Complex speleothem growth processes revealed by trace element mapping and scanning electron microscopy of annual layers

Abstract

Closely-spaced transects measured by excimer laser ablation inductively-coupled plasma mass spectrometry (ELA-ICPMS) at 5 and 32 μm spatial resolution are used to generate trace element composition maps (Ba, Sr, Mg, U, Na, P and Al) from MND-S1, a previously studied modern stalagmite from southwest Australia ( Treble et al., 2003: EPSL 216: 141). Rainfall at the site is highly seasonal, and trace elements in MND-S1 show strong seasonal variation. Trace element maps show that Ba, Sr, U and Na concentrations coherently follow annual growth layers identified from Scanning Electron Microscopy (SEM) images. The SEM images also reveal that stalagmite growth did not proceed uniformly: growth layers vary in thickness and locally pinch out. Highly preferential crystal growth, determined by nucleation sites left by the previous year’s growth, may be responsible for this uneven growth layering. Differential crystal growth apparently causes variability of trace element concentrations along each annual layer, although additional disequilibrium processes affect Mg, which is less distinctly banded than Ba, Sr, U and Na. Uneven and discontinuous growth layers influence the number of annual cycles, their wavelengths and seasonal amplitudes measured in any one transect. This has clear implications for studies that use annual trace element cycles as chronological markers, growth rate or seasonality proxies.