A re-evaluation of the palaeoclimatic significance of Phosphorus variability in speleothems revealed by high-resolution synchrotron radiation micro XRF mapping
S. Frisia ¹, A. Borsato ¹, R. N. Drysdale ², B. Paul ³, A. Greig ³, and M. Cotte 4
 School of Environmental and Life Sciences, University of Newcastle, Australia;  Dep. Resource Management and Geography, University of Melbourne, Australia;  School of Earth Sciences, University of Melbourne, Australia;  European Synchrotron Radiation Facility, Grenoble, France
Phosphorus (P) is potentially an important environmental proxy in speleothem palaeoclimate reconstructions. However, the incorporation of P to a speleothem seems to vary between cave sites. Microbial processes may drive co-precipitation of P-rich phases in coastal caves, which suggest that there may be more than just palaeoclimatic significance for P variability in speleothems. 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 study we investigate the distribution of P in one modern and two early Pliocene speleothems formed in coastal caves on Christmas Island (Indian Ocean) and the Nullabor Plain (southern Australia) by using microscopy and high resolution chemical mapping, combined with a robust knowledge of karst rock.
SMITHS CAVE, CHRISTMAS ISLAND
An actively forming stalagmite (SC4) was collected from the coastal Smiths Cave, located on the NW part of the Christmas Island. Temperature on the island is remarkably constant throughout the year (26ºC) and rainfall (2100 ± 725mm) has seasonal distribution. In the dripwaters that fed SC4, P concentration peaks (but not always) after recharge. P is sourced from apatite minerals in the epikarst (Frisia et al., 2012).
P INCORPORATION IN Sc4
P concentration in SC4 peaks in discrete laminae and reaches up to 8000 ppm in highly corroded crystal surfaces marked by a dark coating, visible in thin section (Fig. A and C). SEM micrographs show that the dark coating consists of tabular, lamellar and globular crystals nested in micrometre-sized hemispheric voids (Fig. B), or lining crystal tips,
where they are associated with fibers, and coating amorphous sheaths between crystals (Fig. D-E).
Synchrotron Radiation micro X ay Fluorescence (SR micro XRF) elemental mapping shows that only a minor fraction of P is incorporated in the solid lattice of the crystals. Most of the P is clearly associated with the micrometre-sized Prich phases associated with the dark coatings on corroded crystals (cfr. Fig. A and C).
P VARIABILITY IN Sc4
The P-rich layers in SC4 precipitated when P and Ca concentration in the solution was high. This occurred when Prior Calcite Precipitation (PCP) was low during aquifer recharge. Phosphate morphologies associated with microbial sheats and corrosion features complicate the scenario.
Periods of cessation of calcite growth allowed microbial colonization of stalagmite top. When drip recommenced after recharge, piston effects fed the drip with waters rich in P, and both organic and inorganic processes may have ultimately resulted in the formation of the dark layers.
WINBIRRA CAVE NULLARBOR PLAIN
At Winbirra, climate is subtropical arid, with surface mean annual temperature of 17.2°C and mean annual rainfall of ca. 270 mm/year. Today, thus, only halite and gypsum speleothems form. In the early Pliocene, reconstructed wetter-than-today climate conditions allowed formation of calcite speleothems. The host rock is a Cainozoic porous limestone containing phosphates. The two speleothems studied here (FS04 and MO1) are ca. 4 Ma old (U/Pb dating).
characterized by several micrite layers, peloid ike and stromatolite-like features.
P-RICH LAYERS IN FS04
The concentration of P is 12 times higher relative to the baseline. The SR micro XRF maps of FS04 show that P peaks in layers where Si, and S (as sulphate) also peak. Si is present as amorphous silica, and S is present as gypsum. There is no evidence of aragonite replacement. Precipitation from marine waters during Pliocene high sea level stands has been excluded.
The thin sections and SR micro XRF maps of MO1 show that P concentration peaks where crystal tips are coated by micrite, and dissolution voids are also filled by micrite. As in FS04, P is associated with Si (amorphous silica) and S (gypsum). Occurrence of micrite is best explained by bacterial mediation, as inorganic precipitation requires high supersaturation and abundant “old” nuclei. Presence of silica and gypsum, suggest precipitation by evaporated waters in dry conditions. P associated with microbial-like fabrics in Winbirra speleothems is here interpreted as marking periods of reduction (or cessation) of abiotic calcite growth, dissolution by microbial metabolism, and bacterially mediated calcite formation during progressive aquifer drainage. The source of P was in the epikarst and in the host rock, rather than from soil-sourced chelates.
- In SC4, high P concentration reflects low or negligible PCP following effective recharge;
- Dark, corroded layers, where bio-mediated phosphate precipitation is suspected are interpreted as a response to piston effects caused by effective recharge following prolonged dry periods
- In two ca. 4 Ma stalagmites from the Nullarbor, microbial processes may have been involved in the P incorporation in the speleothems.
- P-rich stromatolite-like layers are potentially indicative of dry phases of unknown duration (over several thousands of years) within the otherwise wet climate regime of the early Pliocene. Climate instability in the region (and a “Pleistocene climate mode” ) could have commenced at ca. 4 Ma.