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An investigation of the genus \textitMesacanthus (Chordata: Acanthodii) from the Orcadian Basin and Midland Valley areas of Northern and Central Scotland using traditional morphometrics
No full textMesacanthus is a common and speciose genus of acanthodian fish from Lower Old Red Sandstone and Middle Old Red Sandstone assemblages (representing the Lower Devonian and Middle Devonian respectively) and is well represented in many palaeoichthyology collections in the UK. Based upon descriptions given during the 19th century, specimens of the genus Mesacanthus from the Orcadian Basin and Midland Valley areas of Northern and Central Scotland have historically been referred to a number of different species; of these, the most frequently discussed in the literature are M. mitchelli, M. peachi and M. pusillus. In order to test the validity of these three species, traditional morphometric analyses were carried out on over 100 specimens of Mesacanthus, from both the Lower Devonian and the Middle Devonian, that cover the full range of known localities for these taxa in Northern and Central Scotland. Based upon morphological and morphometric comparisons, this investigation has found that at least two species of Mesacanthus are valid (M. mitchelli and M. pusillus) as specimens from the Lower Devonian and Middle Devonian have been shown to differ significantly in a number of important ways. However, no evidence has been found for the validity of the second and distinct Middle Devonian species, M. peachi
Li partitioning in the benthic foraminifera Amphistegina lessonii
No full textThe shallow water benthic foraminifer Amphistegina lessonii was grown in seawater of variable Li and Ca concentration and shell Li/Ca was determined by means of LA-ICPMS. Shell Li/Ca is positively correlated to seawater Li/Ca only when the Li concentration of seawater is changed. If the seawater Ca concentration is changed, shell Li/Ca remains constant. This indicates that Li does not compete with Ca for incorporation in the shell of A. lessonii. A recently proposed calcification model can be applied to divalent cations (e.g., Mg and Sr), which compete for binding sites of ion transporters and positions in the calcite lattice. By contrast, the transport pathway of monovalent cations such as Li is probably diffusion based (e.g., ion-channels), and monovalent cations do not compete with Ca for a position in the calcite lattice. Here we present a new model for Li partitioning into foraminiferal calcite which predicts our experimental results and should also be applicable to other alkali metals
A Late Glacial paleolake record from an up-dammed river valley in northern Transylvania, Romania
No full textLake sediments from the Măgheruş Valley in the lowlands of northern Transylvania provide new evidence of paleoenvironmental development in Romania during the Late Glacial. The studied sediments were deposited as a result of the damming of a small river valley following a series of mass wasting events that occurred during the deglaciation period. A continuous sedimentary sequence belonging to the former lake is preserved in the banks of the stream, and contains evidence of paleoenvironmental changes associated with the Late Glacial Interstadial (Bølling–Allerød or Greenland Interstadial 1, GI-1) and the Late Glacial Stadial (Younger Dryas or Greenland Stadial 1, GS-1), implying that the hallmark climatic episodes of northwestern Europe are also expressed in Eastern Europe. We employ a multiproxy approach based on the analysis of sediment composition and texture, mineral magnetism, organic macrofossils, and radiocarbon dating. The reconstructed paleoenvironmental evolution at the site captures the warm and humid conditions associated with GI-1, as well as the subsequent cooling concomitant with the onset of GS-1. These climatic events are paralleled by vegetation shifts in the region, as deduced from comparisons with pollen sequences from Măgheruş and neighboring locations in Transylvania and the Carpathians. The Late Glacial Interstadial was warmer and wetter, as evidenced by increased organic matter content in the lake, decreased erosion in the catchment, development of palustrine plant communities in the proximity of the lake, and expansion of spruce during the Allerød (GI-1c-a). The Late Glacial Stadial was colder and dryer, as indicated by low sedimentary organic matter content, an increase in erosion markers, and the decline of spruce and its replacement by birch. The landscape was more open, as herbs and grasses also expanded during this time. These fluctuations imply that Late Glacial climatic events are well expressed not only in upland areas of Romania, but also in lowland regions such as the Transylvanian Basin, where climatic effects are expected to be more muted
Constraints on soluble aerosol iron flux to the Southern Ocean at the Last Glacial Maximum
