ALBERT

Description: In the latest Paleocene an abrupt shift to more negative d13C values has been documented at numerous marine and terrestrial sites (Bralower et al., 1997, doi:10.1130/0091-7613(1997)025〈0963:HRROTL〉2.3.CO;2; Cramer et al., 1999; Kaiho et al., 1996, doi:10.1029/96PA01021; Kennett and Stott, 1991, doi:10.1038/353225a0; Koch et al., 1992, doi:10.1038/358319a0; Stott et al., 1996; Thomas and Shackleton, 1996, doi:10.1144/GSL.SP.1996.101.01.20; Zachos et al., 1993). This carbon isotope event (CIE) is coincident with oxygen isotope data that indicate warming of surface waters at high latitudes of nearly 4°-6°C (Kennett and Stott, 1991, doi:10.1038/353225a0) and more moderate warming in the subtropics (Thomas et al., 1999, doi:10.1029/1999PA900031). Here we report 187Os/188Os isotope records from the North Atlantic and Indian Oceans which demonstrate a 〉10% increase in the 187Os/188Os ratio of seawater coincident with the late Paleocene CIE. This excursion to higher 187Os/188Os ratios is consistent with a global increase in weathering rates. The inference of increased chemical weathering during this interval of unusual warmth is significant because it provides empirical evidence supporting the operation of a feedback between chemical weathering rates and warm global climate, which acts to stabilize Earth's climate (Walker et al., 1981). Estimates of the duration of late Paleocene CIE (Bains et al., 1999, doi:10.1126/science.285.5428.724; Bralower et al., 1997, doi:10.1130/0091-7613(1997)025〈0963:HRROTL〉2.3.CO;2; Norris and Röhl, 1999, doi:10.1038/44545; Röhl et al., 2000, doi:10.1130/0091-7613(2000)28〈927:NCFTLP〉2.0.CO;2) in conjunction with the Os isotope data imply that intensified chemical weathering in response to warm, humid climates can occur on timescales of 104-105 years. This interpretation requires that the late Paleocene thermal maximum Os isotope excursion be produced mainly by increased Os flux to the ocean rather than a transient excursion to higher 187Os/188Os ratios in river runoff. Although we argue that the former is more likely than the latter, we cannot rule out significant changes in the 187Os/188Os ratio of rivers.

Description: A novel strip-sampling technique has been applied to the 500-m gabbroic section drilled at site 735 during Leg 118. Twenty-two continuous strips of 1.1- to 4.5-m length were cut longitudinally from the core, allowing for a more representative sampling of this section of the deep ocean crust. A full suite of trace element and isotopic (Sr, Nd, Pb, Os, d18O) analyses were conducted on these strip samples; for comparison, analyses were conducted on a small suite of protolith samples, selected for their fresh and unaltered appearance. Amphibole, diopside, and plagioclase from 18 vein samples were also analyzed for Sr and Nd isotopes. Although the evidence for a seawater component in these gabbros is clear (87/86 Sr up to 0.70316; 206/204 Pb up to 19.3; d18O down to 2.0‰; 187/188 Os up to 0.44), the trace element signatures are dominated by magmatic effects (infiltration and impregnation by late-stage melts derived locally or from deeper levels of the crust). The average upper 500 m 735B gabbro section is somewhat lower than average N-MORB in trace elements such as Ba (30%), Nb (50%), U (40%), and heavy REE (Yb and Lu, 30%), but somewhat enriched in others such as La (23%), Ce (24%), Pb (23%), and Sr (40%). Although the section is largely comprised of cumulate gabbros (Natland et al., 1991), and many of the strip samples show marked Sr and Eu anomalies (plagioclase cumulation), the average composition of the total 500 m section shows no Sr or Eu anomalies (〈1%). This implies that there has been local separation of melt and solids, but no large scale removal of melts from this 500-m gabbro section.

Description: We present osmium isotopic compositions and Os, Re and platinum-group element (PGE) contents in the lower oceanic crust gabbros from ODP Site 735B on the Southwest Indian Ridge. The average gabbros are very depleted in PGEs, e.g. the Os concentrations are very low and range from 0.5 to 38 ppt. One troctolite sample is an exception, containing, e.g. 376 ppt Os, 396 ppt Ir and 8 ppb Pd. Re contents are extremely heterogeneous, ranging from 128 ppt to 28 ppb. The present day 187Os/188Os varies from 0.140 to 0.467, and correlates with Rb/Cs, suggesting that seawater alteration affected the Os isotopic system. The average Os content in the lower oceanic crust is about 9 ppt while median value for Re is about 487 ppt. Such very high Re/Os is relatively similar to the upper MORB-type oceanic crust, indicating that recycled ocean crust basalts and gabbros will rapidly develop very radiogenic Os isotopic signatures.

Description: Correlation of new multichannel seismic profiles across the upper Indus Fan and Murray Ridge with a dated industrial well on the Pakistan shelf demonstrates that ~40% of the Indus Fan predates the middle Miocene, and ~35% predates uplift of the Murray Ridge (early Miocene, ~22 Ma). The Arabian Sea, in addition to the Makran accretionary complex, was therefore an important repository of sediment from the Indus River system during the Paleogene. Channel and levee complexes are most pronounced after the early Miocene, coincident with an increase in sedimentation rates. Middle Eocene sandstones from Deep Sea Drilling Project Site 224 on the Owen Ridge yield K-feldspars whose Pb isotopic composition, measured by in situ ion microprobe methods, indicates an origin in, or north of, the Indus suture zone. This observation requires that India-Asia collision had occurred by this time and that an Indus River system, feeding material from the suture zone into the basin, was active soon after collision. Pleistocene provenance was similar to that during the Eocene, albeit with greater contribution from the Karakoram. A mass balance of the erosional record on land with deposition in the fan and associated basins suggests that only ~40% of the Neogene sediment in the fan is derived from the Indian plate.

Description: Ocean Drilling Program sampling of the distal passive margin of South China at Sites 1147 and 1148 has yielded clay-rich hemipelagic sediments dating to 32 Ma (Oligocene), just prior to the onset of seafloor spreading in the South China Sea. The location of the drill sites offshore the Pearl River suggests that this river, or its predecessor, may have been the source of the sediment in the basin, which accounts for only not, vert, similar ~1.8% of the total Neogene sediment in the Asian marginal seas. A mean erosion depth of not, vert, similar ~1 km over the current Pearl River drainage basin is sufficient to account for the sediment volume on the margin. Two-dimensional backstripping of across-margin seismic profiles shows that sedimentation rates peaked during the middle Miocene (11-16 Ma) and the Pleistocene (since 1.8 Ma). Nd isotopic analysis of clays yielded epsilonNd values of -7.7 to -11.0, consistent with the South China Block being the major source of sediment. More positive epsilonNd values during and shortly after rifting compared to later sedimentation reflect preferential erosion at that time of more juvenile continental arc rocks exposed along the margin. As the drainage basin developed and erosion shifted from within the rift to the continental interior epsilonNd values became more negative. A rapid change in the clay mineralogy from smectite-dominated to illite dominated at not, vert, similar 15.5 Ma, synchronous with middle Miocene rapid sedimentation, mostly reflects a change to a wetter, more erosive climate. Evidence that the elevation of the Tibetan Plateau and erosion in the western Himalaya both peaked close to this time supports the suggestion that the Asian monsoon became much more intense at that time, much earlier than the 8.5 Ma age commonly accepted.