Daniel B. Karner

Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating

The 40Ar/39Ar dating method depends on accurate intercalibration between samples, neutron fluence monitors, and primary 40Ar/40K (or other external) standards. The 40Ar/39Ar age equation may be expressed in terms of intercalibration factors that are simple functions of the relative ages of standards, or equivalently are equal to the ratio of radiogenic to nucleogenic K-derived argon (40Ar/39ArK) values for one standard or unknown relative to another. Intercalibration factors for McClure Mountain hornblende (MMhb-1), GHC-305 biotite, GA-1550 biotite, Taylor Creek sanidine (TCs) and Alder Creek sanidine (ACs), relative to Fish Canyon sanidine (FCs), were derived from 797 analyses involving 11 separate irradiations with well-constrained neutronfluence variations. Values of the intercalibration factors are RFCsMMhb-1 = 21.4876 ± 0.0079; RFCsGA-1550 = 3.5957 ± 0.0038; RFCsTCs = 1.0112 ± 0.0010; RFCsACs = 0.04229 ± 0.00006, based on the mean and standard error of the mean resulting from four or more spatially distinct co-irradiations of FCs with the other standars. Analysis of 35 grains of GHC-305 irradiated in a single irradiation yields RFCsGHC-305 = 3.8367 ± 0.0143. Results at these levels of precision essentially eliminate intercalibration as a significant source of error in 40Ar/39Ar dating. Data for GA-1550 (76 analyses, 5 fluence values), TCs (54 analyses, 4 fluence values), FCs (380 analyses, 40 fluence values) and ACs (86 analyses, 11 fluence values) yield MSWD values showing that the between-grain dispersion of 40Ar∗/39ArK values is consistent with analytical errors alone, whereas MMhb-1 (167 analyses, 4 irradiations) and GHC-305 (34 analyses, 1 fluence value) are heterogeneous and therefore unsuitable as standards for small sample analysis. New K measurements by isotope dilution for two primary standards, GA-1550 biotite (8 analyses averaging 7.626 ± 0.016 wt%) and intralaboratory standard GHC-305 (10 analyses averaging 7.570 ± 0.011 wt%), yield values slightly lower and more consistent than previous data obtained by flame photometry, with resulting 40Ar/40K ages of 98.79 ± 0.96 Ma and 105.6 ± 0.3 Ma for GA-1550 and GHC-305, respectively. Combining these data with the intercalibration approach described herein and using GA-1550 as the primary standard (1.343 × 10−9 mol/g of 40Ar∗; [McDougall, I., Roksandic, Z., 1974. Total fusion 40Ar/39Ar ages using HIFAR reactor. J. Geol. Soc. Aust. 21, 81–89.]) yields ages of 523.1 ± 4.6 Ma for MMhb-1, 105.2 ± 1.1 Ma for GHC-305, 98.79 ± 0.96 Ma for GA-1550, 28.34 ± 0.28 Ma for TCs, 28.02 ± 0.28 for FCs, and 1.194 ± 0.012 Ma for ACs (errors are full external errors, including uncertainty in decay constants). Neglecting error in the decay constants, these ages and uncertainties are: 523.1 ± 2.6 Ma for MMhb-1, 105.2 ± 0.7 Ma for GHC-305, 98.79 ± 0.54 for GA-1550, 28.34 ± 0.16 Ma for TCs, 28.02 ± 0.16 Ma for FCs, and 1.194 ± 0.007 Ma for ACs. Using GHC-305 as the primary standard (1.428 ± 0.004 × 10−9 mol/g of 40Ar∗), ages are 525.1 ± 2.3 Ma for MMhb-1, 105.6 ± 0.3 Ma for GHC-305, 99.17 ± 0.48 Ma for GA-1550, 28.46 ± 0.15 Ma for TCs, 28.15 ± 0.14 Ma for FCs, and 1.199 ± 0.007 Ma for ACs, neglecting decay constant uncertainties. The approach described herein facilitates error propagation that allows for straightforward inclusion of uncertainties in the ages of primary standards and decay constants, without which comparison of 40Ar/39Ar dates with data from independent geochronometers is invalid. Re-examination of 40K decay constants would be fruitful for improved accuracy.

