Laboratory investigation of flow structure and resistance of high- and low-angle dunes

A prominent control on the flow over subaqueous dunes is the slope of the downstream leeside. While previous work has focused on steep (~30°), asymmetric dunes with permanent flow separation, little is known about dunes with lower lee-slope angles for which flow separation is absent or intermittent. Here, we present a laboratory investigation where we systematically varied the dune lee-slope, holding other geometric parameters and flow hydraulics constant, to explore effects on the turbulent flow field and flow resistance. Three sets of fixed dunes (lee-slopes of 10°, 20° and 30°) were separately installed in a 15 m long and 1 m wide flume and subjected to 0.20 m deep flow. Measurements consisted of high-frequency, vertical profiles collected with a Laser Doppler Velocimeter (LDV). We show that the temporal and spatial occurrence of flow separation decreases with dune lee-slope. Velocity gradients in the dune leeside depict a free shear layer downstream of the 30° dunes and a weaker shear layer closer to the bed for the 20° and 10° dunes. The decrease in velocity gradients leads to lower magnitude of turbulence production for gentle lee-slopes. Aperiodic, strong ejection events dominate the shear layer, but decrease in strength and frequency for low-angle dunes. Flow resistance of dunes decreases with lee-slope; the transition being non-linear. Over the 10°, 20° and 30° dunes, shear stress is 8%, 33% and 90 % greater than a flat bed, respectively. Our results demonstrate that dune lee-slope plays an important, but often ignored role in flow resistance.

Mineralogy and structural specification of ODP Leg 125 peridotite samples

Abundant serpentinite seamounts are found along the outer high of the Mariana forearc at the top of the inner slope of the trench. One of them, Conical Seamount, was drilled at Sites 778, 779, and 780 during Leg 125. The rocks recovered at Holes 779A and 780C, respectively, on the flanks and at the summit of the seamount, include moderately serpentinized depleted harzburgites and some dunites. These rocks exhibit evidence of resorption of the orthopyroxene, when present, and the local presence of very calcic-rich diopside in veins oblique to the main high-temperature foliation of the rock. The peridotites, initially well-foliated with locally poikiloblastic textures, show overprints of a two-stage deformation history: (1) a high-temperature (>1000°C), low-stress (0.02 GPa), homogeneous deformation that has led to the present Porphyroclastic textures displayed by the rocks and (2) heterogeneous ductile shearing at a much higher stress (0.05 GPa). This heterogeneous shearing probably describes a single tectonic event because it began at high temperatures, producing dynamic recrystallization of olivine in the shear zone, and ended at low temperatures in the stability field of chlorite and serpentine. In a few samples, olivine shows evidence of quasi-hydrostatic recrystallization at a very high temperature. Here, we propose that this recrystallization was related to fluid/magma percolation, a process that can also account for the resorption of the orthopyroxene and for the late crystallization of diopside veins in the rock. The impregnation by fluid or magma, development of the main high-temperature, low-stress deformation, and subsequent migration recrystallization of olivine probably occurred in a mantle fragment involved in the arc formation. In addition, this mantle has preserved structures that may have formed earlier in the oceanic lithosphere upon which the arc formed. Heterogeneous ductile shear zones in the peridotites may have developed during uplift. The "cold" deformation may have taken place during diapiric rise of hot mantle that underwent subsequent serpentinization or gliding along normal faults associated with the extension of the eastern margin of the forearc.

Analyses of shocked quartz from the K-P boundary

The precise cause and timing of the Cretaceous-Paleocene (K-P) mass extinction 65 Ma ago remains a matter of debate. Many advocate that the extinction was caused by a meteorite impact at Chicxulub, Mexico, and a number of potential kill-mechanisms have been proposed for this. Although we now have good constraints on the size of this impact and chemistry of the target rocks, estimates of its environmental consequences are hindered by a lack of knowledge about the obliquity of this impact. An oblique impact is likely to have been far more catastrophic than a sub-vertical one, because greater volumes of volatiles would have been released into the atmosphere. The principal purpose of this study was to characterize shocked quartz within distal K-P ejecta, to investigate whether the quartz distribution carried a signature of the direction and angle of impact. Our analyses show that the total number, maximum and average size of shocked quartz grains all decrease gradually with paleodistance from Chicxulub. We do not find particularly high abundances in Pacific sites relative to Atlantic and European sites, as has been previously reported, and the size-distribution around Chicxulub is relatively symmetric. Ejecta samples at any one site display features that are indicative of a wide range of shock pressures, but the mean degree of shock increases with paleodistance. These shock- and size-distributions are both consistent with the K-P layer having been formed by a single impact at Chicxulub. One site in the South Atlantic contains quartz indicating an anomalously high average shock degree, that may be indicative of an oblique impact with an uprange direction to the southeast +/- 45°. The apparent continuous coverage of proximal ejecta in this quadrant of the crater, however, suggests a relatively high impact angle of >45°. We conclude that some of the more extreme predictions of the environmental consequences of a low-angle impact at Chicxulub are probably not applicable.

