Evidence from Ar/Ar Ages of Individual Hornblende Grains for Varying Laurentide Sources of Iceberg Discharges 22,000 to 10,500 yr B.P.

The abundance and lithic content of ice rafted detritus in glacial North Atlantic sediment cores vary abruptly on millennial time scales that have been correlated to Dansgaard-Oeschger cycles in the Greenland ice cores. There is growing evidence that various ice sheet outlets contributed increased iceberg fluxes at multiple discrete intervals, and the relative timing of iceberg discharges from different sources is important for understanding interactions between oceans and ice sheets. We present a provenance study based on ⁴⁰Ar/³⁹Ar dates of individual hornblende grains from 20 samples taken at 600 to 700 yr spacing between 10,500 and 22,000 yr B.P., from Orphan Knoll core EW9303-GGC31. Heinrich layers are characterized by a dominant Paleoproterozoic hornblende provenance consistent with published studies. A change in provenance between Heinrich events H2 and H1 indicates contributions of iceberg calving from the Newfoundland and southern Labrador margins. Between H1 and the Younger Dryas interval, Paleoproterozoic ice rafted grains remained dominant. The dominance of Baffin Island (or Greenland?) sources to the ice rafted detritus is ascribed to the retreat of the southern Laurentide ice sheet at about the time of H1—a retreat that isolated Newfoundland and southern Labrador ice from the shelf-slope boundary.     

Telescopes in Education at the IAU

The development and utilization of telescopes for educational purposes were discussed on 18 August at a special session of the IAU in Manchester. The session, which was organized by Barrie Jones and Alan Pickwick of the RAS Education Committee, was entitled "Astronomy Research Projects for School and University Students". Peter Bond summarizes.     

Composition of widespread volcanic glass in deep-sea sediments of the Southern Pacific Ocean: an Antarctic source inferred

Widespread Plio-Pleistocene (2.43-0.06 Ma) tephra zones recognised in deep-sea cores from high latitudes (>60°) in the Southern Pacific Ocean were thought to have originated from calc-alkaline rhyolitic eruptions in New Zealand, some 5000 km distant. Electron microprobe analyses of the glasses reveal a wide diversity of alkalic felsic compositions, as well as minor components of basic and intermediate glasses, incompatible with a New Zealand Neogene source but similar to contemporaneous eruptives from the Antarctic region. Most tephra zones are trachytic; seven zones are peralkaline rhyolite. The rhyolitic zones represent a deep-sea record of widespread silicic eruptions from continental Antarctica, possibly Marie Byrd Land. The extent of these rhyolitic zones suggest a greater frequency of large explosive eruptions in Antarctica than previously documented. The coarse grain size of some of the shards (up to 3 mm), their great distance from the closest sources (>1600 km for some cores), and the presence of nonvolcanic ice-rafted debris indicate some of the glasses, especially the more basic compositions, may have been ice-rafted, contrary to previous suggestions of a fallout origin.     

Climate change and control of the southeastern Bering Sea pelagic ecosystem

We propose a new hypothesis, the Oscillating Control Hypothesis (OCH), which predicts that pelagic ecosystem function in the southeastern Bering Sea will alternate between primarily bottom-up control in cold regimes and primarily top-down control in warm regimes. The timing of spring primary production is determined predominately by the timing of ice retreat. Late ice retreat (late March or later) leads to an early, ice-associated bloom in cold water (e.g., 1995, 1997, 1999), whereas no ice, or early ice retreat before mid-March, leads to an open-water bloom in May or June in warm water (e.g., 1996, 1998, 2000). Zooplankton populations are not closely coupled to the spring bloom, but are sensitive to water temperature. In years when the spring bloom occurs in cold water, low temperatures limit the production of zooplankton, the survival of larval/juvenile fish, and their recruitment into the populations of species of large piscivorous fish, such as walleye pollock (Theragra chalcogramma), Pacific cod (Gadus macrocephalus) and arrowtooth flounder (Atheresthes stomias). When continued over decadal scales, this will lead to bottom-up limitation and a decreased biomass of piscivorous fish. Alternatively, in periods when the bloom occurs in warm water, zooplankton populations should grow rapidly, providing plentiful prey for larval and juvenile fish. Abundant zooplankton will support strong recruitment of fish and will lead to abundant predatory fish that control forage fish, including, in the case of pollock, their own juveniles. Piscivorous marine birds and pinnipeds may achieve higher production of young and survival in cold regimes, when there is less competition from large piscivorous fish for cold-water forage fish such as capelin (Mallotus villosus). Piscivorous seabirds and pinnipeds also may be expected to have high productivity in periods of transition from cold regimes to warm regimes, when young of large predatory species of fish are numerous enough to provide forage. The OCH predicts that the ability of large predatory fish populations to sustain fishing pressure will vary between warm and cold regimes.The OCH points to the importance of the timing of ice retreat and water temperatures during the spring bloom for the productivity of zooplankton, and the degree and direction of coupling between zooplankton and forage fish. Forage fish (e.g., juvenile pollock, capelin, Pacific herring [Clupea pallasii]) are key prey for adult pollock and other apex predators. In the southeastern Bering Sea, important changes in the biota since the mid-1970s include a marked increase in the biomass of large piscivorous fish and a concurrent decline in the biomass of forage fish, including age-1 walleye pollock, particularly over the southern portion of the shelf. Populations of northern fur seals (Callorhinus ursinus) and seabirds such as kittiwakes (Rissa spp.) at the Pribilof Islands have declined, most probably in response to a diminished prey base. The available evidence suggests that these changes are unlikely the result of a decrease in total annual new primary production, though the possibility of reduced post-bloom production during summer remains. An ecosystem approach to management of the Bering Sea and its fisheries is of great importance if all of the ecosystem components valued by society are to thrive. Cognizance of how climate regimes may alter relationships within this ecosystem will facilitate reaching that goal.     

