Strontium isotope dating of ODP Leg 133 sediments

The strontium-isotope dating method, based on the strontium-isotope seawater curve, was used to date stratigraphic events recognized in carbonate sediments drilled during Leg 133 on the Queensland and Marion plateaus. The strontium isotope ages of these events are used to correlate paleoceanographic changes, delineated from oxygen isotope signals, and paleoenvironmental or facies changes recorded in the lithostratigraphy. Results indicate that a strong connection exists between prevailing paleoenvironmental conditions and the developmental style of a carbonate platform. Also, the strontium-isotope ages of discrete dolomite intervals within the sequences were determined, indicating that multiple dolomitization events took place and that a hydrodynamically driven process may be currently active within the modern carbonate platform.

Geochemistry of eolian dust from sediment core LL44-GPC-3

The large-diameter piston core LL44-GPC3 from the central North Pacific Ocean records continuous sedimentation of eolian dust since the Late Cretaceous. Two intervals resolved by Nd and Pb isotopic data relate to dust coming from America (prior to ~40 Ma) and dust coming from Asia (since ~40 Ma). The Intertropical Convergence Zone (ITCZ) separates these depositional regimes today and may have been at a paleolatitude of ~23°N prior to 40 Ma. Such a northerly location of the ITCZ is consistent with sluggish atmospheric circulation and warm climate for the Northern Hemisphere of the early to middle Eocene. Since ~40 Ma, correlations between Nd (~7.55 > epsilon-Nd(t) > ~10.81) and Pb (18.625 < 206/4Pb < 18.879; 15.624 < 207/4Pb < 15.666; 38.611 < 208/4Pb < 38.960; 0.8294 < 207/6Pb < 0.8389; 2.0539 < 208/6Pb < 2.0743) isotopes reflect the progressive drying of central Asia triggered by the westward retreat of the paleo-Tethys. Comparisons between the changes with time in the isotopically well-defined dust flux and Nd and Pb isotopic compositions of Pacific deep water allow one to draw two major conclusions: (1) dust-bound Nd became a resolvable contribution to Pacific seawater only after the one order of magnitude increase in dust flux starting at ~3.5 Ma. Therefore eolian Nd was unimportant for Pacific seawater Nd prior to 3.5 Ma. (2) The lack of a response of Pacific deep water Pb to this huge flux increase suggests that dust-bound Pb has never been important. Instead, mobile Pb associated with island arc volcanic exhalatives probably consists of a significant contribution to Pacific deep water Pb and possibly to seawater elsewhere far away from landmasses.

Calcareous nannofossils of the Soithern Coral Sea

Leg 90 of the Deep Sea Drilling Project drilled 18 holes at eight sites (Sites 587-594) on several shallow-water platforms in the southern Coral Sea, Tasman Sea, and southwestern Pacific Ocean. The results from an additional hole (Hole 586B) drilled at Site 586 during Leg 89 are included in this report. Together, these sites form a latitudinal traverse which extends from the equator (Site 586) to 45°S (Site 594) and includes all the major water masses from tropical to subantarctic. Samples recovered at these sites range in age from middle Eocene to late Quaternary. The calcareous nannoplankton biostratigraphy for Leg 90 has divided into two parts: part 1, the Neogene and Quaternary of Sites 586-594. (this chapter); and part 2, the Paleogene of Sites 588, 592, and 593 (Martini, 1986). A slightly modified version of the Martini (1971) standard Tertiary and Quaternary zonation scheme was used to make age determinations on over 700 samples. All of the relevant Neogene and Quaternary zone-defining nannoplankton are present at Sites 586-591 (0°-30°S) but become increasingly rare or are absent at Sites 592-594 (35°-45°S). Species diversity increases southward from the equator (Site 586) and reaches a peak at 20°S (Site 587). A decrease at 25°S (Site 588) and 30°S (Sites 589-591) is followed by an increase in species diversity at 35°S (Site 592). South of 35°S, species diversity again decreases and reaches a low at 45 °S (Site 594). Species diversity for all sites as a group generally increases through the early, middle, and late Miocene, reaches a peak in the early Pliocene, then gradually decreases through the late Pliocene and Quaternary

10Be data for sediments from DSDP Leg 56 and ODP Leg 170

Sediment accretion and subduction at convergent margins play an important role in the nature of hazardous interplate seismicity (the seismogenic zone) and the subduction recycling of volatiles and continentally derived materials to the Earth's mantle. Identifying and quantifying sediment accretion, essential for a complete mass balance across the margin, can be difficult. Seismic images do not define the processes by which a prism was built, and cored sediments may show disturbed magnetostratigraphy and sparse biostratigraphy. This contribution reports the first use of cosmogenic 10Be depth profiles to define the origin and structural evolution of forearc sedimentary prisms. Biostratigraphy and 10Be model ages generally are in good agreement for sediments drilled at Deep Sea Drilling Project Site 434 in the Japan forearc, and support an origin by imbricate thrusting for the upper section. Forearc sediments from Ocean Drilling Program Site 1040 in Costa Rica lack good fossil or paleomagnetic age control above the decollement. Low and homogeneous 10Be concentrations show that the prism sediments are older than 3-4 Ma, and that the prism is either a paleoaccretionary prism or it formed largely from slump deposits of apron sediments. Low 10Be in Costa Rican lavas and the absence of frontal accretion imply deeper sediment underplating or subduction erosion.

Chemical and isotopic compositions of Archean magmatic and metasedimentary rocks and minerals from the Podolia domain, Ukrainian Shield

Zircons from the oldest magmatic and metasedimentary rocks in the Podolia domain of the Ukrainian shield were studied and dated by the U-Pb method on a NORDSIM secondary-ion mass spectrometer. Age of zircon cores in enderbite gneisses sampled in the Kazachii Yar and Odessa quarries on the opposite banks of the Yuzhnyi Bug River reaches 3790 Ma. Cores of terrigenous zircons in quartzites from the Odessa quarry as well as in garnet gneisses from the Zaval'e graphite quarry have age within 3650-3750 Ma. Zircon rims record two metamorphic events around 2750-2850 Ma and 1900-2000 Ma. Extremely low U content in zircons of the second age group indicates conditions of the granulite facies metamorphism in Paleoproterozoic within the Podolia domain. Measured data on orthorocks (enderbite-gneiss) and metasedimentary rocks unambiguously suggest existence of the ancient Paleoarchean crust in the Podolia (Dniester-Bug) domain of the Ukrainian shield. They contribute in our knowledge of scales of formation and geochemical features of the primordial crust.