Physical properties and AMS characteristics of ODP Site 185-1149 sediments

During Ocean Drilling Program Leg 185, we studied progressive changes of microfabrics of unconsolidated pelagic and hemipelagic sediments in Holes 1149A and 1149B in the northwest Pacific at 5818 m water depth. We paid particular attention to the early consolidation and diagenetic processes without tectonic deformation before the Pacific plate subduction at the Izu-Bonin Trench. Shape, size, and arrangement of pores were analyzed by scanning electron microscope (SEM) and were compared to anisotropy of magnetic susceptibility (AMS) data. The microfabric in Unit I is nondirectional fabric and is characterized by large peds of ~10-100 µm diameter, which are made up of clay platelets (mainly illite) and siliceous biogenic fragments. They are ovoid in shape and are mechanically packed by benthic animals. Porosity decreases from 0 to 60 meters below seafloor (mbsf) in Unit I (from 60% to 50%) in association with macropore size decreases. The microfabric of coarser grain particles other than clay in Unit II is characterized by horizontal preferred orientation because of depositional processes in Subunit IIA and burial compaction in Subunit IIB. On the other hand, small peds, which are probably made of fragments of fecal pellets and are composed of smectite and illite (3-30 µm diameter), are characterized by random orientation of clay platelets. The clay platelets in the small peds in Subunit IIA are in low-angle edge-to-face (EF) or face-to-face (FF) contact. These peds are electrostatically connected by long-chained clay platelets, which are interconnected by high-angle EF contact. Breaking of these long chains by overburden pressure diminishes the macropores, and the clay platelets in the peds become FF in contact, resulting in decreases in the volume of the micropores between clay platelets. Thus, porosity in Subunits IIA and IIB decreases remarkably downward. The AMS indicates random fabric and horizontal preferred orientation fabric in Units I and II, respectively. This result corresponds to that of SEM microfabric observations.In Subunit IIB, pressure solutions around radiolarian tests and clinoptilolite veins with normal displacement sense are seen distinctively below ~170 mbsf, probably in correspondence to the transition zone from opal-A to opal-CT.

Step-heating degassing data for argon-39 from volcanic rocks drilled in the Shikoku Basin and Mariana trench, DSDP Legs 58 and 60

From the experimental data on stepwise thermal release of neutron induced 39Ar (39K (n, p) 39Ar) from rocks and minerals, Arrhenius plots were constructed, which gave activation energies for the thermal release process. The activation energies for DSDP Leg 58 and Leg 60 submarine volcanic rocks range from 12 to 20 kcal/mol, whereas those for granodiorites and the K-feldspar separates have activation energies ranging from 37 to 48 kcal/mol. The smaller activation energies for the submarine volcanic rocks reflect the grain boundary diffusion process, while the thermal diffusion of 39Ar from granodiorites and K-feldspar is essentially controlled by a volume diffusion. The grain boundary diffusion for the submarine volcanic rocks suggests that K resides essentially in the grain boundaries.

Subgrain boundaries and related microstructural features in EDML (Antarctica) deep ice core

Subgrain boundaries revealed as shallow sublimation grooves on ice sample surfaces are a direct and easily observable feature of intracrystalline deformation and recrystallization. Statistical data obtained from the EPICA Dronning Maud Land (EDML) deep ice core drilled in East Antarctica cannot detect a depth region of increased subgrain-boundary formation. Grain-boundary morphologies show a strong influence of internal strain energy on the microstructure at all depths. The data do not support the classical view of a change of dominating recrystallization regimes with depth. Three major types of subgrain boundaries, reflecting high mechanical anisotropy, are specified in combination with crystal-orientation analysis.

