Deep pooling of low degree melts and volatile fluxes at the 85°E segment of the Gakkel Ridge: Evidence from olivine-hosted melt inclusions and glasses

We present new analyses of volatile, major, and trace elements for a suite of glasses and melt inclusions from the 85°E segment of the ultra-slow spreading Gakkel Ridge. Samples from this segment include limu o pele and glass shards, proposed to result from CO₂-driven explosive activity. The major element and volatile compositions of the melt inclusions are more variable and consistently more primitive than the glass data. CO₂ contents in the melt inclusions extend to higher values (167–1596 ppm) than in the co-existing glasses (187–227 ppm), indicating that the melt inclusions were trapped at greater depths. These melt inclusions record the highest CO₂ melt concentrations observed for a ridge environment. Based on a vapor saturation model, we estimate that the melt inclusions were trapped between seafloor depths (～ 4 km) and ～ 9 km below the seafloor. However, the glasses are all in equilibrium with their eruption depths, which is inconsistent with the rapid magma ascent rates expected for explosive activity. Melting conditions inferred from thermobarometry suggest relatively deep (25–40 km) and cold (1240°–1325 °C) melting conditions, consistent with a thermal structure calculated for the Gakkel Ridge. The water contents and trace element compositions of the melt inclusions and glasses are remarkably homogeneous; this is an unexpected result for ultra-slow spreading ridges, where magma mixing is generally thought to be less efficient based on the assumption that steady-state crustal magma chambers are absent in these environments. All melts can be described by a single liquid line of descent originating from a pooled melt composition that is consistent with the aggregate melt calculated from a geodynamic model for the Gakkel Ridge. These data suggest a model in which deep, low degree melts are efficiently pooled in the upper mantle (9–20 km depth), after which crystallization commences and continues during ascent and eruption. Based on our melting model and the assumption that CO₂ is perfectly incompatible, we show that the highest CO₂ concentrations of the melt inclusions (～ 1600 ppm) are consistent with the calculated CO₂ concentrations of primary undegassed melts. The highest measured CO₂/Nb ratio (443) of Gakkel Ridge melt inclusions predicts a mantle CO₂ content of 134 ppm and would result in a global ridge flux of 2.0 × 10¹² mol CO₂/yr.     

Measuring the hydraulic conductivity of shallow submerged sediments

The hydraulic conductivity of submerged sediments influences the interaction between ground water and surface water, but few techniques for measuring K have been described with the conditions of the submerged setting in mind. Two simple, physical methods for measuring the hydraulic conductivity of submerged sediments have been developed, and one of them uses a well and piezometers similar to well tests performed in terrestrial aquifers. This test is based on a theoretical analysis that uses a constant-head boundary condition for the upper surface of the aquifer to represent the effects of the overlying water body. Existing analyses of tests used to measure the hydraulic conductivity of submerged sediments may contain errors from using the same upper boundary conditions applied to simulate terrestrial aquifers. Field implementation of the technique requires detecting minute drawdowns in the vicinity of the pumping well. Low-density oil was used in an inverted U-tube manometer to amplify the head differential so that it could be resolved in the field. Another technique was developed to measure the vertical hydraulic conductivity of sediments at the interface with overlying surface water. This technique uses the pan from a seepage meter with a piezometer fixed along its axis (a piezo-seep meter). Water is pumped from the pan and the head gradient is measured using the axial piezometer. Results from a sandy streambed indicate that both methods provide consistent and reasonable estimates of K. The pumping test allows skin effects to be considered, and the field data show that omitting the skin effect (e.g., by using a single well test) can produce results that underestimate the hydraulic conductivity of streambeds.     

Behavior of gasoline pools following a denatured ethanol spill

In 1999, approximately 72 m3 of denatured fuel-grade ethanol spilled at a bulk fuel terminal that had existing contamination within the subsurface. An unanticipated increase in the measured depth of the light nonaqueous phase liquid (LNAPL) was observed in nearby monitoring wells following the spill. This paper presents results of a laboratory analysis designed to understand the apparent increase in LNAPL mobility at this site. The two-dimensional stainless steel and glass tank allowed visual assessment of the potential effects that the addition of denatured ethanol may have on a site with pre-existing gasoline contamination. Digital images of gasoline and ethanol spill experiments were analyzed for changes in the characteristics of the existing gasoline pool and residual gasoline saturation in the unsaturated zone. Reductions in the surface and interfacial tensions resulted in significant changes in the size, shape, and saturation of the gasoline pool after the addition of ethanol to the system. The final gasoline pool occupied a smaller area and had a higher saturation. In addition, some smearing of the gasoline into the saturated zone occurred as the capillary fringe was depressed.     

