Astrochronology of the Mediterranean Miocene: Linking palaeoenvironmental changes to gateway dynamics. Geologica Ultraiectina (295)

The semi-enclosed land-locked configuration makes the Mediterranean basin an ideal natural laboratory to study both, fundamental plate tectonic processes and astronomically induced variations in the climate and their influences on the marine and continental realms. The prime disruption of the Tethys seaway along the colliding African-Arabian and Eurasian plates disconnected the Mediterranean from the Indian Ocean. This disruption is believed to have triggered the Middle Miocene Climatic Transition. During the late Miocene, Mediterranean-Atlantic marine passage progressively closed leading to restricted conditions in the Mediterranean basins, and eventually to widespread deposition of thick evaporites. To accurate date discrete steps in the constriction of marine gateways and the associated environmental changes, high resolution time frameworks are required. In this PhD project a high resolution time framework was established by extending the astronomical polarity time scale into the middle Miocene in the Mediterranean. This time framework was then used to accurately date environmental changes in the Mediterranean basin, which are reflected in changes in the sediment colour, the internal build-up of basic sedimentary cycles and their patterns, and in geochemical and magnetic proxies. The astronomically dated environmental changes were correlated with geodynamic processes of the gateway closures in the eastern and western Mediterranean, global climatic changes and variations in Earth’s orbital configuration. During the late Miocene, discrete steps in environmental changes at 8.017, 7.616, 7.168 and 6.719 Ma occurred remarkably synchronous in the Mediterranean basin and can be directly linked to steps in the progressive constriction of the marine gateway through northern Morocco and southern Spain. Commonly, the Betic Seaway through southern Spain is thought to have closed in the late Tortonian at ~7.8 Ma, whereas the Rifian Corridor through northern Morocco persisted at least until ~6.8 Ma. Our new chronology for a Neogene basin in southern Spain indicates a stratigraphic hiatus of at least 2 Myr between fully marine conditions at ~7.8 Ma and the first continental deposits. This suggests that a Messinian gateway though southern Spain cannot be completely ruled out. During the middle Miocene, discrete steps in environmental changes in the Mediterranean are for the first time astronomically dated at 15.007 and ~14.485 Ma. They coincide with a time of global climate variability but cannot be linked to processes related to the closure of the Mediterranean-Indian Ocean gateway because it lacks any high resolution and accurate age control. To improve the chronology of this marine passage, Neogene basins in SE Turkey were studied. The new results indicate that basins north of the suture zone emerged during the late Oligocene (~23 Ma), which must have severely constricted this gateway. On the southern side of the suture, foreland basins formed no longer a deep marine connection after ~11 Ma, giving an upper limit to the closure age. Surprisingly, all astronomically dated discrete steps in the Miocene environment occurred during ~400-kyr eccentricity minima. This suggests that the influence of the ~400-eccentrcity cycle occurred superimposed on long-term climatic or directional tectonic trends, such as gateway closures, exerting an additional control on passing certain thresholds at discrete steps in time

Astrochronology of the Mediterranean Miocene: Linking palaeoenvironmental changes to gateway dynamics. Geologica Ultraiectina (295)

The semi-enclosed land-locked configuration makes the Mediterranean basin an ideal natural laboratory to study both, fundamental plate tectonic processes and astronomically induced variations in the climate and their influences on the marine and continental realms. The prime disruption of the Tethys seaway along the colliding African-Arabian and Eurasian plates disconnected the Mediterranean from the Indian Ocean. This disruption is believed to have triggered the Middle Miocene Climatic Transition. During the late Miocene, Mediterranean-Atlantic marine passage progressively closed leading to restricted conditions in the Mediterranean basins, and eventually to widespread deposition of thick evaporites. To accurate date discrete steps in the constriction of marine gateways and the associated environmental changes, high resolution time frameworks are required. In this PhD project a high resolution time framework was established by extending the astronomical polarity time scale into the middle Miocene in the Mediterranean. This time framework was then used to accurately date environmental changes in the Mediterranean basin, which are reflected in changes in the sediment colour, the internal build-up of basic sedimentary cycles and their patterns, and in geochemical and magnetic proxies. The astronomically dated environmental changes were correlated with geodynamic processes of the gateway closures in the eastern and western Mediterranean, global climatic changes and variations in Earth’s orbital configuration. During the late Miocene, discrete steps in environmental changes at 8.017, 7.616, 7.168 and 6.719 Ma occurred remarkably synchronous in the Mediterranean basin and can be directly linked to steps in the progressive constriction of the marine gateway through northern Morocco and southern Spain. Commonly, the Betic Seaway through southern Spain is thought to have closed in the late Tortonian at ~7.8 Ma, whereas the Rifian Corridor through northern Morocco persisted at least until ~6.8 Ma. Our new chronology for a Neogene basin in southern Spain indicates a stratigraphic hiatus of at least 2 Myr between fully marine conditions at ~7.8 Ma and the first continental deposits. This suggests that a Messinian gateway though southern Spain cannot be completely ruled out. During the middle Miocene, discrete steps in environmental changes in the Mediterranean are for the first time astronomically dated at 15.007 and ~14.485 Ma. They coincide with a time of global climate variability but cannot be linked to processes related to the closure of the Mediterranean-Indian Ocean gateway because it lacks any high resolution and accurate age control. To improve the chronology of this marine passage, Neogene basins in SE Turkey were studied. The new results indicate that basins north of the suture zone emerged during the late Oligocene (~23 Ma), which must have severely constricted this gateway. On the southern side of the suture, foreland basins formed no longer a deep marine connection after ~11 Ma, giving an upper limit to the closure age. Surprisingly, all astronomically dated discrete steps in the Miocene environment occurred during ~400-kyr eccentricity minima. This suggests that the influence of the ~400-eccentrcity cycle occurred superimposed on long-term climatic or directional tectonic trends, such as gateway closures, exerting an additional control on passing certain thresholds at discrete steps in time

