Secular variation in the elemental composition of marine shales since 840 Ma: Tectonic and seawater influences

Marine shales, the most common lithology in the stratigraphic record, represent an archive of long-term changes in sediment production, weathering intensity, and seawater chemistry through Earth history whose potential has been underexploited to date. In this study, we compiled compositional data for 14,531 samples from 268 shale and mud units ranging in age from the early Cryogenian (840 Ma) to the Recent. We examined secular variation in the Al-normalized concentrations of ten elements (Mg, Na, K, Ba, Sr, Y, Th, Rb, Nb, and Zr) selected for (1) their presence in typical X-ray fluorescence (XRF) datasets (thus providing sufficient data), and (2) their lack of redox sensitivity (thus excluding elements whose primary control is aqueous redox conditions). Relationships among the study elements were quantified using principal component analysis (PCA) and cluster analysis (CA), revealing several significant groupings: (1) a subset of “detrital elements” (K, Rb, Th, Nb, Y, and Zr) whose concentrations are primarily controlled by subaerial weathering processes; (2) a subset of “hydrogenous elements” (Mg, Na, and Sr) whose concentrations are significantly influenced by seawater chemistry, and (3) Ba, which is subject to multiple controls. The dominant influence on the detrital elements appears to have been not chemical weathering intensity per se but the ratio of Al-poor silt to Al-rich clay in the particulate weathering flux. Although possibly modulated by factors such as climate and land plant evolution, silt:clay ratios appear most closely linked to tectonism, with higher values associated with major orogenic events. The concentrations of the hydrogenous elements in marine shales were likely related to their seawater inventories, which were modulated at long time scales by rates of submarine hydrothermal alteration. Ba in marine shales may have been significantly influenced by atmospheric pO2 and seawater sulfate levels via their control of barite saturation in seawater. In summary, our findings document significant relationships of marine shale composition to first-order geological controls, suggesting that long-term secular changes in sediment and seawater chemistry can be reconstructed from fine-grained siliciclastic sediments.