Renata G. Netto

Colonization of Brackish-Water Systems through Time: Evidence from the Trace-Fossil Record

Abstract Trace fossils in estuarine deposits of different ages have been compared to evaluate colonization history of brackish-water ecosystems and to calibrate trace-fossil, brackish-water models with respect to geologic time. This comparative analysis reveals that, although the colonization of marginal-marine, brackish-water environments was a long-term process that spanned most of the Phanerozoic, this process of invasion of fully marine organisms into restricted, marginal-marine habitats did not occur at a constant rate. Five major colonization phases can be distinguished. The first phase (Ediacaran–Ordovician) represents a prelude to the major invasion that occurred during the rest of the Paleozoic. While Ediacaran–Cambrian ichnofaunas seem to be restricted to the outermost zones of marginal-marine depositional systems, Ordovician assemblages show some degree of landward expansion within brackish-water ecosystems. Intensity of bioturbation and ichnodiversity levels were relatively low during this phase. The second phase (Silurian–Carboniferous) is marked by the appearance of more varied morphologic patterns and behavioral strategies, resulting in a slight increase in ichnodiversity. While previous assemblages were arthropod dominated, brackish-water Silurian–Carboniferous ichnofaunas include structures produced by bivalves, ophiuroids, and polychaetes. Ichnofaunas from the third phase (Permian–Triassic) seem to be characterized by the presence of crustacean burrows, reflecting the late Paleozoic crustacean radiation and adaptation of some groups to brackish-water conditions. The fourth phase (Jurassic–Paleogene) is typified by a remarkable increase in ichnodiversity and intensity of bioturbation of estuarine facies. Colonization occurred not only in softgrounds and firmgrounds, but also in hardgrounds and xylic substrates. The fifth phase (Neogene–Recent) records the onset of modern brackish-water benthos. Although still impoverished with respect to their fully marine counterparts, brackish-water ichnofaunas may reach moderately high diversities, particularly in middle- and outer-estuarine regions, and degree of bioturbation may be high in certain estuarine subenvironments. Comparative analysis of brackish-water ichnofaunas through geologic time provides valuable evidence to understand colonization of marginal-marine environments through the Phanerozoic, and allows for calibration of ichnologic models that may aid in the recognition of estuarine valley-fill deposits in the stratigraphic record.

Extreme freshwater release during the late Paleozoic Gondwana deglaciation and its impact on coastal ecosystems

ABSTRACTStrata in the Paganzo, Tarija, and Parana´ Basins of Argentina and Brazil provideevidence for reconstructing the effects of late Paleozoic glacial retreat. The depositionalenvironment of the transgressive and early highstand fine-grained deposits has been con-troversial, with interpretations ranging from normal-marine shelves to estuaries to lakes.Whereas their counterparts from shallow-marine settings not influenced by glaciation hostdiverse, fully marine ichnofaunas, these fine-grained postglacial deposits are dominatedby nonspecialized grazing trails, simple feeding traces, arthropod trackways, and fishtrails. They are typical of freshwater environments and represent examples of the Mermia and Scoyenia ichnofacies. However, the local presence of acritarchs indicates sporadicmarine influence. These observations suggest a new interpretation, that freshwater con-ditions in fjordlike settings across South America were widespread because Gondwananbasins were overwhelmed by strong meltwater discharge issuing from melting of the con-tinental ice masses.Keywords:

The Mesozoic Marine Revolution

The Mesozoic Marine Revolution (MMR) was a major evolutionary episode involving the large-scale restructuring of shallow-marine benthic communities and the rise to dominance of the Modern Evolutionary Fauna. Although the majority of studies published on the MMR have been based on the body-fossil record, the ichnologic record yields valuable insights into this evolutionary event, most notably regarding the degree of infaunalization, complexity of infaunal tiering structures, and predation intensity. The main groups of bioturbators involved in the MMR were crustaceans, bivalves, echinoids, and “worms,” whereas the most important bioeroders were sponges, gastropods, bivalves, echinoids, and “worms.”

Global deglaciation and the re-appearance of microbial matground-dominated ecosystems in the late Paleozoic of Gondwana.