No full textRelief of iron (Fe) limitation in the Southern Ocean during ice ages, with potentially increased carbon storage in the ocean, has been invoked as one driver of glacial–interglacial atmospheric CO2 cycles. Ice and marine sediment records demonstrate that atmospheric dust supply to the oceans increased by up to an order of magnitude during glacial intervals. However, poor constraints on soluble atmospheric Fe fluxes to the oceans limit assessment of the role of Fe in glacial–interglacial change. Here, using novel techniques, we present estimates of water- and seawater-soluble Fe solubility in Last Glacial Maximum (LGM) atmospheric dust from the European Project for Ice Coring in Antarctica (EPICA) Dome C and Berkner Island ice cores. Fe solubility was very variable (1–42%) during the interval, and frequently higher than typically assumed by models. Soluble aerosol Fe fluxes to Dome C at the LGM (0.01–0.84 mg m−2 per year) suggest that soluble Fe deposition to the Southern Ocean would have been ≥10 × modern deposition, rivalling upwelling supply
Reduced ventilation and enhanced magnitude of the deep Pacific carbon pool during the last glacial period
No full textIt has been proposed that the ventilation of the deep Pacific carbon pool was not significantly reduced during the last glacial period, posing a problem for canonical theories of glacial–interglacial CO2 change. However, using radiocarbon dates of marine tephra deposited off New Zealand, we show that deep- (>2000 m>2000 m) and shallow sub-surface ocean–atmosphere 14C age offsets (i.e. ‘reservoir-’ or ‘ventilation’ ages) in the southwest Pacific increased by ∼1089 and 337 yrs respectively, reaching ∼2689 and ∼1037 yrs during the late glacial. A comparison with other radiocarbon data from the southern high-latitudes suggests that broadly similar changes were experienced right across the Southern Ocean. If, like today, the Southern Ocean was the main source of water to the glacial ocean interior, these observations would imply a significant change in the global radiocarbon inventory during the last glacial period, possibly equivalent to an increase in the average radiocarbon age >2 km>2 km of ∼700 yrs∼700 yrs. Simple mass balance arguments and numerical model sensitivity tests suggest that such a change in the ocean's mean radiocarbon age would have had a major impact on the marine carbon inventory and atmospheric CO2, possibly accounting for nearly half of the glacial–interglacial CO2 change. If confirmed, these findings would underline the special role of high latitude shallow sub-surface mixing and air–sea gas exchange in regulating atmospheric CO2 during the late Pleistocene
Speculations on the formation of cratons and cratonic basins
No full textSurface wave tomography using Rayleigh waves has shown that Tibet and the surrounding mountain ranges that are now being shortened are underlain by thick lithosphere, of simi-
lar thickness to that beneath cratons. Both their elevation and lithospheric thickness can result from pure shear shortening of normal thickness continental lithosphere by about a factor of two. The resulting thermal evolution of the crust and lithosphere is dominated by radioactive decay in the crust. It raises the temperature of the lower part of the crust and of the upper part of the lithosphere to above their solidus temperatures, generating granites and small volumes of mafic alkaline rocks from beneath the Moho, as well as generating high temperature metamorphic assemblages in the crust. Thermal models of this process show that it can match the P, T estimates determined from metamorphic xenoliths from Tibet and the Pamirs, and can also match the compositions of the alkaline rocks. The seismological properties of the upper part of the lithosphere beneath northern Tibet suggest that is has already been heated by the blanketing effect and radioactivity of the thick crust on top. If the crustal thickness is reduced by erosion alone to its normal value at low elevations, without any tectonic extension, over a time scale that is short compared to the thermal time constant of thick lithosphere, of ∼ 250 Ma, thermal subsidence will produce a basin underlain by thick lithosphere. Though this simple model accounts for the relevant observations, there is not yet sufficient information available to be able to model in detail the resulting thermal evolution of the sediments deposited in such cratonic basins
Site characterization and risk assessment in support of the design of groundwater remediation well near a hazardous landfill
No full textThis study was undertaken to support a remediation technique that will be applied to contaminated groundwater in the vicinity of Ghonan landfill, eastern Saudi Arabia. Preliminary field investigations involved hydrogeological characterization of the entire area. The improved understanding of the underlying geology and groundwater movement gained from the preliminary studies helped in determining a test site within the proximity of the landfill. One remediation well and five monitoring wells were constructed at the test site. Optimum pumping rate for the remediation well was determined to range from 2.642 through 7.926 gallons per minute (gpm) (0.01 m3/min through 0.03 m3/min) based on site-specific hydrogeological investigations and mathematical simulation. A concentration of 0.05 mg/L of methyl tertiary-butyl ether (MTBE) contaminated the aquifer in the test site. The simulated