ALBERT

Description: The object of the present study is to introduce a means of comparing the Vostok and marine chronologies. Our strategy has been to use the delta18O of atmospheric O2 (denoted delta18Oatm) from the Vostok ice core as a proxy for the delta18O of seawater (denoted delta18Osw). Our underlying premise in using delta18Oatm as a proxy for delta18Osw is that past variations in delta18Osw (an indicator of continental ice volume) have been transmitted to the atmospheric O2 reservoir by photosynthesizing organisms in the surface waters of the world's oceans. We compare our record of delta18Oatm to the delta18Osw record which has been developed from studies of the isotopic composition of biogenic calcite (delta18Oforam) in deep-sea cores. We have tied our delta18Oatm record from Vostok to the SPECMAP timescale throughout the last 135 kyr by correlating delta18Oatm with a delta18Osw record from V19-30. Results of the correlation indicate that 77% of the variance is shared between these two records. We observed differences between the delta18Oatm and the delta18Osw records during the coldest periods, which indicate that there have been subtle changes in the factors which regulate delta18Oatm other than delta18Osw. Our use of delta18Oatm as a proxy for delta18Osw must therefore be considered tentative, especially during these periods. By correlating delta18Oatm with delta18Osw, we provide a common temporal framework for comparing phase relationships between atmospheric records (from ice cores) and oceanographic records constructed from deep-sea cores. Our correlated age-depth relation for the Vostok core should not be considered an absolute Vostok timescale. We consider it to be the preferred timescale for comparing Vostok climate records with marine climate records which have been placed on the SPECMAP timescale. We have examined the fidelity of this common temporal framework by comparing sea surface temperature (SST) records from sediment cores with an Antarctic temperature record from the Vostok ice core. We have demonstrated that when the southern ocean SST and Antarctic temperature records are compared on this common temporal framework, they show a high degree of similarity. We interpret this result as supporting our use of the common temporal framework for comparing other climate records from the Vostok ice core with any climate record that has been correlated into the SPECMAP chronology.

Description: Although ferromanganese crusts from shallow depths in Lake Baykal have been described previously, the presence of nodules at depths of about 500 m are a new discovery made possible by exploration using manned submersibles. The nodules are comparable to oceanic nodules in composition, but have some significant differences, including a more rapid rate of growth. Sedimentary-diagenetic processes are mainly responsible for their formation, but there is reason to believe hydrothermal fluids play some role.

Description: About 13 m of Cretaceous, tholeiitic basalt, ranging from normal (N-MORB) to transitional (T-MORB) mid-ocean-ridge basalts, was recovered at Ocean Drilling Program Site 843 west of the island of Hawaii. These moderately fractionated, aphyric lavas are probably representative of the oceanic basement on which the Hawaiian Islands were built. Whole-rock samples from parts of the cores exhibiting only slight, low-temperature, seawater alteration were analyzed for major element, trace element, and isotopic composition. The basalts are characterized by enrichment in the high field strength elements relative to N-MORB, by a distinct positive Eu anomaly, and by Ba/Nb and La/Nb ratios that are much lower than those of other crustal or mantle-derived rocks, but their isotope ratios are similar to those of present-day N-MORB from the East Pacific Rise. Hole 843A lavas are isotopically indistinguishable from Hole 843B lavas and are probably derived from the same source at a lower degree of partial melting, as indicated by lower Y/Nb and Zr/Nb ratios and by higher concentrations of light and middle rare earth elements and other incompatible elements relative to Hole 843B lavas. Petrographic and trace-element evidence indicates that the Eu anomaly was the result of neither plagioclase assimilation nor seawater alteration. The Eu anomaly and the enrichments in Ta, Nb, and possibly U and K relative to N-MORB apparently are characteristic of the mantle source. Age-corrected Nd and Sr isotopic ratios indicate that the source for the lavas recovered at ODP Site 843 was similar to the source for Southeast Pacific MORB. An enriched component within the Cretaceous mantle source of these basalts is suggested by their initial 208Pb/204Pb-206Pb/204Pb and epsilon-Nd-206Pb/204Pb ratios. The Sr-Pb isotopic trend of Hawaiian post-shield and post-erosional lavas cannot be explained by assimilation of oceanic crust with the isotopic composition of the Site 843 basalts.

Description: Examination of seismic reflection and core data between Sites 819 and 821 provides information about patterns of sedimentation that result from repetitive fluctuations in relative sea level and climatic perturbations. On the basis of sequencestratigraphic interpretations, we identify nine sequences bounded by sequence boundaries; each sequence contains lithologically distinct and relatively thin units that have been interpreted as condensed sections. Because of very high mixed carbonate-siliciclastic sedimentation rates in this region, resolution of third-, fourth-, and fifth-order changes of sea level is recognized. One predominant condensed section is identified within each sequence and is associated with the maximum flooding surface of that sequence. These condensed sections are marked by increases in abundance of quartz, clay, and shallow-water bioclasts; increases in nonreworked pristine glauconites; decreases in total carbonate abundance; and increases in magnetic susceptibility. Increases in glauconite have been interpreted to indirectly reflect relative decreases in sedimentation rates, whereby sediment residence time is prolonged within suboxic bacterial degradation zones. Increases in bioclasts within transgressive systems tracts are attributed to relatively rapid buildups of shallow-water reefal communities that accompanied punctuated flooding events; during these times shallow-water carbonate growth accelerated as reefal communities built upward rapidly to keep up with rising sea level. Increases in quartz and clay contents during maximum flooding are thought to be tied to sediment starvation, whereby background clays and quartz contents are concentrated. Decreased accommodation potential during relative highstand phases promoted progressive highstand systems tract progradation and sedimentation of reworked glauconites and terrigenous sands, silts, and clays. These depositional phases also are marked by increases in bioclastic sands that accompanied the progradational, seaward building events.