Iron isotope signature of magnetofossils and oceanic biogeochemical changes through the Middle Eocene Climatic Optimum.

21 pagesInternational audienceMagnetotactic bacteria (MTB) intracellularly precipitate magnetite (Fe3O4) crystals that can be preserved in the geological record. When MTB die, the so-called magnetofossils constitute valuable proxies for paleoenvironmental reconstructions and are suspected to represent some of the oldest traces of biomineralization on Earth. Yet, the biogenicity of putative magnetofossils found in ancient terrestrial and extra-terrestrial samples is still largely debated and their significance for past climate still holds uncertainties. Here we studied a sedimentary sequence from the Indian Ocean (ODP Hole 711A) recording the Middle Eocene Climatic Optimum (MECO) through which a magnetofossil-rich interval was deposited. We investigated for the first time the potential of Fe isotopes as a biosignature in magnetofossils and thoroughly describe MECO related paleoenvironmental disruptions based on major and trace element concentrations. Bulk sediment Fe isotopes showed limited variations, with δ56Fe around −0.13 ± 0.04‰ (n = 24), linked to detrital iron rather than MTB activity. Hence, a sequential chemical extraction protocol was applied to determine the specific composition of magnetite. We discuss analytical biases related to this protocol (i.e. partial phyllosilicate and Mn-oxide leaching) and apply corrections to the data. Outside the magnetofossil-rich interval, Fe isotope compositions of oxides (mainly biotic and/or abiotic magnetites and possibly Fe coprecipitated with Mn-oxides) display a small range averaging −0.54 ± 0.05‰, and are interpreted as reflecting dominantly hydrothermal contribution, a conclusion also supported by prominent Eu anomaly. In contrast, the magnetofossil-rich interval shows larger δ56Fe variability in oxides, from −0.12 to −0.94‰, decreasing upwards in the stratigraphic section. This interval likely records enhanced Fe supply from atmospheric fallout, increase in biological productivity (illustrated by increased Ba accumulation rate) and subsequent development of ferruginous conditions in the sediment porewater. Covariations of Fe isotope compositions and Mn/Fe ratios can be explained by a vertical migration of a redox front and associated diagenetic modifications. Precipitation of barite (BaSO4) in the sediments after organic matter decay probably favored the preservation of magnetofossils by decreasing SO42- concentration in porewaters and subsequent H2S production, which usually dissolve magnetite in the sulfidic zone. Finally, we model the evolution of porewater fluid and estimate Fe isotope fractionation between magnetofossils and fluid to Δ56Femag-Fe(II)aq = 0.1–0.3‰, a value significantly different from abiotic magnetite fractionation (~1.5‰). Contrasting with recent results on MTB laboratory culture, no mass independent fractionation of Fe isotopes was observed in the present study. Nevertheless, the diverse geochemical proxies presented here provide important constraints on paleoclimate and magnetofossil biogenicity evaluation

Middle Eocene to early Oligocene magnetostratigraphy of ODP Hole 711A (Leg 115), western equatorial Indian Ocean

Ocean Drilling Program (ODP) Site 711, located in the western equatorial Indian Ocean near the Seychelles Archipelago on Madingley Rise, is an important site for studying middle Eocene to early Oligocene climatic evolution. This site is ideal for studying the impact of Neo-Tethyan gateway closure on Indian Ocean currents and circulation to further understand global climate changes through the greenhouse to icehouse transition. Middle Eocene-to-lower Oligocene strata recovered within Hole 711A (Cores 711A-14X to 21X) primarily consist of clay-bearing nannofossil oozes/chalks, with layers rich in radiolarians. Here, we report a high-resolution magnetostratigraphic record and a new integrated age model for the middle Eocene-to-lower Oligocene section of Hole 711A. Correlation of the polarity pattern to the geomagnetic polarity timescale provides a record from Chron C19r (middle Eocene) to C12r (early Oligocene). Our results extend the existing polarity record down into the middle Eocene and confirm published results from the lower Oligocene section of the hole. Overall, these new results from Hole 711A have important implications for identifying and dating global climate change events, and for reconstructing calcite compensation depth history at this site

The Eocene Thermal Maximum 3: Reading the environmental perturbations at Gubbio (Italy)

