Stable isotopes and origin of Palaeocene fibrous calcite concretions in Patagonia, Argentina

TOURN, SELVIA M.; SCASSO, ROBERTO ADRIÁN; CASTRO, LILIANA; CUITIÑO, JOSÉ IGNACIO

Brasilia

Simposio; VII SSAGI South American Symposium on Isotope Geology; 2010

Laboratorio de Geocronologia. Universidad de Brasilia

Fibrous radial calcite concretions were found in estuarine to palustrine/fluvial strata of the Palaeocene of Patagonia. These deposits record the final regression of the large epicontinental sea that covered most of Patagonia in the Paleocene, and concretions are especially abundant in the “Banco Negro Inferior” of the Río Chico Formation in the Chubut province. Sampling covered a wide latitudinal range, from 37°30’S to 45°36’S. The concretions are 2 to 20 cm in diameter, whitish-gray in colour and espherical to subespherical in shape. Elongated fibrous crystals grew parallel to the “c” crystalographic axis and are grouped in aggregates irradiating out from a slightly spongy, undifferenciated nucleus. Radial agreggates of fibrous calcite don´t show twinning of cleavage, and the fibers bend lateraly along the radius in a feather-like arrangement. The δ18O in the concretions varies between -7.92 and -13.80 o/oo. Dispersion of the values is small within a single concretion or between concretions in the same locality. Most of the concretions show a small decrease in δ18O towards the edges. The δ13C values show wider range, and vary between -25.31 y +0.09 o/oo. In the Banco Negro concretions, the δ13C rises towards the edge but this not true for concretions in other localities, in which variable trends were observed.  Concretions grew progressively from center to edge displacing the host mud. The espherical shape indicates they were formed by crystalline nucleation and difussion in a highly-porous, water-saturated, early-diagenetic environment. Calcite growth along the “c” axis was controlled by the relative abundance of the CO3= ion. Therefore the system was rich in CO3=   up to the end of the concretion development. Negative δ18O values are related to the presence of fresh, meteoric water during the formation of the concretions. Small variations in the δ18O within the concretions suggest rapid growth and large, unlimited source of meteoric waters.  δ18O shows a consistent decrease with paleolatitude, and values -2/-3 o/oo below the estimated values for current meteoric precipitation in Patagonia. The climatic change may account for the difference, because Patagonia is currently a desertic land with arid climate but in the Paleocene the weather was warm and wet. Negative δ13C values reflect carbonate precipitation from solutions rich in dissolved CO2 originated by oxidation of organic matter. Enrichment in the heavy isotope during the concretion growth might reflect increasing contribution of CO2 generated by microbial processes in methanogenic conditions, as it is suggested by the bacterial morphologies preserved within the fibers in the concretions, or by dissolution of fossil shells from the underlying Salamanca Formation.