GSA Bulletin: 40Ar/39Ar Geochronology of Roman Volcanic

Home , Alban Hills, Monti Sabatini

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

Daniel B. Karner* Department of Geology and Geophysics, University of California, Berkeley, California 94720 Paul R. Renne Department of Geology and Geophysics, University of California, Berkeley, California 94720 and Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709

ABSTRACT tifying when coastal sections respond to com- used extensively as the glacial proxy for late Ter- plex (multistep) terminations. tiary time. Before the use of deep-sea cores, The close proximity of the Roman volcanic coastal deposits were studied to determine past cli- province to the Tyrrhenian Sea coastline pro- INTRODUCTION mate changes. Sea-level oscillations due to the vides a unique opportunity to combine clastic temporary storage of water in continental ice stratigraphy with 40Ar/39Ar geochronology to The only radioisotopic geochronometer that sheets have not only shaped the δ18O record, but constrain the timing of Pleistocene sea-level provides reliable ages throughout the entire have altered coastal areas by deposition of clastic oscillations. The main eruptions from the range of middle Pleistocene time is 40K, which sediment during marine transgressions (inter- Monti Sabatini volcanic district occurred dur- partially decays to 40Ar. Both parent and daugh- glacial intervals) and by incising river valleys during the interval 560–280 ka, and the Alban ter of this decay series are well preserved in vol- ing regressions (glacial intervals). Therefore, the Hills volcanic district main eruptions span canic rocks. However, the rarity of volcanogenic coastal stratigraphy, like the δ18O record, can be 560–350 ka. The interfingering of volcanics materials associated with climate proxy records used as a direct proxy for glaciation. Volcanic hori- from these two centers with fluvial and shal- has made it difficult to use K/Ar (and more re- zons interbedded with the coastal deposits provide low-marine sediments of the Tiber River and cently 40Ar/39Ar) dating methods for high-preci- a means by which sea-level variations can be delta provides a datable relative sea-level sion calibration of middle Pleistocene glacial os- dated, and this chronology can then be compared record for this portion of middle Pleistocene cillations. Although recent attempts have been to orbitally based time scales inferred from the time. We calculate the timing of glacial termi- made to date climate proxies using the 40Ar/39Ar δ18O record. nations using analytical errors only, then as- method (e.g., Van den Bogaard et al., 1989), em- Radioisotopic dating of climate events in this sess age uncertainties that include analytical phasis has shifted away from radioisotopic dat- way must deal with the problem of natural plus systematic errors; the latter is required to ing systems such as K/Ar and toward the use of contamination. In volcanically active regions compare 40Ar/39Ar ages with those from other orbital time scales, such as the astronomically where tens to hundreds of eruptions can occur dating methods. Terminations III, V, and VI calibrated (geomagnetic) polarity time scale over geologically short time intervals, entrain- occur at 278 (261, 285) ka (95% confidence in- (APTS, Shackleton et al., 1990; Hilgen, 1991a, ment of older volcanic material in younger flows terval), 430 (422, 442) ka, and 534 (520, 541) 1991b) for dating glacial cycles. Even though can be problematic. Identification of those con- ka, respectively, when only analytical uncer- this approach provides a gross time calibration taminant (xenocrystic) populations can been ex- tainties are used to calculate the ages of brack- of glaciation, it is limited by minimal radioiso- tremely difficult. During the past decade, ad- eting volcanic horizons. The confidence inter- topic verification of the timing of individual vances in the 40Ar/39Ar method, including the use val expands significantly when full external glacial oscillations. The close proximity of the of lasers, low-blank–high-sensitivity mass spec- errors are considered, with predicted ages of Roman volcanic province in Italy to the coastline trometers, electron multipliers, and improvements 276 (258, 289) ka, 430 (416, 448) ka, and 533 of the Tyrrhenian Sea provides a unique oppor- in microextraction techniques, have provided (512, 548) ka for the terminations. The result- tunity to use the 40Ar/39Ar method to date vol- the ability to detect xenocrystic contamination ant 40Ar/39Ar chronology is generally consist- canic horizons interbedded with coastal sed- through single-crystal analysis (Lo Bello et al., ent with the deep-sea δ18O record of sea-level iments and, through facies analysis of those 1987), enabling geochronologists to improve the change tuned to Earth’s obliquity cycle for sediments (Karner and Marra, 1998, this issue), accuracy and precision of ages determined for py- glacial terminations VI, V, and III. In addition, to calibrate the timing of sea-level oscillations roclastic rocks (McDougall and Harrison, 1988). the 40Ar/39Ar constrained Tiber River delta through middle Pleistocene time. This paper pre- sea-level record has the added benefit of iden- sents a detailed geochronology for most of the VOLCANIC STRATIGRAPHY major eruptive events and many smaller ones identified from the Alban Hills and Monti Saba- The Monti Sabatini and Alban Hills eruptive *Present address: Department of Physics, Univer- tini volcanic centers (Table 1). centers of the Roman volcanic province have at- sity of California, Berkeley, California 94720; e-mail: The oxygen isotopic (δ18O) record from pelagic tracted geochronologists since the mid-1960s (see [email protected] sediments recovered in deep-sea cores has been Fornaseri, 1985, for early work). They are located Data Repository item 9845 contains additional material related to this article.

GSA Bulletin; June 1998; v. 110; no. 6; p. 740–747; 3 figures; 1 table.

