Stuart J. Daines

Ocean acidification and the Permo-Triassic mass extinction

Ocean acidification and mass extinction The largest mass extinction in Earths history occurred at the Permian-Triassic boundary 252 million years ago. Several ideas have been proposed for what devastated marine life, but scant direct evidence exists. Clarkson et al. measured boron isotopes across this period as a highly sensitive proxy for seawater pH. It appears that, although the oceans buffered the acidifiying effects of carbon release from contemporary pulses of volcanism, buffering failed when volcanism increased during the formation of the Siberian Traps. The result was a widespread drop in ocean pH and the elimination of shell-forming organisms. Science, this issue p. 229 A rapid injection of massive amounts of carbon into the atmosphere acidified the oceans, causing mass extinction. Ocean acidification triggered by Siberian Trap volcanism was a possible kill mechanism for the Permo-Triassic Boundary mass extinction, but direct evidence for an acidification event is lacking. We present a high-resolution seawater pH record across this interval, using boron isotope data combined with a quantitative modeling approach. In the latest Permian, increased ocean alkalinity primed the Earth system with a low level of atmospheric CO2 and a high ocean buffering capacity. The first phase of extinction was coincident with a slow injection of carbon into the atmosphere, and ocean pH remained stable. During the second extinction pulse, however, a rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota.

Earliest land plants created modern levels of atmospheric oxygen

Significance The rise of atmospheric oxygen over Earth’s history has received much recent interdisciplinary attention. However, the puzzle of when and how atmospheric oxygen reached modern levels remains unresolved. Many recent studies have argued for a major oxygenation event—of uncertain cause—in the Neoproterozoic Era >541 Ma, enabling the rise of animals. Previous modelling work has predicted a late Paleozoic oxygen rise (<380 Ma) due to the rise of forests. Here we show that neither scenario is correct. Instead, the earliest plants, which colonized the land from 470 Ma onward, first increased atmospheric oxygen to present levels by 400 Ma, and this instigated fire-mediated feedbacks that have stabilized high oxygen levels ever since, shaping subsequent evolution. The progressive oxygenation of the Earth’s atmosphere was pivotal to the evolution of life, but the puzzle of when and how atmospheric oxygen (O2) first approached modern levels (∼21%) remains unresolved. Redox proxy data indicate the deep oceans were oxygenated during 435–392 Ma, and the appearance of fossil charcoal indicates O2 >15–17% by 420–400 Ma. However, existing models have failed to predict oxygenation at this time. Here we show that the earliest plants, which colonized the land surface from ∼470 Ma onward, were responsible for this mid-Paleozoic oxygenation event, through greatly increasing global organic carbon burial—the net long-term source of O2. We use a trait-based ecophysiological model to predict that cryptogamic vegetation cover could have achieved ∼30% of today’s global terrestrial net primary productivity by ∼445 Ma. Data from modern bryophytes suggests this plentiful early plant material had a much higher molar C:P ratio (∼2,000) than marine biomass (∼100), such that a given weathering flux of phosphorus could support more organic carbon burial. Furthermore, recent experiments suggest that early plants selectively increased the flux of phosphorus (relative to alkalinity) weathered from rocks. Combining these effects in a model of long-term biogeochemical cycling, we reproduce a sustained +2‰ increase in the carbonate carbon isotope (δ13C) record by ∼445 Ma, and predict a corresponding rise in O2 to present levels by 420–400 Ma, consistent with geochemical data. This oxygen rise represents a permanent shift in regulatory regime to one where fire-mediated negative feedbacks stabilize high O2 levels.

Changing tectonic controls on the long‐term carbon cycle from Mesozoic to present

Tectonic drivers of degassing and weathering processes are key long-term controls on atmospheric CO2. However, there is considerable debate over the changing relative importance of different carbon sources and sinks. Existing geochemical models have tended to rely on indirect methods to derive tectonic drivers, such as inversion of the seawater 87Sr/86Sr curve to estimate uplift or continental basalt area. Here we use improving geologic data to update the representation of tectonic drivers in the COPSE biogeochemical model. The resulting model distinguishes CO2 sinks from terrestrial granite weathering, total basalt weathering, and seafloor alteration. It also distinguishes CO2 sources from subduction zone metamorphism and from igneous intrusions. We reconstruct terrestrial basaltic area from data on the extent of large igneous provinces and use their volume to estimate their contribution to degassing. We adopt a recently published reconstruction of subduction-related degassing, and relate seafloor weathering to ocean crust creation rate. Revised degassing alone tends to produce unrealistically high CO2, but this is counteracted by the inclusion of seafloor alteration and global basalt weathering, producing a good overall fit to Mesozoic-Cenozoic proxy CO2 estimates and a good fit to 87Sr/86Sr data. The model predicts that seafloor alteration and terrestrial weathering made similar contributions to CO2 removal through the Triassic and Jurassic, after which terrestrial weathering increased and seafloor weathering declined. We predict that basalts made a greater contribution to silicate weathering than granites through the Mesozoic, before the contribution of basalt weathering declined over the Cenozoic due to decreasing global basaltic area.

