Geochemistry of DSDP Leg 65 basalts

Leg 65 of the Deep Sea Drilling Project successfully recovered basalts from four sites in the mouth of the Gulf of California, thus completing a transect begun during Leg 64 from the continental margin of Baja California to the east side of the East Pacific Rise (EPR). Sixty-three whole-rock samples from Sites 482, 483, and 485 have been analyzed by X-ray fluorescence techniques, and a further eleven samples by instrumental neutron-activation techniques, in order to assess magma variability within and between sites. Although the major element compositions and absolute hygromagmatophile (HYG) element abundances are quite variable, all of the basalts are subalkaline tholeiites exhibiting strong more-HYG element (e.g., Rb, La, Nb, Ta) depletion (LaN/YbN ~ 0.4; Nb/Zr ~ 0.02; Ba/Zr ~ 0.23; Th/Hf ~ 0.05). These ratios, together with La/Ta ratios of 20 and Th/Ta ratios of 1.25, demonstrate that the Leg 65 basalts resemble the depleted "N-type" ocean ridge basalts recovered from the Mid-Atlantic Ridge (MAR) at 22 °N and other sections of the EPR. Zr/Ti, Zr/Y, and La/Yb ratios increase with increasing fractionation. It is clear that the basalts recovered from Sites 482, 483, and 485 were all derived from a compositionally similar source and that the compositional differences observed between lithological units can be explained by varying degrees of open system fractional crystallization (magma mixing) in subridge magma chambers. The basaltic rocks recovered from Site 474 near the margin of Baja California, and Sites 477, 478, and 481 in the Guaymas Basin, all drilled during Leg 64, have consistently higher Th/Hf, La/Sm, Zr/Ti, and Zr/Y ratios and higher absolute Sr contents than the Leg 65 basalts. While some of these variations may be explained by different conditions of partial melting, it is considered more likely that the mantle source underlying the Guaymas Basin is chemically distinct from that feeding the EPR at the mouth of the Gulf. These source variations probably reflect the complex tectonic setting of the Gulf of California, the magmas formed at the inception of spreading and in the central part of the Gulf containing a minor but significant component of sub-continental (calc-alkaline) material.

Alkenone and boron based Oligocene pCO2 records

The Pliocene period is the most recent time when the Earth was globally significantly (~3°C) warmer than today. However, the existing pCO2 data for the Pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo-pCO2. Moreover, the temporal resolution of the published records does not allow a robust assessment of the role of declining pCO2 in the intensification of the Northern Hemisphere Glaciation (INHG) and a direct comparison to other proxy records are lacking. For the first time, we use a combination of foraminiferal (delta11B) and organic biomarker (alkenone-derived carbon isotopes) proxies to determine the concentration of atmospheric CO2 over the past 5 Ma. Both proxy records show that during the warm Pliocene pCO2 was between 330 and 400 ppm, i.e. similar to today. The decrease to values similar to pre-industrial times (275-285 ppm) occurred between 3.2 Ma and 2.8 Ma - coincident with the INHG and affirming the link between global climate, the cryosphere and pCO2.

Major- and trace-elements at DSDP Legs 68-70 Holes

The basalts recovered from the Costa Rica Rift by drilling at Deep Sea Drilling Project Sites 501, 504, and 505 during Legs 68, 69 and 70 of the Glomar Challenger are the most depleted in the most-hygromagmaphile elements (Th, Ta, Nb, and La) of all MORB recovered to date by the Glomar Challenger. The invariant ratios Nb/Ta, Zr/Hf, and Y/Tb show "chondritic values" (expected for Nb/Ta because of the very low concentrations in these elements). Four samples from a single unit are exceptions: they present a flat to slightly enriched, extended Coryell-Masuda plot, and at the same time their La/Ta ratio is 9 (normalized ratio = 1) instead of 19 (normalized ratio = 2), the value for all other samples. Only one of these two values of the La/Ta ratio had been found so far within a single hole, and moreover within large areas of the oceanic crust (several holes or dredges). The present result shows that local heterogeneity of the upper mantle with respect to the La/Ta ratio may exist.

Over-determined charbonate chemstry dataset from different kinds of manipulation (TA, DIC) and water types (natural and artificial seawater)

The growing field of ocean acidification research is concerned with the investigation of organism responses to increasing pCO2 values. One important approach in this context is culture work using seawater with adjusted CO2 levels. As aqueous pCO2 is difficult to measure directly in small-scale experiments, it is generally calculated from two other measured parameters of the carbonate system (often AT, CT or pH). Unfortunately, the overall uncertainties of measured and subsequently calculated values are often unknown. Especially under high pCO2, this can become a severe problem with respect to the interpretation of physiological and ecological data. In the few datasets from ocean acidification research where all three of these parameters were measured, pCO2 values calculated from AT and CT are typically about 30% lower (i.e. ~300 µatm at a target pCO2 of 1000 µatm) than those calculated from AT and pH or CT and pH. This study presents and discusses these discrepancies as well as likely consequences for the ocean acidification community. Until this problem is solved, one has to consider that calculated parameters of the carbonate system (e.g. pCO2, calcite saturation state) may not be comparable between studies, and that this may have important implications for the interpretation of CO2 perturbation experiments.