Multimillennial synchronization of low and polar latitude ice cores by matching a time constrained Alpine record with an accurate Arctic chronology
Abstract. We present a novel application of empirical methodologies that significantly reduce the chronological uncertainty of a low latitude-high altitude Alpine ice core record obtained in 2011 from the glacier Alto dell’Ortles (3859 m, Eastern Alps, Italy). A preliminary absolute timescale based on a peak in 3H activity, and 210Pb and 14C analyses on carbonaceous particles and organic remains provided evidence of one of the oldest Alpine ice core records spanning the last ~7000 years, back to the last Northern Hemisphere Climatic Optimum. Here we combine three empirical methods that provide an additional number of time markers that corroborate the multimillennial nature of the Alto dell’Ortles ice cores while significantly decreasing the uncertainty of the chronology. First, 14C analysis of an additional organic fragment (a charred spruce needle) discovered next to the basal ice provides an age (232 ± 126 BCE) which agrees with previous 14C dates in the oldest part of the record. Second, a new millennial-scale high resolution atmospheric Pb depositional record was used to synchronize the Alto dell’Ortles cores with an accurately-dated (±5 years) Pb record from an array of Arctic ice cores during the ~200 BCE to ~1900 CE time period. This match resulted in a shift of the initial Alto dell’Ortles timescale ~200 years earlier, but still within the initial time uncertainty. Third, novel seasonally resolved pollen records from the upper firn/ice portion of the Alto dell’Ortles cores were combined with δ18O and dust annual variations to refine the dating for the 20th century by means of an automatic algorithm (Straticounter; between 2011 and 1927 CE) and visual counting (1926–1900 CE). The time markers obtained by these three methods were combined in a continuous timescale by running a Montecarlo based fit (COPRA model). This Alto dell’Ortles revised chronology shows a significantly reduced uncertainty, between ±1 and ±4 years after 1927 CE, and between a maximum of ±100 years to a minimum of ±5 years between 1927 CE and 200 BCE by conservative estimates. An investigation of the revised chronology by means of a simple 1-D flow model suggests that non-steady-state conditions (e.g., changes in past snow accumulation rate) need to be considered to provide a full physical explanation of the age-depth relationship obtained. The new revised chronology will allow the constraint of the Holocene climatic and environmental histories emerging from this high-altitude glacial archive of Central Europe. The novel methodologies may also be adopted to build or improve the chronologies of other ice cores extracted from-low latitude/high-altitude glaciers that typically suffer from larger dating uncertainties compared with well dated polar records.
Paolo Gabrielli et al.
Paolo Gabrielli et al.
Paolo Gabrielli et al.
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Development of well-dated historical records of human impacts and climate are important for a broad range of disciplines, including the natural, physical, and social sciences, as well as the humanities. Polar and alpine ice cores record provide direct, often highly resolved records of past atmospheric and precipitation chemistry that reflect both natural and anthropogenic emissions. The caveat is that they are most useful only if the records can be properly dated.
This manuscript describes the use of standard and non-standard dating techniques to develop a new chronology for the Alto dell’Ortles cores from the Italian Alps. As is typical of cores from relatively thin alpine glaciers, annual layer counting is used in the upper section with constraints provided by known time horizons (e.g., fallout from 1950s and 1960s atmospheric thermonuclear testing, as well as fallout from volcanic eruptions or other highly unusual but well-dated events such as major forest fires or Saharan dust events). In deeper sections of such cores, annual layer counting generally is not possible because of extreme flow thinning and strain so other techniques are needed for establishing the ice age. Here the authors used measurements of lead concentrations in two of the four (or maybe three) Alto dell’Ortles cores collected in 2011 to synchronize the deeper sections to a well-dated, previously published lead record from the Russian Arctic. Additional constraints were provided by radiocarbon dates of both water insoluble organic carbon and discrete organic material from the basal section of one of the cores. The lead synchronization technique used here is relatively new and this is the first effort that I’m aware of to use annual layer counting based on seasonal pollen variations and extreme pollen events as specific time markers. While I support publication and am especially enthusiastic about the better-dated records of climate and human impacts in Europe that will result from the improved chronology, there are number of important issues that need to be resolved first.