Planktonic foraminifera distribution during the mid-Piacenzian warm period in the North Atlantic

The mid-Piacenzian (MP) warm period (3.264-3.025 Ma) has been identified as the most recent time in geologic history during which mean global surface temperatures were considerably warmer than today for a sustained period. This interval has therefore been proposed as a potential (albeit imperfect) analog for future climate change and as such, has received much scientific attention over the past two decades. Central to this research effort is the Pliocene Research, Interpretation, and Synoptic Mapping (PRISM) project, an iterative paleoenvironmental reconstruction of the MP focused on increasing our understanding of warm-period climate forcings, dynamics, and feedbacks by providing three-dimensional data sets for general circulation models. A mainstay of the PRISM project has been the development of a global sea surface temperature (SST) data set based primarily upon quantitative analyses of planktic foraminifer assemblages, supplemented with geochemical SST estimates wherever possible. In order to improve spatial coverage of the PRISM faunal data set in the low and mid-latitude North Atlantic, this study provides a description of the MP planktic foraminifer assemblage from five Ocean Drilling Program sites (951, 958, 1006, 1062, and 1063) in the subtropical gyre, a region critical to Atlantic Ocean circulation and tropical heat advection. Assemblages from each core provide evidence for a temperature- and circulation-driven 5-10° northward displacement of MP faunal provinces, as well as regional shifts in planktic foraminifer populations linked to species ecology and interactions. General biogeographic trends also indicate that, relative to modern conditions, gyre circulation was stronger (particularly the Gulf Stream, North Atlantic Current, and North Equatorial Current) and meridionally broader. A comparison of mid-Piacenzian and modern North Atlantic planktic foraminifer assemblages suggests that low latitude western boundary currents were less than 1 °C warmer while eastern boundary currents were ~1-2 °C warmer, supporting the hypothesis of enhanced northward heat advection along western boundary currents and warming of high latitude Northeast Atlantic source regions for the Canary Current. These findings are consistent with a model of reduced meridional SST gradients, with little-to-no low latitude warming, and more vigorous ocean circulation. Results therefore support the theory that enhanced meridional overturn circulation and associated northward heat advection made an important contribution, in conjunction with elevated atmospheric CO2 concentrations, to the 2-3 °C global surface temperature increase (relative to today) and strong polar amplification of SST warmth during the MP warm period.

Physical properties and velocity measurement comparison for sediments from DSDP Holes 92-597B and 92-597C

The bulk and grain densities, porosity, water content, and ultrasonic compressional- and shear-wave velocities of 25 basalt samples from DSDP Holes 597B and 597C were measured. The velocities were measured at in situ pore and confining pressures. The bulk densities of the samples vary between 2.690 and 3.050 g/cm**3. Porosities of selected samples vary between 2.4 and 9.3%. The grain densities vary between 2.993 and 3.117 g/cm3, a range that suggests that bulk density differences are due primarily to variations in porosity. Compressional-wave velocities range from 5.70 to 6.81 km/s, and shear-wave velocities range from 1.66 to 3.84 km/s. The variation in compressional velocity appears to be due primarily to variations in grain size and the associated greater density of grain-boundary cracks for samples with a smaller average grain size. On the basis of these results we would expect compressional and shear velocities to increase with increasing depth in the shallow crust, primarily as the result of the effects of confining pressure on crack density.

(Table 1) Paleomagnetic of reoriented core pieces of IODP Hole 304-U1309D

Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the "rolling-hinge" model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46° ± 6° anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011° ± 6°. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases.

