Planimetric ice-flow convergence and flow-orthonormal strain rate across the Antarctic Ice Sheet, with links to model result files

Fast-flowing ice streams discharge most of the ice from the interior of the Antarctic Ice Sheet coastward. Understanding how their tributary organisation is governed and evolves is essential for developing reliable models of the ice sheet's response to climate change. Despite much research on ice-stream mechanics, this problem is unsolved, because the complexity of flow within and across the tributary networks has hardly been interrogated. Here I present the first map of planimetric flow convergence across the ice sheet, calculated from satellite measurements of ice surface velocity, and use it to explore this complexity. The convergence map of Antarctica elucidates how ice-stream tributaries draw ice from the interior. It also reveals curvilinear zones of convergence along lateral shear margins of streaming, and abundant convergence ripples associated with nonlinear ice rheology and changes in bed topography and friction. Flow convergence on ice-stream tributaries and their feeding zones is markedly uneven, and interspersed with divergence at distances of the order of kilometres. For individual drainage basins as well as the ice sheet as a whole, the range of convergence and divergence decreases systematically with flow speed, implying that fast flow cannot converge or diverge as much as slow flow. I therefore deduce that flow in ice-stream networks is subject to mechanical regulation that limits flow-orthonormal strain rates. These properties and the gridded data of convergence and flow-orthonormal strain rate in this archive provide targets for ice- sheet simulations and motivate more research into the origin and dynamics of tributarization.

Stress tests of ODP Site 131-808 sediments

Sediments undergoing accretion in trench-forearc systems are subjected to conditions of large lateral thrusting. This stress regime controls the mechanism of faulting as well as the yield and strength properties of the sediment. Understanding them is therefore crucial for the construction of quantitative models of sediment dynamics in convergent margin settings. For this purpose triaxial and oedometer tests were performed on six whole-round core samples recovered from Site 808 from depths between 173 and 705 mbsf. Samples from five depth intervals were subjected to a triaxial test program that was primarily designed to define yield and strength behavior. Test specimens were cut parallel and normal to the core axis. Additional five oedometer tests with similarly prepared specimens were performed on samples from four depth intervals to evaluate the directional state and degree of sediment compaction. Test results show that the degree of sediment compaction is higher than expected from overburden. This overcompaction increases with depth. A well-developed mechanical anisotropy is evident in all samples tested, regardless of their depth and lithology. Values of yield limit, stiffness, and shear strength are up to 40% higher in the horizontal direction compared to the vertical direction. In addition the test data demonstrate that the axis of the volumetric yield loci have rotated into extensional stress field. This verifies that the mechanical state of sediment in the accretionary wedge is controlled by in-situ stress conditions of extensional nature. The coefficients of lateral stress inferred suggest that the extensional stress regime becomes increasingly effective with depth.

Seawater carbonate chemistry, stable carbon isotope fractionation and growth rate during experiments with marine phytoplankton community, 1999

Stable carbon isotope fractionation (%) of 7 marine phytoplankton species grown in different irradiance cycles was measured under nutrient-replete conditions at a high light intensity in batch cultures. Compared to experiments under continuous light, all species exhibited a significantly higher instantaneous growth rate (pi), defined as the rate of carbon fixation during the photo period, when cultivated at 12:12 h. 16:8 h, or 186 h light:dark (L/D) cycles. Isotopic fractionation by the diatoms Skeletonema costatum, Asterionella glacialis, Thalassiosira punctigera, and Coscinodiscus wailesii (Group I) was 4 to 6% lower in a 16:8 h L/D cycle than under continuous light, which we attribute to differences in pi. In contrast, E, in Phaeodactylum tn'cornutum, Thalassiosira weissflogii, and in the dinoflagellate Scrippsiella trochoidea (Group 11) was largely insensitive to day length-related differences in instantaneous growth rate. Since other studies have reported growth-rate dependent fractionation under N-limited conditions in P. tricornutum, pi-related effects on fractionation apparently depend on the factor controlling growth rate. We suggest that a general relationship between E, and pi/[C02,,,] may not exist. For 1 species of each group we tested the effect of variable CO2 concentration, [COz,,,], on isotopic fractionation. A decrease in [CO2,,,] from ca 26 to 3 pm01 kg-' caused a decrease in E, by less than 3%0 This indicates that variation in h in response to changes in day length has a similar or even greater effect on isotopic fractionation than [COz,,,] m some of the species tested. In both groups E, tended to be higher in smaller species at comparable growth rates. In 24 and 48 h time series the algal cells became progressively enriched in 13C during the day and the first hours of the dark period, followed by l3C depletion in the 2 h before beginning of the following Light period. The daily amplitude of the algal isotopic composition (613C), however, was <1.5%0, which demonstrates that diurnal variation in Fl3C is relatively small.

