(Table 1) Stabile isotopes at DSDP Leg 55 Holes

Deep sea drilling on four seamounts in the Emperor Seamount chain revealed that Paleogene shallow-water carbonate sediments of the "bryozoan-algal" facies crown the basalt edifices. According to the biofacies model of Schlanger and Konishi (1966, 1975), this bryozoan- algal assemblage suggests that the seamounts formed in cooler, more northerly waters than those presently occupied by the island of Hawaii; i.e., the paleolatitudes of formation were greater than 20 °N. Moving southward toward the youngest member of the seamount chain, a facies gradient indicative of warmer waters was observed. This gradient is interpreted as a reflection of a northward shift in isotherms during the time span in which the seamounts were progressively formed (Savin et al., 1975). On all seamounts, sedimentation at the drilling sites occurred in a high-energy environment with water depths of approximately 20 meters. Early-stage carbonate diagenesis began in the phreatic zone in the presence of meteoric water, but proceeded after subsidence of the seamounts into intermediate sea waters, where the bulk, stable isotopic composition was determined. The subsidence into intermediate waters was rapid, and permitted establishment of an isotopic equilibrium which, like the facies gradient, reflects the northward shift in isotherms during the Paleogene. Calcite and zeolite cements comprise the later-stage diagenesis, and originated from solutions arising from the hydrolysis of the underlying basalt. In conclusion, the results of this study of the shallow-water carbonate sediments are not inconsistent with a paleolatitude of formation for Suiko Seamount (Site 433) of 26.9 ±3.5 °N, as determined by paleomagnetic measurements (Kono, 1980).

Physical properties and minerals of Leg 65 Holes basalts

Velocities and densities of submarine basalts obtained during Leg 65 are distinguished by their relatively high values (about 6.3 km/s and 2.9 g/cm/**3, respectively). This is consistent with their low degree of alteration. The range of velocities covered by these samples correlates well with porosity, and a comparison with in situ velocities from refraction data suggests maximum porosities ih the upper 100 meters of the crust of about 10%.

d18O, lithologies, mineralogy and determination of temperature and depth of formation of DSDP Site 69-504 siliceous rocks

Seven opal-CT-rich and five quartz-rich porcellanites and cherts from Site 504 have a range in oxygen-isotope values of 24.4 and 29.4 per mil. In opal-CT rocks, d18O becomes larger with sub-bottom depth and with age. Quartz-rich rocks do not show these trends. Boron, in general, increases with decreasing d18O for porcellanites and cherts considered together, supporting the conclusion that boron is incorporated within the quartz crystal structure during precipitation of the SiO2. Silicification of the chalks at Site 504 began 1 m.y. ago - that is, 5 m.y. after sedimentation commenced on the oceanic crust. Temperatures of chert formation determined from oxygen-isotope compositions reflect diagenetic temperatures rather than bottom-water temperatures, and are comparable to temperatures of formation determined by down-hole measurements. Opal-A in the chalks began conversion to opal-CT when a temperature of 50°C was reached in the sediment column. Conversion of opal-CT to quartz started at 55 °C. Silicification occurred over a stratigraphic thickness of about 10 meters when the temperature at the top of the 10 meters reached about 50°C. It took about 250,000 years to complete the silica transformation within each 10-meter interval of sediment at Site 504. Quartz formed over a stratigraphic range of at least 30 meters, at temperatures of about 54 to 60°C. The time and temperatures of silicification of Site 504 rocks are more like those at continental margins than those in deep-sea, open-ocean deposits.

Summary of physical properties on cores used for petrofabric analyses triaxial experiments at DSDP Leg 67 Holes

Deformation features within the cores are studied with a view towards elucidating the structure of the Middle America Trench along the transect drilled during Leg 67. Where possible, inferences are made as to the physical environment of deformation. Extensional tectonics prevails in the area of the seaward slope and trench. Fracturing and one well-preserved normal fault are found mostly within the lower Miocene chalks, at the base of the sedimentary section. These chalks have high porosities (40%-60%) and water content (30%-190%, based on % dry wt.). Experimental triaxial compression tests conducted on both dry and water-saturated samples of chalk from Holes 495 and 499B show that only in the saturated samples is more brittle behavior observed. Brittle failure of the chalks is greatly facilitated by pore fluid pressures that lead to low effective pressures. Additional embrittlement (weakening) can take place as a result of the imposed extensional stress resulting from bending of a subducting elastic oceanic plate. The chalks exhibit, in a landward direction, an increase in density and mechanical strength and a decrease in water content. These changes are attributed to mechanical compaction that may have resulted from tectonic horizontal compression. The structure of the landward slope is not well understood because the slope sites had to be abandoned due to the presence of gas hydrate. The relationship of the chaotic, brittle deformation (observed in the cores from Hole 494A) at the base of the landward slope to tectonic processes remains unclear. The deformation observed on the slope sites (Holes 496 and 497) is mostly fracturing and near-vertical sigmoidal veinlets. These are interpreted as being the result of gas/fluid overpressurization due to the decomposition of the gas hydrate, and not due to tectonic loading of accreted sediments. Aside from four small displacement (less than 1cm) reverse faults observed in the lower Miocene chalks (which may be the product of soft-sediment deformation), there is a noticeable absence of structures reflecting a dominance of horizontal (tectonic) compression along the transect drilled. The absence of such features, the lack of continuity of sediment types across the trench-landward slope, and the normal stratigraphic sequence in Hole 494A do not support any known accretionary model.

