The influence of temperature and seawater carbonate saturation state on 13C-18O bond ordering in bivalve mollusks

The shells of marine mollusks are widely used archives of past climate and ocean chemistry. Whilst the measurement of mollusk delta 18O to develop records of past climate change is a commonly used approach, it has proven challenging to develop reliable independent paleothermometers that can be used to deconvolve the contributions of temperature and fluid composition on molluscan oxygen isotope compositions. Here we investigate the temperature dependence of 13C-18O bond abundance, denoted by the measured parameter Delta 47, in shell carbonates of bivalve mollusks and assess its potential to be a useful paleothermometer. We report measurements on cultured specimens spanning a range in water temperatures of 5 to 25 °C, and field collected specimens spanning a range of -1 to 29 °C. In addition we investigate the potential influence of carbonate saturation state on bivalve stable isotope compositions by making measurements on both calcitic and aragonitic specimens that have been cultured in seawater that is either supersaturated or undersaturated with respect to aragonite. We find a robust relationship between Delta 47 and growth temperature. We also find that the slope of a linear regression through all the Delta 47 data for bivalves plotted against seawater temperature is significantly shallower than previously published inorganic and biogenic carbonate calibration studies produced in our laboratory and go on to discuss the possible sources of this difference. We find that changing seawater saturation state does not have significant effect on the Delta 47 of bivalve shell carbonate in two taxa that we examined, and we do not observe significant differences between Delta 47-temperature relationships between calcitic and aragonitic taxa.

Partial pressure of carbon dioxide along the Transatlantic section from 7 November till 19 December 2000

Along a transatlantic section from 57°N to 60°S that was carried out from November 7 till December 19, 2000 on board R/V Horizont II concentrations of CO2 were measured in the near-water layer of the air and differences between partial pressures in water and air in various climatic zones were calculated. It was shown that variations of CO2 concentrations in the near-water layer of air and those of values of water-air partial pressure difference were from 324x10**-6 to 426x10**-6 and from 150x10**-6 to 100x10**-6 atm, respectively. Maximum value of CO2 partial pressure in air in the near-water layer (426x10**-6 atm) was noted at 45°-47°N; minimum of 324x10**-6 atm was found in Antarctica at 59°S. During measurenents maximum value of CO2 partial pressure difference in water and air (150 x10**-6) was observed at 45°-48°N; maximum flux in this case was directed from the atmosphere to water. Maximum value of CO2 partial pressure difference in water and air for flux directed from the ocean to air (100 x10**-6) was observed at 59°-60°S. Comparison of calculated values of partial pressure difference in water and air with previous data points to more intense exchange of CO2 between the ocean and atmosphere during the survey period was considered. According to values of CO2 partial pressure difference in air and water as compared to 1975, exchange intensity in the Northern Hemisphere (absorption from the atmosphere) increased. A well-pronounced latitudinal effect of distribution of CO2 partial pressure in air was observed. Along the route variations in CO2 concentrations in zones of water divergence and convergence were registered.

Partial melting experiments on oceanic cumulated gabbros

We performed hydrous partial melting experiments at shallow pressures (0.2 GPa) under slightly oxidizing conditions (NNO oxygen buffer) on oceanic cumulate gabbros drilled by ODP (Ocean Drilling Program) cruises to evaluate whether the partial melting of oceanic gabbro can generate SiO2-rich melts with compositions typical of oceanic plagiogranites. The experimental melts of the low-temperature runs broadly overlap those of natural plagiogranites. At 940 °C, the normalized SiO2 contents of the experimental melts of all systems range between 60 and 61 wt%, and at 900 °C between 63 and 68 wt%. These liquids are characterized by low TiO2 and FeOtot contents, similar to those of natural plagiogranites from the plutonic section of the oceanic crust, but in contrast to Fe and Ti-rich low-temperature experimental melts obtained in MORB systems at ~950 °C. The ~1,500-m-long drilled gabbroic section of ODP Hole 735B (Legs 118 and 176) at the Southwest Indian Ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high-temperature shearing. The compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins, while the compositions of the crystals of low-temperature runs correspond to those of minerals from high-temperature microscopic veins occurring in the gabbroic section of the Hole 735B. This suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water-rich fluid phase. If the temperature estimations for hightemperature shear zones are correct (up to 1,000 °C), and a water-rich fluid phase is present, the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust.