Preservation of Total Organic Carbon and evaluation of Corg/Ntot atomic ratio in a sediment outcrop located S-E of the Lago Fagnano (Tierra del Fuego, Argentina).

CAFFAU, M; COMICI, C.; ZECCHIN, M.; PRESTI, M.; LODOLO E.; TASSONE A.; LIPPAI, H,; MENICHETTI, M.

BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA

ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA

Lugar: Trieste; Año: 2010 vol. 51 p. 171 - 174

0006-6729

This study evaluates the content of the Total Organic Carbon (TOC), the Total Nitrogen (TN) and the Corg/Ntot atomic ratio in the sediments outcropping at a cliff in the south-eastern corner of the Lago Fagnano. It consists of muddy to sandy deposits arranged to form a succession about 8 m thick. The sand-mud interlamination of bottomset sediments testifies phases of active hydrodinamics and sediment availability that alternate with quieter periods characterized by the settling of mud by suspension. The abundance of symmetrical ripple profiles in the upper part of the bottomset (Unit 1), the presence of sharp and rounded crests, sigmoidal foresets and of local migration suggest the action of waves combined with currents producing traction on the bed (Yokokawa et al., 1995; Myrow et al., 2002). These bedforms, therefore, can be referred as wave and combined-flow ripples. Climbing patterns are referred to high rates of sediment supply. The observed alternation between high- and low-energy phases may reflect seasonal flood events, probably associated to ice melting, that were also responsible for the avalancing of the large-scale delta foresets. Floods triggered density flows along the delta foresets transporting sandy sediment in the bottomset area. The low average values of TOC (0.07%) and organic matter (3.03) are in accordance with this depositional context, mainly characterized by transport of sediments due to active hydrodinamics, a high sedimentation rate and a high average sand value (49.7%). The vertical sequence from the planar laminated Unit 3 to the rippled Unit 1 suggests a shallowing-upward trend, with the superposition of more energetic facies including traction structures on more distal and deeper facies accumulated below wave base. The described facies show some resemblance with those illustrated by Myrow et al. (2008) in prodeltaic systems. In Unit 2, the TOC average value is 0.21% and the organic matter average value is 8.93. These values testify a quieter period with settling of mud by suspension. The low values of TOC (0.07%) and organic matter (2.87) found in Unit 3 are probably correlated with an alteration of the organic matter content in a post diagenetic phase, as testified by the abundant oxidation found in the sand levels that are present in this unit.