Investigation of diagenesis in sediment core CRP-1 from the Ross Sea, Antarctica

A diagenetic study was carried out on the cored Miocene section in CRP-1 by thin-section, X-ray diffraction, scanning electron microscope, electron microprobe and stable isotopic analysis. Carbonate (calcite, siderite) microconcretions occur locally within intergranular pores and open fractures, and some sands are cemented by microcrystalline calcite. Calcite cement at 115.12 mbsf (metres below sea floor) and possibly microconcretionary calcite at 44.62 mbsf record infiltration of meteoric waters into the section, consistent with sequence stratigraphic evidence for multiple glacial advances over the CRP-1 drillsite. Diagenetic carbonates incorporated carbon derived from both organic matter and marine carbonate. Carbon isotope data are consistent with microconcretion formation at shallow depths. Sandstones are poorly compacted and, despite containing a large component of chemically unstable grains, are virtually unaltered. Preservation of the chemically unstable grain component reflects the cold climate depositional setting and shallow maximum burial depths.

Petrophysical investigation on sediment core CRP-3 from the Ross Sea, Antarctica

A suite of petrophysical properties - velocity, resistivity, bulk density, porosity, and matrix density - was measured on 88 core plugs from the CRP-3 drillhole. Core-plug bulk densities were used to recalibrate both whole-core and downhole bulk density logs. Core-plug measurements of matrix density permit conversion of the whole-core and downhole bulk density logs to porosity. Both velocity and formation factor (a normalized measure of resistivity) are strongly correlated with porosity. The velocity/porosity pattern is similar to that for the lower part of CRP-2A and is consistent with the empirical relationship for sandstones. Core-plug and whole-core measurements of P-wave velocity at atmospheric pressure exhibit excellent agreement. Measurements of velocity as a function of pressure indicate a significantly higher velocity sensitivity to pressure than has been observed at CRP-1 and CRP-2A; rebound or presence of microcracks at CRP-3 may be responsible. The percentage difference between velocities at in situ pressures and atmospheric pressures increases downhole from 0% at the seafloor to 9% at the bottom. This pattern can be used to correct whole-core velocity data, measured at atmospheric pressure, to in situ velocities for depth-to-time conversion and associated comparison to the seismic profile across the drillsite

Geophysical logging on sediment core and hole CRP-3 from the Ross Sea, Antarctica

Cape Roberts drillhole CRP-3 in the northern part of McMurdo Sound (Ross Sea, Antarctica) 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 (Cape Roberts Science Team, 2000). The CRP-3 drillhole extended to 939.42 mbsf (meters below seafloor) at a water depth of 297 m. The first downhole measurements after drilling were the temperature and salinity logs. Both were measured at the beginning and at the end of each of the three logging phases. Although an equilibrium temperature state may not have been fully reached after drilling, the temperature and salinity profiles seem to be scarcely disturbed. The average overall temperature gradient calculated from all temperature measurements is 28.5 K/km; remarkably lower than the temperature gradients found in other boreholes in the western Ross See and the Transantarctic Mountains. Anomalies in the salinity profiles at the beginning of each logging phase were no longer present at the end of the corresponding logging phase. This pattern indicates that drilling mud invaded the formation during drilling operations and flowed back into the borehole after drilling ceased. Thus, zones of temperature and salinity anomalies identify permeable zones in the formation and may be pathways for fluid flow. Radiogenic heat production, calculated from the radionuclide contents, is relatively low, with average values between 0.5 and 1.0 pW/m3. The highest values (up to 2 µW/m3) were obtained for the lower part of the Beacon Sandstone below 855 mbsf. The heat flow component due to radiogenic heat production integrated over the entire borehole is 0.7 mW/m2. Thermal conductivities range from 1.3 to 3 W/mK with an average value of 2.1 W/mK over the Tertiary section. Together with the average temperature gradient of 28.5 K/km this yields an average heat flow value of 60 mW/m2.