Magnetostratigraphic dating of the proposed Rhaetian GSSP at Steinbergkogel (Upper Triassic, Austria): Implications for the Late Triassic time scale

The Global Stratotype Section and Point (GSSP) for the Rhaetian Stage has recently been proposed at Steinbergkogel in Austria. We re-sampled the Steinbergkogel sections (STK-A and STK-B + C) in high-resolution to establish a robust magnetostratigraphy that allows global correlation. The palaeomagnetic signal at Steinbergkogel is composed of three components, which can be separated by thermal demagnetization. The highest temperature component, revealed between 280/300 and maximum 600 °C is of dual polarity and is interpreted as primary. Rock magnetic experiments showed that the signal is carried by magnetite. Our results allow correlation between the two individual Steinbergkogel outcrops. Subsequently, we correlate the two key biostratigraphic horizons for the base of the Rhaetian, the FO of M. hernsteini and the FAD of M. post-hernsteini to other sections of the Tethys domain. The correlation to the astronomically dated continental successions of the Newark basin indicates that these positions for the base of the Rhaetian are most likely determined in chrons E16n and E16r, respectively. This correlation is confirmed by cyclostratigraphic control on the marine Pizzo Mondello (Italy) section, where a combination of long period Milankovitch cycles (~ 175-Myr) and short-eccentricity cycles (~ 100-kyr) provide additional correlation constraints, respectively supporting a long duration of the Rhaetian. Our study implies that the Norian and Rhaetian Stages have durations of ~ 17 and ~ 9 Myr

The Miocene astronomical time scale 9-12Ma: New constraints on tidal dissipation and their implications for paleoclimatic investigations

Orbital tuning and understanding climate response to astronomical forcing in the Miocene require detailed knowledge of the effect of tidal dissipation (Td) and dynamical ellipticity (dE) on astronomical solutions used to compute insolation and orbital target curves for paleoclimatic studies. These Earth parameters affect precession and obliquity; the determination of their effect is of fundamental importance, as phase relations between astronomical forcing and climate response can only be accurately calculated when the relative phasing between precession and obliquity is known. This determination can be achieved through comparison of solutions having different values for Td and/or dE with well-understood paleoclimate data. In this paper we use quantitative color records of precession-obliquity interference recorded in two successive 2.4 Myr eccentricity minima (9–9.6 and 11.5–12.1 Ma) in the Monte dei Corvi section in northern Italy to constrain the effect of Td, using the assumption of a direct response of sapropels to insolation. This quantitative approach results in a minimum uncertainty of astronomically tuned age models of ± 0.8 kyr and Td values 0.95 and 1.05 for the 9–9.6 Ma interval and of +4/−1 kyr (Td values between 0.95 and 1.15) for the 11.5–12.1 Ma interval. This (un)certainty not only limits the precision of determining phase relations but also improves our understanding of the limitations of tuned time scales and determining phase relations in the Miocene

The upper Tortonian - lower Messinian at Monte dei Corvi (Northern Apennines, Italy): Completing a Mediterranean reference section for the Tortonian Stage

