Recovery at Site 1073 was excellent (99.9%; Fig. 10). Sediments range in age from late Eocene to Pleistocene and have been subdivided into three major lithostratigraphic units. Unit I is of Pleistocene age and extends from 0 to 519.8 mbsf. The dominant lithology is silty clay with minor intervals of sandy mud and rare sand beds. The sediment is strongly bioturbated and hydrotroilite stained. The lower portion of this unit is characterized by intervals of soft-sediment deformation and sandy clay with lithic and mud clasts at its base, near seismic reflection pp4(s). Unit II extends from 519.8 to 654.5 mbsf and is late Oligocene to Pliocene in age. The sediment is composed of foraminifer-rich clay, silty clay with numerous discrete burrows, diatomaceous silty nannofossil clay, and clayey to sandy nannofossil chalk. Sand and silt laminae are scattered throughout the unit, and thick beds of glauconite occur toward the base. A major unconformity (late Oligocene to late Eocene) and a sharp contact separates Units II and III. Unit III extends from 654.5 to 663.6 mbsf and is late Eocene in age. The sediment is composed of clay-rich nannofossil chalk and strongly bioturbated nannofossil-rich clay. Tracing of sequence boundaries to Site 1073 permits improved age resolution for boundary pp3(s), which is late Pleistocene (probably less than 0.5 Ma). Shore-based analyses are likely to improve age resolution for the other sequence boundaries. Older surfaces tend to become amalgamated in the slope area. Continued efforts in seismic processing and interpretation, as well as in biostratigraphy, may result in still better estimates.
Biostratigraphic resolution is excellent throughout most of the Pleistocene through late Eocene section. Calcareous nannofossils provide detailed zonations for the stratigraphic interval cored, highlighting the various stratigraphic discontinuities at the base of the cored interval (latest Pliocene through late Eocene). Planktonic foraminifers add to the confidence level of biostratigraphic zonation at Site 1073. Where age calibration is possible using planktonic foraminifers, they are compatible with the nannofossil zonations. Provenance changes indicated by changes in the benthic foraminiferal faunas may be associated with relative sea-level changes in the Pleistocene. When sea level was lowest, the source area of inner neritic benthic foraminifers was closest to Site 1073, facilitating transport of the shallow-water specimens to this location. When sea level was highest, these shallow species migrated landward and may not have been transported across the shelf to the slope. Instead, the source area may have been the outer shelf to upper slope. Early Pleistocene to early Pliocene benthic foraminiferal faunas are dominated by Uvigerina spp., analogous to present-day faunas from the northeast U.S. continental margin, where the highest abundances of U. peregrina coincide with maxima of organic carbon and silt within slope sediments. The early Miocene section yields a diverse, in situ bathyal benthic foraminiferal assemblage that indicates the paleodepth may have been comparable to the present water depth (~600 m). Late Eocene assemblages at Site 1073 are comparable to coeval faunas from lower-upper to middle bathyal paleodepths (~500-1000 m) reported from the Leg 150 New Jersey slope sites.
Despite many voids caused by gas expansion of the cores, high-resolution continuous records of
inclination and magnetization intensity variations were collected for Pleistocene sediments. Two
possible short polarity reversals were found within the Brunhes Chron at 15 and 351 mbsf.
Magnetic polarity is otherwise normal down to 515 mbsf [close to pp4(s)] with magnetization
intensity fluctuating between 0.1 and 20 mA/m. The Brunhes/Matuyama boundary (0.78 Ma)
was not found above 515 mbsf. Between 515 and 524 mbsf, the polarity of magnetic
inclination shows unstable fluctuations, switching between positive and negative, making it
difficult to identify magnetic zones. Between 515 and 519 mbsf, magnetization intensity is ~1
to 5 mA/m. Below 519 mbsf [~pp4(s)], magnetization intensity is generally low, ranging
between 0.05 and 1 mA/m, again making it difficult to identify magnetic polarity zones by
pass-through measurements on biscuited extended core barrel (XCB) archive sections.
