Passive optical retrievals of cloud liquid water path (LWP), like those implemented for Moderate Resolution Imaging Spectroradiometer (MODIS), rely on cloud vertical profile assumptions to relate optical thickness (τ) and effective radius (re ) retrievals to LWP. These techniques typically assume that shallow clouds are vertically homogeneous; however, an adiabatic cloud model is plausibly more realistic for shallow marine boundary layer cloud regimes. In this study a satellite retrieval simulator is used to perform MODIS-like satellite retrievals, which in turn are compared directly to the large-eddy simulation (LES) output. This satellite simulator creates a framework for rigorous quantification of the impact that vertical profile features have on LWP retrievals, and it accomplishes this while also avoiding sources of bias present in previous observational studies. The cloud vertical profiles from the LES are often more complex than either of the two standard assumptions, and the favored assumption was found to be sensitive to cloud regime (cumuliform/stratiform). Confirming previous studies, drizzle and cloud top entrainment of dry air are identified as physical features that bias LWP retrievals away from adiabatic and toward homogeneous assumptions. The mean bias induced by drizzle-influenced profiles was shown to be on the order of 5-10 g/m2. In contrast, the influence of cloud top entrainment was found to be smaller by about a factor of 2. A theoretical framework is developed to explain variability in LWP retrievals by introducing modifications to the adiabatic re profile. In addition to analyzing bispectral retrievals, we also compare results with the vertical profile sensitivity of passive polarimetric retrieval techniques.
This objective of this research was to investigate the potential of using transmission electron microscopy (TEM) to determine sulfide mineral speciation in gabbroic rocks from Atlantis Massif, the site of Integrated Ocean Drilling Program Expedition 304/305.The method takes advantage of TEM analysis techniques and proved successful in sulfide mineral identification.TEM can provide imaging of the sample morphology, crystal structure through the electron diffraction, and chemical compositional information through X-ray energy dispersive spectroscopy.Electron probe microanalysis was also used to determine the chemical composition.
[1] Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100–220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45° rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.
Seismic anisotropy in oceanic layer 2 resulting from a preferred alignment of fractures has been widely recognized, but all experiments to date have sought to measure only the weak azimuthal variation of elastic properties resulting from tectonically controlled systems of vertical fractures. From ocean drilling data, however, especially from DSDP Hole 504B, we know that layer 2 is composed of interleaved massive flows and breccia units, and that the massive units have a very strong concentration of horizontal fractures. Layer 2's pronounced horizontal fabric of low-velocity “layers” (fractures and/or breccia zones) permeating an otherwise high-velocity matrix, will cause P-waves to travel faster horizontally than vertically. This anisotropy has no azimuthal expression, and so cannot easily be recognized in seismic data, but it may lead to overestimation of the thickness of upper crustal layers by as much as 30% in young crust. Further, the anisotropy affects P and S waves differently, so where shear-wave data are available, Poisson's ratio may be substantially underestimated. The widespread observation of a low Poisson's ratio zone in the upper few kilometers of young crust is almost certainly an artifact of ignoring anisotropy. As the crust ages, fractures and voids are filled by chemical alteration and precipitation, the velocity contrast between rock and void-filling material is reduced, and the anisotropy decreases. The errors introduced by assuming isotropy thus show an inverse relationship to crustal age, so that thickness measurements from old crust are probably no more than 10% in error. This explains a long-standing enigma of marine seismology: the apparent thinning of upper crustal layers with age.
Use of 3D-seismic surface mapping has proved to be a rapid and efficient method of identifying the major seafloor features that affect both oil exploration and production processes. Accuracy of this method still remains uncertain where no ground-truth tools are employed. Integrating tools of conventional oceanographic surveys dedicated to near surface geology interpretation and 3D-seismic is the best method to reduce uncertainties and calibrate the inversion of seismic amplitude in lithology. Recognition of the application of 3D-seismic data as a tool for hazard evaluation in deep waters was early performed by the Marine Geology section of Petrobras, which integrated that technique with the latest available tec hnology of data interpretation and visualization. The need of calibrating seismic data induced the application of different methods of ground-truthing, comprising video and still-frame photography of the seafloor, several sample acquisition, surface and deeptowed side-scan sonar imaging and 3.5 kHz subbottom profiles. The gathered data was analyzed to determine the nature and extension of surface and near surface features. Major features such as mass movement deposits, thick shallow sand accumulations, active fault movement and the presence of shallow gas and chemosynthetic communities are investigated by the combination of those different tools with a high accuracy and reliability. This article presents a case of integrating different marine geology tools in order to calibrate 3D-seismic maps, such as physiographic, bathymetric and amplitude/ lithologic maps. In order to better image the attributes some up-to-date visualization methods derived from remote sensing techniques were employed providing astonishing sea-floor landscapes.
