The Project

Climate archives in karst - an integrated approach to the study and modeling

 of abrupt climate oscillations

Synopsis

The project proposes an integrated approach based upon a multi-proxy analysis of the various paleoclimate proxies from caves and karst deposits. Out of these, the most important are the stable isotopes, chemical (trace elements), and mineralogical variations in speleothems (calcite formations in caves), magnetic properties of fluviatile or lacustrine sediments, fossil and sub-fossil species and faunal associations, stable isotopes and composition of the underground glaciers. Most of the above deposits bear information relevant for the paleoclimate evolution at regional scale and their high-resolution radiometric datings allow signal calibration and comparative analysis of the different time-series. The combination of the data measured from various proxies from cave environment (speleothems, sediments, fossil, ice) within the same cave or karst area allow: (1) to overcome the limitations of the some dating methods; (2) to combine different climatic profiles into a composite profile; (3) to carry on time-series analyses taking into account regional or global climatic profiles and to identify the regional constrains of climate oscillations. The data may be calibrated by isotope analysis of newly-precipitated calcite in laboratory-caves correlated with the monitoring of both surface weather conditions and underground microclimate and physical parameters (e.g. drip rate). The conversion of the variations of isotope from cave deposits into variations of temperature or other parameters (precipitation source, vegetation cover, biological activity in soil cover) allow us validate and/or develop new models for abrupt climate oscillations at least at millennial scale.

Basic hypotheses and current researches

The study of the mechanisms that control global climate changes is a scientific topic of present interest and, as such, it is largely dealt with by international (e.g. www.ncdc.noaa.gov/paleo/abrupt), European and national research programs. Understanding these mechanisms relies on the study of climate changes from the recent geological past (the Holocene, the Middle and Late Pleistocene). Most often, paleoclimate proxies (usually, stable isotopes) are measured from deep-sea sediments or, within the continental domain, from lacustrine sediments or ice cores.

        When compared with the deposits from marine or continental domains, the deposits from deep cave settings show several notable advantages. Under a highly stable microclimate such as the one from deep caves and in the absence of most erosional mechanisms that act at grounds’ surface, cave deposits preserve valuable archives such as: the Oxygen isotope signal which is a proxy for regional temperature and/or variations in precipitation sources and paths; the isotopic signal of Carbon and pollen content  which are thought to largely reflect the paleo-composition of vegetation at surface, minor elements signals in fossil remains (e.g. 87Sr/86Sr, 44Ca/40Ca, d15N, d13C), which bear significances related to paleodiet; the succession of aquatic sub-fossil species in lacustrine sediments; magnetic minerals properties relevant for past rainfall regime and paleo-temperatures. Among all these, speleothems, are particularly suitable to paleoclimate analyses since they can be accurately dated using the U-series method which overcomes the ~40 ka limit of the 14C method back to ~700 ka. This limit can be further extended back to ~1.3-1.5 Ma by using the paleomagnetic signal in speleothems and cave sediments coupled with the 238U/234U dating method.

Under these circumstances, the study of paleoclimate-significant cave deposits, especially that of speleothems, has gathered a considerable momentum in the last decade. In 2004 only, the number of papers published in ISI-WOS indexed journals was about five times greater than that corresponding to 1993-1995 time-period, and the figures almost doubled every year since. Despite this upsurge of speleothem-based paleoclimate studies, this is still a frontier scientific field where techniques and analytical procedures are continuously improved. The U-series dating methods, for example, has evolved in only 7 years from alpha spectrometry to thermal-ionization mass spectrometry (TIMS) and currently a growing number of laboratories are using the potential offered by the inductively-coupled plasma mass spectrometers (ICP-MS). In the study of stable isotopes, the innovative approach of the “clumped isotope”  holds promises for the direct use of carbonates as paleothermometers, independently of the d18O in the precipitation waters. New approaches may be noticed in paleontological studies, where fossil remains of cave mammals are studied through a combination of  aDNA sequencing coupled with 14C datings and  d15N/ d13C measurements. This complex analysis allow species dating and the study of their paleodiets, migration paths and, possibly, the assessment of extinction moments and reasons, all bringing new elements for understanding the climate evolution during at least the last 50,000 years.

As any new approach, the study of speleothems (by far the most largely applied) faces many challenges. Some isotope signals (d18O) are better understood, while others (e.g. d13C, d87Sr) require supplemental research in order to discriminate the paleoclimate-significant component. Speleothems climate record of a high resolution which is only limited by the analytical uncertainties range (for example, for the Holocene resolutions smaller than 100 years are routinely reported; for some recent samples annual cyclicity has been recognized and recent researches focus upon inter-annual seasonality). However, calcite precipitation depends upon the supply of percolating water, so speleothem growth may cease during either very cold or very arid epochs. In addition, speleothem growth may cease due to accidental reasons which leads to fragmentary records. The problem of limited duration of speleothem records may be solved either by merging more speleothem profiles or by using supplemental proxies, such as, for instance, the magnetic properties.