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TABLE 1. 40Ar/39Ar AGE SUMMARY Eruptive unit Sample Source Machine, J, Nucleogenic Lab Lat Long Material Method Age ± 2σ# ±2σ∗∗ number Discrimi- corrections † number (north) (east) (No. crystals)§ (ka) nation* Post-TGdS R93-15H2 Monti Sabatini? 2, 7 A 30181 41°47′02′′ 12°26′10′′ Sanidine Wtd. Mn.(4) 251 8 11 Post-TGdS R95-04B Monti Sabatini 2 A 30176 41°51′20′′ 12°19′45′′ Sanidine Wtd. Mn.(5) 266 5 8 TGdS R93-28 Monti Sabatini 6 A 7595 41°56′18′′ 12°30′57′′ Sanidine Wtd. Mn.(6) 283 2 6 Tufo Lionato SPRQ-40 Alban Hills 6 A 7593 41°53′28′′ 12°28′55′′ Leucite Wtd. Mn.(13) 353 4 8 Pozz. Nere R94-20 Alban Hills 2 A 30173 41°54′48′′ 12°33′14′′ Leucite Wtd. Mn.(6) 404 7 13 TSVStorta SPQR-51 Monti Sabatini 1 B 7108 41°53′36′′ 12°29′05′′ Sanidine Wtd. Mn.(7) 413 11 13 TSVStorta R94-30C Monti Sabatini 3, 8 C 8292 41°51′00′′ 12°20′06′′ Sanidine Wtd. Mn.(7) 427 5 11 TSVStorta R95-04H Monti Sabatini 2 A 30175 41°51′20′′ 12°19′45′′ Sanidine Wtd. Mn.(3) 434 8 14 TRaSN R93-26A Monti Sabatini 5 A 7583 42°00′03′′ 12°29′32′′ Sanidine Wtd. Mn.(9) 446 2 9 Pozz. Rosse R95-09A Alban Hills 2 A 30178 41°47′07′′ 12°28′48′′ Leucite Wtd. Mn.(5) 455 8 14 Vallerano R95-10A Alban Hills 2 A 30179 41°47′49′′ 12°28′48′′ Leucite Wtd. Mn.(6) 457 8 14 TRaSN? R94-22B Monti Sabatini 3 C 8299 41°57′15′′ 12°28′00′′ Sanidine Wtd. Mn.(8) 459 7 13 TSVSac R93-17Z Monti Sabatini 5 A 7587 41°40′08′′ 12°28′34′′ Sanidine Wtd. Mn.(13) 474 5 10 TSVSac R93-11M Monti Sabatini 4 A 7577 41°51′43′′ 12°21′23′′ Sanidine Wtd. Mn.(4) 485 3 10 TSVSac R93-22C Monti Sabatini 5 A 7582 41°49′26′′ 12°19′43′′ Sanidine Wtd. Mn.(7) 500 6 11 TSVSac SPQR-5 Monti Sabatini 1 B 7106 41°53′28′′ 12°28′58′′ Leucite Wtd. Mn.(7) 517 15 18 TSVSac R94-28 Monti Sabatini 3, 8 C 8285 41°55′07′′ 12°28′37′′ Sanidine Wtd. Mn.(10) 526 6 13 TGdVT R93-02 Monti Sabatini 6 A 7589 42°05′11′′ 12°32′24′′ Sanidine Wtd. Mn.(7) 538 3 10 TGdVT? R94-26 Monti Sabatini 3, 8 C 8289 41°53′20′′ 12°29′29′′ Sanidine Wtd. Mn.(8) 548 4 13 TGdVT R93-25A Monti Sabatini 5 A 7588 42°12′38′′ 12°29′13′′ Sanidine Wtd. Mn.(6) 557 3 11 Pisolitic Tuff R95-02A Alban Hills 2 A 30188 41°50′08′′ 12°28′58′′ Leucite Youngest(1) 557 14 20 Pre-TGdVT R95-06A Monti Sabatini 2 A 30182 41°50′50′′ 12°17′50′′ Sanidine Wtd. Mn.(5) 605 11 19 Pre-TGdVT R93-06N Monti Sabatini 3, 8 C 8281 41°50′43′′ 12°21′12′′ Sanidine Wtd. Mn.(9) 614 3 14 N.A. R95-16A Vico?/Vulsini? 2 A 30189 41°50′22′′ 12°21′06′′ Sanidine Wtd. Mn.(6) 749 14 24 N.A. R93-12F Vico?/Vulsini? 4 A 7576 41°50′08′′ 12°34′32′′ Sanidine Wtd. Mn.(3) 753 8 16 Note: Full 40Ar/39Ar data set is available from the GSA Data Repository (see text footnote 1). *Machine, J, Discrimination (per amu): 1: MAP2, 3.52 (±0.03) × 10–4, 1.0046 ± 0.0010; 2: MAP1, 1.351 (±0.012) × 10–4, 1.00826 ± 0.00102; 3: MAP2, 3.928 (±0.004) × 10–4, 1.00657 ± 0.00151; 4: MAP1, 3.946 (±0.006) × 10–4, 1.002946 ± 0.001483; 5: MAP1, 3.958 (±0.004) × 10–4, 1.002946 ± 0.001483; 6: MAP1, 3.949 (±0.005) × 10–4, 1.002946 ± 0.001483; 7: MAP1, 1.351 (± 0.012) × 10–4, 1.01011 ± 0.00121; 8: MAP1, 3.928 (±0.004) × 10–4, 1.01011 ± 0.00121. † 39 37 × –4 36 37 × –4 40 39 Corrections for interfering neutron reactions with Ca and K: A: ArCa/ ArCa= 7.64 (±0.56) 10 ; ArCa/ ArCa= 2.705 (±0.105) 10 ; ArK/ ArK= 1.4 (±0.6) × –4 39 37 × –4 36 37 × –4 40 39 × –4 39 37 × –4 36 37 10 .B: ArCa/ ArCa= 6 (±2) 10 ; ArCa/ ArCa= 2.57 (±0.06) 10 ; ArK/ ArK= 0 (±0.02) 10 .C: ArCa/ ArCa= 6.73 (±0.04) 10 ; ArCa/ ArCa= 2.64 × –4 40 39 × –4 (±0.02) 10 ; ArK/ ArK= 7 (±3) 10 . §Method of age calculation: ages calculated from weighted mean 40Ar*/39Ar ratio.Youngest (1) provides maximum age for this sample. #2σ = analytical precision. **2σ = full external errors (see text).

25 km northwest and 20 km southeast, respec- and Ventriglia, 1970; Alvarez, 1972). Small pyro- (not dated here) is a tuff known locally as the tively, of the center of Rome, and their ejecta are clastic and lava-producing events occurred before, Palatino unit because it crops out prominently on interbedded with middle Pleistocene marine and between, and after the three main events and are the Palatine Hill in the center of Rome. Phase 2 alluvial deposits from the Tyrrhenian Sea and named here by stratigraphic position relative to the consists of two ash-flow tuffs, the Pozzolane Nere, Tiber River delta (Fig. 1; Conato et al., 1980). The three main tephra beds. Deposits bracketed by the which is underlain by the Pozzolane Rosse. Sev- volcanic stratigraphy of the region has been stud- Tufo Giallo della Via Tiberina and Tufo Rosso a eral lava flows also occurred during phase 2; the ied thoroughly (Alvarez, 1972, 1975; DeRita Scorie Nere are referred to as Tufi Stratificati Vari- age of one (Vallerano Lava, lab number 30179) et al., 1989, 1992; DeRita and Sposato, 1986; colori di Sacrofano, and those bracketed by the was determined in this study. These lavas are re- Dragone et al., 1963; Mattias and Ventriglia, Tufo Rosso a Scorie Nere and Tufo Giallo de stricted to deeply incised valleys that may indicate 1970). However, due to wide dispersion of pub- Sacrofano are referred to as Tufi Stratificati Vari- times of lower sea level, and so the lavas may have lished ages and conflicting stratigraphic interpre- colori della Storta. Many of these smaller eruptive been emplaced during glacial intervals. By itself tations for many of the volcanic horizons found in events, although volumetrically insignificant, are this evidence is tenuous, and so use of the lavas as the area, regional stratigraphic correlation has areally extensive and thus can provide regional sea-level proxies requires a thorough geochrono- been difficult (Fornaseri, 1985). stratigraphic and geochronologic constraint on in- logic study to verify this interpretation. Phase 3 terbedded marine and nonmarine sediments. consists of ignimbrite, the Tufi di Villa Senni and Monti Sabatini Center the Tufo Lionato. Although the Tufo Lionato Alban Hills Center stratigraphically underlies the Tufi di Villa Senni, Approximately 30 K/Ar and 40Ar/39Ar age de- they are probably different facies of the same erup- terminations have been made previously on tephra Three eruptive phases are distinguished from tion (Fabrizio Marra, 1995, personal commun.). from the Monti Sabatini volcanic center (see the Tuscolano-Artemisio crater of the Alban Hills, Fornaseri, 1985, for early work; Cioni et al., 1993; and volcanics from these events crop out exten- 40Ar/39Ar GEOCHRONOLOGY Barberi et al., 1994). Ignimbrites from three main sively to the south of the Tiber River (DeRita et al., eruptive events are recognized from this center, 1992). Phase 1 of the Tuscolano-Artemisio con- Methodology and crop out extensively to the north of the Tiber sists of two tephra. The older is an ash fall charac- River. They are, from oldest to youngest, the Tufo terized by a thick basal accumulation of accre- The Roman volcanics are greatly enriched in Giallo della Via Tiberina, the Tufo Rosso a Scorie tionary lapilli and is given the descriptive name of potassium; sanidine and/or leucite are found in Nere, and the Tufo Giallo di Sacrofano (Mattias pisolitic tuff (Dragone et al., 1963). The younger all volcanic horizons. Samples were collected

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during four field trips to Rome, and age determi- nations were made on key volcanic horizons following each of those field trips. Hence, the age determination “run” for each field season was irradiated separately, and thus involves distinct nucleogenic corrections (see Table 1). 40Ar/39Ar Monti Sabatini analyses were made at the Berkeley Geochronol- 10' o

ogy Center using facilities and procedures simi- 42 Lago di lar to those described by Deino and Potts (1990). Bracciano Samples were crushed, and crystals ranging in size from 400 µm to several millimeters were handpicked. Grains were selected for clarity un-

der the binocular microscope and were cleaned F

in an ultrasonic bath of 7% HF solution for one o Tiber River 00' ss minute to remove surface alteration and to re- o o 42 duce atmospheric contamination. Samples were G a B B' l placed in aluminum disks along with one or e ri more of the following neutron fluence monitors a er (standards): Fish Canyon Tuff sanidine (FCs, Figure1 Aniene Riv 27.84 Ma; Renne, 1995), Alder Creek sanidine Rome (ACs, 1.186 Ma; Turrin et al., 1994), or Bishop A A'

50' Aventino Hill Tuff sanidine (BTs, 0.772 Ma; recalculated here o

from data in Izett and Obradovitch, 1994), and 41 irradiated at the Triga reactor at Oregon State F T o

y s University for approximately 30 min each. All s Lago di r o r d Albano irradiations were cadmium shielded to minimize h i 40 40 e M the K(n,p) Ar reaction by reducing the ther- a n l i af mal neutron fluence (Tetley et al., 1980). a ed Alban Hills

40' n e Two fully automated microextraction–mass- o S

40 39 41 e spectrometer systems were used for the Ar/ Ar a analyses. MAP1 includes a 6 W continuous Ar-ion laser for sample heating, and a Mass Ana- lyzer Products (MAP) 215 90° sector extended- 0 10 km geometry mass spectrometer. MAP2 includes a 50 W Nd-Yag continuous laser, and a MAP 215-50 Crater rims 90° sector extended-geometry mass spectrometer with electrostatic analyzer. Gettering times of Holocene flood plain and delta 180 s were used. Mass discrimination (1.003– Middle Pleistocene sediments and volcanics 1.008/amu) was monitored during each run by an- Pre-middle Pleistocene sediments alyzing approximately 15 air pipettes, which were o o o o analyzed at the start and end of each run, as well as 12 07'10" 12 17'10" 12 27'10" 12 37'10" bracketing each tephra. Background corrections Figure 1. Regional stratigraphy around Rome, showing the close proximity of the Roman vol- were made from full system blanks, which were canoes to the Tyrrhenian Sea coast and the Tiber River valley. A–A′ and B–B′ are the general run between every three unknowns or when back- lines of section (near coastal and fluvial, respectively) shown in Figure 2. The box titled Figure 1 ground 40Ar current on the electron multiplier ex- refers to Figure 1 in Karner and Marra (1998, this issue). ceeded a specific threshold of 0.01 to 0.05 nA. Typically, 7 to 10 single-crystal total-fusion analyses were made on each tephra, although several samples (lab numbers 30188 and 8292) Error Propagation melt (Webb and McDougall, 1967). As a result, it contained some multiple-grain analyses. Ages has proven practical to calibrate 40Ar/39Ar sani- (Table 1) are calculated from the weighted mean Although 40Ar/39Ar methods can provide very dine standards by 40Ar/39Ar methods against 40Ar*(radiogenic Ar)/39Ar ratio (Renne et al., precise ages, uncertainties in the ages of neutron other materials, the ages of which are determined 1996); a 2σ cutoff was used to identify xenocrysts fluence standards impose fundamental limita- by the K-Ar technique. or alteration. Additional analyses were made on tions on the ultimate uncertainties in age. Con- 40Ar/39Ar dating thus relies on several factors tephra suspected of having xenocrystic contami- ventionally, the ages of 40Ar/39Ar standards are (ages of standards, decay constants for 40K) that nation, in order to characterize thoroughly the determined by K-Ar methods. Unfortunately, the are subject to systematic errors in addition to the youngest age population, presumed to be juvenile. mineral (sanidine) commonly yielding the most analytical uncertainties in relative abundances of In addition, we make the assumption that no sig- precise 40Ar/39Ar data has questionable suitabil- argon isotopes, and uncertainties propagated nificant time lapsed between the time of eruption ity for K-Ar dating because it is often difficult to through the various corrections (i.e., mass discrim- and deposition. extract completely the 40Ar* from the viscous ination, backgrounds, nucleogenic interferences).

