Claudio Gaucher

Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes

Geochemical data suggest that oxygenation of the Earth’s atmosphere occurred in two broad steps. The first rise in atmospheric oxygen is thought to have occurred between ∼2.45 and 2.2 Gyr ago, leading to a significant increase in atmospheric oxygen concentrations and concomitant oxygenation of the shallow surface ocean. The second increase in atmospheric oxygen appears to have taken place in distinct stages during the late Neoproterozoic era (∼800–542 Myr ago), ultimately leading to oxygenation of the deep ocean ∼580 Myr ago, but details of the evolution of atmospheric oxygenation remain uncertain. Here we use chromium (Cr) stable isotopes from banded iron formations (BIFs) to track the presence of Cr(VI) in Precambrian oceans, providing a time-resolved picture of the oxygenation history of the Earth’s atmosphere–hydrosphere system. The geochemical behaviour of Cr is highly sensitive to the redox state of the surface environment because oxidative weathering processes produce the oxidized hexavalent [Cr(VI)] form. Oxidation of reduced trivalent [Cr(III)] chromium on land is accompanied by an isotopic fractionation, leading to enrichment of the mobile hexavalent form in the heavier isotope. Our fractionated Cr isotope data indicate the accumulation of Cr(VI) in ocean surface waters ∼2.8 to 2.6 Gyr ago and a likely transient elevation in atmospheric and surface ocean oxygenation before the first great rise of oxygen 2.45–2.2 Gyr ago (the Great Oxidation Event). In ∼1.88-Gyr-old BIFs we find that Cr isotopes are not fractionated, indicating a decline in atmospheric oxygen. Our findings suggest that the Great Oxidation Event did not lead to a unidirectional stepwise increase in atmospheric oxygen. In the late Neoproterozoic, we observe strong positive fractionations in Cr isotopes (δ53Cr up to +4.9‰), providing independent support for increased surface oxygenation at that time, which may have stimulated rapid evolution of macroscopic multicellular life.

Integrated correlation of the Vendian to Cambrian Arroyo del Soldado and Corumbá Groups (Uruguay and Brazil): palaeogeographic, palaeoclimatic and palaeobiologic implications

Abstract The Corumba Group of SW Brazil and the Arroyo del Soldado Group (ASG) of Uruguay are correlated on the basis of litho-, bio- and chemostratigraphy. Both units represent marine sedimentation with alternating siliciclastics and carbonates developed on a stable continental shelf. In the Corumba basin, sedimentation began in the Varangerian, represented by the glaciomarine Puga Formation. A series of sea-level fluctuations coupled with climatic changes are recorded up section. While uppermost deposits of the ASG are of lowermost Cambrian age, sedimentation ceased in the latest Vendian in the Corumba basin. An assemblage of six species of organic-walled microfossils dominated by Bavlinella faveolata and Soldadophycus bossii, three species of vendotaenids and two species of skeletal fossils (Cloudina and Titanotheca) is described from the Corumba Group. The vendotaenid Eoholynia corumbensis sp. nov is described from siltstones of the Guaicurus Formation. An important diversity of skeletal fossils in the Corumba, Arroyo del Soldado and Nama groups points to favourable Vendian palaeoclimatic conditions in SW-Gondwana. Preliminary carbon isotopic data show a series of alternating positive and negative excursions, corroborating the upper Vendian age indicated by fossils for both units. Previously reported strontium isotopic data are also consistent with this age. It is postulated that the Corumba and ASGs were deposited onto the same shelf, which opened to the east. The Rio de la Plata Superterrane (Craton) extends farther to the north than previously expected, or it was already amalgamated with the Amazonian Craton by Vendian times. Collision of the platform with the Parana Block caused closure of the basin during the Cambrian-Early Ordovician. Finally, models of Neoproterozoic glaciations based on enhanced bioproductivity driven by high nutrient availability are discussed.

