The Ordovician Period records an extraordinary biodiversity increase known as the Great Ordovician Biodiversification Event (GOBE), which coincided with a series of environmental changes to the Earth ...System, notably a cooling global ocean, increased oxygenation, and increased nutrient supply from volcanism and continental weathering. The co-evolution of Earth and its biota during this interval has been studied in various contexts on multiple paleocontinents. Emerging patterns depend on the lens of investigation. Here we summarize the current state of understanding by reviewing and synthesizing the fossil and sedimentary records.
Recent paleontological studies, mainly focused on rhynchonelliform (articulated) brachiopods, bryozoa, cephalopods, trilobites, graptolites, echinoderms, and reef organisms, have documented details of diversification, body size increase, development of ecosystem complexity, and intensification of inter-continental dispersal from the late Cambrian through Late Ordovician. Biomass increased markedly between the Early and Middle Ordovician. Furthermore, diversification rates increase statistically during the mid-Darriwilian Age both globally and regionally.
Coincident with these biotic changes, geochemical proxies record significant changes to Earth's physical system. Oceanic temperatures decreased, and atmospheric oxygen levels increased to near modern levels in concert with the Middle Ordovician diversification of shelly fauna. Anoxic pulses ceased and evidence for deep ocean ventilation prevails in Middle Ordovician strata. Furthermore, a major Middle–Late Ordovician change in oceanic strontium isotopic composition indicates increased weathering of juvenile volcanic rocks and delivery of nutrients to marine settings. This multi-proxy dataset records near-simultaneous changes in fossil-rich shallow marine environments during exactly the interval of greatest diversification.
By integrating biotic and geochemical datasets, a clear picture of the co-evolution of Earth and its biota emerges indicating that the Darriwilian was the critical interval facilitating increased capacity of ecosystems. We, therefore, recommend restricting the term “GOBE” to indicate this short interval of rapid diversification and ecosystem change, and using “Ordovician Radiation” when referencing the sum of diversifications that occurred throughout the Ordovician Period.
•Coordinated physical and biological changes to the Earth System led to the GOBE.•Ecosystem complexity and diversity increased statistically during the Darriwilian.•Ocean ventilation, cooling, and tectonics were necessary preconditions of the GOBE.•Ocean cooling and circulation changes sparked the GOBE.•The GOBE is primarily Darriwilian, facilitated by simultaneous biotic/abiotic events.
Paired measurements of bulk carbonate (δ13Ccarb), organic matter (δ13Corg), and their difference (Δ13C) can be used to estimate changes in isotopic fractionation through time as a function of O2/CO2 ...in the atmosphere. However, because local scale processes can also affect Δ13C, it is essential to compare sections from widely separated water masses. Here we present new δ13Corg data from Ordovician carbonate rocks from the Great Basin, Oklahoma, and Appalachian Basin and compare with published δ13Ccarb records from these sections and paired δ13C values from other carbonate successions around North America. These new data complement previous studies that focused on Upper Ordovician δ13Ccarb excursions and now provide a composite Ordovician δ13Corg record. New Lower Ordovician (Tremadocian Stage) δ13Corg data range from ca. −26 to −28‰, decreasing throughout the Lower–Middle Ordovician (Floian–Dapingian Stages) to ca. −29 to −31‰. δ13Corg values remain at their lowest throughout the Sandbian and are similar to other published Upper Ordovician (Sandbian–Katian) δ13Corg data from North America. Δ13C values from well-preserved intervals generally vary between +26 to +28‰ throughout the Lower to Middle Ordovician (Tremadocian to early Darriwilian), but increase to +31‰ during the mid–late Darriwilian and mid Sandbian, similar to published data from younger Late Ordovician positive δ13C excursions known as the Guttenberg (GICE) and Hirnantian (HICE) events. The overall Δ13C trend shows a ~3‰ increase throughout the Early–Middle Ordovician and coincides with a previously interpreted period of ocean cooling and some of the earliest pulses of global biodiversity of marine invertebrates and planktonic organisms. Modeling studies predict that pCO2 decreased during this time, suggesting that the effect of pCO2 on Δ13C may have been overwhelmed by other controls, such as an in increase in pO2 or a higher O2/CO2 ratio during this biodiversification event.
•New Early–Middle Ordovician paired carbon isotopic analyses are reported here.•New organic isotope data combined with published data make an Ordovician composite.•A 3‰ increase in Δ13C coincides with the onset of the GOBE and possible ocean cooling.•Cooling or pCO2 changes cannot explain the entire Δ13C increase.•Increased pO2 or atmospheric O2/CO2 ratios may explain the long-term Δ13C increase.