Age models, accumulation rates, sand and carbonates at DSDP Leg 74 holes

Accumulation rates for the five sites drilled during Leg 74 of the Glomar Challenger are presented on a common timescale based on calibration of datum levels to paleomagnetic records in Leg 74 sediments for the Paleogene, and a new compilation by Berggren et al. (1985), for the Neogene, and using the seafloor-spreading magnetic anomaly timescale of Kent (1985). We present data on accumulation of total sediment, of foraminifers, of the noncarbonate portion, and of fish teeth that give a history of productivity, winnowing, carbonate dissolution, and nonbiogenic input to what was then a part of the South Atlantic at about 30 deg S.

Geochemistry of basalts fropm DSDP Leg 65 basalts

Samples of basalt collected on Leg 65 near 22°N on the East Pacific Rise all display the depleted light rare-earth pattern of "normal" oceanic crust. Consequently the La/Ta ratio is close to 18, as opposed to the value of 9 associated with the flat or enriched patterns found along parts of the Mid-Atlantic Ridge and the Emperor Seamount chain. The Leg 65 samples are chemically similar to those from the CYAMEX area at 21 °N and to the Leg 54 samples from 9°N, suggesting homogeneity of the upper mantle under the northern part of the East Pacific Rise over a minimum distance of about 1500 km. The geochemistry of the rocks and their field relationships with respect to depth and distance from the axis of the Rise show no pattern of distribution linked to the degree of fractional crystallization and thus cast doubt on any possible model involving large, long-lived magma chambers at the axis of the Rise.

Geochemical history at DSDP Leg 56 Holes

A geological model of subduction postulated by Karig, Ingle, et al. (1975) and Karig and Sharman (1975) proposes that the sedimentary prism at the foot of the landward wall is being actively built as sediment is scraped off the subducting oceanic and plastered onto the base of the wedge, forming an accretionary wedge containing overthrust sedimentary layers or intense sedimentary folding. Because overlying layers must continually be uplifted and compressed to accommodate new matter at the base, the accreting wedge will provide a geochemical record of this process at or near the Japan Trench. Several recent papers have discussed the metalliferous sediments on the active oceanic ridges. The geochemistry of such sediments is now reasonably well known: generally these deposits are considered products of volcanic processes (Boström and Peterson, 1969; Böstrom et al., 1969; Horowitz, 1970, 1974; Cronan et al., 1972; Cronan and Garrett, 1973). The geochemistry of subduction zone sediments, however, is less well known, and the need for studies of these sediments is particularly urgent if such sediments provide a record of the effects of subduction of oceanic plates under continental crust. Because the Japan Trench contains welldeveloped subduction zone deposits, Leg 56 sampling was of utmost importance to the discovery of how they originate.

Neogene and Quaternary paleoenvironmental history at DSDP Leg 71 Holes

The Neogene and Quaternary sedimentary record of Leg 71 and previously drilled sequences from the Southern Ocean reveal evidence of a major late Miocene change of oceanic and glacial conditions in the southern high latitudes during paleomagnetic Chron 9. The characteristics of late Miocene sedimentation and in particular the study of erosional patterns and ice-rafted debris suggest the following conclusions. 1) In the late Miocene, the Polar Front first migrated to the northern latitudes of the Southern Ocean and surface water temperatures became similar to those of today. 2) Extensive ice shelves or ice tongues were not present along the Antarctic margin until late Chron 9 (about 9.0 Ma). 3) Before Chron 9, West Antarctica was occupied by an archipelago and the West Antarctic Sea. 4) Extensive ice shelves formed in the West Antarctic region, eventually coalescing and thickening to form the grounded West Antarctic ice sheet by Chron 9. 5) The newly formed West Antarctic ice sheet was probably unstable and frequently became an ungrounded ice shelf, thus accounting for the scarcity of late Miocene ice-rafted debris. 6) Extensive erosion or nondeposition of sediment was probably the result of increased Antarctic Bottom Water (AABW) formation in the West Antarctic region during the initial formation of extensive West Antarctic ice shelves and during periods when the West Antarctic ice sheet was ungrounded. 7) In the Southwest Atlantic, AABW velocity waned during the latest Miocene. During the late Gilbert Chron a major and permanent change occurred in the pattern of ice-rafting to the South Atlantic, and after 4.35 Ma the increased IRD accumulation rate and frequency of major episodes of IRD accumulation suggest increased stability of the West Antarctic ice sheet. In addition, radiolarian faunas of Hole 514 record at least eight migrations of the Polar Front to the north of the site during the past 4.07 m.y. An apparent increase in the frequency of Polar Front migrations occurred about 2.7-2.6 Ma, possibly in response to oceanic change induced by fluctuations in glacial conditions of the Northern Hemisphere.