The Paleocene-early Eocene interval is punctuated by a series of transient warming events known as hyperthermals that have been associated with changes in the carbon isotope composition of the ocean-atmosphere system. Here we present and discuss a detailed record of calcareous nannofossil and foraminiferal assemblages coupled with high-resolution geochemical, isotopic, and environmental magnetic records across the middle Ypresian at the Contessa Road section (Gubbio, Italy). This allows characterization of the Eocene Thermal Maximum 3 (ETM3, K or X) and recognition of four minor (I1, I2, J, L) hyperthermals. At the Contessa Road section, the ETM3 is marked by short-lived negative excursions in both δ13C and δ18O, pronounced changes in rock magnetic properties, and calcium carbonate reduction. These changes coupled with the moderate to low state of preservation of calcareous nannofossils and planktonic foraminifera, higher FI and agglutinated foraminifera values, along with a lower P/(P + B) ratio (P-planktonic; B-benthic) and coarse fractions provide evidence of enhanced carbonate dissolution during the ETM3. A marked shift toward warmer and more oligotrophic conditions has been inferred that suggests unstable and perturbed environmental conditions both in the photic zone and at the seafloor

Mid-Cretaceous marine Os isotope evidence for heterogeneous cause of oceanic anoxic events

: During the mid-Cretaceous, the Earth experienced several environmental perturbations, including an extremely warm climate and Oceanic Anoxic Events (OAEs). Submarine volcanic episodes associated with formation of large igneous provinces (LIPs) may have triggered these perturbations. The osmium isotopic ratio (187Os/188Os) is a suitable proxy for tracing hydrothermal activity associated with the LIPs formation, but 187Os/188Os data from the mid-Cretaceous are limited to short time intervals. Here we provide a continuous high-resolution marine 187Os/188Os record covering all mid-Cretaceous OAEs. Several OAEs (OAE1a, Wezel and Fallot events, and OAE2) correspond to unradiogenic 187Os/188Os shifts, suggesting that they were triggered by massive submarine volcanic episodes. However, minor OAEs (OAE1c and OAE1d), which do not show pronounced unradiogenic 187Os/188Os shifts, were likely caused by enhanced monsoonal activity. Because the subaerial LIPs volcanic episodes and Circum-Pacific volcanism correspond to the highest temperature and pCO2 during the mid-Cretaceous, they may have caused the hot mid-Cretaceous climate

Integrated magnetostratigraphy, biostratigraphy, and chronostratigraphy of the Paleogene pelagic succession at Gubbio (central Italy)

The Contessa Valley and the Bottaccione Gorge located close to Gubbio (central Italy) include some of the most complete successions of Paleogene sediments known from the Tethyan realm. Owing to the continuous deposition in a pelagic setting, a rather modest tectonic overprint, and the availability of excellent age control through magnetostratigraphy, biostratigraphy, chronostratigraphy, and tephrostratigraphy, and direct radioisotopic dates from interbedded volcaniclastic layers, these sediments have played a prominent role in the establishment of standard Paleogene time scales. We present here a complete and well-preserved Paleogene pelagic composite succession of the Gubbio area that provides the means for a more accurate and precise calibration of the Paleogene time scale. As a necessary step toward the compilation of a more robust database on a wide scale so to improve the magnetostratigraphic, biostratigraphic, and chronostratigraphic framework of the classical Tethyan zonations, enabling regional and supraregional correlations, we have constructed a record of reliable Paleogene planktonic foraminifera, calcareous nannofossil, and dinocyst biohorizons commonly used in tropical to subtropical Cenozoic zonations. In addition, an age model is provided for the Paleogene pelagic composite succession based on magnetostratigraphy, planktonic foraminifera, calcareous nannofossils, and dinocysts that contributes to an integrated chronology for the Paleogene Tethyan sediments from 66 to 23 Ma

Integrated magnetobiostratigraphy of the middle Eocene–lower Oligocene interval from the Monte Cagnero section, central Italy

The Monte Cagnero sedimentary section, which crops out in the northeastern Apennines near Urbania in the Umbria–Marche Basin (Italy), contains well-exposed strata spanning the middle Eocene to lower Oligocene interval. We use an integrated magnetobiostratigraphic approach to generate a high-resolution age model for the Monte Cagnero section, with the goal of obtaining a reliable chronostratigraphic framework for studying Eocene–Oligocene palaeoceanographic changes during the switch from greenhouse to icehouse conditions. The studied sediments consist of alternating reddish and greenish limestones and marlstones. A new integrated age model for the section is based on high-resolution palaeomagnetic analyses, combined with detailed planktonic

