Lake and fjord sediments as Late Glacial to Holocene environmental and climate archives of the southernmost Andes at 53°S, Chile

Environmental and climatic changes during the last 20,000 years were investigated along an east-west transect across the southernmost Andes (53°S) within the interdisciplinary project "Late and Post Glacial Environmental Conditions in the Range of the Southern Hemispheric Westerlies". Twelve coring locations in the different geographical, geological and vegetational zones of the transect, were selected as drill sites on the basis of parametric echo sounding sediment profiles in the fjords of the Gran Campo Nevado area. The stratigraphy of the sediment cores is based on 14C-AMS ages and tephrachronology. Sedimentological and geochemical characteristics were investigated in order to determine sediment sources and their pathways as well as climate and environmental changes. <br>The ice retreat after the Last Glacial Maximum (LGM) is marked in a sediment core from the eastern section of the proglacial lake Seno Skyring by an ice rafted debris layer (IRD), which was formed at around 18,700 to 17,800 cal. years B.P. This indicates a rapid response of the Seno Skyring glaciers to warming of the southeast Pacific soon after the LGM. However, between around 16,000 and 13,000 cal. years B.P. this core still shows relatively high amounts of Andean-derived glacial clay, characterized by high-MgO andesitic composition with low SiO2/Al2O3 ratios. This suggests a still extended glaciation of the Andes and a slowdown of the ice retreat. Further towards the Andes a core from an ancient glacial valley (Estero Vogel) contains an IRD layer formed at around 13,600 cal. years B.P, indicating a still extended glaciation at that time. Moreover, subaquatic ridges located in an echo sounding profile of Estero Vogel are interpreted as a moraine system, which can be related to a moraine system identified in the middle section of the Gajardo Channel. These moraines may be coeval with the moraine E system in the Strait of Magellan dated 15,043±607 to 12,275±201 cal. years B.P. by Clapperton et al. (1995). This relatively extended glaciation was possibly caused by a southward migration of the westerlies and the subsequent higher precipitation. Before the end of the Younger Dryas, at around 12,111±190 cal. years B.P., a rapid increase of the TOC content in the fjord and lake sediments indicates the establishment of vegetation and soils. Simultaneously, a decrease of the Andean glacial clay signature in all sediment cores of the Gran Campo Nevado area indicates a further strong ice retreat, probably partly produced by a further warming in the southeast Pacific. <br>The period between around 11,500 to at least 4500 cal. years is characterized in many sediment cores by high TOC contents, low sedimentation rates and little chemical variations. This reflects a climate optimum with a well&#8211;established Magellanic Rainforest and only little climate fluctuations. Evidence for pronounced Neoglacial conditions, as reported from some glaciers in the Andes (40° to 50°S) and the South Atlantic between 5000 and <1000 cal. years B.P. have not been found in the sediment records of the Andean transect. However, in the superhumid high mountain range (area of the Chandler Island and Muy Profundo Lake) the environment was severely affected by strong eruptions of the Mt. Burney volcano (4281±86 and around 2050±76 cal. years B.P.). Sediment records show that the older eruption produced extensive destruction of vegetation, followed by plant decay. This was probably due to long-term acidification derived from the SO2 from altered tephras. The fine ash of the younger eruption has probably introduced less sulfur and, in contrast to the older eruption, enhanced the plant growth cycle by nutrient input. Sediment cores also show some extreme precipitation events during this period and probably more unstable and colder climatic conditions. The Little Ice Age (LIA) of the last millennium is not evident in most of the sediment records. However, high sedimentation rates, high amounts of clay fraction and relatively low TOC contents in surface sediment cores from the western section of the transect reflect the strong glacier-derived input during the LIA and the present day.

Author dc:creator

• Baeza Urrea, Oscar

Contributors dc:contributor

• Behrmann, Jan H.

• Patagonien
• Patagonia, deglaciation, tephrachronology

Repository record source_url

https://freidok.uni-freiburg.de/data/2415

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:2415