742 Geological Society of America Bulletin, June 1998

Uncertainties in decay constants and the ages by counting and regression statistics rather than λ 40 39 tACs − of standards have traditionally been ignored in ()Ar*ArK ()e 1 by uncertainties arising from corrections for mass 40 39 R≡ ACs = estimating the errors associated with Ar/ Ar 2 λ discrimination, backgrounds, nucleogenic inter- ()40 Ar*39 Ar ()e tFCs −1 dates. Even though this practice can be justified K FCs ferences, or atmospheric contamination. Thus for to some extent in comparing 40Ar/39Ar dates 40 39 based on the same standard, it is appropriate to r ()Ar* ArK ==j j (5) consider systematic errors when comparing λ Ri 40 39 tu − r 40 39 40 39 ()Ar*ArK ()e 1 k ()Ar* Ar Ar/ Ar dates with dates obtained by independ- R≡ u = K k 3 λ ent means, as in the present case. ()40 Ar*39 Ar ()e tACs −1 K ACs the variance is given by Systematic errors become particularly important when they are compounded by several stan- σ 2 σ 2 r rjr dard intercalibrations. In this study we used the Equation 2 can be generalized to an arbitrary σ2 = j+ k (6) Ri r r2 standards Fish Canyon sanidine (FCs), Alder number of intermediate intercalibrations and k  k  Creek Tuff sanidine (ACs), and Bishop Tuff sani- written as dine (BTs). ACs and BTs have ages of 1.186 Ma In the present case, K = 0.03504 ± 0.00013, on (Turrin et al., 1994) and 0.772 Ma (recalculated the basis of data summarized by Samson and here from data in Izett and Obradovitch, 1994), nAlexander (1987); R = 0.046522 ± 0.000207, on 1λ 1 40 39 tK=11n∏R+(3) based on the Ar/ Ar intercalibration with FCs. uiλλthe basis of data reported by Renne (1995); R2 is εi=1 The age of 27.84 Ma (Renne, 1995) for FCs is variable. On the basis of ACs, R2 = 0.042287 ± based on 40Ar/39Ar intercalibration with McClure 0.000362, the arithmetic mean and standard devi- Mountain hornblende (MMhb-1); the age of ation of three values obtained from separate irra-

520.4 Ma for the hornblende is based on a syn- where the Ri are defined as above, and the index diations by Turrin et al. (1994), and on the basis of 40 40 thesis of K/ Ar data reported by Samson and n equals the number of standards involved. This BTs, R2 = 0.027526 ± 0.000180, the arithmetic Alexander (1987). The astronomically consistent expression and the foregoing identities allow mean and standard deviation of 11 values from age of FCs (Renne et al., 1994) was explicitly not combining data from various intercalibration 8 different irradiations reported in Izett and used in order to avoid circularity. studies, so that all the involved standards need not Obradovitch (1994), or are the actual values de-

We estimated systematic errors in age due to be coirradiated with the samples in question. termined for the FCs-unknown coirradiations. R3, uncertainty in decay constants and the ages of Quadratic error propagation (Pugh and as is needed for the ACs- and BTs-unknown coir- standards for the samples of the present study by Winslow, 1969) produces an expression for the radiations, is variable, depending on the actual using quadratic error propagation (Pugh and uncertainty (variance) in age in the absence of values determined for those standards and un- Winslow, 1969). We do so using a single expres- error correlations between the independent knowns. Decay constants are those recommended sion that includes all variables subject to system- variables: by Steiger and Jaeger (1977) with uncertainties atic error, in order to eliminate error correlations.  2 reported by McDougall and Harrison (1988). 40 39  n Writing the Ar/ Ar age equation and substitut- λ KR∏ The external errors calculated from equation 4 β i ing the K-Ar age equation for the age of a stan- σ2=+1 i=1 σ2 are included in Table 1. It is noteworthy that errors tλ λ 40 40 tuuλ 2tuε dard, the age of which is based on K/ Ar mea-  λεe  introduced by uncertainties in the values of decay   surements, it can be shown that   constants and the age of the primary standard, magnified by several intercalibration steps, are 2 40 39    significantly larger than those due to analytical ()Ar* Ark λ  n =1 u +   tKu11n (1)  KR∏ uncertainties alone. The large magnitudes of these λ 40 39 λ ε   i ()Ar* Ar  1 =  k s  ++Ð i1 σ2 errors illustrate the need to apply equation 4 rou- tu λ λ  nβ tinely, as statistical inference based on compar- λλ+KRC ≡ 40 40  ε i 40 39 where K ( Ar*/ K) of the initial standard, in   i=1 isons between Ar/ Ar dates and those deter-   this case MMhb-1, and the subscripts u and s re- (4) mined by other methods are inaccurate otherwise. fer to values associated with the unknown and  2 The approach presented herein can be easily up- n standard. Using ACs as an example, and allowing   dated to account for new calibration and intercali- ∏R  40 39  i 40 39 for multiple intermediate Ar/ Ar standard in- +i=1 σ2 bration data for Ar/ Ar standards (a spreadsheet tercalibration steps (i.e., MMhb-1 → FCs → ACs  nK is available from the authors upon request). It is λλ+KRC  → unknown), it is easily shown that equation 3 ε i also clear that more precisely determined decay  i=1 can be modified to constants would be fruitful for 40Ar/39Ar dating.  2 n    1λ KR∏ DISCUSSION tK=11nRRR+(2) n i uλ λ 123  + i=1 σ2 ε ∑ Rj   = n j1λλ+ The 25 volcanic horizons dated in this study RKjiε ∏R where  i=1 constrain the timing of most of the volumetrically significant volcanic units from the Alban Hills λ 40 39 tFCs − ()Ar*ArK ()e 1 The validity of this expression relies on the ab- and Monti Sabatini volcanoes. Because the vol- R≡ FCs = 1 40 39 λ sence of any error correlations within and be- canics were collected from 24 sections throughout ()Ar*Ar ()e tMMhb −1 K MMhb tween the Ri, which is true because uncertainties the Tiber River delta, they are listed in Table 1 by in these values are overwhelmingly dominated age. As noted in Table 1, the first and second ma-

