astronomical dating method, thereby

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1 doi:.1111/j x Radioisotopic dating of the Tortonian Global Stratotype Section and Point: implications for intercalibration of Ar/ 39 Ar and astronomical dating methods Klaudia F. Kuiper, 1,2 Jan R. Wijbrans 1 and Frederik J. Hilgen 2 1 Department of Isotope Geochemistry, Vrije Universiteit Amsterdam, De Boelelaan 5, 1 HV Amsterdam, The Netherlands; 2 Department of Earth Sciences, Utrecht University, Budapestlaan 4, 354 CD Utrecht, The Netherlands ABSTRACT In this paper we present new Ar/ 39 Ar data of volcanic ash layers intercalated in the astronomically dated sections of Monte dei Corvi and Monte Gibliscemi (Italy) to obtain better radioisotopic time constraints on the Serravallian/Tortonian boundary and to confirm the intercalibration of radioisotopic and astronomical time proposed by Kuiper et al. [04; Fish Canyon Tuff (FCT)-sanidine at 2.21 ± 0.03 Ma]. The latter intercalibration is supported by astronomically calibrated FCT sanidine ages for two ash layers at Monte Gibliscemi (GiF-1: 2.2 ± 0.04; GiD-3: 2. ± 0.04 Ma; ±1 SE). As a consequence, our results support the astronomically calibrated age of 11. Ma for the Tortonian Global Stratotype Section and Point and, hence, the tuning of the Serravallian/ Tortonian boundary interval. The Ancona and Respighi levels at Monte dei Corvi give a more diffuse picture, possibly because of contamination with detrital or xenocrystic material and the inferior quality of biotite for intercalibration purposes. Terra Nova, 17, 35 39, 05 Introduction A systematic discrepancy exists between Ar/ 39 Ar and astronomical ages of late Miocene and Pliocene ash layers in the Mediterranean with Ar/ 39 Ar ages being younger by approximately 1% (Kuiper, 03; Kuiper et al., 04). This discrepancy is largely the result of uncertainties in the values of the decay constants and in the ages of mineral dating standards used in argon geochronology. The total uncertainty can be significantly reduced through the intercalibration of argon standards with the astronomical dating method, thereby eliminating uncertainties connected with the absolute amounts of K and radiogenic Ar in the primary standard and a decrease of the influence of the decay constants (branching ratio is lacking). This approach resulted in astronomically calibrated ages of 2.21 ± 0.03 Ma (Kuiper et al., 04) and 2.24 ± 0.01 Ma (±1 SE) (Kuiper, 03) for the Oligocene Fish Canyon Tuff (FCT) argon dating standard. As these FCT ages are Correspondence: Dr Klaudia Kuiper, Budapestlaan 17, 354 CD, Utrecht, The Netherlands. Tel.: ; fax: ; kkuiper@ geo.uu.nl based on intercalibration with astronomically dated late Miocene ash layers, we choose to confirm the proposed intercalibration by extending our method to the middle Miocene. Several middle Miocene ash layers intercalated in the astronomically dated sections of Monte Gibliscemi and Monte dei Corvi in Italy seem suitable for this purpose. Recently, the Serravallian/Tortonian boundary, or Tortonian Global Stratotype Section and Point (GSSP), was formally designated at the sapropel midpoint of sedimentary cycle 76 in the Monte dei Corvi section (Hilgen et al., 03, 05). The boundary is defined close to the last common occurrence of the calcareous nannofossil Discoaster kugleri and the planktonic foraminifer Globigerinoides subquadratus and is dated astronomically at 11. Ma (Hilgen et al., 03). In addition, the boundary is closely