Growth-rate induced disequilibrium of oxygen isotopes in aragonite: An in situ study

Abstract

Oxygen isotope compositions (δ18O) of biogenic and inorganically precipitated calcium carbonate minerals are being used to reveal marine and terrestrial temperatures from the past. However, experimental and natural observations show factors other than temperature control δ18O in CaCO3via disequilibrium fractionation. Crystallization rate is among these factors, and therefore, its effect on fractionation process has been studied during the last 30years. However, most previous experimental assessments measured δ18O in polycrystalline precipitates using bulk analytical techniques and defined growth rate as the total amount of CaCO3 crystallized during the period of time. Because of this, inter- and intra-crystal isotopic variability has remained poorly constrained. In order to evaluate the effect of crystal extension rate (V) on 18O fractionation between aragonite and fluid an alternative analytical approach was used. Hemispherical bundles of aragonite crystals (spherulites) were measured for δ18O in situ with Secondary Ion Mass Spectrometry (SIMS) at lateral spatial resolution of 10–20μm. The change in V over time was monitored by addition of multiple rare earth element (REE) spikes into the fluid from which the aragonite grew. δ18O in fluid was analyzed with stable isotope ratio mass spectrometer. Results show a decrease in the oxygen isotope fractionation factor (α18O) by 2.4‰ with increasing growth rate of aragonite spherulites from 0.064 to 0.88nm/s (5.5–76μm/day). Crystal growth rate is therefore potentially an important consideration when using δ18O in natural carbonates as a proxy for ocean and terrestrial climate. Growth entrapment model (GEM) was applied to explain experimentally determined α18O-V relationships, assuming lattice-fluid equilibrium α18O values (0% of growth entrapment) at slow growth rates and complete disequilibrium fractionation (100% of growth entrapment) during fast crystallization. Although V values obtained experimentally do not fully cover the V range used in GEM, the combination of experiment and simulations suggests depletion of 18O in the near-surface region of aragonite relative to the bulk crystal lattice. This 18O-depleted zone can be “captured” during rapid (non-equilibrium) crystal growth.