Chemostratigraphy of the Lower Arroyo del Soldado Group (Vendian, Uruguay) and Palaeoclimatic Implications

Abstract New C- and O-isotopic determinations from the Vendian lower Arroyo del Soldado Group are reported and combined with sedimentologic and biostratigraphic data. On the basis of different geochemical and petrographic criteria, the primary nature of the C- and Sr-isotopic signature is shown. Positive d 13 C-values characterize the mainly siliciclastic upper Yerbal Formation, which contains oxide-facies BIF and a diverse assemblage of skeletal fossils, including Cloudina riemkeae. A series of positive and negative d 13 C-excursions occur up-section in the overlying Polanco Formation, mainly composed of limestones and limestone-dolostone rhythmites. The transition to the overlying Barriga Negra and Cerro Espuelitas Formation, which consists of conglomerates and shales/cherts/BIF respectively, is marked by a further negative excursion. On the basis of sedimentary structures, a correlation of d 13 C with palaeobathymetry is established. Positive d 13 C -peaks are associated with high sea-level stand, high organic-carbon burial and relatively higher microfossil diversity, while negative d 13 C -excursions occur in carbonates with less organic matter, less microfossil diversity and are always associated to regressions. These observations can be readily explained by palaeoclimatic models which postulate that enhanced bioproductivity due to higher availability of nutrients (P, N, Fe) was the key factor controlling Neoproterozoic glaciations. The mentioned models are discussed in view of the new data. The occurrence of at least four cold periods in the upper Vendian is envisaged. These cold periods led to sea-level fall and possibly glaciation at higher latitudes. The absence of glacigenic rocks associated to negative d 13 C-excursions in the Arroyo del Soldado Group is probably due to the tropical setting of the basin. Finally, the upper Vendian age of the lower Arroyo del Soldado Group is confirmed, on the basis of C- and Sr-isotopes. The onset of carbonate deposition at the base of the Polanco Formation is estimated at 580 Ma by comparison with existing global isotopic curves.

The Cuchilla Dionisio Terrane, Uruguay: An Allochthonous Block Accreted in the Cambrian to SW-Gondwana

Abstract The Cuchilla Dionisio Terrane (CDT) of Uruguay is a tectonostratigraphic unit defined as the block to the east of the Sierra Ballena Shear Zone (SBSZ). It is composed of a Palaeo- to Mesoproterozoic metamorphic basement (Cerro Olivo and Chafalote Complexes) intensely reworked and intruded by syncollisional to late orogenic granites between 680 and 550 Ma. Dacitic and rhyolitic volcanism is recorded around 570–575 Ma (Cerros de Aguirre and Sierra de Rios Formations). The CDT is correlated with the Pelotas Terrane of southern Brazil. The western boundary of the block in Brazil is defined by the Cangucu and Major Gercino shear zones, which are the northward extension of the SBSZ. The present position of the CDT is not a consequence of orthogonal collision after completion of a Wilson Cycle, but of lateral sinistral accretion along the above mentioned megashear zones. The allochthonous nature of the CDT is postulated on the grounds that: (a) magmatism in the CDT is ca. 120 Ma younger than metamorphism of neighbouring volcanosedimentary successions, (e.g., Porongos Group in Brazil); and (b) while intense volcanism occurred in the CDT in the late Vendian, represented by the Cerros Aguirre and Sierra de Rios Formations, a passive continental margin deepening to the E existed on the eastern edge of the Rio de la Plata Craton, represented by the Arroyo del Soldado Group. Therefore the CDT and Rio del la Plata Craton were separated hundreds or thousands of kilometers by late Vendian times. Accretion of the CDT took place at 530 Ma by tangential collision, and was one of the last events in the complicated amalgamation of W-Gondwana. In Uruguay, the CDT collided with the Nico Perez Terrane, generating the SBSZ and sinistrally reactivating the Sarandi del Yi-Piriapolis Shear Zone. In Brazil, the CDT-Pelotas Terrane collided with the Parana Block and a series of parautochthonous terranes (Curitiba, Apiai) characterized by a Transamazonian basement strongly reworked in the Neoproterozoic. The provenance of the CDT is still uncertain, considering that both Transamazonian and Namaqua-Natal (Kibaran) ages have been reported from its basement. Previously reported Nd model ages (TDM) strongly suggest an African affinity. The Dom Feliciano Belt is thus a collage of diachronous units formed in different geotectonic settings, and not the product of a single orogenic cycle. Terrane-docking at different stages mainly by lateral accretion was the dominant process. Therefore usage of the name “Dom Feliciano Belt” should be restricted only to descriptive purposes.