Sedimentological and paleomagnetic investigations on two gravity cores and ODP sites 113-690 and 693 off Kapp Norvegia, Antarctic continental margin

Oligocene to Quaternary sediments were recovered from the Antarctic continental margin in the eastern Weddell Sea during ODP Leg 113 and Polarstern expedition ANT-VI. Clay mineral composition and grain size distribution patterns are useful for distinguishing sediments that have been transported by ocean currents from those that were ice-rafted. This, in turn, has assisted in providing insights about the changing late Paleogene to Neogene sedimentary environment as the cryosphere developed in Antarctica. During the middle Oligocene, increasing glacial conditions on the continent are indicated by the presence of glauconite sands, that are interpreted to have formed on the shelf and then transported down the continental slope by advancing glaciers or as a result of sea-level lowering. The dominance of illite and a relatively high content of chlorite suggest predominantly physical weathering conditions on the continent. The high content of biogenic opal from the late Miocene to the late Pliocene resulted from increased upwelling processes at the continental margin due to increased wind strength related to global cooling. Partial melting of the ice-sheet occurred during an early Pliocene climate optimum as is shown by an increasing supply of predominantly current-derived sediment with a low mean grain size and peak values of smectite. Primary productivity decreased at ~ 3 Ma due to the development of a permanent sea-ice cover close to the continent. Late Pleistocene sediments are characterized by planktonic foraminifers and biogenic opal, concentrated in distinct horizons reflecting climatic cycles. Isotopic analysis of AT. pachyderma produced a stratigraphy which resulted in a calculated sedimentation rate of 1 cm/k.y. during the Pleistocene. Primary productivity was highest during the last three interglacial maxima and decreased during glacial episodes as a result of increasing sea-ice coverage.

Geochemical composition of minerals obtained from ODP Sites 180-1109, 180-1117 and 180-1118

During the last 8 m.y. the Papuan Peninsula region of Papua New Guinea has been affected by extension which opened the Woodlark Basin. The present-day spreading tip is located at the foot of the Moresby Seamount, a crustal block whose northern flank is an active low-angle normal fault related to this extension. During Ocean Drilling Program Leg 180 (7 June-11 August 1998), 11 sites (1108-1118) were drilled along a north-south-trending transect across the Woodlark Basin just ahead of the spreading tip. Four of these sites (1118, 1109, 1114, and 1117) reached the crystalline basement, which is composed of diabase and gabbro. Sites 1118 and 1109, located on the Woodlark Rise, belong to the hanging wall block, and Sites 1114 and 1117, located on the crest of the Moresby Seamount, belong to the footwall block and the fault zone itself. Most of the basalt, diabase, and gabbro that were recovered show a well-preserved magmatic texture. The diabase, which is the most abundant rock type, has a coarse-grained ophitic texture composed of poikilitic clinopyroxene including radiating, locally skeletal plagioclase laths with interstitial iron oxide grains. Secondary mineralogy consists of chlorite, zeolite, calcite, albite, and quartz. The gabbro shows a medium-grained granular texture. The magmatic mineralogy consists of euhedral laths of plagioclase and anhedral interstitial clinopyroxene. Secondary mineralogy consists of a magnesio to actinolitic hornblende, chlorite, clinozoisite, zeolite, quartz, and calcite. The retrograde metamorphic evolution of both gabbro and diabase occurred under low amphibolite to subgreenschist facies conditions associated mainly with brittle deformation and the development of a local low-temperature shear zone. This shows no evidence for high thermal gradient in the crust during the continental rifting.

(Supplement Table1) Abundances of major elements from KH93-3_DR10 at the Southwest Indian Ridge near the Rodriguez Triple Junction

Petrographic and geochemical analyses of basaltic rocks dredged from the first segment of the Southwest Indian Ridge near the Rodriguez Triple Junction have been completed in order to investigate water-rock interaction processes during mid-ocean ridge (MOR) hydrothermal alteration in the Indian Ocean. In the study area, we have successfully recovered a serial section of upper oceanic crust exposed along a steep rift valley wall which was uplifted and emplaced along a low angle normal fault. On the basis of microscopic observation, dredged samples are classified into three types: fresh lavas, low-temperature altered rocks, and high-temperature altered rocks. The fresh lavas have essentially the same chemical composition as typical N-MORB, although LILE and Nb are slightly enriched and depleted, respectively. Low temperature alteration brought about the enrichment of K2O, Rb, and U due to the presence of K-rich celadonite and U-adsorption onto Fe-oxyhydroxide and clay minerals. On the other hand, chloritization, albitization, and addition of base metals by high temperature hydrothermal alteration result in enrichments of MnO, MgO, Na2O, Cu, and Zn and depletions of CaO, K2O, Cr, Co, Ni, Rb, Sr, and Ba. In addition, U-enrichment is also observable in the high temperature altered rocks probably due to the decrease of uranite solubility in the reducing high-temperature hydrothermal solution. These petrological and geochemical features are comparable to those of the volcanic zone to transition zone rocks in the DSDP/ODP Hole 504B, indicating that our samples were recovered from the upper ~1000 m section of the oceanic crust. Only the alteration minerals related to off-axis alteration are absent in our samples dredged from near the spreading axis. The similarity of alteration between our samples from the Indian Ocean and the Hole 504B rocks from the Pacific Ocean suggests that MOR hydrothermal systems are probably similar across all world oceans.

