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Were transgressive black shales a negative feedback modulating glacioeustasy in the Early Palaeozoic Icehouse
No full textThe Early Palaeozoic Icehouse (Late Ordovician–Early Silurian, c. 455–425 Ma) was
a remarkable event in the Earth’s climatic history, marked by extensive glaciations occurring at a
time of elevated atmospheric CO2. The oceanography of the Early Palaeozoic Icehouse was
markedly different from that of modern oceans, with frequent episodes of oceanic anoxia and
high concentrations of CO2 which may have acidified the oceans and restricted carbonate
burial. Thus, the marine organic carbon reservoir may have more strongly influenced long-term
changes in atmospheric CO2 than at present. We suggest that deposition of black shales represented
a major sink for atmospheric carbon. Sequence stratigraphy reveals that widespread
black shale deposition occurred in transgressions, whereas regressions are characterized by deposition
of bioturbated facies, allowing changes in lithofacies and deep-water redox conditions to be
related to the Early Palaeozoic carbon cycle. Assuming increased temperature is a function of
increased atmospheric CO2, and that glacioeustatic sea-level can serve as a proxy for temperature
due to changing ice volume, we infer that the deposition of transgressive black shales may have
acted as a negative feedback mechanism, drawing down CO2 and preventing the onset of runaway
greenhouse conditions
High-resolution benthic foraminiferal stable isotope stratigraphy across the Oligocene-Miocene boundary at Site 1218.
No full textThe transition from the late Oligocene warm period into the early Miocene was marked by a series of rapid and brief episodes of cryospheric expansion and global cooling. We analyzed benthic foraminifers from nannofossil oozes recovered at Ocean Drilling Program Site 1218 to construct a stable isotope stratigraphy for the deep Pacific
Emerging perspectives on deep-ocean circulation and nutrient storage from Nd isotope records
No full textZircon age constraints on sediment provenance in the Caspian region
No full textSensitive high-resolution ion microprobe (SHRIMP) U–Pb ages for detrital zircons from the Caspian region reveal the age ranges of basement terrains that supplied the sediment. One sample from the modern Volga river has groupings at c. 340–370 Ma, c. 900–1300 Ma and c. 1450–1800 Ma, with a small number of older zircons. This is consistent with derivation from the Precambrian basement of the East European Craton, and Palaeozoic arcs in the Urals. Mid- and Late Proterozoic components may be derived from beyond the present Volga drainage basin, such as the Sveconorwegian orogen. A Bajocian sandstone from the Greater Caucasus has 73% zircons that post-date 350 Ma. Ages cluster at c. 165–185 Ma, c. 220–260 Ma, c. 280–360 Ma and c. 440–460 Ma. This pattern suggests derivation from Palaeozoic basement of the Greater Caucasus itself and/or the Scythian Platform, and igneous rocks generated at a Jurassic arc in the Lesser Caucasus. Four samples from the Lower Pliocene Productive Series of the South Caspian Basin have common Phanerozoic grains, and groups between c. 900–1300 Ma and 1500–2000 Ma. Each sample contains zircons dated to c. 2700 Ma. The overall age patterns in the Productive Series samples suggest a combination of East European Craton and Greater Caucasus source components
Coupled sea surface temperature–seawater δ18O reconstructions in the Arabian Sea at the millennial scale for the last 35 ka
No full textTwo sediment cores from the western (905; 10.46°9′N, 51.56°4′E, water depth 1586 m) and eastern (SK17; 15°15′N, 72°58′E, water depth 840 m) Arabian Sea were used to study past sea surface temperatures (SST) and seawater δ 18O (δ 18Ow) variations for the past 35 ka. We used coupled Mg/Ca-δ 18O calcite variability in two planktonic foraminiferal species: Globigerinoides ruber, which thrives throughout the year, and Globigerina bulloides, which occurs mainly when surface waters contain high nutrients during upwelling or convective mixing. SSTs in both areas based on Mg/Ca in G. ruber were ∼3 to 4°C lower during the Last Glacial Maximum (LGM; ∼21 ka) than today and the Holocene period. The SST records based on G. bulloides also indicate general cooling, down to 18°C in both areas. SSTs in the western Arabian Sea based on G. bulloides were always lower than those based on G. ruber, indicating the presence of strong seasonal temperature contrast during the