Organic-walled microfossils and biostratigraphy of the upper Port Nolloth Group (Namibia): implications for latest Neoproterozoic glaciations

The occurrence of organic-walled microfossils is reported for the first time from the Neoproterozoic Port Nolloth Group, Gariep Belt (southern Namibia). Acritarchs assigned to Bavlinella faveolata occur in the Hilda Subgroup below the younger of two glaciogenic diamictite units (Numees Formation) within the Port Nolloth Group. The microfossil assemblage in the overlying upper Holgat Formation, above the Numees Formation, is characterized by low diversity (six species), dominance of Soldadophycus bossii , and absence of acanthomorphic or large sphaeromorphic acritarchs. The agglutinated foraminifer Titanotheca also occurs in the Holgat Formation. Combined with available chemostratigraphic data and Pb–Pb ages, this microfossil assemblage indicates an upper Ediacaran age of around 555 Ma for the Holgat Formation. Virtually identical microfossil assemblages, negative-to-positive δ 13 C trends, 87 Sr/ 86 Sr values between 0.7080 and 0.7085, as well as Pb–Pb carbonate ages, make it possible to correlate the Holgat Formation with the Buschmannsklippe Formation (Witvlei Group, central Namibia), the Kombuis Member (Cango Caves Group, southern South Africa) and the uppermost Polanco to lowermost Cerro Espuelitas Formation (Arroyo del Soldado Group, Uruguay). Based on these data, the underlying Numees Formation, the age of which has been only loosely constrained so far and subject to considerable debate, can now be assigned to the c . 580 Ma Gaskiers or the possibly younger ( 13 C excursion and a strong sea-level drop. If this correlation is confirmed, lack of glacial deposits there might have implications for the palaeogeographic extent of upper Ediacaran glaciations. Our preliminary studies show that acritarch biostratigraphy can make a significant contribution to unravelling the stratigraphy of Neoproterozoic glacial deposits, especially when combined with C and Sr isotopic data.

Chapter 8 Tectonic Events and Palaeogeographic Evolution of Southwestern Gondwana in the Neoproterozoic and Cambrian

Abstract A new geodynamic model is presented for southwestern Gondwana amalgamation. Rifting of Rodinia was a protracted process, advancing from the boundaries of the supercontinent toward its core, represented by Laurentia. Whereas the Congo-Sao Francisco Craton was likely far away from Rodinia, it is proposed that the Rio de la Plata Craton was part of the supercontinent. We propose the name ‘ Arachania ’ for the block that comprises the Cuchilla Dionisio-Pelotas, Marmora, Tygerberg and correlative terranes, which likely represents a fragment of the Kalahari Craton that a later stage (650–570 l Ma) evolved into a magmatic arc. Available evidence points towards stepwise rifting of Rodinia, and the sequential closure of the thus formed oceanic basins from east to west (present coordinates). These basins were the Damara, Adamastor, Brazilides, Pampean and Iapetus oceans. Final amalgamation of Gondwana took place in the Cambrian (ca. 520 l Ma).

Southernmost Exposures of the Arroyo del Soldado Group (Vendian to Cambrian, Uruguay): Palaeogeographic Implications for the Amalgamation of W-Gondwana