Middle Eocene to early Oligocene magnetostratigraphy of ODP Hole 711A (Leg 115), western equatorial Indian Ocean

Ocean Drilling Program (ODP) Site 711, located in the western equatorial Indian Ocean near the Seychelles Archipelago on Madingley Rise, is an important site for studying middle Eocene to early Oligocene climatic evolution. This site is ideal for studying the impact of Neo-Tethyan gateway closure on Indian Ocean currents and circulation to further understand global climate changes through the greenhouse to icehouse transition. Middle Eocene-to-lower Oligocene strata recovered within Hole 711A (Cores 711A-14X to 21X) primarily consist of clay-bearing nannofossil oozes/chalks, with layers rich in radiolarians. Here, we report a high-resolution magnetostratigraphic record and a new integrated age model for the middle Eocene-to-lower Oligocene section of Hole 711A. Correlation of the polarity pattern to the geomagnetic polarity timescale provides a record from Chron C19r (middle Eocene) to C12r (early Oligocene). Our results extend the existing polarity record down into the middle Eocene and confirm published results from the lower Oligocene section of the hole. Overall, these new results from Hole 711A have important implications for identifying and dating global climate change events, and for reconstructing calcite compensation depth history at this site

Middle Eocene to early Oligocene magnetostratigraphy of ODP Hole 711A (Leg 115), western equatorial Indian Ocean

<p>Ocean Drilling Program (ODP) Site 711, located in the western equatorial Indian Ocean near the Seychelles Archipelago on Madingley Rise, is an important site for studying middle Eocene to early Oligocene climatic evolution. This site is ideal for studying the impact of Neo-Tethyan gateway closure on Indian Ocean currents and circulation to further understand global climate changes through the greenhouse to icehouse transition. Middle Eocene-to-lower Oligocene strata recovered within Hole 711A (Cores 711A-14X to 21X) primarily consist of clay-bearing nannofossil oozes/chalks, with layers rich in radiolarians. Here, we report a high-resolution magnetostratigraphic record and a new integrated age model for the middle Eocene-to-lower Oligocene section of Hole 711A. Correlation of the polarity pattern to the geomagnetic polarity timescale provides a record from Chron C19r (middle Eocene) to C12r (early Oligocene). Our results extend the existing polarity record down into the middle Eocene and confirm published results from the lower Oligocene section of the hole. Overall, these new results from Hole 711A have important implications for identifying and dating global climate change events, and for reconstructing calcite compensation depth history at this site. </p

High-resolution multiproxy cyclostratigraphic analysis of environmental and climatic events across the Cretaceous-Paleogene boundary in the classic pelagic succession of Gubbio (Italy)

We studied a high-resolution multiproxy data set, including magnetic susceptibility (MS), CaCO3 content, and stable isotopes (δ18O and δ13C), from the stratigraphic interval covering the uppermost Maastrichtian and the lower Danian, represented by the pelagic limestones of the Scaglia Rossa Formation continuously exposed in the classic sections of the Bottaccione Gorge and the Contessa Highway near Gubbio, Italy. Variations in all the proxy series are periodic and reflect astronomically forced climate changes (i.e. Milankovitch cycles). In particular, the MS proxy reflects variations in the terrigenous dust input in this pelagic, deep-marine environment. We speculate that the dust is mainly eolian in origin and that the availability and transport of dust are influenced by variations in the vegetation cover on the Maastrichtian-Paleocene African or Asian zone, which were respectively located at tropical to subtropical latitudes to the south or far to the east of the western Tethyan Umbria-Marche Basin, and were characterized by monsoonal circulation. The dynamics of monsoonal circulation are known to be strongly dependent on precession-driven and obliquity- driven changes in insolation. We propose that a threshold mechanism in the vegetation coverage may explain eccentricity-related periodicities in the terrigenous eolian dust input. Other mechanisms, both oceanic and terrestrial, that depend on the precession amplitude modulated by eccentricity, can be evoked together with the variation of dust influx in the western Tethys to explain the detected eccentricity periodicity in the δ13C record. Our interpretations of the δ18O and MS records suggest a warming event ∼400 k.y. prior to the Cretaceous-Paleogene (K-Pg) boundary, and a period of climatic and environmental instability in the earliest Danian. Based on these multiproxy phase relationships, we propose an astronomical tuning for these sections; this leads us to an estimate of the timing and duration of several late Maastrichtian and Danian biostratigraphic and magnetostratigraphic events