formation in galaxies using the cloud density as a variable. The DDE model is most applicable to systems that recycle gas and only slowly remove gas from the system. We propose likely delay mechanisms based on the requirement that the delay time is related to the observationally estimated time between episodic events. The proposed delay time-scale accounting for episodic star formation in galaxy centres on periods similar to   P ∼ 10 Myr  , irregular galaxies with   P ∼ 100 Myr  , and the Milky Way disc with   P ∼ 2  Gyr, could be that for exciting turbulence following creation of massive stars, that for gas pushed into the halo to return and interact with the disc and that for spiral density wave evolution, respectively.     

Flow-induced vegetation uprooting in a meandering bend:Experimental investigation

Vegetation uprooting is a complex process which depends on many interrelated factors. In this paper,attention is focused on the flow-induced uprooting in river bends, where the flow characteristics vary as effected by the channel’s curvature and its continuous changing in the downstream direction. Results are presented by an experimental work done in a high-amplitude meandering flume with mature herbaceous vegetation on the bed. In the first part of the paper, the variation along the bend of the...     

Using in situ cosmogenic 10Be to identify the source of sediment leaving Greenland

We use the concentration of in situ ¹⁰Be in quartz isolated from fluvial and morainal sand to trace sediment sources and to determine the relative contribution of glacerized and deglaciated terrain to Greenland's sediment budget. We sampled along the western, eastern, and southern margins of the Greenland Ice Sheet, and collected sediment sourced from glacerized (n = 19) and non‐glacerized terrain (n = 10), from channels where sediment from glacerized and non‐glacerized terrain is mixed (n = 28), from Holocene glacial‐fluvial terraces (n = 4), and from one sand dune. In situ ¹⁰Be concentrations in sediment range from 1600 to 34 000 atoms g^‐1. The concentration of in situ ¹⁰Be in sediment sourced from non‐glacerized terrain is significantly higher than in sediment sourced from glacerized areas, in mixed channel sediment, and in terrace sediment that was deposited during the Holocene. To constrain the timing of landscape exposure for the deglaciated portion of the Narsarsuaq field area in southern Greenland, we measured in situ ¹⁰Be concentration in bedrock (n = 5) and boulder (n = 6) samples. Paired bedrock and boulder ages are indistinguishable at 1σ uncertainty and indicate rapid exposure of the upland slopes at ~10.5 ka. The isotope concentration in sediment sourced from non‐glacerized terrain is higher than in sediment sourced from glacerized terrain because the non‐glacerized landscape has been exposed to cosmic radiation since early Holocene deglaciation. Sediment from glacerized areas contains a low, but measurable concentration of ¹⁰Be that probably accumulated at depth during a prolonged period of exposure, probably before the establishment of the Greenland Ice Sheet. The concentration of ¹⁰Be in mixed fluvial sediment and in terrace sediment is low, and similar to the concentration in sediment from glacerized areas, which indicates that the Greenland Ice Sheet is the dominant source of sediment moving through the landscape outside the glacial margin in the areas we sampled. Copyright © 2014 John Wiley & Sons, Ltd.     

Detecting inter-aquifer leakage in areas with limited data using hydraulics and multiple environmental tracers,including <Superscript>4</Superscript>He, <Superscript>36</Superscript>Cl/Cl, <Superscript>14</Superscript>C and <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr

The investigation of regionally extensive groundwater systems in remote areas is hindered by a shortage of data due to a sparse observation network, which limits our understanding of the hydrogeological processes in arid regions. The study used a multidisciplinary approach to determine hydraulic connectivity between the Great Artesian Basin (GAB) and the underlying Arckaringa Basin in the desert region of Central Australia. In order to manage the impacts of groundwater abstraction from the Arckaringa Basin, it is vital to understand its connectivity with the GAB (upper aquifer), as the latter supports local pastoral stations and groundwater-dependent springs with unique endemic flora and fauna. The study is based on the collation of available geological information, a detailed analysis of hydraulic data, and data on environmental tracers. Enhanced inter-aquifer leakage in the centre of the study area was identified, as well as recharge to the GAB from ephemeral rivers and waterholes. Throughout the rest of the study area, inter-aquifer leakage is likely controlled by diffuse inter-aquifer leakage, but the coarse spatial resolution means that the presence of additional enhanced inter-aquifer leakage sites cannot be excluded. This study makes the case that a multi-tracer approach along with groundwater hydraulics and geology provides a tool-set to investigate enhanced inter-aquifer leakage even in a groundwater basin with a paucity of data. A particular problem encountered in this study was the ambiguous interpretation of different age tracers, which is attributed to diffusive transport across flow paths caused by low recharge rates.