Payros, Aitor

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 This diachrony most likely also delayed the LO of B. inflatus in historical reference sections of the North Sea area, rendering previous claims that the Lutetian GSSP caused some historical Lutetian sections to be Ypresian in age invalid.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 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 and understand how environmental feedback mechanisms acted on it, an accurate time framework is necessary. In this regard, the astronomically calibrated time scale (ATS) provides the highest possible resolution, but models beyond 40-50 Ma are not fully resolved and actual geological data are incomplete. With the aim of filling this gap, the expanded lower Ypresian Arnakatxa section studied herein offered a potentially valuable orbitally paced geological record. This outcrop displays a well-defined arrangement of strata in couplets and bundles. The spectral analyses of colour data series showed the dominance of two main periodicities, which were related to orbital forcing on sedimentation by precession (20 kyr) and short (100 kyr) eccentricity cycles. Despite not being represented in the spectrograms, the influence of long (405 kyr) eccentricity on sedimentation was also deduced. Moreover, the disruption of the orbital signal in the upper half of the Arnakatxa section correlates with a very long (2.4 Myr) eccentricity node centred at ~54.6 Ma, which could also have caused the amplification of the orbital signal related to obliquity (41 kyr). Taking everything into account, the 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 1.37 Myr). Thus, the Arnakatxa succession could be reliably correlated with Atlantic ODP records, which are the main reference for Ypresian astrochronology, at precessional scale. Furthermore, the results from Arnakatxa also help to identify the astronomical solutions that better match actual geological data, contributing to the construction of a definitive Ypresian ATS. In this regard, the Arnakatxa results are not a good fit for solutions La10a, La10d, La11 and ZB18, but match well with the previously thought to be less reliable solutions La10b and La10c.

Multivariate analysis of the elemental composition of hemipelagic sedimentary successions has provided invaluable information about palaeoenvironmental evolution, including records of short-lived Eocene hyperthermal events. However, few studies have analyzed the sedimentary record of these climatic events in turbidite-rich continental margin successions. In order to test the usefulness of multivariate statistical techniques (factor and cluster analysis) in palaeonvironmental and palaeoclimatic research on turbiditic successions, the lowermost Eocene Solondota section, which accumulated on the North Iberian continental margin, was studied. A prominent negative carbon isotope excursion from Solondota was correlated with the Ypresian (early Eocene) hyperthermal event J, also known as C24n.2rH1. High-resolution sedimentological, geochemical (stable isotopes, major and trace elements) and mineralogical (bulk and clay mineralogy) data show that multivariate statistical analysis helps to manage large-sized quantitative datasets objectively, avoiding arbitrary choice of representative elements and identifying environmental factors (virtual variables) that may not be evident otherwise. Variations in major and minor elements from hemipelagic carbonates across the Solondota carbon isotope excursion suggest a temporarily more humid continental climate, which caused increased terrigenous material input into the marine environment. The finer grained fraction boosted hemipelagic carbonate dilution, whereas the coarser grained sediment was transported by temporarily more frequent and voluminous turbidity currents. Thus, the results from the Solondota carbon isotope excursion revealed similarities with deep marine records of other early Eocene minor hyperthermal events. This demonstrates the validity of deep-marine turbiditic successions for providing reliable sedimentological, mineralogical and geochemical records of global palaeoclimatic significance, complementing the information obtained from other sedimentary environments. Furthermore, the generally expanded nature of turbiditic successions can potentially provide palaeoclimatic information at very high resolution, enriching, and perhaps improving, the commonly condensed and sometimes discontinuous record of hemipelagic- only successions.

The astronomical timescale accuracy generally exceeds other dating methods. Precise age models are pivotal for paleoclimatic research. The middle Eocene astronomical timescale has been poorly constrained due to scarcity of suitable records leading to the so call ‘‘Eocene astronomical timescale gap.’’ We present magnetic susceptibility and color proxy records from an expanded 60 m long cyclic hemipelagic succession from the Oyambre Cape in northern Spain ( 1.3 My long stratigraphic section tuned to the 43.1–44.4 Ma interval in the Lutetian stage). We use the strong eccentricity amplitude modulation of precession in the sedimentary record for orbital tuning. The tuned record is correlated at precession level with previously tuned Ocean Drilling Program (ODP) Site 1260 from the equatorial Atlantic (the only oceanic record that registers geochemical variations in the precession band) and to other lower resolution deep-sea records at eccentricity level from the Southern Atlantic. Our data is consistent with a very long eccentricity minimum (driven by a 2.4 My periodicity) at 43.15 Ma in the orbital solutions and an age for the C20n/ C20r reversal boundary at 43.45 Ma. However, we challenge previous correlations between these Atlantic sites (shifts of one 100 ky eccentricity cycle). Data allows to rule out correlation to either younger or older 405 ky eccentricity cycles, which constrains chronologies for the middle Eocene, emphasizing the need for consistent astrochronological frameworks involving expanded outcrops. This should aid to overcome oceanic drilling shortcomings and sedimentary complexities. Our study highlights this integration need to achieve accuracy and stability of orbital timescales underpinning Eocene paleoclimatic records.

The last hyperthermal event in the Eocene, the Late Lutetian Thermal Maximum or Chron C19r event, took place at ~41.5 Ma, during a long-term global cooling phase which occurred between the warm Early Eocene Climatic Optimum and the icehouse Oligocene Epoch. This paleoclimatic event was first identified in the Equatorial Atlantic Ocean Drilling Program (ODP) Site 1260 as an abrupt peak in bulk Fe content and a short-lived decline in stable isotopes (δ13C, δ18O) and carbonate content. Additional studies have recently been carried from the Southern Atlantic ODP sites 702 and 1263. However, many issues were not addressed at these deep-sea sites and no land-based record of the event had been studied. Therefore, the beach cliff at Cape Oyambre (N Spain) was analyzed with the aim of identifying the C19r event and investigating its paleoenvironmental impact. Using magnetostratigraphic and biostratigraphic information, the astronomically tuned cyclo-stratigraphic record from Oyambre was accurately correlated with ODP Site 1260. This, combined with stable isotope data, allowed identification of the event in a conspicuous dark marl bed. Given that the associated negative carbon isotope excursion extends for 2/3 of a precession-driven hemicouplet, a 7–11 kyr duration was estimated, which agrees with recent estimates from the Atlantic deep-sea sites. Exceptional insolation conditions were found to have accelerated the hydrological cycle, increasing rainfall and runoff on land and terrestrial sediment input to the sea, which resulted in relatively low carbonate content in the deep-sea sediments. The terrestrial input also caused seawater eutrophication and freshening, leading to low δ13C and δ18O values, increased abundance of autochthonous and reworked calcareous nannofossil taxa, peaks in the abundance of opportunistic Reticulofenestra<5μm and opportunistic benthic foraminifera, and a reduction in the abundance of the oligotrophic calcareous nannofossil Zygrhablithus bijugatus. However, neither intensified carbon-gas driven greenhouse effect nor warming over and above natural fluctuations could be demonstrated from the Oyambre data.