Stable carbon and oxygen isotope record of planktonic foraminifera in ODP Site 138-851

This study investigates changes in the upper water column hydrography at Site 851 of the eastern tropical Pacific Ocean since the late Pliocene, using the oxygen and carbon isotopic composition of three species of planktonic foraminifers, each calcifying at different depths in the photic zone. The upper ocean seasonal hydrography in this region responds to the seasonally changing trade winds and thus is expected to respond to past changes in trade winds. One major change occurs at about 1.5 Ma, when the thermocline adjusts from a deep position to a shallower position. The thermocline remains in a relatively shallow position throughout the record up to recent time, with slight variations occurring synchronously with glacial/interglacial stages. In glacials, SSTs are probably a few degrees cooler and the thermocline is slightly deeper. From our knowledge of seasonal and interannual adjustments of the thermocline in this location, a deeper thermocline might be interpreted as either a decrease in the strength of the Equatorial Undercurrent (EUC) that results from lower mean wind strength or an increase in the Equatorial Countercurrent (ECC), which results from an increase in the strength of the southeasterly trade winds. A major shift from higher to lower carbon isotope values occurred at about 1.9 Ma, marking a transition to reduced planktonic-benthic d13C differences after 1.9 Ma. The carbon isotopic data indicate that changes in the carbon isotopic composition of intermediate upwelling water occurs at higher frequencies than the glacial/interglacial changes in ice volume.

(Table 3) Li concentrations and isotopic compositions in foraminiferal shells, carbonate sediments, and associated pore waters from ODP Sites 131-806 and 138-851

An improved procedure for lithium isotope analysis using Li3PO4 as the ion source has been investigated for application to geological samples. The 7Li/6Li ratio is measured using double rhenium filament thermal ionization mass spectrometry in which isotopic fractionation is minimized at high temperatures. The method produces a stable, high intensity Li+ ion beam that allows measurement of nanogram quantities of lithium. This results in a reduction in sample size of up to 1000 times relative to that required for the established Li2BO2+ method while maintaining a comparable precision of better than 1? (1 sigma). Replicate analyses of the NBS L-SVEC Li2CO3 standard yielded a mean value of 12.1047+/-0.0043 (n=21), which is close to the reported absolute value of 12.02+/-0.03. Intercalibration with a wide range of geological samples shows excellent agreement between the Li3PO4 and Li2BO2+ techniques. Replicate analyses of seawater and a fresh submarine basalt display high precision results that agree with previous measurements. Taking advantage of the high ionization efficiency of the phosphate ion source, we have made the first measurements of the lithium concentration (by isotope dilution) and isotopic composition of calcareous foraminiferal tests and other marine carbonates. Preliminary results indicate that substantial lithium exchange occurs between carbonate sediments and their interstitial waters. In addition, a possible link between lithium paleoceanography and paleoclimate during the last 1000 ky may be derived from planktonic foraminiferal tests. This highly sensitive technique can be applied in the examination of low lithium reservoirs and thereby provide insight into some fundamental aspects of lithium geochemistry.

(Table A1) Isotopic ratios of Sr, Nd, and Pb for sediments in ODP Leg 138 sites

The provenance of eolian dust supplied to deep-sea sediments has the potential to offer insights into changes in past atmospheric circulation. Specifically, measuring temporal changes in dust provenance can shed light on changes in the mean position of the Intertropical Convergence Zone (ITCZ), a region acting as a barrier separating wind-blown material derived from northern versus southern hemisphere sources. Here we have analyzed Nd, Sr, and Pb isotope ratios in the operationally-defined detrital component extracted from deep-sea sediments in the eastern equatorial Pacific (EEP) along a meridional transect at 110°W from 3°S to 7°N (ODP Leg 138, sites 848-853). Sr isotope results show that barite Sr has a significant influence on 87Sr/86Sr isotope ratios of samples in the upwelling zone of the EEP. However, sites located >3° or more away from the equator (sites 852 and 853) are believed to not be affected by barite Sr and provide useful detrital Sr signals. 208Pb/206Pb and 207Pb/206Pb ratios in all cores fall into the Pb-isotope space of five potential dust sources (Asia, North and Central/South America, Sahara, and Australia), with no distinct isotopic fingerprinting of the dominant source(s). epsilon-Nd values were most valuable for discerning detrital source provenance, and their values at all sites, ranging from ~5.46 to ~3.25, were more unradiogenic for sediments deposited during the last glacial than for those deposited during the Holocene. There are distinct latitudinal trends in the epsilon-Nd values, with more radiogenic values further south and less radiogenic values further north, excluding site 848. This distinction holds true for both Holocene and last glacial periods. For the most southerly site, 848, we invoke, for the first time, a distinct southern hemisphere Australian source as being responsible for the unradiogenic Nd isotope ratios. Both average last glacial and Holocene epsilon-Nd values show similar sharp gradients along the transect between 5.29°N and 2.77°N, suggesting little movement of the glacial ITCZ in the EEP. However, during the deglacial, this gradient is stronger and shifted further north between 5.29°N and 7.21°N, suggesting a more northerly, possibly stronger, deglacial ITCZ.