Monechi, Simonetta

Biomagnetostratigraphic studies carried out at the Ypresian/Lutetian (Eocene) Gorrondatxe section solved major chronostratigraphic calibration discrepancies that had existed for several decades. The new calibration resulted in the definition of the Global Stratotype Section and Point (GSSP) for the Lutetian Stage at the Gorrondatxe layer containing the lowest occurrence (LO) of the calcareous nannofossil Blackites inflatus (base of Subzone CP12b). However, subsequent studies cast some doubt on the reliability of the Gorrondatxe biomagnetostratigraphic data and, consequently, the appropriateness of the criterion used for the definition of the Lutetian GSSP. In order to address the issues raised, the biomagnetostratigraphy of the Gorrondatxe section was revisited and an additional cyclostratigraphic analysis was undertaken using magnetic susceptibility data series. The present study shows that the Gorrondatxe biomagnetostratigraphy is reliable. The spectral analyses showed the dominance of precession (~20 ky) and short (~100 ky) eccentricity forcing on sedimentation, but the influence of obliquity (~38 and ~55 ky) cycles was also detected and modulation by long (405 ky) eccentricity cycles was deduced. The succession was tuned to the 49.122-48.084 Ma interval and the astronomical ages obtained for a significant number of biomagnetostratigraphic events compared well with those derived from other studies. One of the exceptions is the LO of B. inflatus, which seems to be older in Gorrondatxe than in the Mediterranean and Western Atlantic areas. This diachrony most likely also delayed the LO of B. inflatus in historical reference sections of the North Sea area, rendering previous claims that the Lutetian GSSP caused some historical Lutetian sections to be Ypresian in age invalid. In fact, the definition of the base of the Lutetian Stage by the LO of B. inflatus at Gorrondatxe, now astronomically age dated at 48.455 Ma, best guarantees that the original concepts of the Ypresian and Lutetian historical stratotypes are maintained. Combined with secondary biomagnetostratigraphic marker events and an astronomically tuned cyclostratigraphy, the Ypresian/Lutetian Gorrondatxe section provides the globally significant chronostratigraphic reference model expected of any GSSP.

The early Eocene paleoclimate record provides one of the best analogues for today’s 9 global warming. In order to reconstruct the evolution of the early Eocene paleoclimate, 10 and understand how environmental feedback mechanisms acted on it, an accurate time 11 framework is necessary. In this regard, the astronomically calibrated time scale (ATS) 12 provides the highest possible resolution, but models beyond 40-50 Ma are not fully 13 resolved and actual geological data are incomplete. With the aim of filling this gap, the 14 expanded lower Ypresian Arnakatxa section studied herein offered a potentially 15 valuable orbitally paced geological record. This outcrop displays a well-defined 16 arrangement of strata in couplets and bundles. The spectral analyses of colour data 17 series showed the dominance of two main periodicities, which were related to orbital 18 forcing on sedimentation by precession (20 kyr) and short (100 kyr) eccentricity cycles. 19 Despite not being represented in the spectrograms, the influence of long (405 kyr) 20 eccentricity on sedimentation was also deduced. Moreover, the disruption of the orbital 21 signal in the upper half of the Arnakatxa section correlates with a very long (2.4 Myr) 22 eccentricity node centred at ~54.6 Ma, which could also have caused the amplification 23 of the orbital signal related to obliquity (41 kyr). Taking everything into account, the 24 cyclostratigraphic analyses carried out in Arnakatxa resulted in a precessional scale orbital chronology for the time interval between 55.805 and 54.435 Ma (duration of 26 1.37 Myr). Thus, the Arnakatxa succession could be reliably correlated with Atlantic 27 ODP records, which are the main reference for Ypresian astrochronology, at 28 precessional scale. Furthermore, the results from Arnakatxa also help to identify the 29 astronomical solutions that better match actual geological data, contributing to the 30 construction of a definitive Ypresian ATS. In this regard, the Arnakatxa results are not a 31 good fit for solutions La10a, La10d, La11 and ZB18, but match well with the previously 32 thought to be less reliable solutions La10b and La10c.

