Geochronological database and classification system for age uncertainties in Neotropical pollen records.

The newly updated inventory of palaeoecological research in Latin America offers an important overview of sites available for multi-proxy and multi-site purposes. From the collected literature supporting this inventory, we collected all available age model metadata to create a chronological database of 5116 control points (e.g. 14C, tephra, fission track, OSL, 210Pb) from 1097 pollen records. Based on this literature review, we present a summary of chronological dating and reporting in the Neotropics. Difficulties and recommendations for chronology reporting are discussed. Furthermore, for 234 pollen records in northwest South America, a classification system for age uncertainties is implemented based on chronologies generated with updated calibration curves. With these outcomes age models are produced for those sites without an existing chronology, alternative age models are provided for researchers interested in comparing the effects of different calibration curves and age-depth modelling software, and the importance of uncertainty assessments of chronologies is highlighted. Sample resolution and temporal uncertainty of ages are discussed for different time windows, focusing on events relevant for research on centennial- to millennial-scale climate variability. All age models and developed R scripts are publicly available through figshare, including a manual to use the scripts

Scale-free percolation

Abstract We formulate and study a model for inhomogeneous long-range percolation on Zd. Each vertex x¿Zd is assigned a non-negative weight Wx, where (Wx)x¿Zd are i.i.d. random variables. Conditionally on the weights, and given two parameters a,¿>0, the edges are independent and the probability that there is an edge between x and y is given by pxy=1-exp{-¿WxWy/|x-y|a}. The parameter ¿ is the percolation parameter, while a describes the long-range nature of the model. We focus on the degree distribution in the resulting graph, on whether there exists an infinite component and on graph distance between remote pairs of vertices. First, we show that the tail behavior of the degree distribution is related to the tail behavior of the weight distribution. When the tail of the distribution of Wx is regularly varying with exponent t-1, then the tail of the degree distribution is regularly varying with exponent ¿=a(t-1)/d. The parameter ¿ turns out to be crucial for the behavior of the model. Conditions on the weight distribution and ¿ are formulated for the existence of a critical value ¿c¿(0,8) such that the graph contains an infinite component when ¿>¿c and no infinite component when ¿0, les arêtes sont indépendantes et la probabilité qu’il existe un lien entre x et y est pxy=1-exp{-¿WxWy/|x-y|a}. Le paramètre ¿ est le paramètre de percolation tandis que a caractérise la portée des interactions. Nous étudierons la distribution des degrés dans le graphe résultant et l’existence éventuelle d’une composante infinie ainsi que la distance de graphe entre deux sites éloignés. Nous montrons d’abord que la queue de la distribution des degrés est liée à la queue de la distribution des poids. Quand la queue de la distribution de Wx est à variation régulière d’indice t-1, alors la queue de la distribution des degrés est à variation régulière d’indice ¿=a(t-1)/d. Le paramètre ¿ s’avère crucial pour décrire le modèle. Des conditions sur la distribution des poids et de ¿ sont formulées pour l’existence d’une valeur critique ¿c¿(0,8) telle que le graphe contienne une composante infinie quand ¿>¿c et aucune composante infinie quand

Attracting non-executive directors to the board

Companies that are performing poorly from a commercial perspective and are struggling to tap into the advice and guidance of well-qualified non-executive directors need to consider new board members’ intrinsic motivations and the potential for enhancing their reputation if they are to improve the effectiveness of their board

