Reply: Tephrochronological dating of varved interglacial lake deposits from Piànico-Sèllere (Southern Alps, Italy) to around 400 ka (original) (raw)
We present the first integrated tephrochronological study (major and trace elemental glass composition, Sr and Nd isotope analyses, and 40 Ar/ 39 Ar dating) for the last one tenth (~82 m) of the ~900 m-thick Quaternary lacustrine succession of the Fucino Basin, the largest and probably only Central Apennine intermountain tectonic depression that hosts a continuous lacustrine succession documenting the Plio-Quaternary sedimentary history up to historical times. Major element glass compositions, determined using a wavelength-dispersive electron microprobe (WDS-EMPA), yielded the geochemical fingerprinting needed for a reliable identification of most of the 23 stratigraphically ordered tephra layers under investigation. These include tephra from Italian volcanoes such as Campi Flegrei, Etna, Colli Albani, Ischia, Vico, Sabatini, and undefined volcanic sources in the Neapolitan area and Latium region. The recognition of key Mediterranean marker tephra layers (e.g. X-5 and X-6) is supported by trace element data acquired by Laser Ablation Inductively Couple Plasma Mass Spectrometry (LA-ICP-MS). The Sr and Nd isotope compositions of selected layers where also determined for circumscribing the volcanic source of distal tephra and for supporting correlations with individual eruptive units. We also propose a new, more expeditious covariation diagram (CaO/FeO tot vs Cl) for identifying the volcanic source of trachytic to phonolitic and tephrytic to phonolitic tephra, that are the most common compositions of pyroclastic rocks from volcanoes of Campania and Latium regions. Finally, we present five new 40Ar/39Ar age de-terminations, including a new, analytically well-supported, and more precise 40Ar/39Ar age for the widespread Y-7 tephra, and the first 40Ar/39Ar age determinations for one tephra from the Sabatini volcanic district (~126 ka) and one tephra from Neapolitan volcanic area (Campi Flegrei?; ~159 ka). These newly dated tephra are widely dispersed (e.g. Monticchio, southern Italy, Adriatic Sea and Lake Ohrid, Macedonia-Abania) and have thus the potential to become important Mediterranean MIS 5 and MIS 6 tephrochronological markers. Altogether the new geochemical data and 40Ar/39Ar ages precisely constrain the chronology of the investigated Fucino succession spanning the last ∼190 ka. In light of these results and by considering that this sedimentary succession possibly extends back to ∼2 Ma, Fucino is likely to provide a very long, continuous tephrostratigraphic record for the Mediterranean area and become a key node in the dense network of tephra correlations of this region.
2015
Tephrochronology is a unique method for linking and dating geological, palaeoecological, palaeoclimatic, or archaeological sequences or events. The method relies firstly on stratigraphy and the law of superposition, which apply in any study that connects or correlates deposits from one place to another. Secondly, it relies on characterising and hence identifying or 'fingerprinting' tephra layers using either physical properties evident in the field or those obtained from laboratory analysis, including mineralogical examination by optical microscopy or geochemical analysis of glass shards or crystals (e.g., Fe-Ti oxides, ferromagnesian minerals) using the electron microprobe and other tools. Thirdly, the method is enhanced when a numerical age is obtained for a tephra layer by (1) radiometric methods such as radiocarbon, fission-track, U-series, (U-Th)/He, or Ar/Ar dating, (2) incremental dating methods including dendrochronology or varved sediments or layering in ice cores, or (3) age-equivalent methods such as palaeomagnetism or correlation with marine oxygen isotope stages or palynostratigraphy. Once known, that age can be transferred from one site to the next using stratigraphic methods and by matching compositional characteristics, i.e., comparing 'fingerprints' from each layer. Used this way, tephrochronology is an age-equivalent dating method. Even if a tephra layer is undated, or if it is dated imprecisely, it nevertheless provides an isochron or time-plane that allows the sequence in which it is found to be correlated with other sequences where it occurs. Herein lies the unique power of tephrochronology: deposits and their associated palaeoarchival evidence are thus able to be connected and synchronized positioned precisely on a common time scale using the tephra layer as a stratigraphically fixed tie-point, even where the tephra is poorly or undated. In this situation, the age scale is best envisaged as a length of elastic that can be stretched or contracted when numerical ages are obtained, or age precision is improved, whilst the tephra's stratigraphic juxtaposition with