Calcareous dinocysts and grain size distribution along profile Stevns-Klint

The distribution patterns of calcareous dinoflagellate cysts were studied in the classic Cretaceous Tertiary (K-T) boundary section of Stevns Klint, Denmark, focusing mainly on the response of the cyst association to an abrupt environmental catastrophe. A major part of the Fish Clay, which covers the K-T boundary at its base and is exposed in the investigated section, contains fallout produced by an asteroid impact. Calcareous dinoflagenate cysts are the best preserved remains of carbonate-producing phytoplankton in this layer. The potential of this group of microfossils for the analysis of survival strategies and extinction patterns has been underestimated. The cyst species of the investigated section can be grouped into four assemblages that represent victims, survivors, opportunists, and specially adapted forms. The victims (Pithonelloideae) were an extremely successful group throughout the Upper Cretaceous, but were restricted to the narrow outer shelf. This restriction minimized their spatial distribution, which generally should be large to facilitate escape from unfavorable conditions. Spatial restriction optimized the population decrease by mass mortality, disabling a successful recovery. In contrast, the survivors that became the dominating group in the Danian had a wide spatial range from the shelf environment to the oceanic realm. A unique calcareous dinocyst assemblage in the Fish Clay shows that even under the stressed conditions immediately following the impact event, some species flourished due to special adaptation or high ecological tolerance. The ability of these dinoflagellate species to form calcareous resting cysts in combination with their generally wide spatial distribution in a variety of environments appears to be the main reason for a low extinction rate at the K-T boundary as opposed to the high extinction rate of other phytoplankton groups, such as the coccolithophorids.

Benthic oxygen isotopes, Zr/Al and grain size fractions over the Miocene-Pliocene boundary in the Gulf of Cadiz

During the latest Messinian, hemipelagic sediments exhibiting precession-induced climate variability were deposited. These are overlain by Pliocene sediments deposited at a much higher sedimentation rate, with much higher and more variable XRF-scanning Zr/Al ratios than the underlying sediment, and that show evidence of winnowing, particle sorting and increasing grain size, which we interpret to be related to the increasing flow of MOW. Pliocene sedimentary cyclicity is clearly visible in both the benthic d18O record and the Zr/Al data and is probably also precessionally controlled. On the basis of these results, we conclude that contouritic sedimentation, associated with weak Mediterranean-Atlantic exchange, began in the Gulf of Cadiz virtually at or shortly after the Miocene-Pliocene boundary, with two contouritic bigradational sandy-beds within the fourth precession cycle after the Miocene-Pliocene boundary.

(Table 1) Stable-isotope and eolian grain size data across the Paleocene/Eocene boundary from DSDP Hole 22-215

Deep Sea Drilling Project (DSDP) Site 215 provides an expanded section across the Paleocene/Eocene boundary, the most complete mid-latitude sequence from a Southern Hemisphere location in the Indo-Pacific area. The events of this transition occurred during a span of about 1.2 m.y. Oxygen isotope values derived from benthic foraminiferal calcite decrease by about 1.0 per mil, a decrease most likely related to warming of deep ocean waters. Turnovers of benthic foraminifera accompany d18O changes and culminate in the predominant extinction event at the end of the Paleocene Epoch. Carbon isotope ratios also shift dramatically toward lighter values near the end of the Paleocene, beginning about 0.45 m.y. after oxygen isotope values start to change. The intensity of Southern Hemisphere atmospheric circulation as recorded by grain sizes of eolian particles shows a large and rapid reduction beginning another 0.45 m.y. later. A significant reduction of zonal wind strength at the Paleocene/Eocene boundary, until now observed only at Northern Hemisphere locations, appears to have been a global phenomenon related to decreased latitudinal thermal gradients occasioned by more effective poleward heat transport via the deep ocean.