Physical properties and neutron texture measuerments of deep-sea carbonates of DSDP Holes 62-463 and 62-465A

In weakly indurated, nannofossil-rich, deep-sea carbonates compressional wave velocity is up to twice as fast parallel to bedding than normal to it. It has been suggested that this anisotropy is due to alignment of calcite c-axes perpendicular to the shields of coccoliths and shield deposition parallel to bedding. This hypothesis was tested by measuring the preferred orientation (fabric) of calcite c-axes in acoustic anisotropic, calcareous DSDP sediment samples by X-ray goniometry, and it was found that the maximum c-axis concentrations are by far too low to explain the anisotropies. The X-ray method is subject to a number of uncertainties due to preparatory and technical shortcomings in weakly indurated rocks. The most serious weaknesses are: sample preparation, volume of measured sample (fraction of a mm3), beam defocusing and background intensity corrections, combination of incomplete pole figures, and necessity of recalculation of the c-axis orientations from other crystallographic directions. Goniometry using thermal neutrons overcomes most of these difficulties, but it is time consuming. We test the interferences made about velocity anisotropy by X-ray studies about the concentration of c-axes in deep-sea carbonates by employing neutron texture goniometry to eight DSDP samples comprising mostly nannofossil material. Fabric and sonic velocity were determined directly on the core specimens, thus from the same rock volume and requiring no preparation. The c-axis orientation is obtained directly from the [0006] calcite diffraction peak without corrections. The fabrics are clearly defined, but weak (1.1 to 1.86 times uniform) with the maximum about normal to bedding. They have crudely orthorhombic symmetry, but are not axisymmetric around the bedding normal. The observed c-axis intensities, although higher than determined by the X-ray method on other samples, are by far too low to explain the observed acoustic anisotropies.

Carbon and Sulphur concentrations of sediments from Peru, Oman and Benguela upwelling environments

Abundances of organic carbon, sulfur, and reactive iron in sediments of three upwelling environments (Peru, Oman and Benguela) suggest that organic carbon/reduced sulfur ratios (C/S-ratios) in this category of marine sediments deviate considerably from previously established empirical ratios in normal marine sediments. To clarify the discrepancies, we investigated those components of the diagenetic system that limit the formation of pyrite: sulfate concentrations and reduction rates in pore waters, availability of reactive iron, and the quantity and quality of organic matter. All three limitations are evident in our sample pools. The results suggest that C/S-ratios in recent and fossil marine sediments rich in organic matter may be unsuitable as paleoenvironmental indicators.

Physical properties of sediment cores from the Labrador Sea

Sediment core logs from six sediment cores in the Labrador Sea show millennial-scale climate variability during the last glacial by recording all Heinrich events and several major Dansgaard-Oeschger cycles. The same millennial-scale climate change is documented for surface-water d18O records of Neogloboquadrina pachyderma (left coiled); hence the surface-water d18O record can be derived from sediment core logging by means of multiple linear regression, providing a paleoclimate proxy record at very high temporal resolution (70 yrs). For the Labrador Sea, sediment core logs contain important information about deep-water current velocities and also reflect the variable input of IRD from different sources as inferred from grain-size analysis, benthic d18O, the relation of density and p-wave velocity, and magnetic susceptibility. For the last glacial, faster deep-water currents which correspond to highs in sediment physical properties, occurred during iceberg discharge and lasted for a several centuries to a few millennia. Those enhanced currents might have contributed to increased production of intermediate waters during times of reduced production of North Atlantic Deep Water. Hudson Strait might have acted as a major supplier of detrital carbonate only during lowered sea level (greater ice extent). During coldest atmospheric temperatures over Greenland, deep-water currents increased during iceberg discharge in the Labrador Sea, then surface water freshened shortly after, while the abrupt atmospheric temperature rise happened after a larger time lag of >=1 kyr. The correlation implies a strong link and common forcing for atmosphere, sea surface, and deep water during the last glacial at millennial time scales but decoupling at orbital time scales.

