Dynamics of a mesoscale eddy off Cape Ann, Massachusetts in May 2005

Observations and numerical modeling indicate that a mesoscale anti-cyclonic eddy formed south of Cape Ann at the northern entrance of Massachusetts Bay (MB) during May 2005, when large river discharges in the western Gulf of Maine and two strong Nor'easters passing through the regions led to an unprecedented toxic Alexandrium fundyense bloom (red tide). Both model results and field measurements suggest that the western Maine Coastal Current separated from Cape Ann around May 7–8, and the eddy formed on around May 10. The eddy was trapped at the formation location for about a week before detaching from the coastline and moving slowly southward on May 17. Both model results and theoretical analysis suggest that the separation of the coastal current from the coast and subsequent eddy formation were initiated at the subsurface by an adverse pressure gradient between Cape Ann and MB due to the higher sea level set up by onshore Ekman transport and higher density in downstream MB. After the formation, the eddy was maintained by the input of vorticity transported by the coastal current from the north, and local vorticity generation around the cape by the horizontal gradients of wind-driven currents, bottom stress, and water density induced by the Merrimack River plume. Observations and model results indicate that the anti-cyclonic eddy significantly changed the pathway of nutrient and biota transport into the coastal areas and enhanced phytoplankton including Alexandrium abundances around the perimeter of the eddy and in the western coast of MB.     

Two fundamentally different types of submarine canyons along the continental margin of Equatorial Guinea

Most submarine canyons are erosive conduits cut deeply into the world’s continental shelves through which sediment is transported from areas of high coastal sediment supply onto large submarine fans. However, many submarine canyons in areas of low sediment supply do not have associated submarine fans and show significantly different morphologies and depositional processes from those of ‘classic’ canyons. Using three-dimensional seismic reflection and core data, this study contrasts these two types of submarine canyons and proposes a bipartite classification scheme.The continental margin of Equatorial Guinea, West Africa during the late Cretaceous was dominated by a classic, erosional, sand-rich, submarine canyon system. This system was abandoned during the Paleogene, but the relict topography was re-activated in the Miocene during tectonic uplift. A subsequent decrease in sediment supply resulted in a drastic transformation in canyon morphology and activity, initiating the ‘Benito’ canyon system. This non-typical canyon system is aggradational rather than erosional, does not indent the shelf edge and has no downslope sediment apron. Smooth, draping seismic reflections indicate that hemipelagic deposition is the chief depositional process aggrading the canyons. Intra-canyon lateral accretion deposits indicate that canyon concavity is maintained by thick (>150 m), dilute, turbidity currents. There is little evidence for erosion, mass-wasting, or sand-rich deposition in the Benito canyon system. When a canyon loses flow access, usually due to piracy, it is abandoned and eventually filled. During canyon abandonment, fluid escape causes the successive formation of ‘cross-canyon ridges’ and pockmark trains along buried canyon axes.Based on comparison of canyons in the study area, we recognize two main types of submarine canyons: ‘Type I’ canyons indent the shelf edge and are linked to areas of high coarse-grained sediment supply, generating erosive canyon morphologies, sand-rich fill, and large downslope submarine fans/aprons. ‘Type II’ canyons do not indent the shelf edge and exhibit smooth, highly aggradational morphologies, mud-rich fill, and a lack of downslope fans/aprons. Type I canyons are dominated by erosive, sandy turbidity currents and mass-wasting, whereas hemipelagic deposition and dilute, sluggish turbidity currents are the main depositional processes sculpting Type II canyons. This morphology-based classification scheme can be used to help predict depositional processes, grain size distributions, and petroleum prospectivity of any submarine canyon.     

Mangrove litter fall: Extrapolation from traps to a large tropical macrotidal harbour

Mangrove litter is a major source of organic matter for detrital food chains in many tropical coastal ecosystems, but scant attention has been paid to the substantial challenges in sampling and extrapolation of rates of litter fall. The challenges arise due to within-stand heterogeneity including incomplete canopy cover, and canopy that is below the high tide mark. We sampled litter monthly for three years at 35 sites across eight mapped communities in the macrotidal Darwin Harbour, northern Australia. Totals were adjusted for mean community canopy cover and the occurrence of canopy below the high tide mark. The mangroves of Darwin Harbour generate an estimated average of 5.0 t ha⁻¹ yr⁻¹ of litter. This amount would have been overestimated by 32% had we not corrected for limited canopy cover and underestimated by 11% had we not corrected for foliage that is below the high tide mark. Had we made neither correction, we would have overestimated litter fall by 17%. Among communities, rates varied 2.6-fold per unit area of canopy, and 3.9-fold among unit area of community. Seaward fringe mangroves were the most productive per unit of canopy area but the canopy was relatively open; Tidal creek forest was the most productive per unit area of community. Litter fall varied 1.1-fold among years and 2.0-fold among months though communities exhibited a range of seasonalities. Our study may be the most extensively stratified and sampled evaluation of mangrove litter fall in a tropical estuary. We believe our study is also the first such assessment to explicitly deal with canopy discontinuities and demonstrates that failure to do so can result in considerable overestimation of mangrove productivity.     

