Basin Southwest Research Articles

Methane in an acidic bog lake: The influence of peat in the catchment on the biogeochemistry of methane

Methanogenesis, an anaerobic microbial process in sediments, was investigated in a naturally acidic bog lake, Grosse Fuchskuhle, in northeastern Germany. The lake was artificially divided into four sub-basins: two western basins receiving humic substances from an adjacent Sphagnum bog and two eastern basins isolated from this acidic inflow with installed curtains. Within the northeast (NE) and southwest (SW) basin compartments, one of each group were studied. In the peat influenced western basins the pH-values in the water columns were kept low and increased in the others. The stratification period increased in all four compartments from some weeks to some months, from April to October, with the development of anoxic hypolimnia in all compartments after the installation of the curtains. The layer with the most active methane production moved from a sediment depth of more than 20 cm before separation to close to the surface sediments in all compartments. Methano genic microorganisms were found in the whole sediment core – from surface sediments to a depth of 25 cm. The proportion of methanogens was approximately 15% of total microbial cell numbers, which were approximately 2 × 106 cells/ml sediment. Oligonucleotide probes targeting nearly all families of the phylum Euryarchaeota were tested with fluorescence in-situ hybridization (FISH). In both basins, with and without the influence from peat, oligonucleotide probe (MSMX) targeting Methanosarcinaceae could be detected only as methanogens. The finding of only acetate-using methanogens by FISH indicated acetate as a major methanogenic substrate. Concentration profiles of CH4 as a function of sediment depth were characterized in both basins by high concentrations in the top layers. Methane bubbles were released in the late summer in the eastern sub-basin only, likely from oversaturation in the surface sediments.

The Salina del Fraile pull-apart basin, northwest Argentina

Abstract The Salina del Fraile in northwest Argentina is a Pliocene to Recent pull-apart basin developed at a releasing stepover along the NNE-SSW-trending El Fraile sinistral strike-slip fault. The basin is 35 km long and 12 km wide with a characteristic rhomboidal shape and is starved of synkinematic sediments thus providing unique 3D exposures. Prominent basin side-wall fault systems form scarps 700 m high and large tilted fault blocks form a terraced system along the southwest basin sidewall. A short-cut, basin-floor fault transects the pull-apart basin connecting the northwest strand of the El Fraile Fault to the southeast strand. An anticlinal positive flower structure in the northwest of the basin is a relict of early, segmented, fault growth typical of strike-slip fault evolution. Extension faults in the basin floor indicate a NE-SW intrabasinal extension direction during the Pliocene to Recent. The pull-apart basin accommodates an estimated 7.7 km of sinistral displacement along the El Fraile fault system. The morphology and fault architecture of the Salina del Fraile pull-apart can be directly compared to scaled sandbox models of strike-slip pull-apart basins. This first detailed analysis of the Salina del Fraile pull-apart basin provides a model for 3D architecture and evolution of similar pull-apart basins, and may serve as a template for the interpretation of other pull-apart systems.

Methanogenic archaeal community in the sediment of an artificially partitioned acidic bog lake

Abstract The methanogenic archaeal communities in the sediment of two basins of an artificially partitioned acidic bog lake were studied. In the northeast basin, which was separated from a peat bog, a high methane production rate was measured only in the upper layers of the sediment. In contrast, methane production was detected at various depths of the sediment in the southwest basin, which continuously receiving humic acids from the bog. Ten bands were observed in the denaturing gradient gel electrophoresis (DGGE) fingerprint of the 16S rDNA archaeal amplicons from the NE basin top (0-5 cm) sediment layer, which reflected the presence of at least 10 ribotypes. Seventy clones of the 16S rDNA amplicons were obtained from the NE basin top sediment layer, and were grouped into 10 operational taxonomy units (OTUs) according to their positions on the DGGE gel. Seven of these OTUs could be matched with the bands of the community fingerprint. Phylogenetic analyses indicated that the sequences clustered into three groups: five of the OTUs were related to Methanosaeta, four OTUs to Methanomicrobiales and one OTU to Methanobacterium. Among the OTUs, sequences with high similarities (>96%) were retrieved. The sequence data suggested that the diversity of the methanogenic archaeal community was limited. Bands corresponding to those three phylogenetic groups were found in the DGGE fingerprints of both NE and SW basins, which reflected the presence of the same dominant methanogenic archaeal groups in both basins. However, differences in the distribution of the ribotypes that had high 16S rRNA sequence identities were observed in the fingerprints between the two basins at various depths. The microdiversity decreased along the sediment depth in the NE basin, and vice versa in the SW basin. The greater variety of the archaeal ribotypes, apparently, correlated with the higher methanogenesis rate.

