Dynamic redox and nutrient cycling response to climate forcing in the Mesoproterozoic ocean

Controls on Mesoproterozoic ocean redox heterogeneity, and links to nutrient cycling and oxygenation feedbacks, remain poorly resolved. Here, we report ocean redox and phosphorus cycling across two high-resolution sections from the ~1.4 Ga Xiamaling Formation, North China Craton. In the lower section, fluctuations in trade wind intensity regulated the spatial extent of a ferruginous oxygen minimum zone, promoting phosphorus drawdown and persistent oligotrophic conditions. In the upper section, high but variable continental chemical weathering rates led to periodic fluctuations between highly and weakly euxinic conditions, promoting phosphorus recycling and persistent eutrophication. Biogeochemical modeling demonstrates how changes in geographical location relative to global atmospheric circulation cells could have driven these temporal changes in regional ocean biogeochemistry. Our approach suggests that much of the ocean redox heterogeneity apparent in the Mesoproterozoic record can be explained by climate forcing at individual locations, rather than specific events or step-changes in global oceanic redox conditions

A tectonic-rules-based mantle reference frame since 1 billion years ago – implications for supercontinent cycles and plate–mantle system evolution

Understanding the long-term evolution of Earth's plate–mantle system is reliant on absolute plate motion models in a mantle reference frame, but such models are both difficult to construct and controversial. We present a tectonic-rules-based optimization approach to construct a plate motion model in a mantle reference frame covering the last billion years and use it as a constraint for mantle flow models. Our plate motion model results in net lithospheric rotation consistently below 0.25∘ Myr−1, in agreement with mantle flow models, while trench motions are confined to a relatively narrow range of −2 to +2 cm yr−1 since 320 Ma, during Pangea stability and dispersal. In contrast, the period from 600 to 320 Ma, nicknamed the “zippy tricentenary” here, displays twice the trench motion scatter compared to more recent times, reflecting a predominance of short and highly mobile subduction zones. Our model supports an orthoversion evolution from Rodinia to Pangea with Pangea offset approximately 90∘ eastwards relative to Rodinia – this is the opposite sense of motion compared to a previous orthoversion hypothesis based on paleomagnetic data. In our coupled plate–mantle model a broad network of basal mantle ridges forms between 1000 and 600 Ma, reflecting widely distributed subduction zones. Between 600 and 500 Ma a short-lived degree-2 basal mantle structure forms in response to a band of subduction zones confined to low latitudes, generating extensive antipodal lower mantle upwellings centred at the poles. Subsequently, the northern basal structure migrates southward and evolves into a Pacific-centred upwelling, while the southern structure is dissected by subducting slabs, disintegrating into a network of ridges between 500 and 400 Ma. From 400 to 200 Ma, a stable Pacific-centred degree-1 convective planform emerges. It lacks an antipodal counterpart due to the closure of the Iapetus and Rheic oceans between Laurussia and Gondwana as well as due to coeval subduction between Baltica and Laurentia and around Siberia, populating the mantle with slabs until 320 Ma when Pangea is assembled. A basal degree-2 structure forms subsequent to Pangea breakup, after the influence of previously subducted slabs in the African hemisphere on the lowermost mantle structure has faded away. This succession of mantle states is distinct from previously proposed mantle convection models. We show that the history of plume-related volcanism is consistent with deep plumes associated with evolving basal mantle structures. This Solid Earth Evolution Model for the last 1000 million years (SEEM1000) forms the foundation for a multitude of spatio-temporal data analysis approaches

Palaeolatitudinal distribution of the Ediacaran macrobiota

Macrofossils of the late Ediacaran Period (c. 579–539 Ma) document diverse, complex multicellular eukaryotes, including early animals, prior to the Cambrian radiation of metazoan phyla. To investigate the relationships between environmental perturbations, biotic responses and early metazoan evolutionary trajectories, it is vital to distinguish between evolutionary and ecological controls on the global distribution of Ediacaran macrofossils. The contributions of temporal, palaeoenvironmental and lithological factors in shaping the observed variations in assemblage taxonomic composition between Ediacaran macrofossil sites are widely discussed, but the role of palaeogeography remains ambiguous. Here we investigate the influence of palaeolatitude on the spatial distribution of Ediacaran macrobiota through the late Ediacaran Period using two leading palaeogeographical reconstructions. We find that overall generic diversity was distributed across all palaeolatitudes. Among specific groups, the distributions of candidate ‘Bilateral’ and Frondomorph taxa exhibit weakly statistically significant and statistically significant differences between low and high palaeolatitudes within our favoured palaeogeographical reconstruction, respectively, whereas Algal, Tubular, Soft-bodied and Biomineralizing taxa show no significant difference. The recognition of statistically significant palaeolatitudinal differences in the distribution of certain morphogroups highlights the importance of considering palaeolatitudinal influences when interrogating trends in Ediacaran taxon distributions.NERC DCO Richard Lounsbery Foundation MSCA-I