associated with normal subchron C5r.2n and with oxygen isotope event Mi-5. The Monte Gibliscemi section serves as auxiliary boundary stratotype to overcome the problem of poor preservation of the calcareous plankton and, hence, the lack of a stable isotope record at Monte dei Corvi (Turco et al., 01; Hilgen et al., 03, 05). Volcanic ash layers are intercalated in the boundary interval both at Monte Gibliscemi and Monte dei Corvi and they can be used to confirm the existing astronomical time constraints. Therefore, radioisotopic dating of these ash layers serves two purposes: (i) to confirm the proposed intercalibration of radioisotopic and astronomical time and (ii) to provide additional time constraints on the Tortonian GSSP. Astronomical time frame The Monte dei Corvi and Gibliscemi sections are exposed in the coastal cliffs along the Adriatic coast south of Ancona and in the slopes of Monte Gibliscemi near Mazzarino on Sicily (Fig. 1). Both sections contain deep marine cyclically bedded sediments. Basic precession controlled sedimentary cycles are composed of couplets of light and dark coloured marls and marly limestones, with sapropels frequently or occasionally intercalated (Hilgen et al., 00, 03). These sapropels and the dark coloured marls occur in clusters that define largerscale eccentricity related 0 and 0 kyr cycles. The sections are dated astronomically by tuning the sedimentary cycles to astronomical target curves (Hilgen et al., 00, 03). This tuning starts by correlating sapropel clusters to eccentricity maxima, followed by calibrating the basic cycles Ó 05 Blackwell Publishing Ltd 35

2 Radioisotopic dating of Tortonian GSSP K. F. Kuiper et al. Terra Nova, Vol 17, No. 4, Corsica ITALY Ancona A Gibliscemi D 56 Precession Insolation (w/m 2 ) Age (Ma) 11.0 Ar- 39 Ar ages (Ma) 43 35'30" N La Vedova M. della Nave 175 m La Sardella Sardinia GIB Sicily Landslide M. dei sheds Corvi 236m Urbino Gubbio Assisi MDC Fano Respighi level Ancona level Ancona B Conero Riviera Fabiano Macerata 0 km Gilbliscemi F GiD-4 GiD m 79 GiF GiF m Monte dei Corvi GSSP GiD GiD 3 GSSP Ancona GiF 2.0 GiF 1 Ancona biotite (Montanari et al., 1997) GiD 3 Ancona GiF 1 Ancona biotite 43 35'00" N m 3 Mazzarino 13 34'00" E of Greenwich m 13 34'30" C Respighi biotite (Montanari et al., 1997) S M. Gibliscemi F Gela D D and F subsections Sampling trajectory 13.0 Fig. 1 Geographical locations of studied sections. The figure shows (a) the location of the sections with GIB ¼ Gibliscemi; (b) the Monte dei Corvi section; (c) the Monte Gibliscemi sections. The exact locations are described in Hilgen et al. (00, 03). to precession minima and 65 Nlat summer insolation maxima. For this purpose, the La93 solution with present-day values of dynamical ellipticity and tidal dissipation (La93 (1,1) ; Laskar et al., 1993) is used to compute the astronomical time series (for details see Hilgen et al., 00, 03). Biostratigraphic correlations between Monte Gibliscemi and Monte dei Corvi are relatively straightforward and were used as starting point to correlate the sections cyclostratigraphically (Fig. 2; Hilgen et al., 03). Small differences in the stratigraphic position of some bio-events are because of rareness and poor preservation in Monte dei Corvi. The resulting astronomical time frame provides astronomical ages for each volcanic level, which can be compared directly with Ar/ 39 Ar ages of minerals from the same levels. Analytical techniques Ar/ 39 Ar experiments are performed