Mid Basin Research Articles

A Review of 50 Years of Study of Hydrology, Wetland Dynamics, Aquatic Metabolism, Water Quality and Trophic Status, and Nutrient Biogeochemistry in the Barataria Basin, Mississippi Delta—System Functioning, Human Impacts and Restoration Approaches

Here we review an extensive series of studies of Barataria Basin, an economically and ecologically important coastal basin of the Mississippi Delta. Human activity has greatly altered the hydrology of the basin by decreasing riverine inflows from leveeing of the river and its distributaries, increasing runoff with high nutrient concentrations from agricultural fields, and channelization of wetlands of the basin interior that has altered flow paths to often bypass wetlands. This has resulted in degraded water quality in the upper basin and wetland loss in the lower basin. Trophic state analysis found the upper basin to be eutrophic and the lower basin to be mesotrophic. Gross aquatic primary production (GAPP) was highest in the upper basin, lowest in the mid basin, and intermediate in the lower basin. Forested wetlands in the upper basin have degraded over the past several decades due to increased periods of flooding, while there has been massive loss of emergent wetlands in the lower basin due to increasing water levels and pervasive alteration of hydrology. Restoration will entail reconnection of waterways with surrounding wetlands in the upper basin, and implementation of river sediment diversions, marsh creation using dredged sediments and barrier island restoration. Findings from this review are discussed in terms of the functioning of deltas globally.

Geogenic arsenic contamination of wet-meadows associated with a geothermal system in an arid region and its relevance for drinking water

Arsenic (As) is an important component in thermal springs, which can reach water sources constituting an important hazard for both the environment and people. For this reason, the aim of this paper is to analyze the geologic and geochemical processes that determine the presence and concentration of As in wet-meadows associated to a geothermal field in Patagonia (Argentina) which is used as water supply. To achieve this, during field surveys temperature, pH and electrical conductivity were measured and water and rock samples were taken. Major ions and stable isotopes were determined in water samples while As content was analyzed in both water and rock samples. Due to geological control and chemical analyses, three areas were recognized with respect to major streams in the geothermal field: wet-meadows at headwaters, thermal springs at mid basin and wet-meadows at down basin. Even though, water in wet-meadows have the same origin the obtained results evidence how thermal springs at mid-basin influence the chemistry of these wetlands, particularly those at down basin. In the latter, As raises over two orders of magnitude than the ones at headwaters which surpasses the reference limit, proving that thermal springs are also responsible for the increase of the As content in water changing its quality as a source of potable water. The concentration of this metalloid could be higher but it is retained in travertine and sinter deposits formed near the geothermal discharge area. Understanding processes controlling water quality and the chemistry of As in this type of wetlands is of vital importance, mainly in an arid region where water supply sources are scarce.

Gliders Measure Western Boundary Current Transport from the South Pacific to the Equator*

Abstract “Spray” gliders, most launched from small boats near shore, have established a sustainable time series of equatorward transport through the Solomon Sea. The first 3.5 years (mid-2007 through 2010) are analyzed. Coast-to-coast equatorward transport through the Solomon Sea fluctuates around a value of 15 Sv (1 Sv ≡ 106 m3 s−1) with variations approaching ±15 Sv. Transport variability is well correlated with El Niño indices like Niño-3.4, with strong equatorward flow during one El Niño and weak flow during two La Niñas. Mean transport is centered in an undercurrent focused in the western boundary current; variability has a two-layer structure with layers separated near 250 m (near the core of the undercurrent) that fluctuate independently. The largest variations are in midbasin, confined to the upper layer, and are well correlated with ENSO. Analysis of velocity and salinity on isopycnals shows that the western boundary current within the Solomon Sea consists of a deep core coming from the Coral Sea and a shallow core that enters the Solomon Sea in mid basin. Analysis of the structure of transport and its fluctuations is presented.

The Mid‐Mediterranean Jet Artefact

The Mid‐Mediterranean Jet has been considered to be, since the 1990s, a more or less continuous surface current meandering across most of the eastern basin of the Mediterranean Sea, generating a series of eddies. This relatively recent view of the current, which was not found in earlier circulation diagrams and does not have any theoretical support, emerges more or less from simple statistics applied to any kind of in situ data. Here we use a relatively large set of surface drifter trajectories, which provide reasonable sampling of the area of interest, to show how mesoscale anticyclonic eddies generated elsewhere with specific diameters, locations and displacements, can provide a misleading picture by suggesting the occurrence of a mid basin jet which is actually nothing more than a data analysis artefact.

