Climate vs. Tectonics at the end of the World: the Lago Fagnano Sedimentary Record, Tierra del Fuego, Southernmost South America.

Lacustrine sediments provide one of the best continental archives to reconstruct climate change. Additionally, many lakes are located in tectonically active regions and, therefore, their sediments also store regional tectonic events. Located at 55°S on the island of Tierra del Fuego, Lago Fagnano is the largest (~110 km long), southernmost non-ice covered lake in the world. It occupies the deepest basin in a chain of tectonic depressions along the Magallanes-Fagnano transform system (MFT). Ca. 800 km of reflection seismic data, comprising simultaneously-acquired single-channel 3.5 kHz (pinger) lines and multi-channel 1 in3 airgun lines, revealed an over 100 m-thick sedimentary succession. A preliminary seismic stratigraphic analysis of the high- resolution 3.5 kHz pinger data identified three major seismostratigraphic units defined by different seismic facies. While the lowermost unit is interpreted as product of subglacial to proglacial sedimentation probably deposited at the end of the Late Glacial period, the middle and uppermost unit represent an upward gradual change from a proglacial environment into typical lacustrine facies. The seismic stratigraphic analysis further allows us to recognize several mass-flows and megaturbidite events suggesting a substantial tectonic impact on the most recent lake sedimentation. The mass-flow frequency increases upward probably documenting the onset of Holocene hemipelagic-type sedimentation, thus indicating slope-sediment mobilization in the observed lateral slides. In contrast, Late Glacial mass flows were not initiated on the lateral slopes but rather originated in proglacial settings as reflected by the basinwide distributions of associated redeposits. The internal seismic architecture of the uppermost seismic unit coincides well with the petrophysical properties measured in the cores, allowing a precise core-to-seismic correlation. Preliminary sedimentological analyses of the cores show a diatom-rich and perfectly laminated sequence often interrupted by mass flow sediments that together provide a combined record of decadal changes in regional climate and a suite of tectonic events. The presence of tephras combined with radiocarbon datable remains guarantee a good chronology for this laminated sequence that will allow dating changes in the westerly winds, the Southern Ocean circumpolar flow, and the South Pacific gyre. Ongoing research of the older sedimentary record and comparison to other marine and continental climate archives will improve our understanding of the various forcing mechanisms behind climate change and will provide the first continuous record of tectonic activity for this area of the Southern Hemisphere.