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jor eruptive events from the two volcanoes are in- gle-crystal analysis of the Tufo Rosso a Scorie most paleoclimate researchers have relied on Mi- distinguishable in age. From Monti Sabatini, the Nere (lab number 7583) reported here required 2 lankovitch orbital calculations guided by the ge- Tufo Giallo della Via Tiberina (lab numbers 7588 to 3 heating steps to totally fuse each of 11 large omagnetic polarity time scale (GPTS, Cande and and 7589) has measured ages of 557 ± 3 ka and sanidine crystals. Although this approach does not Kent, 1992) to date climate oscillations as pre- 40 δ18 538 ± 3 ka, and the Tufo Rosso a Scorie Nere (lab offer unequivocal success in detecting Arxs, ob- served in foraminifera tests ( O record). Radio- number 7583) erupted at 446 ± 2 ka. From the Al- serving trends from the initial step through the to- isotopic age control has been relaxed due to de- ban Hills, the “pisolitic tuff” (lab number 30188) tal-fusion step via isotope correlation diagrams velopment of the APTS (Shackleton et al., 1990; 40 possibly erupted at 557 ± 14 ka, and the Poz- provides some ability to detect Arxs. An inverse Hilgen, 1991a, 1991b). During the past decade, zolane Rosse (lab number 30178) has a measured isochron plot of our isotopic data does not suggest paleoclimatologists have produced numerous 40 δ18 age of 455 ± 8 ka. Unfortunately, the tephras from the presence of Arxs in the Tufo Rosso a Scorie time scales for O records by tuning them to a the two volcanoes have not been seen in strati- Nere (isochrons can be constructed from data pro- variety of glacial response models and various graphic contact with each other, so the relative vided in the Supplemental Data Table1). The Milankovitch orbital parameters (a detailed sum- ages of these nearly coeval units cannot be veri- weighted mean age obtained in this study for lab mary can be found in Imbrie et al., 1993). Most fied. The coincident timing may suggest that tec- number 7583 (446 ± 2 ka) is just at the 2σ error of these time scales are more or less consistent tonism has played an important role in triggering range when compared to the work by Cioni with each other for middle Pleistocene time, but eruptions from the two volcanic centers. et al. (1993). Of note, however, is a group of sani- small variations do occur. Cioni et al. (1993) pointed out that the Morlupo dine crystal ages of about 430 ka that we have ob- Evaluating the middle Pleistocene portions of flow (pre–Tufo Giallo della Via Tiberina flow tained (lab numbers 8292 and 30175). Sample these various orbitally based time scales is diffi- from the Monti Sabatini center), which is less sig- 8292 is sanidine from a thin pumice fall, and sam- cult; the GPTS does not adequately limit the tim- nificant volumetrically than the three main Monti ple 30175 is sanidine that has been reworked ing of high-frequency glacial events because only Sabatini flows, yielded an age of 587 ± 9 ka, indi- (along with black scoria that is probably from the one long-lasting geomagnetic reversal occurred cating that Monti Sabatini volcanism began ear- Tufo Rosso a Scorie Nere) into gravel at the base during the middle Pleistocene (the Brunhes- lier than Alban Hills volcanism. Ages obtained in of one aggradational section. This suggests that Matuyama reversal dated at 783 ± 23 ka (2σ), this study for several small tephra (lab numbers like the Tufo Giallo della Via Tiberina, there may Baksi et al., 1992). No radioisotopic age con- 8281 [614 ± 3 ka] and 30182 [605 ± 11 ka]) also have been several eruptions of Tufo Rosso a Scorie straint for the interval 125–783 ka (isotopic stage confirm that the Monti Sabatini activity predates Nere, which may account for the difference in ages 5e as determined by U-Th methods, Mesolella the oldest identified flow from the Alban Hills reported by Cioni et al. (1993) and in this paper. et al., 1969), and the Brunhes-Matuyama rever- 40 (pisolitic tuff) by almost 50 k.y. No identifiable Arxs was found in either sal, is used in orbital tuning procedures. Since six 40 40 Cioni et al. (1993) reported excess Ar ( Arxs) sanidine or leucite analyzed from the Alban full glacial cycles are believed to occur during in sanidine analyzed from the Monti Sabatini vol- Hills. Two of the six layers (lab numbers 7593 this time interval, the validity of using Milan- canic province. Cioni et al. conducted step-heat- and 30188) contained xenocrysts. Villa (1988, kovitch orbital parameters to date these glacial ing analyses on sanidine separates from the 1992), however, reported varying degrees of cycles must be rigorously tested and verified by 40 Morlupo Trachyte, Tufo Giallo della Via Tiberina, Arxs in leucite from the Alban Hills. Villa radioisotopic age constraint on each glacial cycle. 40 Tufo Rosso a Scorie Nere, and an ignimbrite (1992) did not identify Arxs in the Tufi di Villa Only through this process can the circularity (Peperini Listati) from the Tufi Stratificati Vari- Senni, and reported a step-heating plateau age of inherent in development of orbital time scales colori di Sacrofano. The authors reported dis- 355 ± 1 ka (2σ analytical uncertainty) on leucite be reduced. turbed, saddle-shaped spectra for four analyses of from that tephra. The age reported in this paper Our construction of a radioisotopically based the Tufo Giallo della Via Tiberina; integrated ages for the Tufo Lionato (353 ± 4 ka), measured on sea-level history for a portion of the middle Pleis- (including all heating steps) ranged from 687 ka leucite, supports the interpretation that the Tufi tocene is made from the Tyrrhenian Sea coast, to 3542 ka. Cioni et al. did not mention whether di Villa Senni and Tufo Lionato are different where sedimentation in the Tiber River delta is step-heating experiments were conducted on sin- phases of the same eruption, and verifies that no evaluated by comparing older sedimentary cycles 40 gle- or multiple-crystal separates, but the sample Arxs exists in the Tufo Lionato. The strati- to that from the most recent glacial cycle (see weights mentioned (46, 157, 156, and 163 mg) graphically and isotopically consistent ages re- Karner and Marra, 1998, this issue, for a detailed suggest that multiple crystals were used. ported from the Tufo Lionato, as well as the description of the stratigraphy). Throughout the The single-crystal total-fusion ages on sani- other tephra that we have dated from the Alban Tiber River delta, fining-upward sections ranging 40 dine reported here for the Tufo Giallo della Via Hills, suggests that Arxs is less problematic in from fluvial gravel through lacustrine, brackish, Tiberina, conducted on crystals from the basal the larger eruptions from this volcano. or marine clay are found filling incised river val- flow unit (lab number 7588) and the thick, main leys. As in the Mississippi River valley and delta flow (lab number 7589) show no indication of Timing of Sea-Level Oscillations (Fisk, 1944), sedimentation in the Tiber River and 40 Arxs and have produced stratigraphically rea- delta is believed to be a first-order response to eu- sonable ages of 557 ± 3 ka and 538 ± 3 ka for the The difficulty of finding volcanic horizons in- static sea-level rise produced by glacial melting basal and main flow units, respectively. These terfingered with climate proxy records has led (Karner and Marra, 1998, this issue). In addition, ages are consistent with the youngest step ages many researchers to find new approaches to date we interpret the transition from erosion to aggra- obtained by Cioni et al. (1993), suggesting that Pleistocene climate events. Since the early 1980s, dation to occur at, or shortly following, glacial 40 xenocrystic contamination rather than Arxs is terminations (time of maximum glacial melting). the cause of their disturbed age spectra. This inference is made because Holocene aggra- Cioni et al. (1993) also reported a slightly sad- dation in coastal areas receiving high sediment 1 dle-shaped step-heating spectrum for sanidine GSA Data Repository item 9845, supplemental fluxes occurred rapidly, followed by delta progra- 40Ar/39Ar data tables, is available on request from Doc- from the Tufo Rosso a Scorie Nere, for which they uments Secretary, GSA, P.O. Box 9140, Boulder, CO dation. In the Tiber River valley, this occurred in reported an average age of 433 ± 12 ka. The sin- 80301. E-mail: [email protected]. approximately 12 k.y. (Bellotti et al., 1989).

