Interpretation of210Pb profiles and verification of the CRS dating model in PIRLA project lake sediment cores

This paper reports results and analysis of²¹⁰Pb-activity measurements in 51 lake-sediment cores from 32 lakes in the four PIRLA (Paleoecological Investigations of Recent Lake Acidification) project regions (Adirondack Mountains [New York], Northern New England, Northern Florida, and the Northern Great Lakes States). General application of the Constant Rate of Supply (Constant Flux) model for²¹⁰Pb dating is valid for lakes in the PIRLA study, although application of the model is equivocal in a few lakes.²¹⁰Pb inventories and profiles are replicable among closely spaced cores within a lake. Specific²¹⁰Pb activity in surface sediments is negatively correlated with bulk sediment accumulation rate in seepage lakes, but not in drainage lakes. Drainage lakes with lower pH have lower unsupported²¹⁰Pb inventories in sediments, but the relationship does not occur in seepage lakes.²¹⁰Pb profiles in only seven of the cores, all from either the Adirondacks or the northern Great Lakes states, exhibit exponential decay curves. Deviations from an exponential profile include a flattening of the profile in the top few cm or excursions of one or a few measurements away from an exponential curve.²¹⁰Pb dates typically agree with other chronostratigraphic markers, most of which are subject to greater uncertainty. Several hypotheses, including sediment mixing, hydrologic regime, sediment focusing, and acidification, are proposed to explain variation of²¹⁰Pb distribution among lakes and regions. Hydrologic factors exert control on unsupported²¹⁰Pb inventories in PIRLA lakes, and there is a strong focusing effect in drainage lakes but a weak focusing effect in seepage lakes.This is the third of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.     

Epithermal environments and styles of mineralization: Variations and their causes, and guidelines for exploration

Epithermal precious- and base-metal deposits are diverse, reflecting the different tectonic, igneous and structural settings in which they occur, the complexities of their local setting, and the many processes involved in their formation. Most epithermal deposits form at shallow crustal levels where abrupt changes in physical and chemical conditions result in metal deposition and attendant hydrothermal alteration. The principal factors that influence the conditions prevailing in the epithermal environment, and which ultimately determine the sites and character of mineralization, include: geology (structure, stratigraphy, intrusions and rock type, which affect the style and degree of permeability and the reactivity of the host); pressure and temperature (which in the epithermal environment are related on the boiling point with depth curve); hydrology (the relationship between permeability and topography which governs fluid flow, and discharge/recharge characteristics, as well as access of steam-heated waters); chemistry of the mineralizing fluid (which determines the metal-carrying capacity, as well as the associated vein and alteration assemblage); and syn-hydrothermal development of permeability and/or changes in hydraulic gradients.Many attempts have been made to classify epithermal deposits based on mineralogy and alteration, the host rocks, deposit form, genetic models, and standard deposits. All have their strengths and weaknesses. We prefer a simple approach using the fundamental fluid chemistry (high or low sulfidation, reflecting relatively oxidized or reduced conditions, respectively) as readily inferred from vein and alteration mineralogy and zoning, together with the form of the deposit, and using comparative examples to clarify the character of the deposit.Guidelines for exploration vary according to the scale at which work is conducted, and are commonly constrained by a variety of local conditions. On a regional scale the tectonic, igneous and structural settings can be used, together with assessment of the depth of erosion, to select areas for project area scale exploration. At project area scale, direct (i.e. geochemical) or indirect guidelines may be used. Indirect methods involve locating and interpreting hydrothermal alteration as a guide to ore, with the topographic and hydrologic reconstruction of the system being of high priority. These pursuits may involve mineralogic, structural, geophysical or remote sensing methods. On a prospect scale, both direct and indirect methods may be used; however, they can only be effective in the framework of a sound conceptual understanding of the processes that occur in the epithermal environment, and the signatures they leave.     

Effects of soil moisture dynamics on slope failure at Hyrum Reservior,Utah

Field observations of shoreline conditions at Hyrum Reservoir, Utah, were conducted during the summers of 1991 to 1993. A process of bluff retreat is described for a multiple-layered bluff environment of sand and clay layers. Failure is initiated by wetting and drying of clay sediments, which produces horizontal cracks within bluff material. These cracks appear to penetrate to a depth of approximately 100-150 mm before initiating vertical cracking in the sediments. The vertical cracks are propagated by continued drying of the surface sediment, ultimately leading to failure of the bluff material. The physical dimensions of sediment blocks succumbing to this mechanism range from a few hundred millimetres up to 3 m on a side, with a depth of approximately 100-150 mm. The mechanism described here appears to operate optimally when the supply of subsurface moisture is abundant and nearly continuous throughout the spring and early summer. Reservoir draw-down, large capillary fringe effects in the bluff and periodic wetting from upslope undrained hollows are the dominant moisture controls at this site. Moisture delivery to the face is strongly influenced by anisotropy of saturated hydraulic conductivity in the alternating clay and sand layers and related differences in sediment texture.     

Modification of the quality of water injected into Louisiana gulf coast sands: Effects of cation exchange

Interest in artificially recharging selected shallow sands in South Louisiana with fresh water has been stimulated by the desire to retard contamination of municipal groundwater supplies by brackish water, to retard ground subsidence and decrease pumping lifts, and to develop emergency subsurface supplies of potable water for communities dependent on surface waters susceptible to contamination. Results of field experiments, laboratory work, and model calculations demonstrate that ion exchange reactions involving clays dispersed in aquifer sands can be expected to modify significantly the composition of waters injected into Gulf Coast sediments. As little as 0.1 weight percent smectite (montmorillonite) can remove, by exchange with absorbed Na, a significant fraction of the dissolved Ca and Mg present in the injected water. The hardness of the water is thus reduced, which may be a desirable modification in water quality. Exchange occurs as fast as the fluids can be pumped into or out of the aquifer, and the water-softening capacity of the aquifer can be restored by allowing sodium-rich native pore waters to sweep back over the dispersed clays. Each acre of an aquifer 50 feet thick and containing 0.1 wt % smectite could soften half a million gallons of injected Mississippi River water. Many individual Gulf Coast aquifers underlie tens of thousands of acres, and their potential softening capacity is thus enormous. Additional exchange processes involving adjacent aquitard shales presumably will operate over long-term periods. It is possible that Gulf Coast aquifers will be used at some point in the future as processing plants to treat injected water to improve its quality for a variety of municipal and industrial purposes.     

Fish eggs and larvae of the Chesapeake Bay: Observations on the problem of identifying field mortalities of striped bass (Morone saxatilis) larvae from preserved samples

This study considered the rate at which striped bass larvae disintegrate following death as well as the ability of experienced taxonomists to distinguish larvae which were liver or dead on collection from preserved samples. The rate of disintegration was dependent on age (size) of the larvae with pro-larvae being unrecognizable in 8 to 16 h following death (17 to 19°C). Post-larvae decomposed somewhat more slowly, but were unrecognizable as to species after 16 h. Experienced taxonomists were unable to distinguish pro- and post-larvae which had been live or dead prior to preservation. However, the viability of metamorphosed larvae could readily be distinguished.