Seasonal temperatures from δ¹⁸O in Recent Spanish tufa stromatolites: Equilibrium redux!

This study focuses on recent debate over the value of stable-isotope-based environmental proxies recorded in riverine tufa-stromatolites. We recovered a 12-year record (1999-2012) of river-bed tufa stromatolites in the River Piedra (NE Spain) along with a partly overlapping 15-year record (1994 to 2009) of accumulations in a drainage pipe: both deposits formed in water with near identical physico/chemical parameters. Measured water temperature (Tw) data and near constant δ¹⁸Owater composition allowed selection of an ‘equilibrium’ palaeotemperature equation that best replicated actual temperatures. We found, as other have, that both the Epstein et al. (1951) and Kim & O’Neil (1997) formulas for Tw calculation from equilibrium calcite δ¹⁸O compositions were appropriate for the River Piedra where tufa deposition rates are high, means between 5.6 and 10.8 mm in six months. δ¹⁸Ocalcite in both the river and the pipe deposits record essentially the full actual seasonal Tw range. Only the coldest times (Tw < 10ºC), when calcite precipitation mass decreased to minimum, are likely to be unrepresented, an effect most noticeable in the pipe where depositional masses are smaller and below sample resolution. While we cannot rule out kinetic effects on δ¹⁸Ocalcite-based calculated Tw, the good fit between measured Tw and δ¹⁸Ocalcite-calculated Tw indicates that temperature is the principal control. Textural and deposition rate variability between the river and pipe settings are caused by differences in flow velocity and illumination. In the river, calcification of growing cyanobacterial mat occurred throughout the year, producing composite dense and porous laminae, whereas in the pipe, discontinuous cyanobacterial growth in winter promoted more abiogenic calcification. High-resolution δ¹⁸Ocalcite data from synchronous pipe and river laminae show that reversals in Tw occur within laminae, not at lamina boundaries, a pattern consistent with progressive increase in calcite precipitation rate as cyanobacterial growth re-established in spring.