Diagenetic Fate of Biogenic Soft and Hard Magnetite in Chemically Stratified Sedimentary Environments of Mamanguá Ría, Brazil

Magnetotactic bacteria (MTB) synthesize magnetite and greigite crystals under low oxygen conditions in the water column or uppermost sediment (greigite‐producing bacteria are found below the oxic‐anoxic transition). Dissolved iron and oxygen contents in local environments are known to be limiting factors for the production and preservation of biogenic magnetite. Understanding the processes that link MTB to their living environments is fundamental to reconstructing past chemical variations in the water column and sediment, and for using the magnetic properties of biogenic magnetite as environmental proxy indicators. Previous studies have suggested that the frequently identified biogenic soft (BS) and biogenic hard (BH) magnetite types are associated with equant and more elongated morphologies, respectively, and that their abundance varies in accordance with sedimentary oxygen content, where MTB that produce the BH component live in less oxygenated environments. We test this hypothesis in a high‐resolution integrated environmental magnetic and geochemical study of surface sediments from Mamanguá Ría, SE Brazil. Based on magnetic and pore water profiles, we demonstrate that both the BS and BH components occur within microaerobic environments and that as sediment oxygen content decreases with depth, the BS component disappears before the BH component. With continued burial into the sulfidic diagenetic zone, both components undergo progressive dissolution, but the BH component is more resistant to dissolution than the BS component. Our observations confirm previous inferences about the relative stability of these phases and provide a firmer basis for use of these two types of biogenic magnetite as paleoenvironmental proxies.D. R. and L. J. acknowledge funding from FAPESP grants 2012/212123 and 2011/22018‐3, respectively. F. A. acknowledges funding from FAPERJ, CNPq, and CAPES. A. P. R. acknowledges funding from the Australian Research Council (grants DP140104544 and DP160100805)

Magnetostratigraphic Chronology of a Cenozoic Sequence From DSDP Site 274, Ross Sea, Antarctica

New paleomagnetic results from the late Eocene-Middle Miocene samples from Deep Sea Drilling Project Site 274, cored during Leg 28 on the continental rise off Victoria Land, Ross Sea, provide a chronostratigraphic framework for an existing paleoclimate archive during a key period of Antarctic climate and ice sheet evolution. Based on this new age model, the cored late Eocene-Middle Miocene sequence covers an interval of almost 20 Myr (from ∼35 to ∼15 Ma). Biostratigraphic constraints allow a number of possible correlations with the Geomagnetic Polarity Time Scale. Regardless of correlation, average interval sediment accumulation rates above 260 mbsf are ∼6 cm/kyr with the record punctuated by a number of unconformities. Below 260 mbsf (across the Eocene/Oligocene boundary) interval, sedimentation accumulation rates are closer to ∼1 cm/kyr. A major unconformity identified at ∼180 mbsf represents at least 9 Myr accounting for the late Oligocene and Early Miocene and represent non-deposition and/or erosion due to intensification of Antarctic Circumpolar Current activity. Significant fluctuations in grain size and magnetic properties observed above the unconformity at 180 mbsf, in the Early Miocene portion of this sedimentary record, reflect cyclical behavior in glacial advance and retreat from the continent. Similar glacial cyclicity has already been identified in other Miocene sequences recovered in drill cores from the Antarctic margin

Variações paleoceanograficas através do estudo das propriedades magnéticas: a magnetita biogênica como novo indicador paleoambiental

