The role of the Río de la Plata bottom salinity front in accumulating debris

The Río de la Plata, one of the most important South American estuarine environments, is characterized by a bottom salinity front that generates an ecotone between the river and the estuary. Based on bottom trawls and costal sampling we describe the distribution, types, and amount of debris found in the bottom and shoreline across this front. Plastics and plastic bags were the main debris types in both areas. Concentrations of total debris upriver the front were always significantly higher than downriver the front showing that the front acts as a barrier accumulating debris. Moreover, a large part of debris end ups accumulated in the coastal area upriver the frontal position. This area is particularly sensitive because the coastline encompasses an UNESCO Man and the Biosphere Reserve and a Ramsar site, and due to the ecological significance of the front for many valuable species.     

Petrology of the Mecklenburg gabbro-metagabbro complex,North Carolina

The Mecklenburg complex, which crops out in the Carolina Piedmont, consists of older metagabbro and metafelsic rocks of upper amphibolite grade, and a younger postmetamorphic gabbro. On the evidence of field data and gravity measurements, the younger gabbro appears to be a plug-like body with steeply dipping contacts that may converge inward at relatively shallow depth; the pluton may extend only 8,000–15,000 feet into the subsurface.Gabbroic rocks contain plagioclase, olivine, orthopyroxene, clinopyroxene, hornblende, and biotite, and display hypidiomorphic textures characterized by numerous reaction relations among the mafic phases; hydrous minerals are commonly abundant, comprising up to 37% of the rock, and indicate the fugacity of H₂O was relatively high during primary crystallization. In contrast metagabbro lacks olivine and exhibits textures that range from relict igneous varieties to crystalloblastic types that appear to represent equilibrium assemblages. A less common metagabbro type, associated with local zones of deformation, is characterized by faint banding and only trace amounts of anhydrous mafics. Varieties of metagabbro are complexly intermingled in the field, and resultant assemblages and textures largely reflect whole-rock composition and differences in initial H₂O contents, slightly modified locally by addition of fluid along favorable channelways.In general metagabbro is slightly higher in TiO₂, Na₂O, and K₂O, and lower in MgO relative to gabbro; regardless of the degree of recrystallization, the chemistry of the various metagabbro types is nearly identical, thus indicating metamorphism was essentially isochemical.     

Strong Magnetic Levels in Lake Chapala Sediments (western Mexico): Their Mineralogy and Stratigraphic Significance

Lake Chapala, located 120 km northeast of Colima Volcano, lies at the north and northeast of the Citala rift in the Trans-Mexican Volcanic Belt. It belongs to the watershed of the Lerma River, which originates from the Mexico City area, 450 km to the east. Sediment cores, 0.5–2 m long, were collected from the lake. Magnetic susceptibility of the lake sediments generally ranges from 400 to 800 × 10⁻⁹ m³ kg⁻¹; but in some layers it exceeds 1000 or 1500 × 10⁻⁹ m³ kg⁻¹. The magnetic susceptibility vertical profiles display a thin peak (2–5 cm) or a double peak with magnetic susceptibility > 2000 × 10⁻⁹ m³ kg⁻¹. Scanning electron microscope analysis shows that the main mineral responsible for the magnetic susceptibility is titano-magnetite, which is relatively abundant in the magnetic layers. In most of the cores, the layer with magnetic susceptibility > 2000 × 10⁻⁹ m³ kg⁻¹ is coarser grained than the directly under and overlying sediments, which are composed of fine silt. But not all coarser levels are magnetic nor are all magnetic levels bound to coarser layers. The high titano-magnetic concentration probably originates from volcanic ash falls. Sedimentation rates, evaluated for several sites by means of the ²¹⁰Pb and ¹³⁷Cs methods, allow a date approximation (between 1535 and 1670) for the strong magnetic layer deposition. Since important eruptions of Colima Volcano, with ash fall, were reported from 1576 to 1623, the main peak of magnetic susceptibility in the sediment profiles is assumed to be related to these events. The main magnetic layer extends to greater depth in the profiles of the eastern part of the lake plain than at the west. It can, therefore, be suggested that a relative subsidence occurs in the eastern part of the lake; the axis of the eastern plain overlaps the area displaying the maximum subsidence rate and seems therefore to correspond to the prolongation of Citala rift.     

