Preface to the Special Issue on Antarctic Meteorology and Climate:Past,Present and Future

<正>The Antarctic, including the continent of Antarctica and the Southern Ocean, is a critically important part of the Earth system. Research in Antarctic meteorology and climate has always been a challenging endeavor. Studying and predicting weather patterns in the Antarctic are important for understanding their role in local-to-global processes and facilitating field     

The hydrology of glacier-bed overdeepenings: Sediment transport mechanics,drainage system morphology,and geomorphological implications

Evacuation of basal sediment by subglacial drainage is an important mediator of rates of glacial erosion and glacier flow. Glacial erosion patterns can produce closed basins (i.e., overdeepenings) in glacier beds, thereby introducing adverse bed gradients that are hypothesized to reduce drainage system efficiency and thus favour basal sediment accumulation. To establish how the presence of a terminal overdeepening might mediate seasonal drainage system evolution and glacial sediment export, we measured suspended sediment transport from Findelengletscher, Switzerland during late August and early September 2016. Analyses of these data demonstrate poor hydraulic efficiency of drainage pathways in the terminus region but high sediment availability. Specifically, the rate of increase of sediment concentration with discharge was found to be significantly lower than that anticipated if channelized flow paths were present. Sediment availability to these flow paths was also higher than would be anticipated for discrete bedrock-floored subglacial channels. Our findings indicate that subglacial drainage in the terminal region of Findelengletscher is dominated by distributed flow where entrainment capacity increases only marginally with discharge, but flow has extensive access to an abundant sediment store. This high availability maintains sediment connectivity between the glacial and proglacial realm and means daily sediment yield is unusually high relative to yields exhibited by similar Alpine glaciers. We present a conceptual model illustrating the potential influence of ice-bed morphology on subglacial drainage evolution and sediment evacuation mechanics, patterns and yields, and recommend that bed morphology should be an explicit consideration when monitoring and evaluating glaciated basin sediment export rates.     

Compositional inhomogeneities in a single Icelandic tholeiite flow

New trace element analyses are reported for 25 samples of basalt taken from a vertical traverse in an 11 m thick single flow of Icelandic tholeiite. Compositional variations among the samples substantially exceed those expected from analytical uncertainties and are random with respect to height in the flow. These variations in an undifferentiated single flow suggest a short-range segregation model in which the proportions of phenocrysts. groundmass minerals and residual liquid vary randomly among different samples of the flow. A least-squares mixing model is used to determine whether the compositional variations reflect different proportions of crystallizing phases and residual liquid. Most elements (alkalis and alkaline earths excepted) are fit to within their analytical uncertainties, supporting the short-range segregation model. A Monte-Carlo calculation is used to model the phase modes and compositions of various samples of a hypothetical basalt. Most compositional and interelement variations for the Icelandic basalt resemble those of the hypothetical basalt. The calculations show that short-range segregation produces inhomogeneity as large as interflow compositional differences and results in incoherence among elements with different geochemical behaviors while preserving coherence among elements of similar behavior.     

Geology,petrography, and petrology of Pinzon Island,galapagos archipelago

Three stratigraphic units based on geologic relationships and paleomagnetic observations may be distinguished on Pinzon Island. The oldest unit is a broad shield which forms the main body of the island and was erupted during a period of reversed magnetic polarity from an area now occupied by a caldera. Subsequent activity was centered about 1.5 km to the north-northwest from vents later engulfed by the collapse of a younger caldera. The lower portion of this sequence was erupted during a period of transitional pole positions and is overlain by flows of normal polarity. Pinzon has the most diverse suite of differentiated tholeiitic rocks found in the Galapagos Archipelago. Products of eruptive cycles are preserved as sequences of tuffs and flows that have decreasing degrees of differentiation and increasing phenocryst abundance upsection. The sequences may be a consequence of tapping successively deeper levels of compositionally zoned magma chambers. Such a model is consistent with computer calculations utilizing major and trace element data for Pinzon rocks, which suggest that lavas of the island may be related by shallow-level crystal fractionation of observed phenocryst minerals.     

