Surficial deep-sea sediments across the polar frontal system (Southern Ocean, Indian sector): Particulate carbon content and microphyte signatures

Two main subjects are addressed in this study, concerning a complex Sub Antarctic hydrological system during the ANTARES I-MD75 cruise (Southern Ocean, Indian sector): (1) the δ13Corg signature in waters, fluffs and sediments and their relationships with anthropogenic effects, and (2) the degree of preservation of the deep-sea phytodetritus. A North to South gradient was observed for the deep-sea microphytes signatures, with the higher concentrations in the higher latitude: Chaetoceros spores (dominant species: Chaetoceros cf neglectus) were abundant in the fluffy layers from the Permanent Open Ocean Zone (POOZ), and associated with the highest Chl a concentration. The diatom spores were absent in the northern part of that transect. The coccolith repartition and preservation also showed a latitudinal gradient. The presence of coccoliths, more or less intact below the lysocline, may result from different sedimentation rates along this gradient. Undisturbed and thick “fluffs”, in the POOZ and southern Polar Frontal Zone stations, were enriched with coccoliths and Chaetoceros resting spores even at 4700 m, and the revival experiments demonstrated their recent origin, via a possible “in mass” or “in pulse” sedimentation. A latitudinal gradient clearly appeared for δ13Corg, POC and chloropigments, with a relationship between δ13Corg and Chl a, direct/positive for the phytoplankton and indirect/negative for the fluffs and sediments. These observed gradients are in part attributed to the latitudinal effects and to the physical characteristic of the water bodies. Differences between plankton and sediment δ13Corg signatures are attributed to an anthropogenic influence; however, diagenetic processes leading to slight δ13Corg enrichment in sediments, cannot be absolutely rejected at this stage. A stronger deep northward advection to the north of the Polar Front may also in part explain these North to South bio-geochemical deep-sea gradients.