The climatic history of the planet Earth is characterized by long- and short-term climatic variations. Oxygen isotopic data clearly shows how during the Cenozoic (from 65 Ma to the present) our planet shifted over time, from greenhouse to icehouse climate states up to the present. Along this progressive cooling, the record is dotted by rapid warming and cooling transient events, from which the causes are not yet fully understood. One problem with paleoceanographic reconstructions is that the older the event, the harder it is to find a sedimentary record in which the paleoclimatic proxies are preserved enough to be used with confidence. This work has the goal of illustrating the possibility to develop a new paleoenvironmental indicator based on the magnetic properties of magnetite crystals synthetized by magnetotactic bacteria. The sensibility of such crystals to small changes in dissolved oxygen content in the pore water and water column is well known, and can be exploited to reconstruct such changes from the sedimentary record. Qualitative information regarding this topic are defined in scientific literature, but, so far, no quantitative study has been performed. This is the first attempt to quantify the preservation of biogenic magnetite as a function of oxygenation state of waters, and is based on recent sediment extracted from cores collected in the coastal region of Rio de Janeiro (Saco do Mamanguá, Paraty). From these, where it was possible to obtain magnetic data relative to magnetite crystals together as well with as direct measurements of pore water chemistry. The results of this first attempt were used to analyze climatic and oceanographic conditions in two other sites, representative of key past environmental events during the Paleocene. The first case study was performed in a newly descripted sedimentary outcrop in central Turkey, of middle Eocene age, representative of a period of rapid warming (Middle Eocene Climatic Optimum, MECO). The second case study was done using material from a marine sediment core collected in the Ross Sea (Antarctica) that covers from the late Eocene to the Middle Miocene, a period that saw the onset of the modern, permanent ice sheet cover in the Antarctic continent.A historia climática da Terra é caracterizada por variações climáticas de curto e longo prazo, com variações de condições, as vezes extremas. Dados de isótopos de oxigênio mostram claramente que durante o Cenozoico (desde 65 Ma até o presente) o nosso planeta passou de uma fase quente caracterizada por marcado efeito estufa (durante o Paleoceno) para um período progressivamente mais frio em direção ao presente. Ambos esses estados são caracterizados pela alternância de eventos transientes quentes e frios de relativa curta duração, sendo que destes a maioria ainda não é plenamente compreendida. Um dos principais problemas das reconstruções paleoceangráficas é que quanto mais antigo o evento mais difícil encontrar sequências sedimentares cujo registro de proxies paleoambientais seja preservado e confiável. Este trabalho visa explicitar a possibilidade de desenvolver um novo indicador paleoambiental, baseado em propriedades magnéticas de cristais de magnetita biosintetizados por bactérias magnetotáticas. Tais cristais biogênicos são notavelmente sensíveis a pequenas variações de oxigênio dissolvido na coluna de água e na agua intersticial entre os poros dos sedimentos marinhos, e essa sensibilidade pode ser explorada para reconstruir variações climáticas no registro sedimentar do passado. Informações qualitativas estão disponíveis na literatura cientifica mas, até agora, não existem estudos que quantifiquem essa relação. Esta primeira tentativa de quantificar a preservação de magnetita biogênica em função do estado de oxigenação de águas se baseia em testemunhos coletados na costa do estado do Rio de Janeiro (Saco do Mamanguá, Paraty), onde foi possível coletar dados magnéticos relativos aos cristais de magnetita biogênica concomitantemente a medidas diretas de química da água intersticial. Os resultados desse primeiro estudo foram aplicados para reconstruções de variações climáticas e oceanográficas em duas seções que registram importantes mudanças climáticas durante o Paleogeno. No primeiro caso, um afloramento localizado na Turquia central de idade Eocenica e representativo de um período de rápido aquecimento (Ótimo Climático do Eoceno Médio, Middle Eocene climatic Optimum, MECO). No segundo caso, um testemunho coletado no Mar de Ross (Antártica) que abrange o período entre o Eoceno superior e o Mioceno médio, durante o qual houve a formação da primeira cobertura de gelo permanente no continente antártico

Fingerprints of partial oxidation of biogenic magnetite from cultivated and natural marine magnetotactic bacteria using synchrotron radiation

Magnetotactic bacteria are a multi‐phyletic group of bacteria that synthesize membrane‐bound magnetic minerals. Understanding the preservation of these minerals in various environments (e.g., with varying oxygen concentrations and iron supply) is important for understanding their role as carriers of primary magnetizations in sediments and sedimentary rocks. Here we present X‐ray near edge structure (XANES) spectra for Fe in magnetotactic bacteria samples from recent sediments to assess surface oxidation and crystal structure changes in bacterial magnetite during early burial. Our results are compared with a XANES spectrum of cultivated Magnetofaba australis samples, and with magnetic properties, and indicate that oxidation of magnetite to maghemite increases with depth in the sediment due to longer exposure to molecular oxygen. These results are relevant to understanding magnetic signatures carried by magnetofossils in oxic sediments and sedimentary rocks of different ages.A.P.R acknowledges funding from the Australian Research Council (grant DP140104544)

Paleolatitudes of Late Triassic radiolarian cherts from Argolis, Greece: Insights on the paleogeography of the western Tethys