concentration of MTBE at the point of exposure after a period of 2 years was found to be higher than the maximum contaminant level of 0.005 mg/L set by the United States Environmental Protection Agency (U.S. EPA). The results of risk assessment conducted revealed that domestic use of groundwater in the study area through any of the exposure pathways (ingestion, dermal contact, and inhalation in the shower) may lead to development of health risks to human receptors. The landfill, which is being operated as a hazardous landfill and a dump site, may become a source of groundwater pollution in its vicinity in the near future. As a potential health risk, it should be controlled properly by remediating the aquifer and implementing environmental measures to the landfill users
The Earliest History of the Skaergaard Magma Chamber: a Textural and Geochemical Study of the Cambridge Drill Core
No full textThe Cambridge Drill Core provides a continuous sample of the lower part of the floor cumulates of the Skaergaard Intrusion, including ∼150 m of stratigraphy from the unexposed Hidden Zone. Bulk-rock geochemistry together with olivine mineral compositions and augite–plagioclase–plagioclase dihedral angles from the drill core are interpreted as a record of the early history of the Skaergaard magma chamber. A detailed geochemical and microstructural study of the mode and morphology of augite reveals no unambiguous markers that can be used to pinpoint the first appearance of cumulus augite in the stratigraphy sampled by the drill core. The early history of the Skaergaard magma chamber involved the arrival of multiple small batches of magma, each with a variable load of olivine and plagioclase phenocrysts. The region of the core between –120 and –108·8 m records at least three separate, but closely spaced, influxes of new magma, with more magma added at –85 and –65 m. The last influx of magma is recorded by the cumulates just below the lowest exposed horizons of the Layered Series, involving a large volume of magma that inflated the chamber to its final size. The Skaergaard magma chamber therefore formed by the progressive inflation of what was likely to have originated as a sill intruded at the discontinuity between the Precambrian gneisses and the overlying plateau lavas. The ‘Skaergaard parental magma’ should therefore be viewed as the integrated and mixed composition of the various magma influxes that filled the chamber
Anaerobic oxidation of methane by sulfate in hypersaline groundwater of the Dead Sea aquifer
No full textGeochemical and microbial evidence points to anaerobic oxidation of methane (AOM) likely coupled with bacterial sulfate reduction in the hypersaline groundwater of the Dead Sea (DS) alluvial aquifer. Groundwater was sampled from nine boreholes drilled along the Arugot alluvial fan next to the DS. The groundwater samples were highly saline (up to 6300 mm chlorine), anoxic, and contained methane. A mass balance calculation demonstrates that the very low δ13CDIC in this groundwater is due to anaerobic methane oxidation. Sulfate depletion coincident with isotope enrichment of sulfur and oxygen isotopes in the sulfate suggests that sulfate reduction is associated with this AOM. DNA extraction and 16S amplicon sequencing were used to explore the microbial community present and were found to be microbial composition indicative of bacterial sulfate reducers associated with anaerobic methanotrophic archaea (ANME) driving AOM. The net sulfate reduction seems to be primarily controlled by the salinity and the available methane and is substantially lower as salinity increases (2.5 mm sulfate removal at 3000 mm chlorine but only 0.5 mm sulfate removal at 6300 mm chlorine). Low overall sulfur isotope fractionation observed (34ε = 17 ± 3.5‰) hints at high rates of sulfate reduction, as has been previously suggested for sulfate reduction coupled with methane oxidation. The new results demonstrate the presence of sulfate-driven AOM in terrestrial hypersaline systems and expand our understanding of how microbial life is sustained under the challenging conditions of an extremely hypersaline environment
Multiple sulfur isotope constraints on the sulfur cycle in the modern ocean
No full textWe present 28 multiple sulfur isotope measurements of seawater sulfate (δ34SSO4δ34SSO4 and Δ33SSO4Δ33SSO4) from the modern ocean over a range of water depths and sites along the eastern margin of the Pacific Ocean. The average measured δ34SSO4δ34SSO4 is 21.24‰ (±0.88‰,2σ±0.88‰,2σ) with a calculated Δ33SSO4Δ33SSO4 of +0.050‰+0.050‰ (±0.014‰,2σ±0.014‰,2σ). With these values, we use a box-model to place constraints on the gross fraction of pyrite burial in modern sediments. This model presents an improvement on previous estimates of the global pyrite burial flux because it does not rely on the assumed value of δ34Spyriteδ34Spyrite, which is poorly constrained, but instead uses new information about the relationship between δ34Sδ34S and δ33Sδ33S in global marine sulfate. Our calculations indicate that the pyrite burial flux from the modern ocean is between 10% and 45% of the total sulfur lost from the oceans, with a more probable range between 20% and 35%