Relative abundance of pennate diatoms Thalassionema spp. and Thalassiothrix spp. im Upper Miocene-Lower Pliocene bulk sediment at ODP Hole 145-886B, North Pacific (Table 1)

Rapidly deposited Thalassionema-Thalassiothrti pennate diatom oozes previously have been described in Upper Miocene-Lower Pliocene sediment beneath the frontal boundary of the eastern equatorial Pacific. Here we document a new occurrence of Thalassionema-Thalassiothrix ooze in Upper Miocene-Lower Pliocene sediment beneath the frontal boundary of the subarctic North Pacific. The ooze is a 6 m interval of siliceous sediment at Ocean Drilling Program (ODP) sites 885/886 that was rapidly deposited between approximately 5.0 and 5.9 Ma. Bulk sediment in this interval may contain greater than 85% pennate diatom tests. There are also abundant laminae and pockets that are composed entirely of Thalassionema and Thalassiothrix diatoms. The presence of a rapidly deposited ooze dominated by pennate diatoms indicates unusual past conditions in the overlying surface waters. Time coincident deposition of such oozes at two distinct frontal boundary locations of the Pacific suggests that the unusual surface water conditions were causally linked to large-scale oceanographic change. This same oceanographic change most likely involved (1) addition of nutrients to the ocean, or (2) redistribution of nutrients within the ocean. The occurrence and origin of pennate diatom oozes may be a key component to an integrative understanding of late Neogene paleoceanography and biogeochemical cycling.

Age determination of ODP Hole 145-887

Atmospheric carbon dioxide concentrations were significantly lower during glacial periods than during intervening interglacial periods, but the mechanisms responsible for this difference remain uncertain. Many recent explanations call on greater carbon storage in a poorly ventilated deep ocean during glacial periods (Trancois et al., 1997, doi:10.1038/40073; Toggweiler, 1999, doi:10.1029/1999PA900033; Stephens and Keeling, 2000, doi:10.1038/35004556; Marchitto et al., 2007, doi:10.1126/science.1138679; Sigman and Boyle, 2000, doi:10.1038/35038000), but direct evidence regarding the ventilation and respired carbon content of the glacial deep ocean is sparse and often equivocal (Broecker et al., 2004, doi:10.1126/science.1102293). Here we present sedimentary geochemical records from sites spanning the deep subarctic Pacific that -together with previously published results (Keigwin, 1998, doi:10.1029/98PA00874)- show that a poorly ventilated water mass containing a high concentration of respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum. Despite an inferred increase in deep Southern Ocean ventilation during the first step of the deglaciation (18,000-15,000 years ago) (Marchitto et al., 2007, doi:10.1126/science.1138679; Monnin et al., 2001, doi:10.1126/science.291.5501.112), we find no evidence for improved ventilation in the abyssal subarctic Pacific until a rapid transition ~14,600 years ago: this change was accompanied by an acceleration of export production from the surface waters above but only a small increase in atmospheric carbon dioxide concentration (Monnin et al., 2001, doi:10.1126/science.291.5501.112). We speculate that these changes were mechanistically linked to a roughly coeval increase in deep water formation in the North Atlantic (Robinson et al., 2005, doi:10.1126/science.1114832; Skinner nd Shackleton, 2004, doi:10.1029/2003PA000983; McManus et al., 2004, doi:10.1038/nature02494), which flushed respired carbon dioxide from northern abyssal waters, but also increased the supply of nutrients to the upper ocean, leading to greater carbon dioxide sequestration at mid-depths and stalling the rise of atmospheric carbon dioxide concentrations. Our findings are qualitatively consistent with hypotheses invoking a deglacial flushing of respired carbon dioxide from an isolated, deep ocean reservoir periods (Trancois et al., 1997, doi:10.1038/40073; Toggweiler, 1999, doi:10.1029/1999PA900033; Stephens and Keeling, 2000, doi:10.1038/35004556; Marchitto et al., 2007, doi:10.1126/science.1138679; Sigman and Boyle, 2000, doi:10.1038/35038000; Boyle, 1988, doi:10.1038/331055a0), but suggest that the reservoir may have been released in stages, as vigorous deep water ventilation switched between North Atlantic and Southern Ocean source regions.

Nd, Sr and Pb isotopic composition of eolian dust deposited in the central North Pacific

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.

Nd, Sm and carbon isotopic data for samples from ODP Legs 115, 145, 171, and 207

Late Cretaceous fish debris from Demerara Rise exhibits a dramatic positive excursion of 8 e-Nd units during ocean anoxic event 2 (OAE2) that is superimposed on extremely low e-Nd(t) values (-14 to -16.5) observed throughout the rest of the studied interval. The OAE2 e-Nd excursion is the largest yet documented in marine sediments, and the majority of the shift is estimated to have occurred over <20 k.y. Low background e-Nd values on Demerara Rise are explained as the Nd isotopic signature of the South American craton, whereas eruptions of the Caribbean large igneous province or enhanced mixing of intermediate waters in the North Atlantic could have caused the excursion.