Stratigraphy and thermoluminescence dating of Late Glacial cover sand in Denmark

In western Jutland deposits with a texture and structure similar to Dutch cover sands have been found. Investigations have been carried out on the lithology and the lithostratigraphic sequences. Pollen analysis is used for biostratigraphic correlation, and ages have been obtained by the radiocarbon method as well as by thermoluminescence techniques using both quartz and eldspar. The five sites are presented separately and the data are integrated into a general stratigraphic outline of the Late Glacial cover-sand sequence in Jutland. During the latest part of the Upper Pleniglacial and the earliest Late Glacial, older cover-sand type was deposited. Locally this sediment was also deposited during the later parts of the Late Glacial. Younger cover-sand type was deposited primarily during the Early Dryas and Late Dryas, but some sand of this type was also laid down during the Allerød and the earliest Holocene. The palaeobotanical records are discontinuous, but plant growth was probably present throughout the Late Glacial; only during the Altered did the deposition of sand decrease long enough to allow a relatively stable vegetational development. The investigation shows that the sequence of events in Jutland is similar to the Dutch record.     

Rock avalanching into a landslide-dammed lake causing multiple dam failure in Las Conchas valley (NW Argentina) — evidence from surface exposure dating and stratigraphic analyses

Generally landslide dams which exist for several hundreds to thousands of years are considered as stable. We show with an example from the Argentine Andes that such dams can exist for several thousands of years but still may fail catastrophically. Multiple rock avalanches impounded two lakes with surface areas of ~8 km² and ~600 km², respectively, in Las Conchas valley, NW Argentina. Surface exposure dating (SED) by ¹⁰Be of the rock-avalanche deposits or landslide scars indicates that these landslides occurred at 15,300±2,000 yr and 13,550±900 yr. The dams were stable during a strong earthquake, as suggested by seismites within related lake sediments and by multiple coeval landslides in this region, which occurred at ~7.5 kyr. However, when a further rock-avalanche fell into the lower, smaller lake at 4,800±500 yr the dam downriver was destroyed, presumably by the resulting tsunami wave. The resulting flood also destroyed an additional rock-fall dam which had formed at ~5,630 yr ¹⁴C cal BP 30 km downriver. The new dam formed by the second rock avalanche was eroded prior to 3,630 yr ¹⁴C cal BP. This dam erosion coincides with an important climatic shift towards more humid conditions in the Central Andes. Our results show that instead of direct effects of strong seismicity on landslide dams, (1) landsliding into a landslide-dammed lake, (2) abrupt hydrological changes, and (3) climate change towards conditions related to enhanced run-off are processes which can produce failures of quasi-stable natural dams.     

Estimating mortality in natural assemblages of oysters

To evaluate methods for calculating mortality in bivalve molluscs, we analyzed historical data from dredge surveys of both planted and natural oyster (Crassostrea virginica) grounds in Delaware Bay to compare total box-count mortality estimates with those made by accumulating short-term mortality rates obtained from fresh boxes identified by shell condition and degree of fouling. Box-count and cumulative-mortality patterns and values agreed best on grounds with planted oysters, where a cohort with very few dead oysters was broadcast on previously cleaned bottom and was followed over time. This situation is analogous to an artificially created reef with oysters either deployed or naturally set on it. When deaths predominated in late summer and early autumn, the two estimates were similar throughout the following year; when mortality was greatest in spring or early summer, the estimates were similar only through autumn of the same year. Correspondence was much weaker on natural beds, where new individuals constantly recruited to the population and variable numbers of boxes were always present. Nevertheless, total box-count mortality estimates made during autumn stock surveys were significantly correlated with cumulative mortalities calculated for the preceding year. We also estimated disarticulation rates of artificially created boxes by deploying them at three seasons and eight sites in Delaware Bay. Disarticulation time depended on the length of exposure at summer temperatures, with the average time to 50% disarticulation for boxes deployed in spring and in summer being 225 and 345 days, respectively. Disarticulation rates increased with decreasing size, and increasing salinity and temperature. Finally, we compared total box-count and cumulative-mortality estimates with those made using the disarticulation data. Annual averages for the three methods were within 5 percentage points of each other. Our data indicate that total box-count mortality estimates from fall stock surveys can provide a reliable index to total mortality for the previous year, although it cannot describe the seasonal patterns obtainable using the cumulative-mortality method. *** DIRECT SUPPORT *** A02BY003 00003     

