Depositional continuity of seasonally laminated tufas: Implications for δ18O based palaeotemperatures

A ~ 100 ka old laminated tufa (freshwater carbonate) deposit from central Greece was studied for evidence of a seasonal origin of the laminae. Annual laminar couplets (6 mm thick) consist of dense β bands of spring–early summer calcified cyanobacterial bushes and porous α bands of autumn precipitated calcite on un-clustered cyanobacterial filaments. This is evidence of likely seasonal palaeoclimate during the last interglacial in Greece. δ18O variability is explained by seasonal changes in stream water temperature; however, the total range in δ18O translates to ~ 6 °C (± 1 °C) almost certainly underestimating the likely actual range of ~ 17 °C. As abrupt changes in δ18O inferred temperatures (from warming to cooling and vice versa) occur exactly on sharp petrographic lamina boundaries, we infer that mid-summer and mid-winter precipitates are missing. We suggest that when stream conditions (summer drying and winter cold) are unsuited to cyanobacterial substrate development, no tufa build-up occurs. Laminated tufas are thus incomplete records of annual tufa formation, as are the δ18O based palaeoclimate records they preserve. They require careful petrographic study linked to high resolution sampling for δ18O to ensure that palaeotemperature ranges are not seriously underestimated.