respect to the enclosing deposits and associated archival data remains fixed on the 'elastic'. When the tephra age is known, however, that age can be applied directly to the sequence where the tephra has been newly identified. This is because tephra layers are erupted over very short time periods (volcanic eruptions typically last for only hours or days to perhaps weeks or a few months or so at most), and thus each represent an instant in time, geologically speaking (Lowe, 2011). A tephra layer from a powerful eruption can be spread widely over land, sea and ice, hence forming a thin blanket that has exactly the same age wherever it occurs (unless it has been reworked). For example, the Icelandic Fugloyarbanki tephra, identified in the NGRIP ice core from Greenland, has been dated at 26,740 ± 390 (1) calendar (cal.) years before AD 2000 on the basis of multi-parameter counting of annual layers in NGRIP (Davies et al., 2008). It forms a widespread marker horizon or isochron in marine deposits in the North Atlantic and on the distant Faroe Islands between Iceland and Scotland. Thus palaeoarchives at these widely separated localities are now able to be connected precisely. Moreover, the extent of the radiocarbon marine reservoir effect in this region at the time can be examined using the Fugloyarbanki tephra as an independent time-plane.
2016
Tephrochronology is a unique method for linking and dating geological, palaeoecological, palaeoclimatic, or archaeological sequences or events. The method relies firstly on stratigraphy and the law of superposition, which apply in any study that connects or correlates deposits from one place to another. Secondly, it relies on characterising and hence identifying or 'fingerprinting' tephra layers using either physical properties evident in the field or those obtained from laboratory analysis, including mineralogical examination by optical microscopy or geochemical analysis of glass shards or crystals (e.g., Fe-Ti oxides, ferromagnesian minerals) using the electron microprobe and other tools. Thirdly, the method is enhanced when a numerical age is obtained for a tephra layer by (1) radiometric methods such as radiocarbon, fission-track, U-series, (U-Th)/He, or Ar/Ar dating, (2) incremental dating methods including dendrochronology or varved sediments or layering in ice cores, or (3) age-equivalent methods such as palaeomagnetism or correlation with marine oxygen isotope stages or palynostratigraphy. Once known, that age can be transferred from one site to the next using stratigraphic methods and by matching compositional characteristics, i.e., comparing 'fingerprints' from each layer. Used this way, tephrochronology is an age-equivalent dating method. Even if a tephra layer is undated, or if it is dated imprecisely, it nevertheless provides an isochron or time-plane (sometimes referred to as a 'time-parallel' marker bed) that allows the sequence in which it is found to be correlated with other sequences where it occurs. Herein lies the unique power of tephrochronology: deposits and their associated palaeoarchival evidence are thus able to be connected and synchronized positioned precisely on a common time scale using the tephra layer as a stratigraphically fixed tie-point, even where the tephra is poorly or undated. In this situation, the age scale is best envisaged as a length of elastic that can be stretched or contracted when numerical ages are obtained, or age precision is improved, whilst the tephra's stratigraphic juxtaposition with respect to the enclosing deposits and associated archival data remains fixed on the 'elastic'. When the tephra age is known, however, that age can be applied directly to the sequence where the tephra has been newly identified. This is because tephra layers are erupted over very short time periods (volcanic eruptions typically last for only hours or days to perhaps weeks or a few months or so at most), and thus each represent an instant in time, geologically speaking (Lowe, 2011). A tephra layer from a powerful eruption can be spread widely over land, sea and ice, hence forming a thin blanket that has exactly the same age wherever it occurs (unless it has been reworked). For example, the Icelandic Fugloyarbanki tephra, identified in the NGRIP ice core from Greenland, has been dated at 26,740 ± 390 (1) calendar (cal.) years before AD 2000 on the basis of multi-parameter counting of annual layers in NGRIP (Davies et al., 2008). It forms a widespread marker horizon or isochron in marine deposits in the North Atlantic and on the distant Faroe Islands between Iceland and Scotland. Thus palaeoarchives at these widely separated localities are now able to be connected precisely. Moreover, the extent of the radiocarbon marine reservoir effect in this region at the time can be examined using the Fugloyarbanki tephra as an independent time-plane.