at the Vrije Universiteit Amsterdam using a 24 W continuous argon ion laser in combination with a MAP noble gas mass spectrometer (Kuiper, 03). Biotite and sanidine minerals are separated using standard mineral separation techniques. Sanidine (q ¼ g/cm 3 ) is leached with a 1 : 5 HF solution and biotite (q > 3.05 g/cm 3 ) is cleaned with deionized water, both in an ultrasonic bath for 5 min. Samples are irradiated during 7 h in the OSU TRIGA 0 m 5 Respighi 13.5 Respighi biotite Fig. 2 Astronomical tuning of the Gibliscemi and Monte dei Corvi sections. The astronomical tuning is based on Hilgen et al. (00, 03) with the modified tuning of cycles )4 till )79 for Gibliscemi as described in Hilgen et al. (03). The left (right) column represents sapropel layers (outcrop log) for Monte dei Corvi. Ar/ 39 Ar ages and 1r analytical errors including the analytical uncertainty in J are shown in black, 1r total external errors in grey. The ages of Montanari et al. (1997) are recalculated with FCT 2.02 Ma. Selected biostratigraphic events in the overlap between both sections are indicated: (a) Discoaster kugleri FCO, (b) Neogloboquadrina sp. FO, (c) Discoaster kugleri LCO, and (d) Globigerinoides subquadratus LCO. reactor Cd-shielded CLICIT facility. FCT sanidine (FC-2) is used as neutron flux-monitor with an age of 2.02 ± 0.2 Ma (Renne et al., 199). The irradiation parameter J for each unknown is determined by interpolation using a second order weighted polynomial fitting between the standards. System blanks are measured every three steps. Mass discrimination is monitored by frequent analysis of Ar/ 3 Ar reference gas pipette aliquots. Ages are calculated with the decay constants of Steiger and Jäger (1977). Errors are reported as analytical errors excluding the error 36 Ó 05 Blackwell Publishing Ltd

3 Ó 05 Blackwell Publishing Ltd 37 Table 1 Analytical data of Ar/ 39 Ar analyses. Laser power 39 Ar ) moles Ar/ 39 Ar ±1r 3 Ar/ 39 Ar ±1r 37 Ar/ 39 Ar ±1r 36 Ar/ 39 Ar ±1r Ar*/ 39 Ar K ±1r Ar* (%) ±1r K/Ca MdC Respighi biotite VU42-A24 (02M0399b) J ¼ lm approximately 50 grains per step-heating experiment 0. W W W W W W W W > W > fuse > MdC Respighi biotite VU42-A24 (02M0399c) J ¼ lm approximately 50 grains per step-heating experiment 0. W W W W W W W W > W fuse MdC Respighi biotite VU42-A24 (02M06a) J ¼ lm approximately 50 grains per step-heating experiment 0. W W W W W Plateau Plateau Plateau Plateau 1.09 W W W fuse MdC Ancona biotite VU42-A25 (02M00a) J ¼ lm approximately 50 grains per step-heating experiment 0. W W W W W W W W > fuse > Terra Nova, Vol 17, No. 4, K. F. Kuiper et al. Radioisotopic dating of Tortonian GSSP

4 3 Ó 05 Blackwell Publishing Ltd Table 1 Continued Laser power 39 Ar ) moles Ar/ 39 Ar ±1r 3 Ar/ 39 Ar ±1r 37 Ar/ 39 Ar ±1r 36 Ar/ 39 Ar ±1r Ar*/ 39 Ar K ±1r Ar* (%) ±1r K/Ca MdC Ancona biotite VU42-A25 (02M00b) J ¼ lm approximately 50 grains per step-heating experiment 0. W > W W W W W W W fuse MdC Ancona biotite VU42-A57 (02M00d) J ¼ lm approximately 50 grains per step-heating experiment 0. W W W W W W Plateau Plateau Plateau Plateau 1.30 W W fuse MdC Ancona biotite VU42-A57 (02M00e) J ¼ lm approximately 50 grains per step-heating experiment 0. W W W W W W W W fuse MdC Ancona biotite VU42-A25 (02M05a) J ¼ lm approximately 50 grains per step-heating experiment 0. W W W W W W W W >0 35. fuse >0 3.5 MdC GiF-2 feldspar VU42-A33 (02M0369) J ¼ lm approximately 15 grains per analysis 1.7 W W > W Radioisotopic dating of Tortonian GSSP K. F. Kuiper et al. Terra Nova, Vol 17, No. 4, 35 39