Methanogenic archaeal community in the sediment of an artificially partitioned acidic bog lake

The methanogenic archaeal communities in the sediment of two basins of an artificially partitioned acidic bog lake were studied. In the northeast basin, which was separated from a peat bog, a high methane production rate was measured only in the upper layers of the sediment. In contrast, methane production was detected at various depths of the sediment in the southwest basin, which continuously receiving humic acids from the bog. Ten bands were observed in the denaturing gradient gel electrophoresis (DGGE) fingerprint of the 16S rDNA archaeal amplicons from the NE basin top (0–5 cm) sediment layer, which reflected the presence of at least 10 ribotypes. Seventy clones of the 16S rDNA amplicons were obtained from the NE basin top sediment layer, and were grouped into 10 operational taxonomy units (OTUs) according to their positions on the DGGE gel. Seven of these OTUs could be matched with the bands of the community fingerprint. Phylogenetic analyses indicated that the sequences clustered into three groups: five of the OTUs were related to Methanosaeta, four OTUs to Methanomicrobiales and one OTU to Methanobacterium. Among the OTUs, sequences with high similarities (>96%) were retrieved. The sequence data suggested that the diversity of the methanogenic archaeal community was limited. Bands corresponding to those three phylogenetic groups were found in the DGGE fingerprints of both NE and SW basins, which reflected the presence of the same dominant methanogenic archaeal groups in both basins. However, differences in the distribution of the ribotypes that had high 16S rRNA sequence identities were observed in the fingerprints between the two basins at various depths. The microdiversity decreased along the sediment depth in the NE basin, and vice versa in the SW basin. The greater variety of the archaeal ribotypes, apparently, correlated with the higher methanogenesis rate.

Tectonostratigraphy of the Nieuwerkerk Formation (Delfland subgroup), West Netherlands Basin

The Nieuwerkerk Formation is a major Lower Cretaceous synrift and postrift fluvial unit in the West Netherlands Basin (southwest Netherlands) that attains thicknesses in excess of 1 km in places. A strong tectonic overprint on its deposition and a large degree of facies heterogeneity have complicated correlation and greatly hampered understanding reservoir and seal distribution within the unit. The integrated application of fluvial sequence stratigraphic concepts with biostratigraphic dating and the correlation of cycles of changing accommodation to sediment supply ratio (A/S cycles) on three-dimensional (3-D) seismic, well-log, and core data have allowed a much improved understanding of reservoir facies distribution within the Nieuwerkerk Formation. A major intraformational unconformity divides the Nieuwerkerk Formation into two members. The lower of these, the Alblasserdam Member, is predominantly nonmarine and has a significant tectonic depositional overprint. Correlation within this member is dependent on the identification of base-level transit cycles probably induced by pulses of tectonism. The inclusion of 3-D seismic isopach data facilitates mapping thicknesses and reservoir properties of the Alblasserdam Member in areas with no well control. The upper member, the Rodenrijs Claystone Member, was deposited during the postrift stage and is predominantly a coastal-plain succession. Biostratigraphic correlation proved useful in subdividing this unit and correlating key seismostratigraphic markers. Use of biostratigraphic and cyclostratigraphic correlation techniques allowed chronostratigraphically consistent reservoir maps to be made of the constituent members of the Nieuwerkerk Formation. These maps exhibit localized nonmarine syndepositional basins (Alblasserdam Member sand depocenters), followed by gradual southward, landward stepping of facies tracts of the Rodenrijs Claystone Member above the intraformational unconformity. The Rodenrijs Claystone Member is capped by a marine transgression that terminated fluvial deposition in this part of the basin. Interestingly, no major lacustrine facies have been identified in the fluvial units in the West Netherlands Basin, rendering it somewhat anomalous among rift basins.