Eighty-three interstitial water samples were taken to examine potential "high-resolution" variability in alkalinity, NH4+, and HPO42 with depth. Interstitial waters of marine sediment sequences are often characterized by a broad subsurface maximum in alkalinity, NH4+, and HPO42-, which results from bacterial decomposition of organic matter and subsequent diffusion of ions. However, previous work on the New Jersey slope (ODP Leg 150) noted an unusual (if not unique) observation in scientific ocean drilling: at least four peaks in downhole profiles of alkalinity and HPO42-. Limited sampling of interstitial water at Site 903 precluded a detailed investigation of the observation. However, downhole profiles of interstitial water at Site 1073 at significantly higher resolution confirm the observations at Site 903. Five well-defined alkalinity and four HPO42- maxima are observed in the upper 500 mbsf (Fig. 11). The maxima appear to exist in interglacial sediment separated by ~90 m and 100 k.y. The maxima indicate that rates of bacterial decomposition of organic matter on the New Jersey slope are highly heterogeneous in time and/or space. The maxima most likely are preserved because extreme sedimentation rates on the New Jersey slope (~900 m/m.y.) prevent diffusive homogenization of interstitial water chemistry.
Gaseous hydrocarbons were monitored in all cores by headspace gas, and, where possible, by analysis of gas voids using the syringe/vacutainer technique. Sediments contain abundant gas below 10 mbsf, and gaseous voids appear below 34 mbsf. Gas contents in cores diminished below 215 mbsf, probably associated with the switch from advanced hydraulic piston corer (APC) to XCB coring. The composition of gas, as expressed by the C1/C2 value, shows the expected gradual increase in the relative ethane content with increasing depth and temperature, given the prevailing sedimentation rate and geothermal gradient. Total organic carbon content fluctuates between 0.21 and 0.67 wt%, with peaks at 70, 146, 189, and 239 mbsf.
As at other Leg 174A sites, a number of discontinuities and trends in physical properties
measurements coincide with observed lithologic changes, unit boundaries, and interpreted
seismic discontinuities. Several sharp changes in density correlate with changes in velocity in
the same direction (e.g., both increasing downhole); consequently, they reinforce each other in
generating acoustic impedance contrasts. The depths of these physical properties changes are
generally consistent with previously estimated depths of seismic discontinuities. Natural
gamma radiation measurements include a distinct downward decrease within the early-middle
Miocene Subunit IIC, near the predicted level of pp4(s). The Pleistocene section contains
several increasing-upward cycles of natural gamma values that may reflect glacial-interglacial
cyclicity. Velocity measurements include a strongly increasing interval from 541 to 561 mbsf,
which corresponds to an interval of increasing density and thus may define a seismic reflector.
Porosity trends may reflect overall depositional cycles associated with Pleistocene glacial
interglacial deposition. Resistivity appears to correlate mainly with porosity, as at other Leg
174A sites. Changes in pore-water chemistry appear to have little influence on resistivity values.
At first glance, resistivity measurements from cores appear to correlate well with those obtained
by logging, as do velocities and density information.
Acoustic, resistivity, and gamma-ray logging data were obtained successfully to total depth, and a VSP using the WST was conducted to 425 mbsf. Gamma-ray log data show a pronounced cyclicity typified by asymmetric cycles of uphole increases followed by abrupt decreases in natural radioactivity. These cycles in the upper 515 mbsf are interpreted to be a product of sea level change during the Pleistocene, where the terrigenous contribution to the slope during lowstands favored the accumulation of silts and sands, producing lower gamma-ray values, which were then followed by an increase in clay content and higher gamma-ray values during rising sea levels. The tops of the cycles are attributed to an abrupt coarsening in grain size and a corresponding drop in gamma-ray values. A one-dimensional synthetic seismogram was constructed using sonic log data, which compares favorably to actual seismic data in the upper 425 mbsf. Below 425 mbsf, sonic log values, without corresponding checkshot data, were used, and the correlation between the seismic data and the synthetic decreases.
To 174A Conclusions
To 174A Table of Contents