Arctic mercury depletion and its quantitative link with halogens

Gas phase elemental mercury (Hg°) was measured aboard the NASA DC-8 research aircraft during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign conducted in spring 2008 primarily over the North American Arctic. We examined the vertical distributions of Hg° and ozone (O₃) together with tetrachloroethylene (C₂Cl₄), ethyne (C₂H₂), and alkanes when Hg°- and O₃-depleted air masses were sampled near the surface (<1 km). This study suggests that Hg° and O₃ depletions commonly occur over linear distances of ~20–200 km. Horizontally there was a sharp decreasing gradient of ~100 ppqv in Hg° over <10 km in going from the bay near Ellesmere Island to the frozen open ocean. There was a distinct land-ocean difference in the vertical thickness of the Hg°-depleted layer – being variable but around a few 100 meters over the ocean whereas occurring only very near the surface over land. Data support that atmospheric mercury depletion events are driven by Hg° reactions with halogen atoms. Derived from data collected aboard the DC-8, we present mathematical expressions giving the rates of Hg° and O₃ depletions as a function of the radical concentrations. These relationships can be a useful metric to evaluate models that attempt to reproduce springtime Arctic Hg° and/or O₃ depletion events, and they can also be employed to obtain order-of-magnitude estimates of radical concentrations and the ratio [Br]/[Cl].     

Bacteriohopanepolyol biomarker composition of organic matter exported to the Arctic Ocean by seven of the major Arctic rivers

Bacteriohopanepolyols (BHPs) are a diverse group of membrane lipids produced by a wide variety of bacteria and can be used as molecular biomarkers for bacterial processes and populations in both modern and ancient environments. A group of BHPs, including adenosylhopane and structurally related compounds, have been identified as being specific to soils, enabling the transport of terrestrial organic matter (terrOM) to the marine realm to be monitored. Estuary surface sediment samples were obtained from the five Great Russian Arctic Rivers (GRARs; Ob, Yenisey, Lena, Indigirka and Kolyma) and river sediments were obtained from two North American Rivers (Yukon and Mackenzie). Analysis of the BHP signatures, using high performance liquid chromatography–tandem mass spectrometry (HPLC–MSⁿ), indicated the presence of 15 different BHPs originating from a variety of different bacteria, as well as a significant presence of terrestrially derived OM. Total BHP abundance and the contribution of the “soil-marker” BHPs to the total BHP pool increased eastwards among the GRAR sediments. This suggests increasing terrestrial OM or increased preservation of OM as a result of shorter periods of permafrost thawing. The North American rivers showed greatly differing BHP levels between the Yukon and Mackenzie rivers, with a greater BHP input and thus a relatively higher soil OM contribution from the Yukon. The Indigirka River basin in the eastern Siberian Arctic appeared to be the epicentre in the pan-Arctic BHP distribution trend, with the highest “soil-marker” BHPs but the lowest tetrafunctionalised BHPs. Aminobacteriohopanepentol, an indicator of aerobic methane oxidation, was observed in all the sediments, with the source being either the marine environment or methane producing terrestrial environments.     

Bacteriohopanepolyol signatures of bacterial populations in Western Canadian soils

Bacteriohopanepolyols (BHPs) are naturally occurring compounds derived from bacteria. Their quantity and diversity in five Western Canadian soils, which vary mostly in vegetative cover, were examined using high performance liquid chromatography – atmospheric pressure chemical ionization – mass spectrometry (HPLC–APCI–MS). Eighteen BHP compounds including tetra-, penta- and hexa-functionalised components, as well as composite components, were identified. Concentrations were highest in the forest-grassland transition soil [515 μg/g organic carbon (OC)], followed by the forest soil (431 μg/g OC) and the grassland soils (333–306 μg/g OC). The distribution trends measured using HPLC–ACPI–MS agree with hopanoid measurements using gas chromatography–mass spectrometry (GC–MS) but intact BHPs were detected at a concentration that was an order of magnitude higher using HPLC–APCI–MS. Adenosylhopane was the most abundant BHP in all the samples and comprised on average 27% of total BHPs, supporting earlier work indicating that adenosylhopane is an excellent soil-specific biomarker. The soil samples vary in vegetative cover and this is likely one of the main reasons for observing variation in BHP composition, suggesting that BHP biomarkers may be a valuable tool for assessing bacterial community structure in soil when used in cooperation with other molecular microbial ecology methods (such as DNA genotyping).     

