1School of Environmental and Life Sciences, University of Newcastle, Callaghan 2308 NSW, Australia
2Museo delle Scienze, via Calepina 14, 38122 Trento, Italy
3Department of Resource Management and Geography, The University of Melbourne, Parkville 3010 Victoria, Australia
4School of Earth Sciences, the University of Melbourne, 3010, Australia
5CODES ARC Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Australia
6European Synchrotron Radiation Facility, 38043, Grenoble Cedex, France
Abstract. The distribution of phosphorous (P) in one modern and two Early Pliocene speleothems formed in low-lying, Christmas Island and the coastal Nullarbor caves wet settings in Australia is here investigated by microscopy and ultra-high resolution chemical mapping. Monitoring data in the modern setting suggest that co-precipitation of P with calcite occurs when the drip rate decreases, the aquifer is progressively drained and microbial mats possibly aid in the formation of concentrating phosphates. A bulk partition coefficient is proposed, which indicates that the P enrichment in the speleothem could be accounted for by inorganic processes. Our interpretation of the hydrological significance of P incorporation in wet, tropical speleothems is then used to interpret P peaks associated with micritic and stromatolithic layers in the two Early Pliocene stalagmites from the Nullarbor. From these observations it is speculated that dry periods may have interrupted the wet climate regime at ca. 4 Myr ago, hinting at a possible early onset of the Pleistocene climate mode.