Astronomical Polarity Time Scale Research Articles

"Short" or "long" Rhaetian ? Astronomical calibration of Austrian key sections

The establishment of the Late Triassic time scale has given rise to considerable controversy, particularly regarding the Rhaetian duration and the inferred absolute age models. In this respect the astronomical polarity time scale (APTS) established from the continental successions of the Newark Basin (eastern North America) is considered as a reference record, although its completeness is questioned. Numerous magnetostratigraphic correlation schemes have been proposed between the APTS and biostratigraphically well-constrained Tethyan marine sections. This has led to two main contrasting options: a “short” Rhaetian duration (about 4–5 myr), and a “long” one (about 8–9 myr). Astronomical calibration of the Tethyan Rhaetian and estimate of its duration is necessary to help advance this debate. We have undertaken a cyclostratigraphic analysis of a Rhaetian composite record built from four overlapping Austrian reference sections. Magnetic susceptibility variations of the 131.5 m thick record are astronomically paced by the precession and 405-kyr orbital eccentricity cycles. 405-kyr orbital tuning allows to establish a floating time scale, and thus to suggest a minimum duration of 6.69 myr of the Rhaetian stage. Given the well-established radioisotopic age of the Rhaetian-Hettangian boundary of 201.36 Ma, an age no younger than 208.05 Ma for the Norian-Rhaetian boundary can be proposed. This result will contribute to the refinement of the Late Triassic time scale, but it does not solve the long-standing debate on bio-magnetostratigraphic correlations between the continental Newark APTS and the Tethyan marine sections, nor the question of the completeness of the Rhaetian Newark Basin. (Less)

A 9 million-year-long astrochronological record of the early–middle Eocene corroborated by seafloor spreading rates

The early-middle Eocene (ca. 56-41 Ma) is recorded in the pelagic Scaglia Rossa and Variegata Formations of the Umbria-Marche Basin (central Italy). Geochemical and magnetostratigraphic alignment between the Bottaccione section (Gubbio, central Italy) and the Smirra core (Cagli, central Italy) allows us to generate a continuous and wellpreserved new record that, combined with previously published data from the same area, creates a continuous high-resolution record from the Paleocene-Eocene thermal maximum (ca. 56 Ma) to the lower part of chron C21n. Comparison with carbon isotope records from Ocean Drilling Program Sites 1258 and 1263 reveals a satisfactory match, providing further evidence of the global significance of the long-term trend and superposed perturbations captured by the δ13C records. The identification of astronomically forced geochemical cycles allows us to develop a 405 k.y. tuned age model, thereby extending the astrochronology from ca. 56.0 to ca. 47.5 Ma. Marine magnetic anomaly profiles from major oceanic basins characterized by high seafloor spreading rates were used to independently test the astronomical polarity time scale associated with our tuning, as well as other polarity time scales. Our age model suggests the existence of periods of relatively constant seafloor spreading rates separated by rapid changes, while the other time scales generate more gradual variations and also include large and short-term deviations in spreading rates that occur simultaneously in different oceanic basins, implying errors in polarity reversal ages. The Umbria- Marche age model further contributes to the closure of the middle Eocene gap in the astronomical time scale.

Development of the Geomagnetical Polarity Time Scale: A Review

For Middle Mesozoic to Quaternary times,the geomagnetic polarity record is central to the construction of geologic time scales.The precision of the standard geomagnetical polarity time scale (GPTS) strongly influences the development of related disciplines,such as stratigraphy,paleontology,paleoclimatology and so on.We review the history of the developments in GPTS,and mainly present the recent progress in calibration of the astronomical polarity time scale (APTS),as well the shortages of the present APTS.It can be divided into three stages according to the characteristics of the GPTS development:(1) early stage,during the 1960s,is characterized by K/Ar dating and measuring the magnetization of igneous rocks,constructed the recent 5 Ma GPTS;(2) middle stage,from late 1960s to early 1990s,is characterized by analysis of marine magnetic profiles from the world′s ocean basins,constructed the integrated GPTS since Middle Mesozoic;and (3) in the last three decades,with the developments of the Milankovitch theory (astronomical climate theory) and the astronomical calculation of the Earth′s orbit,the geological time scale can be constructed by orbital tuning,which have being successfully developed as a powerful geochronometer in calibration the APTS.The precise APTS is expected to replace the CK95 GPTS and will be widely used in related disciplines.