5 Ó 05 Blackwell Publishing Ltd 39 Table 1 Continued Laser power 39 Ar ) moles Ar/ 39 Ar ±1r 3 Ar/ 39 Ar ±1r 37 Ar/ 39 Ar ±1r 36 Ar/ 39 Ar ±1r Ar*/ 39 Ar K ±1r Ar* (%) ±1r K/Ca 1.7 W W > W W W W > W >999 fuse >999 fuse fuse fuse fuse fuse >999 fuse fuse fuse fuse >999 MdC GiF-2 feldspar VU42-A33 (02M0369) J ¼ lm approximately 15 grains per analysis recomb >999 recomb recomb recomb recomb recomb recomb recomb recomb recomb >999 MdC GiD-3 feldspar VU42-A35 (02M0372) J ¼ lm approximately 15 grains per analysis 1.7 W W W W W W W W W W >999 fuse fuse fuse >999 fuse fuse Terra Nova, Vol 17, No. 4, K. F. Kuiper et al. Radioisotopic dating of Tortonian GSSP

6 390 Ó 05 Blackwell Publishing Ltd Table 1 Continued Laser power 39 Ar ) moles Ar/ 39 Ar ±1r 3 Ar/ 39 Ar ±1r 37 Ar/ 39 Ar ±1r 36 Ar/ 39 Ar ±1r Ar*/ 39 Ar K ±1r Ar* (%) ±1r K/Ca fuse fuse fuse fuse fuse MdC GiD-3 feldspar VU42-A35 (02M0372) J ¼ lm approximately 15 grains per analysis recomb recomb recomb >999 recomb recomb recomb recomb recomb recomb >999 recomb >999 MdC GiD-4 feldspar VU42-A52 (02M0373) J ¼ lm approximately 15 grains per analysis 1.7 W W > W W W W fuse fuse fuse fuse fuse fuse >999 MdC GiD-4 feldspar VU42-A36 (02M0373) J ¼ lm approximately 15 grains per analysis 1.7 W W W W >999 fuse fuse fuse fuse MdC GiD-4 feldspar VU42-A52 (02M0373) J ¼ lm approximately 15 grains per analysis recomb recomb >999 recomb recomb recomb Radioisotopic dating of Tortonian GSSP K. F. Kuiper et al. Terra Nova, Vol 17, No. 4, 35 39

7 Ó 05 Blackwell Publishing Ltd 391 Table 1 Continued Laser power 39 Ar ) moles Ar/ 39 Ar ±1r 3 Ar/ 39 Ar ±1r 37 Ar/ 39 Ar ±1r 36 Ar/ 39 Ar ±1r Ar*/ 39 Ar K ±1r Ar* (%) ±1r K/Ca recomb MdC GiD-4 feldspar VU42-A36 (02M0373) J ¼ lm approximately 15 grains per analysis recomb recomb recomb recomb >999 MdC GiF-1 feldspar VU42-A37 (02M0374) J ¼ lm approximately 15 grains per analysis 1.7 W W W W W W W W W W fuse fuse fuse fuse fuse >999 fuse >999 fuse >999 fuse fuse fuse MdC GiF-1 feldspar VU42-A37 (02M0374) J ¼ lm approximately 15 grains per analysis recomb recomb recomb recomb recomb >999 recomb recomb >999 recomb recomb recomb >999 MdC Ancona feldspar VU42-A29 (02M0367) J ¼ lm approximately 15 grains per analysis 1.7 W W W W W Terra Nova, Vol 17, No. 4, K. F. Kuiper et al. Radioisotopic dating of Tortonian GSSP

8 392 Ó 05 Blackwell Publishing Ltd Table 1 Continued Laser power 39 Ar ) moles Ar/ 39 Ar ±1r 3 Ar/ 39 Ar ±1r 37 Ar/ 39 Ar ±1r 36 Ar/ 39 Ar ±1r Ar*/ 39 Ar K ±1r Ar* (%) ±1r K/Ca 1.7 W W W W W fuse fuse fuse fuse fuse fuse fuse fuse fuse fuse MdC Ancona feldspar VU42-A29 (02M0367) J ¼ lm approximately 15 grains per analysis recomb recomb recomb recomb recomb recomb recomb recomb recomb recomb MdC Ancona feldspar VU42-A32 (02M036) J ¼ lm approximately 15 grains per analysis 1.7 W W W > W W > W W W W W fuse >999 fuse >999 fuse fuse fuse fuse fuse Radioisotopic dating of Tortonian GSSP K. F. Kuiper et al. Terra Nova, Vol 17, No. 4, 35 39

