A sedimentary record of the evolution of the global marine phosphorus cycle.

Phosphorus (P) is typically considered to be the ultimate limiting nutrient for Earth's biosphere on geologic timescales. As P is monoisotopic, its sedimentary enrichment can provide some insights into how the marine P cycle has changed through time. A previous compilation of shale P enrichments argued for a significant change in P cycling during the Ediacaran Period (635-541 Ma). Here, using an updated P compilation-with more than twice the number of samples-we bolster the case that there was a significant transition in P cycling moving from the Precambrian into the Phanerozoic. However, our analysis suggests this state change may have occurred earlier than previously suggested. Specifically in the updated database, there is evidence for a transition ~35 million years before the onset of the Sturtian Snowball Earth glaciation in the Visingsö Group, potentially divorcing the climatic upheavals of the Neoproterozoic from changes in the Earth's P cycle. We attribute the transition in Earth's sedimentary P record to the onset of a more modern-like Earth system state characterized by less reducing marine conditions, higher marine P concentrations, and a greater predominance of eukaryotic organisms encompassing both primary producers and consumers. This view is consistent with organic biomarker evidence for a significant eukaryotic contribution to the preserved sedimentary organic matter in this succession and other contemporaneous Tonian marine sedimentary rocks. However, we stress that, even with an expanded dataset, we are likely far from pinpointing exactly when this transition occurred or whether Earth's history is characterized by a single or multiple transitions in the P cycle.

The tempo of Ediacaran evolution.

The rise of complex macroscopic life occurred during the Ediacaran Period, an interval that witnessed large-scale disturbances to biogeochemical systems. The current Ediacaran chronostratigraphic framework is of insufficient resolution to provide robust global correlation schemes or test hypotheses for the role of biogeochemical cycling in the evolution of complex life. Here, we present new radio-isotopic dates from Ediacaran strata that directly constrain key fossil assemblages and large-magnitude carbon cycle perturbations. These new dates and integrated global correlations demonstrate that late Ediacaran strata of South China are time transgressive and that the 575- to 550-Ma interval is marked by two large negative carbon isotope excursions: the Shuram and a younger one that ended ca. 550 Ma ago. These data calibrate the tempo of Ediacaran evolution characterized by intervals of tens of millions of years of increasing ecosystem complexity, interrupted by biological turnovers that coincide with large perturbations to the carbon cycle.

A long-term record of early to mid-Paleozoic marine redox change.

The extent to which Paleozoic oceans differed from Neoproterozoic oceans and the causal relationship between biological evolution and changing environmental conditions are heavily debated. Here, we report a nearly continuous record of seafloor redox change from the deep-water upper Cambrian to Middle Devonian Road River Group of Yukon, Canada. Bottom waters were largely anoxic in the Richardson trough during the entirety of Road River Group deposition, while independent evidence from iron speciation and Mo/U ratios show that the biogeochemical nature of anoxia changed through time. Both in Yukon and globally, Ordovician through Early Devonian anoxic waters were broadly ferruginous (nonsulfidic), with a transition toward more euxinic (sulfidic) conditions in the mid-Early Devonian (Pragian), coincident with the early diversification of vascular plants and disappearance of graptolites. This ~80-million-year interval of the Paleozoic characterized by widespread ferruginous bottom waters represents a persistence of Neoproterozoic-like marine redox conditions well into the Phanerozoic.

Calibrating the coevolution of Ediacaran life and environment.

The rise of animals occurred during an interval of Earth history that witnessed dynamic marine redox conditions, potentially rapid plate motions, and uniquely large perturbations to global biogeochemical cycles. The largest of these perturbations, the Shuram carbon isotope excursion, has been invoked as a driving mechanism for Ediacaran environmental change, possibly linked with evolutionary innovation or extinction. However, there are a number of controversies surrounding the Shuram, including its timing, duration, and role in the concomitant biological and biogeochemical upheavals. Here we present radioisotopic dates bracketing the Shuram on two separate paleocontinents; our results are consistent with a global and synchronous event between 574.0 ± 4.7 and 567.3 ± 3.0 Ma. These dates support the interpretation that the Shuram is a primary and synchronous event postdating the Gaskiers glaciation. In addition, our Re-Os ages suggest that the appearance of Ediacaran macrofossils in northwestern Canada is identical, within uncertainty, to similar macrofossils from the Conception Group of Newfoundland, highlighting the coeval appearance of macroscopic metazoans across two paleocontinents. Our temporal framework for the terminal Proterozoic is a critical step for testing hypotheses related to extreme carbon isotope excursions and their role in the evolution of complex life.

