No support for widespread surface ocean acidification during Oceanic Anoxic Event 1a

Monday, 15 December 2014: 11:35 AM
Bernhard David Naafs1,2, Jose Manuel Castro3, G.a. De Gea3, M.L. Quijano4, D.N. Schmidt5 and Richard D Pancost1,2, (1)Cabot Institute, University of Bristol, Organic Geochemistry Unit, School of Chemistry, Bristol, United Kingdom, (2)University of Bristol, Organic Geochemistry Unit, School of Chemistry, Bristol, United Kingdom, (3)University of Jaén, Dept. Geología, CEACTierra, Jaén, Spain, (4)University of Jaén, Química Inorgánica y Orgánica, CEACTierra, Jaén, Spain, (5)University of Bristol, School of Earth Sciences, Bristol, United Kingdom
Various studies suggested that changes in morphology and the demise of calcareous nannoplankton around Cretaceous Oceanic Anoxic Event (OAE) 1a were the result of large-scale (surface) ocean acidification caused by a substantial input of CO2. However, the link between extinction, changes in morphology, calcification crisis and surface ocean acidification is heavily debated. Crucially the relative timing, magnitude and duration of CO2 input during OAE 1a and hence potential to change surface ocean pH and calcite saturation state (Ω) are unconstraint. Here we provide the first high-resolution record of atmospheric CO2 across OAE 1a that depicts the often-inferred increase and decrease in pCO2. Crucially, we show that the initial increase in pCO2 was a gradual and sustained process that lasted at least 100 kyr. Earth system modeling indicates that over such timescales buffering of ocean chemistry by dissolution of deep-sea carbonates and weathering on land prevent surface ocean acidification. Widespread surface ocean acidification could therefore not have occurred during OAE 1a. Our results challenge recent suggestion of widespread ocean acidification across OAE 1a and highlight that the currently observed and further predicted anthropogenic decrease in surface ocean pH and Ω is extremely rare if not unique within the geological record.