Вулиця Басейна у Києві

Challenges and pitfalls for developing age models for long lacustrine sedimentary records are discussed and a comparison is made between radiocarbon dating, visual curve matching, and frequency analysis in the depth domain in combination with cyclostratigraphy. A core section of the high resolution 284-ka long temperature record developed from Lake Fuquene in the Northern Andes is used to explore four different age models (a-d). (a) A model based on 46 AMS C-14 dates of bulk sediment is hampered by low concentrations of organic carbon. (b) A model based on the comparison of the radiocarbon dated pollen record to the well-established record from Cariaco Basin using curve matching and visual tie points. For the upper 26 m of the core this approach yields an age interval of 28-59.5 ka. (c) Another age model is based on curve matching and the Intcal09 radiocarbon calibration curve, yielding an age range of 22.5-80.4 ka for the same core interval. (d) Finally, a model is developed based on spectral analysis in the depth domain of the temperature-related altitudinal migrations of the upper forest line. This method identifies periodicities without a pre-conceived idea of age. The main frequency of 9.07 m appears to reflect the 41-kyr orbital signal of obliquity, which is tuned to the filtered 41-kyr temperature signal from the well-constrained LRO4 marine benthic delta O-18 stack record (Lisiecki and Raymo, 2005). Using this last age-modelling approach, the upper 26 m of core Fq-9C yields a temporal interval of 27-133 ka. Problems arising from radiocarbon dating carbon poor sediments from a large lake are addressed and the visual curve matching approach is compared to the analysis of cyclic changes in sediment records in developing an age model. We conclude that the frequency analysis and cyclostratigraphy model is the most reliable one of the four approaches. These results show that cyclostratigraphy may provide a useful method for developing an age model for long terrestrial records including multiple orbital cycles. (C) 2014 Elsevier B.V. Al! rights reserved

Colobanthus quitensis (h.b.k.) bartl. (caryophyllaceae) en los andes colombianos

Colobantbus quitensis y Deschampsia antarctica son las dos únicas fanerogamas nativas presentes hoy dia en el continente antártico (Green, 1970). La primera tiene una distribución amplia en regiones templadas y frías de América Latina, desde la Tierra del Fuego hasta México (Moore, 1972).  En noviembre de 1978 se realizó una exploración geobotánica corta al Páramo Alto del Almorzadero en la Cordillera Oriental, Departamento de Santander; en el piso pantanoso de un vallecito a 3.900 m (El Tutal) se encontraron algunas maticas dispersas de Colobanthus quitensis (biótipo laxa) cerca de una corriente lenta de agua. El hallazgo anterior de Colobanthus quitensis en la región del Nevado del Ruiz en la Cordillera Central motivó la elaboración del presente trabajo, con el fin de dar a conocer los caracteres morfológicos, palinológicos y sociológicos de esta interesante especie

Climate variability and human impact in South America during the last 2000 years: synthesis and perspectives from pollen records

An improved understanding of present-day climate variability and change relies on high-quality data sets from the past 2 millennia. Global efforts to model regional climate modes are in the process of being validated against, and integrated with, records of past vegetation change. For South America, however, the full potential of vegetation records for evaluating and improving climate models has hitherto not been sufficiently acknowledged due to an absence of information on the spatial and temporal coverage of study sites. This paper therefore serves as a guide to high-quality pollen records that capture environmental variability during the last 2 millennia. We identify 60 vegetation (pollen) records from across South America which satisfy geochronological requirements set out for climate modelling, and we discuss their sensitivity to the spatial signature of climate modes throughout the continent. Diverse patterns of vegetation response to climate change are observed, with more similar patterns of change in the lowlands and varying intensity and direction of responses in the highlands. Pollen records display local-scale responses to climate modes; thus, it is necessary to understand how vegetation–climate interactions might diverge under variable settings. We provide a qualitative translation from pollen metrics to climate variables. Additionally, pollen is an excellent indicator of human impact through time. We discuss evidence for human land use in pollen records and provide an overview considered useful for archaeological hypothesis testing and important in distinguishing natural from anthropogenically driven vegetation change. We stress the need for the palynological community to be more familiar with climate variability patterns to correctly attribute the potential causes of observed vegetation dynamics. This manuscript forms part of the wider LOng-Term multi-proxy climate REconstructions and Dynamics in South America – 2k initiative that provides the ideal framework for the integration of the various palaeoclimatic subdisciplines and palaeo-science, thereby jump-starting and fostering multidisciplinary research into environmental change on centennial and millennial timescales