Luca Lanci

A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion

Abstract We hypothesize that the rapid onset of the carbon isotope excursion (CIE) at the Paleocene/Eocene boundary (∼55 Ma) may have resulted from the accretion of a significant amount of 12C-enriched carbon from the impact of a ∼10 km comet, an event that would also trigger greenhouse warming leading to the Paleocene/Eocene thermal maximum and, possibly, thermal dissociation of seafloor methane hydrate. Indirect evidence of an impact is the unusual abundance of magnetic nanoparticles in kaolinite-rich shelf sediments that closely coincide with the onset and nadir of the CIE at three drill sites on the Atlantic Coastal Plain. After considering various alternative mechanisms that could have produced the magnetic nanoparticle assemblage and by analogy with the reported detection of iron-rich nanophase material at the Cretaceous/Tertiary boundary, we suggest that the CIE occurrence was derived from an impact plume condensate. The sudden increase in kaolinite is thus thought to represent the redeposition on the marine shelf of a rapidly weathered impact ejecta dust blanket. Published reports of a small but significant iridium anomaly at or close to the Paleocene/Eocene boundary provide supportive evidence for an impact.

Emergence of Venice during the Pleistocene

The Pleistocene history of sea-level change for the Venice region was reconstructed using an integrated magneto-bio-cyclostratigraphy of lithofacies and a published palynofloral analysis of continuously cored sediments in a 950-meter-deep drill core. The basin in which the Venice region is located collapsed at B1.8 Ma with slow sediment accumulation in the deeper-water starved basin during most of the Matuyama polarity chron but shoaled rapidly in the early and middle Brunhes in response to a major phase of deltaic progradation. The initial transition to continental sediments occurred during a prominent glacioeustatic low-stand that is likely to be MIS 12 (B0.43 Ma) but could be as young as MIS 8 (B0.25 Ma). The Venice area oscillated from below sea level during subsequent major glacioeustatic high-stands to becoming increasingly emergent during major low-stands as the basin continued to fill with marine and continental sediments. Some parts of the Venice area are now emergent for the first time during a glacioeustatic high-stand (i.e., MIS 1 or the Holocene). The total long-term subsidence rate estimated from the VENICE-1 record is less than 0.5 mm/yr, considerably slower than estimates for the Holocene and especially the modern anthropogenic period. r 2002 Elsevier Science Ltd. All rights reserved.

Integrated biomagnetostratigraphy of the Alano section (NE Italy): A proposal for defining the middle-late Eocene boundary

The Alano section has been presented at the International Subcommission on Paleogene Stratigraphy (ISPS) as a potential candidate for defi ning the global boundary stratotype section and point (GSSP) of the late Eocene Priabonian Stage. The section is located in the Venetian Southern Alps of the Veneto region (NE Italy), which is the type area of the Priabonian, being exposed along the banks of the Calcino torrent, near the village of Alano di Piave. It consists of ~120–130 m of bathyal gray marls interrupted in the lower part by an 8-m-thick package of laminated dark to black marlstones. Intercalated in the section, there are eight prominent marker beds, six of which are crystal tuff layers, whereas the other two are bioclastic rudites. These distinctive layers are useful for regional correlation and for an easy recognition of the various intervals of the section. The section is easily accessible, crops out continuously, is unaffected by any structural deformation, is rich in calcareous plankton, and contains an expanded record of the critical interval for defi ning the GSSP of the Priabonian. In order to further check the stratigraphic completeness of the section and constrain in time the critical interval for defi ning the Priabonian Stage, we performed a high-resolution study of integrated calcareous plankton biostratigraphy and a detailed magnetostratigraphic analysis. Here, we present the results of these studies to open a discussion on the criteria for driving the “golden spike” that should define the middle Eocene–late Eocene boundary.

Introduction of thermal activation in forward modeling of hysteresis loops for single‐domain magnetic particles and implications for the interpretation of the Day diagram

[1] Synthetic hysteresis loops were generated by numerically solving the classical Stoner-Wohlfarth model and a thermally activated Stoner-Wohlfarth model for a set of randomly oriented magnetic grains. Although computationally intensive this method allows forward modeling of hysteresis loops of single-domain (SD) and viscous grains. In the classic Stoner-Wohlfarth model the shape of the modeled loops can be modified by changing the distribution of the anisotropy energy but all the loops will all have similar hysteresis parameters M sr /M s and H cr /H c corresponding to that of a theoretical assemblage of SD particles. The thermally activated Stoner-Wohlfarth model, which allows the magnetic moment of each grain to switch between two energy minima according to Boltzmann statistics, extends the SD model toward superparamagnetic (SP) grains and introduces a volume dependency. Numerical simulation using the thermally activated model shows that the shapes of SD loops are modified by the effect of the thermal energy if the particles are sufficiently small. The major effect of the thermal disturbance is observed in highly viscous particles (smaller than approximately 0.03 μm in diameter, for magnetite) where it strongly reduces the coercivity and to a lesser extent the remanent magnetization. The effect on the hysteresis parameters is a large increase in H cr /H c and a decrease in M sr /M s , by factors that vary with anisotropy distribution, grain volume and measurement time. For certain grain sizes, these result in hysteresis parameters that are similar to those attributed to pseudosingle-domain (PSD) grains.

Organic carbon burial following the middle Eocene climatic optimum in the central western Tethys

We present trace metal geochemistry and stable isotope records for the middle Eocene Alano di Piave section, NE Italy, deposited during magnetochron C18n in the marginal Tethys Ocean. We identify a

Evidence for active El Niño Southern Oscillation variability in the Late Miocene greenhouse climate

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Magnetostratigraphy of the Eocene/Oligocene boundary in a short drill-core

500 kyr long carbon isotope perturbation event we infer to be the middle Eocene Climatic Optimum (MECO) confirming the northern hemisphere expression and global occurrence of MECO. Interpreted peak climatic conditions are followed by the rapid deposition of two organic rich intervals (

Magnetostratigraphic evidence that ‘tiny wiggles’ in the oceanic magnetic anomaly record represent geomagnetic paleointensity variations

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Antarctic Ice Sheet variability across the Eocene-Oligocene boundary climate transition.

3\% TOC) and contemporaneous positive

Mineral magnetic record of Holocene environmental changes in Sägistalsee, Switzerland

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