CO<Subscript>2</Subscript> and He degassing at El Chichón volcano,Chiapas, Mexico: gas flux,origin and relationship with local and regional tectonics

During 2007–2008, three CO₂ flux surveys were performed on El Chichón volcanic lake, Chiapas, Mexico, with an additional survey in April 2008 covering the entire crater floor (including the lake). The mean CO₂ flux calculated by sequential Gaussian simulation from the lake was 1,190 (March 2007), 730 (December 2007) and 1,134 g m⁻² day⁻¹ (April 2008) with total emission rates of 164 ± 9.5 (March 2007), 59 ± 2.5 (December 2007) and 109 ± 6.6 t day⁻¹ (April 2008). The mean CO₂ flux estimated from the entire crater floor area was 1,102 g m⁻² day⁻¹ for April 2008 with a total emission rate of 144 ± 5.9 t day⁻¹. Significant change in CO₂ flux was not detected during the period of survey, and the mapping of the CO₂ flux highlighted lineaments reflecting the main local and regional tectonic patterns. The ³He/⁴He ratio (as high as 8.1 R _A) for gases in the El Chichón crater is generally higher than those observed at the neighbouring Transmexican Volcanic Belt and the Central American Volcanic Arc. The CO₂/³He ratios for the high ³He/⁴He gases tend to have the MORB-like values (1.41 × 10⁹), and the CO₂/³He ratios for the lower ³He/⁴He gases fall within the range for the arc-type gases. The high ³He/⁴He ratios, the MORB-like CO₂/³He ratios for the high ³He/⁴He gases and high proportion of MORB-CO₂ (M = 25 ±15%) at El Chichón indicate a greater depth for the generation of magma when compared to typical arc volcanoes.     

Assessing “Urban Karst” Effects from Groundwater–Storm Sewer System Interaction in a Till Aquitard

Storm sewer systems and their associated utility trenches may strongly influence the effects of urbanization on a groundwater system. This study was undertaken to identify the causes of district-wide basement infiltration in an aquitard system. It comprised widespread continuous monitoring of utility trench wells and dye tracing from storm sewer system exfiltration tests. The results indicate that a major effect of urbanization on shallow groundwater is related to storm sewer system exfiltration, which is marked by a characteristic pattern of head variations in the aquitard unrelated to distributed surface infiltration. The aquitard constrains flow from storm sewer system exfiltration to the utility trench, creating an urban flow path for groundwater discharge. Temporary buildup of water levels in the utility trench drives relatively high-velocity flow through the permeable sewer bedding material of the utility trench to a separate foundation drainage collector system, ultimately causing a severe “urban karst” effect that produces system surcharging and widespread basement water infiltration. The main conditions causing the “urban karst” are the large hydraulic conductivity ratio between the utility trench material and the aquitard, and the shallow depth and low gradient of the storm sewer system imposed by a very flat drainage basin.     

Climatic and oceanographic changes in the Northeast Atlantic reflected by magnetic properties of sediments deposited on the Portuguese Margin during the last 340 ka

Rock magnetic parameters measured along two giant piston cores MD95-2040 (40°34′N, 9°51′W) and MD95-2042 (37°47′N, 10°09′W) collected off the Portuguese Margin, related to other proxy-climatic data, have been used to reconstruct magnetic mineralogical changes of, in relation to environmental and climatic conditions over the North Atlantic, Western Europe and Northwest Africa during the last three climatic cycles (since isotope stage 10). Thin discrete layers containing coarse grains of titano-magnetite are associated with events of iceberg discharge during Heinrich events 1-6 [Heinrich, Quat. Res. 29 (1988) 142] that have equivalent events in isotope stages 5-8. Concentrations of fine-grained (Ti-) magnetite and hematite/goethite, varying in phase opposition, are directly linked with alternations of cold and warm climatic periods. Spectral analyses of the rock magnetic signals reveal Milankovitch periods at 100 and 41 ka, confirming the relationship with long-term climatic changes at high latitudes. The nature (Ti-magnetite) and size range of the finest ferrimagnetic fraction as well as its variation, suggest a control by deep currents carrying a colloidal/clayey fraction from remote sources (Iceland, Faeroes, mid-Atlantic Ridge). Variation of hematite/goethite contents is linked with transport by rivers and winds from the neighbouring continent. A tight correlation with the D-O cycles in Greenland ice records confirms that North Atlantic oceanic regimes and continental wind regimes were strongly influenced by millennial scale climatic changes throughout the last 350 ka.     

