A Refined Chronology of Late Eocene to Late Oligocene Coral Reefs: Implications for Coral Calcification Under a Regime of Declining Atmospheric Carbon Dioxide and Changing Ocean Chemistry

Reports: AC2 47576-AC2: A Refined Chronology of Late Eocene to Late Oligocene Coral Reefs: Implications for Coral Calcification Under a Regime of Declining Atmospheric Carbon Dioxide and Changing Ocean Chemistry

Gregoery E. Ravizza, University of Hawaii (Manoa)

Activities: From September 2009 through August 2010 ACS-PRF funds supported 1 undergraduate thesis project with research funds and 1 Ph. D. student with both salary and research support.

Calorimetric investigation of olivine carbonation: Chuen Wing Lam (UH Global Environmental Science B.S. 2010, now at Schlumberger, Houston) conducted this investigation for his senior thesis under the supervision of Greg Ravizza, with the help of Steve Masutani and Chris Kinoshita (UH). The goal of the project was to determine the quantity of heat produced by olivine carbonation (2CO₂ + Mg₂SiO₄ => 2MgCO₃ + SiO₂ ) on a molar basis by reacting a known mass of olivine with carbon dioxide inside a calorimeter cell. The experimental conditions (T = 180 C, P = 760 psi) were chosen to approximate those proposed as the most exothermic by Kelemen and Matter (PNAS 2008).

The impetus for this study was two-fold. First, this reaction provides a possible strategy for long term carbon sequestration. Second, olivine weathering associated with ophiolite obduction in New Caledonia (Reusch, Geology submitted 2011) represents a potential mechanism for CO₂ draw down leading to the late Eocene greenhouse- icehouse transition. The enthalpy of the olivine carbonation reaction is central to these problems because the more exothermic the reaction is the greater the likelihood that this reaction can be self-sustaining without additional energy inputs. Although, the enthalpy of reaction can be calculated from primary thermodynamic data for the reactants and products, to our knowledge it has not be measured directly.

The main results of Lam’s thesis work are as follows: (1) The reaction kinetics are exceedingly slow in the absence of water. (2) In experiments with water present in the reaction cell, total pressure decreased while cell was at constant temperature. Assuming the pressure drop resulted entirely from CO₂ consumption by olivine carbonation, the measured heat of reaction, (3.2 KJ/g olivine) averaged over the course of the experiment is approximately twice the expected value. We believe this high value is an artifact of over correcting for heat exchange between the reaction cell and the external CO₂ reservoir. (3) The reaction rates measured in our experiments imply slower reaction kinetics than those obtained from previous studies that used a much higher fluid:solid ratio. (4) Separate experiments (not performed in the calorimeter) suggest that formation of an iron-rich alteration phase may coat the surface of natural olivine grains where the typical Fe:Mg mole ratio is ≈ 1:9. In the future we hope to support a new graduate student to conduct additional calorimeter experiments with an alternate configuration which better approximate a closed system.

The Eocene Azolla Event and Arctic Carbon Burial: Francois Paquay (UH Ph.D. candidate) abandon efforts to use Sr isotope stratigraphy to date Eocene-Oligocene reef deposits because of problems finding pristine sample material. Instead he has refocused his efforts upon using Osmium isotope as a tracer of the degree of restriction of the Arctic Ocean in the mid-Eocene. During a relatively brief interval of time (≈ 800 kyrs) between 48 and 49 Ma the organic remains of a freshwater fern (Azolla) are unusually abundant within an organic carbon-rich interval (≈5% by weight) at many sites around the Arctic (Brinkhuis et al. Nature 2006). These workers attribute this deposit to a greatly accelerated Arctic hydrologic cycle during the peak of Eocene warmth. Paquay has tested this hypothesis by reconstructing seawater ¹⁸⁷Os/¹⁸⁸Os ratios at sites in the Arctic Basin (IODP 302-4), the Norwegian Greenland Sea (ODP 913) and the South Atlantic (ODP 1263). The Os isotope data display a clear North-South gradient with high ¹⁸⁷Os/¹⁸⁸Os in the Arctic (0.8-0.9), low ratios in the South Atlantic (0.45-0.55) and intermediate values at ODP 913 (0.55-0.65). The high ratios in Arctic sediments are fully consistent with basinal restriction and large Os inputs from weathering of surrounding land masses. This work was presented as poster in June 2010 at the Goldschmidt conference in Knoxville, TN, and a manuscript for publication is in preparation. In addition to support from ACS-PRF, this work was supported by a Schlanger Ocean Drilling Fellowship to Paquay.