Ecology and Status of the Magdalena River Turtle,Podocnemis lewyana, a Colombian Endemic

ABSTRACT We studied distribution, habitat selection, and reproductive ecology of the Magdalena river turtle, Podocnemis lewyana, from January to June 2003 and from January to March 2005 in the mid and low Sinu River, Cordoba, Colombia. The southern limit of its distribution occurs in the town of Carrizola, and the northern limit is near La Doctrina. We found more turtles of bigger size classes in the mid basin than in the low basin. Ninety-two individuals were observed basking in the river. Larger turtles were mostly observed in deep pools (> 3 m) with loamy or sandy bottoms, with easy access to basking sites, such as riverbanks, fallen trees, or beaches, and in areas with all types of shoreline vegetation but especially with riparian forest. Smaller turtles were found in colder, shallower, and more turbid waters than were adults, mainly in areas without riparian forest or in pastures with few trees, usually hidden in pastures flooded by the river. Adults ventured farther from shore than did juveniles. Al...

Using sediments to assess the resistance of a calcareous lake to diffuse nutrient loading

The capacity of lake sediments to retain phosphorus can provide an impor- tant buffer to eutrophication. Concentrations of total phosphorus (TP) in the surface sediment of a shallow calcareous lake (Lough Carra, Ireland) varied significantly across three basins and were correlated strongly with iron and manganese, but corre- lated inversely with calcium carbonate. The concentrations of phosphorus in the surfi- cial sediments of each basin were observed to reflect the mean annual TP of overlying waters, such that the North Basin > South Basin > Mid Basin. Phosphorus sorption experiments on the most TP-enriched sediments predicted a saturation concentration of 0.15 mg P g -1 . Geochemically similar sediments from the northern basin had slightly lower measured TP concentrations (0.10 mg P g -1 ), while those sediments with higher Fe concentrations had up to 0.54 mg P g -1 . Decreasing Fe:P in the upper sections of sediment cores taken from each lake basin show clearly a reduction in the ability of the lake sediments to bind P, reducing the resistance of the lake to eutrophication. The re- duced capacity of the sediments to adsorb P should be viewed as a warning signal that indicates increasing risk to water quality and current high conservation status of Lough Carra. The risk that the lake could flip to an alternative state, with high concentra- tions of phytoplankton and a loss of extensive charophyte beds, requires that measures are effected that will prevent or reduce nutrient loads to the lake. This will necessitate a reversal of the current trend of increasing catchment pressures from intensification of agriculture and increased rural housing.

Aspects of the petroleum geology of the Junggar Basin, Northwest China

Abstract The Junggar Basin is one of a family of productive interior basins in Northwest China. These are generally referred to as ‘flexural’ or ‘orogenic’ basins nucleated by Variscan orogenic episodes. Their uniqueness is enhanced by later rejuvenation and reactivation in the successive orogenic phases affecting Central Asia. The northwest margin of the Junggar Basin has been productive since the discovery of the Karamay-Wuerhe group of fields in the 1950s. The source rocks are generally considered to be Permian lacustrine sediments which provide mostly liquid hydrocarbons to basin margin alluvial fan reservoirs of Permian, Triassic and Jurassic age. The Karamay Thrust Belt, which forms the northwestern margin of the Junggar Basin, was initially formed during Variscan movements and re-activated during several subsequent structural episodes. According to KOC work there are many trap permutations associated with the Thrust Belt in ‘hanging-wall’ and ‘footwall’ situations. Recent studies have focused on the southwestern part of the Junggar Basin. Here marginal production from Himalayan periclinal structures adjacent to the Tien Shan, dates back to the beginning of the century. More recently Carboniferous ‘basement’ rocks on the southward extension of the Karamay Thrust Belt, the Hong Che Fault Belt, have proved productive. The stratigraphy shows the fairly continuous existence of lacustrine conditions in the mid basin since the Late Palaeozoic. Other studies have shown that ‘black mudstone’ conditions are prevalent in the Permian, Triassic, Jurassic and Early Tertiary and that oil-prone algal-rich intervals, indicating permanent or semi-permanent anoxia, are most likely in the Permian. The stratigraphic development also demonstrates the occurrence of suitable reservoir units in marginal alluvial fan-fluviatile clastics at most stratigraphic levels. The source-reservoir spatial relationships demonstrate favourable conditions for primary and secondary migration. The maturation history shows ‘deep’ Permian source rocks becoming mature for oil generation in the Triassic and for gas-generation from Cretaceous times. More gas-prone higher level source rocks generated hydrocarbons progressively from the Cretaceous through the Tertiary. The structural history shows migration routes controlled by regional tilting into the Tien Shan foredeep, and local migration and entrapment by re-activation of the Karamay Thrust Belt along the northwestern basin margin, and of intra-basinal late Variscan transpressional trends. Late Himalayan structuring along the southern margin and basin-wide tilt has effected a phase of re-migration and further funnelling of hydrocarbons to the productive zones of the northwest.