Trace-element of Tuoyun clinopyroxene: Implication for the deep processes of lithospheric mantle beneath the southwest Tianshan, West China

Based on the LAM-ICPMS analytic results on the trace elements of clinopyroxenes in peridotitic xenoliths occurring in early-Cretaceous basalts from western Xinjiang, the properties and the deep processes, including partial melting and mantle metasomatism, of the subcontinental lithospheric mantle beneath the Tuoyun Basin are analyzed. In the northern edge of the Tarim Basin (southwest Tianshan), the Mesozoic subcontinental lithosphere which has experienced the effect of partial melting (<10%) and intricate mantle metasomatism is characterized by Phanerozoic ‘ocean-type’ mantle. The superposed influence of SiO2-unsaturated silicate melt and carbonate melt probably results in the metasomatic medium which resembles the hydrous silicated carbonate melt in some aspects. By comparing Tuoyun mantle with Cenozoic main mantle beneath eastern China, the properties are similar, while the former shows finer grain and higher diopside content of the peridotites and more conspicuous modal metasomatism.

Detachment faulting, mantle serpentinization, and serpentinite- mud volcanism beneath the Porcupine Basin, southwest of Ireland

The Porcupine Basin southwest of Ireland provides an opportunity to study the symmetry of rifting at stretching factors approaching continental breakup. Profiles across the basin image a bright reflection that appears to represent a detachment fault, and may in part be a decollement at the top of partially serpentinized mantle. Although overall the basin appears symmetric, the consistent westward structural dip of the detachment implies that, at high stretching factors, extension was asymmetric. Farther south, the Porcupine median high appears in cross section to be a triangular construction overlying tilted fault blocks and onlapped by postrift sediment. Despite no evidence for synrift magmatism, this high has previously been interpreted as a basaltic structure. However, it may represent a serpentinite-mud volcano or diapir: we suggest that such structures produce the serpentinite breccias found within the rifted continent-ocean transition of nonvolcanic margins.

Neodymium isotopes in basalts of the southwest basin and range and lithospheric thinning during continental extension

The chemical compositions, ages, and Nd and Sr isotopic compositions of Miocene basaltic volcanic rocks from the southwestern Basin and Range Province were measured. The data presented, and other data obtained from the literature show that temporal variations in the Nd and Sr isotopic compositions are typically correlated with the timing of local crustal extension. The isotopic variations cannot be accounted for by crustal contamination, and are best interpreted as indicating the involvement of subcontinental lithospheric mantle as a major source of basalt magma before and during the early stages of extension, giving way to asthenospheric sources as extension proceeds. It is inferred that the basalt isotopic and age data provide information about the amount and timing of changes of the overall lithosphere thickness during extension. If this inference is valid, the inferred lithosphere thickness vs. time histories can be compared to the amount and timing of upper crustal extension to evaluate competing models for lithospheric strain during continental extension. The depth of the lithosphere–asthenospshere boundary is inferred primarily from the isotopic data, but can be refined by accounting for differences in the depth of origin of the basalts as suggested by differences in the degree of silica saturation (nepheline normative vs. hypersthene normative). The transition between silica saturated and undersaturated magmas is dependent on many factors, but for the conditions likely to apply to the Basin and Range Miocene volcanism, it corresponds to a depth of origin of 50±10 km. The data presented show strong support for the inferred mantle geochemical stratigraphy, and the inferred modern depths to the lithosphere–asthenosphere boundary are consistent with available seismic refraction and structural data. In the highly extended regions near the eastern and western margins of the southwestern Basin and Range, temporal changes in basalt isotopic compositions indicate that the subjacent lithospheric mantle thinned contemporaneously with the upper crust, as would be predicted for pure shear. These areas are also characterized by relatively high (although still low) basalt extrusion rates. In contrast, upper crustal extension in the Death Valley area was not accompanied by significant thinning of the subjacent lithospheric mantle, which implies that the location of lithospheric thinning was offset from the location of upper crustal extension as in a simple shear model. The total amount of lithospheric thinning is generally less than would be expected from the magnitude of upper crustal extension and is more evenly distributed; this suggests that there is also a component of distributed shear.

