Anorthite--Forsterite and Anorthite--Enstatite Reactions and their bearing on the Basalt--Eclogite Transformation

The reactions betweeon anorthite and forsterite and betweenanorthite and enstatite have been studied at high pressuresand high temperatures with solid-media and gas-media apparatus.For the 1: 1 (molecular ratio) composition of anorthite andforsterite the following three assemblages have been obtainedwith increasing pressure at a constant temperature: anorthiteforsterite,clinopyroxeneorthopyroxenespinelanorthite, and garnet (pyropegrossulariteseries). Two univariant curves between these three assemblageshave been determined. The amounts of pyroxenes relative to spineland anorthite in the second assemblage increase and the contentsof the Tschermak's components in the pyroxenes increase withincreasing pressure. The 1:2 (molecular ratio) composition of anorthite and forsteritewas also studied, and the univariant curve for the reactionclinopyroxeneorthopyroxenespinel garnet (pyropegrossulariteseries)forsterite has been determined. For the 1: 2 (molecular ratio) composition of anorthite andenstatite the following assemblages have been obtained withincreasing pressure at a constant temperature: anorthiteenstatite,anorthiteorthopyroxeneclinopyroxenequartz, and garnet (pyrope-grossulariteseries)quartz. The univariant curve between the last two assemblageshas been determined. Both the pyroxenes in the second assemblagecontain Tschermak's components, which increase in content withincreasing pressure at the expense of anorthite. These experimental results suggest that basalts, either saturatedor undersaturated with silica, transform into aluminous pyroxene-richassemblages at lower pressures or higher temperatures than thestability field of eclogites, and that pyroxene-spinel symplectitesfound in some metamorphosed gabbro, troctolite, and norite mayhave formed in the stability field of the pyroxene-rich assemblages.The stability field of the pyroxene-rich assemblages is probablyincluded within the pressure-temperature conditions of the granulitefacies. It is also suggested that the garnet-free peridotites,such as the peridotite inclusions in basaltic rocks and theintrusive peridotites in the orogenic belts, are stable at lowerpressures or higher temperatures than garnet peridotites, suchas the inclusions in kimberlite and intrusive garnet peridotitein the orogenic belts.     

Evaluation of the Crystallization Conditions for the Calcalkaline Russian Peak Intrusive Complex, Klamath Mountains, Northern California

The mid-Jurassic calcalkaline Russian Peak intrusive complex,located in the Klamath Mountains of northern California, consistsof an elliptical peridotite-to-quartz diorite suite intrudedby two plutons of granodiorite. Several techniques were usedto decipher the crystallization conditions for ultramafic rocks,quartz diorite, and granodiorite, including comparison of parageneseswith crystallization experiments, application of geothermometersand barometers, and evaluation of phase equilibria. Contactmetamorphic assemblages, hornblende barometry, and amphibolesubstitution schemes indicate that pressures of intrusion were{small tilde}3 kbar. Plagioclase and pyroxene thermometry indicateintrusion temperatures of {small tilde}1000C for quartz dioriteand 900C for granodiorite. Phase equilibrium analysis for thereaction phlogopite+quartz=K-feldspar+enstatite+H₂O, coupledwith an estimate of the water-saturated quartz diorite solidus,suggests that the solidus of two-pyroxene quartz diorite wasat {small tilde}780C with a mole fraction of water of {smalltilde}0•55. The composition of granodiorite is very similarto that used in several crystallization experiments and indicatesa solidus of 70025C. Estimates of oxygen fugacity, obtainedfrom equilibrium relations of olivine, orthopyroxene, and spinelin ultramafic rocks, magnetite and ilmenite in quartz diorite,and magnetite, K-feldspar, and biotite in quartz diorite andgranodiorite are 2•1–2•5 and 1•0–1•3log units above the quartz-fayalite-magnetite (QFM) buffer forgranodiorite and quartz diorite at their respective solidustemperatures; and 1•0–4•0 log units above QFMfor ultramafic rocks and quartz diorite at subsolidus temperatures.Thus, the quartz diorite magma was hotter, drier, and slightlyreduced relative to the grandiorite magma, differences thatset important constraints on the genesis of the Russian Peakmagmas. These results also indicate that quartz diorite wasundersaturated with respect to H₂O as it reached its solidus,a condition that is consistent with the absence of deutericalteration in this unit. In contrast, granodiorite shows extensivedeuteric alteration and features pegmatites, quartz pods, andradial dikes as might be expected for H₂O-saturated conditions. Although calcalkaline plutonic complexes present serious difficultiesin estimating the intensive parameters of crystallization, judiciousapplication of appropriate methods may result in the successfulevaluation of the conditions of crystallization of such complexes.     

Deglacial and Holocene conditions in northernmost Baffin Bay: sediments, foraminifera, diatoms and stable isotopes

A multiproxy study of palaeoceanographic and climatic changes in northernmost Baffin Bay shows that major environmental changes have occurred since the deglaciation of the area at about 12 500 cal. yr BP. The interpretation is based on sedimentology, benthic and planktonic foraminifera and their isotopic composition, as well as diatom assemblages in the sedimentary records at two core sites, one located in the deeper central part of northernmost Baffin Bay and one in a separate trough closer to the Greenland coast. A revised chronology for the two records is established on the basis of 15 previously published AMS ¹⁴C age determinations. A basal diamicton is overlain by laminated, fossil-free sediments. Our data from the early part of the fossiliferous record (12 300–11 300 cal. yr BP), which is also initially laminated, indicate extensive seasonal sea-ice cover and brine release. There is indication of a cooling event between 11 300 and 10 900 cal. yr BP, and maximum Atlantic Water influence occurred between 10 900 and 8200 cal. yr BP (no sediment recovery between 8200 and 7300 cal. yr BP). A gradual, but fluctuating, increase in sea-ice cover is seen after 7300 cal. yr BP. Sea-ice diatoms were particularly abundant in the central part of northernmost Baffin Bay, presumably due to the inflow of Polar waters from the Arctic Ocean, and less sea ice occurred at the near-coastal site, which was under continuous influence of the West Greenland Current. Our data from the deep, central part show a fluctuating degree of upwelling after c . 7300 cal. yr BP, culminating between 4000 and 3050 cal. yr BP. There was a gradual increase in the influence of cold bottom waters from the Arctic Ocean after about 3050 cal. yr BP, when agglutinated foraminifera became abundant. A superimposed short-term change in the sea-surface proxies is correlated with the Little Ice Age cooling.     