Sea ice physics from the southern Weddell Sea

During the German Antarctic Expedition 1979/80, the sea ice conditions in the Weddell Sea were studied along the ice shelf between Cape Fiske (root of the Antarctlc Peninsula) and Atka Bay. Most intensively was the sea ice investigated in an area about 100 km northwest of Berkner Island, where a suitable site for the German station was found. In addition to the drift conditions, ice thickness as weIl as temperature and salinity of the ice were measured and the mechanical properties established. Several ice cores were taken back to Germany, where the compressive strength was measured in respect to strain rate, salinity, depth and temperature.

(Table 2) Organic residue at DSDP Hole 76-534A

The kind, sedimentation rate, and diagenesis of organic particles delivered to the North Atlantic seafloor during the Middle Jurassic-Early Cretaceous were responsible for the presence of carbonaceous sediments in Hole 534A. Organic-rich black clays formed from the rapid supply of organic matter; this organic matter was composed of either abundant, well-preserved, and poorly sorted particles of land plants deposited in clays and silty clays within terrigenous turbiditic sequences (tracheal facies) or abundant amorphous debris (xenomorphic facies) generated through the digestive tracts of marine zooplankton and sedimented as fecal pellets. Evidence for the fecal-pellet origin of xenomorphic debris is illustrated. Black clays were also produced in sediments containing less organic matter as a result of the black color of carbonized particles composing all or most of the residues (micrinitic facies). Slowly sedimented hematitic Aptian clays contain very little carbonized, organic debris that survived diagenetic oxidation. In the red calcareous clay sequence of the Late Jurassic, larger amounts of this oxidized debris turned several clay layers black or blackish red. Carbonized debris also dominates the residues recovered in interbedded black and green Albian clays. Carbonization of organic matter in these sediments either turned them black or provided the diagenetic environment for reduced iron. Carbonized debris is also appreciable in burrow-mottled black-green Kimmeridgian clay. The study of Hole 534A organic matter indicates that during the middle Callovian there was a rapid supply of terrigenous organic matter, followed by a late Callovian episode of rapidly supplied xenomorphic debris deposited as fecal pellets. The Late Jurassic-Berriasian was a time of slower sedimentation of organic matter, primarily of a marine dinoflagellate flora in a poorly preserved xenomorphic facies variously affected by diagenetic oxidation. Several intervals of carbonized tracheal tissue in the Oxfordian and Kimmeridgian suggest episodes of oxidized terrigenous matter. The same sequence of Callovian organic events is evident in much of the Early Cretaceous

(Table 1) Description of structures in cores at DSDP Holes 59-451, 59-448A and 59-447A

Fifteen lengths of Leg 59 cores (primarily from Hole 451 as well as from Holes 447A and 448A) exhibiting macroscopic faults were selected by Dr. R. B. Scott (Co-Chief Scientist, Leg 59) to help us initiate this petrofabric analysis. We proposed to (1) determine what dynamically useful deformation features might be associated with the faults, and (2) infer from these features as much as possible about the physical environment of the deformation (effective pressure, differential stress, temperature, and strain rate), the orientation and relatively magnitudes of the principal stresses at the time of deformation, and the degree of induration of the rocks at the time of deformation. The cores, mainly from Hole 451, had been slabbed on board ship with respect to the trace of bedding so that each cut surface contains the true bedding dip-direction. In general, the cores from Hole 451 are largely calcareous, lithic and vitric, brecciated tuffs, whereas those from Holes 447A and 448A are basalts or basalt breccias.