Lacustrine sedimentation in the diapir-controlled Miocene Bicorb Basin, eastern Spain

The Miocene Bicorb Basin is a small elongated basin developed by normal faulting of a thick Jurassic–Cretaceous carbonate succession and subsequent diapirism of Upper Triassic mudstones and evaporites. The basin fill comprises a sequence over 650 m thick formed by two units. The lower, alluvial unit consists of a complex alternation of conglomerates, sandstones, mudstones, and minor lacustrine limestones. The upper unit comprises marginal alluvial and lacustrine deposits in which seven lacustrine facies associations have been distinguished. The inner lacustrine deposits comprise mudstones and carbonates with minor evaporitic deposits. At the northeast and southwest basin edges, alluvial inputs led to the development of delta and fan-delta environments where sandstone and conglomerate deposits dominate. The latter stages of the lake evolution are represented by an expansive thick limestone sequence which close to the NW fault-bounded margin pass laterally to breccias. The hierarchical arrangement of the upper unit shows five different orders of sequences. The first-order sequence defines a major vertical trend of lake expansion whereas three second-order sequences are linked to major flooding-expansion episodes. All these sequences are related to the tectonic evolution of the diapir. Metre-scale, third- and fourth-order sequences and fifth-order sequences, which consist of finely laminated rhythmite deposits probably record climatically forced processes. The diapir–graben system evolution and the climatic variations have exerted major controls on the sequential arrangement and evolution of the lacustrine system. Both factors have also strongly influenced the changes in the gastropod assemblages.

Microbial Food Webs in an Artificially Divided Acidic Bog Lake

AbstractThe microbial loop of a naturally acidic bog lake, Große Fuchskuhle (Northeastern Germany), that had been artificially divided into 4 basins, was investigated. In the northeast (NE) and southwest (SW) basins, which differ strongly in chemistry and primary production, we conducted intensive studies of the main carbon fluxes through microbial food webs. In the less acidic, NE basin, much higher phytoplankton as well as bacterial biomass and production were found in parallel with negligible numbers of larger zooplankters. Weakly top‐down controlled populations of protists were characterized by an exceptionally low numerical proportion of heterotrophic nanoflagellates (HNF) to ciliates (‐1.5‐3.5). The ciliate community was dominated by a scuticociliate, Cyclidium sp. (&gt;95% of total ciliates), with an estimated grazing rate equal to 46–80% of heterotrophic bacterial production. In contrast, in the more humic, SW basin, both phyto‐ and bacterioplankton dynamics seemed to be top‐down controlled by abundant populations of small fine‐filter feeding cladocerans, Ceriodaphnia quadrangula and Diaphanosoma brachyurum. Consequently, ciliates disappeared from the food web structure of the SW basin, HNF dropped to negligible numbers and bacteria showed very uniform morphology, dominated by small cocci or short rods. Our investigations have shown that the division of the lake into separate compartments can lead to very different microbial food web structures with extreme species compositions.

On the origin age of the Southwest Basin in the South China Sea

This paper propounds that the origin and evolution of the Southwest Basin (SWB) in the South China Sea (SCS) are closely related with those of the SCS, reviews various viewpoints on its origin age with a large gap between the oldest age and the youngest age, offers some suggestions on the SWB’s origin age and gives some proposals to ascertain satisfactorily the origin and evolution, and multi-phasal and multiaxial spreading of the SCS and SWB.

Comparison of the detection of deforested areas using the ERS-1 ATSR and the NOAA-11 AVHRR with reference to ERS-1 SAR data: a case study in the Brazilian Amazon

Abstract Two coarse spatial resolution classifications of nominally 1·1 km resolution of deforested areas over a test site covering part of the south-west Amazon basin are compared. One classification is derived from data acquired by NOAA's Advanced Very High Resolution Radiometer (AVHRR) and the other from the ERS-1 Along Track Scanning Radiometer (ATSR-1). A forest/non-forest classification derived from finer spatial resolution ERS-1 Synthetic Aperture Radar (SAR) data is used as a reference for the comparison. Specific patterns of deforestation in the area are identified and their depiction on each of the coarse spatial resolution datasets is examined in relation to their depiction on the fine spatial resolution classification. The results show that significant differences exist between the two coarse spatial resolution classifications and that special rigorous integration methodologies will have to be developed before the results from either source can be used interchangeably with those from the other.

