J. Steenbrink

Magnetostratigraphy-based astronomical tuning of the early Pliocene lacustrine sediments of Ptolemais (NW Greece) and bed-to-bed correlation with the marine record

Continental deposits from the early Pliocene lacustrine Ptolemais basin in NW Greece display rhythmical alternations of lignite and marl beds. Three parallel sections from this area are studied using magnetostratigraphy and cyclostratigraphy. The presence of the greater part of the Gilbert Chron enables the recognition of astronomical periodicities in the succession. Especially the precessional influence is evident, as it determines the lithological cycles. The continental Ptolemais composite section is correlated to the most recent astronomical time scale — and thus to the marine reference section: the Rossello composite from Sicily [C.G. Langereis, F.J. Hilgen, The Rossello composite: a Mediterranean and global reference section for the Early to early Late Pliocene, Earth Planet. Sci. Lett. 104 (1991) 211‐225] — on a bed-to-bed scale. It is concluded that lignite corresponds to an insolation minimum (beige layer in the Rossello composite), and marl to an insolation maximum (grey layer in the Rossello composite). This implies a precipitation increase during insolation maxima in early Pliocene continental Greece.

Sedimentary cycles and volcanic ash beds in the Lower Pliocene lacustrine succession of Ptolemais (NW Greece): discrepancy between 40 Ar= 39 Ar and astronomical ages

A high-resolution cyclostratigraphy for the rhythmically bedded lignite‐marl sequences of the Lower Pliocene Ptolemais Formation is combined with 40 Ar= 39 Ar dating results of intercalated volcanic ash beds. Detailed field reconnaissance in three open-pit lignite mines reveals three end-member sediment types: lignites, composed primarily of organic material; grey marls, a mixture of carbonate and organic material; and beige marls, almost exclusively composed of carbonate. These lithologies are arranged in two basic types of sedimentary cycles: lignite‐grey marl and lignite‐beige marl cycles. A cyclostratigraphic composite section comprising 56 lignite‐marl cycles is constructed which combines the consistent cycle patterns from three parallel sections. The concordant positions of 20 volcanic ash beds in these sections confirm the cyclostratigraphic correlations and indicate that the lignite‐marl cycles result from regional, basin-wide forcing rather than lateral facies migrations. 40 Ar= 39 Ar ages on sanidine and biotite separates from nine volcanic ash beds were obtained by multiple total fusion and incremental-heating experiments. The 40 Ar= 39 Ar ages range between 5:00 0:05 and 4:04 0:04 Ma and are, in general, consistent with the stratigraphic order. A least-square linear regression using the measured 40 Ar= 39 Ar ages gives an average duration of 21:8 0:8 kyr per lignite‐marl cycle. Evidently, the lignite‐marl cycles in the Ptolemais Formation are linked to the precessional variation in the Earth’s orbit through its influence on Mediterranean climate. For the first time, 40 Ar= 39 Ar dating results, totally independent from any other dating and or tuning technique, confirm the astronomical theory of climate change. The 40 Ar= 39 Ar ages of the volcanic ash beds show a constant200 kyr (4.5%) age discrepancy with the astronomical ages of the same ash beds. This inconsistency remains difficult to explain. The discrepancy is unlikely to have resulted from erroneous astronomical ages, through incorrectness in the astronomical tuning, inaccuracies of the magnetostratigraphic data or the orbital time-series used, and=or errors in the APTS. The 40 Ar= 39 Ar dating results neither give clear indications for a possible source of error. From the excellent data set it is evident that neither loss of radiogenic 40 Ar, nor an underestimation of the contribution of Ca- and K-derived Ar isotopes could have caused the discrepancy. Moreover, the discrepancy is also beyond the errors in the systematic variables, like the decay constants of 40 K or the ages for the neutron-fluence monitors.

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.