Complex polarity pattern at the former Plio–Pleistocene global stratotype section at Vrica (Italy): Remagnetization by magnetic iron sulphides

The Vrica section in Calabria, southern Italy, was the global stratotype for the Pliocene–Pleistocene boundary until this boundary was redefined in 2009. Several paleomagnetic investigations have been carried out at Vrica to determine the age of the formerly defined Pliocene–Pleistocene boundary, which was a key calibration point for the astronomical polarity timescale (APTS). Each study has documented a complex polarity pattern at and above the top of the Olduvai subchron and in relation to the existence of the so-called Vrica subchron. When constructing the APTS, two alternative interpretations for the Vrica section were proposed, neither of which could be conclusively supported. Authigenic growth of magnetic iron sulphide minerals was proposed to explain the complex magnetic polarity record. Availability of a fresh 50-m sediment core enabled us to test this possibility. Our magnetostratigraphic record is similar to that of previous studies, but it is also complex above the Olduvai subchron. We confirm abundant occurrences of authigenic greigite and pyrrhotite, along with detrital titanomagnetite. Authigenic monoclinic pyrrhotite indicates growth significantly later than deposition, and greigite can grow at any time during diagenesis, depending on the availability of dissolved iron and sulphide. The spatially variable magnetic polarity pattern at Vrica is therefore interpreted to have resulted from post-depositional magnetic iron sulphide formation at variable times. Tectonism along the Calabrian arc provides a plausible mechanism for forcing reducing fluids through the sediments, thereby supplying the dissolved ions needed to produce late diagenetic sulphide growth and remagnetization. The complex magnetostratigraphic record at Vrica was taken into account when the APTS was developed, and alternative interpretations result in a maximum age difference of 50 kyr for the upper Olduvai reversal. Our results therefore do not undermine the APTS. Rather, they explain the complex magnetic polarity pattern at this globally important location and highlight the importance of remagnetization processes in such sediments.

Rhaetian magneto-biostratigraphy from the Southern Alps (Italy): Constraints on Triassic chronology

New Late Triassic–earliest Jurassic magneto-biostratigraphic data have been obtained from three overlapping sections in the Southern Alps, Italy (Costa Imagna, Brumano, Italcementi Quarry), composed of ~ 520 m of shallow-marine carbonates outcropping in stratigraphic continuity. Characteristic magnetic components of presumed depositional age record a sequence of 9 normal and reverse polarity magnetozones (as defined by at least three stratigraphically superposed samples) linked by conodont and palynofloral evidence from this study and the literature to Rhaetian to Triassic–Jurassic boundary age. This represents a significantly larger number of polarity zones than previously recognized in more condensed Rhaetian sections from the literature, and by inference represents more time. These data are placed in a broader Late Triassic temporal framework by means of correlations to published magneto-biostratigraphic data from the Tethyan marine Pizzo Mondello section and the Newark astronomical polarity time scale (APTS). This framework is consistent with a position of the Norian–Rhaetian boundary (as defined at Brumano and Pizzo Mondello by the first appearance of Misikella posthernsteini) within Newark magnetozones E17r–E19r in the ~ 207–210 Ma time interval, in basic agreement with the position originally estimated in the Newark using pollen biostratigraphy (E18 at 208–209 Ma). This framework is also consistent with the position of the Triassic–Jurassic boundary interval (placed at Italcementi Quarry at the acme of Kraeuselisporites reissingeri coincident with a negative carbon isotope excursion) correlative to just above Newark magnetozone E23r and just below the oldest CAMP lavas dated at ~ 202 Ma. Hence, we estimate the duration of the Rhaetian to be ~ 5.5–8.5 Myr (or even longer if the Triassic–Jurassic boundary is instead placed above the negative carbon isotope excursion as at Kuhjoch, which is the designated GSSP for the base of the Hettangian), and encompassing 9 magnetozones. This duration contrasts with a duration of ~ 2 Myr and only ~ 4 magnetozones in several alternative published magneto-biostratigraphic schemes.

