Capelinhos: The disappearing volcano

The 1957–58 eruption of Capelinhos, on Faial, in the Azores, built a new volcano at the west end of the island. Erosion by the sea has subsequently destroyed over two-thirds of this structure and will probably continue to remove much of the rest of the volcano over the next few decades.     

Oil-weathering behavior in Arctic environments

Oil-weathering processes in ice-free subarctic and Arctic waters include spreading, evaporation, dissolution, dispersion of whole-oil droplets into the water column, photochemical oxidation, water-in-oil emulsification, microbial degradation, adsorption onto suspended particulate material, ingestion by organisms, sinking, and sedimentation. While many of these processes also are important factors in ice-covered waters, the various forms of sea ice (depending on the active state of ice growth, extent of coverage and/or decay) impart drastic, if not controlling, changes to the rates and relative importance of different oil-weathering mechanisms. Flow-through seawater wave-tank experiments in a cold room at −35°C and studies in the Chukchi Sea in late winter provide data on oil fate and effects for a variety of potential oil spill scenarios in the Arctic. Time-series chemical weathering data are presented for Prudhoe Bay crude oil released under and encapsulated in growing first-year columnar ice through spring breakup.     

Petrography and textural development of inorganic and biogenic lithotypes in a relict barite tufa deposit at Flybye Springs, NT, Canada

A relict mound of Holocene barite (BaSO₄) tufa underlies the Flybye Springs, a small, barium‐rich, cold sulphur spring system in the Northwest Territories of Canada. The tufa is composed of relatively pure barite with ≤0·34 wt% Ca²⁺ and ≤0·77 wt% Sr²⁺. The mound is made up of coated bubble, raft, undulatory sheet, stromatolitic, coated grain and detrital conglomerate barite tufa. Although previously unreported in barite, these lithotypes are akin to facies found in many carbonate spring deposits. Raft and ooid‐coated grain tufa was formed via ‘inorganic’ barite precipitation in spring water ponds and tributaries where rapid oxidation of sulphide to sulphate established barite supersaturation. Undulatory sheet tufa may have formed by the reaction of dissolved barium with sulphate derived from the oxidation of extracellular polysaccharide‐rich colloidal sulphur films floating in oxygenated, barite‐saturated spring water ponds. Coated bubble, oncoid‐coated grain and stromatolitic tufa with filamentous microfossils was formed in close association with sulphur‐tolerant microbes inhabiting dysoxic and oxygenated spring water tributaries and ponds. Adsorption of dissolved barium to microbial extracellular polysaccharide probably facilitated the development of these ‘biogenic’ lithotypes. Detrital conglomerate tufa was formed by barite cementation of microdetrital tufa, allochthonous lithoclasts and organic detritus, including caribou hair. Biogenic textures, organic artefacts and microfossils in the Flybye barite tufa have survived diagenetic aggradational recrystallization and precipitation of secondary cements, indicating the potential for palaeoecological information to be preserved in barite in the geological record. Similarities between the Flybye barite tufa and carbonate spring deposits demonstrate that analogous textures can develop in chemical sedimentary systems with distinct mineralogy, biology and physiochemistry.     

Late Quaternary contourites and glaciomarine sedimentation in the Fram Strait

Two sites in the eastern Fram Strait, the Vestnesa Ridge and the Yermak Plateau, have been surveyed and sampled providing a depositional record over the last glacial‐interglacial cycle. The Fram Strait is the only deep‐water connection from the Arctic Ocean to the North Atlantic and contains a marine sediment record of both high latitude thermohaline flow and ice sheet interaction. On the Vestnesa Ridge, the western Svalbard margin, a sediment drift was identified in 1226 m of water. Gravity and multicores from the crest of the drift recovered turbidites and contourites. ¹⁴C dating indicates an age range of 8287 to 26 900 years BP (Early Holocene to Late Weichselian). The Yermak Plateau is characterized by slope sediments in 961 m of water. Gravity and multicores recovered contourites and hemipelagites. ¹⁴C ages were between 8615 and 46 437 years BP (Early Holocene to mid‐Weichselian). Downcore dinoflagellate cyst analyses from both sites provide a record of changing surface water conditions since the mid‐Weichselian, suggesting variable sea ice extent, productivity and polynyas present even during the Last Glacial Maximum. Four layers of ice‐rafted debris were also identified and correlated within the cores. These events occurred ca at 9, 24 to 25, 26 to 27 and 43 ka, asynchronous with Heinrich layers in the wider north‐east Atlantic and here interpreted as reflecting instability in the Svalbard/Barents Ice sheet and the northward advection of warm Atlantic water during the Late Weichselian. The activity of the ancestral West Spitsbergen Current is interpreted using mean sortable silt records from the cores. On the Vestnesa Ridge drift the modern mass accumulation rate, calculated using excess ²¹⁰Pb, is 0·076 g cm^?2 year^?1. On the Yermak Plateau slope the modern mass accumulation rate is 0·053 g cm^?2 year^?1.     

