Climate change in lowland Central America during the late deglacial and early Holocene

The transition from arid glacial to moist early Holocene conditions represented a profound change in northern lowland Neotropical climate. Here we report a detailed record of changes in moisture availability during the latter part of this transition (～11 250 to 7500 cal. yr BP) inferred from sediment cores retrieved in Lake Petén Itzá, northern Guatemala. Pollen assemblages demonstrate that a mesic forest had been largely established by ～11 250 cal. yr BP, but sediment properties indicate that lake level was more than 35 m below modern stage. From 11 250 to 10 350 cal. yr BP, during the Preboreal period, lithologic changes in sediments from deep‐water cores (>50 m below modern water level) indicate several wet–dry cycles that suggest distinct changes in effective moisture. Four dry events (designated PBE1‐4) occurred centred at 11 200, 10 900, 10 700 and 10 400 cal. yr BP and correlate with similar variability observed in the Cariaco Basin titanium record and glacial meltwater pulses into the Gulf of Mexico. After 10 350 cal. yr BP, multiple sediment proxies suggest a shift to a more persistently moist early Holocene climate. Comparison of results from Lake Petén Itzá with other records from the circum‐Caribbean demonstrates a coherent climate response during the entire span of our record. Furthermore, lowland Neotropical climate during the late deglacial and early Holocene period appears to be tightly linked to climate change in the high‐latitude North Atlantic. We speculate that the observed changes in lowland Neotropical precipitation were related to the intensity of the annual cycle and associated displacements in the mean latitudinal position of the Intertropical Convergence Zone and Azores–Bermuda high‐pressure system. This mechanism operated on millennial‐to‐submillennial timescales and may have responded to changes in solar radiation, glacial meltwater, North Atlantic sea ice, and the Atlantic meridional overturning circulation (MOC). Copyright © 2005 John Wiley & Sons, Ltd.     

Future trends in transport and fate of diffuse contaminants in catchments,with special emphasis on stable isotope applications

A summary is provided of the first of a series of proposed Integrated Science Initiative workshops supported by the UNESCO International Hydrological Programme. The workshop brought together hydrologists, environmental chemists, microbiologists, stable isotope specialists and natural resource managers with the purpose of communicating new ideas on ways to assess microbial degradation processes and reactive transport at catchment scales. The focus was on diffuse contamination at catchment scales and the application of compound‐specific isotope analysis (CSIA) in the assessment of biological degradation processes of agrochemicals. Major outcomes were identifying the linkage between water residence time distribution and rates of contaminant degradation, identifying the need for better information on compound specific microbial degradation isotope fractionation factors and the potential of CSIA in identifying key degradative processes. In the natural resource management context, a framework was developed where CSIA techniques were identified as practically unique in their capacity to serve as distributed integrating indicators of process across a range of scales (micro to diffuse) of relevance to the problem of diffuse pollution assessment. Copyright © 2006 John Wiley & Sons, Ltd.     

The Relative Lyapunov Indicator: An Efficient Method of Chaos Detection

A recently introduced chaos detection method, the relative Lyapunov indicator (RLI) is investigated in the cases of symplectic mappings and of a continuous Hamiltonian system. It is shown that the RLI is an efficient and easy-to-calculate numerical tool in determining the true nature of individual orbits, and in separating ordered and regular regions of the phase space of dynamical systems. An application of the RLI for stability investigations of some recently discovered exoplanetary systems is also presented.     

Ni/S/Cl systematics and the origin of impact‐melt glasses in Martian meteorite Elephant Moraine 79001