The extensive matgrounds in Carboniferous-Permian open-marine deposits of western Argentina constitute an anachronistic facies, because with the onset of penetrative bioturbation during the early Paleozoic microbial mats essentially disappeared from these settings. Abundant microbially induced sedimentary structures in the Argentinean deposits are coincident with the disappearance of trace and body fossils in the succession and with a landward facies shift indicative of transgressive conditions. Deposits of the Late Carboniferous-Early Permian glacial event are well developed in adjacent basins in eastern Argentina, Brazil, South Africa and Antarctica, but do not occur in the western Andean basins of Argentina. However, the deglaciation phase is indirectly recorded in the studied region by a rapid rise in sea level referred to as the Stephanian-Asselian transgression. We suggest that an unusual release of meltwater during the final deglaciation episode of the Gondwana Ice Age may have dramatically freshened peri-Gondwanan seas, impacting negatively on coastal and shallow-marine benthic faunas. Suppression of bioturbation was therefore conducive to a brief re-appearance of matground-dominated ecosystems, reminiscent of those in the precambrian. Bioturbation is essential for ecosystem performance and plays a major role in ocean and sediment geochemistry. Accordingly, the decimation of the mixed layer during deglaciation in the Gondwana basins may have altered ecosystem functioning and geochemical cycling.

Age constraints of the glaciation in the Paraná Basin: evidence from new U–Pb dates

The absence of absolute dating in glacial deposits of the Paraná Basin prevents better understanding of the complete history of the Late Palaeozoic Ice Age in Western Gondwana and the climatic change that took place in the late Carboniferous. In this study, new U–Pb radiometric ages date the topmost glacial deposits of the Itararé Group at 307.7 ± 3.1 Ma (Kasimovian−Moscovian) and the base of post-glacial deposits of the Rio Bonito Formation at 298.8 ± 1.9 Ma (Ghzelian−Asselian). Correlation of glacial and post-glacial deposits of the Paraná Basin with other southwestern Gondwana basins shows that the end of the Late Palaeozoic Ice Age is asynchronous. Supplementary material: Laser ablation inductively coupled plasma mass spectrometry U–Pb zircon data and methods for the ash sample of the Itararé Group and the tonstein sample of the Rio Bonito Formation are available at https://doi.org/10.6084/m9.figshare.c.3278273.

Paleoicnologia e a palinologia dos ritmitos do Grupo Itararé na pedreira de Águas Claras (Santa Catarina, Brasil)

O presente estudo faz uma analise integrada da paleoicnologia e da palinologia dos ritmitos siltico-argilosos da Formacao Rio do Sul aflorantes na pedreira de Aguas Claras, com vistas a sua caracterizacao paleoambiental. A assembleia icnofossilifera e dominada por pistas de moluscos e por trilhas de crustaceos pericarideos ou sincarideos e merostomados, preservadas como subimpressoes. Protovirgularia isp., seguida por Diplichnites gouldi e Diplopodichnus biformis sao as icnoespecies mais comuns, alem de Maculichna varia e Umfolozia sinuosa . Escavacoes rasas de organismos vermiformes ( Cochlichnus anguineus, Hormosiroidea meandrica e Treptichnus isp.) e impressoes de repouso de artropodes ( Gluckstadella cooperi ) complementam a assembleia icnofossilifera. A analise palinologica realizada em amostras dos mesmos niveis onde se registra a icnofauna revelou elementos palinologicos escassos, com dominância de graos de polen monossacados e fitoclastos opacos. A presenca de Vittatina vittifera , Protohaploxypinus goraiensis e Illinites unicus permite sugerir idade permiana inferior, correspondente a Subzona Protohaploxypinus goraiensis base da Zona Vittatina costabilis . A dominância de Protovirgularia isp. sobre as demais icnoespecies, sua ocorrencia na maior parte dos niveis ritmicos da sucessao e a abundância de marcas de enrugamento geradas por esteiras microbianas sugerem deposicao em planicies marginais marinhas, na zona de intermare. A ocorrencia localizada de escavacoes rasas de organismos dulciaquicolas revela que essas planicies eram periodicamente inundadas pelas aguas de degelo. Por outro lado, a presenca comum de trilhas de artropodes de habito terrestre por sobre as suites subaquaticas sugere periodos de exposicao subaerea desses substratos. O dominio de graos de polen e a grande quantidade de fitoclastos opacos reforcam um posicionamento mais distal a area fonte, com forte influencia continental. Palavras-chave: paleoicnologia, palinologia, Formacao Rio do Sul, Grupo Itarare, Bacia do Parana.