Phosphorus is a nutrient fundamental to life and when it precipitates in modern environments bacteria are intimately involved in its release, concentration, and mineralization. Preserved fossil ...bacteria in phosphate crusts and grains from the ca. 1850 million-year-old Bijiki Iron Formation Member of the Michigamme Formation, Michigan provide insight into the longevity and nature of this relationship. The Michigamme Formation accumulated near the end of the Earth's initial phosphogenic episode (ca. 2.2 and 1.8Ga) to produce one of the first granular phosphorites. Phosphatic lithofacies consist of fine- to medium-sand-sized francolite peloids concentrated on bedding surfaces in peritidal facies. Granular beds are up to 2cm thick and peloids are often partially to completely replaced by dolomite and chert. The grains contain organic matter and pyrite framboids that suggest bacterial breakdown of organic matter and bacterial sulfate reduction.
The peritidal nature of phosphorite in the Michigamme Formation is in sharp contrast to Phanerozoic phosphogenic environments in deeper coastal upwelling settings. Peritidal settings were well suited for phosphogenesis under the very low oxygen and low dissolved sulfate levels of the Paleoproterozoic as cyanobacteria produced oxygen in shallow water and evaporation led to increased sulfate concentrations. Such concomitant processes helped establish focused redox interfaces in the sediment that chemosynthetic bacterial communities (sulfur oxidizers, reducers, forms that concentrate P, and possibly iron oxidizers) could exploit. Phosphate released from organic matter by heterotrophic bacteria and Fe-redox pumping was further concentrated by these chemotrophs; a process that forms late Neoproterozoic to Phanerozoic phosphorites but on a much larger scale.
This early example of a granular phosphorite demonstrates that, like their Phanerozoic counterparts, Paleoproterozoic phosphorites are the concentrated indirectly biomineralized products of bacterial communities. But unlike younger analogs, which accumulated across subtidal shelves and shelf margins, these ancient deposits formed only in tidal flat settings where phosphogenic redox processes could be established in the sediment. From this early beginning, the zone of phosphogenesis likely migrated into deeper water settings as oxygen and sulfate levels rose, expanding the zone of chemosynthetic bacterial and associated phosphogenesis across the shelf.
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•Chemosynthetic organisms caused phosphate mineralization.•Chemosynthetic bacteria create and exploit redox conditions in near-shore settings.•Phosphate mineralization began in Paleoproterozoic peritidal settings.•Post-middle Neoproterozoic this microbial-sedimentary system shifted to deep water.•Francolite is a signpost for early life in Proterozoic marine sedimentary rocks.
Middle–Late Ordovician sequences from the Appalachian Basin and Arbuckle Mountain regions of North America were analyzed for carbonate-associated sulfate (δ34SCAS) and pyrite (δ34Spyr) paired with ...carbonate (δ13Ccarb) and organic matter (δ13Corg) chemostratigraphy. Two major negative drops in δ34SCAS (12‰ excursions) are recognized: the older decline in δ34SCAS occurs within the Histiodella holodentata–Phragmodus polonicus Conodont Zones and the younger drop is within the Cahabagnathus sweeti–Amorphognathus tvaerensis (Baltoniodus gerdae subzone) Zones. These overall these negative shifts in δ34SCAS have an antithetical relationship with positive shifts in δ34Spyr (~+10‰) and δ13Ccarb (~+2‰) recorded in the same successions. The older negative δ34SCAS shift is coincident with the widely documented mid-Darriwilian δ13C excursion (MDICE), and the younger negative δ34SCAS shift is coincident with another positive δ13Ccarb shift in the early Sandbian. Geochemical modeling of these sulfur isotope shifts suggests that a decrease in the global rate of pyrite burial or isotope fractionation between seawater sulfate and sedimentary pyrite could account for these negative δ34SCAS trends. Additionally, a substantial increase in the weathering flux of pyrite to the global oceans could also explain these secular sulfur isotope trends. While increased crustal weathering is broadly consistent with a sea-level lowstand, and the seawater 87Sr/86Sr isotope record of change in continental weathering in the late Darriwilian Stage of the Ordovician, geologic and geochemical proxy evidence do not support distinct pulses of continental weathering required to generate two separate negative shifts in δ34SCAS. These antithetical isotope trends may be best explained by changes in the marine redox state that significantly reduced microbially mediated pyrite burial and organic matter remineralization rates. Pulses of oceanic ventilation would have expanded habitable environments for marine organisms, and thus is broadly consistent with major increases in biodiversification during this period of the Ordovician.
