Aluminum‐26 in H4 chondrites: Implications for its production and its usefulness as a fine‐scale chronometer for early solar system events

Abstract— In order to investigate whether or not ²⁶Al can be used as a fine‐scale chronometer for early solar system events we measured, with an ion microprobe, Mg isotopes and Al/Mg ratios in separated plagioclase, olivine, and pyroxene crystals from the H4 chondrites Ste Marguerite (SM), Forest Vale (FV), Beaver Creek and Quenggouk and compared the results with the canonical ²⁶Al/²⁷Al ratio for calcium‐aluminum‐rich inclusions (CAIs). For SM and FV, Pb/Pb and Mn‐Cr ages have previously been determined (Göpel et al., 1994; Polnau et al., 2000; Polnau and Lugmair, 2001). Plagioclase grains from these two meteorites show clear excesses of ²⁶Mg. The ²⁶Al/²⁷Al ratios inferred from these excesses and from isotopically normal Mg in pyroxene and olivine are (2.87 ± 0.64) × 10^?7 for SM and (1.52 ± 0.52) × 10^?7 for FV. The differences between these ratios and the ratio of 5 times 10^?5 in CAIs indicate time differences of 5.4 ± 0.1 Ma and 6.1 ± 0.2 Ma for SM and FV, respectively. These differences are in agreement with the absolute Pb/Pb ages for CAIs and SM and FV phosphates but there are large discrepancies between the U‐Pb and Mn‐Cr system for the relative ages for CAIs, SM and FV. For example, Mn‐Cr ages of carbonates from Kaidun are older than the Pb/Pb age of CAIs. However, even if we require that CAIs are older than these carbonates, the time difference between this “adjusted” CAI age and the Mn‐Cr ages of SM and FV require that ²⁶ Al was widely distributed in the early solar system at the time of CAI formation and was not mostly present in CAIs, a feature of the X‐wind model proposed by Shu and collaborators (Gounelle et al., 2001; Shu et al., 2001). From this we conclude that there was enough ²⁶Al to melt small planetary bodies as long as they formed within 2 Ma of CAIs, and that ²⁶Al can serve as a fine‐scale chronometer for early solar system events.     

Evaporation and Sublimation of Boric Acid: Application for Boron Purification from Organic Rich Solutions

We report in this paper a series of experiments whose purpose is to test the long held idea of boron volatility when neutral to acidic solutions containing traces of boron are evaporated. Boron recoveries have been measured precisely by isotopic dilution and boron isotopic ratios have also been determined. Most of the evaporations have been conducted at 60-65 °C. It was found that no loss of boric acid occurs when c. 1 μg of B is evaporated in water, HCl, HF and acetone solutions. At the same time, we did not observe any associated B isotopic fractionation. In contrast to previous studies on B volatilisation, it was not found necessary to use mannitol to prevent B losses during the evaporation of acidic solutions. Important losses of boron were observed from solutions of methanol, ethanol and organic-rich natural solutions. In the presence of methanol and ethanol, a volatile methyl (or ethyl) borate is likely to be formed. In contrast to these studies, we show that boron is highly volatile when the dried residues of an evaporation are heated further. The sublimation of boron that then occurs can be moderated by the use of mannitol, but a slight increase of the temperature allows B volatilisation. Despite the substantial losses of B by sublimation, no B isotopic fractionation was observed at 60-65 C. The property of boron to sublime was used to separate B from an organic matrix. A miniaturized sublimation apparatus, comprising a 5 ml Teflon beaker, and the associated microsublimation technique are described. The 100% boron recovery and the absence of B isotopic fractionation make this method suitable for extracting B from organic-rich samples.     

