A critical evaluation of the boron isotope-pH proxy: The accuracy of ancient ocean pH estimates

The boron isotope-pH technique is founded on a theoretical model of carbonate δ¹¹B variation with pH that assumes that the boron isotopic composition of carbonates mirrors the boron isotopic composition of borate in solution (δ¹¹B_carb = δ¹¹B_borate). Knowledge of the fractionation factor for isotope exchange between boric acid and borate in solution (α_4-3), the equilibrium constant for the dissociation of boric acid (pK_B*), as well as the isotopic composition of boron in seawater (δ¹¹B_sw) are required parameters of the model.The available data suggests that both the value of α_4-3 and the history of δ¹¹B_sw are poorly constrained. However, if one assumes that δ¹¹B_carb = δ¹¹B_borate, an empirical value for α_4-3 can be estimated from the results of inorganic carbonate precipitation experiments. This exercise yields an α_4-3 value of ∼0.974 in accordance with recent theoretical estimates, but substantially deviates from the theoretical value of 0.981 often used to estimate paleo-ocean pH. Re-evaluation of ocean pH using an α_4-3 value of 0.974 and published foraminiferal δ¹¹B values for the Cenozoic yield pH estimates that are relatively invariant, but unrealistically high (∼8.4-8.6). Uncertainty increases as foraminiferal ‘vital effects’ are considered and different models for secular changes in seawater δ¹¹B are applied.The inability to capture realistic ocean pH possibly reflects on our understanding of the isotopic relationship between carbonate and borate, as well as the mechanism of boron incorporation in carbonates. Given the current understanding of boron systematics, pH values estimated using this technique have considerable uncertainty, particularly when reconstructions exceed the residence time of boron in the ocean.     

Comment on ‘Magnetic studies of magma-supply and sea-floor metamorphism: Troodos ophiolite dikes’, by G.J. Borradaile and D. Gauthier

Anisotropy of magnetic susceptibility is used as a proxy for the determination of magmatic flow direction in mafic dykes. Here we take advantage of the dataset from G.J. Borradaile and D. Gauthier to comment three points: (1) the sampling strategy; (2) the geometric relationship between magnetic axis dyke, and, (3) an alternative interpretation to obtain a flow direction. The magnetic lineations published by Borradaile and Gauthier correspond to the zone axis of the dyke and magnetic foliation poles, questioning the reliability of the magnetic lineation as a flow estimate. An alternative interpretation is based on the use of the tiling of the magnetic foliation plane against the dyke wall.     

Comment on: “Origin, timing, and temperature of secondary calcite-silica mineral formation at Yucca Mountain, Nevada” by N. S. F. Wilson, J. S. Cline, and Y. V. Amelin

Yucca Mountain in southern Nevada is being evaluated as a potential site for the geological disposal of high-level nuclear waste. A reliable assessment of the future performance of the repository will require detailed paleohydrogeological information. Hydrogenic secondary minerals from the vadose zone of Yucca Mountain are being studied as paleohydrogeological indicators. A phenomenological model envisaging the deposition of secondary minerals by meteoric fluids infiltrating downward though the vadose zone was proposed in the reviewed paper. Our evaluation reveals that the model is not supported by empiric evidence reported in the paper.