Reports: ND250755-ND2: Boron Isotope Effects in Synthetic Calcium Carbonates: For a Better Reconstruction of Paleo-Ocean pH

Sang-Tae Kim, PhD, McMaster University

The relative abundance of two stable isotopes of boron in marine carbonates has been used to reconstruct ancient ocean pH changes.  This so called “boron isotope paleo-ocean pH proxy" is based upon the four hypotheses that the pH of seawater dictates the boron isotope composition of marine calcium carbonates: (1) boric acid (B(OH)3) and borate (B(OH)4-) are the two most dominant boron-bearing aqueous species in seawater, (2) their relative abundances and respective boron isotope compositions are a function of ocean pH, (3) borate is isotopically heavier than boric acid at equilibrium, and finally (4) borate is preferentially incorporated into the crystal structure of carbonate minerals without a significant kinetic effect.  However, the application of the paleo-ocean pH proxy has been hampered by a number of complications, but most of the issues have recently been resolved with the exception of the last assumption. 

The goal of my ACS-PRF research proposal is to gain a basic understanding of the fourth assumption of the boron isotope paleo-ocean pH proxy.  In order to achieve this research goal, my research group at McMaster University has been conducting a series of experiments under precisely-controlled laboratory conditions which enable us to systematically evaluate the speciation of boron-bearing aqueous species and their incorporation into the two most common polymorphs of calcium carbonates.  In particular, we have focused on the development of experimental methods for preparing kinetic and vital effects-free calcium carbonates from three different pH values over the last 18 months.  It should be noted that synthesizing calcium carbonate minerals from a different, but a stable pH value is essential for the successful completion of the research.  So far, we have successfully established an experimental protocol for synthesizing kinetic and vital effects-free aragonite (one of the polymorphs of calcium carbonates) from a stable mid-pH range saline solution.  We are also in the final stage of optimizing experimental protocols for synthesizing aragonite samples from two additional stable pH values.  Several calcium carbonates that were synthesized using our new experimental techniques have already been sent out for testing to our collaborators at the University of Southampton in the U.K.

As a junior faculty member from a research-intensive university in Canada, the ACS-PRF New Directions (ND) grant has provided me with unique opportunities and flexibility to pursue new research with both my undergraduate and graduate students.  Two graduate students are currently working on my ACS-PRF research project and I have also hired both part-time and full-time undergraduate students to assist me in the laboratory on various tasks for the project.  Students who are involved in my ACS-PRF research project learn many analytical techniques that are essential in the field of geochemistry.  Therefore, this hands-on laboratory experience is an invaluable asset for their future careers as scientists. The outcome of my ACS-PRF research is expected to provide a basis for the development/calibration of reliable paleo-ocean pH proxies for biogenic carbonates.  Eventually, this will lead to a more accurate reconstruction of past ocean pH and past atmospheric CO2 concentrations.