9 Terra Nova, Vol 17, No. 4, K. F. Kuiper et al. Radioisotopic dating of Tortonian GSSP Table 1 Continued Ar* (%) ±1r K/Ca 39 Ar ) moles Ar/ 39 Ar ±1r 3 Ar/ 39 Ar ±1r 37 Ar/ 39 Ar ±1r 36 Ar/ 39 Ar ±1r Ar*/ 39 ArK ±1r Laser power fuse >999 fuse >999 fuse MdC Ancona feldspar VU42-A32 (02M036) J ¼ lm approximately 15 grains per analysis recomb recomb >999 recomb recomb recomb recomb recomb recomb recomb >999 recomb Isotope ratios are corrected for blanks, mass discrimination and radioactive decay. Total system blanks were in the range of 2 6 )17 moles for mass, )1 moles for mass 39, 3 and 36, and 1 2 )17 moles for mass 37. Mass discrimination factor ranged from to per atomic mass unit. Ages are given with ±1r analytical error including uncertainties in intercept values, blank corrections, mass discrimination and interference reactions. The correction factors used were ( 39 Ar/ 37 Ar) Ca ¼ , ( 36 Ar/ 37 Ar) Ca ¼ and ( Ar/ 39 Ar) K ¼ ratios are calculated by multiplying 39 Ar K / 3 Ar Cl with This factor is based on K 2 O content of MMhb-1 of Samson and Alexander (197), Cl content of Roddick (193) and Ar/ 39 Ar data for MMhb-1 irradiated in OSU TRIGA CLICIT of Renne et al. (199). Biotite plateau steps are indicated in grey. Feldspar samples have been measured in two steps, which are recombined afterwards (see text). Data of the two separate and recombined (recomb) steps are given. in J at the 6% confidence level, unless stated otherwise. Ar/ 39 Ar ages In the Monte dei Corvi section two volcanic levels (Ancona and Respighi) were collected along the beach section. In the Monte Gibliscemi section four volcanic layers were sampled (GiF-1, GiF-2, GiD-3, GiD-4). All data are given in Table 1. Figure 3 shows the biotite age spectra and Fig. 4 the age probability distributions and underlying individual experiments for sanidine. Monte dei Corvi: Respighi level The three incremental heating spectra of Respighi biotite all seem to define a plateau age (Fig. 3a c). Two of them can be combined to a weighted mean plateau age of ± 0.01 Ma. Sample 02M0399b yields a younger plateau age of ± 0.03 Ma. Isochrons are poorly constrained because of clustering of the data, but intercepts do not deviate from the atmospheric Ar/ 36 Ar ratio at the 95% confidence level. Radiogenic Ar * yields are constant around 90% apart from the first and final steps (Fig. 3). This indicates that the biotite is not or only marginally altered. A previous study reported an integrated isochron age of.94 ± 0.0 Ma (Montanari et al., 1997, recalculated to FCT of 2.02 Ma). This age is c Ma younger than the results presented here (Fig. 2). This discrepancy cannot be explained easily, because both studies are able to define plateaus, and isochron intercepts are indistinguishable from atmospheric argon. Age differences are not the result of systematic errors in decay constants and standard ages (all ages are calculated with the same set of parameters, both studies use FCT standard). Montanari et al. (1997) report times lower ratios and the radiogenic Ar * release pattern is slightly different in that the Ar* yields are lower in the first % of 39 Ar K released and then remain constant around 90%. Apparently, both studies dated different biotite populations from the Respighi interval. However, it must Ó 05 Blackwell Publishing Ltd 393

10 Radioisotopic dating of Tortonian GSSP K. F. Kuiper et al. Terra Nova, Vol 17, No. 4, ± 0.03 Ma; MSWD M0399B E TF: ± 0.03 Ma II:.06 ± 0.19 Ma 11.2 ± 0.03 Ma; MSWD M00D A C G ± 0.02 Ma; MSWD 0.32 TF: ± 0.03 Ma II: ± 0.26 Ma TF: 13. ± 0.02 Ma II: ± 0.13 Ma TF: 11.0 ± 0.03 Ma II: ± 0.07 Ma ± 0.02 Ma; MSWD M05A M06A Cumulative 39 Ar Released (%) Cumulative 39 Ar Released (%) be remarked that the age of ± 0.11 Ma considered discrepant by Montanari et al. (1997) and excluded from their age determination falls in between the ages reported here. Ar * (%) Ar * (%) Ar * (%) Ar * (%) 1 1 F TF: ± 0.03 Ma II: ± 0. Ma 11.3 ± 0.03 Ma; MSWD M00E B H 11.4 ± 0.02 