Experimental evidence for kinetic effects on B/Ca in synthetic calcite: Implications for potential B(OH) 4  and B(OH) 3 incorporation Joji Uchikawa a,⇑ , Donald E. Penman b , James C. Zachos b , Richard E. Zeebe a a Department of Oceanography, SOEST, University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822, United States b Department of Earth and Planetary Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95060, United States Received 25 July 2014; accepted in revised form 26 November 2014; Available online 5 December 2014 Abstract The boron to calcium ratio (B/Ca) in biogenic CaCO 3 is being increasingly utilized as a proxy for past ocean carbonate chemistry. However, B/Ca of cultured and core-top foraminifers show dependence on multiple physicochemical seawater properties and only a few of those have been inorganically tested for their impacts. Accordingly, our understanding of the controls on foraminiferal B/Ca and thus how to interpret B/Ca in fossil shells is incomplete. To gain a clearer understanding of the B incorporation mechanism, we performed inorganic calcite precipitation experiments using a pH-stat system. As pre- viously reported, we confirm that B/Ca in calcite increases with both fluid pH and total B concentration (denoted as [B T ], where [B T ] = [B(OH) 3 ] + [B(OH) 4  ]). We provide the first evidence that B/Ca also increases with the concentration of total dissolved inorganic carbon (DIC) and calcium ion. With the exception of the [B T ] experiments, these chemical manipulations were accompanied by an increase in calcite saturation, and accordingly precipitation rate (denoted as R). But when pH and [Ca 2+ ] were jointly varied at a fixed saturation level to maintain relatively constant R at different pH and [Ca 2+ ] combinations, B/Ca was insensitive to both pH and [Ca 2+ ] changes. These experimental results unequivocally suggest kinetic effects related to R on B/Ca. Furthermore, with a suite of chemical manipulations we found that the B/Ca variability is explicable by just R and the [B T ]/[DIC] ratio in the parent fluids. This observation was particularly robust for relatively rapidly precipitated samples, whereas for relatively slowly precipitated samples, it was somewhat ambiguous whether the [B T ]/[DIC] or [B(OH) 4  ]/[HCO 3  ] ratio provides a better fit to the experimental data. Nonetheless, our experimental results can be considered as indirect evidence for incorporation of both B(OH) 4  and B(OH) 3 into calcite. We propose a simple mathematical expression to describe the mode of B incorporation into synthetic calcite that depends only on the fluid [B T ]/[DIC] ratio and the precip- itation rate R. This novel finding has important implications for future calibrations and applications of the B/Ca proxy as well as the d 11 B paleo-pH proxy. Ó 2014 Elsevier Ltd. All rights reserved. 1. INTRODUCTION The change in the Earth’s surface temperature per doubling of the atmospheric CO 2 concentration is referred to as climate sensitivity, which depends on the background climate state and an intricate balance of numerous feedback mechanisms operating on various timescales (Rohling et al., 2012; Zeebe, 2013). Climate sensitivity derived from contin- uous paleotemperature and paleo-CO 2 reconstruction is of crucial importance to reliably predict future climate change from anthropogenic CO 2 emissions. Ancient atmosphere trapped in Antarctic ice-cores provides direct CO 2 records http://dx.doi.org/10.1016/j.gca.2014.11.022 0016-7037/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: uchikawa@hawaii.edu (J. Uchikawa). www.elsevier.com/locate/gca Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 150 (2015) 171–191