Universita` degli Studi di Urbino

We have conducted high-resolution paleomagnetic and rock magnetic studies, in addition to stable isotope analyses of the Massignano sedimentary section, which is the Global Stratotype Section and Point (GSSP) for the Eocene-Oligocene boundary. Our research builds upon the many past studies of the Massignano section in seeking to understand the timing and nature of the paleoenvironmental variations that occurred during the transition for the Earth’s climate system from greenhouse to icehouse. The new paleomagnetic results provide a refined magnetostratigraphy of the section and new age for the Eocene-Oligocene boundary at 33.7 Ma. Abrupt and large alternations in magnetic, concentration, composition, and grain sizes that occur in the high-resolution rock magnetic record are interpreted to be the result of rapid bimodal shifts in deep-sea circulation that affect sediment sources or transport. We speculate that currents flowing through the gateway between the Atlantic and Indo-pacific Oceans may have turned on and off as the gateway was progressively closing, resulting in the deposition of two different assemblages of magnetic minerals at Massignano. Finally, stable isotope (δ18O and δ13C) data collected on ostracod valves also suggest significant changes in sea bottom circulation in the Neo-Tethys Ocean at and about 2 m.y. before the Eocene-Oligocene boundary.

We report a high-resolution paleomagnetic investigation constrained by new qualitative and semi-quantitative analyses of planktic and benthic foraminifera, nannofossil assemblages, integrated with oxygen and carbon isotope measurements, for the middle Eocene Scaglia limestones of the Contessa Highway section, central Italy. Calcareous plankton assemblages enable recognition of several biostratigraphic events from planktic foraminiferal Zone P11 to the lower part of Zone P15 and from calcareous nannofossil Zone NP15 to the upper part of Zone NP17, which results in refinement of the magnetobiostratigraphy of the Contessa Highway section. Correlation of the paleomagnetic polarity pattern with the geomagnetic polarity timescale provides a direct age interpretation for strata around the middle Eocene Scaglia limestones of the Contessa Highway section, from Chrons C21n (47 Ma) through to Subchron C18n.1n (38.5 Ma). Bulk carbon isotope values indicate a distinct carbon isotopic shift at 40 Ma that is interpreted to represent the first evidence in the northern hemisphere of the middle Eocene climatic optimum (MECO) that has recently been observed as a stable isotope anomaly in multiple records from the Indian-Atlantic sector of the Southern Ocean. This demonstrates a global response of the carbon cycle to the proposed transient increased pCO2 levels during the late middle Eocene and a consequent global CO2-driven climate change.