Holocene and LGM. We interpret the consistent presence of this seasonal temperature contrast to reflect a combination of seasonal summer upwelling (SW monsoon) and winter convective mixing (NE monsoon) in the western Arabian Sea. In the eastern Arabian Sea, G. bulloides-based SSTs were slightly lower (about 1°C) than G. ruber-based SSTs during the Holocene, indicating the almost absence of a seasonal temperature gradient, similar to today. However, a large seasonal temperature contrast occurred during the LGM which favors the assumption that strong NE monsoon winds forced winter upwelling or convective mixing offshore Goa. SST and δ 18Ow reconstructions reveal evidence of millennial-scale cycles, particularly in the eastern Arabian Sea. Here, the stadial periods (Northern Hemisphere cold periods such as Younger Dryas and Heinrich events) are marked by increasing SSTs and salty sea surface conditions relative to those during the interstadial periods. Indeed, the δ 18Ow record shows evidence of low-saline surface waters during interstadial periods, indicating increased freshwater runoff from the Western Ghats as a consequence of enhanced SW monsoon intensity
Quantification of soil erosion rates related to ancient Maya deforestation
No full textWe used seismic and sediment core data to quantify soil erosion rates for the past ~6000 yr in the closed catchment of Lake Salpetén, in the tropical lowlands of northern Guatemala. The region was affected by ancient Maya land use from before ca. 1000 B.C. to A.D. 900. This period of human impact coincided with deposition in the lake of a detrital unit (Maya Clay) as much as 7 m thick that contrasts sharply with the relatively organic-rich gyttja deposited both before and after Maya occupation of the watershed. The greatest soil loss, with mean sustained values of ~1000 t/km2yr–1, occurred in the Middle and Late Preclassic Periods (700 B.C. to A.D. 250), associated with relatively low Maya population densities. Soil erosion slowed during the period of maximum population density in the Late Classic Period (A.D. 550–830), indicating a decoupling between human population density and soil erosion rate. The most rapid soil loss occurred early during initial land clearance, suggesting that even low numbers of people can have profound impacts on lowland tropical karst landscapes
The origin of dolomite in the Asmari Formation (Oligocene-Lower Miocene), Dezful Embayment, SW Iran
No full textDolomitisation is an important factor controlling reservoir quality in the Asmari Formation in many producing fields in SW Iran. Dolostones have higher average porosities than limestones. Petrographic and geochemical studies have been used to determine the causes of Asmari dolomitisation at the Bibi Hakimeh and Marun fields and at the Khaviz anticline. The formation is generally characterized by a large-scale trend of upward-decreasing accommodation. Basal strata were deposited under relatively open-marine, high-energy conditions, whereas the Middle to Upper Asmari succession was deposited in relatively protected settings with more frequent evidence of exposure and evaporitic conditions. There is a general upward increase in the abundance of both anhydrite (occurring as nodules and cement) and dolomite.
Two main types of dolomite fabric are recognised, reflecting the textures of the precursor limestones: (1) finely crystalline pervasive dolomite (commonly <20μ) replacing mud-rich facies; and (2) combinations of finely crystalline replacive dolomite and surrounding areas of coarser dolomite cement (crystals up to 100μ) in grain-supported facies. Fluid inclusion data indicate that finely crystalline dolomites formed at low temperatures (ca. <50°C), while the coarser dolomite formed at higher temperatures (50–;140°C).
Whole rock-carbonate oxygen and carbon isotope analyses of pure dolostone samples show no apparent correlation with either depositional or diagenetic textures: δ18O is generally 0 to 2.7‰ PDB, and δ13C is −1 to 4‰ PDB. The importance of evaporated seawater to Asmari dolomitisation is indicated by the ubiquitous occurrence of felty-textured anhydrite nodules in dolostone beds and the presence of high-salinity fluid inclusions in dolomite. The derivation of dolomitising fluids from contemporaneous seawater is supported by the general correspondence between age estimates derived from the strontium isotope composition of anhydrites and dolomites and those derived from stratigraphic considerations. This suggested synsedimentary dolomitisation.