Consolidation properties and strength characteristics of ODP Site 204-1244 sediments

Eight whole-core samples from Ocean Drilling Program Site 1244, Hydrate Ridge, Cascadia continental margin, were provided to Massachusetts Institute of Technology (Cambridge, Massachusetts, USA) for geotechnical characterization. The samples were collected from depths ranging from 5 to 136 meters below seafloor (mbsf). Seven of the eight whole-core samples were located within the gas hydrate stability zone, whereas the eighth sample was located in the free gas zone. Atterberg limits testing showed that the average liquid limit of the soil is 81% and the average plastic limit is 38%, giving an average plasticity index of 43%. The liquid limit is sensitive to oven drying, shown by a drop in liquid limit to 64% when tests were performed on an oven-dried sample. Loss on ignition averages 5.45 wt%. Constant rate of strain consolidation (CRSC) tests were performed to obtain the compression characteristics of the soil, as well as to determine the stress history of the site. CRSC tests also provided hydraulic conductivity and coefficient of consolidation characteristics for these sediments. The compression ratio (Cc) ranges from 0.340 to 0.704 (average = 0.568). Cc is fairly constant to a depth of 79 mbsf, after which Cc decreases downhole. The recompression ratio (Cr) ranges from 0.035 to 0.064 (average = 0.052). Cr is constant throughout the depth range. In situ hydraulic conductivity varies between 1.5 x 10**-7 and 3 x 10**-8 cm/s and shows no trend with depth. Ko-consolidated undrained compression/extension (CKoUC/E) tests were also performed to determine the peak undrained shear strength, stress-strain curve, and friction angle. The normalized undrained strength ranges from 0.29 to 0.35. The friction angle ranges from 27 to 37. Because of the limited amount of soil, CRSC and CKoUC/E tests were also conducted on resedimented specimens.

Compaction curves and sediment characteristics of ODP Leg 165 and Leg 130 sites

Compaction curves for 11 samples from the mixed sediments and calcareous chalk with clay from the Caribbean Sites 999 and 1001 are discussed with reference to compaction curves for calcareous ooze and chalk of the Ontong Java Plateau (Leg 130). The burial history is discussed from preconsolidation data and present burial conditions and suggests a removal of ~400 m of sediment at the hiatus 166 meters below seafloor (mbsf) at Site 1001. This interpretation predicts a previous burial to >500 mbsf for depth intervals containing microstylolites, which corresponds to observations at Sites 999 and 807 (Ontong Java Plateau). Thus, data from three sites from two widely separate regions indicate that microstylolites in carbonates form at minimum burial depths deeper than 500 m. No direct link between formation of microstylolites and cementation was found, suggesting that dissolution and precipitation are not necessarily related. Porosity rebound during core retrieval could not be detected for soft sediments, whereas a porosity rebound of ~2% was deduced for deeper, cemented intervals. Comparing the compaction curves, two distinct rates of porosity loss are noted: (1) samples dominated by clay (>45% insoluble residue) compact at a higher rate than samples dominated by fine-grained carbonate and (2) fine-grained carbonate supported samples (with <45% insoluble residue) compact at the same rate irrespective of the content of nonsupporting microfossils or pore-filling clay.

Physiological and biochemical characteristics of Calanoides carinatus in waters off the coast of Namibia

Biochemical composition, feeding and oxygen uptake rate of mass planktic copepod Calanoides carinatus were studied off the coast of Namibia in January 1986. Population of this species in the area had two parts: the surface group inhabiting the 0-100 m layer and the deep part inhabiting depths greater than 200 m. Individuals in the surface and deep parts of the population differed in food content of guts, lipid content of bodies, oxygen uptake rate and behavior. Differences in biochemical composition and rate of physiological processes indicate that individuals in the deep part of the population are in diapause. Nature of changes in biochemical composition of C. carinatus in surface and deep waters in relation to life cycle characteristics in upwelling waters are discussed.