Intriguing climatic shifts in a 90 kyr old lake record from northern Russia

A 22 m long sediment core from Lake Yamozero on the Timan Ridge in northern Russia has provided evidence of intriguing climatic shifts during the last glacial cycle. An overall shallowing of the lake is reflected in the lower part of the cores, where pollen indicates a transition from glacial steppe vegetation to interstadial shrub-tundra. These beds are capped by a well-defined layer of compact clay deposited in relatively deep water, where pollen shows surrounding spruce forests and warmer-than-present summer temperatures. The most conservative interpretation is that this unit represents the last interglacial period. However, a series of Optical Stimulated Luminescence (OSL) dates suggests that it corresponds with the Early Weichselian Odderade interstadial (MIS 5a). This would imply that the Odderade interstadial was just as warm as a normal interglacial in this continental part of northern Europe. If correct, then pollen analysis, as a correlation tool, is less straightforward and the definition of an interglacial is more complex than previously thought. We discuss the validity and possible systematic errors of the OSL dates on which this age model is based, but conclude they really indicate a MIS 5a age for the warm period. Above the clay is an unconformity, most likely reflecting a period of subaerial exposure implying dry conditions. Deposition of silt under fluctuating cold climates in the Middle Weichselian continued until a second gap in the record at c . 40 kyr BP. The lake basin started to fill up again around 18 kyr BP.     

Metamorphism, Fluid Flow and Partial Melting in Pelitic Rocks from the Onawa Contact Aureole, Central Maine, USA

Field, petrologic and geochemical data were used to characterizefluid infiltration and partial melting during metamorphism ofpelitic rocks in the contact aureole of the Onawa pluton, centralMaine, USA. Mineral assemblages delineate five metamorphic zoneswithin the contact aureole: chlorite zone, andalusite–cordierite(a–c) zone, alkali feldspar zone, sillimanite zone andleucocratic-vein (l–v) zone. The sequence of observedmineral assemblages and mineral–fluid reactions calculatedby mass balance is similar to those observed in other contactaureoles. Pressure of contact metamorphism is 3 kbar, on thebasis of optimum geothermobarometry calculations. Metamorphictemperatures vary from 500C in the andalusite–cordieritezone to 65OC in the leucocratic-vein zone. Data from fieldobservations, mineral textures, observed reaction stoichiometry,geothermometry and major-element geochemistry suggest that theleucocratic veins of the l-v zone represent crystallized, partialmelts. Two overall calculated mineral reactions are responsiblefor vein formation: which can be modeled as combinations of two NKFMTASH meltingreactions: Progress of (M1) and (M2) was measured in eight samples, andreaction (M1) is the dominant melt-forming reaction in all samples.Partial melting (and vein formation) was therefore driven byinfiltration of the l-v zone by H²O-rich fluids. Calculatedtime-integrated fluid fluxes for l-v zone samples range from09 10⁴ to 31 10⁴ mol/cm², and flow was in the directionof increasing temperature. KEY WORDS: pelites; contact metamorphism; fluid infiltration; partial melting; Onawa Pluton; Maine; USA ^*Corresponding author. Telephone:(516) 632–8192. Fax (516)632–8240 e-mail: gsymmes{at}ccmail.sunysb.edu     

Lithologic transition and bed thickness periodicities in turbidite successions of the Antola Formation, Northern Apennines, Italy