Monte Carlo sampling for error propagation in linear regression and applications in isochron geochronology.

Geochronology is essential for understanding Earth's history. The availability of precise and accurate isotopic data is increasing; hence it is crucial to develop transparent and accessible data reduction techniques and tools to transform raw mass spectrometry data into robust chronological data. Here we present a Monte Carlo sampling approach to fully propagate uncertainties from linear regressions for isochron dating. Our new approach makes no prior assumption about the causes of variability in the derived chronological results and propagates uncertainties from both experimental measurements (analytical uncertainties) and underlying assumptions (model uncertainties) into the final age determination. Using synthetic examples, we find that although the estimates of the slope and y-intercept (hence age and initial isotopic ratios) are comparable between the Monte Carlo method and the benchmark "Isoplot" algorithm, uncertainties from the later could be underestimated by up to 60%, which are likely due to an incomplete propagation of model uncertainties. An additional advantage of the new method is its ability to integrate with geological information to yield refined chronological constraints. The new method presented here is specifically designed to fully propagate errors in geochronological applications involves linear regressions such as Rb-Sr, Sm-Nd, Re-Os, Pt-Os, Lu-Hf, U-Pb (with discordant points), Pb-Pb and Ar-Ar.

Tracking the rise of eukaryotes to ecological dominance with zinc isotopes.

The biogeochemical cycling of zinc (Zn) is intimately coupled with organic carbon in the ocean. Based on an extensive new sedimentary Zn isotope record across Earth's history, we provide evidence for a fundamental shift in the marine Zn cycle ~800 million years ago. We discuss a wide range of potential drivers for this transition and propose that, within available constraints, a restructuring of marine ecosystems is the most parsimonious explanation for this shift. Using a global isotope mass balance approach, we show that a change in the organic Zn/C ratio is required to account for observed Zn isotope trends through time. Given the higher affinity of eukaryotes for Zn relative to prokaryotes, we suggest that a shift toward a more eukaryote-rich ecosystem could have provided a means of more efficiently sequestering organic-derived Zn. Despite the much earlier appearance of eukaryotes in the microfossil record (~1700 to 1600 million years ago), our data suggest a delayed rise to ecological prominence during the Neoproterozoic, consistent with the currently accepted organic biomarker records.

Controlled hydroxyapatite biomineralization in an ~810 million-year-old unicellular eukaryote.

Biomineralization marks one of the most significant evolutionary milestones among the Eukarya, but its roots in the fossil record remain obscure. We report crystallographic and geochemical evidence for controlled eukaryotic biomineralization in Neoproterozoic scale microfossils from the Fifteenmile Group of Yukon, Canada. High-resolution transmission electron microscopy reveals that the microfossils are constructed of a hierarchically organized interwoven network of fibrous hydroxyapatite crystals each elongated along the [001] direction, indicating biological control over microstructural crystallization. New Re-Os geochronological data from organic-rich shale directly below the fossil-bearing limestone constrain their age to <810.7 ± 6.3 million years ago. Mineralogical and geochemical variations from these sedimentary rocks indicate that dynamic global marine redox conditions, enhanced by local restriction, may have led to an increase in dissolved phosphate in pore and bottom waters of the Fifteenmile basin and facilitated the necessary geochemical conditions for the advent of calcium phosphate biomineralization.

Re-Os geochronology and coupled Os-Sr isotope constraints on the Sturtian snowball Earth.

After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitudes at least twice between 717 and 635 Mya. Although the initiation mechanism of these Neoproterozoic Snowball Earth events has remained a mystery, the broad synchronicity of rifting of the supercontinent Rodinia, the emplacement of large igneous provinces at low latitude, and the onset of the Sturtian glaciation has suggested a tectonic forcing. We present unique Re-Os geochronology and high-resolution Os and Sr isotope profiles bracketing Sturtian-age glacial deposits of the Rapitan Group in northwest Canada. Coupled with existing U-Pb dates, the postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological constraints for both the onset and demise of a Cryogenian glaciation from the same continental margin and suggests a 55-My duration of the Sturtian glacial epoch. The Os and Sr isotope data allow us to assess the relative weathering input of old radiogenic crust and more juvenile, mantle-derived substrate. The preglacial isotopic signals are consistent with an enhanced contribution of juvenile material to the oceans and glacial initiation through enhanced global weatherability. In contrast, postglacial strata feature radiogenic Os and Sr isotope compositions indicative of extensive glacial scouring of the continents and intense silicate weathering in a post-Snowball Earth hothouse.