Oxygen and carbon isotopes of planktic foraminifera and ageprofile datum description at DSDP Hole 69-504

A detailed oxygen isotope record (resolution: about 2500 years) has been obtained for the Pleistocene sediments at Hole 504. Preliminary measurements made deeper in the section suggest that at least the upper Pliocene section is also amenable to detailed stable isotope work. The record for the middle Pleistocene resembles that obtained previously from piston cores in the western equatorial Pacific, although the superior resolution of this high-accumulation-rate site reveals a greater amplitude of isotope variation than previously observed. The record for the lower Pleistocene reveals variation that is both greater in amplitude and higher in frequency than apparent from previously analyzed piston cores. The site provides the best material recovered to date for the study of the evolution of climatic variability during the past few million years.

Beta-sitosterol and stigmasterol in slope sediments at DSDP Leg 67 Holes

Sediments collected from three continental slope sites of Leg 67 have been examined for total organic carbon content and amount of the sterols beta-sitosterol and stigmasterol. The amount of beta-sitosterol relative to stigmasterol is found to increase landward in the sediment, despite differing ages of the samples studied. The ratio of beta-sitosterol to stigmasterol in near-shore sediments is proposed as a source proximity indicator.

Thickness and duration of biostratigraphic zones, assemblages, ages and sedimentation rates at DSDP Sites 76-534 and 44-391

In this Initial Report of the Deep Sea Drilling Project, detailed studies of Sites 533 (gas hydrates) on the Blake Outer Ridge and 534 (oldest ocean history) in the Blake-Bahama Basin have provided answers to many geological and geophysical questions posed over the decade that deep drilling has been undertaken in this part of the western North Atlantic. The history of drilling and a historical review of key scientific accomplishments have been presented in the Introduction (Gradstein and Sheridan, this volume). In this final chapter we review highlights of new geological, geophysical and paleoceanographic interpretations presented in this volume, and offer a critical review of this information. We conclude with a listing of some outstanding problems and recommendations for future research, including data collection.

Datum events and their estimated magnetochronology at DSDP Site 72-516

Calcareous plankton biostratigraphy (foraminifers and nannoplankton) and magnetostratigraphy of the upper Oligocene to Pleistocene have been studied in hydraulic piston Cores 516-1 to 516-44, 516A-5 to 516A-11, and 516F-1 to 516F-11, Rio Grande Rise (water depth 1313 m). Some 80 biostratigraphic datum events have been correlated to the magnetic polarity stratigraphy over an interval representing the Matuyama to Chron 5, and Chrons 16 to 23. Coring disturbance and biostratigraphic evidence of a condensed section preclude unambiguous identification of polarity or biostratigraphic events over an approximately 30-m interval in the middle and upper Miocene. Sedimentation rates varied considerably during the Neogene, but an abnormally thick upper Oligocene and lower Miocene section allows a high degree of magnetobiochronologic resolution. A new planktonic foraminiferal zonation for the Miocene completes the midlatitude Neogene zonation of the South Atlantic. Important magnetobiostratigraphic correlations at Site 516 and their estimated magnetochronology include: (1) Oligocene/ Miocene boundary = first appearance datum (FAD) Globorotalia kugleri = last appearance datum (LAD) Reticulofenestra bisecta = mid-Anomaly 6C (Chron 23) = 23.7 Ma; (2) Aquitanian/Burdigalian boundary = LAD G. kugleri = between base Anomaly 6A and top of unnumbered anomaly between 6A and 6B (Chron 21) = 21.8 Ma; (3) Zone N6/N7 boundary = LAD Catapsydrax dissimilis (= FAD G. pseudomiozea and G. zealandica) = Chron 16/17 boundary = 17.6 Ma; (4) early/middle Miocene (= Burdigalian/Langhian) boundary = FAD Praeorbulina sicana = midpart of Anomaly 5C (Chron 16) = 16.6 Ma or FAD P. glomerosa = just above Anomaly 5C (inferred) = 16.3 Ma; (5) Zone N8/N9 boundary = FAD Orbulina suturalis above Anomaly 5C (later part Chron 16, inferred); (6) Miocene/ Pliocene boundary = LAD Globoquadrina dehiscens LAD Globorotalia lenguaensis = basal Gilbert Chron = 5.3 Ma.

Migration of C1 to C8 volatile organic compounds in sediments at DSDP Hole 75-530A

The distribution of C1 to C8 hydrocarbons in sediment samples from DSDP Leg 75, Hole 530A, indicates that significant amounts of methane and ethane have migrated from organic-rich to organic-lean shales in close proximity. Most compounds larger than ethane are not migrating out of black shales, where they occur in high concentrations. These results lead to a general model for assessing migration. In addition, three shale types are identified on the basis of organic carbon and pyrolysis products and patterns.