Life: the first two billion years Knoll, Andrew H.; Bergmann, Kristin D.; Strauss, Justin V.
Philosophical transactions of the Royal Society of London. Series B. Biological sciences,
11/2016, Volume:
371, Issue:
1707
Journal Article
Peer reviewed
Open access
Microfossils, stromatolites, preserved lipids and biologically informative isotopic ratios provide a substantial record of bacterial diversity and biogeochemical cycles in Proterozoic (2500–541 Ma) ...oceans that can be interpreted, at least broadly, in terms of present-day organisms and metabolic processes. Archean (more than 2500 Ma) sedimentary rocks add at least a billion years to the recorded history of life, with sedimentological and biogeochemical evidence for life at 3500 Ma, and possibly earlier; phylogenetic and functional details, however, are limited. Geochemistry provides a major constraint on early evolution, indicating that the first bacteria were shaped by anoxic environments, with distinct patterns of major and micronutrient availability. Archean rocks appear to record the Earth's first iron age, with reduced Fe as the principal electron donor for photosynthesis, oxidized Fe the most abundant terminal electron acceptor for respiration, and Fe a key cofactor in proteins. With the permanent oxygenation of the atmosphere and surface ocean ca 2400 Ma, photic zone O2 limited the access of photosynthetic bacteria to electron donors other than water, while expanding the inventory of oxidants available for respiration and chemoautotrophy. Thus, halfway through Earth history, the microbial underpinnings of modern marine ecosystems began to take shape.
This article is part of the themed issue ‘The new bacteriology’.
Atmospheric CO2 levels and global climate are regulated on geological timescales by the silicate weathering feedback. However, this thermostat has failed multiple times in Earth's history, most ...spectacularly during the Cryogenian (c. 720–635 Ma) Snowball Earth episodes. The unique middle Neoproterozoic paleogeography of a rifting, low-latitude, supercontinent likely favored a globally cool climate due to the influence of the silicate weathering feedback and planetary albedo. Under these primed conditions, the emplacement and weathering of extensive continental flood basalt provinces may have provided the final trigger for runaway global glaciation. Weathering of continental flood basalts may have also contributed to the characteristically high carbon isotope ratios (δ13C) of Neoproterozoic seawater due to their elevated P contents. In order to test these hypotheses, we have compiled new and previously published Neoproterozoic Nd isotope data from mudstones in northern Rodinia (North America, Australia, Svalbard, and South China) and Sr isotope data from carbonate rocks. The Nd isotope data are used to model the mafic detrital input into sedimentary basins in northern Rodinia. The results reveal a dominant contribution from continental flood basalt weathering during the ca. 130 m.y. preceding the onset of Cryogenian glaciation, followed by a precipitous decline afterwards. These data are mirrored by the Sr isotope record, which reflects the importance of chemical weathering of continental flood basalts on solute fluxes to the early–middle Neoproterozoic ocean, including a pulse of unradiogenic Sr input into the oceans just prior to the onset of Cyrogenian glaciation. Hence, our new data support the hypotheses that elevated rates of flood basalt weathering contributed to both the high average δ13C of seawater in the Neoproterozoic and to the initiation of the first (Sturtian) Snowball Earth.
•Dominant contribution from flood basalt weathering preceding the onset of Cryogenian glaciation.•High rates of flood basalt weathering contributed to high average δ13C through P fertilization.•Weathering of flood basalt provinces provided the final trigger for Snowball glaciation.
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.
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.
Calibrating the Cryogenian Macdonald, Francis A; Schmitz, Mark D; Crowley, James L ...
Science (American Association for the Advancement of Science),
03/2010, Volume:
327, Issue:
5970
Journal Article
Peer reviewed
The Neoproterozoic was an era of great environmental and biological change, but a paucity of direct and precise age constraints on strata from this time has prevented the complete integration of ...these records. We present four high-precision U-Pb ages for Neoproterozoic rocks in northwestern Canada that constrain large perturbations in the carbon cycle, a major diversification and depletion in the microfossil record, and the onset of the Sturtian glaciation. A volcanic tuff interbedded with Sturtian glacial deposits, dated at 716.5 million years ago, is synchronous with the age of the Franklin large igneous province and paleomagnetic poles that pin Laurentia to an equatorial position. Ice was therefore grounded below sea level at very low paleolatitudes, which implies that the Sturtian glaciation was global in extent.
Neoproterozoic iron formation (NIF) provides evidence for the widespread return of anoxic and ferruginous basins during a time period associated with major changes in climate, tectonics and ...biogeochemistry of the oceans. Here we summarize the stratigraphic context of Neoproterozoic iron formation and its geographic and temporal distribution. It is evident that most NIF is associated with the earlier Cryogenian (Sturtian) glacial epoch. Although it is possible that some NIF may be Ediacaran, there is no incontrovertible evidence to support this age assignment. The paleogeographic distribution of NIF is consistent with anoxic and ferruginous conditions occurring in basins within Rodinia or in rift-basins developed on its margins. Consequently NIF does not require whole ocean anoxia. Simple calculations using modern day iron fluxes suggest that only models that invoke hydrothermal and/or detrital sources of iron are capable of supplying sufficient iron to account for the mass of the larger NIF occurrences. This conclusion is reinforced by the available geochemical data that imply NIF record is a mixture of hydrothermal and detrital components. A common thread that appears to link most if not all NIF is an association with mafic volcanics.