Abstract The occurrence of the Vendian to lowermost Cambrian Arroyo del Soldado Group (ASG) is reported from an area located to the SW of Minas, which was formerly mapped as part of the Lavalleja Group. The Yerbal, Polanco and Cerro Espuelitas Formations of the ASG occur there as a rather continuous, microfossil-rich sedimentary cover, intruded by granites of probable Cambrian age (Minas Granite). Organic-walled microfossils recovered from the Yerbal and Polanco Formations include Bavlinella faveolata, Leiosphaeridia tenuissima, Soldadophycus bossii, Soldadophycus major, Siphonophycus solidum, Glenobotrydion aenigmatis, and filament mats. The acritarch Dyctiotidium sp. is described for the first time from the Yerbal Formation. Essentially the same microfossil assemblage occurs in five samples of this unit collected immediately to the north of Minas. No identifiable microfossils were found in fifteen samples of organic-rich lithologies of the Lavalleja Group. Based on detailed geological mapping, litho- and biostratigraphic data, we show that the contact between the Arroyo del Soldado and Lavalleja Groups is of tectonic nature (Arroyo La Plata Lineament), the latter being pre-Vendian in age. The Villa Serrana Block is defined, and consists of the Carape and Lavalleja Groups, Las Ventanas and Playa Hermosa Formations, an unnamed siliciclastic-ultrabasic succession showing low-grade metamorphism, and various granitic intrusions mostly of Neoproterozoic to Cambrian age. Its boundaries are the Sarandi del Yi-Piriapolis shear zone to the W, the Sierra Ballena shear zone to the E, the Arroyo La Plata Lineament to the N, and the Rio de la Plata to the S. The Villa Serrana Block could be an allochthonous terrane, or alternatively, a part of the Nico Perez Terrane that was thrusted onto the ASG. No continuity exists between the Brusque and Porongos groups of southern Brazil and the Lavalleja Group. According to available data these units are probably not coeval, and do not represent the suture between the cratonic areas to the W (Rio de la Plata Craton) and the remnants of a Neoproterozoic magmatic arc to the E (Cuchilla Dionisio-Pelotas Terrane). Accretion of these blocks was controlled by sinistral, continental-scale megashears. The geology of the area is far too complex to be explained in terms of a single orogeny following one Wilson cycle.

Stromatolitic and Trace Fossils Community of the Cerro Victoria Formation, Arroyo del Soldado Group (Lowermost Cambrian, Uruguay)

Abstract The Cerro Victoria Formation of the Arroyo del Soldado Group (Uruguay) comprises 400 m of mainly stromatolitic, micritic and oolitic limestones. In this low-diversity stromatolite paleocommunity, moundlike and planar laminites dominate, but columnar forms, ooids and pisoids occur as well. Stromatolites played a major role in the construction of the carbonate ramp, which developed onto a passive (eastern) margin of the Rio de la Plata Craton. A paleobiological reconstruction of the stomatolite community, which includes subtidal, intertidal and supratidal environments, is presented. A low-diversity trace fossil association, dessignated as “Thalassinoides-Gyrolithes-Palaeophycus” assemblage, is preserved in syngenetic concretions. The stromatolite community, ichnofossil assemblage and ichnofabrics are typical for the lowermost Cambrian, corroborating earlier age estimations. The unit represents a shallowing-upward sequence deposited under predominantly subtidal, normal marine, high energy conditions. Tropical climate prevailed, as shown by thick carbonate deposits, evaporitic minerals (halite, gypsum) and abundance of ooids. The Cambrian of western Gondwana is characterized by scarcity of carbonate ramps. This was mainly determined by high siliciclastic input related to erosion of the Pan African-Rio Doce orogenic belts. Carbonate ramps developed in blocks located at tropical paleolatitudes and characterized by low clastic input, such as the Nico Perez Terrane and Argentine Precordillera.

Comment on "Bilaterian Burrows and Grazing Behavior at >585 Million Years Ago"

Pecoits et al. (Reports, 29 June 2012, p. 1693) describe bilaterian trace fossils and assign them an Ediacaran age based on the age of a granite interpreted as intrusive. We argue that the granite is not intrusive but in fact represents the basement of the sedimentary succession. Moreover, we show that identical trace fossils occur in nearby Carboniferous-Permian glacigenic rocks.

Origin and impact of the oldest metazoan bioclastic sediments

The emergence of soft-bodied metazoans and the radiation of the earliest skeletal organisms substantially changed the ecological dynamics of Ediacaran environments, leading to the genesis of biogenic hard-part deposits for the first time in Earth’s history. The impact of bioclast origin on sedimentary processes is analyzed herein, focusing on the sedimentology and taphonomy of shell concentrations dominated by the Ediacaran index fossil Cloudina from the Itapucumi Group, Paraguay. Skeletal concentrations include both dense accumulations of parautochthonous, disarticulated specimens (“Type 1 deposits”) and in situ specimens preserved as loosely packed assemblages (“Type 2 deposits”). At that time, Cloudina was the critical source of durable biomineralized hard parts in an environment nearly free of other bioclasts. The simple fabric and geometry of these accumulations are typical of Cambrian-style shell beds. Despite their Precambrian age, these deposits indicate that the establishment of the Phanerozoic style of marine substrates and preservation in early shell beds was determined more by the acquisition of hard parts than by environmental changes.