Precessional forcing of lacustrine sedimentation in the late Cenozoic Chemeron Basin, Central Kenya Rift, and calibration of the Gauss/Matuyama boundary

The fluviolacustrine sedimentary sequence of the Chemeron Formation exposed in the Barsemoi River drainage, Tugen Hills, Kenya, contains a package of five successive diatomite/fluvial cycles that record the periodic development of freshwater lakes within the axial portion of the Central Kenya Rift. The overwhelming abundance in the diatomite of planktonic species of the genera Aulacoseira and Stephanodiscus, and the virtual absence of benthic littoral diatoms and detrital material indicate areally extensive, deep lake systems. A paleomagnetic reversal stratigraphy has been determined and chronostratigraphic tie points established by 40Ar/ 39Ar dating of intercalated tuffs. The sequence spans the interval 3.1–2.35 Ma and bears a detailed record of the Gauss/Matuyama paleomagnetic transition. The 40Ar/ 39Ar age for this boundary of 2.589 ± 0.003 Ma can be adjusted to concordance with the Astronomical Polarity Time Scale (APTS) on the basis of an independent calibration to 2.610 Ma, 29 kyr older than the previous APTS age. The diatomites recur at an orbital precessional interval of 23 kyr and are centered on a 400-kyr eccentricity maximum. It is concluded that these diatomite/fluvial cycles reflect a narrow interval of orbitally forced wet/dry climatic conditions that may be expressed regionally across East Africa. The timing of the lacustrine pulses relative to predicted insolation models favors origination of moisture from the northern Africa monsoon, rather than local circulation driven by direct equatorial insolation. This moisture event at 2.7–2.55 Ma, and later East African episodes at 1.9–1.7 and 1.1–0.9 Ma, are approximately coincident with major global climatic and oceanographic events.

Late Miocene to Early Pliocene chronostratigraphic framework for the Dacic Basin, Romania

New magnetostratigraphic results and calcareous nannofossil data of seven sections in the Dacic Basin of southern Romania, covering the Latest Maeotian to Early Romanian time span (6.4–4.0 Ma), allow a correlation with the current Astronomical Polarity Time Scale. This establishes a chronological framework for the Pontian and Dacian stages of the Paratethys and facilitates to correlate the Paratethys stages with those of the Mediterranean. The Maeotian–Pontian boundary is estimated at 6.15 ± 0.11 Ma, within subchron C3An.1n, while the age of the Pontian–Dacian boundary is 5.30 ± 0.1 Ma, within subchron C3r. The Odessian–Portaferrian and Portaferrian–Bosphorian substage boundaries can be placed at about 6.0 and 5.6 Ma, at the base and near the end of chron C3r, respectively. Consequently, the Pontian stage has a duration of approximately 0.85 Myr and is coeval with the upper half of the Messinian stage. The base of the Dacian stage corresponds to the onset of the Pliocene and is coeval with the Pontian–Kimmerian boundary of the Euxinic (Black Sea) Basin. The Dacian–Romanian boundary is recorded either at 4.58 ± 0.05 Ma, in subchron C3n.2n (Nunivak), or at 4.25 ± 0.05 Ma in subchron C3n.1n (Cochiti). The length of the Dacian stage is therefore about 0.7 Myr or approximately 1 Ma. The transition of the Getian into the Parscovian substage can be dated at ∼ 4.83 Ma within subchron C3n.3n (Sidufjall), or at ∼ 4.55 Ma within subchron C3n.2n (Nunivak). The occurrence of calcareous nannofossil assemblages in the lower Portaferrian and near the Portaferrian–Bosphorian transition, belonging to the upper part of the Discoaster quinqueramus–NN11 Zone, and in the middle Bosphorian, belonging to the Amaurolithus tricorniculatus–NN12 Zone, indicates the presence of ephemeral marine connections between the Paratethys Basin and the Mediterranean during the Late Messinian. Another marine influx, inferred from nannofossil species characteristic of the NN12 Zone and found in lower Dacian sediments near the top of subchron C3n.4n (Thvera), post-dates the basal-Pliocene transgression of the Mediterranean Basin.

