Index properties, consolidation characteristics and sediment rebound of ODP Sites 167-1020 and 167-1021 sediments

Drilling during Leg 167 at the California margin was scheduled to recover continuous sedimentary sections. Multiple advanced piston core (APC) holes drilled at different depth offsets provided core overlap in successive APCs. Correlation of high-resolution laboratory physical properties data from adjacent APC holes was used to compile composite depth sections for each site. The composite depth sections were used to confirm continuous recovery and enable high-resolution sampling. The meters composite depth (mcd) scale differs from the shipboard meters below seafloor (mbsf) scale because of (1) core expansion following recovery (MacKillop et al., 1995, doi:10.2973/odp.proc.sr.138.118.1995), (2) coring gaps, and (3) stretching/compression of sediment during coring (Lyle, Koizumi, Richter, et al., 1997, doi:10.2973/odp.proc.ir.167.1997). Moran (1997, doi:10.2973/odp.proc.sr.154.132.1997) calculated that sediment expansion accounted for 90%-95% of the Leg 154 depth offset between shipboard mbsf and the mcd scales. Terzaghi's one-dimensional theory of consolidation (Terzaghi, 1943) describes the response of sediments to stress loading and release. Mechanical loading in marine environments is provided by the buoyant weight of the overlying sediments. The load increases with depth below seabed, resulting in sediment volume reduction as water is "squeezed" out of the voids in the sediment. Stress release during core recovery results in expansion of the sediment and volume increase as water returns to the sediment. The sediment expansion or rebound defines the elastic properties of the sediment. In this study we examine the elastic deformation properties of sediments recovered from Sites 1020 and 1021. These results are used to (1) correct the laboratory index properties measurements to in situ values and (2) determine the contribution of sediment rebound to the depth offset between the mbsf and mcd scales.

Diatom abundance in sediments from ODP Leg 167 sites

The Pliocene and Pleistocene periods are known for the onset and consequent amplification of glacial-interglacial cycles. The California margin, situated in the mid-latitudes of the northern Pacific Ocean, is expected to be one of the most interesting regions for Pliocene to Pleistocene paleoceanography because this area occupies a unique position in the ocean-atmosphere system over the region. In this study, we investigated paleoceanographic history, using fossil diatoms, since the Brunhes/Matuyama (B/M) paleomagnetic boundary in which glacial and interglacial periods began to alternate in 100-yr cycles. In Hole 1018A, to a depth corresponding to the beginning of Northern Hemisphere glaciation (late Pliocene), we investigated the responses of the ocean-atmosphere system to stepwise cooling in the California margin. Although the work is still continuing, this data report shows that fossil diatoms of Pliocene and Pleistocene sediments significantly changed both in quality and quantity and implies a possible relationship to global climatic changes.

Relative abundances of stratigraphically useful planktonic foraminifers from ODP Leg 167 sites

Late Neogene biostratigraphy of planktonic foraminifers has been investigated from 13 sites cored during Ocean Drilling Program Leg 167 off the coast of California. The planktonic foraminiferal biostratigraphy of six of these sites is presented here at higher stratigraphic resolution for the interval that encompasses the late early Pliocene through the Quaternary (~3.5 Ma to present day). The sites form a transect along the California margin from 31°N to 41°N within the California Current system. A new planktonic foraminiferal zonation has been established largely on evolutionary changes within the Neogloboquadrina plexus, supported by other taxa. A total of eight zones are recognized, most of which are broadly applicable throughout the region, thus providing a biostratigraphic zonation of the sequence at ~0.5-m.y. intervals. The new zonation appears to be unique to the California Current system. The diversity of planktonic foraminiferal assemblages during the late Neogene appears to have remained relatively constant despite large-scale paleoclimatic change. The assemblages are consistently dominated by few taxa that almost always include the neogloboquadrinids and Globigerina bulloides. Low diversity and high dominance of the assemblages favored these and other taxa well adapted to upwelling systems exhibiting high seasonal surface ocean variability. Apparently the oceanographic conditions that favor such assemblages have persisted at least for the duration of the late Neogene (~3.5 Ma to present day). The biostratigraphically important forms have been illustrated with scanning electron micrographs.