Paleoecology of the Southern Chilean Lake District-Isla Grande de Chiloé during Middle-Late Llanquihue Glaciation and Deglaciation

Abstract

Subantarctic Parkland and Subantarctic-North Patagonian Evergreen Forest, embracing >40,000 14 C years of middle and late Llanquihue glaciation. are reconstructed from pollen contained in multiple interdrift deposits and cores of lake sediments. The subantarctic plant communities at low elevations have since been replaced by temperate Valdivian Evergreen Forest. Data in support of the vegetation reconstruction derive from close-interval sampling (>1400 pollen analysed stratigraphic levels) and high-resolution chronology (>200 AMS and conventional radiocarbon-dated horizons). Pollen sequences are from 15 sites, eight of which are exposures and seven mires, located in relation to lobes of piedmont glaciers that occupied Lago Llanquihue, Seno Reloncavi, Golfo de Ancud, and the east-central sector of Isla Grande de Chiloe at the northern limit of the Golfo Corcovado lobe. Recurring episodes of grass maxima representing Subantarctic Parkland, when grass and scrub became widespread among patches of southern beech (Nothofagus), bear a relationship to glacial advances. The implication of the maxima, prominent with advances at 22,400 and 14,800 14 C yr BP during late Llanquihue glaciation in marine oxygen-isotope Stage 2, is of successive intervals of cold climate with summer temperatures estimated at 6-8°C below the modern mean. The earliest recorded maximum at >50,000 14 C yr BP is possibly during late Stage 4. At the time of middle Llanquihue glaciation in Stage 3, cool, humid interstades on Isla Grande de Chiloe with Subantarctic Evergreen Forest, which under progressive cooling after 47,000 14 C yr BP was in) creasingly replaced by parkland. During stepwise deglaciation, when transitional beech woodland communities supplanting parkland became diversified by formation of thermophilous North Patagonian Evergreen Forest, warming in the order of 5-6°C was abrupt alter 14,000 14 C yr BP. Closed-canopy North Patagonian Evergreen Forest was established by 12,500 14 C yr BP. Later, after c. 12,000 until 10.000 14 C yr BP, depending on localion, forest at low elevations became modified by expansion of a cold-tolerant element indicative of ≤2-3°C cooler climate. This stepwise climatic sequence is seen at all late-glacial sites. Cool, humid interstadial conditions, punctuated by cold stadial climate, are characteristic of the last >40,000 14 C years of the Pleistocene at midlatitude in the Southern Hemisphere. Pollen sequences from southern South America and terrestrial-marine records from the New Zealand-Tasmania sector express a broad measure of synchrony of vegetational/climatic change for marine oxygen-isotope Stages 2-3. The data, combined with the timing of glacial maxima in the Southern Andes, Southern Alps of New Zealand, and in the Northern Hemisphere, are indicative of synchronous, millennial-scale, midlatitude climatic changes in the polar hemispheres.