Laboratory studies of methane and ethane adsorption and nucleation onto organic particles: Application to Titan's clouds

Titan, Saturn's largest moon, has a thick nitrogen/methane atmosphere. The temperature and pressure conditions in Titan's atmosphere are such that the methane vapor should condense near the tropopause to form clouds. Several ground-based measurements have observed sparse cloud-like features in Titan's atmosphere, while the Cassini mission to Saturn has provided large scale images of the clouds. However, Titan's cloud formation conditions remain poorly constrained. Heterogeneous nucleation (from the vapor phase onto a solid or liquid aerosol surface) greatly enhances cloud formation relative to homogeneous nucleation. In order to elucidate the cloud formation mechanism near the tropopause, we have performed laboratory measurements of the adsorption of methane and ethane onto solid organic particles (tholins) representative of Titan's photochemical haze. We find that monolayers of methane adsorb onto tholin particles at saturation ratios less than unity. We also find that solid methane nucleates onto the adsorbed methane at a saturation ratio of S=1.07±0.008. This implies that Titan's methane clouds should form easily. This is consistent with recent measurements of the column of methane ruling out excessive methane supersaturation. In addition, we find ethane adsorbs onto tholin particles in a metastable phase prior to nucleation. However, ethane nucleation onto the adsorbed ethane occurs at a relatively high saturation ratio of S=1.36±0.08. These findings are consistent with the recent report of polar ethane clouds in Titan's lower stratosphere.     

Optical properties of Titan and early Earth haze laboratory analogs in the mid-visible

Scattering and absorption of sunlight by aerosols are integral to understanding the radiative balance of any planetary atmosphere covered in a haze, such as Titan and possibly the early Earth. One key optical parameter of an aerosol is its refractive index. We have simulated both Titan and early Earth organic haze aerosols in the laboratory and measured the real and imaginary portion of their refractive index at λ = 532 nm using cavity ringdown aerosol extinction spectroscopy. This novel technique allows analysis on freely-floating particles minutes after formation. For our Titan analog particles, we find a real refractive index of n = 1.35 ± 0.01 and an imaginary refractive index k = 0.023 ± 0.007, and for the early Earth analog particles we find n = 1.81 ± 0.02 and k = 0.055 ± 0.020. The Titan analog refractive index has a smaller real and similar imaginary refractive index compared to most previous laboratory measurements of Titan analog films, including values from Khare et al. (Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W. [1984]. Icarus 60, 127-137). These newly measured Titan analog values have implications for spacecraft retrievals of aerosol properties on Titan. The early Earth analog has a significantly higher real and imaginary refractive index than Titan analogs reported in the literature. These differences suggest that, for a given amount of aerosol, the early Earth analog would act as a stronger anti-greenhouse agent than the Titan analog.     

Advances in landslide nowcasting: evaluation of a global and regional modeling approach

The increasing availability of remotely sensed data offers a new opportunity to address landslide hazard assessment at larger spatial scales. A prototype global satellite-based landslide hazard algorithm has been developed to identify areas that may experience landslide activity. This system combines a calculation of static landslide susceptibility with satellite-derived rainfall estimates and uses a threshold approach to generate a set of ‘nowcasts’ that classify potentially hazardous areas. A recent evaluation of this algorithm framework found that while this tool represents an important first step in larger-scale near real-time landslide hazard assessment efforts, it requires several modifications before it can be fully realized as an operational tool. This study draws upon a prior work’s recommendations to develop a new approach for considering landslide susceptibility and hazard at the regional scale. This case study calculates a regional susceptibility map using remotely sensed and in situ information and a database of landslides triggered by Hurricane Mitch in 1998 over four countries in Central America. The susceptibility map is evaluated with a regional rainfall intensity–duration triggering threshold and results are compared with the global algorithm framework for the same event. Evaluation of this regional system suggests that this empirically based approach provides one plausible way to approach some of the data and resolution issues identified in the global assessment. The presented methodology is straightforward to implement, improves upon the global approach, and allows for results to be transferable between regions. The results also highlight several remaining challenges, including the empirical nature of the algorithm framework and adequate information for algorithm validation. Conclusions suggest that integrating additional triggering factors such as soil moisture may help to improve algorithm performance accuracy. The regional algorithm scenario represents an important step forward in advancing regional and global-scale landslide hazard assessment.     

