Os isotope ratios of Late Eocene sediments from the tropical Pacific

High resolution records (ca. 100 kyr) of Os isotope composition (187Os/188Os) in bulk sediments from two tropical Pacific sites (ODP Sites 1218 and 1219) capture the complete Late Eocene 187Os/188Os excursion and confirm that the Late Eocene 187Os/ 188Os minimum, earlier reported by Ravizza and Peucker-Ehrenbrink (2003, doi:10.1016/S0012-821X(03)00137-7), is a global feature. Using the astronomically tuned age models available for these sites, it is suggested that the Late Eocene 187Os/188Os minimum can be placed at 34.5 +/- 0.1 Ma in the marine records. In addition, two other distinct features of the 187Os/188Os excursion that are correlatable among sections are proposed as chemostratigraphic markers which can serve as age control points with a precision of ca. +/-0.1 Myr. We propose a speculative hypothesis that higher cosmic dust flux in the Late Eocene may have contributed to global cooling and Early Oligocene glaciation (Oi-1) by supplying bio-essential trace elements to the oceans and thereby resulting in higher ocean productivity, enhanced burial of organic carbon and draw down of atmospheric CO2. To determine if the hypothesis that enhanced cosmic dust flux in the Late Eocene was a cause for the 187Os/188Os excursion can be tested by using the paired bulk sediment and leachate Os isotope composition; 187Os/188Os were also measured in sediment leachates. Results of analyses of leachates are inconsistent between the south Atlantic and the Pacific sites, and therefore do not yield a robust test of this hypothesis. Comparison of 187Os/188Os records with high resolution benthic foraminiferal delta18O records across the Eocene-Oligocene transition suggests that 187Os flux to the oceans decreased during cooling and ice growth leading to the Oi-1 glaciation, whereas subsequent decay of ice-sheets and deglacial weathering drove seawater 187Os/188Os to higher values. Although the precise timing and magnitude of these changes in weathering fluxes and their effects on the marine 187Os/188Os records are obscured by recovery from the Late Eocene 187Os/188Os excursion, evidence of the global influence of glaciation on supply of Os to the ocean is robust as it has now been documented in both Pacific and Atlantic records.

Radiocarbon ages and stable isotope measurements on sediment cores of the subarctic Pacific and its marginal seas

Under modern conditions only North Pacific Intermediate Water is formed in the northwest Pacific Ocean. This situation might have changed in the past. Recent studies with general circulation models indicate a switch to deep-water formation in the northwest Pacific during Heinrich Stadial 1 (17.5-15.0 ka) of the last glacial termination. Reconstructions of past ventilation changes based on paleoceanographic proxy records are still insufficient to test whether a deglacial mode of deep-water formation in the North Pacific Ocean existed. Here we present deglacial ventilation records based on radiocarbon-derived ventilation ages in combination with epibenthic stable carbon isotopes from the northwest Pacific including the Okhotsk Sea and Bering Sea, the two potential source regions for past North Pacific ventilation changes. Evidence for most rigorous ventilation of the intermediate-depth North Pacific occurred during Heinrich Stadial 1 and the Younger Dryas, simultaneous to significant reductions in Atlantic Meridional Overturning Circulation. Concurrent changes in d13C and ventilation ages point to the Okhotsk Sea as driver of millennial-scale changes in North Pacific Intermediate Water ventilation during the last deglaciation. Our records additionally indicate that changes in the d13C intermediate-water (700-1750 m water depth) signature and radiocarbon-derived ventilation ages are in antiphase to those of the deep North Pacific Ocean (>2100 m water depth) during the last glacial termination. Thus, intermediate- and deep-water masses of the northwest Pacific have a differing ventilation history during the last deglaciation.