Lithofacies,depositional environments,regional biostratigraphy and age of the Chitarwata Formation in the Bugti Hills,Balochistan, Pakistan

The Oligocene-early Miocene Chitarwata Formation records a critical interval of terrestrial sedimentation that predates the Siwalik deposits on the Potwar Plateau of north-central Pakistan. This Oligocene-early Miocene time interval has long been considered as lacking in the entire Indo-Pakistan region. The Chitarwata Formation is widely exposed in the Sulaiman Range, but has never been described in detail in the Sulaiman Lobe, where the famous fossiliferous strata called ‘Bugti Bone Beds’ have been known for over a century and half. The Chitarwata Formation represents coastal-delta at the base, and plain and fluvial environments at the top. Lithofacies and sedimentary structures of the Chitarwata Formation in the Bugti area are described in detail, and show a clearly distinct lithologic pattern, different from that reported from the Zinda Pir area. The Chitarwata Formation also records an important transition in the evolution of the drainage systems in the area during the late Paleogene and early Neogene. This transition from the west-flowing paleo-Indus fluvial system to the development of the ancestral Indus drainage system may explain the numerous hiatuses that characterize the Chitarwata Formation. The abundance of fossil mammals from the Chitarwata and overlying Vihowa formation in the Bugti Hills provides critical biochronologic information that sheds new light on biostratigraphic correlation with the Zinda Pir area and for the entire Sulaiman Range.     

Geostatistical analysis of arsenic concentration in groundwater in Bangladesh using disjunctive kriging

The National Hydrochemical Survey of Bangladesh sampled the water from 3,534 tube wells for arsenic throughout most of Bangladesh. It showed that 27% of the shallow tube wells (less than 150 m deep) and 1% of the deep tube wells (more than 150 m deep) exceeded the Bangladesh standard for arsenic in drinking water (50 µg L^–1). Statistical analyses revealed the main characteristics of the arsenic distribution. Concentrations ranged from less than the detection limit (0.5 µg L^–1), to as much as 1,600 µg L^–1, though with a very skewed distribution, and with spatial dependence extending to some 180 km. Disjunctive kriging was used to estimate concentrations of arsenic in the shallow ground water and to map the probability that the national limit for arsenic in drinking water was exceeded for most of the country (the Chittagong Hill Tracts and the southern coastal region were excluded). A clear regional pattern was identified, with large probabilities in the south of the country and small probabilities in much of the north including the Pleistocene Tracts. Using these probabilities, it was estimated that approximately 35 million people are exposed to arsenic concentrations in groundwater exceeding 50 µg L^–1 and 57 million people are exposed to concentrations exceeding 10 µg L^–1 (the WHO guideline value).     

A high resolution palaeoclimatic record for the last 27,500 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustrine organic matter

Variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of Lake Bosumtwi, Ghana reflect climatically induced changes to the lake and the catchment flora. Cores spanning the last 27.5 kyr of sedimentation in the lake show large oscillations inδ¹³C_org PDB andδ¹⁵N_org Air. The late Pleistocene record is particularly detailed, showing changes of ca. 20‰ in carbon and ca. 10‰ in nitrogen isotopic compositions. These variations are of complex origin. Although different in magnitude, major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the Last Glacial Maximum. The Allerød-Younger Dryas period also seems to have been marked by generally dry conditions in this part of tropical West Africa. Nitrogen isotopic evidence suggests that during the period 9.2–3.2 kyr the lake had an extremely stable water column, probably due to the absence of a windy or cool, dry season, or both. Regular circulation of the water column recommenced during the late Holocene and has persisted until the present day.     

A new late Pleistocene-Holocene water-level curve for Lake Bosumtwi,Ghana

Field data, supported by 28¹⁴C dates, show that during the late Pleistocene Lake Bosumtwi suffered a major regression and probably remained low for some time, the exposed lacustrine sediments undergoing considerable pedogenesis. Just after 13,000 B.P. the lake was rising again, and thereafter the crater has generally been characterised by a lake significantly higher than it is at present. These periods of high water level, of 2000–2500 years duration, were interrupted by short, but intense regressions centred around 10,500, 8000, 4000–4500 and just after 1000 years B.P. During the highstands the local climate is likely to have been broadly similar to today; the nature of the climatic changes responsible for the regressions is not known with any certainty.     

Reconstruction of tectonically deformed sedimentary structures

A method of reconstructing the initial shape of tectonically deformed sedimentary structures is presented. This method makes no assumptions regarding the state of strain in the rock. Examples are given in which the observed asymmetry of convolute structures is shown to be solely of tectonic origin.