Chemical mass balances in metalliferous deposits from the Atlantis II Deep, Red Sea

In order to assess the quantitative distribution of mineral species within the sedimentary series of the Atlantis II Deep, we have examined the chemical composition, mineralogy, and physical properties of 120 sediment samples from two cores that sampled the entire sediment sequences in the West and South-West basins. Biostratigraphic correlations and chemical budget calculations indicate that the nonmetalliferous solid fractions (i.e., detrital and biogenic particles) in the older sedimentary unit (unit 1 in the West basin) represent 46% wt of the total, and that they were deposited between 23,000 and 15-12,000 years bp with a mass accumulation rate between 109 and 150 kg per thousand years per square metre. The muddy sediment in the upper part of the West basin core (units 2, 3, and 4) consists mainly of metalliferous particles which account for less than 15% wt of the mud, and yield a mass accumulation rate (130–162 kg/k.y./M 2) close to that of nonmetalliferous particles deposited simultaneously. Nonmetalliferous particles were probably the major source of Si and a significant source of Pb through dissolution in the brine system and diagenesis in the metalliferous mud. The metalliferous sediments in the upper unit of the South-West basin (unit U, 1100 cm) were deposited at the same time as those of the upper unit in the West basin (unit 4,335 cm). The calculated mass accumulation rate is about 700 kg/k.y./m 2 for unit U. The recent sediments of the South-West basin are more enriched in Zn and Cu, and more depleted in Mn relative to Fe than the contemporary sediments in the West basin, suggesting that the hydrothermal source has been in, or near, the South-West basin during the last 2,250 years. Assuming that a Salton Sea-like solution supplies all the Fe contained in the West basin metalliferous sedimentary pile, the mineralizing brine flowed at a minimum rate of 30 L/s. Except for the period of Mn-oxide deposition, the difference between the metal/Fe ratios in oxide-rich and sulphide-rich facies is attributed to changes in the chemical composition of the mineralizing fluid rather than to chemical processes acting in the brine system of the Atlantis II Deep.

3-D inversion of deep tow magnetic data on the Atlantis II Deep (Red Sea): hydrothermal and geodynamic interpretation

Abstract To study the magnetic signature of hydrothermal oceanic crust areas, data were collected, using a magnetometer towed a few hundred meters above the sea floor, within an acoustic transponder net. The experiment was performed in the Red Sea over the Atlantic II Deep (near 21°20′N 38°05′E). This deep, created during the Brunnes Epoch, is well known for its intense hydrothermal activity and the presence of hot brines overlying metalliferous sediments. The magnetic field was inverted to obtain the distribution pattern of crustal magnetization, including the effects of topography and irregular depth of the magnetic sensor. The procedure, however, does not produce any unique solution. It is approximated, using probabilistic linear methods (Tarantola and Valette, 1982), which require the introduction of constraints, in the form of gaussian probability density functions. The resultant magnetization pattern then follows a gaussian law; the mean and standard deviation are then computed. The results show, contrary to what has been suggested formerly (Rona, 1978a, b), that the strongest magnetization, as well as the most frequent hydrothermal and magmatic occurrences are found in the Southwest Basin. The high magnetic intensities are thus associated with a recent magmatism, which initiates the hydrothermal circulation. Two hypotheses explain this: 1. (1)The freshness of the basalts and the low-temperature alteration are the principal causes of magnetization contrasts and represent, respectively, the discharge and recharge zones of the hydrothermal plumbing. 2. (2)In the general framework of the Red Sea opening by progressive tearing, the formation of the Atlantis II was extremely fast, according to the elastic rebound theory (Reid, 1911). This hypothesis is applicable to some other deeps.

Geological interpretation of DEKORP deep seismic reflection profiles 1C and 9N across the variscan Saar-Nahe Basin southwest Germany

Deep seismic reflection results across the Variscan Saar-Nahe Basin in southwest Germany, recorded as parts of DEKORP lines 1C and 9N, provide excellent images of the geometry of the basin and also of the structures in the underlying basement. This basement consists of several discrete units, some of which are highly-reflective, whereas others are essentially non-reflective; two of these units consist of well-layered dipping reflectors that are interpreted as having been telescoped by thrusting. Adjacent to the South Hunsrück Fault, the sedimentary pile within the Saar-Nahe Basin is over 8 km thick, but it thins dramatically towards the southeast and east. The position of continuous reflectors, both in the basin and the basement, indicate that the South Hunsruck Fault is subvertical and not a listric normal fault as proposed in several earlier interpretations. This implies that the basin did not form by normal extension but that it is transtensional in character, its geometry predominantly being controlled by strike-slip movements on the South Hunsruck Fault.