High-precision U-Pb zircon age from the Triassic of Italy: Implications for the Triassic time scale and the Carnian origin of calcareous nannoplankton and dinosaurs

The Triassic time scale is poorly constrained due to a paucity of high-precision radiometric ages. We present a 206 Pb/ 238 U age of 230.91 ± 0.33 Ma (error includes all known sources) for zircon from an ash bed in the upper Carnian (Upper Triassic) of southern Italy that requires a major revision of the Triassic time scale. For example, the Norian stage is lengthened to more than 20 m.y. The section containing the ash bed is correlated with other Tethyan sections and, indirectly, with the Newark astronomical polarity time scale (APTS). The dating provides also a minimum age for some important climatic and biotic events that occurred during the Carnian. We note a coincidence between these events and the eruption of the large igneous province of Wrangellia, but the possible link between volcanism and climatic and biotic events requires further scrutiny.

Implications for the early shield-stage evolution of Tenerife from K/Ar ages and magnetic stratigraphy

The combined use of field geology, radioisotopic dating and magnetic stratigraphy applied to the old shield volcanoes of Tenerife provides a reliable time framework for the early, shield-stage evolution of the island. The greater part of this new set of ages, obtained from sequences of lava flows is in agreement with the astronomical polarity time scale. This approach illustrates that previous K–Ar data collected without a comprehensive stratigraphy should be viewed with caution, and in some cases discarded altogether. The shield volcanoes of Tenerife encompass a relatively small number of magnetozones, an observation consistent with the relatively short periods of growth shown by the new ages (1–2 my). The island was constructed by the aggregation of three successive shields: the Roque del Conde (Central shield), between about 11.9 and 8.9 Ma, and the Teno (6.2–5.6 Ma) and Anaga (4.9–3.9 Ma) volcanoes. This new oldest subaerial age of Tenerife fits with the others obtained in the Canaries in a clear west to east monotonous age progression, one of the main restrictions for hotspot-related island chains.

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 40Ar/ 39Ar 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 poor preservation of the calcareous microfossils and the lack of a reliable magnetostratigraphy across the boundary interval. Finally, our study reveals that a single dominantly precession controlled oscillatory climate system is responsible for late Neogene sapropel formation in the Mediterranean during the last 13.5 million years.

The age and duration of the Matuyama–Brunhes transition from new K–Ar data from La Palma (Canary Islands) and revisited 40Ar/ 39Ar ages

Paleomagnetic investigations conducted on lava sequences from La Palma (Canary Islands) yielded several independent records of the Matuyama–Brunhes transition (MBT). Seven K–Ar ages of flows sampled across the MBT suggest a duration between 0 and 11 kyr, and provide a weighted mean age of 786±3 ka (analytical uncertainty only) for the transition. This value is significantly older than the previously admitted age of 779±2 ka, derived from volcanic or sedimentary sequences. When the 1% uncertainty on the calibration standard used in the present study is considered, our age for the transition recorded at La Palma becomes 786±8 ka. We have recalculated the 40Ar/ 39Ar determinations from the previous studies using recent revisited ages for 40Ar/ 39Ar standards. This yields a global dataset of 23 K–Ar and 40Ar/ 39Ar determinations. Our best estimate for the MBT age derived from this compilation is 789±2 ka (analytical error), or 789±8 ka (total error). Within this 8 kyr uncertainty, which could be considered as a minimum estimate, this age remains compatible with recent determinations made by the astronomical time scale. However, the present study suggests that better control of the lock-in depth, as well as better quantification of uncertainties involved in the tuning of δ 18O records to orbital forcing models, are needed to improve further the astronomical polarity time scale derived from sedimentary records. In addition, absolute dating by K–Ar and 40Ar/ 39Ar will gain in accuracy when uncertainty determinations on standards will be significantly improved.

