”The potential of upper ocean alkalinity controls for atmospheric carbon dioxide changes” Christoph Heinze University of Bergen Geophysical Institute and Bjerknes Centre for Climate Research Co-sponsored by: EU Integrated Project CARBOOCEAN

higher atmospheric pCO 2 ↓ higher ocean pCO 2 /acidification ↓ suspended particulate CaCO 3 in upper water column may dissolve Apart from adverse effects on biota: Is there a significant negative feedback to CO 2 climate forcing associated with this? The question

Heinze, C., 2004, Simulating oceanic CaCO3 export production in the greenhouse, Geophysical Research Letters, 31, L16308 acidification feedbacks, A1B scenario: less CaCO 3 production ca. -20 ppm and less particle ballast ca. +20 ppm compensate each other POTENTIAL LIMIT? CaCO 3 export production ballast effect

CO 3 2- standard run vs. observations Atlantic Pacific atmosphere 283 μatm GEOSECSMODEL

↓ Biological export production rates ↓ ↑ Sediment ↑ C org CaCO 3 biogenic silica clay atm. dust

scenario 1 – enhanced CaCO 3 dissolution Dissolution rate constant – standard: R = k (1-Ω) 3 k=1 day -1 Ω=1-[Ca 2+ ][CO 3 2- ]/K sp R(min) = const., where water column is oversaturated for CaCO 3 Dissolution rate constant – scenario 1: R = as above but R = R(min) x 10, where water column is oversaturated for CaCO 3 in the standard run

scenario 1 – enhanced CaCO 3 dissolution alkalinity in upper water increases →

scenario 1 – CaCO 3 accumulation 0 years 100 years 1000 years years

scenario 2 – enhanced CaCO 3 dissolution no riverine alkalinity delivery anymore Dissolution rate constant – standard: R = k (1-Ω) 3 k=1 day -1 Ω=1-[Ca 2+ ][CO 3 2- ]/K sp R(min) = const., where water column is oversaturated for CaCO 3 Dissolution rate constant – scenario 2: R = as above but R = R(min) x 10, where water column is oversaturated for CaCO 3 in the standard run

scenario 2 – enhanced CaCO 3 dissolution no riverine alkalinity delivery anymore alkalinity in upper water increases less →

scenario 2 – CaCO 3 accumulation 0 years 100 years 1000 years years

Conclusions years: ocean acidification is not likely to induce a significant negative CO 2 feedback through upper ocean CaCO 3 dissolution alone Longer time scales: mismatch between delivery of alkalinity by rivers and enhanced CaCO 3 dissolution in the water column → potentially significant negative feedback In any case: ocean acidification may perturb sediment mixed layer globally and quasi-instantaneously

Sediment column 130 kyrBP up to preindustrial:

A synthetic sediment core from the equatorial Pacific

A synthetic sediment core from the western North Atlantic Heinze, 2001, GRL, 28(22),