Determination of the Surface and Bed Topography in Central Greenland

AbstractThe surface and bottom topography of the central Greenland ice sheet was determined from airborne ice-radar soundings over a 180 km by 180 km grid centered on the 1974 “Summit” site (lat. 72°18′N., long. 37°55′W.), using the Technical University of Denmark 60 MHz ice radar. Over 6100 km of high-quality radar data were obtained, covering over 99'% of the grid, along lines spaced 12.5 km apart in both north-south and east-west directions. Aircraft location was done with an inertial navigation system (INS) and a pressure altimeter, with control provided by periodically flying over a known point at the center of the grid. The ice radar was used to determine ice thickness; the surface topography was determined independently using height-above-terrain measurements from the aircraft’s radar altimeter. The calculated surface topography is accurate to about ±6 m, with this error arising mostly from radar-altimeter errors. The ice thickness and bottom topography are accurate to about ±50 m, with this error dominated by the horizontal navigation uncertainties due to INS drift; this error increases to about ±125 m in areas of rough bottom relief (about 12% of the grid).The highest point on Greenland is at lat. 72°34′ N., long. 37°38′W., at an altitude of 3233 ± 6 m a.s.l. The ice surface at this point divides into three sectors, one facing north, one east-south-east, and one west-south-west, with each having a roughly uniform slope. The ice divide between the last two sectors is a well-defined ridge running almost due south. The ice is about 3025 m thick at the summit. Excluding the mountainous north-east corner of the grid, where the ice locally reaches a thickness of about 3470 m and the bed dips to about 370 m below sea-level, the maximum ice thickness, approximately 3375 m, occurs about 97 km south-south-west of the summit. The average bed altitude over the entire grid is 180 m and the average ice thickness is 2975 ± 235 m. The ice in most of the south-west quadrant of the grid is over 3200 m thick, and overlies a relatively smooth, flat basin with altitudes mostly below sea-level. There is no predominant direction to the basal topography over most of the grid; it appears to be undulating, rolling terrain with no obvious ridge/valley structure. The summit of the ice sheet is above the eastern end of a relatively large, smooth, flat plateau, about 10–15 km wide and extending about 50 km to the west. If the basal topography were the sole criterion, then a site somewhere on this plateau or in the south-west basin would be suitable for the drilling of a new deep ice core.

Determination of the Surface and Bed Topography in Central Greenland

AbstractThe surface and bottom topography of the central Greenland ice sheet was determined from airborne ice-radar soundings over a 180 km by 180 km grid centered on the 1974 “Summit” site (lat. 72°18′N., long. 37°55′W.), using the Technical University of Denmark 60 MHz ice radar. Over 6100 km of high-quality radar data were obtained, covering over 99'% of the grid, along lines spaced 12.5 km apart in both north-south and east-west directions. Aircraft location was done with an inertial navigation system (INS) and a pressure altimeter, with control provided by periodically flying over a known point at the center of the grid. The ice radar was used to determine ice thickness; the surface topography was determined independently using height-above-terrain measurements from the aircraft’s radar altimeter. The calculated surface topography is accurate to about ±6 m, with this error arising mostly from radar-altimeter errors. The ice thickness and bottom topography are accurate to about ±50 m, with this error dominated by the horizontal navigation uncertainties due to INS drift; this error increases to about ±125 m in areas of rough bottom relief (about 12% of the grid).The highest point on Greenland is at lat. 72°34′ N., long. 37°38′W., at an altitude of 3233 ± 6 m a.s.l. The ice surface at this point divides into three sectors, one facing north, one east-south-east, and one west-south-west, with each having a roughly uniform slope. The ice divide between the last two sectors is a well-defined ridge running almost due south. The ice is about 3025 m thick at the summit. Excluding the mountainous north-east corner of the grid, where the ice locally reaches a thickness of about 3470 m and the bed dips to about 370 m below sea-level, the maximum ice thickness, approximately 3375 m, occurs about 97 km south-south-west of the summit. The average bed altitude over the entire grid is 180 m and the average ice thickness is 2975 ± 235 m. The ice in most of the south-west quadrant of the grid is over 3200 m thick, and overlies a relatively smooth, flat basin with altitudes mostly below sea-level. There is no predominant direction to the basal topography over most of the grid; it appears to be undulating, rolling terrain with no obvious ridge/valley structure. The summit of the ice sheet is above the eastern end of a relatively large, smooth, flat plateau, about 10–15 km wide and extending about 50 km to the west. If the basal topography were the sole criterion, then a site somewhere on this plateau or in the south-west basin would be suitable for the drilling of a new deep ice core.

