Large-scale spheroidal redoximorphic features around plinthite nuclei in Orinoco River sediments reflect mean seasonal fluctuation in river stage and ENSO-related anomalies

Abstract

In sediments emerging on the scarp of Lower Orinoco River, Venezuela, we have discovered and studied large (0.5 m) features formed by a plinthite nucleus surrounded by evenly spaced concentric spheroidal structures of iron oxides separated by depletion zones. These features are located in sediments subjected to the mean annual river fluctuation (approx. 14 m) and hence are submerged for several months each year. To the best of our knowledge, structures like these have never been reported or studied. The nuclei and concentric ring formations found in the Orinoco sediments represent an extreme case of regular currently alternating redox conditions. Here we show that the concentric ferric rings surrounding the nuclei could be the result of repeated cycles of diffusion of ferrous ions during flood and subsequent precipitation as ferric oxide during the dry period, thus reflecting the seasonal fluctuation in river level. Our results are consistent with a simple proposed model of ferrous iron diffusion/oxidation according to the flood/dry intervals imposed by river dynamics. This paper is a contribution toward understanding these redoximorphic features (RFs) by: (1) describing their composition and mineralogy; (2) suggesting a possible mode of their formation using the switch to reducing conditions and diffusion of soluble Fe during flood, and its subsequent oxidation and hence immobilization and partial crystallization once exposed to the air; (3) by confirming how this theoretical approach fits the actual pattern of river hydrology in Orinoco, both under average conditions, and as modified by climatic extreme conditions influenced by the El Nino/La Nina Southern Oscillation (ENSO) affecting the ratio of flood/emergence.