Reconstruction of Holocene climate dynamics in the Dolomites from a peat bog core : the first multi-proxy study

Abstract

Paleoclimate and paleoenvironmental studies in the North-Eastern Italian Alps are hampered by the rarity of well-preserved high-altitude deposits and the lack of high-resolution multi-proxy records with adequate chronological control. Records from peat-bogs have been demonstrated to be among the best tools in paleoenvironmental studies to reconstruct past climate conditions and variations in atmospheric composition.

An ombrotrophic peat bog is a domed peatland hydrologically isolated from the influence of local groundwaters and surface waters, in which the surface layers are supplied only by atmospheric depositions. For this reason, they constitute an authentic records of information about past and present patterns in global climatic change.

Here we present the first complete Late Glacial to Holocene peat bog succession from the Dolomites (Danta di Cadore, Belluno, Italian Alps).

In 2011 a 7.0 m deep peat bog core was drilled at Val di Ciampo (Belluno province, 1400 m a.s.l.) and its potential as a paleoclimatic and paleoenvironmental archive has been evaluated. The depth-age scale is based upon independent 14C and 210Pb dates, modelled with ''Clam'' method, this combined with peat stratigraphy, demonstrates that the peat core covers more than 13,200 years (cal BP), extending back to the end of the last part of the Late Glacial.

We determined bulk density, inorganic matter content, and several parameters of pore water such as pore water pH, conductivity, Ca/Mg ratios, and Ca and Ti trends, to identify changes in trophic conditions through the entire profile. This multi-proxy approach confirms that the uppermost 400 cm of the bog are ombrotrophic, and demonstrates that this core is the longest Eastern Alpine ombrotrophic record yet obtained, covering the last 7,000 years.

Chronological constraints of the course of deglaciation in the Southern Alps are fewer than those available for the northern slope of the Alps. For the Piave basin, the mode and timing of deglaciation are well-defined only for the its mid-part, while no data are available for the upper section. In such a context of very limited data the oldest radiocarbon age (13,110-13,330 years cal BP) represents a very valuable result, providing clear evidence that, during the Bolling-Allerod interstadial, the upper part of the Piave Glacier was ice free, and confirming that the retreat process of Piave Glacier from the Last Glacial Maximum was very rapid. Pollen assemblages at the transition from the Late Glacial to the Early Holocene were studied at high resolution. In this time frame pollens show that denser forests of Gymnospermeae were present during the Bolling-Allerod interstadial (at approximately around 13,200 years cal BP), and were reduced by the climatic cooling of the Younger Dryas (12,600 - 11,500 years cal BP), when a more open type of vegetation spread. Then, with the beginning of the Holocene, forests developed again with the expansion of species such as Corylus, a warmth-requiring tree.

The investigation of atmospheric deposition during the Holocene is extremely important because it provides information about the climate-related changes in the atmospheric composition as well as the impact of human activities on the environment. With this aim, the concentration of 44 trace elements have been determined at a resolution of 1 cm on the first meter of the bog.

X-ray Fluorescence Core Scanner (XRF-CS) analysis was here applied for the first time on peat bog sequences. Results were combined with those obtained by Inductively Coupled Plasma Mass Spectrometry (ICP-MS), providing information about geochemical processes occurring in the bog, and about their influence on major and trace elements distribution along the profile. XRF-CS signals were calibrated using ICP-MS results, showing very high correlation and demonstrating that even the XRF-CS technique, when applied to peat samples, provides reliable quantitative results.

Particular attention was directed to elements related to mining activity, that has characterized the history of the Cadore region since the Middle Ages. Pb, Ag, Cd concentrations and Enrichment factors (EFs) were determined in the upper 100 cm of the record. In addition lead isotopes were also measured. Concentration levels and EFs of several trace elements such as Pb, Ag and Cd, fit very well the documented chronology about mining activity in Cadore region, indicating that the Val di Ciampo bog recorded the development and the history of mining exploitation at least at a regional scale. In particular, the Pb, Ag and Cd maximum concentrations are recorded between 1950s and 1980s, time interval that corresponds to the highest activity of the mining sites.

Lead isotopes ratios were measured to identify natural and anthropogenic sources of Pb emissions in the first metre of the bog. The isotopic composition of lead shows an increase of Pb deriving from fuel combustion over the last decades that gradually overlie the impacts of mining activity. In fact the uppermost 28 cm (corresponding to the period between AD 1940 and 2011) are characterized by more radiogenic inputs due to leaded gasoline combustion, with a signature closer to coal and fuel. Specifically, the 206Pb/207Pb decreasing trend, indicator of increasing leaded gasoline combustion, reached the minimum value of 1.153 in the 1990s (that correspond to maximum value of Pb flux, 0.75 g cm-2y-1 and then increases again. In these years, in fact, Italy started to follow EU rules to limit global pollutants in the atmosphere, and finally in 2002 banned leaded-fuels.

Val di Ciampo demonstrates a record not only of global (e.i. leaded gasoline combustion) or local (e.i. mining activity) changes in atmospheric inputs, but also more regional trends. In fact, both 206Pb/207Pb and Pb flux show a particular event between 1975 and 1980: this behaviour is characteristic of the ILE experiment (Isotopic Lead Experiment), a large scale isotopic tracer experiment that was carried out in the Piedmont region of Northwest Italy to study the metabolic paths of this element into the environment. All the gasoline used was labelled with a well isotopically-defined Pb.

The high-resolution physical, chemical and biological data obtained from the analysis of this peat archive improve our understanding of European Alpine Holocene climate and environmental variability and the relationship between natural fluctuations and anthropogenic impacts.