Mineralogy of serpentine muds of ODP Leg 125 holes (Table 1)

Large serpentinite seamounts are common in the forearc regions between the trench axis and the active volcanic fronts of the Mariana and Izu-Bonin intraoceanic arcs. The seamounts apparently form both as mud volcanoes, composed of unconsolidated serpentine mud flows that have entrained metamorphosed ultramafic and mafic rocks, and as horst blocks, possibly diapirically emplaced, of serpentinized ultramafics partially draped with unconsolidated serpentine slump deposits and mud flows. The clayand silt-sized serpentine recovered from three sites on Conical Seamount on the Mariana forearc region and from two sites on Torishima Forearc Seamount on the Izu-Bonin forearc region is composed predominantly of chrysotile, brucite, chlorite, and clays. A variety of accessory minerals attest to the presence of unusual pore fluids in some of the samples. Aragonite, unstable at the depths at which the serpentine deposits were drilled, is present in many of the surficial cores from Conical Seamount. Sjogrenite minerals, commonly found as weathering products of serpentine resulting from interaction with groundwater, are found in most of the samples. The presence of aragonite and carbonate-hydroxide hydrate minerals argues for interaction of the serpentine deposits with fluids other than seawater. There are numerous examples of sedimentary serpentinite deposits exposed on land that are very similar to the deposits recovered from the serpentine seamounts drilled on ODP Leg 125. We suggest that Conical Seamount may be a type locality for the study of in situ formation of many of these sedimentary serpentinite bodies. Further, we suggest that both the deposits drilled on Conical Seamount and on Torishima Forearc Seamount demonstrate that serpentinization can continue in situ within the seamounts through interaction of the serpentine deposits with both seawater and subduction-related fluids.

(Table 1) Boron-isotope measurements of shallow-dwelling planktonic foraminifera of ODP Holes 144-817A, 144-872C and 143-865C

Knowledge of the evolution of atmospheric carbon dioxide concentrations throughout the Earth's history is important for a reconstruction of the links between climate and radiative forcing of the Earth's surface temperatures. Although atmospheric carbon dioxide concentrations in the early Cenozoic era (about 60 Myr ago) are widely believed to have been higher than at present, there is disagreement regarding the exact carbon dioxide levels, the timing of the decline and the mechanisms that are most important for the control of CO2 concentrations over geological timescales. Here we use the boron-isotope ratios of ancient planktonic foraminifer shells to estimate the pH of surface-layer sea water throughout the past 60 million years, which can be used to reconstruct atmospheric CO2 concentrations. We estimate CO2 concentrations of more than 2,000 p.p.m. for the late Palaeocene and earliest Eocene periods (from about 60 to 52 Myr ago), and find an erratic decline between 55 and 40 Myr ago that may have been caused by reduced CO2 outgassing from ocean ridges, volcanoes and metamorphic belts and increased carbon burial. Since the early Miocene (about 24 Myr ago), atmospheric CO2 concentrations appear to have remained below 500 p.p.m. and were more stable than before, although transient intervals of CO2 reduction may have occurred during periods of rapid cooling approximately 15 and 3 Myr ago.