Animals originated in the Neoproterozoic and ‘exploded’ into the fossil record in the Cambrian. The Cambrian also represents a high point in the animal fossil record for the preservation of soft tissues that are normally degraded. Specifically, fossils from Burgess Shale-type (BST) preservational windows give paleontologists an unparalleled view into early animal evolution. Why this time interval hosts such exceptional preservation, and why this preservational window declines in the early Paleozoic, have been long-standing questions. Anoxic conditions have been hypothesized to play a role in BST preservation, but recent geochemical investigations of these deposits have reached contradictory results with respect to the redox state of overlying bottom waters. Here, we report a multi-proxy geochemical study of the Lower Cambrian Mural Formation, Alberta, Canada. At the type section, the Mural Formation preserves rare recalcitrant organic tissues in shales that were deposited near storm wave base (a Tier 3 deposit; the worst level of soft-tissue preservation). The geochemical signature of this section shows little to no evidence of anoxic conditions, in contrast with published multi-proxy studies of more celebrated Tier 1 and 2 deposits. These data help confirm that ‘decay-limited’ BST biotas were deposited in more oxygenated conditions, and support a role for anoxic conditions in BST preservation. Finally, we discuss the role of iron reduction in BST preservation, including the formation of iron-rich clays and inducement of sealing seafloor carbonate cements. As oceans and sediment columns became more oxygenated and more sulfidic through the early Paleozoic, these geochemical changes may have helped close the BST taphonomic window.
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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.
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- Award ID(s):
- 1922966
- NSF-PAR ID:
- 10280568
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 7
- Issue:
- 28
- ISSN:
- 2375-2548
- Page Range / eLocation ID:
- eabf4382
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Careful evaluation of the local geochemical conditions in past marine settings can provide a window to the average redox state of the global ocean during episodes of extensive organic carbon deposition. These comparisons aid in identifying the interplay between climate and biotic feedbacks contributing to and resulting from these events. Well‐documented examples are known from the Mesozoic Era, which is characterized by episodes of widespread organic carbon deposition known as Oceanic Anoxic Events. This organic carbon burial typically leads to coeval positive carbon‐isotope excursions. Geochemical data are presented here for several palaeoredox proxies (Cr/Ti, V, Mo, Zn, Mn, Fe speciation, I/Ca and sulphur isotopes) from a section exposed at Furlo in the Marche–Umbrian Apennines of Italy that spans the Cenomanian–Turonian boundary. Here, Oceanic Anoxic Event 2 is represented by a
ca 1 m thick radiolarian‐rich millimetre‐laminated organic‐rich shale known locally as the Bonarelli Level. Iron speciation data for thin organic‐rich intervals observed below the Bonarelli Level imply a local redox shift going into the Oceanic Anoxic Event, with ferruginous conditions (i.e. anoxic with dissolved ferrous iron) transiently developed prior to the event and euxinia (i.e. anoxic and sulphidic bottom waters) throughout the event itself. Pre‐Oceanic Anoxic Event enrichments of elements sensitive to anoxic water columns were due to initial development of locally ferruginous bottom waters as a precursor to the event. However, the greater global expanse of dysoxic to euxinic conditions during the Oceanic Anoxic Event greatly reduced redox‐sensitive trace‐metal concentrations in seawater. Pyrite sulphur isotopes document a positive excursion during the Oceanic Anoxic Event. Carbonate I/Ca ratios were generally low, suggesting locally reduced bottom‐water oxygen conditions preceding the event and relatively increased oxygen concentrations post‐event. Combined, the Furlo geochemical data suggest a redox‐stratified water column with oxic surface waters and a shallow chemocline overlying locally ferruginous bottom waters preceding the event, globally widespread euxinic bottom waters during the Oceanic Anoxic Event, followed by chemocline shallowing but sustained local redox stratification following the event. -