Ma; MSWD M00B TF: ± 0.03 Ma II: ± 0. Ma ± 0.03 Ma; MSWD M0399C D TF: ± 0.03 Ma II: 11. ± 0.04 Ma ± 0.02 Ma; MSWD M00A TF: 11.4 ± 0.02 Ma II: ± 0.0 Ma Fig. 3 Incremental heating spectra of Respighi (A-C) and Ancona (D-H) biotite. Plateau steps are shown with 1r analytical errors excluding the error in J. TG ¼ total gas age and II ¼ inverse isochron age. To be eligible for inclusion in the plateau steps must yield Ar * >%. Plateaus should contain at least 50% of the 39 Ar K released and MSWD values of plateaus should be smaller than the statistical T-distribution at a 95% confidence level (Koppers, 02). Dashed (continuous) line represents ratio ( Ar * yield). 0 Monte dei Corvi: Ancona level Ancona biotite yields five plateaus ages (Fig. 3d h) with a combined plateau age of 11.2 ± 0.01 Ma. Isochron intercepts do not deviate from the Ar * (%) Ar * (%) Ar * (%) Ar * (%) atmospheric Ar/ 36 Ar ratio. Montanari et al. (1997) reported a weighted plateau age of ± 0. Ma (recalculated to FCT of 2.02 Ma) for one biotite incremental heating experiment. Again our age is significantly older, our ratios are times higher and Ar * release patterns are slightly different. We either dated different biotite populations or some kind of systematic error (e.g. heterogeneity of samples, recoil, etc.) must have occurred. Multigrain feldspar fractions from two samples obtained during different field campaigns are fused in two steps (a pre-heating step with a defocused laser beam with an output of 2 W and a fusion step). We expect that the preheating steps, initially performed to remove undesirable atmospheric argon adsorbed to the crystal surface, yield low amounts of radiogenic Ar. But no substantial difference in the radiogenic Ar yield between both steps is detected. Therefore, data of both steps are re-combined afterwards (Table 1). Isochrons are poorly constrained, but do not deviate from atmospheric argon at the 95% confidence level. Ar * yields are higher than 96% and Cl can hardly be detected (low 3 Ar Cl signals because of Cdlining in the reactor). The combined weighted mean age of all analyses (excluding outliers, Fig. 4a,b) yields an age of ± 0.01 Ma (Table 2). Gibliscemi The two-step fusion procedure is also applied to multigrain feldspar fractions of the volcanic layers in Gibliscemi. Samples show no significant differences in Ar * contents between the two steps (apart from GiF-2, which is also heterogeneous in age), although there is a tendency that the fusions steps are slightly older (but not significant at the 95% confidence level). Therefore both steps are re-combined (Table 1). Two ash levels (GiD-4 and GiF-2) are extremely heterogeneous (Table 2; Fig. 4e h) and no reliable ages can be deduced. The two youngest GiF-2 ages coincide more or less with the astronomical age (Fig. 4g), but the youngest age of GiD-4 is c. 0.5 Ma younger than the astronomical age (Fig. 4h). GiD-3 shows a normal age distribution (Fig. 4c) and yields a weighted mean age of 11.4 ± 0.01 Ma. The age 394 Ó 05 Blackwell Publishing Ltd