Rift and plate boundary evolution across two supercontinent cycles

The extent of continental rifts and subduction zones through deep geological time provides insights into the mechanisms behind supercontinent cycles and the long term evolution of the mantle. However, previous compilations have stopped short of mapping the locations of rifts and subduction zones continuously since the Neoproterozoic and within a self-consistent plate kinematic framework. Using recently published plate models with continuously closing boundaries for the Neoproterozoic and Phanerozoic, we estimate how rift and peri-continental subduction length vary from 1 Ga to present and test hypotheses pertaining to the supercontinent cycle and supercontinent breakup. We extract measures of continental perimeter-to-area ratio as a proxy for the existence of a supercontinent, where during times of supercontinent existence the perimeter-to-area ratio should be low, and during assembly and dispersal it should be high. The amalgamation of Gondwana is clearly represented by changes in the length of peri-continental subduction and the breakup of Rodinia and Pangea by changes in rift lengths. The assembly of Pangea is not clearly defined using plate boundary lengths, likely because its formation resulted from the collision of only two large continents. Instead the assembly of Gondwana (ca. 520 Ma) marks the most prominent change in arc length and perimeter-to-area ratio during the last billion years suggesting that Gondwana during the Early Palaeozoic could explicitly be considered part of a Phanerozoic supercontinent. Consequently, the traditional understanding of the supercontinent cycle, in terms of supercontinent existence for short periods of time before dispersal and re-accretion, may be inadequate to fully describe the cycle. Instead, either a two-stage supercontinent cycle could be a more appropriate concept, or alternatively the time period of 1 to 0 Ga has to be considered as being dominated by supercontinent existence, with brief periods of dispersal and amalgamation.Andrew S.Merdith, Simon E.Williams, Sascha Brune, Alan S.Collins, R. Dietmar Mülle

Global Hydrogen Production During High‐Pressure Serpentinization of Subducting Slabs

Abstract Serpentinization is among the most important, and ubiquitous, geological processes in crustal–upper mantle conditions (<6 GPa, <600°C), altering the rheology of rocks and producing H2 that can sustain life. While observations are available to quantify serpentinization in terrestrial and mid‐ocean ridge environments, measurements within subduction zone environments are far more sparse. To overcome this difficulty, we design a methodology to quantify and offer a first‐order estimate of the magnitude of “slab‐serpentinization” that has occurred over the last 5 Ma within the world's subduction zones by coupling four discrete tectonic and geophysical datasets—(a) raster grids of relic abyssal peridotite (peridotite exhumed from slow spreading mid‐ocean ridges but unaffected by pre‐subduction serpentinization) within ocean basins, (b) slab geometry, (c) thermal profiles and a (d) plate‐tectonic model. Averaged per year, our results suggest that 4.2–24 • 107 kg of H2 per annum could be generated from “slab‐serpentinization” within a subduction zone. Our estimate is 3–4 orders of magnitude lower than what is thought to be produced at mid‐ocean ridges, and 1–2 orders of magnitude lower than what could occur through serpentinization at trench flexure and when including possible mantle wedge serpentinization. Higher hydrogen production is correlated most strongly with the spreading history of ocean basins, underlaying the importance of the tectonic history of a slab prior to subduction

Duration of Sturtian "Snowball Earth" glaciation linked to exceptionally low mid-ocean ridge outgassing

The Sturtian “Snowball Earth” glaciation (ca. 717–661 Ma) is regarded as the most extreme interval of icehouse climate in Earth’s history. The exact trigger and sustention mechanisms for this long-lived global glaciation remain obscure. The most widely debated causes are silicate weathering of the ca. 718 Ma Franklin large igneous province (LIP) and changes in the length and degassing of continental arcs. A new generation of two independent Neoproterozoic full-plate tectonic models now allows us to quantify the role of tectonics in initiating and sustaining the Sturtian glaciation. We find that continental arc length remains relatively constant from 850 Ma until the end of the glaciation in both models and is unlikely to play a role. The two plate motion models diverge in their predictions of the timing and progression of Rodinia break-up, ocean-basin age, ocean-basement depth, sea-level evolution, and mid-ocean ridge (MOR) carbon outflux. One model predicts MOR outflux and ocean basin volume–driven sea level lower than during the Late Cenozoic glaciation, while the other predicts outgassing and sea level exceeding those of the Late Cretaceous hothouse climate. The second model would preclude a major glaciation, while the first model implies that the trigger for the Sturtian glaciation could have been a combination of an extremely low MOR outflux (∼9 Mt C/yr) and Franklin LIP weathering. Such minimal outflux could have maintained an icehouse state for 57 m.y. when silicate weathering was markedly reduced, with a gradual build-up of MOR CO2 in the atmosphere paired with terrestrial volcanism leading to its termination.Adriana Dutkiewicz, Andrew S. Merdith, Alan S. Collins, Ben Mather, Lauren Ilano, Sabin Zahirovic, and R. Dietmar Mülle

