A re-evaluation of the palaeoclimatic significance of phosphorus variability in speleothems revealed by high-resolution synchrotron micro XRF mapping 1School of Environmental and Life Sciences, University of Newcastle, Callaghan 2308 NSW, Australia
14 Dec 2012
2Department of Resource Management and Geography, The University of Melbourne, Parkville 3010 Victoria, Australia
3School of Earth Sciences, The University of Melbourne, 3010 Australia
4CODES ARC Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Australia
5European Synchrotron Radiation Facility, 38043, Grenoble Cedex, France
Received: 16 Jun 2012 – Published in Clim. Past Discuss.: 11 Jul 2012 Abstract. Phosphorus (P) is potentially a very important environmental proxy in
speleothem palaeoclimate reconstructions. However, the transfer of P to a
speleothem seems to vary between cave sites. Therefore, it is important to
investigate the source of P and the way it is incorporated into a speleothem
on a site-by-site basis before it can be used as a robust palaeoclimate
Revised: 22 Nov 2012 – Accepted: 22 Nov 2012 – Published: 17 Dec 2012
In this paper, the distribution of P in one modern and two Early Pliocene
speleothems formed in coastal caves on Christmas Island (Indian Ocean) and
the Nullarbor Plain (southern Australia) is investigated using microscopy
and ultra-high resolution chemical mapping.
Phosphorus has been found to be both incorporated in the lattice and present
as diverse P-rich phases. Monitoring data from Christmas Island suggest that
co-precipitation of P-rich phases occurs when "prior calcite precipitation"
decreases following recharge, even if the drip rate decreases. Microbial
mediation may also play a role, which complicates a direct climate
relationship between P and hydrology. We find that some P-enriched layers
contain dissolution features, with possible involvement of microbial mats
which colonise pores during reduced drip rates associated with prolonged dry
In the two Early Pliocene speleothems the relationship between P and
microbial laminae is clearer. Both petrographic and chemical data suggest
that phosphorus-rich phases in the microbial laminae mark intervals of
reduced drip rates, which may indicate dry intervals during the otherwise
wet palaeoclimate of the Early Pliocene.
We develop a speleothem distribution coefficient for phosphorus (SKP) rather than the thermodynamic
partition coefficient (KP) to account for the presence of crystalline
phosphate inclusions. SKP describes P enrichment in speleothems
regardless of the process, as similar mechanisms of phosphate
co-precipitation may be in operation in biotic and abiotic conditions.
The most important implication of our study is that variability in P
concentration may be related to diverse processes which can be recognized
through petrographic observations and chemical mapping. In particular, there
may not be a direct relation between an increase in P concentration and
seasonal infiltration as has been found in some previous studies, especially
if the source of this element is not the labile phosphate released through
leaching during seasonal vegetation dieback in temperate climates.
Citation: Frisia, S., Borsato, A., Drysdale, R. N., Paul, B., Greig, A., and Cotte, M.: A re-evaluation of the palaeoclimatic significance of phosphorus variability in speleothems revealed by high-resolution synchrotron micro XRF mapping, Clim. Past, 8, 2039-2051, doi:10.5194/cp-8-2039-2012, 2012.