A sediment chronology of the eutrophication of Chesapeake Bay

Chesapeake Bay sediments were examined for biogeochemical evidence of eutrophication trends using two mesohaline sediment cores. Measurements of ²¹⁰Pb geochronology and sediment profiles of organic carbon, nitrogen, organic phosphorus, inorganic phosphorus, and biogenis silica (BSi) were used used to develop temporal concentration trends. Recent sediments have 2–3 times as much organic carbon and nitrogen as sediments from 80 to 100 yr ago, but the increases result from both changes in organic matter deposition and time-dependent changes in organic matter decomposition rates. Despite increases in phosphorus loading, no major changes in phosphorus concentration were noted throughout most of the century; anthropogenic phosphorus deposition, though not evident in sulfidic mid-bay sediments, must occur in more oxidizing sediment environments in both the northern and southern bays. Temporal trends in BSi concentrations are much less evident and the lack of substantial increases in this century suggest that BSi inputs may be capped by late spring-summer Si limitation.     

Rare earth geochemistry of fused ophiolitic and alpine lherzolites

Partial fusion hypotheses have been proposed for the origin of lherzolite-harzburgite alpine peridotite associations. Analyzed lherzolites from Othris, Ronda, Lanzo and Beni Bouchera, have light REE depleted to chondritic REE abundances, and clinopyroxenes contain most of the REE relative to depleted olivine and orthopyroxene. Variation in the level of REE enrichment within these lherzolites indicates mantle heterogeneity probably caused by partial melting processes. The Beni Bouchera spinel lherzolite and the Othris plagioclase lherzolite are the best candidates for relatively undepleted mantle based on REE studies. Fractional fusion calculations (15–25%) reveal that partial melts have REE characteristics somewhat similar to oceanic tholeiites. Conversely, computed source peridotites from oceanic tholeiites (Schilling, 1975) are similar to the alpine lherzolites reported here. Alpine lherzolites are, however, depleted in trace elements (K, Rb, Sr and Ba, Menzies and Murthy 1976). Since the lherzolites have an undepleted major, minor and REE chemistry close to that of pyrolite, the lost trace element-rich fraction must represent a small degree of melting. It is proposed that alpine lherzolites are residue left after the loss of a nephelinitic/alkalic fraction, ([Ce/Yb]_N=2.0–4.01) representing a small degree of partial fusion. This labile fraction may have existed as an intergranular phase or hydrous mineral prior to melting.     

The Reykjanes Ridge: structure and tectonics of a hot-spot-influenced, slow-spreading ridge, from multibeam bathymetry, gravity and magnetic investigations

We report a comprehensive morphological, gravity and magnetic survey of the oblique- and slow-spreading Reykjanes Ridge near the Iceland mantle plume. The survey extends from 57.9°N to 62.1°N and from the spreading axis to between 30 km (3 Ma) and 100 km (10 Ma) off-axis; it includes 100 km of one arm of a diachronous ‘V-shaped' or ‘chevron' ridge. Observed isochrons are extremely linear and 28° oblique to the spreading normal with no significant offsets. Along-axis there are ubiquitous, en-echelon axial volcanic ridges (AVRs), sub-normal to the spreading direction, with average spacing of 14 km and overlap of about one third of their lengths. Relict AVRs occur off-axis, but are most obvious where there has been least axial faulting, suggesting that elsewhere they are rapidly eroded tectonically. AVRs maintain similar plan views but have reduced heights nearer Iceland. They are flanked by normal faults sub-parallel to the ridge axis, the innermost of which occur slightly closer to the axis towards Iceland, suggesting a gradual reduction of the effective lithospheric thickness there. Generally, the amplitude of faulting decreases towards Iceland. We interpret this pattern of AVRs and faults as the response of the lithosphere to oblique spreading, as suggested by theory and physical modelling. An axial, 10–15 km wide zone of high acoustic backscatter marks the most recent volcanic activity. The zone's width is independent of the presence of a median valley, so axial volcanism is not primarily delimited by median valley walls, but is probably controlled by the lateral distance that the oblique AVRs can propagate into off-axis lithosphere. The mantle Bouguer anomaly (MBA) exhibits little mid- to short-wavelength variation above a few milliGals, and along-axis variations are small compared with other parts of the Mid-Atlantic Ridge. Nevertheless, there are small axial deeps and MBA highs spaced some 130 km along-axis that may represent subdued third-order segment boundaries. They lack coherent off-axis traces and cannot be linked to Oligocene fracture zones on the ridge flanks. The surveyed chevron ridge is morphologically discontinuous, comprising several parallel bands of closely spaced, elevated blocks. These reflect the surrounding tectonic fabric but have higher fault scarps. There is no evidence for off-axis volcanism or greater abundance of seamounts on the chevron. Free-air gravity over it is greater than expected from the observed bathymetry, suggesting compensation via regional rather than pointwise isostasy. Most of the observed variation along the ridge can be ascribed to varying distance from the mantle plume, reflecting changes in mantle temperature and consequently in crustal thickness and lithospheric strength. However, a second-order variation is superimposed. In particular, between 59°30′N and 61°30′N there is a minimum of large-scale faulting and crustal magnetisation, maximum density of seamounts, and maximum axial free-air gravity high. To the north the scale of faulting increases slightly, seamounts are less common, and there is a relative axial free-air low. We interpret the 59°30′N to 61°30′N region as where the latest chevron ridge intersects the Reykjanes Ridge axis, and suggest that the morphological changes that culminate there reflect a local temperature high associated with a transient pulse of high plume output at its apex.     