The liminality of training spaces: Places of private/public transitions

This paper draws upon research, conducted for the London West Learning and Skills Council, on the training experiences of women with dependent children. One of the striking revelations of the research, we suggest, is the way in which training spaces are used and perceived by women, which are often at odds with government intentions. To help make sense of women’s use of, and motivation for, training we utilise the concept of ‘liminality’ and the private/public imbrication to explain the ways in which women use, or are discouraged from using, training spaces. Further, how the varied and multiple uses women in our research have put training to in their own lives has encouraged us to rethink the relationship between the private and the public more generally. In the light of this, we suggest that training and the places in which training take place, have been neglected processes and spaces within feminist geography and might usefully be explored further to add to an extensive literature on women’s caring and domestic roles and their role in the paid workplace.     

Oxygen consumption rates in pelagic sediments from the Central Pacific: First estimates from microelectrode profiles

Oxygen profiles in pelagic sediments from the Manganese Nodule Program calcareous and siliceous ooze sites (MANOP sites C and S) in the central Pacific Ocean were measured with microelectrodes and are used to predict oxygen consumption rates beneath the sediment-water interface. We explain possible artifacts which occur during sample recovery and argue that minimum estimates of 0.083 and 0.025 μ moles $\text{O}{}_2\text{cm}{}^2\text{day}$ (C and S, respectively) can be calculated from the data. These oxygen consumption rates are in good agreement with in situ respirometer measurements previously reported for comparable sediments in the north Pacific, but previous estimates based on modeling of pore water nitrate profiles at both sites are two to five times smaller than our minimum fluxes. The differences in oxygen fluxes calculated by the two methods are probably in part due to uncertainty in the assumptions inherent in the nitrate model. However, non-steady state fluctuations in particulate organic matter fluxes could also be a reason for the disparity.     

Symposium über Fragen der Cyanophytentaxonomie im Seenforschungsinstitut der ETHZ/EAWAG in Kastanienbaum (Schweiz), 2.–15. August 1979 Verhandlungsbericht

The 8th Symposium of the International Association for Cyanophyte Research IAC was attended by 29 specialists from 11 countries. The 19 oral presentations and discussions centered on the following themes: The basic principles of cyanophyte taxonomy (3 lectures), systematic problems of certain taxonomic groups (5 lectures), morphology (6 lectures) and ecology (5 lectures). The intensive discussions between representatives of the classical (european) taxonomic treatment of cyanophytes (based on morphological and ecological characteristics) and those representing the bacteriological study of ‘cyanobacteria’ were especially valuable. The central theme of the meeting was, however, the mutual microscopic study and discussion of problematic blue-greens from natural habitats, that were mostly collected during the three field trips into different areas of central Switzerland (Grimsel-Gotthard-Nufenen. Klewenalp. Gerzensee).      

Factors Influencing Recreational Boaters’ Intentions to Comply with Right Whale Regulations in the Southeastern United States

Regulations for boaters can help mitigate adverse effects on threatened marine mammals. One management tool to protect endangered North Atlantic right whales is a 460-m distance restriction rule for all vessels. This study is the first effort to analyze factors that influence recreational boaters’ intentions to comply with this regulation. Using the theory of planned behavior, we analyzed 362 mail surveys of recreational boaters using the offshore waters of the southeastern United States. We found that two constructs of the theory significantly explained 58% of the intention to comply with the rule: (1) positive attitude toward the rule and (2) stronger belief that other people are complying. Boaters recommended increasing knowledge about whales to improve compliance, but they were divided with respect to increasing fines for violators to increase compliance. This information can be useful for designing outreach strategies to protect whales.