Biogeochemistry: Early phosphorus redigested

Atmospheric oxygen was maintained at low levels throughout huge swathes of Earth's early history. Estimates of phosphorus availability through time suggest that scavenging from anoxic, iron-rich oceans stabilized this low-oxygen world

Model based Paleozoic atmospheric oxygen estimates: a revisit to GEOCARBSULF

Geological redox proxies increasingly point towards low atmospheric oxygen concentrations during the early Paleozoic Era, with a subsequent protracted rise towards present-day levels. However, these proxies currently only provide qualitative estimates of atmospheric O₂ levels. Global biogeochemical models, in contrast, are commonly employed to generate quantitative estimates for atmospheric O₂ levels through Earth’s history. Estimates for Paleozoic pO₂ generated by GEOCARBSULF, one of the most widely implemented carbon and sulfur cycle models, have historically suggested high atmospheric O₂ levels throughout the Paleozoic, in direct contradiction to competing models. In this study, we evaluate whether GEOCARBSULF can predict relatively low Paleozoic O₂ levels. We first update GEOCARBSULF by adopting the recent compilation of the δ¹³C value of marine buried carbonate and replacing the old formulation of the sulfur isotope fractionation factor with empirical sulfur isotope records. Following this we construct various O₂ evolution scenarios (with low O₂ levels in the early Paleozoic) and examine whether GEOCARBSULF can reproduce these scenarios by varying the weathering/degassing fluxes of carbon and sulfur, or carbonate δ¹³C. We show that GEOCARBSULF can, in fact, maintain low-O₂ (even 1–5% atm) levels through the early Paleozoic by only varying the carbonate δ¹³C within 2 standard deviation (SD) bounds permitted by the geological record. In addition, it can generate a middle–late Paleozoic rise in O₂ concentration, coincident with the diversification of land plants. However, we also argue that tracking atmospheric O₂ levels with GEOCARBSULF is highly dependent on carbonate carbon isotope evolution, and more accurate predictions will come from an improved C isotope record

Chromium isotopes in marine hydrothermal sediments

Hydrothermal chromium (Cr) cycling contributes to marine Cr inventories and their Cr isotopic composition, yet Cr isotope effects associated with this cycling remain poorly documented. Here we determine the distribution, isotopic composition, and diagenetic mobility of Cr in hydrothermal sediments from the distal flank of the South East Pacific Rise (SEPR, DSDP-site 598). We find that Cr is primarily associated with the metalliferous iron (oxyhydr) oxide and detrital components of the sediment (0.4–3.6 mg kg⁻¹), whereas Cr concentrations are much lower in the dominant carbonate phase (80% Cr from the sediment relative to Fe. We propose this loss is tied to oxidation of authigenic Cr(III) to Cr(VI) followed by diagenetic remobilization and efflux from the sediment pile. The bulk δ⁵³Cr composition of the SEPR sediments is isotopically light (−0.24 to −0.57 ± 0.05‰) and the authigenic δ⁵³Cr is as light as −1.2 ± 0.2‰, and we argue that this light Cr isotopic composition results from the partial reduction of oxic seawater-bearing Cr(VI) by reduced hydrothermal vent fluids enriched in Fe(II)aq. Diagenetic oxidation of the reactive Cr pool by Mn-oxides and loss of Cr(VI) from the sediment may further deplete the sediment in ⁵³Cr during diagenesis. The δ⁵³Cr composition of the detrital Cr fraction of the sediment (average δ⁵³Cr composition = −0.05 ± 0.04‰) falls within the igneous silicate earth (ISE) range, revealing that detrital Cr delivered to this region of the Pacific ocean is unfractionated, and has carried a relatively constant δ⁵³Cr composition over the last 5.7 million years. Together our results show that light δ⁵³Cr compositions in hydrothermal sediments are imparted through a combination of processes previously overlooked in the marine Cr biogeochemical cycle, and that the δ⁵³Cr composition of such sediments may provide a rich source of information on paleo-marine redox conditions

Evolution of the structure and impact of Earth’s biosphere

Life on Earth has existed for over 3.5 billion years and has caused fundamental changes in Earth’s biogeochemistry. However, the timing and impact of major events in the evolution of the biosphere are hotly contested, owing partially to the inherent difficulty in studying events that occurred in deep time. In this Review, we discuss the evolving structure of Earth’s biosphere and major changes in its capacity to alter geochemical cycles. We describe evidence that oxygenic photosynthesis evolved relatively early, but contend that marine primary productivity was low, surface oxygen was scarce and marine anoxia was prevalent for the majority of Earth’s history. Anoxygenic phototrophs were likely a key part of the marine biosphere in these low-oxygen oceans, and nutrient uptake by these organisms was one factor limiting the extent of marine oxygenic photosynthesis. Moreover, there are potential issues with the commonly held idea that the diversification of eukaryotes fundamentally altered ocean nutrient cycling and transformed the marine biological pump. Furthermore, we argue that terrestrial primary productivity was a substantial mode of biological carbon fixation following the widespread emergence of continental land masses, even before the rise of land plants, impacting carbon cycling on a global scale

Highly dynamic marine redox state through the Cambrian explosion highlighted by authigenic δ²³⁸U records

