High‐resolution synchrotron X‐ray fluorescence investigation of calcite coralloid speleothems: Elemental incorporation and their potential as environmental archives

Abstract

Synchrotron high-resolution and micro-X-ray fluorescence elemental mapping of two coeval coralloid speleothems from Lamalunga Cave (Italy) are complemented with petrographic, morphological and microstratigraphic studies. The importance of these speleothems relies on their direct and indirect association with a complete Neanderthal skeleton ('Altamura Man') found inside the cave. The coralloids grew discontinuously between 64Á6 ka and the Holocene and reveal exceptionally high concentrations of Mg, Sr and Si, particularly on convex surfaces, where evaporation is more intense. The incorporation of trace elements depends on several factors including location, shape and geometrical evolution during their growth, as well as climate and environmental parameters. This resulted in calcite precipitation with Sr compositions from 100 to 1200 ppm and an average concentration of 7000 ppm Mg. An unusually high Si content (up to 16%) is possibly derived from volcanic ash transported as particulate and in solution inside the cave. The most common fabrics observed consist of non-fluorescent elongated columnar calcite forming clean isopachous bands and fluorescent fibre-like crystals associated with laminated, lenticular bands high in Sr, Mg and Si. Variability in Sr, Mg and Si concentrations appears to induce fabric changes in the coralloids. Elongation and lattice distortion of the crystals was found to coincide with high Mg concentrations. The transition from compact elongated to open to fibre-like, is here interpreted as due to high concentrations of Si and Sr, which are preferentially incorporated in the speleothem at crystal boundaries and intra-laminae. It is here inferred that coralloid fabric changes and their elemental content potentially record local rainfall variations through time, with the clean compact calcite marking high infiltration and open fibre-like and micrite fabrics recording dry periods.

Accurate dating of stalagmites from low seasonal contrast tropical Pacific climate using Sr 2D maps, fabrics and annual hydrological cycles

“…Commonly, this is the exception, because crystal growth processes and the presence of extra-lattice particulates yields trace element distribution inhomogeneities along the same seasonal/annual layer. Chronologies built upon annual trace element cycles that can be precisely compared with the physical aspects of layering in speleothems are the most reliable, because peaks and troughs along sampling transects at high spatial resolution can be visually superimposed to fabrics 16 , 17 , 23 , 24 . However, annual laminae-specific heterogeneities due to crystal fabrics and presence of nano-particulate can only be detected by two-dimensional, continuous, high resolution trace element mapping 25 , which can be performed by using Synchrotron radiation-based micro X-Ray fluorescence over entire speleothem slabs 26 , 27 .…”

Tools for uranium characterization in carbonate samples: case studies of natural U–Pb geochronology reference materials

“…Laser ablation mapping (Woodhead et al, 2010(Woodhead et al, , 2007Piccione et al, 2019;Drost et al, 2018;Roberts et al, 2020), synchrotron XRF mapping (Cole et al, 2004;Piccione et al, 2019;Frisia et al, 2008;Vanghi et al, 2019), PIXE (Ortega et al, 2005(Ortega et al, , 2003, µXRF (de Winter et al, 2017), and EMPA (Suzuki et al, 2016) permit tying petrographic observations of different phases to in situ chemical maps of U and other elements that inform fluid chemistry. For the purpose of U-Pb dating, knowledge of the mineral lattice location of U and its oxidation state and speciation provide a fundamental framework for interpreting ages, as well as revealing details of the fluid chemistry (Kelly et al, 2006(Kelly et al, , 2003Sturchio, 1998;Cole et al, 2004).…”

Brucite-Aragonite Precipitates as Weathering Products of Historic Non-MgO-Based Geomaterials

“…When these waters flow over the cone-crust, silica precipitates after intense evaporation. Moreover, microbial mediation in silica precipitation in this kind of speleothems cannot be entirely ruled out as it discussed in [30].…”

“…These microclimatic conditions are characterized by moderate luminosity and high connection between internal and external environments through The evolution from massive to fibrous aragonite firstly and from massive to flaky brucite secondly indicate an initial increase of the Mg/Ca ratio in the solution (associated to the aragonite precipitation) followed by a pH decrease as the brucite formation implies a consumption of hydroxyl ions (OH − ). On the one hand, the presence of Mg 2+ ions favors the development of fibrous aragonite crystals [24,29,30]. On the other hand, the size and morphology of brucite crystals depends strongly on pH [31].…”