Abstract Emissions of SO 2 by volcanic eruptions have been shown to be important for short-term environmental and climate changes. Stratospheric sulphur mass-loading by explosive silicic eruption is commonly considered to be the principal forcing factor for these changes. The SO 2 emissions from basaltic flood lava eruptions have not featured strongly in the discussions on volcano-climate interactions, notwithstanding the fact that basaltic magma is typically richer in sulphur (by a factor of two to four), than silicic magmas, as well as the evidence of widespread atmospheric impact associated with historical flood lava eruption. Fourteen Holocene flood lava eruptions are known from the Veidivötn, Grímsvötn, and Katla volcanic systems of the Eastern Volcanic Zone in South Iceland, which include the three largest of its kind in Iceland; the 1783–1784 Laki, 934–40 Eldgjá, and c. 8600 years BP Thjórsá events. We present new data on the sulphur content in melt inclusions from the Veidivötn system and use this information, along with existing inclusion data from the Grímsvötn and Katla volcanic systems, to establish an empirical method for estimating the sulphur mass release from these basaltic flood lava eruptions. The results show that these eruptions released a total of c. 700 Mt SO 2 into the atmosphere in four 600- to 850-year-long eruption periods. During each period, between 98 and 328 Mt SO 2 were emitted into the atmosphere, and the mass loadings from individual eruptions ranged from 5 to 210 Mt SO 2 . These flood lava eruptions are likely to have resulted in widespread atmospheric perturbations and, by analogy with the 1783–1784 Laki eruption, the effects of the largest eruptions may have been felt on a hemispheric scale.
Leg 209 of the Ocean Drilling Program will be devoted to coring mantle peridotite along the Mid-Atlantic Ridge (MAR) from 14° to 16°N.This area was identified at the 1996 Workshop on Oceanic Lithosphere and Scientific Drilling into the 21st Century as the ideal region for drilling of a strike line of short holes to sample the upper mantle in a magma-starved portion of a slow-spreading ridge.In this area, igneous crust is locally absent and the structure and composition of the mantle can be determined at sites over ~100 km along strike.A central paradigm of Ridge Interdisciplinary Global Experiments (RIDGE) program studies is the hypothesis that mantle flow, or melt extraction, or both, are focused in three dimensions toward the centers of magmatic ridge segments, at least at slow-spreading ridges such as the MAR.This hypothesis has essentially reached the status of accepted theory, but it has never been subject to a direct test.A strike line of oriented mantle peridotite samples extending for a significant distance within such magmatic segments offers the possibility of directly testing this hypothesis.Continued dredging and submersible studies cannot provide the spatial information required to make such a test.The primary aim of drilling is to characterize the spatial variation of mantle deformation patterns, residual peridotite composition, melt migration features, and hydrothermal alteration along axis.Hypotheses for focused solid or liquid upwelling beneath ridge segments make specific predictions regarding the spatial variation of mantle lineation or the distribution of melt migration features.These predictions may be directly tested by drilling.
We present a multidisciplinary approach to investigating the physical properties of rocks interpreted to have been exhumed from the lower oceanic crust and upper mantle.Laboratory-measured compressional-wave velocities have been examined with respect to geochemical and petrological characteristics to help explain the seismic velocity structure of the oceanic lithosphere beneath the fast-spreading ridge near Hess Deep.Samples analyzed indicate that no bulk-rock chemical indices exhibit apparent correlation with velocity behavior.Differences between mean atomic weight predicted from velocity-density systematics and calculated from bulk-rock geochemistry can be ascribed to alteration effects and preferred mineral orientation.The primary controls on velocity behavior are similar in the gabbroic rocks sampled at Site 894 and the ultramafic rocks sampled at Site 895.In the gabbroic rocks, velocity behavior is controlled by alteration of clinopyroxene and possibly preferred mineral orientation.In the serpentinized peridotites, the primary control on velocity behavior is intensity of serpentinization.Comparison of velocity-depth profiles and mineral composition profiles highlights compositional trends that are not obvious from compositional data alone.Similar-scale, high-velocity gradients in cores from the fast-spreading ridge near Hess Deep, and the slow-spreading Southwest Indian Ridge, suggest that similar scale structural controls (on -150 m wavelength) may be present in both environments.
