ALBERT

Description: Mixing models are statistical tools that use biotracers to probabilistically estimate the contribution of multiple sources to a mixture. These biotracers may include contaminants, fatty acids, or stable isotopes, the latter of which are widely used in trophic ecology to estimate the mixed diet of consumers. Bayesian implementations of mixing models using stable isotopes (e.g. MixSIR, SIAR) are regularly used by ecologists for this purpose, but basic questions remain about when each is most appropriate. In this study, we describe the structural differences between common mixing model error formulations in terms of their assumptions about the predation process. We then introduce a new parameterization that unifies these mixing model error structures, as well as implicitly estimates the rate at which consumers sample from source populations (i.e. consumption rate). Using simulations and previously published mixing model datasets, we demonstrate that the new error parameterization outperforms existing models and provides an estimate of consumption. Our results suggest that the error structure introduced here will improve future mixing model estimates of animal diet. This article is protected by copyright. All rights reserved.

Description: Seismicity was monitored beneath the Krafla central volcano, NE Iceland, between 2009 and 2012 during a period of volcanic quiescence, when most earthquakes occured within the shallow geothermal field. The highest concentration of earthquakes is located close to the rock-melt transition zone as the IDDP-1 wellbore suggests, and decays quickly at greater depths. We recorded multiple swarms of microearthquakes, which coincide often with periods of changes in geothermal field operations, and found that about one third of the total number of earthquakes are repeating events. The event size distribution, evaluated within the central caldera, indicates average crustal values with b = 0.79 ± 0.04. No significant spatial b -value contrasts are resolved within the geothermal field nor in the vicinity of the drilled melt. Besides the seismicity analysis, focal mechanisms are calculated for 342 events. Most of these short-period events have source radiation patterns consistent with double-couple (DC) mechanisms. A few events are attributed to non-shear faulting mechanisms with geothermal fluids likely playing an important role in their source processes. Diverse faulting styles are inferred from DC events, but normal faulting prevails in the central caldera. The best-fitting compressional and tensional axes of DC mechanisms are interpreted in terms of the principal stress or deformation-rate orientations across the plate boundary rift. Maximum compressive stress directions are near-vertically aligned in different study volumes, as expected in an extensional tectonic setting. Beneath the natural geothermal fields, the least compressive stress axis is found to align with the regional spreading direction. In the main geothermal field both horizontal stresses appear to have similar magnitudes causing a diversity of focal mechanisms.

Description: ABSTRACT A series of radiocarbon dates from two Epipalaeolithic sites – Kharaneh IV and Ayn Qasiyya – in the Azraq Basin of eastern Jordan provide a new perspective on the chronology and settlement patterns of this part of southwest Asia during the Late Pleistocene. We discuss the implications to our understanding of the chronology of Late Pleistocene lithic industries, particularly in regard to current hypotheses for the abandonment of eastern Jordan's ‘mega-sites’, Kharaneh IV and Jilat 6. Modelling a series of accelerator mass spectrometry dates from Kharaneh IV indicates a much shorter span of occupation for the site than previously assumed by the size and density of its deposits. Given the high density of material accumulated over a relatively short time span, we show that Kharaneh IV was an aggregation site occupied intensively by a significant number of people, providing new perspectives on the east Jordanian phenomenon of Epipalaeolithic ‘mega-sites’.

Description: Apatite incorporates variable and significant amounts of halogens (mainly F and Cl) in its crystal structure which can be used to determine the initial F and Cl concentrations of magmas. The amount of chlorine in the apatite lattice also exerts an important compositional control on the degree of fission track annealing. Chlorine measurements in apatite have conventionally required electron probe microanalysis (EPMA). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is increasingly used in apatite fission-track dating to determine U concentrations and also in simultaneous U-Pb dating and trace element measurements of apatite. Apatite Cl measurements by ICP-MS would remove the need for EPMA but the high (12.97 eV) first ionisation potential makes analysis challenging. Apatite Cl data were acquired using two analytical set-ups: a Resonetics M-50 193 nm ArF Excimer laser coupled to an Agilent 7700x quadrupole ICP-MS (using a 26 μm spot with an 8 Hz repetition rate) and a Photon Machines Analyte Excite 193 nm ArF Excimer laser coupled to a Thermo Scientific iCAP Qc (using a 30 μm spot with a 4 Hz repetition rate). Chlorine concentrations were determined by LA-ICP-MS (1140 analyses in total) for nineteen apatite occurrences, and there is a comprehensive EPMA Cl and F dataset for thirteen of the apatite samples. The apatite sample suite includes different compositions representative of the range likely to be encountered in natural apatites, along with extreme variants including two end-member chlorapatites. Between twenty-six and thirty-nine isotopes were determined in each apatite sample corresponding to a typical analytical protocol for integrated apatite fission track (U and Cl contents) and U-Pb dating, along with REE and trace element measurements. 35 Cl backgrounds (present mainly in the argon gas) were ~ 45–65 kcps in the first set-up and ~ 4 kcps in the second set-up. 35 Cl background-corrected signals ranged from ~ 0 cps in end-member fluorapatite to up to ~ 90 kcps in end-member chlorapatite. Use of a collision cell in both analytical set-ups decreased the low mass sensitivity by approximately an order of magnitude without improving the 35 Cl signal to background ratio. A minor Ca isotope was used as the internal standard to correct for drift in instrument sensitivity and variations in ablation volume during sessions. The 35 Cl/ 43 Ca values for each apatite (10–20 analyses each) when plotted against the EPMA Cl concentrations yield excellently constrained calibration relationships, demonstrating the suitability of the analytical protocol and that routine apatite Cl measurements by ICP-MS are achievable. This article is protected by copyright. All rights reserved.

