Sedimentary evidence of deglacial megafloods in the northern Gulf of Mexico (Pigmy Basin)

Cored sediments from the Pigmy Basin, northern Gulf of Mexico, were analyzed in order to better constrain late deglacial and early Holocene paleoenvironmental and sedimentary changes in response to North American climate evolution. Mineralogical and geochemical proxies indicate the succession of two sedimentary regimes: dominantly detrital during the deglaciation (15–12.9 cal ka BP) whereas biogenic contribution relatively increased later on during the Younger Dryas and early Holocene (12.9 and 10 cal ka BP). Geochemical data reveal that the deglacial record mainly reflects variations of terrigenous supply via the Mississippi River rather than modifications of redox conditions in the basin. Specific variations of almost all the parameters measured in this paper are synchronous with the main deglacial meltwater episode (Meltwater Spike) described or modeled in previous marine or continental studies. During this episode, most parameters display “stair-step-like” – pattern variations highlighting three successive steps within the main meltwater flow. Variations in grain-size and clay mineral assemblage recorded in the Pigmy Basin indicate that the erosional regime was very strong on land during the first part of the Meltwater Spike, and then milder, inducing more subtle modifications in the sedimentary regime in this part of the Gulf. Specific geochemical and mineralogical signatures (notably, clay minerals and trace metal geochemistry) pinpoint a dominant origin from NW North America for detrital particles reflecting meltwater outflow from the south-western Laurentide Ice Sheet (LIS) margin during the most intense freshwater discharge. The observed decrease of the sedimentation rate from about 200 to 25 cm/ka at ca 12.9 ka evidenced a drastic decrease of erosional processes during late phase of discharge, consistently with the hypotheses of major reduction of meltwater flow. The major modification at 12.9 cal ka BP is interpreted to result from both modifications of the main Mississippi fluvial regime due to eastward and northward rerouting of meltwater flow at the onset of the Younger Dryas, and the increase of sea-surface temperature linked to insolation. Finally, slight grain-size modifications suggest that some freshwater discharges may have episodically reached the Gulf of Mexico after the Younger Dryas reflecting possible small adjustments of the postglacial hydrological regime.     

Pliocene extensional tectonics in the Eastern Central Patagonian Cordillera: geochronological constraints and new field evidence

Recent field work and review of radiometric data obtained from Neogene lavas and plutonic rocks exposed in the Eastern Central Patagonian Cordillera (46–48ºS), which overlie subducted segments of the South Chile Ridge, suggest important Late Miocene to Pleistocene morphological changes in relation to base level variations and/or tectonic events. We present new field observations from a region south of Lago General Carrera‐Buenos Aires, between the main Cordillera and the Meseta del Lago Buenos Aires, demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5–4 Ma and after 3 Ma. We also show that the 12 Ma basaltic flows of the Meseta del Lago Buenos Aires (∼2000 m a.s.l.) have been subjected to deep incision, with younger lavas dated at 1.2 Ma partially filling the valleys. These incisions are thought to reflect progressive eastward tilting of the entire meseta. Our new observations, together with additional features from Central Patagonia, strongly suggest that tectonic events led to a regional widespread morphological change after 5 Ma. The coincidence in time and space between the subduction of segments of the South Chile Ridge at 6 and 3 Ma causing opening of a slab window, and strong base level variations in the studied area, suggests a cause‐and‐effect relationship. In Central Patagonia, compressional tectonics ended well before extensional events reported here. Causes of uplift and further extension are probably completely disconnected. The uplift is purely tectonic in origin and occurred prior to the subduction of the South Chile Ridge. Extension should be a consequence of this subduction.     

Multi-scale slope instabilities along the Nile deep-sea fan,Egyptian margin: A general overview

The Nile deep-sea fan (NDSF), turbiditic system reaching a size of about 90,000 km², has been investigated since 1998 by several geophysical methods (multibeam bathymetry, backscatter imagery, seismic data, 3–5 kHz echo-sounding). The analysis of this important data set evidenced that the NDSF is the locus of numerous multi-scale slope instabilities. Three main types of instabilities have been defined, mainly on the basis of their size or origin. (1) First type of instabilities related to the generalized gravity spreading of the Plio-Quaternary deep-sea fan on Messinian salt layers. This global spreading is accommodated by numerous localized slides. (2) Second type of instabilities correspond to giant mass movements probably triggered either by earthquakes, fluids, or climate and eustatic oscillations. Finally, (3) third type of instabilities correspond either to localized levee liquefactions or to thin-skinned slides on the steep slopes of the Eratosthenes seamount. The deposits generated by these slope movements greatly participate in the building of the NDSF. The characterization of these different instabilities, in a petroleum province as the NDSF, has important implications in terms of risk assessments when considering drilling operations.     

Changes of coastal sedimentation in the Gulf of Tehuantepec,South Pacific Mexico,over the last 100 years from short-lived radionuclide measurements

Short-lived radionuclides (²¹⁰Pb and ¹³⁷Cs) were used to document sedimentation regime changes over the last ～100 years, from analysis of a single sedimentary sequence collected in the coastal zone of the Gulf of Tehuantepec, South Pacific of Mexico. Sedimentation rates found in the core Tehua II-21 varied from 0.03 to 0.21 cm yr^?1, whereas mass accumulation rates ranged from 0.05 to 0.29 g cm^?2 yr^?1. ¹³⁷Cs data validate results obtained from ²¹⁰Pb measurements and confirm significant changes in the sedimentation regime between 1950 and 1970. We associate these alterations to land use changes, including deforestation for agriculture development and industrialization, as well as the regimentation of the Tehuantepec River from the early 1960s to the late 1970s. This interval has been marked by a major demographic expansion that followed the industrial development of the area.     