The Antola Formation of Upper Cretaceous age crops out extensively in the Northern Apennines and consists of graded units of calcareous sandstones, sandstones, marlstones, and shales. It can be subdivided into the Cerreto, Antola Marlstone, Bruggi, and S. Donato Members on the basis of bed thicknesses and percentage of shales. Although the whole formation is interpreted as a deep-sea basin plain deposit, the members constitute lateral facies subdivisions which range from proximal, thick-bedded turbidities that show a prevalence of thinning upward cycles in bed thicknesses to distal turbidites that show predominantly thickening upward cycles and have a high percentage of shale. Repetitive patterns in the lithological sequence of the turbidite association are generally distinctive and are satisfactorily described as first order Markov chains. Only the Antola Marlstone Member has an additional second order Markov property. Imaginary eigenvalues of the transition probability matrices of all but the Bruggi Member demonstrate a strong cyclic character in the lithologic ordering within the formation. The behaviour of the Antola Marlstone and of the Bruggi may reflect the influence of a secondary ophiolitic intra-basinal source of clastics that contributed sandy turbidites and olistostromes. Systematic long-term variations in the sequence of bed thickness development in some sections of the Antola Formation are often subtle and equivocal, and pose special problems in interpretation. Fourier analysis was applied to the task of partitioning fundamental wavelengths from “background noise” introduced by essentially random depositional processes. In all members there is (1) strong short-term wavelength of two to three beds indicative of alternating thin and thick beds and judged to be typical of turbidite sequences; (2) an intermediate wavelength ranging from about five beds (proximal facies), eight beds (distal) to nine beds (very distal), which have both thinning and thickening upward trends, interpreted respectively as valley fill due to shifting talwegs of low density turbidity currents, and to progradational, flat turbidite lobes; (3) a poorly defined long-term wave-length of from thirty to greater than sixty beds that may be related to an unspecified trend in the evolution of the sedimentary basin. Phase angles associated with the coniputed power spectra give indications as to the asymmetry (thickening or thinning upward) or symmetry of the representative units.     

Relict magnesian calcite oolite and subsidence of the Amazon shelf

Oolitic sediments occur over large portions of the outer continental shelf off northernmost Brazil and southern French Guiana. Carbon-14 dates, stable isotope values and analogies with similar deposits from other areas in the world indicate that the ooids were formed in shallow water (possibly protected hypersaline environments) during low stands of sea level. In contrast to both modern ooids and other relict ooids, however, the Amazon shelf ooids are composed entirely of magnesian calcite (12 mol % MgCO₃) and possess radial crystal microstructures. A comparison of water depths and ages of the Amazon ooids with the generally accepted eustatic sea level curve indicates that the Amazon shelf subsided more than 100 m 16-21 thousand years ago. Prior to and subsequent to the subsidence, however, the outer shelf remained relatively stable.     

Sequence stratigraphy of a Late Devonian ramp-situated reef system in the Western Canada Sedimentary Basin: dynamic responses to sea-level change and regressive reef development

The Alexandra Formation, located in the Northwest Territories of Canada, is formed of a Late Devonian (Frasnian) reef system that developed on a gently sloping, epicontinental ramp in the Western Canada Sedimentary Basin. High‐resolution sequence stratigraphic analysis of its deposits delineates two reef complexes that are separated by a Type I sequence boundary. The second reef complex developed on the outer ramp, basinward of the first, after sea‐level fell ≈17 m. Stratigraphic complexity of the second reef complex was a result of its initiation during forced regression, and its development through an entire cycle of sea‐level rise followed by sea‐level fall. Its highstand systems tract was not characterized by high rates of carbonate production or sediment shedding. Rather, these features took place as sea‐level fell, after its highstand systems tract. The sequence stratigraphic framework of this regressive reef system highlights a number of depositional parameters that differ from high‐relief, shelf‐situated reef systems with steep, narrow margins. These have implications for understanding the controls on the development of ramp‐situated reef systems, and the nature of reef systems with gently sloping profiles. This study demonstrates that the development of stromatoporoid reef systems may be far more complex than generally realized, and that high‐resolution sequence stratigraphy may provide the tools for better understanding of complex, often enigmatic, aspects of these systems.