Palaeoclimatic implications of the growth history and stable isotope (δ18O and δ13C) geochemistry of a Middle to Late Pleistocene stalagmite from central-western Italy
Introduction
Cave speleothems (e.g. stalagmites and flowstones) are secondary carbonates formed by the precipitation of calcite or aragonite from CaCO3-rich percolation waters via the principal mechanism of CO2-outgassing [1]. They generally develop in regions of above-freezing temperatures where there is sufficient rainfall and soil biological activity to promote karst bedrock dissolution and the transport of solutes downwards to the cave environment [2]. The chemistry of speleothem source waters is sensitive to changes in climate-driven environmental processes at the Earth's surface [3]. These changes are incorporated into a speleothem as the calcite is deposited, producing archives capable of yielding palaeoenvironmental records over 102- to 105-year timescales [4].
Much of the recent palaeoclimate research utilising speleothems has focused on extracting high-resolution (annual to decadal) records from Holocene-age deposits [5], [6], [7]. However, speleothems also provide a rare opportunity to investigate pre-Holocene climate change at glacial–interglacial timescales [8]. The Devils Hole vein calcite, for example, has demonstrated that speleothems can preserve long, continuous to quasi-continuous, well-dated records of terrestrial palaeoclimate [4], and can thus be used to test theories of climate forcing and reveal the timing, magnitude, duration and geographic extent of global and regional climatic events [9], [10], [11].
Past speleothem research focusing on the Mediterranean basin has provided valuable insights into regional climate history over the past ∼200 kyr, including the timing of sea-level changes [12], [13], onshore responses to sapropel events [14], [15], dust flux [16] and changes in precipitation regimes [15]. However, detailed knowledge of regional terrestrial palaeoclimate prior to Marine Isotope Stage (MIS) 7 is restricted to a few lake cores with limited chronological control [17]. Consequently, further palaeoclimate data are required from datable archives sensitive to climate change so that the regional response to individual glacial–interglacial cycles can be resolved.
In this paper, we examine the growth history and stable isotope geochemistry of a stalagmite from Antro del Corchia, a cave located in an alpine karst of central-western Italy. It provides a rare radiometrically dated terrestrial record of climate history from the central Mediterranean pre-dating MIS 7.
Section snippets
Geological and hydrological setting
Antro del Corchia is situated in the Alpi Apuane massif, which rises ∼2000 m above the coastal plain of northern Tuscany, Italy (Fig. 1). Much of the massif comprises intensively karstified, steeply dipping to overturned Mesozoic marbles and metadolostones [18], [19]. Rainfall reaching the cave is received along the main Apuane watershed and drains to a series of springs near the village of Stazzema (∼400 m a.s.l.) [19]. In spite of its proximity to the coast, the karst hydrological base level
Sample composition
CC1 is the top 450 mm broken section of an ∼800 mm tall stalagmite. It consists almost entirely of slightly opaque, macrocrystalline, columnar calcite which is interrupted at three locations. Hiatuses are clearly visible at two of these (H2 and H3 in Web Fig. 2), providing evidence of periodic drip-water cessation. A third feature (H1 in Web Fig. 2) comprises a 2–8-mm-thick band of white calcium carbonate which tapers towards the central axis. Thin sections show a further hiatus at this point
Age data and age–depth models
Corrected radiometric ages are shown in Table 1. CC1 contains relatively high U concentrations (2–18 ppm), unusually low 234U/238Uinitial ratios and very little detrital Th (230Th/232Th>2500) (Web Table 1). Preliminary investigations of the U geochemistry of the source bedrock confirmed earlier research [25] that the Brecce di Serravezza unit, which forms part of the recharge area bedrock, is the most likely source of the high U concentrations. On the other hand, all bedrock 234U/238Umeasured
Palaeoclimatic implications of growth phases
The growth phases for CC1 suggest a strong degree of forcing by climatic shifts of glacial-to-interglacial magnitude. Growth ceased during two glacial periods, MIS 10 and MIS 8. In the former, the timing (∼348 kyr) is close to the glacial maximum proposed by the SPECMAP group [29], [30] and the Vostok ice record based on a recent revised age scale [32]. The hiatus during MIS 8 (∼279 kyr) appears to precede glacial maximum conditions by ∼10 kyr, suggestive of the onset of a more extreme
Conclusions
The growth history and stable isotope geochemistry of stalagmite CC1 from the Alpi Apuane of central-western Italy preserve an intermittent record of climate changes between ∼380 and ∼43 kyr. The stalagmite provides the first well-dated record of pre-MIS 7 terrestrial environmental change from the central Mediterranean region, particularly the MIS 10–MIS 8 period. The major features of its growth history can be explained by climate changes during glacial/interglacial transitions. Cessation of
Acknowledgements
We thank the Gruppo Speleologico Lucchese and the Federazione Speleologica Toscana for financial and logistical support, T. Donnelly at SUERC for technical support with stable isotope analyses, K. Collerson (University of Queensland) for providing access to TIMS Th–U dating, L. Piccini (University of Florence) for information on Antro del Corchia, and O. Rey-Lescure and D. Redwood for the back trajectory analysis. RND acknowledges the University of Pisa for hosting a sabbatical visit in 1999,
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