744 Geological Society of America Bulletin, June 1998

A Ponte Galeria A' Marine B Rome B' 70 (Fosso Galeria Valley) Terraces (Tiber River Valley)

± 60 474 5 ± 251±8 605 11 485±3

50 17 ± alley 614 3 15 427±5 404±7 no age 13 7 283±2 constraint 40 266±5 11 9 13 11 9 7 alley 500±6 7 PGf 749±14 526±5 30 753±8 353±4 ± 413±11 ± 455 8 434±8 no age 557 14 5 19constraint 457±8 Fosso Galeria V 20 15 er River V ± b no age 517±15 353 4 i T constraint 538±2 10 Pliocene-Pleistocene Substrate 446±2 Meters above sea level 548±4 459±7 Volcanic horizon 557±3 0

Lacustrine or marine clay Fluvial or marine sand Holocene Pliocene-Pleistocene Substrate

-10 Holocene

-20 Eolian sand Fluvial or marine gravel

Figure 2. Generalized sections with elevations in meters above sea level for near-coastal (A–A′) and fluvial (B–B′) clastic aggradational sections in the Tiber River valley and delta. Horizontal dimension is not to scale. Relative stratigraphic positions and ages of tephra (±2σ analytical) are used to determine the time span of erosion and aggradation. The transition from erosion to aggradation is interpreted to occur during, or shortly after, a glacial termination. Numbers in bold refer to corresponding δ18O isotopic stages for interglacial periods (positive δ18O stages occur during erosional events and are not shown). Marine terrace elevation data is from Hearty and Dai-Pra (1986) and Sorgi (1994) and is discussed in Karner and Marra (1998, this issue).

We use ages of volcanic horizons deposited positions and approximate elevations of the 25 Figure 3 represents the obliquity clock (Berger et within these fining-upward sections to date the volcanic horizons used to determine the time al. state a 5 k.y. uncertainty for the Site 806 time initiation of interglacial periods. Similarly, vol- span of these glacial and interglacial periods. It is scale). The fact that an approximate 100 k.y. cycle canics that are reworked into the basal gravel, or stressed that the lines of section in Figure 1 are shows up in this core after tuning it to the 41 k.y. are found filling the bottoms of the incised val- only meant to show the general locations of sam- cycle indicates that the 100 k.y. cycle is not an leys, are interpreted to have been deposited during pling (either from within Rome or in the coastal artifact of tuning the data to eccentricity or preglacial periods (although the volcanics reworked sections near the mouth of the Tiber River), as cession (which is modulated by eccentricity). into the gravel could be from older interglacial or sampling was done throughout the region. Exact Also shown in Figure 3 is the time history of glacial intervals). In all cases, the volcanics re- sampling locations are listed by latitude and the Tiber River delta developed from the 25 vol- worked into the basal gravel have been traced longitude in Table 1. canic horizons dated in this study, shown with an- back to their original sources (and their strati- Deciding which orbitally tuned deep-sea alytical (Fig. 3A) and full external (Fig. 3B) un- graphic relationship determined from there) or record provides the best comparison for the certainties. Since calibration of glacial cycle have been determined to postdate the aggrada- 40Ar/39Ar dated Tiber River delta sea-level history timing rather than amplitude is the focus of this tional sections from the previous interglacial epi- is challenging. In light of recent questions about work, the tephra age data in Figure 3 are posi- sode (Karner and Marra, 1998, this issue). the nature of the 100 k.y. cycle (Farley and Patter- tioned based on their relative sea-level positions, We limit our interpretation of the stratigra- son, 1995; orbital inclination in Muller and Mac- and amplitudes are arbitrarily set to provide easy phy with this simple binary (interglacial or Donald, 1995, 1997, which produces a single 100 comparison to the δ18O record. glacial) method in order to account for local k.y. spectral peak, versus eccentricity, which For the tephra ages, the time axis of Figure 3 variations in sedimentation rate. The volcanics should produce 95, 125, and 400 k.y. spectral represents the radioisotopic decay clock for 40K. were collected from 24 different locations and peaks), we rely on a deep-sea record tuned only to The goodness of fit of the two independent time so their exact stratigraphic relationship to each the Earth’s obliquity (41 k.y.) cycle, because the records can be used to check their reliability for other can not be determined, but their relative effects of the obliquity cycle remain uncontested providing accurate ages for glacial events. Sub- positions within aggradation or eroding sec- (the Rome section is compared to several other stantial disagreement could call either or both of tions are clear. With this limitation, we simply time scales in Karner and Marra, 1998, this issue). the time scales into question. group volcanics that occur in common aggrad- Figure 3 shows the δ18O record of Ocean Drill- Comparing the 40Ar/39Ar ages to the δ18O ing or eroding sections in order to provide a ing Program (ODP) Site 806 recovered from the record suggests that the Tyrrhenian Sea under- general time range for those interpreted glacial Ontong-Java Plateau (Berger et al., 1992). This went six sea-level oscillations during the time and interglacial intervals. δ18O record has been tuned only to the obliquity recorded by the volcanic horizons. Single-crystal Figure 2 is a summary diagram of the aggra- cycle, rather than to a full Milankovitch template 40Ar/39Ar ages have bracketed the timing of some dational sections found near the coast (A–A′, that includes eccentricity, obliquity, and preces- of these events, showing that sea-level change is Fig. 1) and in the Tiber River valley in Rome sion terms (as shown in Berger et al., 1995). dominated by a quasi–100 k.y. cycle. The similar- (B–B′, Fig. 1), showing the relative stratigraphic Therefore, for the δ18O record, the time axis of ity of sea-level history recorded in the delta sedi-

