A re-evaluation of the palaeoclimatic significance of phosphorus variability in speleothems revealed by high-resolution synchrotron micro XRF mapping

Abstract

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 proxy.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 spells.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 (SK P ) rather than the thermodynamic partition coefficient (K P ) to account for the presence of crystalline phosphate inclusions. SK P 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.

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

“…During dry episodes, speleothem surfaces are also more likely to be colonized by microbes as discussed by Frisia et al . () and this could explain the apparent correlation between fluorescence and Sr content.…”

“…The authors demonstrated that high Si/Ca content in the stalagmite coincides with more positive δ 18 O values and, thus, reduced rainfall amount. Furthermore, microbial mediation in silica precipitation cannot be entirely ruled out as amorphous silica was observed associated with micrite laminae in stalagmites from the Nullarbor (Australia) showing strong fluorescence and associated with S and P (Frisia et al ., ). Thus, the presence of intercrystalline silica may indicate microbial colonization of the coralloid surface, most likely when the surface of the speleothem was relatively dry.…”

Dripwater organic matter and trace element geochemistry in a semi-arid karst environment: Implications for speleothem paleoclimatology

“…In a pioneering study in SE Australia, McDonald et al (2007) presented 5-yrs of trace metal data from cave dripwater and noted a more complex response of dripwater Mg/Ca, Sr/Ca and Ba/Ca to climate than observed in temperate regions. More recently, Frisia et al (2012) demonstrated that the P record preserved in Australian speleothems could be both a function of within-cave microbial processes as well as soil-derived infiltration. However, despite these two studies, review of available literature indicate that karst infiltration processes in arid and semi-arid zones are rarely investigated and thus require further research.…”

Reconstructing seasonality through stable-isotope and trace-element analyses of the Proserpine stalagmite, Han-sur-Lesse cave, Belgium: indications for climate-driven changes during the last 400 years

“…An explanation proposed by Frisia et al (2012) is that P is not derived from soil leaching but from other sources such as phosphate minerals present in the epikarst or microbiological activity. However, no data are available on the P concentrations in the host rock and no microbially induced petrographic features were observed in the Proserpine speleothem, making it hard to test this hypothesis.…”