Cambrian–Devonian sedimentary rocks of the northern Canadian Cordillera record both the establishment and demise of the Great American Carbonate Bank, a widespread carbonate platform system that fringed the ancestral continental margins of North America (Laurentia). Here, we present a new examination of the deep-water Road River Group of the Richardson Mountains, Yukon, Canada, which was deposited in an intra-platformal embayment or seaway within the Great American Carbonate Bank called the Richardson trough. Eleven detailed stratigraphic sections through the Road River Group along the upper canyon of the Peel River are compiled and integrated with geological mapping, facies analysis, carbonate and organic carbon isotope chemostratigraphy, and new biostratigraphic results to formalize four new formations within the type area of the Richardson Mountains (Cronin, Mount Hare, Tetlit, and Vittrekwa). We recognize nine mixed carbonate and siliciclastic deep-water facies associations in the Road River Group and propose these strata were deposited in basin-floor to slope environments. New biostratigraphic data suggest the Road River Group spans the late Cambrian (Furongian) – Middle Devonian (Eifelian), and new chemostratigraphic data record multiple global carbon isotopic events, including the late Cambrian Steptoean positive carbon isotope excursion, the Late Ordovician Guttenberg excursion, the Silurian Aeronian, Valgu, Mulde (mid-Homerian), Ireviken (early Sheinwoodian), and Lau excursions, and the Early Devonian Klonk excursion. Together, these new data not only help clarify nomenclatural debate centered around the Road River Group, but also provide critical new sedimentological, biostratigraphic, and isotopic data for these widely distributed rocks of the northern Canadian Cordillera.more » « less
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Abstract Uranium isotopes (238U/235U) have been used widely over the last decade as a global proxy for marine redox conditions. The largest isotopic fractionations in the system occur during U reduction, removal, and burial. Applying this basic framework, global U isotope mass balance models have been used to predict the extent of ocean floor anoxia during key intervals throughout Earth's history. However, there are currently minimal constraints on the isotopic fractionation that occurs during reduction and burial in anoxic and iron‐rich (ferruginous) aquatic systems, despite the consensus that ferruginous conditions are thought to have been widespread through the majority of our planet's history. Here we provide the first exploration of δ238U values in natural ferruginous settings. We measured δ238U in sediments from two modern ferruginous lakes (Brownie Lake and Lake Pavin), the water column of Brownie Lake, and sedimentary rocks from the Silurian‐Devonian boundary that were deposited under ferruginous conditions. Additionally, we provide new δ238U data from core top sediments from anoxic but nonsulfidic settings in the Peru Margin oxygen minimum zone. We find that δ238U values from sediments deposited in all of these localities are highly variable but on average are indistinguishable from adjacent oxic sediments. This forces a reevaluation of the global U isotope mass balance and how U isotope values are used to reconstruct the evolution of the marine redox landscape.
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null (Ed.)Low oxygen conditions in the modern Baltic Sea are exacerbated by human activities; however, anoxic conditions also prevailed naturally over the Holocene. Few studies have characterized the specific paleoredox conditions (manganous, ferruginous, euxinic) and their frequency in southern Baltic sub-basins during these ancient events. Here, we apply a suite of isotope systems (Fe, Mo, S) and associated elemental proxies (e.g., Fe speciation, Mn) to specifically define water column redox regimes through the Baltic Holocene in a sill-proximal to sill-distal transect (Lille Belt, Bornholm Basin, Landsort Deep) using samples collected during the Integrated Ocean Drilling Program Expedition 347. At the sill-proximal Lille Belt, there is evidence for anoxic manganous/ferruginous conditions for most of the cored interval following the transition from the Ancylus Lake to Littorina Sea but with no clear excursion to more reducing or euxinic conditions associated with the Holocene Thermal Maximum (HTM) or Medieval Climate Anomaly (MCA) events. At the sill-distal southern sub-basin, Bornholm Basin, a combination of Fe speciation, pore water Fe, and solid phase Mo concentration and isotope data point to manganous/ferruginous conditions during the Ancylus Lake-to-Littorina Sea transition and HTM but with only brief excursions to intermittently or weakly euxinic conditions during this interval. At the western Baltic Proper sub-basin, Landsort Deep, new Fe and S isotope data bolster previous Mo isotope records and Fe speciation evidence for two distinct anoxic periods but also suggest that sulfide accumulation beyond transient levels was largely restricted to the sediment-water interface. Ultimately, the combined data from all three locations indicate that Fe enrichments typically indicative of euxinia may be best explained by Fe deposition as oxides following events likely analogous to the periodic incursions of oxygenated North Sea waters observed today, with subsequent pyrite formation in sulfidic pore waters. Additionally, the Mo isotope data from multiple Baltic Sea southern basins argue against restricted and widespread euxinic conditions, as has been demonstrated in the Baltic Proper and Bothnian Sea during the HTM or MCA. Instead, similar to today, each past Baltic anoxic event is characterized by redox conditions that become progressively more reducing with increasing distance from the sill.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/sciadv.abf4382