Millennial-scale climate variations recorded in Early Pliocene colour reflectance time series from the lacustrine Ptolemais Basin (NW Greece)

Abstract Quaternary climate proxy records show compelling evidence for climate variability on time scales of a few thousand years. The causes for these millennial-scale or sub-Milankovitch cycles are still poorly understood, not least due to the complex feedback mechanisms of large ice sheets during the Quaternary. We present evidence of millennial-scale climate variability in Early Pliocene lacustrine sediments from the intramontane Ptolemais Basin in northwestern Greece. The sediments are well exposed in a series of open-pit lignite mines and exhibit a distinct millennial-scale sedimentary cyclicity of alternating lignites and lacustrine marl beds that resulted from precession-induced variations in climate. The higher-frequency, millennial-scale cyclicity is particularly prominent within the grey-coloured marl segment of individual cycles. A stratigraphic interval of ∼115 ka, covering five precession-induced sedimentary cycles, was studied in nine parallel sections from two open-pit lignite mines located several km apart. High-resolution colour reflectance records were used to quantify the within-cycle variability and to determine its lateral continuity. Much of the within-cycle variability could be correlated between the parallel sections, even in fine detail, which suggests that these changes reflect basin-wide variations in environmental conditions related to (regional) climate fluctuations. Interbedded volcanic ash beds demonstrate the synchronicity of these fluctuations and spectral analysis of the reflectance time series shows a significant concentration of within-cycle variability at periods of ∼11, ∼5.5 and ∼2 ka. The occurrence of variability at such time scales at times before the intensification of the Northern Hemisphere glaciation suggests that they cannot solely have resulted from internal ice-sheet dynamics. Possible candidates include harmonics or combination tones of the main orbital cycles, variations in solar output or periodic motions of the Earth and Moon.

Refinement of the Messinian APTS from sedimentary cycle patterns in the lacustrine Lava section (Servia Basin, NW Greece)

A high-resolution cyclostratigraphy and magnetostratigraphy is presented for the Messinian lacustrine Lava section from the Servia Basin in NW Greece, constraining more precisely the absolute ages of magnetic polarity subchrons C3An.1n and C3An.2n. The section contains 15 distinct sedimentary cycles of alternating dark- and light-coloured marls, while the gamma-ray attenuation record reveals an additional five to six cycles. The cycles in the lower half of the section are on average 5.3 m thick, as opposed to the cycles in the upper part, which have an average thickness of 3.1 m. Palynological results define the lithological alternations in both the lower and upper cycles in terms of periodic changes in humidity, where the light marls represent the humid periods and the dark marls the relatively dry periods. Changes in cycle thickness and shifts in average gamma-ray values suggest a rather abrupt decrease in sedimentation rate at V60 m in the section. This is confirmed by the magnetostratigraphy, which recorded four reversals, which ^ given the biostratigraphic constraints from the Lava locality ^ could be correlated unambiguously to subchrons C3An.1n and C3An.2n of the geomagnetic polarity time scale. With this magnetostratigraphic time control, the average duration of the cycles can be calculated to be constant in the entire section, and similar to precession. The astronomical origin of the cycles is confirmed by the results of spectral analyses of gamma-ray and susceptibility time series. The sedimentary cycles in the upper part of the Lava section are unambiguously tuned to insolation using the typical clustering of the cycles that follows the eccentricity cycle. The filtered gamma-ray record centred at 41 kyr confirms the tuning in the upper part and allows tuning of the lower part. The tuning results in accurate ages for the sedimentary cycles and polarity reversals that confirm the astronomical tuning of [Krijgsman et al., Nature 400 (1999) 652^655], but define more precisely the astronomical polarity time scale. fl 2000 Elsevier Science B.V. All rights reserved.

A short-term cooling event, 4.205 million years ago, in the Ptolemais Basin, northern Greece

Abstract A distinct clay-rich layer in the otherwise regular succession of alternating lignites and marls of the early Pliocene Ptolemais Formation reveals an equally distinct palynomorph assemblage; only in this layer (fresh water) dinoflagellate cysts, Spiniferites cruciformis and Gonyaulax apiculata, are encountered. These co-occurring dinoflagellate species may be taken to indicate that surface water temperatures must have been remarkably low for early Pliocene mid-latitudes, contrasting other indications that overall warm humid climates prevailed at this time. A multi-component study of this so-called ‘dinolayer’ with an astrochronologically derived age of 4.205±0.01 Ma was undertaken to further investigate the apparently contrasting climatic signals. Integrated sedimentological, micropaleontological, palynological and geochemical data indicate that the dinolayer was deposited in a shallow fresh water lake. The pollen record points to slightly cooler conditions during deposition of the dinolayer in comparison with conditions just below and above. An anomalously low winter temperature is proposed as a possible cause for the recorded phenomena.