Giuliana Villa

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

Late Neogene evolution of the Taza-Guercif Basin (Rifian Corridor, Morocco) and implications for the Messinian salinity crisis

Abstract Magnetostratigraphic and biostratigraphic results are presented from Neogene deposits in the Taza–Guercif Basin, located at the southern margin of the Rifian Corridor in Morocco. This corridor was the main marine passageway which connected the Mediterranean with the Atlantic during Messinian times. Correlation of the biostratigraphy and polarity sequence of the Taza–Guercif composite section to the astronomical time scale, allows an accurate dating of three subsequent events in the Rifian Corridor. (1) The oldest marine sediments marking the opening of the Rifian Corridor were deposited at 8 Ma. At this age, a deep (600 m) marine basin developed in the Taza–Guercif area, marked by deposition of precession-controlled turbidite–marl cycles. (2) Paleodepth reconstructions indicate that a rapid (5 m/ka) shallowing of the marine corridor took place at the Tortonian/Messinian boundary, at an age of 7.2 Ma. This shallowing phase is primarily related to active tectonics, although a small glacio-eustatic sea level lowering also took place. (3) The Taza–Guercif Basin was emergent at an age of 6.0 Ma and, subsequently, continental sedimentation continued well into the Early Pliocene. We suggest that shallowing and restricting the marine passageway through the Rifian Corridor actually initiated the Messinian salinity crisis, well before the deposition of the Messinian evaporites in the Mediterranean.

Integrated stratigraphy and astrochronology of the Messinian GSSP at Oued Akrech (Atlantic Morocco)

A much improved high-resolution integrated stratigraphy (calcareous plankton biostratigraphy, magnetostratigraphy, cyclostratigraphy) is presented for the classic section of Oued Akrech (Atlantic Morocco) straddling the Tortonian^Messinian boundary. Magnetobiostratigraphic correlations with time-equivalent and astronomically dated sections in the Mediterranean indicate that cyclic alternations of indurated light beige coloured marls and softer, more clayey and reddish coloured marls are dominantly precession-controlled. Characteristic sedimentary cycle patterns, in particular those reflecting precession^obliquity interference, allow for one possible tuning, thus providing accurate astronomical ages for cycles, calcareous plankton events and magnetic reversals. The tuning further indicates that the reddish layers are the equivalent of sapropels in the Mediterranean. The Messinian Global boundary Stratotype Section and Point (GSSP) has recently been formally defined at the base of the reddish layer of cycle No. 15 in section Oued Akrech. This level coincides closely with the first regular occurrence of the Globorotalia miotumida group and is astronomically dated at 7.251 Ma. The global correlation potential is guaranteed by the straightforward calibration of the Oued Akrech magnetostratigraphy to the geomagnetic polarity time scale, locating the GSSP within C3Br.1r. In the marine realm the calcareous nannofossil genus Amaurolithus provides a series of extremely useful events to delimit the boundary on a global scale. The astronomical tuning guarantees a direct first-order calibration of the Messinian GSSP to the standard geological time scale once, as anticipated, the late Miocene part of the astronomical time scale has been incorporated. fl 2000 Elsevier Science B.V. All rights reserved.

Mid-late Pleistocene glacimarine sedimentary processes of a high-latitude, deep-sea sediment drift (Antarctic Peninsula Pacific margin)

The effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the Antarctic Peninsula Pacific margin. Nineteen sediment cores were visually described, logged for magnetic susceptibility, and X-radiographed. About 1000 analyses were performed for grain size, clay minerals and biostratigraphy (foraminifera, nannofossils and diatoms). Four sediment types associated with distinct sedimentary processes are recognised based on textural/compositional analysis. (1) Hemipelagic mud forms the bulk of the interglacial sediment, and accumulated from the pelagic settling of bioclasts and ice-rafted/wind-transported detritus. (2) Terrigenous mud forms the bulk of the glacial sediment, and accumulated from a combination of sedimentary processes including turbidity currents, turbid plumes, and bottom current reworking of nepheloid layers. (3) Silty deposits occurring as laminated layers and lenses, represent the lateral spillout of low-density turbidity currents. (4) Lastly, glacial/interglacial gravelly mud layers derive from settling of ice-rafted detritus. Five depositional settings are interpreted within sediment Drift 7, each characterised by the dominance/interaction of one or several depositional processes. The repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages (glaciation, glacial, deglaciation, and interglacial), each one characterised by a distinctive clay mineral assemblage and bioclastic content. Variations in clay mineral assemblage within interglacial stage 5 (core SED-06) suggest minor colder climatic fluctuations, possibly correlatable with substages 5a to 5e.

Integrated chronostratigraphic calibration of the Oligocene-Miocene boundary at 24.0 ± 0.1 Ma from the CRP-2A drill core, Ross Sea, Antarctica

An expanded Oligocene-Miocene boundary interval recovered in the Cape Roberts Project CRP-2A core from beneath the Ross Sea, Antarctica, has yielded a high-resolution integrated chrono stratigraphy that has, in turn, enabled a new, more direct, calibra tion of magnetic polarity and biostratigraphic events. The Oligocene-Miocene boundary interval in the CRP-2A core comprises three ∼60-m-thick, rapidly deposited (>0.5 m/k.y.) sedimentary sequences (sequences 9, 10, and 11). In sequences 10 and 11, single-crystal, laser-fusion 40Ar/39Ar analyses of anorthoclase phenocrysts from two tephra horizons independently calibrate the CRP-2A magnetic-polarity stratigraphy and age model. Sequences 10 and 11 encompass subchron C6Cn.3n, which is dated as 24.3 ± 0.1 to 24.16 ± 0.1 Ma. Sequence 9 is interpreted to encompass subchron C6Cn.2n and the Oligocene-Miocene boundary, which is dated as 24.0 ± 0.1 Ma. These ages are ∼0.2 m.y. older than those of the geomagnetic polarity time scale calibrated from seafloor-spreading ridges and ∼0.9–1.3 m.y. older than the newly proposed astronomically calibrated ages. We contend that the discrepancy with the astronomically calibrated ages arises from a mismatch of three 406 k.y. eccentricity cycles or a 1.2 m.y. modulation of obliquity amplitude in the astronomical calibration of the Oligocene–Miocene time scale.

Biostratigraphic characterization and Quaternary microfossil palaeoecology in sediment drifts west of the Antarctic Peninsula – implications for cyclic glacial–interglacial deposition

Abstract The SEDANO Project recovered 19 gravity cores on sediment drifts from the Pacific continental margin of the Antarctic Peninsula [Camerlenghi et al. (1997a) High-resolution terrigenous sedimentary record of the sediment drifts on the Antarctic Peninsula Pacific margin. In: Ricci, C.A. (Ed.), The Antarctic region. Museo Nazionale dell’Antartide, Siena, pp. 705–710]. Fifteen cores were sampled with the aim of developing an integrated biostratigraphy and palaeoecology based on calcareous nannofossils, diatoms, planktonic and benthic foraminifera, framed in a depositional process reconstruction. Barren gray laminated and brown bioturbated hemipelagic sediments characterize glacial and interglacial cycles, respectively. Analyses from both intervals allow comparison between microfossil occurrence in glacial and interglacial cycles. The unit boundaries were drawn more accurately by means of the microfossil distribution. On the basis of micropalaeontological and sedimentological evidence, Interglacial Unit C is correlated to Oxygen Isotope Stage 5, thus dating the unit boundaries at 127 and 70 ka. Peaks in diatom abundance correlate well with interglacial units and indicate high productivity and an open ocean environment. A calcareous nannofossil cold-taxa association is present in most cores examined, and its consistent distribution within Interglacial Unit C indicates key environmental relationships. The occurrence of calcareous nannofossils has been related to temperature tolerance, sea-ice cover reduction, nutrient availability, and factors limiting primary productivity. Our results confirm that coccolithophorids occurred at southern high latitudes, in the western marginal basins of the Antarctic Peninsula, during short periods of the Late Quaternary. A foraminiferal assemblage, made up of sinistral Neogloboquadrina pachyderma and few benthics, occurs only in interglacial units. Correlation of microfossil occurrences with climatic cycles adds information on their palaeoecology and palaeoproductivity in the southern high latitudes.