Martian meteorite Elephant Moraine A79001 (EET 79001) has received considerable attention for the unusual composition of its shock melt glass, particularly its enrichment in sulfur relative to the host shergottite. It has been hypothesized that Martian regolith was incorporated into the melt or, conversely, that the S‐enrichment stems from preferential melting of sulfide minerals in the host rock during shock. We present results from an electron microprobe study of EET 79001 including robust measurements of major and trace elements in the shock melt glass (S, Cl, Ni, Co, V, and Sc) and minerals in the host rock (Ni, Co, and V). We find that both S and major element abundances can be reconciled with previous hypotheses of regolith incorporation and/or excess sulfide melt. However, trace element characteristics of the shock melt glass, particularly Ni and Cl abundances relative to S, cannot be explained either by the incorporation of regolith or sulfide minerals. We therefore propose an alternative hypothesis whereby, prior to shock melting, portions of EET 79001 experienced acid‐sulfate leaching of the mesostasis, possibly groundmass feldspar, and olivine, producing Al‐sulfates that were later incorporated into the shock melt, which then quenched to glass. Such activity in the Martian near‐surface is supported by observations from the Mars Exploration Rovers and laboratory experiments. Our preimpact alteration model, accompanied by the preferential survival of olivine and excess melting of feldspar during impact, explains the measured trace element abundances better than either the regolith incorporation or excess sulfide melting hypothesis does.     

Snow accumulation and ablation response to changes in forest structure and snow surface albedo after attack by mountain pine beetle

This study quantified changes in snow accumulation and ablation with forest defoliation in a young pine stand attacked by mountain pine beetle, a mature mixed species stand, and a clearcut in south‐central British Columbia. From 2006 to 2012, as trees in the pine stand turned from green to grey, average canopy transmittance increased from 27% to 49%. In the mixed stand, transmittance remained constant at 19%. In 2009, the year of greatest needle loss, average snow surface litter cover in the pine stand was 29% (range 4 – 61%), compared to ≤9% in other years and over double that in the mixed stand. By 2012, litter accumulation in the now‐grey pine stand was only a sixth of that in the mixed stand. Inter‐annual variability in the weather had the greatest effect on snow accumulation and ablation, with the greatest differences between both forested stands and the clearcut occurring in 2010, the year of lowest SWE. Differences in snow accumulation between the pine and mixed stand increased in 2010 as a result of decreased snow interception in the young stand after needlefall. Average ablation rates in the attacked stand were most different from the mixed stand in 2009 and 2012, the years with the largest and smallest over‐winter needle loss, respectively. This study shows that grey, non‐pine, and understory trees moderate snow response to changes in the main canopy. It also highlights the complex interrelationships between ecohydrological processes key to assessing watershed response to forest cover loss in snow dominated hydrologic regimes. Copyright © 2012 John Wiley & Sons, Ltd.     

Were last glacial climate events simultaneous between Greenland and France? A quantitative comparison using non‐tuned chronologies

Several large abrupt climate fluctuations during the last glacial have been recorded in Greenland ice cores and archives from other regions. Often these Dansgaard–Oeschger events are assumed to have been synchronous over wide areas, and then used as tie‐points to link chronologies between the proxy archives. However, it has not yet been tested independently whether or not these events were indeed synchronous over large areas. Here, we compare Dansgaard–Oeschger‐type events in a well‐dated record from southeastern France with those in Greenland ice cores. Instead of assuming simultaneous climate events between both archives, we keep their age models independent. Even these well‐dated archives possess large chronological uncertainties that prevent us from inferring synchronous climate events at decadal to multi‐centennial time scales. If possible, comparisons between proxy archives should be based on independent, non‐tuned time‐scales. Copyright © 2009 John Wiley & Sons, Ltd.     

Assessing the effects of post‐pine beetle forest litter on snow albedo

The effect of forest litter on snow surface albedo has been subject to limited study, mainly in the hardwood‐dominated forests of the northeastern United States. Given the recent pine beetle infestation in Western North America and associated increases in litter production, this study examines the effects of forest litter on snow surface albedo in the coniferous forests of south‐central British Columbia. Measured changes in canopy transmittance provide an indication of canopy loss or total litterfall over the winter of 2007–2008. Relationships between percent litter cover, an index of albedo, snow depth, and snow ablation during the 2008 melt season are compared between a mature, young, and clearcut coniferous stand. Results indicate a strong feedback effect between canopy loss and subsequent enhanced shortwave transmittance, and litter accumulation on the snow surface from that canopy loss. However, this relationship is confounded by other variables concurrently affecting albedo. While results suggest that a relatively small percent litter cover can have a significant effect on albedo and ablation, further research is underway to extract the litter signal from that of other factors affecting albedo, particularly snow depth. Copyright © 2010 John Wiley & Sons, Ltd.