•Newly paired sulfur and carbon isotope data of Ordovician carbonates from Laurentia•Two large drops in δ34SCAS correspond to positive shifts in δ13Ccarb values.•First isotopic evidence for Ordovician decoupling of carbon and sulfur cycles•These data support intervals of oceanic ventilation in the Middle–Late Ordovician.•Oceanic oxygenation provides bolstering conditions for coincident GOBE pulses.
The Great Ordovician Biodiversification Event (GOBE) represents a four-fold increase of genus-level diversity that post-dates the diversification of the ‘Cambrian explosion’ by 40–50 Myr. A major ...increase in atmospheric oxygen (O2) levels is thought to be a leading cause of the ‘Cambrian explosion’ that lowered metabolic costs associated with skeletal and collagen biomineralization. The cause(s) of the GOBE, however, is (are) less well understood, and may include a cooling climate, increased nutrient availability, higher sea levels and increased ecospace, and further oxygenation of shallow marine environments. Atmospheric O2 levels are difficult to quantitatively estimate, particularly whether oxygenation was a plausible driver of the GOBE, because O2 estimates are hampered in part by the coarse time resolution of redox proxy records (e.g., iron-based geochemical data) and isotope mass balance models. Newly published high-resolution geochemical trends are used to better constrain the timing and degree of oxygenation across the GOBE, which include traditional methods such as stable carbon (δ13C) and sulfur (δ34S) isotope trends to estimate O2 levels on a global scale, and local-scale redox-sensitive proxies such as iodine (I/Ca) and trace metal concentrations and isotopes (e.g., Mo and U). Taken together, geochemical evidence suggests that Ordovician environments became progressively more oxygenated following the end of recurrent anoxic events and extinctions of late Cambrian and Early Ordovician marine faunas. A sluggishly circulating Cambrian global ocean could have maintained an oxycline in the water column that impinged upon the continental shelf, which extended into wide expanses of shallow shelf settings during relative sea level rises that caused positive δ13C and δ34S excursions and mass extinctions. Ordovician oxygenation roughly corresponds to cessation of positive δ13C excursions (i.e., shallow water anoxia) and the major pulses of biodiversification that comprise the GOBE, suggesting that oxygenation was an important driver of Ordovician biodiversity. Anoxic conditions below an oxycline that impinged upon the continental shelf likely persisted until the Devonian when a cooler climate invigorated circulation and ventilated the global ocean, with the possible exception of some isolated basins that were persistently anoxic.
A Tremadocian (Early Ordovician, base Stairsian North American Stage) mass extinction event is recorded globally in rocks from several ancient continents and is accompanied by a globally correlated ...positive carbon isotope excursion (CIE; the largest during the Early Ordovician). In this study, elemental concentrations and uranium isotope compositions (δ238U) were measured for carbonate samples from three sections (along a proximal-to-distal transect: Ibex area, Shingle Pass, Meiklejohn Peak, respectively) in the Great Basin to test the role of ocean anoxia/euxinia on the base Stairsian mass extinction event. Dysoxic/suboxic to oxic water columns and non-sulfidic porewaters are inferred for these sections based on multiple local redox proxies (e.g., Mo/Al, U/Al, Mo and U concentrations, Mo/U ratios).
Carbonate δ238U data exhibit different δ238U trends in the three sections. The proximal Ibex area exhibits a small negative δ238U excursion (a magnitude of 0.2–0.3‰) and the Shingle Pass section has one sample with unusually low δ238U, in both cases within the middle of the CIE. Samples with the lowest δ238U in each section also have the highest Mn/Sr ratios, suggesting diagenetic alteration associated with adsorption of isotopically light U onto Mn oxides, followed by dissolution of Mn oxides by reducing porewaters and capture of the isotopically light U by sediments. The distal Shingle Pass and Meiklejohn Peak sections better preserve global ocean signals and the limited δ238U variations in these sections imply no significant changes in global seawater δ238U and thus no significant changes in the extent of global ocean euxinia. Given the non-sulfidic porewaters inferred for these carbonates, we apply a sediment–seawater δ238U offset of 0–0.27‰ (the 0.27 ± 0.14‰ isotopic offset observed for modern Bahamas carbonates with sulfidic porewaters is used as a maximum constraint) to the average carbonate δ238U from the two distal sections (−0.48 ± 0.09‰) to derive a coeval seawater δ238U of between −0.75 ± 0.09‰ and −0.48 ± 0.09‰. A three-sink U isotope mass balance model suggests 0.2–15.8% global euxinic seafloor area existed during deposition of these Tremadocian carbonates. Our study implies that a small biotic crisis during a small CIE was not associated with an expansion of global ocean euxinia, in contrast to major Phanerozoic mass extinctions that were commonly associated with significant expansion of ocean euxinia during large carbon perturbations (e.g., the Late Devonian). In addition, we propose that the post-SPICE Cambrian and Early Ordovician oceans were characterized by limited changes in ocean euxinia and only a few episodes of expanded ocean suboxia-anoxia during carbon perturbations. The “biomere” events within this period, including the base Stairsian event, were probably related to these episodes of expanded suboxic-anoxic oceans and/or other ecological stresses.