A high-resolution integrated stratigraphy including biostratigraphy, magnetostratigraphy, tephrostratigraphy and cyclostratigraphy is presented for the upper Tortonian and lower Messinian (Upper Miocene) at Monte dei Corvi. Numerical age control comes from a combination of magnetobiostratigrapic dating and astronomical tuning, while radioisotope dating of ash layers produced less reliable ages. The deep marine succession is characterized by large-scale non-repetitive stratigraphic changes allowing the discrimination between a Lower, Brownish, Rossini and Euxinic Shale Interval. The sedimentary cycles, which occur superimposed on these large-scale changes, are related to astronomical climate forcing. Astronomical tuning of the sedimentary cycles provides not only absolute ages for biostratigraphic events, reversal boundaries and volcanic ash layers, but also for the observed non-cyclic changes. These can be linked to environmental and geodynamic changes in the Mediterranean region preceding the Messinian Salinity Crisis. The beginning of the Brownish Interval marked by tripartite cycles at 8.017 Ma coincides with tectonic re-arrangements in the Mediterranean, such as compression in the Betics and opening of the Rifian corridor. At 7.616 Ma the change to the calcareous Rossini Interval can be linked to tectonic uplift in the northern Apennines and the deepening of the Rifian corridor, while the Betic corridor became restricted coinciding with the end of evaporite deposition in the eastern Betics. The change to euxinic shales at 7.168 Ma reflects basin-wide restriction marked by diatomite and sapropel formation in the Mediterranean basin related to the severely constricted Betic and Rifian corridors. The occurrence of a tripartite construction of cycles in the euxinic shales at 6.719 Ma coincides with diatomite formation in the eastern Mediterranean, uplift and shallowing in the Northern Apennines and further constriction of the Atlantic gateways. We conclude that late Tortonian–early Messinian tectonic and environmental events occur remarkably synchronously in the Mediterranean basin and most likely result from a combination of geodynamic processes and eccentricity-controlled climatic changes. In combination with the upper Serravallian and lower Tortonian interval, which includes the formal Tortonian Global Stratotype Section and Point (GSSP), the Monte dei Corvi is the only complete section in the Mediterranean covering the entire Tortonian Stage. We therefore propose the Monte dei Corvi section as Tortonian reference section, particularly for the Mediterranean region. In addition, it may serve as a unit-stratotype for the Tortonian

On the Late Miocene continentalization of the Guadix Basin: More evidence for a major Messinian hiatus

The chronology of the Late Miocene Mediterranean-Atlantic gateways through southern Iberia is a key issue to better understand the geodynamic processes that lead to the Messinian salinity crisis. The timing of the North Betic corridor continentalization has recently been constrained by integrated magnetobiostratigraphic dating of the La Lancha section in the Guadix basin (Hüsing et al., 2010: Palaeogeography Palaeoclimatology Palaeoecology 291, 167–179). This work showed that the continentalization of the Guadix basin encompasses an approximately 2 myr hiatus, with the interval between ∼7.7 to ∼5.5 (or 5.0) Ma missing in the stratigraphic record. Minwer-Barakat and colleagues comment that this hiatus could be slightly shorter, but they base their hypotheses solely on an interval without any reliable magnetostratigraphic data. Their key localities Negratín 1 and Rambla de Chimeneas 3 (MN13), however, confirm that the oldest faunas found at La Lancha section correlate with the upper part of chron C3r (∼5.5 Ma). These continental faunas thus provide more evidence on the presence of a major Messinian hiatus in the Guadix record

Astronomically-calibrated magnetostratigraphy of the Lower Jurassic marine successions at St. Audrie's Bay and East Quantoxhead (Hettangian-Sinemurian; Somerset, UK)

Astronomically-calibrated magnetostratigraphies are becoming the standard in the Geomagnetic Polarity Time Scale (GPTS) because in time these records can be accurately compared for consistency. The Lower Jurassic GPTS is however still poorly resolved with major inconsistencies between different records from the Paris Basin and the continental Newark and Hartford basins, while only the latter have been astronomically-calibrated. Here, an independent cyclostratigraphy, astronomical calibration to the 405-kyr eccentricity cycle and magnetostratigraphy for the lower Jurassic from coastal outcrops in Somerset, United Kingdom, covering the Hettangian and lower Sinemurian Stages is presented. These results fully confirm the magnetostratigraphic patterns derived in the terrestrial Hartford Basin, eastern USA, with a dominant normal polarity interrupted by three short reverse chrons: H24r, H25r, and H26r. The cyclostratigraphic analysis of the marine strata from the St. Audrie's Bay/East Quantoxhead section is further extended and improved yielding an independent astronomical calibration to the 405-kyr, long eccentricity cycle and a floating astrochronology for the new magnetostratigraphy. The astronomical tuning shows consistency with the Hartford Basin at the 405-kyr, long eccentricity time scale and minor differences at the 100-kyr, short eccentricity time scale. These inconsistencies will have to be resolved to further improve the Geologic Time Scale 2012, in which the Hartford Basin magnetostratigraphy forms the basis for the Lower Jurassic. The St. Audrie's Bay/East Quantoxhead composite section could serve as a unit stratotype section for the Hettangian Stage