A study of interacting multi-scale wind systems, Canterbury plains, New Zealand

Summary The wind regime of the Canterbury region, New Zealand, is composed of several interacting multi-scale wind systems all of which show strong diurnal periodicity. The dynamic orographic effect of the Southern Alps on the prevailing westerly flow results in perturbations to the pressure field and localized antitriptic airflow. Superimposed on this larger scale process are thermotopographic effects resulting from both regional and local land-sea thermal contrasts and slope heating. These processes act within an hierarchy of scales to produce a complex wind regime characterized by marked temporal variability, a layered vertical structure and the frequent occurrence of convergence lines and shear zones. The synergistic nature of the forcing mechanisms and the tendency for nocturnal decoupling of the boundary layer due to stability variations makes it difficult to differentiate and label discrete wind components.Attempts to simulate this regime using the Colorado State University mesoscale model showed that the model was unable to adequately resolve both the dynamic orographic effect and the local thermotopographic effect because of their differing scales of influence. These results suggest that a more holistic approach to both empirical and theoretical studies in such environments is required if more accurate wind field forecasting is to be achieved.

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Eine Studie zusammenwirkender Windsysteme verschiedener Größenordnungen, Canterbury Plains, Neuseeland

Zusammenfassung Das Windregime des Gebiets von Canterbury, Neuseeland, setzt sich aus verschiedenen zusammenwirkenden Windsystemen verschiedener Größenordnungen zusammen, die alle einen starken Tagesgang aufweisen. Der dynamisch-orographische Effekt der neuseeländischen Alpen auf die vorherrschende Westströmung führt zu Störungen im Druckfeld und lokalen Luftbewegungen im Lee. Diesem großräumigen Prozeß sind thermisch-topographische Effekte überlagert, die sowohl durch regionale als auch lokale thermische Unterschiede zwischen Land und Meer und die Erwärmung der Hangregion hervorgerufen werden. Die Vorgänge spielen sich in einer Hierarchie von Größenordnungen ab. Sie erzeugen ein kompliziertes Windsystem, das durch hohe zeitliche Variabilität, eine schichtweise thermische Struktur und häufige Konvergenz- und Scherungszonen gekennzeichnet ist. Die synergetische Natur der Antriebe und die Tendenz zum nächtlichen Entkoppeln der planetaren Grenzschicht aufgrund von Stabilitätsschwankungen macht es schwer, die einzelnen Windkomponenten zu trennen und zuzuordnen.Die Versuche, dieses Regime mit Hilfe des Mesoscale-Modells der Colorado State University zu simulieren, zeigten, daß es aufgrund der verschiedenen Größenordnungen des Einflusses nicht geeignet war, gleichzeitig den dynamisch-orographischen und den thermo-topographischen Effekt zu reproduzieren. Diese Ergebnisse legen sowohl für empirische wie für theoretische Untersuchungen einen holistischeren Ansatz nahe, um eine genauere Prognose des Windfeldes zu ermöglichen.