Biological,physical and chemical properties at the Subtropical Shelf Front Zone in the SW Atlantic Continental Shelf

The physical aspects of the Subtropical Shelf Front (STSF) for the Southwest Atlantic Continental Shelf were previously described. However, only scarce data on the biology of the front is available in the literature. The main goal of this paper is to describe the physical, chemical and biological properties of the STSF found in winter 2003 and summer 2004. A cross-section was established at the historically determined location of the STSF. Nine stations were sampled in winter and seven in summer. Each section included a series of conductivity-temperature-depth (CTD) stations where water samples from selected depths were filtered for nutrient determination. Surface samples were taken for chlorophyll a (Chl-a) determination and plankton net tows carried out above and below the pycnocline. Results revealed that winter was marked by an inner-shelf salinity front and that the STSF was located on the mid-shelf. The low salinity waters in the inner-shelf indicated a strong influence of freshwater, with high silicate (72 μM), suspended matter (45 mg l⁻¹), phosphate (2.70 μM) and low nitrate (1.0 μM) levels. Total dissolved nitrogen was relatively high (22.98 μM), probably due to the elevated levels of organic compound contribution close to the continental margin. Surface Chl-a concentration decreased from coastal well-mixed waters, where values up to 8.0 mg m⁻³ were registered, to offshore waters. Towards the open ocean, high subsurface nutrients values were observed, probably associated to South Atlantic Central Waters (SACW). Zooplankton and ichthyoplankton abundance followed the same trend; three different groups associated to the inner-, mid- and outer-shelf region were identified. During summer, diluted waters extended over the shelf to join the STSF in the upper layer; the concentration of inorganic nutrients decreased in shallow waters; however, high values were observed between 40 and 60 m and in deep offshore waters. Surface Chl-a ranged 0.07–1.5 mg m⁻³; winter levels were higher. Three groups of zoo and ichthyoplankton, separated by the STSF, were also identified. Results of the study performed suggest that the influence of freshwater was stronger during winter and that abundance distribution of Chl-a, copepods and ichthyoplankton was related to the Plata Plume Waters (PPW), rather than to the presence of the STSF. During summer, when the presence of freshwater decreases, plankton interactions seem to take place in the STSF.     

Early to Middle Devonian granitic and volcanic rocks from the central Gulf of Maine

Cashes Ledge igneous suite in the central Gulf of Maine is represented by 10 granitic and two felsic tuff samples collected from bedrock outcrops using the submersible Alvin in 1971–1972 and archived at the Woods Hole Oceanographic Institute. Laser ablation ICP-MS analyses of zircon grains yielded crystallization ages of 414.9 ± 1.1 Ma and 399.7 ± 1.5 Ma for two alkali feldspar granite samples, 407.0 ± 1.9 Ma for a syenogranite sample, and 384.4 ± 2.3 Ma and 383.9 ± 1.6 Ma for two felsic tuff samples. The samples contain iron-rich mafic minerals, including aegirine-augite, grunerite/ferroedenite, and annite. Most of the samples are alkaline to slightly peralkaline, with high concentrations of SiO₂, Y, Zr, Nb, and REE, strong negative Eu anomalies, and positive epsilon Nd values (1.8 to 3.7). The suite resembles part of a belt of similar Silurian–Devonian rocks with ages between 426 and 370 Ma now recognized in the central part of Avalonia in southeastern New England. They formed in a long-lived, likely extensional regime linked to subduction and subsequent complex transcurrent motions among Ganderia, Avalonia, and Meguma, culminating in the closure of the Rheic Ocean.     

Radiocarbon age inversions and progression: source and causes in Late Holocene sediments from Lake Chapala,western Mexico

The results of eight radiocarbon datings of Lake Chapala sediments (site T46) are presented, the age inversions (AI) observed and their age progression discussed. As deduced from some AIs and the ²¹⁰Pb activity (site CHP4), the bioturbation zone in the lake varies over a depth of 5–25 cm. The linear sedimentation rates (LSRs) calculated from ¹⁴C ages do not match the LSR calculated from unsupported ²¹⁰Pb activity for the upper sediments. This demonstrates the usefulness of dating sediments with complementary radiometric techniques such as short-lived isotope counting (SLIC), i.e., ²¹⁰Pb and ¹³⁷Cs. This approach leads to the following conclusions: (1) The incorporation of detrital particles with ancient carbon into the sedimentary column of the lake occurred by a combination of: (a) the presence of outcrops of hydrothermal petroleum with ages >40 ka (ka = thousands of years) in the lake, and (b) mass transport due to the presence of two elongated gyre circulation patterns integrated by cyclonic circulation (counterclockwise) in the north portion of the lake and anticyclonic circulation in the southern part. (2) Consequently, the ¹⁴C ages of shallow lake sediments have geologic ages one order of magnitude greater compared to their ages determined by the ²¹⁰Pb method. (3) A bioturbation mechanism is not necessary to explain the ¹⁴C AI in the top 70 cm and from 110 to 150 cm depth of the sediments. (4) According to the biological proxies data for the last 600 years B.P., the paleoclimate at Lake Chapala has changed from sub-humid to dry environmental conditions, and eutrophication has increased over the past 100 years due to local input from ongoing agricultural activities.