Mineralogy, parageneses, and phase relations of copper-iron sulfides in the Atlantis II deep, red sea

New investigations are carried out on the mineralogy and mineral chemistry of sulfide assemblages obtained in samples from one core in the hydrothermally active, southwest basin of the Atlantis II deep, Red Sea. The most abundant sulfide phases are the exsolved intermediate solid solution (ISS) and chalcopyrite. Sphalerite, pyrrhotite, marcasite, mackinawite, and presumably wurtzite are also observed. Two distinct groups of paragenesis were encountered: (a) Intermediate solid solution with sphalerite incrustations and intergrowths, and (b) intermediate solid solution barren of sphalerite intergrowths. The first group is confined to the upper part of the Co zone and the SOAN zone (Backer and Richter 1973), and the second is present in the entire core 100-3-7. An optically isotropic chalcopyrite is found for the first time as a natural mineral in Atlantis II, Red Sea. Yet its existence as a novel phase needs x-ray confirmation. It exhibits a lower reflectivity than normal chalcopyrite and is isotropic. Chalcopyrite occurs either as a single phase or in association with tetragonal chalcopyrite. Our investigations indicate that the formation of Atlantis II deposits is a result of complex processes. These processes are characterized by compositional changes in the ore-bearing fluids and the change in sulfur fugacity (especially with depth). The presence of exsolved chalcopyrite lamellae in ISS indicates slow cooling below 450°C. However, it is difficult to understand why the cubic chalcopyrite is not converted to the tetragonal form even though the temperature of transformation lies above 450°C (470° – 500°C, Cabri 1973). The Cu/Fe ratio changes in the exsolved chalcopyrite lamellae from core to rim of the composite grains. The ratio is higher in the rims. This suggests that primary inhomogenous ISS grains formed from solutions with a continuous increase in the Cu/Fe ratio. Slow cooling is also required to account for the exsolution of chalcopyrite lamellae in ISS. The low sulfur content in isotropic chalcopyrite is also suggestive of low fs2. The low S content in the chalcopyrite may be the controlling factor for the sluggish conversion from cubic to tetragonal chalcopyrite. Mackinawite lamellae show the same orientation in ISS and exsolved isotropic chalcopyrite indicating that mackinawite exsolved before the breakdown of ISS. This strongly suggests that mackinawite is stable above 300°C (contrary to experimental results by Zoka et al. 1973). Pyrrhotite was probably formed by the sulfurization of ilvaite. The pyrrhotite grains with several complex successive zones show the sequence of the sulfurization episodes.

Native American basketry: an annotated bibliography

Northeast Southeast Great Lakes Plains Great Basin Southwest California Northwest Plateau Subarctic Arctic

Responses of Lake Whitefish (Coregonus clupeaformis) to Fertilization of Lake 226, the Experimental Lakes Area

The responses of an unexploited population of lake whitefish (Coregonus clupeaformis) to fertilization of Lake 226 (L226), the Experimental Lakes Area, northwestern Ontario, were measured for 4 yr (1973–77). A vinyl sea curtain separated the lake into a northeast (NE) basin, which received P, N, and C additions, and a southwest (SW) basin, which received only N and C additions. Lake whitefish in the NE basin grew faster, had higher coefficients of condition, and had greater recruitment and production than those in the SW basin during the 2nd through 4th yr of fertilization. No differences between basins were detected in annual survival for fish [Formula: see text], but survival of age 0 fish was greater in the NE basin. Whitefish production in L226NE averaged twice that in L226SW from the 2nd to the 4th yr of fertilization, though significant variations occurred between years in both the NE and SW populations.