Geological Society of America Bulletin, June 1998 745

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Isotopic stages: (283 ± 2 ka) and 30176 (266 ± 5 ka); a predicted A 7 9 11 13 15 17 19 age of 278 (261, 285) ka (95% confidence interval [CI]) is based on analytical errors. Using full ex- -2 ? ternal errors, the predicted age of termination III is

Interglacial 276 (258, 289) ka (95% CI). Termination IV has

Term. VI Term. Term. III Term. Term. IV Term. Term. V Term. only one-sided age constraint (lab number 7593,

353 ± 4 ka), which indicates that the termination

) οο

/ occurred some time after emplacement of this ig- ο

nimbrite. Termination V is best constrained by lab Ο ( Ο

18 numbers 30175 (434 ± 8 ka) and 8292 (427 ± 5

δ -1 ka); a predicted termination age of 430 (422, 442) ka (95% CI) is based on analytical errors, and an age of 430 (416, 448) ka (95% CI) is based on full external errors. Termination VI is most precisely

Glacial bracketed by lab numbers 8285 (526 ± 6 ka) and 7589 (538 ± 3 ka); a predicted termination age of 0 534 (520, 541) ka (95% CI) is based on analytical 200 300 400 500 600 700 800 errors, and an age of 533 (512, 548) ka (95% CI) is Age (ka) based on full external errors. Termination VII predates emplacement of lab numbers 8281 and 30182, suggesting an approximate age of 620 ka B Isotopic stages: for that termination. Older terminations do not 7 9 11 13 15 17 19 have good age constraints. Lab numbers 7576 and ? δ18 -2 30189 occur during a glacial stage ( O stage 18), but additional control is needed to reliably con-

Interglacial strain the age of these older terminations.

Term. V Term. Term. VI Term. Term. III Term. Term. IV Term. Comparison of these predicted glacial termina-

) tions with those from Site 806 indicates that there οο

/ is relatively good agreement between the two time ο

scales. Termination V is very clear in the Site 806

Ο ( Ο 18

δ -1 record, and has an approximate age of 430 ka, which matches well the predicted age of 430 ka from the Tiber River delta section. However, the positions of terminations III and VI are not particularly clear for Site 806, because both terminations δ18 Glacial have a two-step rise in negative O. The uncer- tain termination positions in the deep-sea record 0 200 300 400 500 600 700 800 provide an ideal opportunity to use the Tiber River delta section to determine which step is manifested Age (ka) in the coastal areas as a sea-level rise. The two possible ages for termination III in the 40 39 Figure 3. (A) Sea-level interpretation and Ar/ Ar ages with 2σ (analytical) errors from Ro- Site 806 record are ca. 260 and 225 ka. Using an- 40 39 man volcanics. Positions of Ar/ Ar ages are simplified to a sea-level lowstand (glacial) and alytical and full error estimations, only the older highstand (interglacial) interpretation. Open squares are the predicted ages of glacial termina- step is consistent with the 95% confidence inter- tions; 1σ and 2σ limits are determined from analytical errors of bracketing tephra ages. Heavy val of the Tiber River delta section, albeit at the dashed lines approximate timing of glacial terminations. Shaded bars are the 2σ confidence in- extreme limit of this interval (CIs 261–285 and tervals for the terminations. Site 806 δ18O record and time scale are from Berger et al. (1992). 258–289 ka, respectively). (B) The same diagram as A, but using full 2σ external errors of 40Ar/39Ar ages. The confidence Similarly, termination VI has a two-step termi- intervals for the timing of glacial terminations expand significantly when full external errors are nation that occurs at about 535 and 505 ka in Site considered, overlapping termination III in the Site 806 record. 806. Using analytical or full external uncertainties (CIs 520–541 ka and 512–549 ka, respectively), ments and the ODP Site 806 δ18O record supports nature), they can be treated statistically by the only the older step is consistent with the 95% con- the interpretation that erosion and aggradation in method used to estimate the age of geomagnetic fidence intervals of the radioisotopically predicted the delta resulted from eustatic fluctuations rather reversals (Cox and Dalrymple, 1967), using the termination age from the Tiber River delta. This than local (tectonic) effects, and hence can pro- most precise ages of bracketing volcanic layers in suggests an age of ca. 535 ka for termination VI. vide radioisotopic constraints for this portion of order to obtain predicted ages for the glacial term- the middle Pleistocene. inations. This technique has shown to be adaptable CONCLUSIONS Because the stratigraphic interpretations of the to a variety of geologic events (e.g., Harland et al., tephra shown in Figure 3 are limited to preglacial 1982; Hall and Farrell, 1995). With this approach, The highly potassic nature of tephra from the and postglacial termination status (i.e., binary in termination III is bracketed by lab numbers 7595 Roman volcanic province has allowed us to use

