SEASONALLY BANDED FRESHWATER TUFAS FROM CENTRAL GREECE: DO d18O VALUES REFLECT A COMPLETE SEASONAL PALAEOCLIMATE RECORD?
Laminar tufas (ambient temperature freshwater spring and stream carbonates) are the subject of recent terrestrial palaeoclimate studies. Claims that currently active and older Holocene deposits preserve intra-annual records of stream temperature variability1,2 assume such records are complete. This is disputed here using high resolution (~ 0.2mm) d18O analysis and petrography of a Greek Pleistocene perched springline deposit. This tufa (currently being dated) formed during a comparatively wet interglacial interval up to 400ka BP. Petrography shows this tufa is not significantly diagenetically altered.
Annual laminar couplets (6mm thick) consist of dense bands of carbonate coated cyanobacterial shrubs', and porous bands of un-clustered cyanobacterial filaments. Cavities formed by caddis fly larvae within dense bands provide seasonal (spring) marker horizons. Fabric changes are sharp, with structures at the top of one band often truncated by the base of the next. Abrupt fabric changes coincide with sharp turns in the d18O record. Changes in stream water temperature (based on tufa d18O) from warming during deposition of dense tufa to cooling (porous tufa) are instant, occurring exactly on sharp petrographic boundaries.
It is important to note that this truncated record reflects the absence of summer precipitate. Fall spring boundaries are equally sharp, reflecting the absence of winter precipitate. Laminar tufa d18O clearly records stream temperature, but apparent failure to properly link d18O to petrography in previous studies has led to over optimistic interpretations of full intra-annual records. d18O variability commonly accounts for just half the observed temperature variation in Recent records. We infer that this is due to the absence of summer and winter precipitates. The locus and timing of tufa precipitation (although purely physico-chemical) is dictated by seasonal growth (and perhaps partial self-destruction) of the cyanobacterial substrate.
1: Matsuoka, J., Kano, A., Oba, T., Watanabe, T., Sakai, S. & Seto, K. (2001) Seasonal variation of stable isotopic compositions recorded in a laminated tufa, SW Japan. EPSL, 192, 31-44.
2: Andrews, J.E., & Brasier, A.T. (2005) Seasonal records of climatic change in annually laminated tufas: short review and future prospects. J Quat Sci 20 (5) In Press