The Hellenides fold-and-thrust range comprises two subparallel ophiolite belts of Triassic to Jurassic age – the external ophiolite belt (e.g., Mirdita, Pindos, Argolis) in the west, and the internal ophiolite belt in the east – broadly separated by the continental crust units of the Korabi–Pelagonian Zone. It is still a matter of debate whether these ophiolites derived from a single ocean (Meliata–Maliac–Vardar) or mark the suture of two distinct oceanic seaways (Pindos and Meliata–Maliac–Vardar). We contribute to the resolution of this controversy by studying the Migdalitsa Ophiolitic Complex in Argolis (Greece), which contains Triassic oceanic basalts and pertains to the external (western) ophiolite belt. Three key areas were mapped and several sites were targeted for structural, biostratigraphic, and paleomagnetic analyses. Radiolarian cherts in primary or tectonic contact with oceanic basalts were dated to the Late Triassic (Carnian–Norian) using radiolarians, and provided paleomagnetic directions of primary origin carried by magnetite and hematite. The derived mean direction was corrected for sedimentary inclination shallowing and yielded a paleolatitude of ~ 22°N that was placed in a broader paleogeographic context by reconstructing Pangea at ~ 225 ± 5 Ma using a recent apparent polar wander path corrected for sedimentary inclination shallowing. The Late Triassic paleogeography of the Tethys Ocean was constrained using additional paleolatitude estimates from the literature, which we checked and corrected (when possible) for sedimentary inclination shallowing. According to our reconstruction, the Meliata–Maliac–Vardar Ocean between Adria – the promontory of Africa – and Europe represents the locus of origin of the Late Triassic Argolis ophiolitic rocks of the external ophiolite belt, presently resting in tectonic contact on the Pindos–Subpelagonian zones, which represent a deep-water trough with no substantial evidence of in situ oceanization in the Triassic

Carbon cycle instability and orbital forcing during the Middle Eocene Climatic Optimum

The Middle Eocene Climatic Optimum (MECO) is a global warming event that occurred at about 40 Ma. In comparison to the most known global warming events of the Paleogene, the MECO has some peculiar features that make its interpretation controversial. The main peculiarities of the MECO are a duration of ~500 kyr and a carbon isotope signature that varies from site to site. Here we present new carbon and oxygen stable isotopes records (δ13C and δ18O) from three foraminiferal genera dwelling at different depths throughout the water column and the sea bottom during the middle Eocene, from eastern Turkey. We document that the MECO is related to major oceanographic and climatic changes in the Neo-Tethys and also in other oceanic basins. The carbon isotope signature of the MECO is difficult to interpret because it is highly variable from site to site. We hypothesize that such δ13C signature indicates highly unstable oceanographic and carbon cycle conditions, which may have been forced by the coincidence between a 400 kyr and a 2.4 Myr orbital eccentricity minimum. Such forcing has been also suggested for the Cretaceous Oceanic Anoxic Events, which resemble the MECO event more than the Cenozoic hyperthermals

Environmental magnetic implications of magnetofossil occurrence during the Middle Eocene Climatic Optimum (MECO) in pelagic sediments from the equatorial Indian Ocean

Magnetic properties of pelagic marine sediments that record the Middle Eocene Climatic Optimum (MECO) at ~. 40. Ma provide information about major environmental changes. The main variations observed during this transient warming event reflect a bacterial magnetofossil signal, but the cause of the linkage between bacterial production and climate remains unclear. We present an environmental magnetic study of middle Eocene deep-sea sediments from the northern edge of Madingley Rise (Ocean Drilling Program Hole 711A, equatorial Indian Ocean) to investigate the origin of the increased magnetic mineral concentration concomitant with subchron C18n.2n, which corresponds to the MECO interval in ODP Hole 711A. This magnetic mineral peak also coincides with a change in lithofacies from calcareous nannofossils to radiolarian ooze, and a slight increase in clay concentration. Magnetite is the main magnetic mineral in the MECO sediments, which occurs as magnetically non-interacting single domain biogenic particles. The increased magnetic mineral concentration across the MECO event is likely to have been caused by increased eolian iron fertilization. This is interpreted to have given rise to increased surface ocean productivity, where increased delivery of iron and nutrients to the seafloor enhanced magnetotactic bacterial populations during the MECO event

Paleoenvironmental signature of the Selandian-Thanetian Transition Event (STTE) and Early Late Paleocene Event (ELPE) in the Contessa Road section (western Neo-Tethys)