The Role of Mid-Latitude Pacific Cyclones in the Winter Precipitation of California*

The characteristics of the mid-latitude Pacific cyclones that produce precipitation over California are investigated statistically to determine their role in governing precipitation receipt. From all cyclones occurring over the Pacific Ocean from December through March and between 1965–1990 inclusive, we identify a subset of cyclones likely to produce precipitation over California. The characteristics analyzed are track, frequency, duration, speed, central pressure, and proximity. These are related to monthly precipitation for the entire period and are also used to explain the difference in precipitation received during the earlier (wet) and latter (dry) halves of the 1980s. Results indicate that the winter cyclones responsible for precipitation over California originate in the southeast quadrant of the region influenced by the Aleutian Low and decay south of the Gulf of Alaska. The extent of cyclonic activity over the east Pacific Ocean near California diminished during the dry period. There were fewer cyclones, they were significantly weaker, and they traveled along a more meridional track during the dry period.     

Carbon and hydrogen isotopic compositions of algae and bacteria from hydrothermal environments,Yellowstone National Park

Stromatolites forming today on a small scale in hydrothermal environments are chemical and biological analogues of much larger Precambrian formations. Carbon isotopic composition varied as a function of CO₂ concentration, pH, and species composition. Stratiform, layered stromatolites grew in silica-depositing springs at 55° to 70°C; they consisted mainly of a unicellular alga, Synechococcus, and a filamentous, photosynthetic bacterium, Chloroflexus. These thermophiles become enriched in ¹²C as the concentration of carbon dioxide in the effluent waters increases. At a concentration of 40 ppm total inorganic C, and δ¹³C of organic carbon was ～ ?12%., whereas at 900 ppm total inorganic C, the δ¹³C of similar species was ～ ?25%.. Conical stromatolites or conophytons (principally a filamentous, blue-green alga Phormidium and Chloroflexus) grew at 40°-55°C. In older, broader conophytons, Chloroflexus was the dominant organism. Their δ¹³C values were ～ ?18%. in a variety of hot springs. In carbonate-depositing springs, i.e., carbon dioxide saturated, conophytons and stromatolites consisting of a variety of blue-green algae and photosynthetic bacteria had the most negative δ¹³C values (to ?30%.). These carbon isotope ratios are directly comparable to carbon isotope ratios of kerogen from Precambrian stromatolites. The presence and activity of methanogenic bacteria or heterotrophic, aerobic and anaerobic bacteria did not alter significantly the δ¹³C of the original organic matter.The hydrogen isotopic fractionation between thermophilic organisms and water is 0 to ?74 for temperatures of 85° to 46°C, respectively. Acidophilic algae fractionated hydrogen isotopes to a lesser extent than did the photosynthetic organisms inhabiting neutral pH springs. Because organic matter retains some of its original isotopic signature, relationships of CO₂ levels, pH, temperature, and species composition between modern stromatolites and their environment and those of the Precambrian can be inferred.     

Cycling of dissolved and particulate nitrogen and carbon in the Framvaren Fjord, Norway: stable isotopic variations

The reaction pathways of nitrogen and carbon in the Framvaren Fjord (Norway) were studied through stable isotope analysis (δ¹⁵N and δ¹³C) of dissolved inorganic and particulate organic matter (POM). The variations in the isotopic compositions of the various C and N pools within the water column were use to evaluate the historical deposition of material to the sediments. The high δ¹⁵N-NH₄⁺ at the O₂/H₂S interface, as a consequence of microbial uptake between 19 and 25 m, results in extremely depleted δ¹⁵N-particulate nitrogen (PN) of approximately 1‰ within the particulate maximum at approximately 19 m. The carbon isotopic distribution of dissolved inorganic carbon (DIC) and particulate organic carbon (POC) within the interface suggests that the distinct microbial flora (Chromatium sp. and Chlorobium sp.) fractionate inorganic carbon to different degrees. The extremely light δ¹³C-POC within the interface (−31‰) appears to be a result of carbon uptake by Chromatium sp. while δ¹³C-POC of −12‰ is more indicative of Chlorobium sp. Nitrogen isotopic mass balance calculations suggested that approximately 75% of the material sinking to the sediments was derived from the dense particulate maximum between 19 and 25 m. The sediment distribution of nitrogen isotopes varied from 2‰ at the surface to approximately 6‰ at 30 cm. The nitrogen isotopic variations with depth may be an indicator of the depth or position of the O₂/H₂S interface in the fjord. Low sediment δ¹⁵N indicated that the interface was within the photic zone of the water column, while more enriched values suggested that the interface was lower in the water column potentially allowing for less fractionation during biological incorporation of dissolved inorganic nitrogen. Results indicate that the dense layers of photo-autotrophic bacteria in the upper water column impart unique carbon and nitrogen isotopic signals that help follow processes within the water column and deposition to the sediments.