Middle Eocene to Late Oligocene Antarctic glaciation/deglaciation and Southern Ocean productivity

During the Eocene-Oligocene transition, Earth cooled significantly from a greenhouse to an icehouse climate. Nannofossil assemblages from Southern Ocean sites enable evaluation of paleoceanographic changes and, hence, of the oceanic response to Antarctic ice sheet evolution during the Eocene and Oligocene. A combination of environmental factors such as sea surface temperature and nutrient availability is recorded by the nannofossil assemblages of and can be interpreted as responses to the following changes. A cooling trend, started in the Middle Eocene, was interrupted by warming during the Middle Eocene Climatic optimum and by short cooling episodes. The cooling episode at 39.6 Ma preceded a shift toward an interval that was dominated by oligotrophic nannofossil assemblages from ~39.1 to ~36.2 Ma. We suggest that oligotrophic conditions were associated with increased water mass stratification, low nutrient contents, and high efficiency of the oceanic biological pump that, in turn, promoted sequestration of carbon from surface waters, which favored cooling. After 36.2 Ma, we document a large synchronous surface water productivity turnover with a dominant eutrophic nannofossil assemblage that was accompanied by a pronounced increase in magnetotactic bacterial abundance. This turnover reflects a response of coccolithophorids to changed nutrient inputs that was likely related to partial deglaciation of a transient Antarctic ice sheet and/or to iron delivery to the sea surface. Eutrophic conditions were maintained throughout the Oligocene, which was characterized by a nannofossil assemblage shift toward cool conditions at the Eocene-Oligocene transition. Finally, a warm nannofossil assemblage in the Late Oligocene indicates a warming phase.

New magnetobiostratigraphic chronology and paleoceanographic changes across the Oligocene‐Miocene boundary at DSDP Site 516 (Rio Grande Rise, SW Atlantic)

New magnetobiostratigraphic data for the late Oligocene through early Miocene at Deep Sea Drilling Project (DSDP) Hole 516F provide a significantly revised age model, which permits reevaluation of developments that led to the Mi-1 glacial event at the Oligocene-Miocene boundary. Our new high-resolution paleomagnetic study, which is supported by quantitative calcareous nannofossil and planktonic foraminiferal analyses, significantly refines previous age models for Oligocene-Miocene sediments from DSDP Hole 516F, with ages that are systematically younger than those previously determined. In some parts of the Oligocene, the discrepancy with previous studies exceeds 450 kyr. Based on this new age model, we infer a progressive increase in sedimentation rate and paleoproductivity between circa 23.9 Ma and circa 22.9 Ma, with the highest rate coinciding with the Mi-1 glacial event at the Oligocene-Miocene boundary. This productivity increase would have resulted in higher rates of carbon burial and in turn a drawdown of atmospheric CO2. Immediately afterward, an abrupt decrease in sedimentation rate and paleoproductivity suggests that the Mi-1 deglaciation was associated with decreased carbon input into the ocean. Elevated sedimentation rates are also documented at ~24.5 Ma, coincident with the Oi2D glacioeustatic event. The presence of volcanic material within the sediments during these glacial events is interpreted to have resulted from redeposition of sediment scoured from nearby sites on the Rio Grande Rise due to transient variations in bottom water flow patterns.