The Hominoidea (apes and, eventually, humans) and Cercopithecoidea (Old World monkeys) diverged from a common ancestor during the late Oligocene (~25 Ma) in East Africa. Subsequently, both catarrhine ...groups diversified in the early Miocene (~23-16 Ma), making this time period an extremely important one for understanding our (deep) ancestral roots and the link between our hominoid ancestors and the environment. A remote region of west Turkana known as Loperot provides an exciting opportunity to study early Miocene primate paleoecology and has the potential to reveal the phylogenetic relationship between cercopithecoid monkeys and stem hominoids. The site of LpM4 is particularly fossiliferous and has produced a diverse fauna that includes several catarrhine fossils that await description. Radioisotopic dating indicates that the Loperot fauna are older than ~17 Ma (early Miocene). Using geochemical data from paleosols and stable isotope values from rhizolith calcite, coupled with detailed sedimentology and stratigraphy, we reconstruct the early Miocene Critical Zone of Loperot to reveal a large perennial river system fed by ephemeral streams that created localized riparian forest microhabitats within a larger open ecosystem under semi-arid climate conditions, making Loperot's environmental reconstruction unlike many other early Miocene catarrhine-bearing localities. The perennial river system supported corridors of C3 forests; however, because these forests were restricted, they could not support a high diversity of forest indicator taxa. This is the most comprehensive analysis of the environmental conditions at Loperot to date, and provides new empirical evidence to suggest that African climate varied significantly by region and that arid habitats certainly existed during the early Miocene.
The carbon isotopic composition of the global dissolved inorganic carbon (DIC) reservoir is best estimated from open ocean pelagic carbonate sediments (δ13Ccarb). However, this is not practical for ...most of geologic time because seafloor subduction has removed the pre-Jurassic record and these time periods may have lacked planktonic calcifying organisms, and therefore shallow water carbonate platform or periplatform sediments are utilized. Shallow water deposits are susceptible to a wide range of post-depositional alteration processes and syn-depositional controls on δ13Ccarb that include carbonate mineralogy, water mass restriction, and a host of related variables (e.g., pH, temperature, organic decomposition, evaporation and CO2 solubility) that can produce local gradients in DIC. The degree to which shallow water δ13C curves diverge from open marine deposits may be critical to understanding how well global carbon cycle isotope mass balance models can predict organic carbon burial rates, but documentation of such divergence is often hindered by factors that limit chronostratigraphic correlation in restricted water masses (e.g., endemic faunas). Here we integrate strontium isotope (87Sr/86Sr) stratigraphy and biostratigraphy to compare δ13C curves in a case study along a depth transect in Middle–Late Ordovician carbonate platform settings. The restricted tidal flat and more open marine deposits are offset by a maximum of ∼2‰ during sea level drop and ∼0‰ during highstand flooding of the platform. Global carbon cycle models such as GEOCARBSULF use published δ13Ccarb curves to drive organic carbon burial rates under the assumption that δ13Ccarb reflects a global seawater signal. We show here the potential pitfalls of using a published δ13Ccarb curve that violates this global assumption. For the 460 million year Middle–Late Ordovician time bin in GEOCARBSULF, improper usage of our locally depleted δ13C curve to drive global organic carbon burial would result in erroneous atmospheric O2 (∼10.8% O2, equal to about a 30% reduction from what it should be at ∼15.2% O2 using global δ13C) and CO2 (∼400 ppm too high, equal to about a 13% increase from what it should be at 2570 ppm CO2). With detailed sedimentologic analysis, it may be possible to identify and exclude δ13C samples that record the influences of local carbon cycling from global carbon cycle models such as GEOCARBSULF.
•87Sr/86Sr and bio-stratigraphy reveals δ13C gradient along depth transect in Ordovician.•δ13C gradient as large as 2‰ during sea level drop, approaches 0 during flooding.•2‰ lighter δ13C input to global carbon cycle models is a pitfall that lowers O2 by 5%.•Use of 2‰ lighter δ13C introduces less error in CO2 but levels still rise by 400 ppm.
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