(1) In the abstract, the authors describe as “novel” the approaches used to redate the cores. The most significant relatively new part of their approach is lead-based synchronization to a well-dated polar ice core which also is the manuscript title. In fact, exactly this lead synchronization approach has been used previously to date rapidly thinning ice cores over the Common Era and beyond. Specifically, Osman et al., (2021) used this technique of annual layer counting in the upper section and lead synchronization in the lower section on a coastal dome core from Greenland. Similarly, Preunkert et al. (2019) used this techniques on a core from the French Alps, including additional constraints from radiocarbon dating of organic material in the deep ice corresponding to antiquity. Therefore, “novel” should be removed from the abstract and these earlier applications at least mentioned and briefly discussed to provide context for the current study. Citations to these earlier publications obviously should be added as well.
Preunkert, S., J.R. McConnell, H. Hoffmann, M. Legrand, A. Wilson, S. Eckhardt, A. Stohl, N. Chellman, M. Arienzo, & R. Friedrich (2019) Lead and antimony in basal ice from Col du Dome (French Alps) dated with radiocarbon: A record of pollution during antiquity, Geophys Res Lett, doi:10.1029/2019GL082641.
Osman, M., B.E. Smith, L.D. Trusel, S.B. Das, J.R. McConnell, N. Chellman, M. Arienzo, & H. Sodemann (2021)Abrupt Common Era hydroclimate shifts drive west Greenland ice cap change, Nature Geoscience, doi:10.1038/s41561-021-00818-w.
(2) The citation for the published Russian Arctic lead record is incorrect. It should be McConnell et al., 2019, not McConnell et al., 2018 (these are different publications and not simply the result of typos in their text).
McConnell, J.R., N.J. Chellman, A.I. Wilson, A. Stohl, M.M. Arienzo, S. Eckhardt, D. Fritzsche, S. Kipfstuhl, T. Opel, P.F. Place, & J.P. Steffensen(2019) Pervasive Arctic lead pollution suggests substantial growth in Medieval silver production modulated by plague, climate and conflict, Proc Natl Acad Sci U.S.A., doi:10.1073/pnas.1904515116.
(3) The use of SZ (the island of Severnaya Zemlya) rather than AN (the ice cap Akademii Nauk which is one of several glaciers/ice caps on SZ) as the ice core name for the Russian Arctic lead record is somewhat confusing. This may be because some earlier publications from the German/Russian team that collected and first analyzed the core referred to it both as SZ (e.g., Fritzsche et al., Annals of Glaciology, 2006) and AN (e.g., Opel et al., Journal of Glaciology, 2009; Opel et al., Climate of the Past, 2012). However, the lead record used here was published as the AN record in McConnell et al., 2019 and so the core name AN should be used here as well.
(4) It appears from Fig. 2 (bottom graph) that there are sometimes very large differences (nearly an order of magnitude for some periods) in the lead concentrations measured in the two Alto dell’Ortles cores. These aren’t just short term differences but decadal or longer differences that I find is quite unexpected in two nearby cores. Please elaborate. Do these differences mean that the lead fluxes measured in these Alpine cores are not regionally or even locally representative? In addition, the tie point at ~69.5 m between the two Alto dell’Ortles lead records is incorrect – or at least not optimal. Correcting it would improve the agreement between the lead records and so make the chronologies more consistent.
(5) The new age scale is quite different from TC2016 even in the upper 100 m where both chronologies presumably are based largely on annual layer counting (albeit with constraints). This seems quite surprising. I understand that the new chronology incorporated pollen records in the upper part of the core but what caused the annual layer counting in the original chronology to be so far off? Presumably TC2016 was based on the same δ18O and dust measurements as in the current study. Please elaborate.
(6) I find the use a logarithmic age axis in the flow modeling sections (Fig. 6 showing annual layer thickness vs age) rather confusing. Why did you use a logarithmic scale? The main point of the manuscript is the redating of the deeper core (below 100 m) so shouldn’t that be emphasized rather than the top 100 m?
(7) In the third paragraph of the introduction, you say that four cores were collected from the Alto dell’Ortles site in 2011. After that, however, I find only a discussion of three cores. Did I miss something?
(8) I don’t find Fig. 1 particularly compelling or informative. Is the point to show that the water isotopes are in better agreement once the new lead-based and other improvements in the tie points between cores are made? If so, it would be much clearer to show this by overplotting the original and improved water isotope records or by using cross plots. Improvements could be quantified by showing how the correlations between different core records have improved either overall or for specific depth/time sections.