Dispersal of natural and anthropogenic lead through submarine canyons at the Portuguese margin

Submarine canyons represent natural conduits for preferential transport of particulate material, including anthropogenic contaminants, from coastal zones directly to the deep sea. To assess related dispersal of natural and anthropogenic lead (Pb), we analyzed Pb concentrations and stable isotope ratios in surface sediments and sediment trap particulate material from the Portuguese margin Nazaré and Setúbal/Lisbon canyons. Geochemical data are integrated with previously obtained data on near-bottom hydrodynamics and processes and pathways of sediment transport. The two canyon systems are located in close geographic proximity to each other, but represent contrasting settings in terms of sediment input and down-canyon sediment transport processes.Concentration–isotope diagrams and three-isotope plots (²⁰⁶Pb/²⁰⁷Pb vs. ²⁰⁸Pb/²⁰⁶Pb) suggest binary mixing between natural and anthropogenic end members. The inferred isotopic signature of pollutant Pb (²⁰⁶Pb/²⁰⁷Pb=1.143 [1.134–1.149, 95% confidence interval]) is most consistent with industrial Pb; ongoing influence from gasoline Pb additives is at most of minor importance. Two proposed natural end members most likely bracket the isotopic signature of natural Pb. Accordingly, binary mixing calculations indicate that on average 20–45% vs. 35–55% of total Pb is derived from anthropogenic sources in the Nazaré and Setúbal–Lisbon canyon systems, respectively. Enhanced anthropogenic influence in the latter area is consistent with its proximity to heavily populated and industrialized areas and with sediment input from the Tagus and Sado rivers, potential major carriers of pollutant particles. In both canyon systems, the anthropogenic component generally decreases with increasing water depth.Isotopic signatures of sediment trap particulate material are generally consistent with surface sediment data at similar water depth, but show large variability in the upper Nazaré canyon and major deviations from surface sediments in the lower canyon. In the lower canyon, Pb isotopic ratios of sediment trap particulate material mostly reflect low pelagic fluxes from the overlying water column, whereas surface sediment signatures are dominated by episodic down-canyon mass transport events. Such gravity flows appear to incorporate older (pre-industrial) material masking the isotopic signature of pollutant Pb. Large variability in the upper canyon reflects continuous sediment resuspension by bottom currents. Stronger average bottom currents are associated with higher ²⁰⁶Pb/²⁰⁷Pb ratios of sediment trap particulate material and hence decreased influence of pollutant Pb. This may reflect preferential resuspension of natural Pb at the canyon floor and/or additional remobilization of older, less-polluted sediment in adjacent areas such as the canyon walls.     

Near-bed particle deposition and resuspension in a cold-water coral mound area at the Southwest Rockall Trough margin,NE Atlantic

Cold-water coral reefs and mounds are observed mainly on slopes and topographic highs, in areas with high current speeds. Previous investigations of the near-bed hydrodynamic regime around cold-water coral mounds at the Southwest Rockall Trough margin have revealed the presence of internal waves with a diurnal tidal frequency. Hitherto only short-term measurements existed on the particle supply to the corals and data are lacking on the seasonal variability. Bottom landers equipped with sensors recording near-bottom current dynamics were deployed at two sites in a mound area on the Southwest Rockall Trough margin, one with a dense coral cover and one without coral cover. At both sites a similar seasonal variation in internal-wave activity was recorded with high activity during winter and summer months and less dynamic conditions in spring and autumn. Increased intensity of internal-wave activity, reflected in higher average near-bottom current speed and amplitude of daily temperature fluctuations, results in higher mass fluxes as recorded in the sediment traps. On the site without coral cover, mass fluxes are two times higher, compared to the site with dense coral cover. During periods of high mass fluxes a predominance of resuspended material was observed at both sites, as indicated by reduced ²¹⁰Pb activity and low organic matter concentrations. The flux of resuspended material largely masked the primary pelagic signal. However, low δ¹⁵N values in early spring and summer marked the arrival of fresh particles on both sites. A dense coral framework, baffling a large amount of particles settling between the coral branches, results in differences in particle flux, chemical composition and freshness of the trapped material. On the long term the presence of a coral framework plays a crucial role in the build-up of cold-water coral mounds.     

Twist transport in strongly torsioned astrophysical flux tubes

Riemannian geometrical effects on the expansion of the electron magnetohydrodynamical (EMH) superconductivity modeled twisted nonplanar thin magnetic flux tubes are considered. A solution is found which represents almost incompressible plasma flows, where the twist of flux tube is computed in terms of the continuous variation of its cross-section. It is shown that the twist increases in regions where twisted flux tube expands as in Parker’s conjecture. From computation of compression along the tube we show that when the torsion is weak a centrifugal or vorticity effect on the longitudinal direction of the tube enhances the screening effect on the “superconductor”. Throughout the paper we consider helical flux tubes where torsion and curvature of the tube are constants. Thus we show that the Parker’s conjecture is valid in a continuos manner for these type II superconducting twisted flux tubes. Throughout the paper we adopt the approximation that the radial component of the magnetic field varies so slowly along the tube axis that it can be approximated to zero along the tube. It is suggested that the models discussed here may also be applied to DNA and nanotubes.