The continental crust is central to the biological and geological history of Earth. However, crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks-volcanic and plutonic rocks-and the processes by which they differentiate to felsic compositions. Our analysis of a comprehensive global data set of volcanic and plutonic whole-rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction-zone settings, but are divergent in continental rifts. Offsets in major- and trace-element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics. In both tectonic settings, our results indicate that fractional crystallization, rather than crustal melting, is predominantly responsible for the production of intermediate and felsic magmas, emphasizing the role of mafic cumulates as a residue of crustal differentiation.
Atmospheric oxygen concentration has increased over Earth history, from ∼0 before 2.5 billion years ago to its present-day concentration of 21%. The initial rise in pO 2 approximately 2.3 billion years ago required oxygenic photosynthesis, but the evolution of this key metabolic pathway was not sufficient to propel atmospheric oxygen to modern levels, which were not sustained until approximately two billion years later. The protracted lag between the origin of oxygenic photosynthesis and abundant O 2 in the surface environment has many implications for the evolution of animals, but the reasons for the delay remain unknown. Here we show that the history of sediment accumulation on continental crust covaries with the history of atmospheric oxygen concentration. A forward model based on the empirical record of net organic carbon burial and oxidative weathering of the crust predicts two significant rises in pO 2 separated by three comparatively stable plateaus, a pattern that reproduces major biological transitions and proxy-based pO 2 records. These results suggest that the two-phased oxygenation of Earth's surface environment, and the long delays between the origin of life, the evolution of metazoans, and their subsequent diversification during the Cambrian Explosion, were caused by step-wise shifts in the ability of the continents to accumulate and store sedimentary organic carbon. The geodynamic mechanisms that promote and inhibit sediment accumulation on continental crust have, therefore, exerted a first-order control on the evolution of Earth's life and environment.
SignificanceIt has long been observed that the sequence of sedimentary rocks deposited in the past half-billion years often sharply overlies older igneous or metamorphic basement at an erosional surface known as the Great Unconformity. We provide evidence that this unconformity may record rapid erosion during Neoproterozoic “snowball Earth” glaciations. We show that the extent of Phanerozoic sedimentation in shallow continental seas can be accurately reproduced by modeling the accommodation space produced by the proposed glacial erosion, underlining the importance of glaciation as a means for lowering erosional base level. These results provide constraints on the sedimentary and geochemical environment in which the first multicellular animals evolved and diversified in the “Cambrian explosion” following the unconformity.
Sedimentary rocks are often described as declining in quantity with increasing age due to the cumulative effects of crustal deformation and erosion. One important implication of such a model is that the geological record becomes progressively less voluminous and less complete with increasing age. Here we show that the predictions of a model in which the destruction of sedimentary rock is the predominant process signal are borne out only among sediments deposited on oceanic crust and among sediments deposited above sea level in non-marine environments. Most of the surviving volume of sedimentary rock (~75%) was deposited in and adjacent to shallow seas on continental crust and does not exhibit any steady decrease in quantity with increasing age. Instead, shallow marine sediments exhibit large fluctuations in quantity that were driven by shifting global tectonic boundary conditions, such as those that occur during the breakup and coalescence of supercontinents. The accumulation of sediments on the continents has not been uniform in rate, but it does record a primary signal of net growth that has many implications for the long-term evolution of Earth's surface environment.
Characterizing the lithology, age, and physical-chemical properties of rocks and sediments in the Earth's upper crust is necessary to fully assess energy, water, and mineral resources and to address many fundamental questions in the Earth sciences. Although a large number of geological maps, regional geological syntheses, and sample-based measurements have been produced, there is no openly available database that integrates rock record-derived data while facilitating large-scale, quantitative characterization of the volume, age, and material properties of the upper crust. Here we describe Macrostrat, a relational geospatial database and supporting cyberinfrastructure that is designed to enable quantitative spatial and geochronological analyses of the entire assemblage of surface and subsurface sedimentary, igneous and metamorphic rocks. Macrostrat now contains general, comprehensive summaries of the age and properties of 33,903 lithologically and chronologically-defined geological units distributed across 1,474 regions in North and South America, the Caribbean, New Zealand, and the deep sea. Sample-derived data, including fossil occurrences in the Paleobiology Database, more than 180,000 geochemical and outcrop-derived measurements, and more than 2.1 million bedrock geologic map units from over 170 map sources, are linked to specific Macrostrat units and/or lithologies. Macrostrat has generated numerous quantitative results and its data infrastructure is being used in several independently developed mobile applications. However, it is necessary to expand geographic coverage and to refine age models and material properties to arrive at a more precise characterization of the upper crust globally.
Stromatolites are abundant in shallow marine sediments deposited before the evolution of animals, but in the modern ocean they are restricted to locations where the activity of animals is limited. Overall decline in the abundance of stromatolites has, therefore, been attributed to the evolution of substrate-modifying metazoans, with Phanerozoic stromatolite resurgences attributed to the aftermaths of mass extinctions. Here we use a comprehensive stratigraphic database, the published literature, and a machine reading system to show that the rock record-normalized occurrence of stromatolites in marine environments in North America exhibits three phases: an initial Paleoproterozoic (ca. 2500 Ma) increase, a sustained interval of dominance during the Proterozoic (2500-800 Ma), and a late Neoproterozoic (700-541 Ma) decline to lower mean prevalence during the Phanerozoic (541-0 Ma). Stromatolites continued to exhibit large changes in prevalence after the evolution of metazoans, and they transiently achieved Proterozoic-like prevalence during the Paleozoic. The aftermaths of major mass extinctions are not well correlated with stromatolite resurgence. Instead, stromatolite occurrence is well predicted by the prevalence of dolomite, a shift in carbonate mineralogy that is sensitive to changes in water-column and pore-water chemistry occurring during continent-scale marine transgressive-regressive cycles.
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