A low-latitude Upper Pleistocene oxygen isotope stack

Below oxygen isotope stage 16, the orbitally derived time-scale developed by Shackleton et al. (1990) from ODP site 677 in the equatorial Pacific differs significantly from previous ones (e.g. Kominz and Pisias, 1979 doi:10.1126/science.204.4389.171; Morley and Hays, 1981 doi:10.1016/0012-821X(81)90034-0, Imbrie et al. 1984), yielding estimated ages for the last Earth magnetic reversals that are 5-7% older than the K/Ar values (Mankinen and Dalrymple, 1979 doi:10.1029/JB084iB02p00615; Berggren et al., 1985; Harland and Armstrong, 1989) but are in good agreement with recent Ar/Ar dating (Baksi et al., 1991; 1992 doi:10.1126/science.256.5055.356; Spell and McDougall, 1992 doi:10.1029/92GL01125). These results suggest that in the lower Brunhes and upper Matuyama chronozones most deep-sea climatic records retrieved so far apparently missed or misinterpreted several oscillations predicted by the astronomical theory of climate. To test this hypothesis, we studied a high-resolution oxygen isotope record from giant piston core MD900963 (Maldives area, tropical Indian Ocean) in which precession-related oscillations in delta18O are particularly well expressed, owing to the superimposition of a local salinity signal on the global ice volume signal (Rostek et al., 1993 doi:10.1038/364319a0). Three additional precession-related cycles are observed in oxygen isotope stages 17 and 18 of core MD900963, compared to the SPECMAP composite curves (Imbrie et al., 1984; Prell et al., 1986 doi:10.1029/PA001i002p00137), and stage 21 clearly presents three precession oscillations, as predicted by Shackleton et al. (1990). The precession peaks found in the delta18O record from core MD900963 are in excellent agreement with climatic oscillations predicted by the astronomical theory of climate. Our delta18O record therefore permits the development of an accurate astronomical time-scale. Based on our age model, the Brunhes-Matuyama reversal is dated at 775 +/- 10 ka, in good agreement with the age estimate of 780 ka obtained by Shackleton et al. (1990) and recent radiochronological Ar/Ar datings on lavas (Baksi et al., 1991; 1992; Spell and McDougall, 1992). We developed a new low-latitude, Upper Pleistocene delta18O reference record by stacking and tuning the delta18O records from core MD900963 and site 677 to orbital forcing functions.

(Table 1) Oxygen and carbon isotope ratios of benthic foraminifera of DSDP Holes 12-119 and 48-401

We present here oxygen and carbon isotopic records of Eocene to Oligocene benthic foraminifera from two Bay of Biscay Deep Sea Drilling Project (DSDP) sites (119 and 401). d18O of benthic foraminifera increases 1.9 per mil from a middle Eocene minimum (Zones P10-P11) to an earliest Oligocene maximum (Zone NP21). Approximately 1.4 per mil of the increase in benthic foraminiferal d18O occurs during the late Eocene to earliest Oligocene (Zones P15/16-NP21). Previous results from other North Atlantic DSDP sites (400A and 398) have significantly lower d18O values of benthic foraminifera, some by as much as 2 per mil (Vergnaud-Grazzini et al., 1978; 1989, doi:10.2973/dsdp.proc.48.119.1979; Vergnaud-Grazzini, 1979, doi:10.2973/dsdp.proc.47-2.117.1979 ). We believe that these differences result from diagenetic alteration of the sediments in the deeper-buried Sites 400A and 398.

In-situ Iron isotope measurements and mineral compositions from ODP Hole 206-1256D and IODP Hole 335-U1256D

In-situ Fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the Fe isotopic composition of Fe-Ti-oxides and Fe-sulfides of the different lithologies of the drilled rocks from IODP Hole 1256D (eastern equatorial Pacific; 15 Ma crust formed at the East Pacific Rise). Most igneous rocks normally have a very restricted range in their 56Fe/54Fe ratio. In contrast, Fe isotope compositions of hot fluids (> 300 °C) from mid-ocean-ridge spreading centers define a narrow range that is shifted to lower delta 56Fe values by 0.2 per mil - 0.5 per mil as compared to igneous rocks. Therefore, it is expected that mineral phases that contain large amounts of Fe are especially affected by the interaction with a fluid that fractionates Fe isotopes during exsolution/precipitation of those minerals. We have used a femtosecond UV-Laser ablation system to determine mineral 56Fe/54Fe ratios of selected samples with a precision of < 0.1 per mil (2 sigma level) at micrometer-scale. We have found significant variations of the delta 56Fe (IRMM-014) values in the minerals between different samples as well as within samples and mineral grains. The overall observed scale of delta 56Fe (magnetite) in 1256D rocks ranges from - 0.12 to + 0.64 per mil, and of delta 56Fe (ilmenite) from - 0.77 to + 0.01 per mil. Pyrite in the lowermost sheeted dike section is clearly distinguishable from the other investigated lithological units, having positive delta 56Fe values between + 0.29 and + 0.56 per mil, whereas pyrite in the other samples has generally negative delta 56Fe values from - 1.10 to - 0.59 permil. One key observation is that the temperature dependent inter-mineral fractionations of Fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values, while synthesized, well equilibrated magnetite-ilmenite pairs are compatible with the theoretical predictions. Theoretical considerations including beta-factors of different aqueous Fe-chlorides and Rayleigh-type fractionations in the presence of a hydrous, chlorine-bearing fluid can explain this observation. The disagreement between observed and theoretical equilibrium fractionation, the fact that magnetite, in contrast to ilmenite shows a slight downhole trend in the delta 56Fe values, and the observation of small scale heterogeneities within single mineral grains imply that a general re-equilibration of the magnetite-ilmenite pairs is overprinted by kinetic fractionation effects, caused by the interaction of magnetite/ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling, or incomplete re-equilibration at low temperatures. Furthermore, the observation of significant small-scale variations in the 56Fe/54Fe ratios of single minerals in this study highlights the importance of high spatial-resolution-analyses of stable isotope ratios for further investigations.