Late Pleistocene to Holocene marine transgression and thermohaline control on sediment transport in the western Magellanes fjord system of Chile (53°S)

In the Western Strait of Magellan in southernmost Chile marine transgression occurred between 14,500 and 13,500 cal. BP. This is indicated by strongly increased accumulation of biogenic carbonate and first appearance of foraminifers in sediment records. From that time until 11,500 cal. BP, sedimentation in the western fjords became predominant autochthonous, due to higher salinity and clay flocculation, and Late Glacial glacier retreat. Present day thermohaline zonation pattern, extensively representative for the Holocene, and sedimentation rates indicate that westerlies hampered westward outflow of superficial (0–30 m water depth) glacial clay-rich freshwater from glaciated areas. During the Holocene, isostatic uplift of the Andes overcompensated sea level rise. In areas with high Glacial glacier loading this led to shallowing fjord sills and restricted exchange with marine water, especially since high freshwater inflow produced strong pycnoclines and preserved old saline water in fjord bottoms. To the east of the climate divide the Seno Skyring fjord system shows a year-round stable stratification, despite a strong wind-induced eastward superficial current in the upper 30–50 m of the water column. Such currents enabled significant Late Glacial eastward transport of glacial clay. Sediment cores from this area indicate that east-ward sediment flux slowed down during the Holocene, probably due to less intense westerlies. Investigated present day thermohaline characteristics of the fjord system across the superhumid climate divide of the Southern Andes indicate details of the exchange between marine and freshwater which are fundamental for evaluation of sediment transport pathways, biogenic productivity and interpretation of paleoclimate records in this area.