No abstract

The elemental composition of river sediments is determined by the chemistry of the eroding substrate (e.g., bedrock and soils) and modified by processes including chemical weathering, cation exchange, and

The composition of continental crust records the balance between construction by tectonics and destruction by physical and chemical erosion. Quantitative constraints on how igneous addition and chemical weathering have modified the continents' bulk composition are essential for understanding the evolution of geodynamics and climate. Using novel data analytic techniques we have extracted temporal trends in sediments' protolith composition and weathering intensity from the largest available compilation of sedimentary major element compositions: ∼15,000 samples from 4.0 Ga to the present. We find that the average Archean upper continental crust was silica-rich and had a similar compositional diversity to modern continents. This is consistent with an early Archean, or earlier, onset of plate tectonics. In the Archean, chemical weathering sequestered ∼25 % more CO 2 per mass eroded for the same weathering intensity than in subsequent time periods, consistent with carbon mass balance despite higher Archean outgassing rates and more limited continental exposure. Since 2.0 Ga, over long (>0.5 Gyr) timescales, crustal weathering intensity has remained relatively constant. On shorter timescales over the Phanerozoic, weathering intensity is correlated to global climate state, consistent with a weathering feedback acting in response to changes in CO 2 sources or sinks.

Sediments contained in river channels are the products of physical erosion and chemical weathering of rocks outcropping in upstream catchments (e.g., Caracciolo, 2020;Weltje, 2012;Weltje & Eynatten, 2004). During transport, sedimentary geochemistry is altered by processes including chemical weathering (i.e., reaction of primary minerals with natural waters to form secondary minerals and solutes), sorting, cation-exchange, and selective transport/deposition (e.g., Bouchez et al., 2012;Tipper et al., 2021). As fluvial sediments can be transported on timescales of order 10 10 2 3  years, their geochemistry probably represents a spatial and temporal integration of catchment processes (Repasch et al., 2020). Consequently, they are frequently studied to understand the rates and location of chemical weathering, physical erosion and sediment transport (e.g.,

The elemental composition of a sediment is set by the composition of its protolith and modified by weathering, sorting, and diagenesis. An important problem is deconvolving these contributions to a sediment's composition to arrive at information about processes that operate on the Earth's surface. We approach this problem by developing a predictive and invertible model of sedimentary major element composition. We compile a data set of sedimentary rock, river sediment, soil, and igneous rock compositions. Principal component analysis of the data set shows that most variation can be simplified to a small number of variables. We thus show that any sediment's composition can be described with just two vectors of igneous evolution and weathering. We hence define a model for sedimentary composition as a combination of these processes. A 1:1 correspondence is observed between predictions and independent data. The log ratios ln(K 2 O∕MgO) and ln(Al 2 O 3 ∕Na 2 O) are found to be simple proxies for, respectively, the model's protolith and weathering indices. Significant deviations from the model can be explained by sodium-calcium exchange. Using this approach, we show that the major element composition of the upper continental crust has been modified by weathering, and we calculate the amount of each element that it must have lost to modify it to its present composition. By extrapolating modern weathering rates over the age of the crust, we conclude that it has not retained a significant amount of the necessarily produced weathering restite. This restite has likely been subducted into the mantle, indicating a crust-to-mantle recycling rate of 1.33 ± 0.89 ×10 13 kg•year −1 .