•Most Neoproterozoic iron formation is Cryogenian in age and associated with glaciation.•Neoproterozoic iron formation does not require whole ocean anoxia.•Mafic volcanism and glaciation tipped the balance towards ferruginous conditions.
Constraints on marine phosphate availability and cycling directly inform our understanding of long‐term biological evolution. However, early Earth phosphate records are sparse, biased toward ...siliciclastic samples, and susceptible to post‐depositional modification. Well‐preserved shallow marine inorganic carbonate precipitates provide a complementary yet understudied record of phosphate cycling. We combined micro‐X‐ray fluorescence mapping, X‐ray absorption, and Nuclear Magnetic Resonance spectroscopy on samples of Precambrian syndepositional herringbone calcite (HBC) and microspar to characterize phosphorus speciation and distribution in these carbonate fabrics. Phosphorus spectroscopy from synthetic calcite, HBC, and microspar, is qualitatively consistent with a disordered distribution of phosphate. These characteristics are diagnostic of calcite‐hosted phosphate, which is pervasive at low concentrations in HBC and microspar. This study provides evidence that ancient, well‐preserved carbonate fabrics retain phosphate sourced from seawater and highlights the potential for an unaltered archive of marine phosphate concentration over geologic time.
Plain Language Summary
Phosphorus is a critical nutrient to sustain life on Earth as we know it. Understanding the form and source of phosphorus on the early Earth will enhance our knowledge of the origins and early evolution of life. However, there is no way to directly measure the source, form and abundance of phosphorus in the environment when life was evolving over 2.5 billion years ago. The rock record provides a potential archive to indirectly measure phosphorus from ancient marine environments, yet there are a number of considerations, for example; (a) which minerals are able to record information about marine phosphorus? (b) How do we know those minerals have not been altered over time? In this work, we have identified well‐preserved, unaltered carbonate minerals from ∼2.5 billion and 800 million years ago, that preserve marine phosphate within their mineral structure using micron‐scale techniques. The characterization of this form of phosphorus is important to our understanding of the evolution of the phosphorus cycle over Earth's history.
Key Points
Synchrotron analyses identify calcite‐hosted phosphate in well‐preserved Precambrian carbonate fabrics
Herringbone calcite and synsedimentary microspar serve as an unaltered archive for ancient marine dissolved inorganic phosphate
Micrite retains abundant apatite mineral micro‐inclusions that may bias bulk calcite‐hosted phosphate or P/Ca records
A mechanistic understanding of relationships between global glaciation, a putative second rise in atmospheric oxygen, the Shuram carbon isotope excursion, and the appearance of Ediacaran-type fossil ...impressions and bioturbation is dependent on the construction of accurate geological records through regional stratigraphic correlations. Here we integrate chemo-, litho-, and sequence-stratigraphy of fossiliferous Ediacaran strata in northwestern Canada. These data demonstrate that the FAD of Ediacara-type fossil impressions in northwestern Canada occur within a lowstand systems tract and above a major sequence boundary in the informally named June beds, not in the early Ediacaran Sheepbed Formation from which they were previously reported. This distinction is substantiated by δ13Ccarb chemostratigraphy of the Sheepbed carbonate, which overlies the Sheepbed Formation, and the Gametrail Formation, which overlies the June beds. The Sheepbed carbonate hosts heavy δ13Ccarb values whereas the Gametrail Formation contains a large δ13Ccarb excursion, which we correlate with the globally recognized Shuram excursion. Stratigraphically above the Gametrail excursion, the first bilaterian burrows are present in the basal Blueflower Formation. Together, these data allow us to construct an age model for Ediacaran strata in northwestern Canada and conclude that a purported shift in Fe speciation in the Sheepbed Formation significantly predates the shift recorded above the ca. 582Ma Gaskiers glaciation in Newfoundland and the first appearance of Ediacaran biota.
The Gametrail excursion shares many characteristics with Shuram negative δ13Ccarb excursion: 1) δ13Ccarb and δ18Ocarb covary; 2) δ13Ccarb and δ13Corg do not covary; 3) the excursion is developed during a transgressive systems tract and recovers in an highstand systems tract; and 4) values in some sections are well below mantle δ13C input values but are variable between sections. We relate regional lateral variability in the magnitude and character of this excursion to condensation and diachronous deposition during the transgression and local authigenic carbonate production. In light of these observations, we explore a variety of models for the genesis of the Shuram excursion and suggest that the location and amount of authigenic carbonate production played a role in the excursion.
•The first appearance of Ediacaran fossils in NW Canada occurs above the Sheepbed Formation.•Fe-speciation shifts are not correlative with first appearance of Ediacaran fossils.•A Shuram-like carbon isotope excursion occurs above the first appearance of Ediacaran fossils.•The first bilaterian burrows are present directly above the Shuram-like excursion.•We suggest that Shuram-like excursions form in global authigenic events.
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