Predicted relative sea-level changes (18,000 years B.P. to present) caused by late-glacial retreat of the Antarctic Ice Sheet

Predictions of global changes in relative sea level caused by retreat of the Antarctic Ice Sheet from its 18,000 yr B.P. maximum to its present size are calculated numerically. When combined with the global predictions of relative sea-level change resulting from retreat of the Northern Hemisphere ice sheets, the results may be compared directly to observations of sea-level change on the Antarctic continent as well as at distant localities. The comparison of predictions to the few observations of sea-level change on Antarctica supports the view that the Antarctic Ice Sheet was larger 18,000 years ago than at present. The contribution of the Antarctic Ice Sheet to the total eustatic sea-level rise is assumed to be 25 m (25% of the assumed total eustatic rise). If as little as 0.7 m of this 25-m rise occurred between 5000 yr B.P. and the present, few mid-oceanic islands would emerge. If the Antarctic Ice Sheet attained its present dimensions by 6000 yr B.P., however, and if the volume of the ocean has remained constant for the past 5000 years, numerous islands throughout the Southern Hemisphere would emerge. It is suggested that a thorough study of Pacific islands, believed by some to have slightly emerged shorelines of Holocene age, would yield useful information about ocean volume changes during the past 5000 years, and hence on the glacial history of the Antarctic Ice Sheet.     

Systematics of the spinel structure type

Systematic trends in the geometry of 149 oxide and 80 sulfide binary and ternary spinels have been examined from the standpoint of ionic radius and electronegativity. The mean ionic radii of the octahedral and tetrahedral cations, taken together, account for 96.9 and 90.5% of the variation in the unit cell parameter, a, of the oxides and sulfides, respectively, with the octahedral cation exerting by far the dominant influence in sulfides. The mean electronegativity of the octahedral cation exerts an additional, but small, influence on the cell edge of the sulfides. The equation a=(8/3√d)d _tet+(8/3)d _oct, where d _tet and d _oct are the tetrahedral and octahedral bond lengths obained from the sum of the ionic radii, accounts for 96.7 and 83.2% of the variation in a in the oxides and sulfides, respectively, again testifying to the applicability of the hard-sphere ionic model in the case of the spinel structure. Comparison of observed and calculated u values for 94 spinels indicates that up to 40% of the experimentally measured anion coordinates may be significantly in error. In addition to these compounds, u values are given for 52 spinels for which no data have previously been determined. Diagrams are presented for the rapid interpretation of the internal consistency of published data and the prediction of the structural parameters of hypothetical or partially studied spinels.     

Measurement of the effect of particle size variation on the detrital remanent magnetization to anhysteretic remanent magnetization ratio in some abyssal sediments

Deposition experiments have been carried out to measure the effect of particle size variation on the relationship between detrital remanent magnetization (DRM), anhysteretic remanent magnetization (ARM), and geomagnetic field intensity in sediments.Foraminiferal ooze from a box core taken in the Columbian Basin south of Jamaica was separated into several particle size ranges and redeposited in the laboratory in known magnetic fields. The intensity and alternating field (AF) demagnetization characteristics of the DRM and a subsequently applied ARM were compared for the various particle size ranges.The results show a variation of DRM/ARM ratios with particle size. The DRM intensities and directions indicate that particle sizes greater than 38 μm do not contribute significantly to the DRM of the total sediment. ARM intensities for larger particle sizes and particle size analysis of the whole sediment indicate that the fraction greater than 38 μm does make a significant contribution to the total ARM of a sample. Use of the DRM/ARM ratio in experimental measurements of magnetic paleointensity indicates that the method is unsatisfactory for sediments having a significant fraction of magnetic particles larger than 38 μm. It is also shown that, for sediments having a significant fraction of high-coercivity magnetic grains, the relative orientation of the ARM and DRM will affect ARM intensities, making necessary the use of corrected ARM and DRM intensities for ratio calculations.     

Biogeochemical and hydrographic controls on chromophoric dissolved organic matter distribution in the Pacific Ocean

