Porosity and hydric behavior of typical calcite microfabrics in stalagmites

Abstract

Petrophysical techniques commonly used for material characterization are applied for the first time to speleothem samples to investigate the porosity and hydric behavior of calcite stalagmites used in paleoclimatology. These techniques allow the determination of the stalagmites' potential to undergo diagenetic transformations when substantial changes in drip waters occur in the cave environment. The petrophysical techniques include water absorption under vacuum and by capillarity, nuclear magnetic resonance, environmental scanning electron microscopy, and mercury intrusion porosimetry. The studied samples comprise five common calcite microfabrics, which have markedly different porosities and hydric behaviors and, as a consequence, different sensibilities to diagenetic processes related to the influx of water. The experiments show that stalagmites can behave as complex, small-scale hydrological systems and that the circulation of water through them by complex nets of interconnected pores might be common. As the circulation of water favors diagenetic transformations that involve geochemical and isotopic changes, the characterization of flow patterns is key for outlining areas that are susceptible to such modifications, which is critical to paleoclimatic studies that are based on speleothems because geochemical and stable isotopic data are used as paleoenvironmental proxies and absolute ages are obtained by using radioactive isotope ratios. These potential modifications also have obvious implications for studies based on fluid inclusions in speleothems.

The integrated methodology, which uses primarily non-destructive techniques, shows a high potential for characterization of any type of speleothem and other continental carbonates such as tufas or sinters.

Introduction

Speleothems are widely used in climate change studies. They show a great potential for use in the reconstruction of high-resolution paleoclimatic series due to the integration of multi-proxy analysis, the characterization of their internal stratigraphy, and absolute radiometric dating (i.e., U-series). Using this approach, many types of geochemical data (e.g., elemental geochemistry, stable isotopes, and radioactive isotopes) are used and interpreted under the basic assumption that speleothems behave essentially as closed geochemical systems.

However, the results of previous petrographical studies suggest that we should be cautious with this assumption. Speleothems often undergo diagenetic processes, such as corrosion and dissolution, micritization, neomorphism (inversion), and recrystallization (e.g., Frisia, 1996, Railsback et al., 1997, Railsback et al., 2002, McDermott et al., 1999, Martín-García et al., 2009, Martín-García et al., 2011). These processes can modify speleothem geochemistry and isotopic ratios and, thus, have critical implications for the interpretations of environmental proxies and radiometric age-dating (e.g., Whitehead et al., 1999, Borsato et al., 2003, Muñoz-García et al., 2007).

Diagenetic alterations occurring in stalagmites are commonly related to changes in the cave environment and in the hydrochemistry of drip waters (e.g., Frappier, 2008). Eventually, these changes can induce the cessation of carbonate precipitation, and a new interaction between drip waters and the carbonate minerals previously deposited can develop. This interaction geochemically alters the surface of the speleothem, and the resulting diagenetic features may be preserved as “altered” stratigraphic intervals if carbonate deposition resumes after a period of no deposition (e.g., Railsback et al., 2011). In some situations, the speleothem surface is permeable enough to allow water percolation, or the waters are corrosive enough to generate new percolation paths through the dissolution of existing carbonate. Then, the water enters the speleothem, promoting diagenetic transformations as it flows downwards via a network of connected pores (e.g., Frisia et al., 2000, Frisia et al., 2002, Borsato et al., 2003).

This suggests that the distribution of diagenetic processes within speleothems may be controlled strongly by primary porosity and the existence of internal heterogeneities, which determine the favorable pathways for secondary porosity development, water percolation, and water–speleothem interaction. These petrophysical properties should be a function mainly of speleothem microfabrics and internal microstratigraphy.

In this important task of recognizing and interpreting diagenetic patterns in speleothems, it is surprising how little attention has been given to their petrophysical characterization. Petrophysical non-destructive techniques (NDT) commonly used in other research fields (such as material characterization, historical building preservation, or geoarcheology) can be easily applied to speleothems, offering new methods to apply to the study of these rocks. This work is the first attempt of the application of such an approach to speleothems, and it should be of great benefit to future research.

In the context described above, this paper investigates the porosity and hydric behavior of the speleothems that are commonly used in paleoclimate studies, i.e., calcite stalagmites. These stalagmites, despite their homogeneous mineralogical composition, exhibit a wide variety of microfabrics that depend on the factors affecting their formation (e.g., Frisia and Borsato, 2010). For this study, five different microfabrics, all commonly found in calcite stalagmites, have been investigated using NDTs that include water absorption under vacuum conditions and under atmospheric pressure by capillarity, nuclear magnetic resonance relaxometry and imaging (NMR–MRI), and environmental scanning electron microscopy (ESEM). The results obtained using the NDTs have been compared with those obtained by polarized light optical microscopy (PLOM) and mercury intrusion porosimetry (MIP).

The aim of this research is thus to investigate the porosity and hydric behavior of different microfabrics in calcite stalagmites, to determine their potential to be diagenetically modified, and to discuss their capacity to remain behaving as closed geochemical systems when substantial changes in drip waters occur. The paper demonstrates the potential of applying petrophysical techniques to the study of speleothems. The integrated methodology introduced herein can be applied to any type of speleothem, independently of its mineralogy or stratigraphy, and has great potential to be used for the characterization of other continental carbonate deposits, such as tufas and sinters, with which speleothems share many characteristics.