40Ar/39Ar geochronology of Roman volcanic province tephra in the Tiber River valley: Age calibration of middle Pleistocene sea-level changes

The close proximity of the Roman volcanic province to the Tyrrhenian Sea coastline provides a unique opportunity to combine clastic stratigraphy with 40 Ar/ 39 Ar geochronology to constrain the timing of Pleistocene sea-level oscillations. The main eruptions from the Monti Sabatini volcanic district occurred during the interval 560–280 ka, and the Alban Hills volcanic district main eruptions span 560–350 ka. The interfingering of volcanics from these two centers with fluvial and shallow-marine sediments of the Tiber River and delta provides a datable relative sea-level record for this portion of middle Pleistocene time. We calculate the timing of glacial terminations using analytical errors only, then assess age uncertainties that include analytical plus systematic errors; the latter is required to compare 40 Ar/ 39 Ar ages with those from other dating methods. Terminations III, V, and VI occur at 278 (261, 285) ka (95% confidence interval), 430 (422, 442) ka, and 534 (520, 541) ka, respectively, when only analytical uncertainties are used to calculate the ages of bracketing volcanic horizons. The confidence interval expands significantly when full external errors are considered, with predicted ages of 276 (258, 289) ka, 430 (416, 448) ka, and 533 (512, 548) ka for the terminations. The resultant 40 Ar/ 39 Ar chronology is generally consistent with the deep-sea δ 18 O record of sea-level change tuned to Earth9s obliquity cycle for glacial terminations VI, V, and III. In addition, the 40 Ar/ 39 Ar constrained Tiber River delta sea-level record has the added benefit of identifying when coastal sections respond to complex (multistep) terminations.

CORRELATION OF FLUVIODELTAIC AGGRADATIONAL SECTIONS WITH GLACIAL CLIMATE HISTORY : A REVISION OF THE PLEISTOCENE STRATIGRAPHY OF ROME

Volcanic horizons interbedded with fluviodeltaic clastic sediments of the Tiber and Fosso Galeria River valleys near Rome provide a unique opportunity to date five middle Pleistocene fining-upward aggradational sections. 40 Ar/ 39 Ar ages from the interbedded volcanic beds illustrate that aggradation occurred on an approximate 100 k.y. cycle and was initiated by sea-level rise associated with glacial melting. The age constraint additionally shows that aggradation occurs rapidly in response to sea-level change, and thus ages from volcanic strata within the aggradational sections provide high-resolution constraint on the timing of relative sea-level rise. The established chronology is consistent with other glacial proxies and their (radioisotopically or orbitally based) time scales for the timing of glacial terminations VI and V, but is significantly older than most records for the timing of termination III. Ultimately, the age of each termination should be determined by radioisotopic means, and to that end the work here shows that fluviodeltaic sedimentology provides valuable information for middle Pleistocene time-scale calibration.

Pulsed uplift estimated from terrace elevations in the coast of Rome: evidence for a new phase of volcanic activity?

Abstract The most recent phase of activity from the Roman volcanoes, since 45 ka, has been interpreted to be the waning stage of the older explosive activity that peaked between 550 and 250 ka. Yet substantial regional uplift from approximately 250 ka suggests that magma chambers beneath the area are growing rather than shrinking. We have estimated the uplift history of the region using radioisotopically and paleomagnetically constrained coastal terrace elevation data that allow us to correlate these terraces with marine oxygen isotopic stages 1–22. Elevations have been corrected for glacio-eustatic sea-level change. Our study shows that over the last million years, regional uplift of coastal terraces occurred in pulses that preceded volcanic activity at roughly 800 ka (in the Monti Sabatini) and 600 ka (in the Alban Hills). The recent pulse of uplift since 250 ka occurred primarily during a period of relative volcanic dormancy that lasted from 250 to 45 ka. We hypothesize that the most recent volcanic activity since 45 ka may reflect the completion of magma chamber recharge, suggesting the start of a new volcanic epoch. It is also possible that the local tectonic stress field changed recently from one dominated by transpression to one dominated by extension, allowing magma to more easily reach the surface via normal faults.