Rodinian devil in disguise: Correlation of 1.25-1.10 Ga strata between Tasmania and Grand Canyon

Locating the continuation of the ca. 1.30-1.00 Ga Grenville orogen on continents formerly adjacent to Laurentia is central to resolving the paleogeography of the supercontinent Rodinia. Here we emphasize a correlation of late Mesoproterozoic foreland basins that, prior to truncation by Neoproterozoic rift margins, may have extended west of Laurentia within Rodinia. We propose correlation of the Unkar Group (Grand Canyon, Arizona, USA) with the upper Rocky Cape Group (Tasmania, southeast Australia) based on their similar stratigraphy, 1.25-1.10 Ga depositional age, and detrital zircon U-Pb age distribution and Hf isotope composition. This correlation places Tasmania adjacent to southwest Laurentia in the late Mesoproterozoic, which supports a new paleogeographic model for Rodinia. In this model, Tasmania and crustal fragments of Laurentia comprising the South Tasman Rise and the Coats Land block form key links between the Grenville orogen in southwest Laurentia and the Maud orogen (East Antarctica). A 1.14-1.07 Ga connection between the combined Grenville-Maud orogen and the Musgrave orogen of central Australia is compatible with paleomagnetic data but requires ~4000 km of relative motion between Australia-Antarctica and Laurentia prior to the final assembly of Rodinia at ca. 0.90 Ga. We hypothesize that the final assembly of Rodinia was achieved by dextral motion between the crust of Australian and Laurentian affinity along a plate boundary concealed beneath ice cover in East Antarctica

Taking Be proud! Be responsible! to the Suburbs: A Replication Study

CONTEXT: An important phase of HIV prevention research is replicating successful interventions with different groups and in different settings. METHODS: Be Proud! Be Responsible!, a successful intervention originally targeting black urban males and carried out in nonschool settings, was presented in health classes at urban and suburban schools with diverse student bodies. A group-randomized intervention study, which included 1,357 ninth and 10th graders from 10 paired schools in a Midwestern metropolitan area, was conducted in 2000-2002. Half the schools received the intervention, and half received a general health promotion program. Students\u27 reports of their sexual behavior and selected cognitive mediators were analyzed immediately following the programs and four and 12 months later. RESULTS: Compared with students who received the control curriculum, students exposed to the intervention reported significantly greater knowledge of HIV, other STDs and condoms; greater confidence in their ability to control sexual impulses, to use condoms and to negotiate the use of condoms; and stronger intentions to use condoms. Stratified analyses revealed that the strongest intervention impacts were on knowledge and efficacy among males and students attending suburban schools. The intervention had no impact on sexual initiation, frequency of intercourse or condom use. CONCLUSIONS: Schools are a logical and viable setting for the dissemination and acquisition of information about HIV, including prevention strategies. However, the behavioral impact of an intervention may not be easily transferable when the program is taught outside a carefully controlled, nonschool setting

The extinction of land mammals during the future assembly of the next supercontinent

Mammals have dominated Earth for approximately 55 Myr thanks to their adaptations and resilience to warming and cooling during the Cenozoic. All life will eventually perish in a runaway greenhouse once absorbed solar radiation exceeds the emission of thermal radiation in several billions of years. However, conditions rendering the Earth naturally inhospitable to mammals may develop sooner because of long-term processes linked to plate tectonics (short-term perturbations are not considered here). In ~250 Myr, all continents will converge to form Earth’s next supercontinent, Pangea Ultima. A natural consequence of the creation and decay of Pangea Ultima will be extremes in due to changes in volcanic rifting and outgassing. Here we show that increased, solar energy (F⨀; approximately +2.5% W m−2 greater than today) and continentality (larger range in temperatures away from the ocean) lead to increasing warming hostile to mammalian life. We assess their impact on mammalian physiological limits (dry bulb, wet bulb and Humidex heat stress indicators) as well as a planetary habitability index. Given mammals’ continued survival, predicted background levels of 410–816 ppm combined with increased F⨀ will probably lead to a climate tipping point and their mass extinction. The results also highlight how global landmass configuration, and F⨀ play a critical role in planetary habitability