11 Terra Nova, Vol 17, No. 4, K. F. Kuiper et al. Radioisotopic dating of Tortonian GSSP A C E G Fig. 4 Ar/ 39 Ar age probability distributions. Cumulative age probability distributions are shown with underlying individual experiments and 1r analytical errors. The vertical shaded bars represent the astronomical age and uncertainty. Ancona, GiF-2 and Gid-4 are heterogeneous in age and their age distribution near their astronomical age is shown in more detail. probability distribution of GIF-1 shows outliers towards older ages (Fig. 4d). The oldest age is removed from the distribution (based on MSWD < T-test statistic at the 95% confidence level; Table 2) yielding a weighted mean age of 11.9 ± 0.02 Ma for GiF-1. B D F H Implications and conclusions As stated before the Serravallian Tortonian boundary (Tortonian GSSP) is defined in the Monte dei Corvi section at the sapropel midpoint of cycle 76 (Hilgen et al., 05). The Ancona level is intercalated in the marl just below the sapropel of cycle 72, located approximately 2 m below the boundary. At Monte Gibliscemi the Serravallian Tortonian boundary is bracketed by four volcanic levels: approximately 9 m (GiF-2) and m (Gif-1) below and approximately 4 m (GiD-3) and m (GiD-4) above the boundary (Fig. 2). As all these volcanic levels occur in astronomically dated sections the Ar/ 39 Ar and astronomical dating methods can be directly compared. When two independent dating methods are compared all sources of error must be included in the age error estimates. Here, we assume that the tuning is correct based on the good to excellent correspondence between characteristic cycle patterns and the insolation target curve (Hilgen et al., 00, 03). Taking the remaining small uncertainties in the astronomical ages into account, astronomical ages for ash layers are reported with an uncertainty of ± 0.0 Ma (Kuiper, 03). An extra uncertainty of one precession cycle (i.e. ±0.0 Ma) is added to the astronomical age of GiF-2, because the correspondence of the sedimentary cycle pattern and calculated insolation is less straightforward in this interval. Table 2 Ar/ 39 Ar ages are also reported with total external errors, i.e. including analytical uncertainties in samples and standards, decay constant uncertainties and uncertainties in the potassium and radiogenic argon contents in primary standards. Biotite ages overlap with the astronomical ages of the ash levels (Fig. 2) apart from the Respighi level in Montanari et al. (1997), which is younger. Although the total external errors cause the Ar/ 39 Ar ages to overlap with the astronomical ages, it is difficult to explain internal inconsistencies in biotite ages (note that Fig. 2 shows total external errors as well as errors including analytical uncertainties in sample and J). We do not find evidence for excess argon in this study, but biotite may yield well-defined but meaningless plateau ages even if severely contaminated with excess argon (Foland, 193). Additionally, biotite is not the most suitable mineral for intercalibration purposes (see e.g. Roberts et al., 01; DiVincenzo et al., 03) and is further neglected here. Ó 05 Blackwell Publishing Ltd 395

12 Radioisotopic dating of Tortonian GSSP K. F. Kuiper et al. Terra Nova, Vol 17, No. 4, Table 2 Summary of Ar/ 39 Ar data. Ar/ 39 Ar age (Ma) Identity Analytical error External error N MSWD Ar K % Isochron intercept Isochron age (Ma) K/Ca MdC Respighi biotite m0399b ± (7) ± ± m0399c ± (3) ± ± m06a ± (2) ± ± m0399c/02m06a ± (5) ± ± MdC Ancona biotite m00a 11.6 ± (2) ± ± m00b 11.4 ± (4) ± ± m00d 11.2 ± (4) ± ± m00e 11.3 ± (3) ± ± m05a ± (2) ± ± Combined plateau 11.3 ± (15) ± ± MdC Ancona feldspar 11.6 Ar*% 02m ± (3) ± ± m ± ± ± Combined ± (4) ± ± GiD-4 feldspar m ± GiD-3 feldspar m ± ± ± GiF-2 feldspar m0369. ± m ± () GiF-1 feldspar m ± m ± (1) ± ± Weighted mean Ar/ 39 Ar ages and standard errors age are given at the 6% confidence level as analytical errors (excluding uncertainty in J) and total external errors (i.e. including the analytical uncertainty, uncertainty in J, uncertainties in intercalibration between primary and secondary standards, errors in the absolute amount of K and radiogenic Ar * contents of the primary standard and uncertainties in decay constants). N is the number of experiments, with the number of omitted data/steps given in parentheses. 