ALBERT

Beschreibung: An abrupt lithofacies change between calcareous shale and noncalcareous shale occurs in strata deposited in the mid-Cretaceous Greenhorn Seaway in the southeastern corner of Montana. The facies were correlated lithostratigraphically using bentonites and calcarenites. The lithocorrelations were then refined using ammonites, foraminifera, and calcareous nannofossils. Twenty-five time slices were defined within the upper middle and lower upper Cenomanian strata. Biofacies analysis indicate that lithofacies changes record the boundary or oceanic front between two water masses with distinctly different paleoceanographic conditions. One water mass entered the seaway from the Arctic and the other from the Gulf of Mexico/Tethys. The microfauna and microflora permit interpretation of the environmental conditions in each water mass. At times when the front was near vertical, the two water masses were of the same density but of different temperatures and salinities.

Beschreibung: Alteration patterns in the lavas and dykes of the Troodos Ophiolite, Cyprus, record a complex history of axial hydrothermal alteration, crustal aging, and subsequent uplift and emplacement of the ophiolite. Field mapping shows that distribution of five alteration zones, each with distinct mineralogical, geochemical, and hydrologie characteristics, is influenced by igneous stratigraphy, structure, and the nature and thickness of the overlying sediments. Paragenetic sequences of secondary minerals indicate that alteration conditions changed progressively as the crust cooled and moved off-axis. Along spreading axes, low temperatures (≤50°C) were maintained by the rapid flow of seawater in and out of the lavas, and only minimal alteration took place. In contrast, lower water/rock ratios and higher temperatures (〉200°C) in the dykes promoted extensive seawater-rock interaction. Although the sharp rise in temperature between the two regimes generally coincides with the lava-dyke transition, late-stage intrusions or hydrothermal upwelling zones locally cause high-temperature alteration to extend upward into the lavas. As a segment of crust moved off-axis, temperatures remained low in the lavas and progressively decreased, from 〉250° to 〈80°C, in the dykes. High permeability in the uppermost lavas led to the downward migration of an oxidative alteration front whose thickness and spatial distribution was dependent upon the rate and nature of sedimentation and, thus, the original seafloor morphology. Although field relations show that alteration has a consistent vertical pattern in Troodos, the alteration zones are not laterally continuous, and the stratigraphie depth of their boundaries varies considerably.

Beschreibung: The morphology and structure of the submarine flanks of the Canary Islands were mapped using the GLORIA long-range side-scan sonar system, bathymetric multibeam systems, and sediment echosounders. Twelve young (〈2 Ma) giant landslides have been identified on the submarine flanks of the Canary Islands up to now. Older landslide events are long buried under a thick sediment cover due to high sedimentation rates around the Canary Islands. Most slides were found on the flanks of the youngest and most active islands of La Palma, El Hierro, and Tenerife, but young giant landslides were also identified on the flanks of the older (15–20 Ma) but still active eastern islands. Large-scale mass wasting is an important process during all periods of major magmatic activity. The long-lived volcanic constructive history of the islands of the Canary Archipelago is balanced by a correspondingly long history of destruction, resulting in a higher landslide frequency for the Canary Islands compared to the Hawaiian Islands, where giant landslides only occur late in the period of active shield growth. The lower stability of the flanks of the Canaries is probably due to the much steeper slopes of the islands, a result of the abundance of highly evolved intrusive and extrusive rocks. Another reason for the enhanced slope instability is the abundance of pyroclastic deposits on Canary Islands resulting from frequent explosive eruptions due to the elevated volatile contents in the highly alkalic magmas. Dike-induced rifting is most likely the main trigger mechanism for destabilization of the flanks. Flank collapses are a major geological hazard for the Canary Islands due to the sector collapses themselves as well as triggering of tsunamis. In at least one case, a giant lateral blast occurred when an active magmatic or hydrothermal system became unroofed during flank collapse.

Beschreibung: In the western equatorial Pacific, the El Niño/Southern Oscillation (ENSO) phenomenon is characterized by precipitation variability associated with the migration of the Indonesian low pressure cell to the region of the date line and the equator. During ENSO events, Tarawa Atoll (1°N, 172°E) experiences heavy rainfall which has an estimated δ18O of about −8 to −10‰ δ18OSMOW. At Tarawa, sufficient precipitation of this composition falls during ENSO events to alter the δ18O and the salinity of the surface waters. Oxygen isotope records from two corals collected off the reef crest of Tarawa reflect rainfall variations associated with both weak and strong ENSO conditions, with approximately monthly resolution. Coral skeletal δ18O variations due to small sea surface temperature (SST) changes are secondary. These records demonstrate the remarkable ability of this technique to reconstruct variations in the position of the Indonesian Low from coral δ18O records in the western equatorial Pacific, a region which has few paleoclimatic records. The coral isotopic data correctly resolve the relative magnitudes of recent variations in the Southern Oscillation Index. Combining the Tarawa record with an oxygen isotopic history from a Galápagos Islands coral demonstrates the ability to distinguish the meteorologic (precipitation) and oceanographic (SST) anomalies that characterize ENSO events across the Pacific Basin over the period of common record (1960–1979). Comparison of the intensity of climatic anomalies at these two sites yields insight into the spatial variability of ENSO events. Isotope records from older corals can provide high-resolution, Pacific-wide reconstructions of ENSO behavior during periods of different climate boundary conditions.