Quaternary Geochronology, 2021
Distal tephra from the major Somma-Vesuvius Avellino (AV) eruption is widespread in the coastal basins of Southern Lazio (Central Italy). Dated to 1995 ± 10 cal yr BC in 2011, later on doubts arose about the reliability of this frequently cited age. This led to a major effort to date AV tephra holding sections, based on a thorough methodological approach. Various aspects were studied to identify sections yielding reliable 14 C ages, including bioturbation, inbuilt age, and variable sediment accumulation rate. Lowered rates upon deposition of tephra, particularly in anoxic marshy environments and attributed to toxic F contents, showed up as sharp increases in pollen density. The 'sampling error' was quantified for specific sedimentary environments and derived from coring data and published data on accumulation rates for similar Central Mediterranean sites. Next, two Bayesian analyses were performed, a traditional using the full set of samples and a novel, based on samples that were deemed as suitable (no bioturbation, inbuilt age, etc.) and of which the age was corrected for the sampling error. The age obtained by the novel analysis had the smallest range (1909-1868 cal yr BC), differs about a century, and is virtually identical to the ages published by Passariello et al. (2009) and Alessandri (2019). The earlier found age (2011) is ascribed to a statistical coincidence. The results solve a long debate on the age of the AV eruption, which is the youngest of the three major eruptions in the Central Mediterranean Bronze Age. Ages of the other two, the Agnano Mt Spina (Phlegrean) and FL eruption (Etna), are still uncertain and disputed. This study illustrates the need for a thorough approach in 14 C dating tephra holding sediment archives in the Central Mediterranean, and employed a methodology that can be applied in such approach. Attention is called for potentially toxic fluorine concentrations in Campanian tephra, which may have had a serious impact on the contemporary environment and induced chronological hiatuses, but hitherto were not reported for the early tephra.
Quaternary Science Reviews, 2013
We describe the diagnostic lithological and chemical features of distal tephras from major Middle-Late Pleistocene (560e36 ka) explosive eruptions of the Colli Albani volcanic district, central Italy. In particular, we explore the time-dependent variability of the Sr and Nd isotope compositions as a tool for recognising and pinpointing individual Colli Albani tephra in distal settings. The distal tephras investigated are in lacustrine and fluvial sediments of central Apennine intermountain basins located 70 kme100 km east of Colli Albani. The recognition of the Colli Albani tephras is essentially based on the K-foiditic composition of their glass, which, within the Italian volcanological framework, is a distinctive character of the Colli Albani pyroclasts. In detail, these tephras are attributed to the following eruptive units: Tufo Pisolitico di Trigoria (561 AE 2 ka); Tufo del PalatinoeTufo di Bagni Albule (530 AE 2/527 AE 2 ka), Tufo di Bagni AlbuleePozzolane Rosse air-fall sequence (517 AE 1 to 500 AE 3 ka), Pozzolane Rosse (457 AE 4 ka), Villa Senni (365 AE 4 ka), and Albano 5e7 (41 AE 7 to 36 AE 1 ka). These correlations are supported by 40 Ar/ 39 Ar dating of the distal tephras correlated to the Pozzolane Rosse (457.4 AE 1.7 ka), Villa Senni (365 AE 2 ka) and Albano 5e7 (41 AE 9 ka) and by 87 Sr/ 86 Sr measured on clinopyroxene crystals and fresh glassy scoria from distal Colli Albani tephras that range from w0.711 to w0.709. These ratios are similar to those that characterise the individual proximal correlative units, and show the same decreasing trend over time. In contrast, the 143 Nd/ 144 Nd ratios for proximal and distal bulk samples and clinopyroxene increase from w0.51212 to w0.51215 from the oldest to the youngest tephra deposit. In summary, the study of Sr and Nd isotope compositions that is here applied on products from the Colli Albani volcanic district is a powerful, complementary tool to the more traditional tephrostratigraphic methods (e.g., componentry and electron microprobe analysis) for fingerprinting of distal tephras over a large region of the central Mediterranean, and over a large time interval, such as from 560 ka to 36 ka.