Authigenic carbonates from the Cascadia subduction zone and their relation to gas hydrate stability

Authigenic carbonates are intercalated with massive gas hydrates in sediments of the Cascadia margin. The deposits were recovered from the uppermost 50 cm of sediments on the southern summit of the Hydrate Ridge during the RV Sonne cruise SO110. Two carbonate lithologies that differ in chemistry, mineralogy, and fabric make up these deposits. Microcrystalline high-magnesium calcite (14 to 19 mol% MgCO3) and aragonite are present in both semiconsolidated sediments and carbonate-cemented clasts. Aragonite occurs also as a pure phase without sediment impurities. It is formed by precipitation in cavities as botryoidal and isopachous aggregates within pure white, massive gas hydrate. Variations in oxygen isotope values of the carbonates reflect the mineralogical composition and define two end members: a Mg-calcite with δ18O =4.86‰ PDB and an aragonite with δ18O =3.68‰ PDB. On the basis of the ambient bottom-water temperature and accepted equations for oxygen isotope fractionation, we show that the aragonite phase formed in equilibrium with its pore-water environment, and that the Mg-calcite appears to have precipitated from pore fluids enriched in 18O. Oxygen isotope enrichment probably originates from hydrate water released during gas-hydrate destabilization

Physical properties of sediment core CRP-3

Drillhole CRP-3 in northern McMurdo Sound (Ross Sea, Antarctica) targeted the western margin of the Victoria Land basin to investigate Neogene to Palaeogene climate and tectonic history by obtaining continuous core and downhole logs. Well logging of CRP-3 has provided a complete and comprehensive data set of in situ geophysical measurements down to nearly the bottom of the hole (920 m below sea floor (mbsf)). This paper describes the evaluation and interpretation of the downhole logging data using the multivariate statistical methods of factor and cluster analysis. The factor logs mirror the basic geological controls (i.e., grain size, porosity, clay mineralogy) behind the measured geophysical properties, thereby making them easier to interpret geologically. Cluster analysis of the logs delineates individual logging or sedimentological units with similar downhole geophysical properties. These objectively and independently defined units help differentiate lithological and sedimentological characteristics (e.g. grain size, sediment provenance, glacial influence). For CRP-3, the three factor logs derived from the downhole measurements reflect sediment grain size (proxy for lithology), the occurrence of diamict and/or conglomerate (glacial influence), and sediment provenance. It is possible to divide the borehole into three main sections on the basis of cluster analysis of the three factor logs. The top section down to about 200 mbsf is dominated by mudstone with clearly different physical properties from the mudstones occurring below this depth. Beneath 200 mbsf sandstones dominate the lithology. Two types of sandstones were characterised in the early Oligocene/late Eocene sequence, with a division between the two types occurring at about 630 tnbsf. These two types of sandstones, which are differentiated mainly on the basis of their magnetic and radiogenic properties, can be correlated by detrital mode provenance analysis. Comparison of the results of the factor and cluster analyses with the reflection seismic profiles shows that the major change in sediment source from the Victoria Group to the Taylor Group at 630 mbsf (earliest Oligocene) is not seen by seismic sequence analysis. This observation may have important consequences for the entire Ross Sea seismic stratigraphy

Temperature and resistivity profile of drill hole CRP-2A

In the northern part of McMurdo Sound (Ross Sea, Antarctica), drillhole CRP-2/2A targeted he western margin of the Victoria Land basin to investigate Neogene to Palaeogene climatic and tectonic history by obtaining continuous core and downhole logs. The background of the project and its detailed aims, methods used and results so far are summarized in Cape Roberts Science Team (1999). The CRP-2A drillhole extended to 625 mbsf (meters below seafloor) with an average of 95 % recovery of Oligocene to Quaternary sediments. Most of the downhole logging tools were run to the bottom of the hole. The first measurement after drilling operations was the temperature and salinity measurements, which were completed in two phases down to 166 mbsf and down to the bottom of the hole. Although an equilibrium temperature state had not been reached after drilling operations, the temperature profiles provide interesting and important data on formation temperature and fluid movement. The average overall temperature gradient is 24 K/km, somewhat lower than the temperature gradients found in the DVDP, MSSTS, and CIROS boreholes. Active permeable zones could be detected at least at two depths (150 and 580 mbsf) by falling temperatures and salinities, suggesting cold water influx. The driving forces for fluid movements detected by the anomalies in CRP-2 are still unknown