Onset of Maikop sedimentation and cessation of Eocene arc volcanism in the Talysh Mountains, Azerbaijan

Abstract The Maikop Series forms an important source rock in the former Paratethys. Deposition is often interpreted as anoxic, linked to restriction of the Paratethys. The Pirembel formation in the Talysh Mountains (Azerbaijan) is attributed to the Maikop Series and was deposited above the Eocene volcanic Peshtasar formation. Dating the onset of anoxia could help to distinguish glacio-eustatic from tectonic causes of restriction. We integrated magnetostratigraphy and biostratigraphy to date the onset of Pirembel sedimentation and used geochemistry to characterize the tectonic setting of the Peshtasar volcanic rocks. The onset of Maikop sedimentation in the Talysh was determined to be 37.7 Ma, ruling out a link with the major sea-level drop at the Eocene–Oligocene Transition (33.9 Ma) and favouring a tectonic cause. Extrapolating the average sedimentation rate (34 cm kyr) suggests that the entire Pirembel formation belongs to the Late Eocene. We hypothesize that the end of volcanism is important in the transition to Pirembel sedimentation. The palaeomagnetic and geochemistry results for the volcanic rocks cluster in three groups, suggesting three distinct episodes of volcanism. Volcanic sills within the Eocene Arkevan formation plot exactly on these groups, confirming the relationship between the Arkevan and Peshtasar formations. Volcanic rocks of the Talysh show continental-arc signatures and may be related to an Eocene volcanic belt extending towards southeastern Iran. Supplementary material: The full analytical data of the Ar–Ar dating are available at http://www.geolsoc.org.uk/SUP18851

Extreme depositional events on the NW-Barents Sea continental margin

Lucchi, Renata G. ... et. al.-- 87° Congresso della Societa Geologica Italiana e 90° Congresso della Societa Italiana di Mineralogia e Petrologia, The Future of the Italian Geosciences - The Italian Geosciences of the Future, 10-12 September 2014, Milan, Italy.-- 1 pageThe Montellina Spring (370 m a.s.l.) represents an example of groundwater resource in mountain region. It is a significant source of drinking water located in the right side of the Dora Baltea Valley (Northwestern Italy), SW of Quincinetto town. This spring shows a morphological location along a ridge, 400 m from the Renanchio Torrent in the lower sector of the slope. The spring was investigated using various methodologies as geological survey, supported by photo interpretation, structural reconstruction, NaCl and fluorescent tracer tests, discharge measurements. This multidisciplinary approach, necessary due to the complex geological setting, is required for the importance of the Montellina Spring. It is interesting in the hydrogeological context of Western Alps for its high discharge, relatively constant over time (average 150 l/s), and for its location outside a fluvial incision and suspended about 40 m above the Dora Baltea valley floor (Lasagna et al. 2013). According to the geological setting, the hydrogeological reconstruction of the area suggests that the large amount of groundwater in the basin is essentially favoured by a highly fractured bedrock, covered by wide and thick bodies of glacial and gravitational sediments. The emergence of the water along the slope, in the Montellina Spring, is essentially due to a change of permeability between the deep bedrock and the shallow bedrock and/or surficial sediments. The deep bedrock, showing closed fractures and/or fractures filled by glacial deposits, is slightly permeable. The shallow bedrock, strongly loosened as result of gravitational phenomena, and the local gravitational sediments are, on the contrary, highly permeable. The concentration of water at the spring is due to several reasons. a) The spring is immediately downward a detachment niche, dipping towards the spring, that essentially drains the water connected to the change of permeability in the bedrock. b) It is along an important fracture, that carries a part of the losses of the Renanchio Torrent. c) Finally, it is favored by the visible and buried morphology. Although it is located along a ridge, the spring occurs in a small depression between a moraine and a landslide body. It also can be favored by the likely concave trend of buried base of the landslide. At last, tracer tests of the Renanchio Torrent water with fluorescent tracer are performed, with a continuous monitoring in the Montellina Spring. The surveys permit to verify and quantify the spring and torrent hydrogeological relationship, suggesting that only a small fraction of stream losses feeds the spring.