The adsorption of boron on detrital particles like clay or metal oxides is thought to be a major mechanism driving changes in the boron isotopic composition of seawater on geologic timescales. However, the sensitivity of adsorption parameters to long-term changes in the seawater concentration of major ions (Mg2+, Ca2+, SO42-) and dissolved inorganic carbon (HCO3-, CO32-) is not known. We conducted multiple sets of adsorption experiments that consist of suspending pretreated clay minerals (either kaolinite, smectite or illite) in artificial seawater with a modified chemical composition. Specifically, we investigate adsorption in seawater with a major ion composition resembling that of the Cretaceous (100 Ma) and the Eocene (50 Ma), as well as modern seawater with either reduced or elevated concentrations of dissolved inorganic carbon. We finally combine the results with modeled values for the mineral assemblage of detrital sediment to constrain boron adsorption fluxes in the past. The dataset consists of two sheets that store (1) the results of our adsorption experiments and (2) the modeled sediment properties.

Experiments were performed on KGa-1b kaolinite, SWy-3 smectite and IMt-2 illite obtained from the Clay Mineral Society. For each of these clays, a consistent particle size fraction of 2 – 0.2 μm was extracted by repeated centrifugation and decantation. As a result, clay samples used in the experiments have a high mineralogical purity of 95% (in the case of kaolinite and illite) and 50% (in the case of smectite).
Pretreated clays were submerged in one of four different boron-containing artificial seawater solutions. These seawater solutions were prepared by mixing trace element-grade salts with ultrapure water according to the recipe of Millero (2013). Specifically, the amounts of added MgCl2, CaCl2, Na2SO4 and NaHCO3 were varied to produce four different seawater stock solutions that have (i) a major ion concentration similar to Eocene seawater; (ii) a major ion concentration similar to Cretaceous seawater; (iii) a DIC concentration half as high as in modern seawater; (iv) a DIC concentration twice as high as in modern seawater. Clay and seawater were allowed to interact for 48h through continuous agitation, after which solution samples were extracted.