Core derived downhole logs for holes CRP-2 and CRP-2A

In the northern McMurdo Sound (Ross Sea, Antarctica), the CRP-2/2A drillhole targeted the western margin of the Victoria Land Basin to investigate Neogene to Palaeogene climatic and tectonic history by obtaining continuous core and downhole logs. Well logging of CRP-2/2A has provided a complete and comprehensive dataset of in situ geophysical measurements. This paper describes the evaluation and interpretation of the downhole logging data using multivariate statistical methods. Two major types of multivariate statistical methods were each yielding a different perspective: (1) Factor analysis was used as an objective tool for classification of the drilled sequence based on physical and chemical properties. The factor logs are mirroring the basic geological controls (i.e., grain size, porosity, clay mineralogy) behind the measured geophysical properties, thereby making them easier to interpret geologically. (2) Cluster analysis of the logs groups similar downhole geophysical properties into one cluster, delineating individual logging or sedimentological units. These objectively and independently defined units, or statistical electrofacies, are helpful in differentiating lithological and sedimentological characterisations (e.g. grain size, provenance). The multivariate statistical methods of factor and cluster analysis proved to be powerful tools for fast, reliable, and objective characterisation of downhole geophysical properties at CRP-2/2A, resulting in interpretations which are consistent with sedimentological findings

The impact of dynamic data reshaping on adjoint code generation for weakly-typed languages such as Matlab

Productivity-oriented programming languages typically emphasize convenience over syntactic rigor. A well-known example is Matlab, which employs a weak type system to allow the user to assign arbitrary types and shapes to a variable, and it provides various shortcuts in programming that result in implicit data reshapings. Examples are scalar expansion, where a scalar is implicitly expanded to a matrix of the appropriate size filled with copies of the scalar value, the use of row vectors in place of column vectors and vice versa, and the automatic expansion of arrays when indices outside of the previously allocated range are referenced. These features need to be addressed at runtime when generating adjoint code, as Matlab does not provide required information about types, shapes and conversions at compile time. This fact, and the greater scope of reshaping possible, is a main distinguishing feature of Matlab compared to traditional programming languages, some of which, e.g. Fortran 90, also support vector expressions. In this paper, in the context of the AdiMAT source transformation tool for Matlab, we develop techniques generally applicable for adjoint code generation in the face of dynamic data reshapings occurring both on the left- and right-hand side of assignments. Experiments show that in this fashion correct adjoint code can be generated also for very dynamic language scenarios at moderate additional cost

A New User Interface for ADiMat: Toward Accurate and Efficient Derivatives of Matlab Programs with Ease of Use

Various techniques in computational science and engineering benefit from accurate and efficient derivative computation. ADiMat is a software tool that transforms a numerical program written in Matlab into another Matlab program for the computation of derivatives that are free from truncation error. We introduce a new and easy-to-use interface for ADiMat and present case studies from geophysics and fluid mechanics to quantify the performance of the code generated by ADiMat

Porosity and sound velocity measured in sediment core CIROS-1

The CIROS-1 drillhole, which in 1986 reached a depth of 700 m below the seafloor, is still the only deep hole that can provide information on the velocity structure of the upper crust in McMurdo Sound and the Ross Sea, Antarctica. A careful review and quality control of the downhole logging data of CIROS-1 resulted in a new porosity depth function that is consistent with porosity data from the MSSTS-1 and CRP-1 drillholes. Using existing porosity-velocity equations, it was possible for the first time to obtain reliable velocity information for the upper 700 m of strata off the Victoria Land coast. The calculated synthetic seismograms, based on downhole velocity and density data, fit very well with the existing seismic lines IT90A-71, PD90-12, and NBP9601-89. The quality of the correlation confirms that the average velocity of the top 700 m of strata is about 2 000-2 300 m/s, and not 2 800-3 000 m/s, as was previously assumed. In consequence, these distinctly lower velocities result in shallower depths for the seismic unconformities V3/V4 andV4/V5 and thus may have important implications for further drilling off Cape Roberts

Physical properties of sediment core CRP-1

The relationship between whole-core compressional wave velocities and gamma-ray attenuation porosities of sediments cored at CRP-1 is examined and compared with results from core-plug samples and global models. Both core-plug and whole-core velocities show a strong dependence on porosity: this relationship appears to be independent of lithology. In the range from 0.1 to 0.4 of fractional porosity (Miocene strata), plug velocities are generally 0.2 - 0.5 km s-1 higher than whole-core velocities. Possible reasons include decreased rigidity in the whole core and diagenetic changes in the plugs. Possibly both velocity measurements are correct but neither is fully representative for in situ conditions. It appears that the core-plug results are more compatible with data from other regions than the whole-core data. After removing first-order compaction control from the whole-core porosity record, a second-order control by clay content can be quantified as a simple positive linear regression (R=0.6). In contrast, after correction for first-order control, porosity and velocity are not significantly influenced by lonestone abundance except for rare, very large lonestones