The Abad composite (SE Spain): a Messinian reference section for the Mediterranean and the APTS

A high-resolution integrated stratigraphy is presented for the Abad marls of the Sorbas and Nijar basins in SE Spain (pre-evaporitic Messinian of the Western Mediterranean). Detailed cyclostratigraphic and biostratigraphic analyses of partially overlapping subsections were needed to overcome stratigraphic problems in particular encountered at the complex transition from the Lower to the Upper Abad. The resulting Abad composite section contains a continuous stratigraphic record from the Tortonian/Messinian boundary up to the transition to the Messinian evaporites of the Yesares Member. All together, 18 calcareous plankton events were recognized which were shown to be synchronous throughout the Mediterranean by means of detailed (bed-to-bed) cyclostratigraphic correlations. The magnetostratigraphy allowed the identification of the four magnetic reversals of chron C3An in the Upper Abad. Details in the sedimentary cycle patterns allowed the Abad composite to be astronomically calibrated. This calibration to the 65°N summer insolation curve of solution La90 (1,1) yielded astronomical ages for all sedimentary cycles, calcareous plankton bioevents, ash layers and paleomagnetic reversals. Up to now, the Abad composite is the only astronomically well-calibrated section that provided a reliable cyclostratigraphy, magnetostratigraphy and calcareous plankton biostratigraphy. As such it will serve as a reference section both for the pre-evaporite Messinian in the Mediterranean as well as for the Messinian interval in the Astronomical Polarity Time Scale.

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 ∼60 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.

Astronomical forcing of sedimentary cycles in the middle to late Miocene continental Calatayud Basin (NE Spain)

A high-resolution cyclostratigraphic and magnetostratigraphic study was carried out on cyclically bedded successions of middle Miocene lacustrine to distal alluvial fan-floodplain deposits from the Calatayud basin, in northeast Spain. Eight (partially overlapping) subsections near the village of Orera are correlated in detail using distinct sedimentary cycle patterns or by following marker beds in the field. Together they form the Orera Composite Section (OCS). Sedimentary cycles are recognised on at least two different scales. The basic small-scale cycles in the OCS consist of an alternation of grey and, occasionally, red clays with white, dolomite-rich, carbonate beds. They are arranged in larger-scaled, so-called large-scale cycles based on repetitive changes in the overall carbonate–clay lithology. Two other, but less distinct, types of intermediate scale cyclicity are also recognised. In terms of depositional environment, the cycle hierarchy is interpreted to represent periodic lake expansion over the palaeo-alluvial fan-floodplain area. The palaeomagnetic results yield a reliable magnetostratigraphic record, which confirms the cyclostratigraphic correlations between the subsections in detail. Rock magnetic experiments reveal that haematite is the main magnetic carrier of the primary component. The magnetostratigraphy of the OCS is correlated straightforwardly with the geomagnetic polarity time scale. This resulted in an age of 10.7–12.8 Ma for the entire succession, which is supported by fossil micromammal findings. In addition, it also reveals the presence of two, possibly three, short new polarity intervals. Based on the number of cycles in the OCS the average periodicity of the basic small-scale cycles is approximately 23 000 yr, while the large-scale cycle indicates a periodicity of 400 000 yr. This suggests that these sedimentary cycles are controlled by astronomically induced climate changes causing lake-level fluctuations and thus resulting in the deposition of carbonate-clay cycles. The continental sequences of the OCS provide a unique opportunity to extend the astronomical polarity time scale into the middle Miocene. The overlap of such continental sequences in the Mediterranean area with time-equivalent astronomically induced marine sequences is fundamental for establishing marine–continental, bed-to-bed, correlations and for understanding regional climate change.