Beschreibung: As shown by the work of Dansgaard and his colleagues, climate oscillations of one or so millennia duration punctuate much of glacial section of the Greenland ice cores. These oscillations are characterized by 5°C air temperature changes, severalfold dust content changes and 50 ppm CO2 changes. Both the temperature and CO2 change are best explained by changes in the mode of operation of the ocean. In this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50°N. Based on this finding, we suggest that the Greenland climate changes are driven by oscillations in the salinity of the Atlantic Ocean which modulate the strength of the Atlantic's conveyor circulation.

Beschreibung: Current measurements from two consecutive yearlong deployments of three moored stations at the western end of the equator in the Atlantic, along 44°W, are used to determine the northwestward flow of warm water in the upper several 100 m and of the southeastward counterflow of North Atlantic Deep Water (NADW). Measurements from three acoustic Doppler current profilers (ADCPs) looking upward from 300 m toward the surface allowed calculation of a time series of upper layer transports over 1 year. Mean transport through the array for the upper 300 m is 23.8 Sv with an annual cycle of only ±3 Sv that has its maximum in June-August and minimum in northern spring. Estimated additional mean northwestward transport in the range 300–600 m is 6.7 Sv, based on moored data and shipboard Pegasus and lowered ADCP profiling. In the depth range 1400–3100 m a current core with maximum annual mean southeastward speed of 30 cm s−1 is found along the continental slope that carries an estimated upper NADW transport of 14.2–17.3 Sv, depending on the extrapolation used between the mooring in the core and the continental slope. This transport is higher than off-equatorial estimates and suggests near-equatorial recirculation at the upper NADW level, in agreement with northwestward mean flow found about 140 km offshore. Below 3100 m and above the 1.8°C isotherm, only a small core of lower NADW flow with speeds of 10–15 cm s−1 is found over the flat part of the basin near 1.5°N, clearly separated from the continental slope by a zone of near-zero mean speeds. Estimated transport of that small current core is about 4.5 Sv, which is significantly below other estimates of near-equatorial transport of lower NADW and suggests that a major fraction of lower NADW may cross the 44°W meridian north of the Ceara Rise. Intraseasonal variability is large, although smaller than observed at 8°N near the western boundary. It occurs at a period of about 1 month when it is dominant in the near-surface records and corresponds to earlier observations in the equatorial zones of all oceans and at a period of about 2 months when it is dominant at the NADW level and could be imported either from the north along the boundary or from the east along the equator. The existence of an annual cycle in the deep currents of a few centimeters per second amplitude, as suggested by high-resolution numerical model results, could neither be proven nor disproven because of the high amount of shorter-period variability.

Beschreibung: The carbon isotopic composition of methane emitted by the Alaskan emergent aquatic plants Arctophila fulva, a tundra mid-lake macrophyte, and Carex rostrata, a tundra lake margin macrophyte, was −58.6 ± 0.5 (n=2) and −66.6±2.5 (n= 6) ‰ respectively. The methane emitted by these species was found to be depleted in 13C by 12‰ and 18‰, relative to methane withdrawn from plant stems 1 to 2 cm below the waterline. As the macrophyte-mediated methane flux represented approximately 97% of the flux from these sites, these results suggest the more rapid transport of 12CH4 relative to 13CH4 through plants to the atmosphere. This preferential release of the light isotope of methane, possibly combined with CH4 oxidation, caused the buildup of the heavy isotope within plant stems. Plant stem methane concentrations ranged from 0.2 to 4.0% ( math formula, 1.4; standard deviation (sd), 0.9; n=28) in Arctophila, with an isotopic composition of −46.1±4.3 ‰ (n = 8). Carex stem methane concentrations were lower, ranging from 150 to 1200 ppm ( math formula, 500; standard deviation, 360; n = 8), with an isotopic composition of −48.3±1.4‰ (n=3). Comparisons of the observed isotopic fractionations with those predicted from gas phase effusion and diffusion coefficients suggest a combination of one or both of these gas transport mechanisms with bulk (non-fractionationating) flow.

Beschreibung: Burrowing activities of the only European fiddler crab, Uca tangeri, and its resulting influence on biotope in mudflats were investigated during 1989-1990 at Ria Formosa, Portugal. Individuals use the same burrow for ca 1 wk, then occupy another or dig a new one. Overall a burrow is inhabited for ca 3 mo by several individuals before it is abandoned. Vacated burrows decay within 2 to 3 wk. Burrow size and number vary with the season. Burrow density was highest in spring and early summer with ca 17 burrows m-2, and then decreased. Deepest burrows (up to 90 cm long) were found in winter, the shallowest (up to 40 cm long) in summer. Volume of the sediment moved by U. tangen varied monthly between 3000 and 6000 cm3 per m2 of mudflat. Water is only found in the lower third of the burrow. Burrow water contains less oxygen and more nitrate than the surrounding water of the Ria Formosa.

Beschreibung: Respiration and activity of eelpouts Zoarces viviparus L. were measured in an underwater respiration chamber in Kiel Bay (Germany) under short-term hypoxia. Respiration and swimming activity both declined almost continuously with decreasing oxygen saturation...