The composition and weathering of the continents over geologic time

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 CO2 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. Onshorter timescales over the Phanerozoic, weathering intensity is correlated to global climate state, consistent with a weathering feedback acting in response to changes in CO2 sources or sinks.AGL is funded by the Natural Environment Research Council Grantham Institute SSCP DTP (grant number NE/L002515/1). OS acknowledges support from NERC grants NE/T012455/1 and NE/T00696X/1. This work was supported by CAS

The composition and weathering of the continents over geologic time

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 CO2 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 CO2 sources or sinks

The Bowland Shale Formation in the Blacon Basin: syngenetic processes, stacking patterns and heat productivity

We conducted a high-resolution multi-disciplinary analysis of two core sections in the borehole Ellesmere Port-1, Cheshire, UK. Biostratigraphic analysis indicates the core sections are Kinderscoutian and late Arnsbergian-Chokerian in age, respectively. Both cores are assigned to the Bowland Shale Formation (Holywell Shale). Coupled core scan and discrete geochemical analysis enables interpretation of syngenetic processes at a high stratigraphic resolution. Both cores exhibit the classic cyclicity of limestones, calcareous to non-calcareous mudstones and siltstones, interpreted to represent sediment deposition during fourth-order sea level fluctuation. Machine learning of the well log data coupled to the core scan data enabled prediction of the key lithofacies through the entire Bowland Shale interval in Ellesmere Port-1. The machine predictions show the Bowland Shale is interfingered with three turbiditic leaves of the Cefn-y-fedw Sandstone Formation and contains at least 12 complete fourth-order cycles. The Bowland Shale exhibits high radiogenic heat productivity (RHP) in comparison to other sedimentary rocks, due primarily to relative enrichment in U under intermittently euxinic conditions. Thermal modelling, however, shows Bowland Shale RHP contributes a negligible source of additional heat at the scale of 100s m

Cross-basin Mo and U analysis of the Upper Mississippian Bowland Shale UK

The Bowland sub-basin is a target for hydrocarbon exploration but to a large extent it remains unexplored. To determine the economic potential of the Bowland sub-basin, it is important to identify the oceanographic processes involved in the deposition of the Bowland Shale Formation in the Late Mississippian (ca. 330 Ma). Palaeoceanographic processes are known to be a major control on the development of hydrocarbon source rocks. This study investigates core (Preese Hall-1 and Becconsall-1Z) materials from the Upper Bowland Shale, and compares to previously published data (outcrop Hind Clough), all from the Bowland sub-basin, Lancashire, UK. The sedimentology and geochemistry of this formation was determined via a multi-technique approach including x-ray fluorescence (XRF), sedimentology, gamma ray spectra, x-ray diffraction (XRD) and RockEval(6)TM pyrolysis. Key trace metal abundances and enrichment factors were used to assess sediment provenance and to determine the bottom water redox conditions during the deposition of the Upper Bowland Shale. Our results support interpretations that contemporaneous anoxia developed in bottom waters in at least three sites in the Bowland sub-basin. In a comparison with the Fort Worth Basin (Barnett Shale, USA), the Bowland sub-basin was apparently less restricted and deposited under a much higher mean sediment accumulation rate compared to the Fort Worth Basin. Knowledge from this study improves future resource estimates of the Bowland Shale Formation, and challenges the early assumptions that the Barnett Shale is an analogue to the Bowland Shale