ALBERT

Beschreibung: The silicate fractions of recent pelagic sediments in the central north Pacific Ocean are dominated by eolian dust derived from central Asia. An 11 Myr sedimentary record at ODP Sites 885/886 at 44.7°N, 168.3°W allows the evaluation of how such dust and its sources have changed in response to late Cenozoic climate and tectonics. The extracted eolian fraction contains variable amounts (〉70%) of clay minerals with subordinate quartz and plagioclase. Uniform Nd isotopic compositions (epsilon-Nd =38.6 to 310.5) and Sm/Nd ratios (0.170-0.192) for most of the 11 Myr record demonstrate a well-mixed provenance in the basins north of the Tibetan Plateau and the Gobi Desert that was a source of dust long before the oldest preserved Asian loess formed. epsilon-Nd values of up to 36.5 for samples 62.9 Ma indicate 〈=35 wt% admixture of a young, Kamchatka-like volcanic arc component. The coherence of Pb and Nd in the erosional cycle allows us to constrain the Pb isotopic composition of Asian loess devoid of anthropogenic contamination to 206Pb/204Pb =18.97 +/- 0.06, 207Pb/204Pb =15.67 +/- 0.02, 208Pb/204Pb =39.19 +/- 0.11. 87Sr/86Sr (0.711-0.721) and Rb/Sr ratios (0.39-1.1) vary with dust mineralogy and provide an age indication of ~250 Ma. 40Ar/39Ar ages of six dust samples are uniform around 200 Ma and match the K-Ar ages of modern dust deposited on Hawaii. These data reflect the weighted age average of illite formation. Changes from illite- smectite with significant kaolinite to illite- and chlorite-rich, kaolinite-free assemblages since the late Pliocene document changes in the intensity of chemical weathering in the source region. Such weathering evidently did not disturb the K-Ar systematics, and only induced scatter in the Rb-Sr data. We propose that when smectite forms at the expense of illite, K and Ar are quantitatively lost from what becomes smectite, but are quantitatively retained in adjacent illite layers. 40Ar/39Ar age data, therefore, are insensitive to smectite formation during chemical weathering but date the diagenetic growth of illite, the major K-bearing phase in the dust. Over the past 12 Myr, the dust flux to the north Pacific increased by more than an order of magnitude, documenting a substantial drying of central Asia. This climatic change, however, did not alter the ultimate source of the dust, and neoformational products of chemical weathering always remained subordinate to assemblages reworked by mechanical erosion in dust deposited in eastern Asia and the Pacific Ocean.

Beschreibung: In samples from 1575 to 1982 mbsf anhydrite leached from whole-rock powders. 87Sr/86Sr and Sr/Ca determined following methods described in (Teagle et al., 1996, doi:10.2973/odp.proc.sr.148.113.1996, Teagle et al., 1998, doi:10.2973/odp.proc.sr.158.223.1998). For the period of the analysis (Jan. 1994-Dec. 1995) NIST SRM 987 gave 87Sr/86Srs0.710244 +/- 0.000018 (2 sigma, n = 115). Full analytical procedural blanks were 〈50 pg for Sr. delta18O following (Pickthorn and O´Neil, 1985). Repeated extractions and measurements of samples and standards were reproducible within +/- 0.5.

Beschreibung: Drilling at Bougainville Guyot (Ocean Drilling Program Site 831), New Hebrides Island Arc during Leg 134 revealed that 727.5 m of carbonate overlies an andesite basement. The carbonate cap at Site 831 consists of 20 m of pelagic carbonate overlying 707.5 m of neritic carbonates. The neritic section consists of ~230 m of largely unaltered aragonite sediment that overlies ~497 m of totally calcitized limestone. The deeper portion of the calcitized interval has been pervasively altered by diagenesis. Prior to this study the age distribution of sediments at Bougainville Guyot was poorly known because age diagnostic fossils are sparsely and discontinuously distributed in the sequence. We have used Sr isotopes to provide temporal constraints on the deposition of carbonates at Site 831; these constraints are critical in reconstructing the vertical movement of Bougainville Guyot before its collision with the New Hebrides Island Arc. Overall, the chronostratigraphy of Bougainville Guyot can be subdivided into three intervals: (1) a Pleistocene interval (102.4 to 391.11 meters below sea floor [mbsf]); (2) a Miocene interval (410.31 to 669.53 mbsf); and (3) an Oligocene interval (678.83 to 727.50 mbsf). Strontium isotopic ages of samples increase with increasing depth in the carbonate sequence, except near the bottom of the sequence, where several samples exhibit a consistent reversed age vs. depth trend. Such age reversals are most likely the product of post-depositional rock-water interaction. Preliminary stable isotope data are consistent with diagenetic alteration in the marine and meteoric environments. Several abrupt decreases in d87Sr, and hence age, of sediments are recognized in the carbonate cap at Bougainville Guyot. These disconformities are most likely the product of subaerial exposure in response to relative sea-level fall. Indeed, Sr-isotope ages indicate that 2 to 9 m.y. of sediment deposition is missing across these d87Sr disconformities.