Description: Numerical models of ocean biogeochemistry are relied upon to make projections about the impact of climate change on marine resources and test hypotheses regarding the drivers of past changes in climate and ecosystems. In large areas of the ocean, iron availability regulates the functioning of marine ecosystems and hence the ocean carbon cycle. Accordingly, our ability to quantify the drivers and impacts of fluctuations in ocean ecosystems and carbon cycling in space and time relies on first achieving an appropriate representation of the modern marine iron cycle in models. When the iron distributions from thirteen global ocean biogeochemistry models are compared against the latest oceanic sections from the GEOTRACES programme we find that all models struggle to reproduce many aspects of the observed spatial patterns. Models that reflect the emerging evidence for multiple iron sources or subtleties of its internal cycling perform much better in capturing observed features than their simpler contemporaries, particularly in the ocean interior. We show that the substantial uncertainty in the input fluxes of iron results in a very wide range of residence times across models, which has implications for the response of ecosystems and global carbon cycling to perturbations. Given this large uncertainty, iron-fertilisation experiments based on any single current generation model should be interpreted with caution. Improvements to how such models represent iron scavenging and also biological cycling are needed to raise confidence in their projections of global biogeochemical change in the ocean.

Description: Based on hierarchical multi‐species models applied to avian mist‐netting data from six sites along a montane elevation gradient in a large protected area, we show advancement of avian breeding phenology (mean first capture date of juvenile birds) and increase in productivity (probability of capturing a juvenile bird) over 25 years with variable but declining snowfall and increasing spring temperatures. Breeding phenology depended on elevation and tracked climatic conditions. Productivity was higher in relatively warm springs, while productivity–elevation responses were variable among species; species with higher productivity at higher elevations tended to be species with recent range retractions. Abstract Climate variation has been linked to historical and predicted future distributions and dynamics of wildlife populations. However, demographic mechanisms underlying these changes remain poorly understood. Here, we assessed variation and trends in climate (annual snowfall and spring temperature anomalies) and avian demographic variables from mist‐netting data (breeding phenology and productivity) at six sites along an elevation gradient spanning the montane zone of Yosemite National Park between 1993 and 2017. We implemented multi‐species hierarchical models to relate demographic responses to elevation and climate covariates. Annual variation in climate and avian demographic variables was high. Snowfall declined (10 mm/year at the highest site, 2 mm at the lowest site), while spring temperature increased (0.045°C/year) over the study period. Breeding phenology (mean first capture date of juvenile birds) advanced by 0.2 day/year (5 days); and productivity (probability of capturing a juvenile bird) increased by 0.8%/year. Breeding phenology was 12 days earlier at the lowest compared to highest site, 18 days earlier in years with lowest compared to highest snowfall anomalies, and 6 d earlier in relatively warm springs (after controlling for snowfall effects). Productivity was positively related to elevation. However, elevation–productivity responses varied among species; species with higher productivity at higher compared to lower elevations tended to be species with documented range retractions during the past century. Productivity tended to be negatively related to snowfall and was positively related to spring temperature. Overall, our results suggest that birds have tracked the variable climatic conditions in this system and have benefited from a trend toward warmer, drier springs. However, we caution that continued warming and multi‐year drought or extreme weather years may alter these relationships in the future. Multi‐species demographic modeling, such as implemented here, can provide an important tool for guiding conservation of species assemblages under global change.