GEOCHEMICAL RECORDS OF CLIMATIC CHANGE IN TUFA CARBONATE: A CRITICAL REVIEW

There has recently been much interest in tufa carbonate geochemical records as potential palaeoclimatic archives. Most interest has focussed on stable isotopes of oxygen as possible palaeotemperature recorders. Sable carbon isotope data have largely been interpreted as reflecting changes in sources of carbon to aquifer waters, combined with effects of carbon dioxide degassing.

Some recent and fossil freshwater tufas (tufa stromatolites) contain millimetre-scale, alternating laminae of dense micrite and more porous or sparry crystalline calcites. These alternating laminae are thought to represent seasonally controlled differences in the biotic activity of microbes, and /or seasonally controlled changes in calcification rate. Either way, couplets of these microbially-mediated alternating calcified laminae are generally agreed to represent annual seasonality. Combined stable isotope (δ¹⁸O and δ¹³C) and trace element (Mg, Sr, Ba) geochemistry from recent tufa stromatolites show that seasonal climatic information is available from these calcites.

Variability in δ¹⁸O is controlled by stream temperature change and /or seasonal evaporation on sub-annual timescales, but by changes in recharge isotopic composition on millennial timescales. Variability in δ¹³C on all timescales results from changes in soil-zone carbon inputs modulated by (1) variable degassing of ¹²C-enriched CO₂ in the aquifer system, and (2) precipitation of calcite along the aquifer flow path, a process that increases δ¹³C of dissolved inorganic carbon in the remaining water. Mechanisms 1 and 2 are linked because calcite precipitates in aquifers where degassing occurs, e.g. air pockets. The latter mechanism for δ¹³C enrichment also causes sympathetic variation between Me/Ca ratios and δ¹³C because trace elements with partition coefficients <<1 (e.g. Sr, Ba) remain preferentially in solution. Since degassing in air pockets will be enhanced during decreased recharge when water saturation of the aquifer is lowest, sympathetic variation in Me/Ca ratios and δ¹³C is a possible index of recharge and therefore precipitation intensity.

Geochemical information in well-dated fossil tufas have great potential for Quaternary palaeoclimate reconstructions, potentially allowing recovery of long-term changes in recharge temperatures, annual seasonal water temperature variation and change in rainfall intensity. However, careful consideration of facies context and diagenetic effects, particularly aggrading neomorphism, are required.