39 Ar K is the percentage of 39 Ar K contributing to the biotite plateaus, Ar * is the percentage of radiogenic Ar in the sample. All ages are reported relative to FCT of 2.02 Ma (Renne et al., 199) and consensus decay constants are used (Steiger and Jäger, 1977). Astronomical ages for the tephras are given in bold italic. Inverse isochron ages and intercepts are given with 1r analytical error. When MSWDs of isochrons >statistical F-test at the 95% confidence level, reported isochrons are error chrons (Koppers, 02) and are not given in the table. For all plateaus MSWDs <Student s T distribution at 95% confidence level. Data are excluded from weighted mean age for Ancona and GiF-1 feldspar data until MSWD <Student s T-test at the 95% confidence level. Sanidine is considered more suitable for Ar/ 39 Ar dating, because of its higher K-content and a lower mineral/fluid partition coefficient for argon than micas (Kelley, 02). The proposed intercalibration of radioisotopic and astronomical dating methods is based on sanidine (Kuiper, 03; Kuiper et al., 04), resulting in astronomically calibrated ages of 2.21 ± 0.04 Ma and 2.24 ± 0.01 Ma for FCT sanidine, one of the most widely used secondary standards (Fig. 5). The astronomical calibrated FCT ages based on the new feldspar data are listed in Table 3 and compared with a selection of FCT ages based on other methods (Fig. 5). In fact, the most reliable Ar/ 39 Ar ages from Monte Gibliscemi (GiF-1 and GiD-3) support the astronomical intercalibration proposed by Kuiper et al. (04) and Kuiper (03) (Fig. 5). Nevertheless contamination with older detrital or xenocrystic grains, clearly visible in GiF-2 and GiD-4, but also observed in the Ancona layer, may affect all our multigrain samples (Fig. 4). Such a contamination results in Ar/ 39 Ar ages older than the eruption age and consequently younger intercalibrated FCT ages. Due to the analyses of multigrain fractions we cannot completely exclude this contaminating component in our data. Contamination can be excluded for the single crystal sanidine ages from the Melilla Basin (Kuiper, 03), but might explain the younger intercalibrated FCT age based on the Ancona ash layer (Fig. 5). Further, the astronomically calibrated FCT ages are within error consistent but slightly older than the ages of Renne et al. (199) and Spell and McDougall (03) with larger total uncertainties in the latter age estimates related to the uncertainties in decay constants and absolute amounts of Ar * and K (Fig. 5). Kwon et al. (02) combined pairs of Ar * / 39 Ar K ratios with historical or U/Pb reference ages and used a series of statistical methods to infer an age for FCT (and simultaneously the total the decay constant) which is in remarkable agreement with the astronomically calibrated FCT ages. The U/Pb data of Schmitz and Bowring (01) are slightly older, but might suffer from residence times in the magma chamber. Therefore, the intercalibration between the astronomical and Ar/ 39 Ar methods proposed by Kuiper et al. (04) and Kuiper (03) is supported 396 Ó 05 Blackwell Publishing Ltd