Can glacial erosion limit the extent of glaciation?

In southern South America, the maximum areal extent of ice during the Quaternary Period, the Greatest Patagonian Glaciation (GPG, [Mercer, J.H., 1983. Cenozoic glaciation in the southern hemisphere. Annual Reviews of Earth and Planetary Science 11, 99–132.]), occurred at 1.1 Ma and subsequent glaciations were overall less extensive. The GPG preceded global minimum temperatures and maximal volume of ice, which occurred in the last ~ 800 kyr, as recorded in the marine δ¹⁸O record. Significant modification of the drainage morphology of the southern Andes from a non-glaciated to glaciated landscape occurred throughout the Quaternary Period. We infer a non-climatic relationship between glacial modification of the mountains and the decreasing extent of ice and we discuss processes of landscape development that could have caused the trend. Specifically, these include modification of valleys, such as development from a V- to a U-shape, and lowering of mass-accumulation areas. Such changes would strongly affect glacial dynamics, the mass balance profile and mass-flux during succeeding glaciations, especially for low-gradient outlet glaciers occupying low areas. Other areas around Earth (at least where ice has been warm-based) also may exhibit a non-random trend of decreasing extent of ice with time, ultimately because of glacial erosion in the Quaternary Period.     

Mechanisms controlling the air–sea flux in the North Sea

The mechanisms driving the air–sea exchange of carbon dioxide (CO₂) in the North Sea are investigated using the three-dimensional coupled physical–biogeochemical model ECOHAM (ECOlogical-model, HAMburg). We validate our simulations using field data for the years 2001–2002 and identify the controls of the air–sea CO₂ flux for two locations representative for the North Sea's biogeochemical provinces. In the seasonally stratified northern region, net CO₂ uptake is high () due to high net community production (NCP) in the surface water. Overflow production releasing semi-labile dissolved organic carbon needs to be considered for a realistic simulation of the low dissolved inorganic carbon (DIC) concentrations observed during summer. This biologically driven carbon drawdown outcompetes the temperature-driven rise in CO₂ partial pressure (pCO₂) during the productive season. In contrast, the permanently mixed southern region is a weak net CO₂ source (). NCP is generally low except for the spring bloom because remineralization parallels primary production. Here, the pCO₂ appears to be controlled by temperature.     

Local food activity in the Republic of Ireland and Great Britain

Recent changes in local food supply systems have attracted substantial research interest, but little consideration has been paid to exactly where they occur. This article combines data from three studies to compare local food system development in England, Wales, Scotland and Ireland using a single index of food relocalisation, thereby exploring the usefulness of the Index across different social and political contexts. Four diagnostic indicators suggest that local food systems in the south west of Ireland and Britain are particularly well developed. The Index itself is a useful tool for making international comparisons, being easy to replicate and allowing the integration of different data sets. Perhaps its greatest utility is that it opens up new avenues for further research.     

GEOMORPHOLOGY OF THE LAKE SHEWA LANDSLIDE DAM,BADAKHSHAN, AFGHANISTAN,USING REMOTE SENSING DATA

Lake Shewa in northeastern Badakhshan, Afghanistan, was dammed sometime in antiquity when a large rock avalanche (sturzstrom) from the fault‐shattered and strongly weathered Archean gneisses of the Zirnokh peaks to the north moved into the Arakht River valley. This rock avalanche dammed up the river and its tributaries to a dam thickness of c. 400 m, producing a 12‐km‐long lake that is as much as 270 m deep, leaving c. 80 m of freeboard to the top of the dam. At least four separate instances of slope failure have been mapped at the site of the landslide dam, as well as a rock glacier, using remotely sensed data, historical maps, and Google Earth?. Spring seepage through the dam face has caused several recent subsidiary debris slides, which if continued at a large enough scale for long enough, or with additional seismicity from the active strike‐slip faults that cross beneath the landslide dam, could threaten its integrity. Otherwise the clean water that emerges from the dam face could be the source of an unvarying mini‐hydroelectric power source, in addition to the agricultural irrigation that it provides at the present time.     

The pelagic distribution of marine birds in a heterogeneous environment

In this paper I review recent, small-scale, process-oriented studies of the pelagic distribution of marine birds. I examine the roles played by a variety of hydrographic features, including steep flow gradients (fronts) and water column stratification, in determining the abundance or availability of avian prey. The ice edge/marginal ice zone is a frontal area of particular interest in polar regions. In most oceanic systems we have examples of avian use of a feature, hut only poor information on the importance of the feature for supporting the population as a whole. I review recent studies of the spatial and numerical concordance of marine birds and their prey, and find that these studies have yielded mixed results, with correlations stronger for piscivorous birds, particularly murres foraging on capelin, than for planktivores. Review of investigations of multispecies interactions during foraging shows that flock foraging has both beneficial and negative aspects. Interspecific facilitation of foraging occurs in numerous interactions involving both other species of birds and marine mammals. However, co-occurrence of predators may occur because both seek the same prey, and caution must be used in assessing mutualistic interactions between predatory species.