Extraterrestrial accretion from the GISP2 ice core

The rate of extraterrestrial accretion for particles in the size range 0.45 μm to ∼20 μm was determined from dust concentrates extracted from Greenland Ice Sheet Project 2 (GISP2) ice core samples. Using instrumental neutron activation analysis (INAA), we determined the iridium (Ir) content of the dust. Following a core-specific correction for terrestrial Ir and assuming a chondritic Ir abundance of 500 ppb, we measure an average accretion rate for 0.45 μm to ∼20 μm particles over the entire Earth of 0.22 (± 0.11) × 109 g/yr (kton/yr) for 317 years of ice through the interval 6 to 20 ka. This is consistent with the interplanetary dust accretion rate of 0.17 (± 0.08) x 109 g/yr that we derive from published 3He data for the GISP2 core. Accounting for particles that are larger and smaller than those detected by or experiment, our best estimate of the total accretion rate (including particle sizes up to about 4 cm in diameter) is 2.5 × 109 g/yr. The uncertainty in this estimate is dominated by statistical fluctuations in the number of particles expected to end up in the ice core and not by measurement error. Based on Monte Carlo simulations, we estimate the upper limit for total extraterrestrial accretion to Earth of 6.25 × 109 g/yr (95% confidence level). This accretion rate is consistent with some estimates from micrometeorite concentrations in polar ice, estimates from ground-based radar studies, and with accretion estimates of 3He-bearing interplanetary dust particles, assuming that 3He is correlated with particle surface area. It is, however, lower than estimates based on platinum group element studies of marine sediments. The conflict may indicate systematic errors with either the marine or the non-marine samples, departures from the assumed particle spectrum of Grun and coauthors, or time-variable accretion rates, with the early Holocene period being characterized by low levels of accretion.

Paleomagnetism and geochronology of early Middle Pleistocene depositional sequences near Rome: comparison with the deep-sea δ18O record

Abstract A sequence-stratigraphic analysis was carried out on clastic coastal deposits (Ponte Galeria Formation – PGF) of the Tiber River delta, cropping out to the southwest of Rome, in order to correlate these sequences to Pleistocene eustasy. Tectonic subsidence affected sedimentation, but is distinguished from the glacio-eustatic signal, showing that the PGF consists of four depositional sequences that correlate to two fourth-order cycles. A paleomagnetic investigation of clay horizons deposited before and after the Brunhes–Matuyama reversal, combined with 40 Ar/ 39 Ar ages from a tuff horizon within the PGF, constrain the timing of the relative sea-level changes and allow us to tie it to the astronomically calibrated deep-sea δ 18 O record. The Brunhes–Matuyama boundary in the PGF postdates a set of highstand systems tract sediments, suggesting that the reversal occurs late in δ 18 O stage 19. This position is consistent with the recent study of deep-sea cores by Bassinot et al. [Earth Planet. Sci. Lett. 126 (1994) 91–108]. The assumption of a common continental stage marking the start of the Middle Pleistocene ( δ 18 O stage 22) in the Mediterranean region is not supported by the Tiber River sections which, in our interpretation, begin at stage 20.

Quaternary fluvial-volcanic stratigraphy and geochronology of the Capitoline Hill in Rome

In and around Rome, coastal and fluvial sediments influenced by Quaternary sea-level changes interfinger with accurately datable, highly potassic pyroclastic volcanic units. We describe the complex internal geology of the Capitoline Hill, for centuries a place of leading importance in ancient Rome and more recently a focus of archaeological research. Outcrops and many new drill cores reveal at least four cycles of valley cutting and filling, probably recording the history of the Tiber River in response to changes in sea level. Single-crystal 40 Ar/ 39 Ar dates on sanidine give ages of 518 ± 6, 413 ± 5, and 353 ± 2 ka for three prominent volcanic units filling separate paleovalleys. The Roman region offers the potential for direct age calibration of the Quaternary sea-level record.

40Ar/39Ar dating of glacial termination V and the duration of Marine Isotopic Stage 11

Using 40 Ar/ 39 Ar data from volcanic horizons intercalated with sea level high stand deposits of the Tiber River Delta, we determine the timing of Glacial Termination V and the duration of Marine Isotopic Stage (MIS) 11. Termination V occurred in the interval 421-447 ka (95% confidence), and the duration of MIS 11 is 11-39 kyr (95% confidence). This termination age is older than the time scale developed by Shackleton et al. [1990] for ODP Site 677, and is consistent with the SPECMAP time scale [Imbrie et al., 1984]. These results verify that, if forced by insolation, Termination V was caused by the fourth lowest (out of 32) insolation maxima of the last 700 kyr.