Index properties, shear strenghts and consolidation characteristics of ODP Hole 164-995A sediments

Geotechnical properties of sediment from Ocean Drilling Program Leg 164 are presented as: (1) normalized shipboard strength ratios from the Cape Fear Diapir, the Blake Ridge Diapir, and the Blake Ridge; and (2) Atterberg limit, vane shear strength, pocket-penetrometer strength, and constant-rate-of-strain consolidation results from Hole 995A, located on the Blake Ridge. This study was conducted to understand the stress history in a region characterized by high sedimentation rates and the presence of gas hydrates. Collectively, the results indicate that sediment from the Blake Ridge exhibits significant underconsolidated behavior, except near the seafloor. At least 10 m of additional overburden was removed by erosion or mass wasting at Hole 993A on the Cape Fear Diapir, compared to nearby sites.

Heat flow measurements in the Weddell Sea

From January to March 1987, heat flow measurements were tried at four sites (Sites 689, 690, 695, and 696) during ODP Leg 113, in the Weddell Sea, Antarctica. At Site 690 (Maud Rise), a convex upward shaped temperature vs. depth profile was observed. This profile cannot be explained by steady-state conduction through solid materials only. We conclude that the minimum heat flow value at Site 690 is 45 mW/m2. A prominent bottom simulating reflector (BSR) was observed at 600 mbsf at Site 695. However, the observed temperature is too high to explain the BSR as a gas hydrate. The origin of the BSR remains unknown, although it is probably of biogenic origin as observed in the Bering Sea during DSDP Leg 19. After correcting for the effects of sedimentation, heat flow values at Sites 695 and 696 are 69 and 63 mW/m2, respectively. Furthermore, we compiled heat flow data south of 50°S. In the Weddell Sea region, the eastern part shows relatively low heat flow in comparison with the western part, with the boundary between them at about 15°W longitude.

Seismic properties of sheeted dikes from ODP Hole 111-504B

Seismic velocities have been measured at confining pressures to 600 MPa for eight samples of sheeted dike rock obtained from Hole 504B during Leg ODP 111. The compressional- and shear-wave velocities are, in general, higher than the velocities measured in overlying dike rocks obtained from the hole during DSDP Leg 83. The velocity gradients observed in Layer 2C result from decreasing porosity with depth and increasing metamorphic grade. The laboratory-measured velocities of the Leg 111 dike rocks are similar to those of dike rocks reported for the Bay of Islands, Samail, and Troodos ophiolites.

Organic matter preservation in sediments of the Peru and Oman continental margin

Detailed petrographical and bulk geochemical investigations of organic matter (OM) have been performed on sediments deposited below or close to upwelling areas offshore Peru (ODP-Leg 112; Sites 679, 681, 688) and Oman (ODP-Leg 117; Sites 720, 723, 724) in order to obtain a quantitative understanding of its accumulation and degradation. Microscopical as well as nanoscopical investigations reveal that the OM in sediments affected by upwelling mechanisms mainly (up to 98%) consists of unstructured (amorphous) organic aggregates without any apparent biological structures. In sediments which are not or to a lesser extent affected by upwelling (Site 720) terrestrial OM predominates. Organic carbon (TOC) contents are highly variable and range between 9.8% in sediments deposited below upwelling cells and 0.2% in sediments outside the upwelling zone. The TOC/sulphur ratios of the sediments scatter widely. The samples from the deep-water locations (Sites 688 and 720), show C/S-ratios of "normal" marine sediments, whereas at the other locations no correlation or even a negative correlation between sulphur and TOC concentration exists. In most of the upwelling-influenced sediments OM contains a significant amount of sulphur. The incorporation of sulphur into the OM followed microbial sulphate reduction and occurred in the upper meters of the sedimentary column. Below, OM is still present in vast amounts and relatively hydrogen-rich, but is nevertheless non-metabolizable and becomes the limiting factor for bacterial sulphate reduction. According to mass balance calculations 90-99% of the OM produced in the photic zone was remineralized and 1-3% was consumed by microbial sulphate reduction. The aerobic and anaerobic processes have greatly affected degradation and conservation of OM.

