ALBERT

Description: Calcareous nannofossils are sufficiently numerous in the upper 900 m of the Neogene sediment drift cored beneath the lower continental rise at DSDP Site 603 to permit delineation of zones, correlations with the paleomagnetic data (Pliocene-Pleistocene only), and the detection of major Miocene hiatuses and their correlation with seismic stratigraphy. Holes 603, 603B, and 6O3C were spudded in lower Pleistocene sediments just east of the crest of the Hatteras Outer Ridge, and all nannofossil zones and subzones are accounted for down to a hiatus within the middle Tortonian (late Miocene) Zone CN8. This hiatus lies some 30 m above a more extensive disconformity between 661 and 672 m where sediments of Subzone CN7a and a portion of Zone CN6 have been removed. The resulting hiatus is correlated with local reflection Horizon M2, which is considered equivalent to the regional Reflector Merlin. The hiatus between 661 and 672 m dates Merlin at this site between about 9.6 and 10.4 Ma. A strong, parallel, unnamed reflector is correlated with the superjacent hiatus within CN8, and is dated between 8.5 and 9 Ma. These disconformities help delineate a "condensed" interval, which falls within the Vail et al. (1980) cycle TM3.1. This eustatic event has been characterized as the sharpest and most profound sea-level drop of the late Miocene. The lower Tortonian "condensed" interval at Site 603 is closely correlative with spectacular debris flows cored in presumed canyon fill deposits immediately above reflection Horizon M2/Merlin at DSDP Site 604 on the upper rise off New Jersey. We suggest that the erosion along the lower rise at Site 603 and the synchronous canyon cutting evidenced by the debris flows on the upper rise at Site 604, both associated with the regional Reflector Merlin, are linked closely to Southern Hemisphere glacial activity which led to the formation of the West Antarctic Ice Sheet. Apart from the lower Tortonian "condensed" interval, the drift sediments of the Hatteras Outer Ridge are primarily muddy contourites, augmented to some extent by fine turbidites in the lower portion. Most were deposited at a rate of about 87 m/Ma. Just after the late Miocene erosional events, sedimentation rates during nannofossil Zone CN8b time were 192 m/Ma, about double that for the overlying section. This suggests that the site was then the locus of deposition for material eroded during canyon-cutting events along the slope and shelf. The lowest sample dated (911 m) is assigned to Subzone CN5b (not older than 13.1 Ma). A rare glauconitic silty sand turbidite at 834.8 m contains upper Eocene coccoliths, probably eroded from submarine outcrops along the slope, perhaps during a brief middle Miocene canyon-cutting episode.

Description: An Eocene-Oligocene oxygen and carbon isotope history based on planktonic and benthic foraminifers from Deep Sea Drilling Project Leg 71 cores has been constructed for the Maurice Ewing Bank of the eastern Falkland Plateau, Southwest Atlantic Ocean. Specifically, the cores cover portions of the middle Eocene, upper Eocene, and lower Oligocene. Surface water isotopic temperatures postulated for the middle Eocene at Site 512 fluctuated within about four degrees but generally averaged about 9°C. Bottom isotopic temperatures at Site 512 (water depth, 1846 m) were generally a degree lower than surface water temperatures. Surface water isotopic temperatures at Site 511 initially averaged about 11°C during the late Eocene, but dropped to an average of 7°C in the early Oligocene. Bottom isotopic temperatures at Site 511 (water depth, 2589 m) generally record temperatures between 12.5°C and 8°C, similar to the range in the surface water isotopic temperatures. During the early Oligocene, bottom isotopic temperatures dropped sharply and averaged about 2°C (very close to present-day values). Surface water temperature values also decreased to an average of about 7°C, therefore leading to a significant divergence between surface and bottom water isotopic temperatures during the early Oligocene. Comparisons among Southern Ocean DSDP Sites 511, 512, and 277, and between these and other DSDP sites from central and northern latitudes (Sites 44, 167, 171, 292, 357, 398, 119, and 401) show that much of the Eocene was characterized by relatively warm temperatures until sometime in either the middle Eocene, late Eocene, or early Oligocene. At each site, conspicuous 18O enrichments occur in both the benthic and planktonic foraminifers over a relatively short period of time. Although a general trend toward a climatic deterioration is evident, the density of data points among the various studies is still too sparse to determine either synchrony or time-transgression between the major isotopic events. A close correlation could be made between the Site 511 oxygen isotope temperature curve and paleoclimatic trends derived independently from radiolarian studies. The sharp temperature drop and the divergence between bottom and surface water temperatures during the early Oligocene apparently reflect a major expansion of the antarctic water mass. The migration of the boundary between the subantarctic and antarctic water masses over the site at this time would account in part for the sharp temperature changes. Sharp changes of this nature would not necessarily be noted in other geographic areas, particularly those to the north which have different oceanographic regimes.