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.

Bridging the gap between omics and earth system science to better understand how environmental change impacts marine microbes

Abstract The advent of genomic‐, transcriptomic‐ and proteomic‐based approaches has revolutionized our ability to describe marine microbial communities, including biogeography, metabolic potential and diversity, mechanisms of adaptation, and phylogeny and evolutionary history. New interdisciplinary approaches are needed to move from this descriptive level to improved quantitative, process‐level understanding of the roles of marine microbes in biogeochemical cycles and of the impact of environmental change on the marine microbial ecosystem. Linking studies at levels from the genome to the organism, to ecological strategies and organism and ecosystem response, requires new modelling approaches. Key to this will be a fundamental shift in modelling scale that represents micro‐organisms from the level of their macromolecular components. This will enable contact with omics data sets and allow acclimation and adaptive response at the phenotype level (i.e. traits) to be simulated as a combination of fitness maximization and evolutionary constraints. This way forward will build on ecological approaches that identify key organism traits and systems biology approaches that integrate traditional physiological measurements with new insights from omics. It will rely on developing an improved understanding of ecophysiology to understand quantitatively environmental controls on microbial growth strategies. It will also incorporate results from experimental evolution studies in the representation of adaptation. The resulting ecosystem‐level models can then evaluate our level of understanding of controls on ecosystem structure and function, highlight major gaps in understanding and help prioritize areas for future research programs. Ultimately, this grand synthesis should improve predictive capability of the ecosystem response to multiple environmental drivers.

Biogeochemical Transformations in the History of the Ocean

The ocean has undergone several profound biogeochemical transformations in its 4-billion-year history, and these were an integral part of the coevolution of life and the planet. This review focuses on changes in ocean redox state as controlled by changes in biological activity, nutrient concentrations, and atmospheric O2. Motivated by disparate interpretations of available geochemical data, we aim to show how quantitative modeling-spanning microbial mats, shelf seas, and the open ocean-can help constrain past ocean biogeochemical redox states and show what caused transformations between them. We outline key controls on ocean redox structure and review pertinent proxies and their interpretation. We then apply this quantitative framework to three key questions: How did the origin of oxygenic photosynthesis transform ocean biogeochemistry? How did the Great Oxidation transform ocean biogeochemistry? And how was ocean biogeochemistry transformed in the Neoproterozoic-Paleozoic?

The effects of marine eukaryote evolution on phosphorus, carbon and oxygen cycling across the Proterozoic–Phanerozoic transition

A ‘Neoproterozoic oxygenation event’ is widely invoked as a causal factor in animal evolution, and often attributed to abiotic causes such as post-glacial pulses of phosphorus weathering. However, recent evidence suggests a series of transient ocean oxygenation events ∼660–520 Ma, which do not fit the simple model of a monotonic rise in atmospheric oxygen (pO2). Hence, we consider mechanisms by which the evolution of marine eukaryotes, coupled with biogeochemical and ecological feedbacks, potentially between alternate stable states, could have caused changes in ocean carbon cycling and redox state, phosphorus cycling and atmospheric pO2. We argue that the late Tonian ocean ∼750 Ma was dominated by rapid microbial cycling of dissolved organic matter (DOM) with elevated nutrient (P) levels due to inefficient removal of organic matter to sediments. We suggest the abrupt onset of the eukaryotic algal biomarker record ∼660–640 Ma was linked to an escalation of protozoan predation, which created a ‘biological pump’ of sinking particulate organic matter (POM). The resultant transfer of organic carbon (Corg) and phosphorus to sediments was strengthened by subsequent eukaryotic innovations, including the advent of sessile benthic animals and mobile burrowing animals. Thus, each phase of eukaryote evolution tended to lower P levels and oxygenate the ocean on ∼104 year timescales, but by decreasing Corg/P burial ratios, tended to lower atmospheric pO2 and deoxygenate the ocean again on ∼106 year timescales. This can help explain the transient nature and ∼106 year duration of oceanic oxygenation events through the Cryogenian–Ediacaran–Cambrian.

OMEN-SED 0.9: A novel, numerically efficient organic matter sediment diagenesis module for coupling to Earth system models

1. Comment: Model formulation The model assumes no overlap of mineralization reactions with different terminal electron acceptors, and assumes that secondary redox reactions can be collapsed onto the interfaces between different mineralization zones (p.9). This is probably ok in environments typically encountered at greater water depth, but there is ample evidence of ’overlapping’ mineralization pathways in surficial sediments, in particular in permeable or bioturbated settings.