Eocene to Quaternary planktonic foraminifers from the Northwest Pacific

Eocene through Quaternary planktonic foraminifers were identified in cores recovered during Leg 126. Turbidites and volcanic ash beds are intercalated with hemipelagic sediments. Preservation of foraminifers is variable, ranging from excellent to poor and appears to have been affected by fluctuations in the carbonate compensation depth (CCD), depth of burial, changes in bottom water temperature, current velocity, sediment accumulation rates and seafloor topography. Preservation of foraminifers in Quaternary sediments is generally good, however, species abundance varies by a factor of I05-106 and reflects dilution by volcanogenic as well as terrigenous constituents and cannot be used for paleoceanographic reconstructions. In pre-Quaternary deposits planktonic foraminiferal tests frequently exhibit dissolution effects; biostratigraphic zonation and placement of zonal boundaries is difficult owing to hiatuses, dissolution facies, extraneously deposited sediments, and discontinuous coring. The Eocene foraminiferal faunas include specimens of the Globorotalia cerroazulensis plexus, markers of Zone P16 as well as Globigerina senni and Globigerinatheka spp., which became extinct before the end of the Eocene. Six hiatuses and/or dissolution periods, probably reflecting global cooling events and/or changes in oceanic circulation patterns were recorded at Site 792. Recrystallized, poorly preserved, possibly reworked Eocene species (Globigerina senni and Globigerapsis sp.) were recorded in sediments at Site 793.

Organic geochemistry of Lower Cretaceous sediments from the northwetsern Pacific guyits

Organic geochemical data of Lower Cretaceous shallow water sediments from two sites (865 and 866) drilled during ODP leg 143 are presented. The organic matter is mainly terrestrial at the bottom of the sedimentary column at site 865, whereas algal and/or bacterial organic matter is dominant at site 866. This is the first evidence of shallow water deposition of organic matter during the Early Cretaceous in the Northwestern Pacific. The lower Aptian organic carbon-rich layers from the shallow water sediments of site 866 are geochemically similar to coeval mid-water sediments of site 463.

Age marks in bottom sediments of the Pacific Ocean

Distribution of K-Ar age marks in Pacific bottom sediments is shown on the base of large analytical material. Dependence of K-Ar age marks on abundance of potassium-bearing authigenic diagenetic minerals (glauconite, phillipsite) widely spread in the studied part of the World Oceans.

Pb concentrations, Pb isotopic composition, and amount of metalliferous components in pelagic sediments from the Pacific Ocean

The amount of lead annually transferred from oceanic crust to metalliferous sediments was estimated in order to test the hypothesis that a non-magmatic flux of lead causes the Pb surplus in the continental crust. A Pb surplus has been inferred from global crust-mantle lead mass balances derived from lead concentration correlations with other trace elements and from lead isotope systematics in oceanic basalts. DSDP/ODP data on the amount of metalliferous sediments in the Pacific Ocean and along a South Atlantic traverse are used to calculate the mean worldwide thickness of 3 (+/-1) m for purely metalliferous sediment componens. Lead isotope ratios of 39 metalliferous sediments from the Pacific define mixing lines between continent-derived (seawater) and mantle-derived (basaltic) lead, with the most metal-rich sediments usually having the most mantle-like Pb isotope composition. We used this isotope correlation and the Pb content of the 39 metalliferous sediments to derive an estimate of 130 (+/-70) µg/g for the concentration of mantle-derived lead in the purely metalliferous end-member. Mass balance calculations show that at least 12 (+/-8)% of the lead, annually transferred from upper mantle to oceanic crust at the ocean ridges, is leached out by hydrothermal processes and re-deposited in marine sediments. If all of the metalliferous lead is ultimately transferred to the continental crust during subduction, the annual flux of this lead from mantle to continental crust is 2.6 (+/-2.0) * 10**6 kg. Assuming this transfer rate to be proportional to the rate of oceanic plate production, one can fit the lead transfer to models of plate production rate variations through time. Integrating over 4 Ga, hydrothermal lead transfer to the continental crust accounts for a significant portion of the Pb surplus in the continental crust. It therefore appears to be one of the main reasons for the anomalous behavior of lead in the global crust-mantle system.

