Publication Date:
2021-03-18
Description:
The trace-element geochemistry of speleothems is becoming increasingly used for reconstructing palaeoclimate, with a particular
emphasis on elements whose concentrations vary according to hydrological conditions at the cave site (e.g. Mg, Sr, Ba
and U). An important step in interpreting trace-element abundances is understanding the underlying processes of their incorporation.
This includes quantifying the fractionation between the solution and speleothem carbonate via partition coefficients
(where the partitioning (D) of element X (DX) is the molar ratio [X/Ca] in the calcite divided by the molar ratio [X/Ca] in the
parent water) and evaluating the degree of spatial variability across time-constant speleothem layers. Previous studies of how
these elements are incorporated into speleothems have focused primarily on stalagmites and their source waters in natural
cave settings, or have used synthetic solutions under cave-analogue laboratory conditions to produce similar dripstones. However,
dripstones are not the only speleothem types capable of yielding useful palaeoclimate information. In this study, we
investigate the incorporation of Mg, Sr, Ba and U into a subaqueous calcite speleothem (CD3) growing in a natural cave pool
in Italy. Pool-water measurements extending back 15 years reveal a remarkably stable geochemical environment owing to the
deep cave setting, enabling the calculation of precise solution [X/Ca]. We determine the trace element variability of ‘modern’
subaqueous calcite from a drill core taken through CD3 to derive DMg, DSr, DBa and DU then compare these with published
cave, cave-analogue and seawater-analogue studies. The DMg for CD3 is anomalously high (0.042 ± 0.002) compared to previous
estimates at similar temperatures ( 8 C). The DSr (0.100 ± 0.007) is similar to previously reported values, but data
from this study as well as those from Tremaine and Froelich (2013) and Day and Henderson (2013) suggest that [Na/Sr] might
play an important role in Sr incorporation through the potential for Na to outcompete Sr for calcite non-lattice sites. DBa in
CD3 (0.086 ± 0.008) is similar to values derived by Day and Henderson (2013) under cave-analogue conditions, whilst DU (0.013 ± 0.002) is almost an order of magnitude lower, possibly due to the unusually slow speleothem growth rates
(〈1 lm a
1), which could expose the crystal surfaces to leaching of uranyl carbonate. Finally, laser-ablation ICP-MS analysis
of the upper 7 lm of CD3, regarded as ‘modern’ for the purposes of this study, reveals considerable heterogeneity, particularly
for Sr, Ba and U, which is potentially indicative of compositional zoning. This reinforces the need to conduct 2D mapping
and/or multiple laser passes to capture the range of time-equivalent elemental variations prior to palaeoclimate interpretation.
Description:
the Australian
Research Council (Discovery Project number DP160102969,
awarded to RD, GZ, ER and JW; Laureate Fellowship
FL160100028 awarded to JW; and Future Fellowship
FT130100801 awarded to JH.
Description:
Published
Description:
67-91
Description:
5A. Ricerche polari e paleoclima
Description:
JCR Journal
Keywords:
Trace-element geochemistry
;
Speleothems;
;
Calcite
;
Partition coefficients
;
Caves
;
Palaeoclimate
;
fractionation between the solution and speleothem carbonate
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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