IMPLICATIONS OF CARBONATE MINERAL SURFACE DYNAMICS FOR STUDIES OF PAST OCEANS AND CLIMATES

Growth of calcite from aqueous solution has been a subject of investigation for decades because of their importance to marine life and geochemical cycles that regulate Earth’s climate. Carbonate minerals are viewed as recorders of past Earth surface conditions. Recent work on the effects of mineral surface complexity on growth kinetics, the incorporation of impurities, and the fractionation of isotopes, produces new perspectives on how carbonate mineral growth responded to changing conditions in oceans and soils over the history of the Earth. The isotopic composition of the Ca, C, and O, plus the concentrations and isotopic compositions of cation Mg, Sr, Mn, U, Cd, and anion impurities SO₄and BO₃are all used to provide information about past mineral growth conditions or seawater composition. All are subject to kinetic fractionation effects due to mineral surface processes.

It has been established that seawater composition has shifted on ca. 100 million year timescales^1,2. The modern ocean has Mg/Ca = 5.3, SO₄/Ca = 2.7, pH ≈ 8.1, and Ca/CO₃^2-≈ 50. In the mid-Cretaceous and in much of the Paleozoic these values may have been 1, 0.3, 7.6, and 500. All of these variables affect calcite growth and partitioning of impurity elements and isotopes during growth^2,3, and they also tend to correlate roughly with changes in global ocean temperatures and atmospheric CO₂.² For example, 100 million years ago, calcite growth would have been less inhibited by Mg and SO₄, which coupled with lower pH and higher temperature means that calcite could precipitate at the same rate at lower supersaturation. Changing seawater composition also affects fractionation of ⁴⁴Ca/⁴⁰Ca,¹⁸O/¹⁶O, Sr/Ca, and Mg/Ca during calcite growth^3,4.

Bulk carbonate Sr/Ca in Cretaceous marine carbonate sediments is about 1/3 the value in analogous more recent sediments, which could be prima facieevidence that marine carbonate formation happened at closer-to-equilibrium conditions. It follows that O isotopes and Mg/Ca may underestimate Cretaceous seawater temperatures. Recent experimental results continue to challenge existing models, but paleoseawater and temperature reconstruction presents an interesting test of our understanding of the mineral surface controls on calcite growth and composition.

(1) Lowenstein et al. (2014) Treatise on Geochemistry(2) Turchyn and DePaolo (2019) Ann. Rev. Earth. Planet. Sci.(3) Watkins et al (2017) RiMG(4) Lammers and Mitnick (2019) GCA