Stable isotope record of foraminifera from the subarctic Pacific

Benthic (Uvigerina spp., Cibicidoides spp., Gyroidinoides spp.) and planktonic (N. pachyderma sinistral, G. bulloides) stable isotope records from three core sites in the central Gulf of Alaska are used to infer mixed-layer and deepwater properties of the late glacial Subarctic Pacific. Glacial-interglacial amplitudes of the planktonic delta18O records are 1.1-1.3 per mil, less than half the amplitude observed at core sites at similar latitudes in the North Atlantic; these data imply that a strong, negative deltaw anomaly existed in the glacial Subarctic mixed layer during the summer, which points to a much stronger low-salinity anomaly than exists today. If true, the upper water column in the North Pacific would have been statically more stable than today, thus suppressing convection even more efficiently. This scenario is further supported by vertical (i.e., planktic versus benthic) delta18O and delta13C gradients of >1 per mil, which suggest that a thermohaline link between Pacific deep waters and the Subarctic Pacific mixed layer did not exist during the late glacial. Epibenthic delta13C in the Subarctic Pacific is more negative than at tropical-subtropical Pacific sites but similar to that recorded at Southern Ocean sites, suggesting ventilation of the deep central Pacific from mid-latitude sources, e.g., from the Sea of Japan and Sea of Okhotsk. Still, convection to intermediate depths could have occurred in the Subarctic during the winter months when heat loss to the atmosphere, sea ice formation, and wind-driven upwelling of saline deep waters would have been most intense. This would be beyond the grasp of our planktonic records which only document mixed-layer temperature-salinity fields extant during the warmer seasons. Also we do not have benthic isotope records from true intermediate water depths of the Subarctic Pacific.

South Pacific dissolved Nd isotope compositions, rare earth element and nutrient concentrations from SONNE cruise SO213

Despite its enormous extent and importance for global climate, the South Pacific has been poorly investigated in comparison to other regions with respect to chemical oceanography. Here we present the first detailed analysis of dissolved radiogenic Nd isotopes (epsilon-Nd) and rare earth elements (REEs) in intermediate and deep waters of the mid-latitude (~40°S) South Pacific along a meridional transect between South America and New Zealand. The goal of our study is to gain better insight into the distribution and mixing of water masses in the South Pacific and to evaluate the validity of Nd isotopes as a water mass tracer in this remote region of the ocean. The results demonstrate that biogeochemical cycling (scavenging processes in the Eastern Equatorial Pacific) and release of LREEs from the sediment clearly influence the distribution of the dissolved REE concentrations at certain locations. Nevertheless, the Nd isotope signatures clearly trace water masses including AAIW (Antarctic Intermediate Water) (average epsilon-Nd = -8.2 ± 0.3), LCDW (Lower Circumpolar Deep Water) (average epsilon-Nd = -8.3 ± 0.3), NPDW (North Pacific Deep Water) (average epsilon-Nd = -5.9 ± 0.3), and the remnants of NADW (North Atlantic Deep Water) (average epsilon-Nd = -9.7 ± 0.3). Filtered water samples taken from the sediment-water interface under the deep western boundary current off New Zealand suggest that boundary exchange processes are limited at this location and highlight the spatial and temporal variability of this process. These data will serve as a basis for the paleoceanographic application of Nd isotopes in the South Pacific.

Chemical and mineral compositions of basalts from the Pacific Ocean, DSDP data

Mineral and chemical alterations of basalts were studied in the upper part of the ocean crust using data of deep-sea drilling from D/S Glomar Challenger in the main structures of the Pacific floor. Extraction of majority of chemical elements (including heavy metals) from basalts results mainly from their interaction with heated sea water. As a result mineralized hydrothermal solutions are formed. On entering the ocean they influence greatly on ocean sedimentation and ore formation.