Recent in situ observations of chromophoric dissolved organic material (CDOM) in the Pacific Ocean reveal the biogeochemical controls on CDOM and indicate predictive potential for open-ocean CDOM in diagnosing particulate organic matter (POM) remineralization rates within ocean basins. Relationships between CDOM and concentrations of dissolved oxygen, nutrients and inorganic carbon in the subthermocline waters of the Pacific reflect the relative influences of water mass ventilation and water-column oxidative remineralization. Apparent in situ oxygen utilization (AOU) accounts for 86% and 61% of variance in CDOM abundance, respectively, in Antarctic Intermediate Water and North Pacific Intermediate Water. In the deep waters of the Pacific below the zone of remineralization, AOU explains 26% of CDOM variability. The AOU–CDOM relationship results from competing biogeochemical and advective processes within the ocean interior. Dissolved organic carbon (DOC) is not statistically linked to the CDOM or AOU distributions, indicating that the majority of CDOM production occurs during the remineralization of sinking POM and thus potentially provides key information about carbon export. Once formed in the ocean interior, CDOM is relatively stable until it reaches the surface ocean where it is destroyed by solar bleaching. Susceptibility to bleaching confers an additional tracer-like quality for CDOM in water masses with active convection, such as mode waters that appear as subsurface CDOM minima. In the surface ocean, atypically low CDOM abundance highlights a region of unusually extreme oligotrophy: the subtropical South Pacific gyre. For these hyper-oligotrophic waters, the present CDOM observations are consistent with analysis of in situ radiometric observations of light attenuation and reflectance, demonstrating the accuracy of the CDOM spectrophotometric observations. Overall, we illustrate how CDOM abundance in the ocean interior can potentially diagnose rates of thermohaline overturning as they affect regional biogeochemistry and export. We further show how relative surface ocean CDOM abundances are driven in large part by processes occurring in the deep layers of the ocean. This is particularly significant for the interpretation of the global surface distribution of CDOM using satellite remote sensing.     

Three-dimensional benchmark for variable-density flow and transport simulation: matching semi-analytic stability modes for steady unstable convection in an inclined porous box

This benchmark for three-dimensional (3D) numerical simulators of variable-density groundwater flow and solute or energy transport consists of matching simulation results with the semi-analytical solution for the transition from one steady-state convective mode to another in a porous box. Previous experimental and analytical studies of natural convective flow in an inclined porous layer have shown that there are a variety of convective modes possible depending on system parameters, geometry and inclination. In particular, there is a well-defined transition from the helicoidal mode consisting of downslope longitudinal rolls superimposed upon an upslope unicellular roll to a mode consisting of purely an upslope unicellular roll. Three-dimensional benchmarks for variable-density simulators are currently (2009) lacking and comparison of simulation results with this transition locus provides an unambiguous means to test the ability of such simulators to represent steady-state unstable 3D variable-density physics.     

Assessment of channel dynamics,in-stream structures and post-project channel adjustments in North Carolina and its implications to effective stream restoration

Site assessment and monitoring data were analyzed for 26 stream restoration projects in North Carolina where the channel was reconfigured. Post-project changes in channel capacity were highly variable from site to site, but more than 60% of the projects underwent, on average at a given site, at least a 20% change in channel capacity. An analysis of site and basin geomorphology revealed that large post-construction adjustments were associated with highly dynamic stream channels characterized by a combination of high sediment transport capacity, large sediment supply, and/or easily eroded bank materials. In-stream structures along dynamic, reconfigured channels also exhibited high incidences of damage. Thus, the design and construction of channels in a state of equilibrium, which do not exhibit excessive erosion or deposition along highly dynamic rivers is currently problematic. In light of these findings, a conceptual framework based on geomorphic parameters is put forth to assess the likelihood of project success early in the design process to (1) eliminate high risk sites from consideration of channel reconfiguration and (2) improve upon the implemented management strategies that are ultimately used. It is also argued that where space permits an enhanced natural channel, adjustment approach is likely to be more effective than projects based on natural channel design.     

Modern influences on chironomid distribution in western Ireland: potential for palaeoenvironmental reconstruction

Ireland provides a unique setting for the study of past climates, as its climate is dominated by westerly airflow from the North Atlantic and readily responsive to changes in North Atlantic circulation patterns. Although there has been substantial research on Ireland’s past environments, quantitative palaeolimnological research, especially chironomid-based research, has been lacking. In order to further develop chironomid-based palaeolimnological investigations, a calibration set was constructed to determine the dominant environmental controls on modern chironomids in western Ireland. Chironomid subfossils were collected from surface sediments of 50 lakes. The lakes were characterised with 36 environmental variables, including physical attributes, lake water characteristics, lake sediment characteristics and land cover within each catchment. In this exploratory study, no specific environmental variable was targeted and lakes were chosen to span gradients of latitude, elevation, depth and trophic status. Redundancy analysis showed that six environmental variables—mean July air temperature, lake depth, dissolved organic carbon, and percentage catchment land cover of agriculture, peat bog and scrubland—captured a large and statistically significant portion of the variance in the chironomid data. July temperature and agricultural land cover were the most dominant environmental variables, with July temperature proving the most suitable for inference model development. A classical weighted-averaging model was developed to estimate July air temperature, with a coefficient of determination (r _jack ² ) of 0.60 and root mean square error of prediction (RMSEP) of 0.57 °C. Results suggest that summer temperature is the dominant influence on chironomid distribution across a wide variety of lake types, and the relatively small RMSEP should allow for more accurate reconstructions of Ireland’s relatively subdued Holocene temperature fluctuations.