BORON INCORPORATION INTO SYNTHETIC ARAGONITE: REFINING THE BORON ISOTOPE-pH PROXY

Abstract

<p>By studying the boron isotope composition of marine carbonates, the effectiveness of the tracer to reconstruct the ancient seawater pH and, in turn, atmospheric pCO₂ can be assessed. The boron isotope-pH proxy relies on the hypothesis that only B(OH)₄^-, which has a known boron isotope composition with respect to pH, is incorporated into the carbonate crystal lattice. This research synthesized inorganic aragonite from a range of stable pH values to quantify the dependence of aragonite <em>δ</em>¹¹B on the pH of the precipitating solution. The increasing boron isotope composition of aragonite with increasing pH is consistent with sole incorporation of B(OH)₄^- into the carbonate. The sensitivity of the acid dissociation and isotope equilibrium constants make it difficult to confirm whether or not B(OH)₄^- is the only species contributing to the boron isotope composition of aragonite. The relationship between the boron isotope composition of marine carbonates and ocean pH has wide appeal and, if properly understood, could provide tremendous insight into the history of Earth’s climate.</p> <p>This research also evaluated carbon isotope fractionation between aragonite and dissolved inorganic carbon (DIC) in high ionic strength systems at 25 °C and found it to be consistent with carbon isotope fractionation in low ionic strength environments. The analysis of various isotopic compositions within this study led to the development of new methodology to simultaneously measure the oxygen isotopes of water and carbon isotopes of DIC from small solution samples using continuous flow isotope ratio mass spectrometry (CF-IRMS).</p>