Influence of clay-mineral assemblages on the B/Ga salinity proxy

Boron/gallium (B/Ga) ratios have proven useful in reconstructing salinity conditions in ancient shale and marl formations. Boron is adsorbed from the watermass in proportion to its aqueous concentration, yielding sedimentary concentrations many times greater than background levels, whereas Ga is present mainly in the detrital fraction. However, the amount of B uptake by a sample depends not only on aqueous B concentrations but potentially also on the mineralogical composition of the clay fraction of the sediment, which is the main host of adsorbed B. Clay minerals have varying capacities to adsorb borate (B(OH)₄⁻), the main ionic species of boron in natural aqueous systems: highest for illite, intermediate for smectite and chlorite, and lowest for kaolinite. Thus, the salinity signal recorded by a given formation may depend on the nature of its clay-mineral assemblage. In this study, we analyzed relationships of B/Ga to clay-fraction compositions, as determined by X-ray diffraction (XRD) and geochemical (e.g., K/Al) data, for four suites of samples: (i) Modern samples from lacustrine, estuarine, coastal plain and continental shelf settings in the USA; (ii) Eocene samples from the Bohai Bay Basin of NE China; (iii) Pennsylvanian samples from the Midcontinent and Illinois basins of the USA; and (iv) Devonian samples from the Illinois Basin. Our results show that B content covaries nearly ubiquitously with total clay content, an effect explicitly compensated for by normalization of B concentration data to Ga. A key finding of our study is that, after normalization for total clay content, B/Ga ratios show no systematic relationship to any clay-mineral component in either the whole-rock or the clay-fraction data. This is good news for general application of the B/Ga paleosalinity proxy in that it suggests that the clay-mineral assemblage generally does not exert a strong influence on sediment B/Ga ratios. However, it does not preclude the existence of such influence in individual formations, and clay-fraction mineralogical analysis is a recommended auxiliary tool in paleosalinity studies. It should also be noted that all of the present study units have experienced burial heating to no higher than the oil window (∼100–180 °C), and that the influence of clay-mineral assemblages on B/Ga ratios might be manifested in other formations of greater thermal maturity. These findings will prove valuable in guiding the interpretation of elemental salinity data in future paleoenvironmental studies.