The history of oceanic oxygenation from the late Neoproterozoic to the early Cambrian is currently debated, making it difficult to gauge whether, and to what extent environmental triggers played a role in shaping the trajectory of metazoan diversification. Uranium isotope (U) records from carbonates have recently been used to argue for significant swings in the global marine redox state from the late Neoproterozoic to the early Cambrian. However, geochemical signatures in carbonates—the U isotope archive most commonly employed to argue for redox shifts—are susceptible to diagenetic alteration and may have variable offsets from seawater values. Therefore, there is an impetus to reconstruct seawater U isotopic evolution using another sedimentary archive, in order to verify that these excursions can indeed be linked to global shifts in marine redox landscape. Here we report new U isotope data from two fine-grained siliciclastic upper Ediacaran to lower Cambrian (ca. 551–515 Ma) successions in South China. We find large δ²³⁸U swings between -0.63‰ and +0.39‰ for calculated values of authigenic U in the siliciclastic rocks, consistent with correlative records from the carbonates. The replication of these patterns in both carbonate and siliciclastic units provides confirmatory evidence that the early Cambrian seawater was characterized by highly variable U isotope compositions. These new δ²³⁸U data also provide higher-resolution records of global oceanic redox conditions during Cambrian Age 3, coeval with a critical interval of the Cambrian explosion. These δ²³⁸U data bolster the case that the Ediacaran-Cambrian transition experienced massive swings in marine redox state, providing a dynamic environmental backdrop for and potentially even a key driver of the emergence and radiation of metazoans

Atmospheric oxygen regulation at low Proterozoic levels by incomplete oxidative weathering of sedimentary organic carbon

It is unclear why atmospheric oxygen remained trapped at low levels for more than 1.5 billion years following the Paleoproterozoic Great Oxidation Event. Here, we use models for erosion, weathering and biogeochemical cycling to show that this can be explained by the tectonic recycling of previously accumulated sedimentary organic carbon, combined with the oxygen sensitivity of oxidative weathering. Our results indicate a strong negative feedback regime when atmospheric oxygen concentration is of order pO2∼0.1 PAL (present atmospheric level), but that stability is lost at pO2<0.01 PAL. Within these limits, the carbonate carbon isotope (δ13C) record becomes insensitive to changes in organic carbon burial rate, due to counterbalancing changes in the weathering of isotopically light organic carbon. This can explain the lack of secular trend in the Precambrian δ13C record, and reopens the possibility that increased biological productivity and resultant organic carbon burial drove the Great Oxidation Event

A lithium-isotope perspective on the evolution of carbon and silicon cycles

The evolution of the global carbon and silicon cycles is thought to have contributed to the long-term stability of Earth's climate. Many questions remain, however, regarding the feedback mechanisms at play, and there are limited quantitative constraints on the sources and sinks of these elements in Earth's surface environments. Here we argue that the lithium-isotope record can be used to track the processes controlling the long-term carbon and silicon cycles. By analysing more than 600 shallow-water marine carbonate samples from more than 100 stratigraphic units, we construct a new carbonate-based lithium-isotope record spanning the past 3 billion years. The data suggest an increase in the carbonate lithium-isotope values over time, which we propose was driven by long-term changes in the lithium-isotopic conditions of sea water, rather than by changes in the sedimentary alterations of older samples. Using a mass-balance modelling approach, we propose that the observed trend in lithium-isotope values reflects a transition from Precambrian carbon and silicon cycles to those characteristic of the modern. We speculate that this transition was linked to a gradual shift to a biologically controlled marine silicon cycle and the evolutionary radiation of land plants

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 Ga) 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

Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event

The early Earth was characterized by the absence of oxygen in the ocean–atmosphere system, in contrast to the well-oxygenated conditions that prevail today. Atmospheric concentrations first rose to appreciable levels during the Great Oxidation Event, roughly 2.5–2.3 Gyr ago. The evolution of oxygenic photosynthesis is generally accepted to have been the ultimate cause of this rise, but it has proved difficult to constrain the timing of this evolutionary innovation. The oxidation of manganese in the water column requires substantial free oxygen concentrations, and thus any indication that Mn oxides were present in ancient environments would imply that oxygenic photosynthesis was ongoing. Mn oxides are not commonly preserved in ancient rocks, but there is a large fractionation of molybdenum isotopes associated with the sorption of Mo onto the Mn oxides that would be retained. Here we report Mo isotopes from rocks of the Sinqeni Formation, Pongola Supergroup, South Africa. These rocks formed no less than 2.95 Gyr ago in a nearshore setting. The Mo isotopic signature is consistent with interaction with Mn oxides. We therefore infer that oxygen produced through oxygenic photosynthesis began to accumulate in shallow marine settings at least half a billion years before the accumulation of significant levels of atmospheric oxygen

Low-oxygen waters limited habitable space for early animals

The oceans at the start of the Neoproterozoic Era (1,000–541 million years ago, Ma) were dominantly anoxic, but may have become progressively oxygenated, coincident with the rise of animal life. However, the control that oxygen exerted on the development of early animal ecosystems remains unclear, as previous research has focussed on the identification of fully anoxic or oxic conditions, rather than intermediate redox levels. Here we report anomalous cerium enrichments preserved in carbonate rocks across bathymetric basin transects from nine localities of the Nama Group, Namibia (~550–541 Ma). In combination with Fe-based redox proxies, these data suggest that low-oxygen conditions occurred in a narrow zone between well-oxygenated surface waters and fully anoxic deep waters. Although abundant in well-oxygenated environments, early skeletal animals did not occupy oxygen impoverished regions of the shelf, demonstrating that oxygen availability (probably >10 μM) was a key requirement for the development of early animal-based ecosystems