The Global Stratotype Section and Point (GSSP) for the base of the Bartonian, currently remains undefined. The Bartonian unit stratotype is located at the Barton coastal section in the Hampshire Basin, on the South Coast of the UK. The base of the “Barton beds” was originally placed at the lowest occurrence of Nummulites prestwichianus, and this is still the basis of the recognition of the unit Bartonian Stage as a formal chronostratigraphic unit of the Paleogene. However, this biostratigraphic marker is not widely applicable elsewhere. The base of the lithostratigraphic unit, the Barton Clay Formation, also extends below this level creating further complication. The parastratotype section is located at Alum Bay, 7 km away, on the Isle of Wight. Despite a number of studies carried out in 1970s and ‘80s on both sections, global correlation remains problematic. Here we present an integrated (micropalaeontological, stratigraphic, palaeomagnetic) study of the Lutetian-Bartonian transition at Alum Bay, and aim at improving the stratigraphy of this section to better define the base of the Bartonian and contribute towards a decision on the GSSP.

During the middle Eocene to early Oligocene Earth transitioned from a greenhouse to an icehouse climate state. The interval comprises the Middle Eocene Climatic Optimum (MECO; ~40 Ma) and a subsequent long-term cooling trend that culminated in the Eocene-Oligocene transition (EOT; ~34 Ma) with the Oi-1 glaciation. Here, we present a refined calcareous nannofossil biostratigraphy and an orbitally tuned age model for the Monte Cagnero (MCA) section spanning the middle Eocene to the early Oligocene (~41 to ~33 Ma). Spectral analysis of magnetic susceptibility (MS) data displays strong cyclicities in the orbital frequency band allowing us to tune the identified 405 kyr eccentricity minima in the MS record to their equivalents in the astronomical solution. Our orbitally tuned age model allows us to estimate the position and duration of polarity chrons (C18 to C13) and compare them with other standard and orbitally tuned ages. We were also able to constrain the timing and duration of the MECO event, which coincides with a minimum in the 2.4 Myr and 405 kyr eccentricity cycles. Our study corroborates the previous estimated age for the base of the Rupelian stage (33.9 Ma) and estimates the base of the Priabonian stage in the MCA section to be at 37.4 Ma. Finally, calcareous nannofossils with known paleoenvironmental preferences suggest a gradual shift from oligotrophic to meso-eutrophic conditions with an abrupt change at ~36.8 Ma. Besides, nannofossil assemblages suggest that enhanced nutrient availability pre- ceded water cooling at the late Eocene. Altogether, this evidence points to a poorly developed water column stratification prior to the cooling trend.

The GSSP for the base of the Chattian Stage (Paleogene System, Oligocene Series) is defined at meter level 197 in the Monte Cagnero section, which belongs to the pelagic succession of the Umbria–Marche basin (Urbania, central Italy: 43°38′47.81″N–12°28′03.83″E). This level with an astronomical age of 27.82 Ma coincides with the highest common occurrence of the planktonic foraminifer Chiloguembelina cubensis at the base of planktonic foraminiferal O5 Zone and falls in the upper part of calcareous nannofossil NP24 Zone, in the lower part of dinocyst Dbi Zone, and in the lower Chron C9n. The proposal was approved by the International Subcommission of Paleogene Stratigraphy in July 2015, approved by the International Commission of Stratigraphy in August 2016, and ratified by the International Union of Geological Sciences in September 2016.

The Global Stratotype Section and Point (GSSP) for the base of the Bartonian (middle Eocene) stage is as yet undefined. Here, we assess the GSSP potential of the La Acebosa Formation exposed on the eastern side of the Cape of Oyambre (San Vicente de la Barquera, province of Cantabria, northern Spain). Sedimentological and benthic foraminiferal data indicate that the La Acebosa Formation represents a bathyal environment related to a deepening, eastwardfacing slope. Preliminary planktic foraminiferal and calcareous nannofossil results show that the upper part of the section corresponds to zones E11 and CP14a, respectively, and thus could include the Lutetian–Bartonian boundary. Although the quality of demagnetization is quite poor, preliminary magnetostratigraphic data suggest that Chron C19n, which is most likely to define the Bartonian GSSP, could be recorded in the upper part of the La Acebosa Formation. A denser magnetostratigraphic and biostratigraphic sampling is required in the upper part of this unit in order to accurately identify Chron C19n and to establish the sequence of events across the Lutetian–Bartonian transition.

The Global Stratotype Section and Point (GSSP) for the base of the Bartonian (middle Eocene) stage is as yet undefined. Herein we assess the GSSP potential of the successions found in the Basque–Cantabrian and Aquitanian basins (western Pyrenees). On the basis of the available data, no outcrop in the Biarritz and Pamplona areas fulfilled the requirements outlined by the International Commission on Stratigraphy. However, the succession exposed on the eastern side of the Cape of Oyambre (San Vicente de la Barquera, province of Cantabria, northern Spain) did so, and yielded positive preliminary results.

