Rosalind Rickaby

The origin of carbon isotope vital effects in coccolith calcite

Calcite microfossils are widely used to study climate and oceanography in Earths geological past. Coccoliths, readily preserved calcite plates produced by a group of single-celled surface-ocean dwelling algae called coccolithophores, have formed a significant fraction of marine sediments since the Late Triassic. However, unlike the shells of foraminifera, their zooplankton counterparts, coccoliths remain underused in palaeo-reconstructions. Precipitated in an intracellular chemical and isotopic microenvironment, coccolith calcite exhibits large and enigmatic departures from the isotopic composition of abiogenic calcite, known as vital effects. Here we show that the calcification to carbon fixation ratio determines whether coccolith calcite is isotopically heavier or lighter than abiogenic calcite, and that the size of the deviation is determined by the degree of carbon utilization. We discuss the theoretical potential for, and current limitations of, coccolith-based CO2 paleobarometry, that may eventually facilitate use of the ubiquitous and geologically extensive sedimentary archive.

Deglacial rearrangement of carbon and nutrients in surface and intermediate depth waters [Abstract]

In this study we used in situ AFM imaging of biotite (001) surfaces to observe dissolution processes in oxalic acid and hydrochloric acid solutions in real time. In oxalic acid solutions at pH 2 and 1 we observe the combined dissolution processes of slow etch pit formation, followed by relatively fast etch pit growth. The etch pit depth is found to be consistent with the thickness of 1 tot layer (~1 nm). Measurements of the fractional surface area covered by etch pits over time provide dissolution rates in the acid solutions. Experiments over a range of temperature, 10°C < T < 35 °C, in the pH 1 solution allow an estimate of the apparent activation energy, Ea= 49 ± 2 kJmol using the Arrhenius equation. We find that the average radius of etch pits grows linearly with time for all T, and use these growth rates to obtain a second value of Ea = 53 ± 3 kJmol. We also find a linear relationship between the dissolution rate and the density of step edges on the surface for all values of T. However, Ea is found to vary between ~ 40 kJmol and >100 kJmol at low and high step density, respectively. This variation may reflect the greater energy required for etch pit formation compared with etch pit growth. In an HCl solution of pH 1 we do not observe the formation of discrete etch pits, but a more general degradation over the entire biotite (001) surface. We conclude that etch pit occurrence in the oxalic acid solution proceeds through chelation of aluminium ions in the (001) surface by the oxalate ligand. This may destabilize the silicate structure, leading to the removal of the entire tot layer, as observed. In the HCl solution the aluminium cations are less soluble due to the lack of organic ligand so the silicate structure remains intact. Metal silicate partitioning of Ge, Mo, Ga and P: Constraints on core formationI will discuss three types of mass transport related isotope fractionations. 1. Isotope fractionation by evaporation of silicate liquids. 2. Isotope fractionation by chemical diffusion in liquids. 3. Isotope fractionation by thermal diffusion in molten basalt. We (Richter et al., 2002 and work in progress) have carried out a large number of experiments on isotope fractionation during evaporation and find that it follows a Rayleigh fractionation law of the form RiJ/RiJo=fJ where RiJ is the ratio of isotopes i and j when a fraction fJ of isotope j remains in the condensed phase and RiJo is the original isotope ratio of the of the condensed phase. The fractionation parameter α is the ratio of the isotopic composition of the evaporation flux to that of the substrate. In the case of magnesium evaporating from silicate liquid at 1400°C, (relevant to the evaporation history of Ca-Al-rich inclusions found in primitive meteorites), we find α=0.9910, which is significantly different from the often assumed value of α=(24/25)= 0.9798 for the fractionation of Mg/Mg in the evaporation residue. We (Richter et al., 1999, 2003) reported the results of experiments for the isotopic fractionation of Li, Ca, and Ge (used as a Si analogue) during diffusion between molten basalt and rhyolite, and now have new data on the isotopic fractionation of Mg is this system. We have also carried out experiments on the isotopic fractionation of Li, Mg, and Cl during diffusion in water (Richter et al., 2006). The results are reported in terms of the exponent β in Di/DJ=(mJ/mi), where the Dk is the self diffusion coefficients of isotope k and mk is its atomic mass. For molten silicate we found β7Li/6Li ≈ 0.215, β44Ca/40Ca ≈ 0.075, β26Mg/24Mg ≈ 0.05, β76Ge/70Ge ≈ 0. In water β7Li/6Li ≈ 0.015, β37Cl/35Cl ≈ 0.025, β26Mg/24Mg ≈ 0. Note the very much smaller fractionations in water. Our most recent experiments involve isotopic fractionations by thermal diffusion (often also called Soret diffusion). We are finding surprisingly large isotopic fractionations (~8x89 for Mg/Mg) associated with a change of only about 150ûC across molten basalt. We are in the process of determining the thermal isotopic fractionation factors of the other major elements in molten basalt.