Magnetostratigraphy of the Zobzit and Koudiat Zarga sections (Taza-Guercif basin, Morocco): implications for the evolution of the Rifian Corridor

Magnetostratigraphic analyses for two Neogene ‘post-nappes’ successions of the Taza-Guercif basin enable a reliable correlation of the sedimentary sequence to the astronomical polarity time scale. Rock magnetic analyses indicate that hematite is the dominant carrier of the magnetisation in the marine marls of the Melloulou Formation, whereas both magnetite and hematite contribute to the NRM of the near-shore and continental sediments of the Kef Ed Deba and Bou Irhardaiene Formations. Anisotropy of magnetic susceptibility (AMS) measurements indicate that the maximum axes of the magnetic fabric are aligned in a direction SW–NE. This suggests that the AMS is tectonically induced, related to SE–NW compression, in agreement with the major fold and fault systems in the basin. Our magnetostratigraphic correlation shows that the oldest marine sediments in the basin, which are related to the development of the Rifian Corridor, are dated at approximately 8 Ma. Between 7.2 and 7.1 Ma, just after the Tortonian/Messinian boundary, an important shallowing of the Taza-Guercif basin takes place. This shallowing phase is primarily related to active tectonics, although a small glacio-eustatic sea level lowering also took place. Our results indicate that at least the Taza-Guercif basin, and perhaps the entire Rifian Corridor, became emerged at an age between 6.7 and 6.0 Ma. Continental deposits, separated from the underlying deposits by a considerable hiatus of 700 kyr, extend into the Pliocene (up to 4.7 Ma).

Orbital tuning of geomagnetic polarity time-scales

Milankovitch climate cyclicity and magnetic polarity stratigraphy are being successfully combined as a powerful geochronometer in the astronomical polarity timescale (APTS). The APTS for 0{5.23 Ma has been rapidly accepted as the denitive chronology for the Pliocene and Pleistocene against which even high precision radiometric dating is now calibrated. Extensions of astronomical calibration to the late Miocene (5.23 Ma to ca. 10 Ma) in Mediterranean and Pacic marine sections agree to within about a 100 ka eccentricity, which amounts to an uncertainty of only ca .1 % of the age. Orbital eccentricity periods are thought to remain stable over very long times and thus provide the possibility of precise relative age control in the preNeogene. An APTS, for example, has been developed in a thick lacustrine section of Late Triassic age (ca. 202{233 Ma) on the basis of the 404 ka orbital eccentricity cycle modulating the expression of the precession climate cyclicity. Finally, there has been renewed speculation about an obliquity-modulated precessional geodynamo based on periodicities in relative palaeointensity data from Ocean Drilling Program sediment cores.

Cyclicity and NRM acquisition in the armantes section (Miocene, Spain): Potential for an astronomical polarity time scale for the continental record

The results are presented of a detailed study of a short (250 kyr) interval from a cyclically bedded continental sequence that is composed of alternating reddish silts and whitish limestones (Armantes section, Central Spain). The lithology as well as the carbonate and gamma‐ray records indicate that five small‐scale cycles are developed in one large‐scale cycle, implying that the small‐scale cyclicity is related to precession and thus caused by climate forcing. Rock magnetic experiments show that the NRM is carried by hematite and magnetite/maghemite and that the relative contribution of hematite is related to lithology. The positions of the reversal boundaries can reliably be determined, even though reversals of the magnetite and hematite components appear to be recorded at slightly different levels. We conclude that the well‐developed cyclicity and the good paleomagnetic properties of the sediments will enable the construction of an astronomical polarity time scale (APTS) for the continental realm.

40Ar/39Ar ages from the rhyolite of Alder Creek, California: Age of the Cobb Mountain Normal-Polarity Subchron revisited

New 40 Ar/ 39 Ar age determinations on sanidine from the rhyolite of Alder Creek, California, indicate a 1.186 ±0.006 Ma age for the Cobb Mountain Normal-Polarity Subchron. The new age is statistically older (α = 0.05) than the previously reported K-Ar age (1.12 ±0.02 Ma) and agrees with the age suggested by the astronomical polarity time scale. Incomplete extraction of radiogenic 40 Ar ( 40 Ar*) from the sanidine is the most likely reason for the disparity between the 40 Ar/ 39 Ar and K-Ar ages. Because the Cobb Mountain subchron is a worldwide, short-duration event, and because no widely used interlaboratory 40 Ar/ 39 Ar standard younger than 27 Ma exists, we propose that sanidine from the rhyolite of Alder Creek be considered for use as a new Quaternary 40 Ar/ 39 Ar mineral standard.