Heuristic numerical and analytical models of the hydrologic controls over vertical solute transport in a domed peat bog,Jura Mountains,Switzerland

We report the results of numerical and analytical simulations to test the hypothesis that downward vertical flow of porewater from the crests of domed alpine and kettle bogs controls vertical porewater distributions of major solutes such as Ca and Mg. The domed Etang de la Gruère bog (EGr), Switzerland, characterized by a vertical downward gradient of 0·04 and stratified layers of peat, is chosen as a field site for the model calibration and evaluation. The middle 4‐m section of the 6·5 m thick bog peat is heavily humified and has a hydraulic conductivity of ～10^?5·6 cm s^?1. Above and below, peat is less humified with a hydraulic conductivity of ～10^?3 cm s^?1. Heuristic finite difference simulations, using Visual MODFLOW, of the bog hydraulics show that the higher conductivity peat at the bog base is critical to create the observed deep, local flow cells that substantively recharge porewater. Model results and Peclet number calculations show that before ～7000 ¹⁴C yr BP diffusion of solutes from underlying mineral soils controlled the vertical distribution of porewater chemistry. From 7000 to ～1250 ¹⁴C BP the porewater chemistry was probably controlled by both upward diffusion and downward advection, and after ～1250 ¹⁴C yr BP porewater chemistry was probably controlled by downward advection. Concentrations of conservative major solutes in the porewaters of alpine, ombrotrophic bogs are the net effect of both downward vertical porewater movement and upward vertical diffusion, the magnitudes of which are delicately poised to the configuration of the bog water table over time and subsurface peat stratigraphy. Copyright © 2002 John Wiley & Sons, Ltd.     

Chandra and his students at Yerkes Observatory

S. Chandrasekhar’s interactions with graduate students in his more than a quarter century at Yerkes Observatory are described. His graduate teaching, Ph.D. thesis students, colloquia and colloquium series, and seminar series were all important aspects of this side of his scientific research career. His managing editorship ofThe Astrophysical Journal, his one experience in observational astrophysics, a second paper he wrote describing some of the early observational work at Yerkes Observatory, and a third on “the case for astronomy” are all discussed. A famous myth about one of his courses is corrected, and the circumstances under which the “S. Candlestickmaker” parody was written are recounted. Chandra’s computers, recruited in the Williams Bay community, are mentioned. A complete or nearly complete table of all the thesis students who received their Ph.D. degrees under his supervision, at Yerkes and on the campus in Chicago up through his last one in Astronomy and Astrophysics in 1973, is presented, with references to their published thesis papers.     

Brachinites: Igneous rocks from a differentiated asteroid

Abstract— We have done petrologic studies of brachinites Allan Hills (ALH) 84025, Elephant Moraine (EET) 99402, and EET 99407; bulk geochemical studies of EET 99402 and EET 99407; Ar‐Ar studies of Brachina and EET 99402; and a Xe isotopic study of Brachina. Textural, mineral compositional, and bulk compositional evidence show that EET 99402 and EET 99407 are paired. ALH 84025, EET 99402, and EET 99407 have igneous textures. Petrofabric analyses of ALH 84025 and EET 99407 demonstrate the presence of lineations and probable foliations of olivine grains that support formation as igneous cumulates. Mineral minor element chemistry and bulk rock incompatible lithophile element contents of the brachinites are distinct from those of acapulcoitelodranite clan meteorites, a suite of high‐grade metamorphic rocks and anatectic residues. The differences demonstrate a higher blocking temperature of equilibration for the brachinites and that cumulus plagioclase is present in EET 99402, EET 99407, and probably ALH 84025, thus indicating an igneous origin. Brachinites are differentiated, ultramafic achondrites, and are not part of a suite of primitive achondrites. We infer that their parent asteroid is a differentiated body. Brachina has an excess of ¹²⁹Xe correlated with reactor‐produced ¹²⁸Xe, demonstrating that short‐lived ¹²⁹I was present at the time of formation. This, plus literature data, attests to early formation of the brachinites, within a few Ma of the formation of chondrites. Ar‐Ar age data show that Brachina and EET 99407 were degassed about 4.13 Ga ago, possibly by a common impact event. EET 99402 and EET 99407 show petrographic evidence for shock, including possible conversion of plagioclase to maskelynite followed by devitrification. Brachina is unshocked, making a direct association between the Ar‐Ar age and textures ambiguous.