Elena Turco

Integrated stratigraphy and astronomical tuning of the Serravallian and lower Tortonian at Monte dei Corvi (Middle^Upper Miocene, northern Italy)

An integrated stratigraphy (calcareous plankton biostratigraphy, magnetostratigraphy and cyclostratigraphy) is presented for the Serravallian and lower Tortonian part (Middle^Upper Miocene) of the Monte dei Corvi section located in northern Italy.The detailed biostratigraphic analysis showed that both the Discoaster kugleri acme and the first influx of Neogloboquadrina acostaensis are recorded at Monte dei Corvi; these events, which passed unobserved in previous studies, play an important role in delineating the Serravallian^Tortonian boundary.Thermal and alternating field demagnetization revealed a characteristic low-temperature component marked by dual polarities.The resultant magnetostratigraphy for the upper part of the section can be unambiguously calibrated to the GPTS ranging from C5n.2n up to C4r.2r. Unfortunately, the lower part of the section, including the Serravallian^Tortonian boundary interval, did not produce a reliable magnetostratigraphy despite the fact that some short reversed intervals and a single normal interval are recorded.Using sedimentary cycle patterns in combination with the calcareous plankton biostratigraphy the section can be correlated cyclostratigraphically in detail to the partially overlapping and previously tuned section of Monte Gibliscemi on Sicily.The Monte dei Corvi section is dated astronomically by calibrating the basic small-scale sedimentary cycles to the precession and 65‡N lat.summer insolation time series of the La93 solution following an initial tuning of larger-scale cycles to eccentricity.An almost perfect fit is found between the cycle patterns and intricate details, especially precession^obliquity interference, in the insolation target between 8.5 and 10 Ma. The tuning to precession remains robust for most intervals back to the base of the section dated at 13.4 Ma and shows that the section is continuous apart from a possible short hiatus in the Tortonian. It provides accurate astronomical ages for all sedimentary cycles, calcareous plankton events, polarity reversals and ash layers and marks a significant improvement of the recently proposed astronomical calibrations of the Monte dei Corvi section and of parallel sections in the Mediterranean.Astronomical ages for the Ancona and Respighi ashbeds are significantly older than previously reported 40 Ar/ 39 Ar biotite ages, even if the revised older age for the FCT-san dating standard of 28.02 Ma is applied. The astronomical dating of the magnetic reversals in the Monte dei Corvi section results in the completion of the astronomical polarity time scale for the last 13 Myr.The Monte dei Corvi section has recently been proposed as the stratotype section for the Serravallian^Tortonian boundary despite the moderate to

Punctuated evolution of global climate cooling during the Late Middle to Early Late Miocene: High‐resolution planktonic foraminiferal and oxygen isotope records from the Mediterranean

High-resolution planktonic foraminiferal and oxygen isotope records are presented from a Mediterranean deep marine succession, dated astronomically between 12.12 and 9.78 Ma. Planktonic and benthic oxygen isotope records are punctuated by two episodes of δ18O increase, which have astronomical ages of 11.4 and 10.4 Ma and correspond to the Mi5 and Mi6 events of Miller et al. [1991a]. These ice growth events coincide with low-amplitude variations in the 1.2 Myr obliquity cycle and are accompanied by significant faunal changes in the Mediterranean, such as the arrival of neogloboquadrinids, the increase in abundance of the G. apertura-G. obliquus group, and the areal differentiation between N. atlantica and N. acostaensis. Short-term variations in the planktonic foraminiferal and oxygen isotope records correspond to dominantly precession-controlled sedimentary cycles. Features of the sapropel/grey marl layers indicate that the short-term astronomically controlled circum-Mediterranean climate changes remained basically the same over the last 12 Myr.

CALCAREOUS PLANKTON HIGH RESOLUTION BIO-MAGNETOSTRATIGRAPHY FOR THE LANGHIAN OF THE MEDITERRANEAN AREA

High-resolution quantitative and qualitative analyses of the planktonic foraminifer and calcareous nannofossil content have been carried out on three Middle Miocene sections, from the Mediterranean area. Such sections (Cretaccio section, Tremiti Islands, Southern Italy; Moria section, Marche Region, Central Italy; DSDP Site, 372 succession, Balearic Basin), all well known in the literature, have been chosen because of their high-quality biostratigraphic potential. Remarkable magnetostratigraphic data were provided by the Site 372 succession where all chrons and subchrons of the interval C5Br-C5AAn have been recognised. The investigated interval falls between the First Occurrence (FO) of Praeorbulina glomerosa sicana and the Last Occurrences (LO) of Sphenolithus heteromorphus and Globorotalia peripheroronda. The LO of S. heteromorphus was detected in the uppermost part of the investigated sequence of Site 372 at the same stratigraphic level as the G. peripheroronda LO. A drastic decrease in abundance of S. heteromorphus (Last Common Occurrence -LCO) was detected slightly below its last occurrence; this event is well correlatable with the same event astronomically calibrated at Ras-il Pellegrin section (Malta Island), which has been recently ratified as the Global Stratotype Section and Point (GSSP) for the base of the Serravallian by the International Union of Geological Sciences. The stratigraphic correlation of the studied sections is based on first and last occurrences, abundance fluctuations of selected taxa and additional biohorizons. In particular the peculiar distribution pattern of some taxa, e.g. Paragloborotalia siakensis and Helicosphaera waltrans, offered the opportunity to increase the biostratigraphic resolution of the Langhian interval. The resulting integrated calcareous plankton bio-magnetostratigraphic scheme represents the downward extension of that one previously established for the Serravallian - Tortonian interval. The biostratigraphic correlation of the studied sections with the Langhian historical Stratotype pointed out its low degree of reliability. On the other hand, none of the sections here studied is suitable to be proposed as candidate for defining the Langhian GSSP. Thus the problem of finding, in the Mediterranean area, a valid section which could yield a new GSSP for the Langhian Stage is still open.

PRESENT STATUS OF THE ASTRONOMICAL (POLARITY) TIME-SCALE FOR THE MEDITERRANEAN LATE NEOGENE

Sedimentary cycles may reflect orbitally induced climate oscillations and can then be used to construct astronomical time–scales. Following the initial tuning of the Late Pleistocene, the ‘anchored’ astronomical time–scale was extended to the base of the Pliocene, using palaeoclimatic records from Ocean Drilling Project (ODP) sites in the eastern equatorial Pacific and North Atlantic and sedimentary cycle patterns in marine successions exposed onland in the Mediterranean. In this paper we present a review of the progress subsequently made in establishing a Late Neogene astronomical (polarity) time–scale (A(P)TS) in the Mediterranean region. Major steps forward are (1) the evaluation of the initial time–scale, using high–resolution climatic proxy records, different astronomical solutions and the additional influence of obliquity on sedimentary cycle patterns, (2) the extension of the A(P)TS into the Middle Miocene, i.e. back to about 12 3Ma, (3) the closure of the Messinian gap in the A(P)TS, (4) the incorporation of the continental record, and (5) the intercalibration of astronomical and radioisotopic time.

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.