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: 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: A non-Fickian physico-chemical model for electrolyte transport in high-ionic strength systems is developed and tested with laboratory experiments with copper sulfate as an example electrolyte. The new model is based on irreversible thermodynamics and uses measured mutual diffusion coefficients, varying with concentration. Compared to a traditional Fickian model, the new model predicts less diffusion and asymmetric diffusion profiles. Laboratory experiments show diffusion rates even smaller than those predicted by our non-Fickian model, suggesting that there are additional, unaccounted for processes retarding diffusion. Ionic diffusion rates may be a limiting factor in transporting salts whose effect on fluid density will in turn significantly affect the flow regime. These findings have important implications for understanding and predicting solute transport in geologic settings where dense, saline solutions occur.

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: New light is shed on Worthington's concept of the North Atlantic circulation, postulating the existence of two anticyclonic gyres. This concept, which seems to have been laid to rest in the last decade, has now been reinforced by the results of a simple linear Sverdrup circulation model yielding a band of westward transport all across the North Atlantic at about the Azores latitude. This narrow band is called the Azores Countercurrent (AzCC) and matches the position of westward flow required by Worthington's “northern gyre.” An anomaly in the meridional change of the wind-stress curl in the eastern North Atlantic has been identified as the driving mechanism. A comparison with observations shows that the AzCC is verified in many analyses of historical datasets and synoptic surveys. A lack of the AzCC in other analyses is probably due to missing meridional sections, strong smoothing, and the superimposed Ekman flow close to the sea surface directed to the southeast. The AzCC has not been verified in low-resolution general circulation models applying simplified wind-stress fields and large friction coefficients, but there is evidence for its existence in recent high-resolution models driven by realistic wind stresses. Based on these findings, a new pattern for the wind-driven upper ocean circulation of the midlatitude North Atlantic is presented.

Beschreibung: The Multitracers Experiment studied a transect of water column, sediment trap, and sediment data taken across the California Current to develop quantitative methods for hindcasting paleoproductivity. The experiment used three sediment trap moorings located 120 km, 270 km, and 630 km from shore at the Oregon/California border in North America. We report here about the sedimentation and burial of particulate organic carbon (Corg) and CaCO3. In order to observe how the integrated CaCO3 and Corg burial across the transect has changed since the last glacial maximum, we have correlated core from the three sites using time scales constrained by both radiocarbon and oxygen isotopes. By comparing surface sediments to a two-and-a-half year sediment trap record, we have also defined the modern preservation rates for many of the labile sedimentary materials. Our analysis of the Corg data indicates that significant amounts (20–40%) of the total Corg being buried today in surface sediments is terrestrial. At the last glacial maximum, the terrestrial Corg fraction within 300 km of the coast was about twice as large. Such large fluxes of terrestrial Corg obscure the marine Corg record, which can be interpreted as productivity. When we corrected for the terrestrial organic matter, we found that the mass accumulation rate of marine Corg roughly doubled from the glacial maximum to the present. Because preservation rates of organic carbon are high in the high sedimentation rate cores, corrections for degradation are straightforward and we can be confident that organic carbon rain rate (new productivity) also doubled. As confirmation, the highest burial fluxes of other biogenic components (opal and Ba) also occur in the Holocene. Productivity off Oregon has thus increased dramatically since the last glacial maximum. CaCO3 fluxes also changed radically through the deglaciation; however, they are linked not to CaCO3 production but rather to changes in deepwater carbonate chemistry between 18 Ka and now.