746 Geological Society of America Bulletin, June 1998

40 39 Bellotti, P., Carboni, M. G., Milli, S., Tortora, P., and Valeri, P., Matuyama-Brunhes geomagnetic boundary: Journal of the Ar/ Ar method to date with high precision 1989, La piana deltizia del Tevere: Analisi di facies ed Geophysical Research, v. 99B, p. 2925–2934. most of the major eruptions from the Alban Hills ipotesi evolutiva dall’ultimo low stand glaciale all’attuale: Karner, D. B., and Marra, F., 1998, Correlation of fluviodeltaic and Monti Sabatini volcanoes. Coeval ages from Giornale di Geologia, v. 51, no. 1, p. 71–91. aggradational sections with glacial climate through his- Berger, W. H., Bickert, T., Schmidt, H., and Wefer, T., 1992, tory: A revision of the Pleistocene stratigraphy of Rome: tephra from the two centers suggest that tectonism Quaternary oxygen isotope record of pelagic foraminifers; Geological Society of America, v. 110, p. 748–758. may have controlled some of the major eruptions, Site 806, Ontong Java Plateau: Proceedings of the Ocean Lo Bello, P., Feraud, G., Hall, C. M., York, D., Lavina, P., and and our data suggest that no identifiable 40Ar ex- Drilling Program, Scientific Results,Volume 130: College Bernat, M., 1987, 40Ar/39Ar step-heating and laser fusion xs Station, Texas, Ocean Drilling Program, p. 381–395. dating of a Quaternary pumice from Neschers, Massif ists in any of the tephra dated in this study. Berger, W. H., Bickert, T., Wefer, G., and Yasuda, M. I., 1995, Central, France: The defeat of xenocrystic contamination: 40Ar/39Ar ages from tephra intercalated with Brunhes-Matuyama boundary; 790 k.y. date consistent Chemical Geology, Isotope Geoscience Section, v. 66, with ODP Leg 130 oxygen isotope records based on fit to p. 61–71. sediments from the Tiber River valley and delta Milankovitch template: Geophysical Research Letters, Mattias, P. P., and Ventriglia, U., 1970, La regione vulcanica dei show that transitions from erosion to aggradation, v. 22, p. 1525–1528. Monti Sabatini e Cimini: Memorie della Società Geolog- presumably caused by glacial terminations, oc- Cande, S. C., and Kent, D. V., 1992, A new geomagnetic polar- ica Italiana, v. 9, p. 331–384. ity time scale for the Late Cretaceous and Cenozoic: Jour- McDougall, I., and Harrison, T. M., 1988, Geochronology and curred at ca. 535, 430, and 280 ka. The relative nal of Geophysical Research, v. 97B, p. 13917–13951. thermochronology by the 40Ar/39Ar method: Oxford sea-level history of the Tiber River delta is gener- Cioni, R., Laurenzi, M. A., Sbrana, A., and Villa, I. M., 1993, Monographs on Geology and Geophysics, Volume 9: Ox- ally consistent with the deep-sea δ18O record at 40Ar/39Ar chronostratigraphy of the initial activity in the ford, United Kingdom, Oxford University Press, 212 p. Monti Sabatini volcanic complex (Italy): Bollettino della Mesolella, K. J., Matthews, R. K., Broecker, W. S., and Thurber, Site 806 tuned to the obliquity cycle, although Società Geologica Italiana, v. 112, p. 251–263. D. L., 1969, The astronomical theory of climate change: full external errors are required for the timing of Conato, V., Esu, D., Malatesta, A., and Zarlenga, F., 1980, New Barbados data: Journal of Geology, v. 77, p. 250–274. data on the Pleistocene of Rome: Quaternaria, v. 22, Muller, R. A., and MacDonald, G. 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R., 1995, Excess 40Ar in biotite and hornblende from cially helpful when the δ18O curve shows com- of the Olorgesailie Formation, southern Kenya Rift: Jour- the Noril’sk 1 Intrusion, Siberia; implications for the age nal of Geophysical Research, v. 95B, p. 8453–8470. of the Siberian Traps: Earth and Planetary Science Let- plex (multistep) terminations. De Rita, D., and Sposato, A., 1986, Correlazioni tra eventi es- ters, v. 131, p. 165–176. As similar studies are conducted on coastal plosivi e assetto strutturale del substrato sedimentario del Renne, P. R., Deino, A. L., Walter, R. C., Turrin, B. D., Swisher, complesso vulcanico Sabatino: Memorie della Società C. C., III, Becker, T. A., Curtis, G. H., Sharp, W. D., and sections in other regions, a database of high-pre- Geologica Italiana, v. 35, p. 727–733. Jaouni, A.-R., 1994, Intercalibration of astronomical and cision measurements can be constructed to fur- De Rita, D., Di Filippo, M., and Sposato, A., eds., 1989, Carta radioisotopic time: Geology, v. 22, p. 783–786. ther refine the timing of Pleistocene eustasy. With geologica del complesso vulcanico Sabatino: Rome, Renne, P. R., Deckart, K., Ernesto, M., Féraud, G., and Piccir- Italy, Consiglio Nazionale delle Ricerche, 1 sheet, scale illo, E. M., 1996, Age of the Ponta Grossa Dike Swarm this high-resolution age control on relative sea 1:50 000. (Brazil) and implications for Paraná flood volcanism: level, the 40K and orbital chronometers can truly De Rita, D., Funiciello, R., and Rosa, C., 1992, Volcanic activ- Earth and Planetary Science Letters, v. 144, p. 199–211. be tested for high-precision agreement. ity and drainage network evolution of the Alban Hills area Samson, S. D., and Alexander, E. C., Jr., 1987, Calibration of (Rome, Italy): Acta Vulcanologica, Marinelli Volume, the interlaboratory 40Ar–39Ar dating standard, MMhb-1: no. 2, p. 185–198. Chemical Geology, Isotope Geoscience Section, v. 66, ACKNOWLEDGMENTS Dragone, F., Malatesta, A., and Segre, A. G., 1963, Cerveteri: p. 27–34. Carta Geologica D’Italia, Foglio 149, 1 sheet, scale Shackleton, N. J., Berger, A., and Peltier, W. R., 1990, An alter- 1:25 000. native astronomical calibration of the lower Pleistocene This work was supported by California Space Farley, K. A., and Patterson, D. B., 1995, A 100-k.y. periodicity time scale based on ODP Site 677: Royal Society of Ed- Institute (CalSpace) Minigrant CS-93-21, Na- in the flux of extraterrestrial 3He to the sea floor: Nature, inburgh Transactions, Earth Sciences, v. 81, p. 251–261. v. 378, p. 600–603. Sorgi, C., 1994, La successione morfo-litostratigrafica in destra tional Aeronautics and Space Administration Fisk, H. N., 1944, Geological investigation of the alluvial valley Tevere dell’ambito dell’evoluzione geologica quaternaria Global Change Fellowship #NGT-30191, Na- of the lower Mississippi River: Vicksburg, Mississippi dell’area romana [Laurea thesis]: Rome, Italy, University tional Science Foundation grant EAR-9405347, River Commission, 78 p. of Rome “La Sapienza.” Fornaseri, M., 1985, Geochronology of volcanic rocks from Steiger, R. H., and Jaeger, E., 1977, Subcommission on and by the Division of Environmental Sciences, Latium (Italy): Rendiconti della Societa Italiana di Min- geochronology: Convention on the use of decay constants U.S. Department of Energy, under contract DE- eralogia e Petrologia, v. 40, p. 73–106. in geo- and cosmochronology: Earth and Planetary Sci- Hall, C. M., and Farrell, J. W., 1995, Laser 40Ar/ 39Ar ages of ence Letters, v. 36, p. 359–362. AC03-76SF00098. 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