Dolomitisation of the upper half of the Asmari Formation may have occurred as a result of two syn-sedimentary mechanisms: (1) by the reflux of evaporative brines concentrated in shallow lagoons or sabkhas, through immediately underlying strata (mainly during highstands); and (2) by the flushing of platform-top carbonates by basinal evaporated waters during lowstand/early transgression. Continued dolomitisation during deeper burial is supported by the presence of high-temperature fluid inclusions and iron-rich crystal rims. Dolomite within the lower part of the Asmari Formation probably mostly formed during burial as a result of compaction of, and fluid exclusion from, the underlying Pabdeh marls and shales
Crustal Structure of the British Isles and its Epeirogenic Consequences
No full textCrustal receiver functions have been calculated for a network of 51 three-component broadband seismometers distributed across the British Isles and NW Europe. Over 3200 receiver functions were assembled for 1055 events. For each station, preliminary estimates of crustal thickness and Vp/Vs ratio were obtained from H−κ plots. Stacked receiver functions were then inverted to determine shear wave velocity as a function of depth. Each result was checked by guided forward modelling and by Monte Carlo error analysis. In this way, the robustness of our final calculated velocity profiles was carefully tested. A set of depth migrated profiles was also constructed using an average of 50 events for each station over a range of backazimuths. These profiles agree well with legacy wide-angle crustal models. Our results show that crustal thickness varies between 24 and 36 km across the British Isles. Thicker crust is found beneath north Wales and beneath central Scotland. Thinner crust occurs beneath northwest Scotland and northwest Ireland. By combining our database with the results of controlled source, wide-angle experiments and with depth-converted reflection profiles, we have produced a detailed crustal thickness map for a region encompassing the British Isles. Our synthesis of crustal thickness and structure has important implications for the tectonic and magmatic histories of this region. Complex Moho structure with lower crustal P-wave velocities of >7 km s−1 occurs beneath regions of Cenozoic magmatism, which may be consistent with magmatic underplating. Thin crust beneath northern Britain suggests that present-day long wavelength topography is maintained by regional dynamic support, originating beneath the lithospheric plate
Rock fragments from mud volcanic deposits of the Gulf of Cadiz: An insight into the Eocene-Pliocene sedimentary succession of the basin
No full textTwo hundred clasts from the mud breccia extruded by the Yuma mud volcano in the Gulf of Cadiz are used to establish an offshore stratigraphy. The clasts are a very complex mixture of material from the sedimentary successions through which the mud volcano erupted and provide important information about the composition and genesis of deep-seated strata. We deal with fragments of rocks that were deposited millions of years ago, subsequently buried and later erupted on to the seafloor. Rock fragments from the mud breccia, studied in thin section under a polarizing microscope, provide genetic indications suggestive of deep-sea conditions during the accumulation of strata. A reconstruction of the sedimentary succession through which the Yuma mud volcano erupted and the depositional evolution of the Gulf of Cadiz was made on the basis of lithology and age determinations of the rock. Age and depositional environment documented by the clasts indicate that a marine basin existed in the Gulf of Cadiz area at least from the Eocene, and that in the Early and Middle Eocene a deep-sea fan depositional environment prevailed in the region, resulting in the accumulation of thick turbidites. The Late Eocene and Oligocene are not represented among rock fragments from the mud breccia, suggesting regional uplift and non-deposition in the area. Sedimentation was re-established in the Miocene with the accumulation of a clayey Aquitanian-Burdigalian succession. Pelagic carbonate sedimentation became predominant in the Langhian time. The Serravallian-Early Tortonian is characterized by a high supply of terrigenous material, resulting in the deposition of turbiditic clays and sands. Shallow water sedimentation with accumulation of carbonate rocks prevailed in the basin during the Middle Tortonian. © 2003 Elsevier Science B.V. All rights reserved
Nanoscale properties of thin twin walls and surface layers in piezoelectric WO3-x
No full textReduced WO3 single crystals have been investigated by conductive and piezoresponse force microscopy. The reduced crystals show a piezoelectric active surface layer with a noncentrosymmetric tetragonal structure which is different from the bulk of the crystal. The domain walls carry a high current while the bulk remains insulating. Twin-related interfaces are atomically thin, the upper bound being less than 10 nm. The reduced surface layer remained in the piezoelectric state even after several weeks, while the current in the twin boundaries was reduced by re-oxidization. This layer shows a significant piezoelectric activity with a piezoelectric coefficient of about 7.9 pm/V