How soil moisture mediates the influence of transpiration on streamflow at hourly to interannual scales in a forested catchment

The water balance equation dictates that streamflow may be reduced by transpiration. Yet temporal disequilibrium weakens the relationship between transpiration and streamflow in many cases where inputs and outputs are unbalanced. We address two critical knowledge barriers in ecohydrology with respect to time, scale dependence and lags. Study objectives were to correlate components of the water balance equation at hourly to annual scales, quantify time lags, and simplify critical components of the water budget during wet and dry conditions. We tested interrelationships among precipitation, vapour pressure deficit, transpiration, soil moisture, and streamflow within the confines of a 60‐hectare forested watershed in the western Cascades of Oregon. The Pacific Northwest is an ideal location to compare wet and dry seasons because of its Mediterranean climate. Soil moisture explained more than 80% of the variation in streamflow at all temporal scales investigated. Streamflow was most strongly coupled to soil moisture in the wet season because of gravitational drainage patterns; strong coupling of transpiration to vapour pressure deficit was dominant in the dry season and driven by low humidity. We observed progressively longer hourly time lags between soil moisture and streamflow in the dry season, which relates to an increasing soil moisture deficit that took an average of 48 days to refill after the onset of winter rains. We propose that transpiration drives seasonal patterns in soil moisture that relate to patterns in streamflow only after long time lags. In other words, soil moisture mediates the influence of transpiration on streamflow. Copyright © 2011 John Wiley & Sons, Ltd.     

Towards an understanding of the Of?p star HD 191612: optical spectroscopy

We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe–O8fp). The Balmer and He  i lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. He  ii absorptions and metal lines (including many selective emission lines but excluding He  ii λ4686 Å emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with   P _orb= 1542 d, e = 0.45  . We conduct a model-atmosphere analysis of the spectrum, and find that the system is consistent with a ∼O8 giant with a ∼B1 main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying 'clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.     

Source Specific Station Corrections for Regional Phases at Fennoscandian Stations

—?The IASPEI91 global travel-time curves are used as the default for event location at the Prototype International Data Center (PIDC). In order to improve event location, a 1-D Baltic travel-time model was implemented at the PIDC in 1997 for locating events using regional phases from Fennoscandian stations. Where a single model is insufficient for characterizing the regional geology, path-dependent corrections, or Source Specific Station Corrections (SSSCs), are more appropriate for event locations. We have developed SSSCs for regional phases at the Fennoscandian stations by interpolating travel times through different 1-D models. SSSCs for stations NRIS and SPITS are also derived, given the fact that paths from both stations to high latitude events are within the Fennoscandia regionalization as Baltic.¶Validation testing of the SSSCs demonstrates that using SSSCs in event location is superior to not using SSSCs, a nd, in most cases, to using the 1-D model directly when locating events. For a ground-truth data set which includes events in the Baltic Shield with location accuracy better than 2?km, the average improvement in location due to SSSCs is 9?km, and the median coverage ellipse is reduced by 2710?km² (from 3830 to 1120?km²). These results are similar to those obtained using the 1-D Baltic model. For a CEB (Calibration Event Bulletin) data set which includes events along the North Atlantic oceanic ridge and in central/southern Europe, using SSSCs the ridge events move closer to the ridge axis, and the European events move closer to CEB locations than 1-D Baltic locations. For a constrained JHD (Joint Hypocenter Determination) data set of events in the Novaya Zemlya region, when using SSSCs or the 1-D Baltic model, relative to the JHD locations mislocations are less or similar to those without SSSCs. All coverage ellipses are smaller but sti ll contain the JHD solutions.¶Our SSSCs are strongly dependent on the 1-D regional models and regionalization. Future development in 1-D velocity models and travel-time curves should improve such SSSCs, event locations, and uncertainties. It is hoped that the implementation and demonstration of SSSCs in the PIDC software will encourage these further developments. These SSSCs were implemented at the PIDC for Reviewed Event Bulletin (REB) location in April 1999.