Sedimentology and geochemistry of a human-induced tufa deposit: Implications for palaeoclimatic research

Abstract

Geochemical variations across laminated tufas and travertines may reflect the growth style of the carbonate build-up and not just climate-related changes. This work presents the study of a carbonate deposit, formed on a ravine wall in Gran Canaria Island (Spain), from a broken pipe system used for irrigation of banana plantations. The deposit is a few tens of metres long and has a stepped morphology formed by successive cascade-barriers and pools. The main facies are framestones of coated stems, laminated bindstones, phytoclastic wackestones and silty mudstones, all of which display micritic or coarse crystalline textures. Lamination from a framestone with coarse crystalline texture was studied from a petrological-sedimentological and geochemical perspective, and water palaeotemperatures were calculated. Lamination displaying five orders of magnitude, from daily to annual or higher, was controlled by the discontinuous supply of water. Lamination consists of crystalline laminae-discontinuity couplets at all observed scales. Estimated mean precipitation rates are 0·7 mm year-1, but discontinuity of sedimentation at all lamination orders may have involved greater precipitation rates. Whereas elemental geochemistry suggests variable conditions not far from chemical equilibrium, stable isotopes suggest that calcite precipitated under disequilibrium conditions. However, the small dimensions of the deposit and the relatively high flow velocities allowed lack of δ13C and δ18O isotope fractionation in CO2-HCO3- nor in HCO3--calcite, leading to independent temperature calculations, both with mean values of 25°C. Isotopic trends found throughout lamination cannot be explained by strong changes in water temperature nor in δ13CDICor δ18Ow. The correction made to eliminate these isotopic trends yielded narrower temperature ranges. This paper discusses the accuracy of temperature estimations despite the difficulties coming from disequilibrium and how isotopic trends through time could be explained by the growth of the deposit and not by climate-related changes.