Late Neogene sediment composition and accumulation rates of ODP Hole 120-751A

Through scanning electron microscope analysis of sediment microfabric, we have evaluated variations in high-resolution shipboard physical properties (index properties and shear strength), sediment components (smear slide determinations), and shore-based calcium carbonate and biogenic silica data from Site 751 (Kerguelen Plateau). The stratigraphic section at this site records a change in biogenic ooze composition from predominantly calcareous (nannofossil) to siliceous (diatom) ooze from ~23 Ma to the present, reflecting expansion of Antarctic water masses during the late Neogene. The profound change in physical properties and sediment character at 40.1 mbsf (~5-6 Ma) evidently records the northward movement of the Polar Front and a change in absolute accumulation rates of sediment at this site. Trends in geotechnical properties with depth at Site 751 allowed us to subdivide the sedimentary column into a number of geotechnical units that reflect changes in depositional and postdepositional processes with time. Geotechnical properties are sensitive to changing sedimentary inputs of primarily siliceous and calcareous microfossils. This allows us to study the physical nature of biostratigraphically-identified hiatuses and variations in environmental conditions linked to the migration of the Polar Front across this region. The analysis of geotechnical properties permits a more detailed division of the sedimentary column than is possible from shipboard lithologic descriptions alone. Our study of the sedimentary microfabric indicates that randomly oriented, elongate pennate diatom valves compose the sediments with highest porosity and water content values, and the lowest density values (wet bulk, dry bulk, and grain density). Conversely, sediments composed of nannofossils and disassociated nannofossil crystallites and little or no siliceous remains have the lowest porosity and water content values, and the highest density values. Samples of mixed siliceous/calcareous composition have intermediate physical property values, but these vary according to the nature of the sedimentary matrix and the state of preservation of individual skeletal elements.

Physics, geochemistry and bulk sedimentology on cores from the Peru Basin

Empirical relationships between physical properties determined non-destructively by core logging devices and calibrated by carbonate and opal measurements determined on discrete samples allow extraction of carbonate and opal records from the non-destructive measurements in biogenic settings. Contents of detrital material can be calculated as a residual. For carbonate and opal the correlation coefficients (r) are 0.954 and ?0.916 for sediment density, ?0.816 and 0.845 for compressional-wave velocity, 0.908 and ?0.942 for acoustic impedance, and 0.886 and ?0.865 for sediment color (lightness). Carbonate contents increase in concert with increasing density and acoustic impedance, decreasing velocity and lighter sediment color. The opposite is true for opal. The advantages of deriving the sediment composition quantitatively from core logging are: (i) sampling resolution is increased significantly, (ii) non-destructive data can be gathered rapidly, and (iii) laboratory work on discrete samples can be reduced. Applied to paleoceanographic problems, this method offers the opportunity of precise stratigraphic correlations and of studying processes related to biogenic sedimentation in more detail. Density is most promising because it is most strongly affected by changes in composition.

Geotechnical properties at DSDP Leg 66 Holes

Delineating the interrelationships between tectonics, sedimentation, and geotechnical properties is particularly important for areas subjected to the dynamic affects of convergence. DSDP Leg 66 drilling within the Middle America Trench complex provided a unique opportunity to investigate these interrelationships along a transect of eight drill sites beginning on the trench outer slope and traversing the trench, trench inner slope, and upper continental slope. Investigations of other convergent margins suggest that deformation occurs most rapidly along the lower trench inner slope and is reflected by the geotechnical properties (Carson, 1977; Seely, 1977; and von Huene, 1979). This study focuses on the geotechnical properties of Middle America Trench sediments and the possible affects of convergence on these properties.

Seismic properties of volcanic rocks from DSDP Hole 62-465A

Hess Rise, a major structural feature in the northern Pacific, is one of several oceanic plateaus which apparently possess anomalous seismic structures (Sutton et al., 1971). Hence, Laboratory measurements of compressional- and shear-wave velocities in rocks from oceanic plateau regions are of considerable interest. Several questions come to mind: (1) Are compressional - wave velocities of volcanic rocks from oceanic plateaus similar to basalts of equivalent age from normal oceanic crust? (2) Do velocity-density relations for plateau rocks fit the well-established trends for Layer 2 basalts? (3) How do Poisson's ratios, determined from compressional- and shear-wave velocities, of oceanic plateau rocks compare with those of normal sea-floor basalts? To answer these questions, we have selected for velocity measurements five volcanic samples from different depths from Hole 465A, on southern Hess Rise. It is particularly significant that all the rocks are vesicular and have been highly altered, which significantly affects their elastic properties.