Impact of beaver dam analogues on hydrology in a semi-arid floodplain

Natural beaver ponds help connect the stream to the floodplain, maintain late summer low flows and reduce peak flow during high flow events by offering temporary surface water (SW) storage. When beavers are extirpated from the landscape, stream degradation often ensues. This study assesses the impact of beaver dam analogues (BDA) as a stream restoration technique to help maintain low flow water levels and enhance stream-floodplain interactions on a seasonal basis in Red Canyon Creek, Lander, WY. BDAs increased SW and groundwater (GW) levels, favoured the occurrence of flow reversals (i.e., stream-to-floodplain GW flow) during high flow events associated with mid-winter and early-spring thaw events, and reduced the groundwater-to-stream hydraulic gradient on an annual basis. Although GW temperatures varied seasonally, relatively cooler GW temperatures were observed in the BDA impacted reach compared to the control reach away from BDA influence. BDAs however did not significantly impact stream temperatures. Overall, results suggest that when installed in sequence, BDA complexes can successfully reconnect the stream to its floodplain, and ultimately increase SW-GW exchange at the floodplain scale by allowing flow reversals to occur and by reducing the GW to stream hydraulic gradient. Although BDAs built with fence posts, willow branches, sediments and small boulders are naturally porous and require regular maintenance, this study also highlights the viability of small BDAs as a restoration practice to enhance landscape resilience to drought and high flow events in deeply incised channels where beavers would not come back naturally.     

Exploring the application of topographic indices in urban areas as indicators of pluvial flooding locations

In urban areas, the presence of impervious surfaces limits natural drainage and routes water to stormwater infrastructure with finite capacity, making these areas especially prone to flooding. Though large floods are responsible for endangering lives and causing extensive damage, there is growing evidence that more frequent floods with shallow water depths, termed nuisance flooding, can have a high cumulative cost and many direct and indirect damages. To determine whether locations of nuisance flooding may be linked to topography, we took a parsimonious, spatially distributed approach to explore whether high topographic index values co-occur with citizen-reported nuisance flooding. We obtained nuisance flood reports from the municipal data service 311 for several watersheds in New York City and Baltimore, USA. Our analysis tested two topographic indices (TI)—topographic wetness index (TWI) and sink depth—both calculated from high-resolution (~1 m) digital elevation models. Generally, our findings suggest that not all but many locations of reported flooding tend to coincide with deep sinks or large TWI. However, nuisance flooding reports most commonly coincided with deep sinks and high TWI when using a maximum, instead of coincident, TI value extracted around each reported location of flooding, an approach we used due to the uncertainty in location accuracy of flooding reports. Overall, our results show promise for application of topographic indices, typically applied in more natural settings, as indicators of nuisance flooding areas in urbanized environments. Although limitations to this approach exist, the application of TIs and crowd-sourced reporting in tandem could provide a useful starting point for mapping flood-prone areas in many cities with technologically adept community members and ample geospatial data.     

Ecotoxicity assessment of natural attenuation effects at a historical dumping site in the western Baltic Sea

During the late 1950s and early 1960s of the past century, industrial waste material highly enriched in various contaminants (heavy metals, PAHs) was dumped in the inner Mecklenburg Bay, western Baltic Sea. Large-scale shifts in the spatial distribution of heavy metals in surface sediments were mapped by geochemical monitoring in the mid-1980s and 12 years later in 1997. A further study in 2001 was designed to investigate the small-scale spatial distribution of contaminants inside, on top of, and around the historical dumping ground and to examine possible effects to benthic organisms (Arctica islandica, microbiological toxicity tests). The site is located within an area characterized by a discontinuous deposition of fine sediments, but net sedimentation rate of about 3mm/y led ultimately to a gradual coverage of the dumped material. During the first decades after the dumping, about half of the originally dumped material was already spread over the surrounding area as a result of sediment dynamics and re-suspension processes. Recent sediments of the historical dump site are still significantly enriched in heavy metals and PAHs. Microbiological toxicity test results were positive, though uptake of contaminants in mussels was found to deviate only slightly from that of a non-contaminated reference station due to a gradual dilution and decrease in bioavailability of the contaminants.