Description: Authigenic gypsum, pyrite, and glauconite are disseminated throughout an unusually long (346 m) Miocene section of mixed biogenic carbonate and diatomaceous ooze drilled on the Falkland Plateau at DSDP Site 329 (water depth, 1519 m). The present organic carbon content of the sediment is low, ranging between 0.1 and 0.7%. Gypsum occurs as euhedral single or twinned crystals of selenite up to 5 mm in diameter, sometimes in the form of gypsum rosettes. These crystals are intact and unabraded, comprising up to 4% of the washed sample. The authigenic nature of the gypsum is demonstrated by the presence of diatoms and radiolarians embedded within the gypsum crystals. The gypsum co-occurs with pyrite and glauconite in these samples. The pyrite occurs as framboids, foraminiferal infillings, rods, and granular sheetlike masses composed of pyrite octahedra. The glauconite occurs as foraminiferal infillings and as free grains. The gypsum and pyrite were identified by energy-dispersive X-ray analysis and scanning electron micrographs. Some of the gypsum has grown on pyrite, indicating that it precipitated after the pyrite, perhaps in response to a change in pH conditions. The formation of the mineral suite can be explained by current models of in situ sulfide and sulfate precipitation coincident with diagenesis and oxidation of much of the original organic carbon.

Description: Three Pleistocene, five Pliocene, and thirteen late and middle Miocene calcareous nannofossil datums have been identified in the Leg 170 cored sequences collected from a transect across the Middle America Trench off the Nicoya Peninsula. Although some nannofossil zones could not be delineated, particularly in the Pliocene and upper Miocene, there appears to be a complete or very nearly complete Pleistocene through lower Miocene section at Sites 1039 and 1040. The oldest assemblages, observed at Site 1039 and 1040, are latest early Miocene in age (nannofossil Zone NN4). These assemblages are associated with gabbro intrusions into the basal sediments (one contact metamorphic hornfels sample contains relict nannofossils), indicating an age for the intrusion event of between 15.6 and 18.2 Ma at both Sites 1039 and 1040. Reference Site 1039, located on the Cocos plate, provides the best-preserved sequence of sediments of late Pleistocene to latest early Miocene age. The sediments cored in the prism sections at Sites 1040, 1041, 1042, and 1043 all indicate that the age of nannofossil assemblages in the prism sediments, including the toe, wedge, and apron, are all Pleistocene with a considerable amount of upper Miocene reworking. A period of low sediment accumulation rates (~5.3 m/m.y.) is recorded for Pliocene and upper Miocene sediments at Sites 1039, 1040, and 1043. Pliocene calcareous nannofossil assemblages characteristic of the ~2.5- to 3.75-m.y. time interval (nannofossil Zones NN16 and equivalent nannofossil Subzones CN12b and CN12a) were not resolved at any site. Nannofossil Zones NN15, NN14, NN13, and NN12 (early late Pliocene to early Pliocene) could not be resolved at any site either because of the absence of marker species. Within the Miocene at Sites 1039 and 1040, nannofossil Zones NN10-NN6 were difficult to differentiate because of the absence of several species that define the zonal boundaries. These intervals, where the nannofossil zones have not been resolved or are partially resolved, are primarily composed of carbonate ooze deposited during an ~8.5-m.y. (2.5-11 Ma) low sediment accumulation rate time interval. The absence of many of the marker species is attributed to warmer water conditions during those periods. Many of the same marker species are absent in the sediments recovered from nearby Deep Sea Drilling Project Site 155 in the Panama Basin.