Sedimentary records of the Early Cenozoic indicate a series of events with climatic and carbon cycle variability known as hyperthermals. A similar to 350-kyr-long event of environmental disruption during the Paleocene, not described before and here named Selandian Thanetian Transition Event (SITE), has been recognized and well constrained in the western Tethys Contessa Road section (Gubbio, Italy) through high-resolution biostratigraphic, geochemical, and rock-magnetic data. The SITE exhibits peculiar stressed ecological responses among calcareous nannofossils and foraminifera, which highlight marked environmental perturbation affecting the biosphere. The environmental instability is not confined within the photic zone but extends to the seafloor leading to little more trophic conditions of the sea surface waters with an enhanced, but of short measure, nutrient availability on the seafloor conditions and marked rise of lysocline. Magnetic Susceptibly (MS) is dominantly controlled by the balance between carbonate productivity and detrital supply, as evidenced by the strong correlation between MS and CaCO3 (%) (r(2) = 0.72). However, we also document two components in the isothermal remanent magnetization (IRM) and first-order reversal curves (FORC) diagrams that prove the occurrence of biogenic magnetite throughout the SITE. Systematic variations in bio-geochemical and magnetic parameters show the relative abundance of carbonate production (or inversely dissolution of carbonate) versus detrital supply during the SITE, which induced higher populations of magnetotactic bacteria through increased terrigenous input and, therefore, increased nutrient supply. Noteworthy, the uppermost part of the SITE includes the equivalent of the suspected hyperthermal, short-lived Early Late Paleocene Event (ELPE). The ELPE event shows an episode of increase in magnetic properties of the sediments, including an increase in magnetofossil concentration, as indicated by IRM components and FORC diagrams. The comparison of biotic and abiotic records throughout the STTE at Contessa Road section with available data across the ELPE from former investigated ocean and land-based sites provides lines of evidence that this latter event might be indeed only the terminal part of a long-lasting environmental change than hitherto supposed

Diversions of the Ribeira River Flow and Their Influence on Sediment Supply in the Cananeia-Iguape Estuarine-Lagoonal System (SE Brazil)

The Cananéia-Iguape system is a combined estuarine-lagoonal sedimentary system, located along the SE coast of Brazil. It consists of a network of channels and islands oriented mainly parallel to the coast. About 165 years ago, an artificial channel, the Valo Grande, was opened in the northern part of this system to connect a major river of the region, the Ribeira River, to the estuarine-lagoon complex. The Valo Grande was closed with a dam and re-opened twice between 1978 and 1995, when it was finally left open. These openings and closures of the Valo Grande had a significant influence on the Cananéia-Iguape system. In this study we present mineralogical, chemical, palaeomagnetic, and geochronological data from a sediment core collected at the southern end of the 50 km long lagoonal system showing how the phases of the opening and closure of the channel through time are expressed in the sedimentary record. Despite the homogeneity of the grain size and magnetic properties throughout the core, significant variations in the mineralogical composition showed the influence of the opening of the channel on the sediment supply. Less mature sediment, with lower quartz and halite and higher kaolinite, brucite, and franklinite, corresponded to periods when the Valo Grande was open. On the other hand, higher abundance of quartz and halite, as well as the disappearance of other detrital minerals, corresponded with periods of absence or closure of the channel, indicating a more sea-influenced depositional setting. This work represented an example of anthropogenic influence in a lagoonal-estuarine sedimentary system, which is a common context along the coast of Brazil

Movement of carbon among estuarine habitats and its assimilation by invertebrates

We measured the extent of movement of carbon and its assimilation by invertebrates among estuarine habitats by analysing carbon stable isotopes of invertebrates collected along transects crossing the boundary of two habitats. The habitats were dominated by autotrophs with distinct isotope values: (1) mudflats containing benthic microalgae (mean −22.6, SE 0.6‰) and (2) seagrass and its associated epiphytic algae (similar values, pooled mean −9.8, 0.5‰). Three species of invertebrates were analysed: a palaemonid shrimp, Macrobrachium intermedium, and two polychaete worms, Nephtys australiensis and Australonereis ehlersi. All species had a similar narrow range of isotope values (−9 to −14‰), and showed no statistically significant relationship between position along transect and isotope values. Animals were relying on carbon from seagrass meadows whether they were in seagrass or on mudflats hundreds of metres away. Particulate organic matter collected from superficial sediments along the transects had similar values to animals (mean −11.1, SE 1.3‰) and also showed no significant relationship with position. The isotope values of these relatively immobile invertebrates and the particulate detritus suggest that carbon moves from subtidal seagrass meadows to mudflats as particulate matter and is assimilated by invertebrates. This assimilation might be direct in the case of the detritivorous worm, A. ehlersi, but must be via invertebrate prey in the case of the carnivorous worm, N. australiensis and the scavenging shrimp, M. intermedium. The extent of movement of carbon among habitats, especially towards shallower habitats, is surprising since in theory, carbon is more likely to move offshore in situations such as the current study where habitats are in relatively open, unprotected waters.Rod M. Connolly, Daniel Gorman and Michaela A. Gues