Significance: Seawater elemental composition, namely Mg:Ca and Sr:Ca ratios are widely used in marine sciences. Our new single-laboratory global dataset, combined with past data of seawater Mg:Ca and Sr:Ca ratios, suggests that modern ocean variability is significant across different environments, being similar to the changes during the Neogene Period (20 Ma). Because there is large variability in seawater ratios, we cannot assume that secular changes derived from fossilized taxa with varied ecological niches and from diverse ecosystems provide an unbiased representation of past global seawater chemistry. We suggest that the problem of inconsistencies in previous studies of, for example, paleo conditions from different fossil taxa may be reduced by accounting for taxonomic differences, ecosystems, and abiotic processes to overcome the regional environmental effect.

Rice hull ash (RHA) is an effective and low-cost adsorbent for water purification. In this study, we systematically investigated the effects of ashing temperature and atmosphere on the physic-chemical characteristics of RHA and its adsorption for methylene blue (MB). XRD, SEM, and BET analyses indicate that all RHA samples are porous materials that consist of carbon and amorphous silica. RHA ashed in nitrogen (BRHA) exhibit higher BET surface areas and lower mesopore fractions than that of ashed in air (WRHA) due to the retention of carbon. The pore volumes and surface area of BRHA increase with ashing temperature, while that of WRHA exhibit a first increase and subsequently decrease. The adsorption kinetics of MB adsorption by RHA fit pseudo-second-order model. The calculated q _e values exhibit linear relationships with the mesopore volumes of RHA. The adsorption isotherms fit the Langmuir isotherm rather than the Freundlich equation, indicating the monolayer adsorption of MB. WRHA that ashed at 450°C and BRHA that ashed at 750°C present the highest adsorption capacities for MB with q ₀ values of greater than 45 mg g⁻¹. This systematic study will provide essential information on the effects of ashing conditions on the physico-chemical characteristics and dye removal of RHA.