The global stratotype sections and points for the bases of the Selandian (Middle Paleocene) and Thanetian (Upper Paleocene) stages have been defined in the coastal cliff along the Itzurun Beach at the town of Zumaia in the Basque Country, northern Spain. In the hemipelagic section exposed at Zumaia the base of the Selandian Stage has been placed at the base of the Itzurun Formation, ca. 49 m above the Cretaceous/ Paleogene boundary. At the base of the Selandian, marls replace the succession of Danian red limestone and limestone-marl couplets. The best marine, global correlation criterion for the basal Selandian is the second radiation of the important calcareous nannofossil group, the fasciculiths. Species such as Fasciculithus ulii, F. billii, F. janii, F. involutus, F.pileatus and F. tympaniformis have their first appearance in the interval from a few decimetres below up to 1.1 m above the base of the Selandian. The marker species for nannofossil Zone NP5, F. tympaniformis, first occurs 1.1 m above the base. Excellent cyclostratigraphy and magnetostratigraphy in the section creates further correlation potential, with the base of the Selandian occuring 30 precession cycles (630 kyr) above the top of magnetochron C27n. Profound changes in sedimentology related to a major sea-level fall characterize the Danian-Selandian transition in sections along the margins of the North Atlantic. The base of the Thanetian Stage is placed in the same section ca. 78 m above the Cretaceous/Paleogene boundary. It is defined at a level 2.8 m or eight precession cycles above the base of the core of the distinct clay-rich interval associated with the Mid-Paleocene Biotic Event, and it corresponds to the base of magnetochron C26n in the section. The base of the Thanetian is not associated with any significant change in marine micro-fauna or flora. The calcareous nannofossil Zone NP6, marked by the first occurrence of Heliolithus kleinpelli starts ca. 6.5 m below the base of the Thanetian. The definitions of the global stratotype points for the bases of the Selandian and Thanetian stages are in good agreements with the definitions in the historical stratotype sections in Denmark and England, respectively.

The GSSP for the base of the Lutetian Stage (early/ middle Eocene boundary) is defined at 167.85 metres in the Gorrondatxe sea-cliff section (NW of Bilbao city, Basque Country, northern Spain; 43º22'46.47" N, 3º 00' 51.61" W). This dark marly level coincides with the lowest occurrence of the calcareous nannofossil Blackites inflatus (CP12a/b boundary), is in the middle of polarity Chron C21r, and has been interpreted as the maximumflooding surface of a depositional sequence that may be global in extent. The GSSP age is approximately 800 kyr (39 precession cycles) younger than the beginning of polarity Chron C21r, or ~47.8 Ma in the GTS04 time scale. The proposal was approved by the International Subcommission on Paleogene Stratigraphy in February 2010, approved by the International Commission of Stratigraphy in January 2011, and ratified by the International Union of Geological Sciences in April 2011.

The Oligocene represents an important time period from a wide range of perspectives and includes significant climatic and eustatic variations. The pelagic succession of the Umbria-Marche Apennines (central Italy) includes a complete and continuous sequence of marly limestones and marls, with volcaniclastic layers that enable us to construct an integrated stratigraphic framework for this time period. We present here a synthesis of detailed biostratigraphic, magnetostratigraphic, and chemostratigraphic studies, along with geochronologic results from several biotite-rich volcaniclastic layers, which provide the means for an accurate and precise radiometric calibration of the Oligocene time scale. From this study, the interpolated ages for the Rupelian/Chattian stage boundary, located in the upper half of Chron 10n at meter level 188 in the Monte Cagnero section, and corresponding to the O4/O5 planktonic foraminiferal zonal boundary, are 28.36 Ma (paleomagnetic interpolation), 28.27 ± 0.1 Ma (direct radioisotopic dating), and 27.99 Ma (astrochronological interpolation). These ages appear to be slightly younger than those reported in recent chronostratigraphic time scale compilations. The Monte Cagnero section is a potential candidate for defining the Chattian Global Stratotype Section and Point (GSSP) and some reliable criteria are here proposed for marking the Rupelian/Chattian boundary according to International Union of Geological Sciences (IUGS) recommendations.