Calcium isotope (δ 44/40 Ca) variations of Neogene planktonic foraminifera

Measurements of the calcium isotopic composition (δ 44/40 Ca) of planktonic foraminifera from the western equatorial Pacific and the Indian sector of the Southern Ocean show variations of about 0.6‰ over the past 24 Myr. The stacked δ 44/40 Ca record of Globigerinoides trilobus and Globigerina bulloides indicates a minimum in δ 44/40 Ca sw (seawater calcium) at 15 to 16 Ma and a subsequent general increase toward the present, interrupted by a second minimum at 3 to 5 Ma. Applying a coupled calcium/carbon cycle model, we find two scenarios that can explain a large portion of the observed δ 44/40 Ca sw variations. In both cases, variations in the Ca input flux to the ocean without proportional changes in the carbonate flux are invoked. The first scenario increases the riverine calcium input to the ocean without a proportional increase of the carbonate flux. The second scenario generates an additional calcium flux from the exchange of Ca by Mg during dolomitization. In both cases the calcium flux variations lead to drastic changes in the seawater Ca concentrations on million year timescales. Our δ 44/40 Ca sw record therefore indicates that the global calcium cycle may be much more dynamic than previously assumed.