CPOD successful in trial for detecting Māui dolphin outside harbours

Critically endangered Māui dolphin (Cephalorhynchus hectori maui) are found exclusively off the west coast of New Zealand’s North Island. One important data gap troubling efforts to reduce the current unsustainable level of bycatch is the limited information on the offshore and alongshore limits of their range. Passive acoustic monitoring studies could address this data gap, but Māui dolphin habitat is dominated by a sandy substrate that can shift greatly in the strong local currents. In February 2016, a short pilot CPOD deployment was thus attempted as a first step to see if the equipment could be deployed and recovered in the area and if the data collected would be viable, with a second unit deployed inside a harbour for comparison with earlier TPOD data. The pilot was largely successful, and roughly 11% of all minutes in the core habitat location contained detections of clicks attributed to Māui dolphin. A visual inspection of the data suggests some trends with diel and tidal patterns. Further work of this type is thus recommended.     

U and Th concentrations and isotope ratios in modern carbonates and waters from the Bahamas

The short residence times of Th and Pa in seawater make them very responsive to changes in the ocean environment. We use a new multi-ion-counting technique to make Th and Pa isotope measurements in seawaters from a near-shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs (the Bahamas). An unusual feature of the Bahamas setting is the shallow depth of water residing on the bank tops. These waters have significantly lower ²³²Th/²³⁰Th (∼10,000) than those immediately adjacent to the banks (24,000-31,000) and a (²³¹Pa/²³⁰Th) near the production ratio (∼0.1). The change in ²³²Th/²³⁰Th and (²³¹Pa/²³⁰Th) on the bank tops is explained by almost quantitative removal of Th and Pa by scavenging, and their replacement with a mixture of ²³⁰Th and ²³¹Pa alpha-recoiled from the underlying carbonates, together with Th from dust dissolution. Analysis of a water profile in the Tongue of the Ocean, which separates the Great and Little Bahama Banks, allows us to trace the movement of bank-top water to depth. A distinct minimum in both ²³²Th/²³⁰Th (∼13,000) and (²³¹Pa/²³⁰Th) (∼0.5) is observed at ∼430 m and is interpreted to reflect density cascading of bank-top water with entrained carbonate sediment. These results suggest that Th and Pa can be used as water-mass tracers in near-shore environments. Uranium concentration measurements on the same waters demonstrate that U is conservative across a range in salinity of 2 psu, with a concentration of 3.33 ppb (at a salinity of 35).The incorporation of U and Th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these Bahamas waters. Such incorporation is critical for U-Th geochronology. U isotope analyses demonstrate that seawater δ²³⁴U averages 146.6 and does not vary by more than 2.5%o, and that carbonates capture this value. Additional high precision measurements (≈±1%o) on modern carbonates confirm that all oceans have identical δ²³⁴U. Modern marine carbonates are shown to have ²³²Th/²³⁰Th ratios that reflect the local seawater in which they formed.     

Contrasting flow pathways within tropical forest slopes of Ultisol soils

There are very few experimental studies identifying hydrological pathways within rain forest slopes. Such knowledge is, however, necessary to understand why forest disturbance affects rainfall–riverflow response and nutrient migration. This study examines flow pathways within lowland rain forest slopes comprising Udults of the Ultisol soil order. Experimentation was conducted on four SE Asian hillslope units (each 5 × 5 m in plan) in the Bukit Timah catchment (Singapore Island), and in the W8S5 catchment (Sabah, Borneo Island). The flow pathways were identified by artificial tracer experiments. We evaluated how well hydrometric calculations based on tensiometry and permeametry measurements predicted the tracer patterns. The tracer work indicated much faster subsurface flows at Bukit Timah than W8S5 for the storms studied. Some explanation of the greater subsurface waterflows at Bukit Timah in comparison to W8S5 is afforded by the less steep moisture release curves which maintain hydraulic conductivity as the soil dries. Vertical flow of the tracer through the upper 1 m of soil predominated (>90 per cent of percolation) in the Bukit Timah slopes. In some contrast, a major component (approximately 60 per cent) of the tracer percolation was directed laterally within the W8S5 slopes. The flow vectors calculated using the hydrometric methods did, however, grossly under‐estimate the degree of lateral deflection of waterflow generated at W8S5 and to a lesser extent over‐estimated it at Bukit Timah. In part, these errors may relate to the inability of traditional hydrometric techniques to fully characterize the effect of the large and small ‘natural soil pipes’ present within both catchments. In conclusion, the study indicates that marked variations in flow vectors exist within the Udult great group of SE Asian soils and hydrometric calculations may be poor predictors of these dominant pathways. Copyright © 2005 John Wiley & Sons, Ltd.     

Oceanic electromagnetic studies: A review

The review covers the main results of the oceanic electromagnetic studies presented during last 4–5 years. Seafloor electromagnetic observations are dedicated to studies in solid Earth geophysics and oceanography. Thus, a large range of objectives and targets are under investigation. Technological and theoretical advances provide an increase of quantity and quality of the collected seafloor data. The host conductivity models for oceanic crust and mantle are being developed; the possibility of obtaining useful structural constraints in heterogeneous environment are discussed. Significant oceanographic results were derived from electromagnetic field observations; these appeared to be a new powerful method for study of long-term large-scale ocean variability.