(Table 1) Carbon content and petroleum potential at DSDP Hole 73-520

At the end of the Leg 73 cruise, at Site 520, a thick upper Miocene facies of laminated diatomite was discovered. It was hypothesized that the sediment was derived from nutrient-rich waters in a young Cenozoic anoxic basin in the open Atlantic Ocean near the Mid-Atlantic Ridge.

Distribution of planktonic foraminifers, d18O and d13C in Globigerinoides sacculifer as well as Cibicidoides kullenbergi at DSDP Holes 90-590A and 90-590B

High biogenic sedimentation rates in the late Neogene at DSDP Site 590 (1293 m) provide an exceptional opportunity to evaluate late Neogene (late Miocene to latest Pliocene) paleoceanography in waters transitional between temperate and warm-subtropical water masses. Oxygen and carbon isotope analyses and quantitative planktonic foraminiferal data have been used to interpret the late Neogene paleoceanographic evolution of this site. Faunal and isotopic data from Site 590 show a progression of paleoceanographic events between 6.7 and 4.3 Ma, during the latest Miocene and early Pliocene. First, a permanent depletion in both planktonic and benthic foraminiferal d13C, between 6.7 and 6.2 Ma, can be correlated to the globally recognized late Miocene carbon isotope shift. Second, a 0.5 per mil enrichment in benthic foraminiferal d18O between 5.6 and 4.7 Ma in the latest Miocene to early Pliocene corresponds to the latest Miocene oxygen isotopic enrichment at Site 284, located in temperate waters south of Site 590. This enrichment in d18O coincides with a time of cool surface waters, as is suggested by high frequencies of Neogloboquadrina pachyderma and low frequencies of the warmer-water planktonic foraminifers, as well as by an enrichment in planktonic foraminiferal d18O relative to the earlier Miocene. By 4.6 Ma, benthic foraminiferal d18O values become depleted and remain fairly stable until about 3.8 Ma. The early Pliocene (~4.3 to 3.2 Ma) is marked by a significant increase in biogenic sedimentation rates (37.7 to 83.3 m/m.y.). During this time, heaviest values in planktonic foraminiferal d18O are associated with a decrease in the gradient between surface and intermediate-water d13C and d18O, a 1.0 per mil depletion in the d13C of two species of planktonic foraminifers, and a mixture of warm and cool planktonic foraminiferal elements. These data suggest that localized upwelling at the Subtropical Divergence produced an increase in surface-water productivity during the early Pliocene. A two-step enrichment in benthic foraminiferal d18O occurs in the late Pliocene sequence at Site 590. A 0.3 per mil average enrichment at about 3.6 Ma is followed by a 0.5 per mil enrichment at 2.7 Ma. These two events can be correlated with the two-step isotopic enrichment associated with late Pliocene climatic instability and the initiation of Northern Hemisphere glaciation.

(Table 1) Results from XRD analysis as well as sediment type and selected results from ring shear experiments

The location of the seaward tip of a subduction thrust controls material transfer at convergent plate margins, and hence global mass balances. At approximately half of those margins, the material of the subducting plate is completely underthrust so that no accretion or even subduction erosion takes place. Along the remaining margins, material is scraped off the subducting plate and added to the upper plate by frontal accretion. We here examine the physical properties of subducting sediments off Costa Rica and Nankai, type examples for an erosional and an accretionary margin, to investigate which parameters control the level where the frontal thrust cuts into the incoming sediment pile. A series of rotary-shear experiments to measure the frictional strength of the various lithologies entering the two subduction zones were carried out. Results include the following findings: (1) At Costa Rica, clay-rich strata at the top of the incoming succession have the lowest strength (µres = 0.19) while underlying calcareous ooze, chalk and diatomite are strong (up to µres = 0.43; µpeak = 0.56). Hence the entire sediment package is underthrust. (2) Off Japan, clay-rich deposits within the lower Shikoku Basin inventory are weakest (µres = 0.13-0.19) and favour the frontal proto-thrust to migrate into one particular horizon between sandy, competent turbidites below and ash-bearing mud above. (3) Taking in situ data and earlier geotechnical testing into account, it is suggested that mineralogical composition rather than pore-pressure defines the position of the frontal thrust, which locates in the weakest, clay mineral-rich (up to 85 wt.%) materials. (4) Smectite, the dominant clay mineral phase at either margin, shows rate strengthening and stable sliding in the frontal 50 km of the subduction thrust (0.0001-0.1 mm/s, 0.5-25 MPa effective normal stress). (5) Progressive illitization of smectite cannot explain seismogenesis, because illite-rich samples also show velocity strengthening at the conditions tested.

Elemental composition and petroleum-generating potential of the methylene chloride extracts from DSDP Leg 66 Holes

Organic geochemical studies on samples from Holes 487, 488, and 490 in the southern Mexico Middle America Trench provided an opportunity to characterize the organic fraction of the sedimentary section in an active trench environment and to project the petroleum-producing potential of the extracted lipid fractions. The samples were geologically young and of shallow burial history. Samples from Hole 487, located on the oceanic plate, range in age from late Miocene to middlelate Pleistocene. Samples from Hole 488, representing undifferentiated Quaternary sediment, were collected on the landward side of the lower trench slope. Miocene(?) to Quaternary sediments from Hole 490 were obtained from the upper slope immediately seaward of the inferred location of the continental crust.

(Table 1) Strontium concentration and isotope ratios of carbonates from DSDP Holes 69-504B as well as 83-504B and fish debris samples from Indian and Pacific Ocean sediments

Recent revisions of the geological time scale by Kent and Gradstein (in press) suggest that, on the average, Cretaceous magnetic anomalies are approximately 10 m.y. older than in Larson and Hilde's (1975) previous time scale. These revised basement ages change estimates for the duration of alteration in the ocean crust, based on the difference between secondary-mineral isochron ages and magnetic isochron-crustal ages, from 3 to approximately 13 m.y. In addition to the revised time scale, Burke et al.'s (1982) new data on the temporal variation of 87Sr/86Sr in seawater allow a better understanding of the timing of alteration and more realistic determinations of water/rock ratios during seawater-basalt interaction. Carbonates from all DSDP sites which reached Layer 2 of Atlantic crust (Sites 105, 332, 417, and 418) are deposited within 10-15 m.y. of crustal formation from solutions with 87Sr/86Sr ratios identical to unaltered or contemporaneous seawater. Comparisons of the revised seawater curve with the 87Sr/86Sr of basement carbonates is consistent with a duration of approximately 10-15 m.y. for alteration in the ocean crust. Our preliminary Sr and 87Sr/86Sr data for carbonates from Hole 504B, on 5.9-m.y.-old crust south of the Costa Rica Rift, suggest that hydrous solutions from which carbonates precipitated contained substantial amounts of basaltic Sr. For this reason, carbonate 87Sr/86Sr cannot be used to estimate the duration of alteration at this site. A basalt-dominated alteration environment at Hole 504B is consistent with heat-flow evidence which indicates rapid sediment burial of crust at the Costa Rica Rift, sealing it from access by seawater and resulting in unusually low water/rock ratios during alteration.