The pre-Colombian footprint on terrestrial nutrient cycling in Costa Rica: insights from phosphorus in a lake sediment record

The impact of pre-Columbian subsistence agriculture on soil nutrient cycling in the American tropics is poorly quantified. Paleolimnological research can address this deficit by documenting the temporal evolution of nutrient cycling in lake watersheds over different time scales. Here we describe our use of a chemical sequential extraction technique adapted from soil fertility research to discern geochemical fractions of phosphorus (P) in lake sediments that serve as proxies for landscape-scale soil nutrient status. These P fractions are mineral P (the original lithic source of bioavailable P), occluded P (mainly bound to soil oxides), and organic P (remains of organic matter production by plants). We applied the P fractionation technique to a lake sediment core from a small lake in southern Costa Rica, Laguna Zoncho. Prior analyses of microfossils and stable carbon isotopes in this core documented an approximately 3,000 year history of human occupation and agricultural activity in the Zoncho watershed, and shifts in diatom communities in the lake associated both with human impacts and with climate-driven changes in lake level. Our P analyses revealed relatively constant P geochemistry during the first approximately 2,500 years of the record, when other sedimentary proxies reveal forest clearance and maize agriculture of varying intensity. However, the period from approximately 500 to 100 years BP is marked by a drastic shift toward a P geochemistry dominated by occluded forms, with a concomitant decrease in the relative content of both the organic and mineral P forms. This interval coincides with post-Conquest depopulation and forest regeneration at the site, and with an apparent deepening of the lake caused by a shift toward a wetter climate. The dominance of the occluded P fraction during this interval is the opposite of the trend expected with such a climate shift, implicating human dynamics as the principal driver of the changes in soil nutrient status indicated by the P fractions in the Zoncho core. We propose that the entire P geochemical record is dominated by human-induced alteration of the soil nutrient cycles via agriculture and occupation, and that the only interval that reveals the “natural” nutrient status in the region is the short interval when the site is abandoned and surrounding forests regrow. These results for Laguna Zoncho reveal the close connection between even relatively low-technology human activities and soil nutrient status.     

Effect of fault reactivation on deformation of off-fault fractures near a generic deep geological repository in crystalline rock in Canada

Journal of Seismology - In this study, fault rupture and its effect on the deformation of the off-fault fractures are numerically simulated. The purpose of the analysis is to determine the distance...     

Experimental Study of the Effects of DNAPL Distribution on Mass Rebound

The release of stored dissolved contaminants from low permeability zones contributes to plume persistence beyond the time when dense nonaqueous phase liquid (DNAPL) has completely dissolved. This is fundamental to successfully meeting acceptable low concentrations in groundwater that are driven by site‐specific cleanup goals. The study goals were to assess the role of DNAPL entrapment morphology on mass storage and plume longevity. As controlled field studies are not feasible, two‐dimensional (2D) test tanks were used to quantify the significance of mass loading processes from source dissolution and stored mass rebound. A simple two‐layer soil domain representing a high permeable formation sand overlying a zone of lower permeability sand was used in the tests. DNAPL mass depletion through dissolution was monitored via X‐ray photon attenuation, and effluent samples were used to monitor the plume. These data enabled analysis of the DNAPL distribution, the dissolved plume, and the dissolved phase distribution within the low permeability layer. Tests in an intermediate tank showed that mass storage contributes substantially to plume longevity. Detectable effluent concentrations persisted long after DNAPL depletion. The small tank results indicated that the DNAPL morphology influenced the flow field and caused distinctive transport mechanisms contributing to mass storage. Zones of high DNAPL saturation at the interface between the low and high permeability layers exhibited flow bypassing and diffusion dominated transport into the low permeability layer. In the absence of a highly saturated DNAPL zone near the soil interface the contaminant penetrated deeper into the low permeability layer caused by a combination of advection and diffusion.     

Precision astrometry with SuperCOSMOS

We describe the new, fast, high-precision microdensitometer SuperCOSMOS. Some aspects of hardware and software design that enable high-precision astrometry from photographic plates are explained. We show that the positioning repeatability of the measuring machine is less than 0.1 μ μ m standard error in either coordinate, and the absolute positional accuracy is about 0.15 μ m standard error. Furthermore, measurements of the same plate in different orientations show that the sampling errors are small (e.g. ∼ 0.2 μ m, rising to ∼ 1.0 μ m at the plate limit, for stellar images in a IIIaJ emulsion), thus allowing the extraction of relative positional information from Schmidt plates at accuracies less than 1 μ m. We demonstrate that SuperCOSMOS is capable of measuring the positions of bright stars (i.e. those more than ∼ 4 mag above the plate limit) to a precision ∼ 0.5 μ m with survey–grade photographic plates employing fine–grained emulsions.     

A six-square-degree survey for Pleiades low-mass stars and brown dwarfs

We present the results of a six-square-degree Pleiades survey in I and Z , which is photometrically complete to approximately I _KP=19.2 ( I _Cou=19.6 in the Pleiades). We remove non-cluster contamination on the basis of proper motions and infrared photometry, and present 339 candidate cluster members, 30 of which are fainter than I _KP=17.5, and are thus strong brown-dwarf candidates.     

L dwarfs in the Hyades

We present the results of a proper motion survey of the Hyades to search for brown dwarfs, based on UKIRT Deep Sky Survey (UKIDSS) and Two-Micron All Sky Survey (2MASS) data. This survey covers  ∼275 deg²  to a depth of   K ∼ 15  mag, equivalent to a mass of  ∼0.05 M_⊙  assuming a cluster age of 625 Myr. The discovery of 12 L dwarf Hyades members is reported. These members are also brown dwarfs, with masses between  0.05 < M < 0.075 M_⊙  . A high proportion of these L dwarfs appear to be photometric binaries.