Oxygen-isotope composition, temperature and depth of formation of chert at DSDP Leg 62 Holes

We measured oxygen-isotope compositions of 16 siliceous rocks from Deep Sea Drilling Project Sites 463, 464, 465, and 466 (Leg 62). Samples are from deposits that range in age from about 40 to 103 m.y. and that occur at sub-bottom depths of 9 to 461 meters. Mean d18O values range from 28.4 to 36.8 per mil and 36.0 ± 0.3 per mil for quartz-rich and opal-CTrich rocks, respectively. d18O values in chert decrease with increasing sub-bottom depth; the slope of the d18O/depth curve is less steep for Site 464 than for the other sites which indicates that chert at Site 464 formed at higher temperatures than chert at Sites 463, 465, and 466. Temperatures of formation of cherts were 7 to 42°C, using the silica-water fractionation factor of Knauth and Epstein (1976), or 19 to 56°C, using the equation of Clayton et al. (1972). Temperatures in the sediment where the cherts now occur are lower than their isotopically determined temperatures of formation, which means that the cherts record an earlier history when temperatures in the sediment section were greater. Estimated sediment temperatures when the cherts formed are comparable to, but generally slightly lower than, those calculated from Knauth and Epstein's equation. The isotopic composition of cherts is more closely related to environment of formation (diagenetic environment) or paleogeothermal gradients, than to paleoclimates (bottom-water temperatures). Opal-CT-rich rocks may better record paleo-bottom-water temperature. In Leg 62 cherts, better crystallinity of quartz corresponds to lower d18O values; this implies progressively higher temperatures of equilibration between quartz and water during maturation of quartz. The interrelationship of d18O and crystallinity is noted also in continental-margin deposits such as the Monterey Formation - but for higher temperatures. The apparent temperature difference between open-ocean and continental-margin deposits can be explained by the dominant control of temperature on silica transformation in the rapidly deposited continental-margin deposits, whereas time, as well as temperature, has a strong influence on the transformations in open-ocean deposits. Comparisons between the chemistry and d18O values of cherts reveal two apparent trends: both boron and SiO2 increase as d18O increases. However, the correspondence between SiO2 and d18O is only apparent, because the two cherts lowest in SiO2 are also the most deeply buried, so the trend actually reflects depth of burial. The correspondence between boron and d18O supports the conclusion that boron is incorporated in the quartz crystal structure during precipitation

Seawater carbonate chemistry and carbon and oxygen isotopes during experiments with planktonic foraminifera Orbulina universa and Globigerina bulloides, 1997

Stable oxygen and carbon isotope measurements on biogenic calcite and aragonite have become standard tools for reconstructing past oceanographic and climatic change. In aquatic organisms, 18O/16O ratios in the shell carbonate are a function of the ratio in the sea water and the calcification temperature (Epstein et al., 1953). In contrast, 13C/12C ratios are controlled by the ratio of dissolved inorganic carbon in sea water and physiological processes such as respiration and symbiont photosynthesis (Spero et al., 1991, doi:10.1029/91PA02022). These geochemical proxies have been used with analyses of foraminifera shells to reconstruct global ice volumes (Shackleton and Opdyke, 1973, doi:10.1016/0033-5894(73)90052-5), surface and deep ocean temperatures (Broecker, 1986, doi:10.1016/0033-5894(86)90087-6; Labeyrie et al., 1987, doi:10.1038/327477a0), ocean circulation changes (Duplessy et al., 1988, doi:10.1029/PA003i003p00343) and glacial-interglacial exchange between the terrestrial and oceanic carbon pools (Sackleton, 1977). Here, we report experimental measurements on living symbiotic and non-symbiotic plankton foraminifera (Orbulina universa and Globigerina bulloides respectively) showing that the 13C/12C and 18O/16O ratios of the calcite shells decrease with increasing seawater [CO3 2-]. Because glacial-period oceans had higher pH and [CO3 2-] than today (Sanyal et al., 1995, doi:10.1038/373234a0), these new relationships confound the standard interpretation of glacial foraminiferal stable-isotope data. In particular, the hypothesis that the glacial-interglacial shift in the 13C/12C ratio was due to a transfer of terrestrial carbon into the ocean(Shackleton ,1977) can be explained alternatively by an increase in ocean alkalinity (Lea et al., 1996). A carbonate-concentration effect could also help explain some of the extreme stable-isotope variations during the Proterozoic and Phanerozoic aeons (Kaufman et al., 1993, doi:10.1016/0012-821X(93)90254-7).