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With 11 Figures     

Isotopic composition of surface-correlated chromium in Apollo 16 lunar soils

We have analyzed by thermal ionization mass spectrometry (TIMS) the isotopic composition of Cr in five progressive etches of size-sorted plagioclase grains separated from lunar soils 60601 and 62281. Aliquots of the etch solutions were spiked for isotopic dilution (ID) analysis of Cr and Ca. The Ca ID data indicate that the initial etch steps represent dissolution of an average 0.1 to 0.2 μm depth from the grain surfaces, the approximate depth expected for implanted solar wind. The Cr/Ca ratio in the initial etches is several fold higher than that expected for bulk plagioclase composition, but in subsequent etches decreases to approach the bulk value. This indicates a source of Cr extrinsic to the plagioclase grains, surface-correlated and resident in the outermost fraction of a μm, which we provisionally identify as solar wind Cr. The surface-correlated Cr is isotopically anomalous and by conventional TIMS data reduction has approximately 1 permil excess ⁵⁴Cr and half as great excess ⁵³Cr. In successive etches, as the Cr/Ca ratio decreases and approaches the bulk plagioclase value, the magnitude of the apparent anomalies decreases approaching normal composition. If these results do indeed characterize the solar wind, then either the solar wind is enriched in Cr due to spallation in the solar atmosphere, or the Earth and the various parent bodies of the meteorites are isotopically distinct from the Sun and must have formed from slightly different mixes of presolar materials. Alternative interpretations include the possibility that the anomalous Cr is meteoritic rather than solar or that the observed (solar) Cr is normal except for a small admixture of spallation Cr generated on the Moon. We consider these latter possibilities less likely than the solar wind interpretation. However, they cannot be eliminated and remain working hypotheses.     

Impacts of Climate Change on the Global Forest Sector

The path and magnitude of future anthropogenic emissions of carbon dioxide will likely influence changes in climate that may impact the global forest sector. These responses in the global forest sector may have implications for international efforts to stabilize the atmospheric concentration of carbon dioxide. This study takes a step toward including the role of global forest sector in integrated assessments of the global carbon cycle by linking global models of climate dynamics, ecosystem processes and forest economics to assess the potential responses of the global forest sector to different levels of greenhouse gas emissions. We utilize three climate scenarios and two economic scenarios to represent a range of greenhouse gas emissions and economic behavior. At the end of the analysis period (2040), the potential responses in regional forest growing stock simulated by the global ecosystem model range from decreases and increases for the low emissions climate scenario to increases in all regions for the high emissions climate scenario. The changes in vegetation are used to adjust timber supply in the softwood and hardwood sectors of the economic model. In general, the global changes in welfare are positive, but small across all scenarios. At the regional level, the changes in welfare can be large and either negative or positive. Markets and trade in forest products play important roles in whether a region realizes any gains associated with climate change. In general, regions with the lowest wood fiber production cost are able to expand harvests. Trade in forest products leads to lower prices elsewhere. The low-cost regions expand market shares and force higher-cost regions to decrease their harvests. Trade produces different economic gains and losses across the globe even though, globally, economic welfare increases. The results of this study indicate that assumptions within alternative climate scenarios and about trade in forest products are important factors that strongly influence the effects of climate change on the global forest sector.     

The social impact of anthropogenic landscape modification in the Río Verde drainage basin,Oaxaca, Mexico

This article examines the effects of anthropogenic landscape modification in the upper drainage basin of the Río Verde on environments and populations of the lower Río Verde Valley, Oaxaca, Mexico. Archaeological and geomorphological research conducted by the Río Verde Formative Project indicates that Late Formative (400–100 B.C.) population growth and agricultural intensification in the Oaxaca and Nochixtlán valleys increased erosion and runoff from the Verde's upper drainage. This geomorphic change in the highland valleys altered the drainage system and led to increased flooding and alluviation in the lower Río Verde Valley. The environmental effect in the lower valley may have increased the agricultural potential of the region, leading to population growth, and at least indirectly to social change.     

Coherence of Coronal Mass Ejections in Near-Earth Space

Time–distance helioseismology is a set of powerful tools to study localized features below the Sun’s surface. Inverse methods are needed to robustly interpret time–distance measurements, with many examples in the literature. However, techniques that utilize a more statistical approach to inferences, and that are broadly used in the astronomical community, are less-commonly found in helioseismology. This article aims to introduce a potentially powerful inversion scheme based on Bayesian probability theory and Monte Carlo sampling that is suitable for local helioseismology. We first describe the probabilistic method and how it is conceptually different from standard inversions used in local helioseismology. Several example calculations are carried out to compare and contrast the setup of the problems and the results that are obtained. The examples focus on two important phenomena that are currently outstanding issues in helioseismology: meridional circulation and supergranulation. Numerical models are used to compute synthetic observations, providing the added benefit of knowing the solution against which the results can be tested. For demonstration purposes, the problems are formulated in two and three dimensions, using both ray- and Born-theoretical approaches. The results seem to indicate that the probabilistic inversions not only find a better solution with much more realistic estimation of the uncertainties, but they also provide a broader view of the range of solutions possible for any given model, making the interpretation of the inversion more quantitative in nature. The probabilistic inversions are also easy to set up for a broad range of problems, and they can take advantage of software that is publicly available. Unlike the progress being made in fundamental measurement schemes in local helioseismology that image the far side of the Sun, or have detected signatures of global Rossby waves, among many others, inversions of those measurements have had significantly less success. Such statistical methods may help overcome some of these barriers to move the field forward.