Depositional continuity of seasonally laminated tufas: implications for delta O-18 based palaeotemperatures

A similar to 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 beta bands of spring-early summer calcified cyanobacterial bushes and porous alpha bands of autumn precipitated calcite on unclustered cyanobacterial filaments. This is evidence of likely seasonal palaeoclimate during the last interglacial in Greece. delta O-18 variability is explained by seasonal changes in stream water temperature: however, the total range in delta O-18 translates to similar to 6 degrees C (1 degrees C) almost certainly underestimating the likely actual range of similar to 17 degrees C. As abrupt changes in delta O-18 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 delta O-18 based palaeoclimate records they preserve. They require careful petrographic study linked to high resolution sampling for delta O-18 to ensure that palaeotemperature ranges are not seriously underestimated. (C) 2009 Elsevier B.V. All rights reserved.