Heavy boron isotopic composition of Pennsylvanian brachiopod calcite: implications for pH, CO2 and secular variation of seawater δ11B

The Late Carboniferous (Pennsylvanian) icehouse is thought to have been a period of low, rapidly fluctuating CO2 concentrations coincident with major episodes of glaciation, as evidenced by proxy measurements, carbon budget models, and the temporal distribution of glacial deposits and cyclothems. The B-isotope paleo-pH proxy has proven to be a valuable tool for estimating paleo-atmospheric pCO2. However, applications of the proxy to deep time are complicated by a probable long-term secular variation in the B-isotopic composition of seawater, and potential diagenetic alteration of ancient samples. Our data show that very low δ11B values for Mississippian and Permian brachiopods appear to require significant secular variation in the B isotope composition of seawater. We measured δ11B of Late Pennsylvanian brachiopod calcite with negative thermal ionization mass spectrometry (NTIMS). Museum specimens were screened for diagenetic alteration using scanning electron microscopy and cathodoluminescence microscopy. The Pennsylvanian values are higher than Neogene values, consistent with high pH and low CO2, if changes in seawater δ11B are ignored. While we cannot directly calculate a pH (pCO2) from these results, they appear to require a higher pH than today's ocean and/or a high δ11B of Pennsylvanian seawater. Some models have suggested secular variability related to hydrothermal alteration at mid-ocean ridges while others call on variation of river run-off as the dominant control. We suggest another control may be exerted by aqueous B speciation. During periods of low pCO2 (high pH), we expect that seawater δ11B will increase due to the relatively higher abundance of the isotopically lighter B(OH)4-, which is chemically reactive and sorbs to clays and carbonates, thus removing isotopically light boron. On the other hand, during periods of high pCO2 (low pH), the relative increase in the abundance of the uncharged B(OH)3 species will result in less adsorption and removal of B, resulting in a decrease in seawater δ11B, approaching river input values. Therefore measurements of δ11B cannot be used to reconstruct absolute values for ocean pH and atmospheric CO2 concentration without knowing the secular variation reconstruction of seawater pH, but relative shifts can provide important paleo-climate information.