Chapter 11 – Glacial Environments

Glacial environments are subject to drastic oscillations in energy regime that rapidly modify the local environment. The impact of glacial phenomena on the distribution, abundance, and evolution of biota based on trace-fossil evidence is the focus of this chapter. Arthropod trackways, shallow horizontal burrows, and fish trails dominate the glacial and periglacial trace-fossil assemblages preserved in terrestrial and glaciolacustrine sedimentary sequences, while nonspecialized feeding burrows that are diminutive when compared to normal-marine settings comprise glaciomarine ichnofaunas. As a consequence of rapid meltwater discharge, freshwater conditions prevail in some fjord settings during deglaciation, allowing for the establishment of suites more typical of freshwater or brackish-water conditions. Despite changes in the composition of the trace making community through time, ichnofacies relationships and ecological niche occupation are similar between the Paleozoic and Cenozoic, an indication of the constancy of the interplay between the biotic community and glacial processes.

Assinaturas icnológicas da sucessão sedimentar Rio Bonito no bloco central da jazida carbonífera de Iruí, Cachoeira do Sul (RS)

The subsurface deposits of the Rio Bonito sedimentary succession in the central block of the Irui coal mine (Cachoeira do Sul, Rio Grande do Sul State, S Brazil) is composed of sedimentary facies associations that represent deposition in dominantly marginal-marine and shallow-marine settings, the later in minor scale. Ichnofabrics of Chondrites , Chondrites-Helmintopsis-Planolites , Cylindrichnus-Thalassinoides , Helminthopsis , Macaronichnus , Ophiomorpha , Palaeophycus, Planolites , Thalassinoides , and Thalassinoides-Palaeophycus occur in the marginal marine settings , whereas the Thalassinoides-Palaeophycus-Helminthopsis ichnofabric is present in the shallow marine deposits. The marginal-marine ichnofabrics are characterized by the low bioturbation index, the reduced size of burrows, and the low ichnodiversity, whereas the marine ones show a moderate to high degree of bioturbation and low ichnodiversity. The ichnofabric pattern, its stratigraphic distribution, and its sedimentological relationships allow recognizing four ichnological signatures, three suggesting stress caused by changes in the salinity gradient, and one substrate-controlled. Ichnofabrics of Helmithopsis, Palapeophycus , and Planolites in lithofacies SiltArg suggest the dominance of oligohaline to freshwater conditions. Ichnofabrics of Thalassinoides-Palaeophycus and Cylindrichnus-Thalassinoides refl ect the dominance of mesohaline conditions, each one representing, respectively, more quiet and stable environments (impoverished Cruziana Ichnofacies suite), and moderate to high energy settings (mixed Skolithos-Cruziana Ichnofacies suite). The Thalassinoides-Palaeophycus-Helminthopsis ichnofabric represents an arquetypical Cruziana Ichnofacies suite, indicating shallow marine settings. The reduced size of the burrows, however, suggests dominance of polyhaline rather than stenohaline conditions and its local distribution allow infer deposition in embayments, which mimic the environmental conditions found in the lower shorefce / offshore transition zones. The Thalassinoides ichnofabric is substrate-controlled, representing a Glossifungites Ichnofacies suite. It demarks two autogenic stratigraphic surfaces, and one allogenic stratigraphic surface interpreted as a sequence boundary. The integrated ichnological and sedimentological analysis in high resolution scale, allowed refining the existent paleoenvironmental and stratigraphic interpretations of the Rio Bonito sedimentary succession in the study area. Key words: ichnology, sedimentology, ichnofabrics, ichnological signatures, coastal deposits.

MICROBE-MEDIATED PRESERVATION OF INVERTEBRATE FECAL PELLETS: EVIDENCE FROM THE ICHNOFOSSIL PHYMATODERMA BURKEI, PERMIAN SHALLOW-MARINE, TERESINA FORMATION, SOUTHERN BRAZIL

Abstract Fecal pellet-filled trace fossil Phymatoderma burkei from the Permian shallow-marine Teresina Formation (southern Brazil) was microscopically and geochemically investigated to reveal the significance of physicochemical processes in the preservation of invertebrate fecal pellets. Scanning electron microscope (SEM) analysis shows micron-sized spherules only in the pelletal infill of P. burkei, but not within the surrounding host sandstone. These spherules show highly uniform morphology and size (approximately 1 &mgr;m), and generally have hollow structures occasionally occurring as aggregates. Geochemically, such spherules are characterized by presence of Fe and Mn, occasionally with Ba. The evidence suggests that the spherules are mineral-replaced or mineral-encrusted microbial cells, which are most likely coccoid bacteria based on their morphological similarity. Within the fecal pellets excreted by the trace-maker of P. burkei, microbially mediated precipitation of Fe and Mn minerals probably occurred on or just near the outer surface of coccoid bacterial cells. Such microbial processes are important in the preservation of invertebrate fecal pellets excreted in shallow tiers, especially under wave- or current-influenced shallow-marine sedimentary conditions.