The contribution of peatlands to the study of Quaternary and geomorphological problems in Tierra del Fuego, southernmost Argentina. Past and on-going research.

CORONATO, A., RABASSA, J., ROIG, C.,HEUSSER, C., ROIG, F., BORROMEI, A., PONCE, F., FRANZÉN, L., XABIER PONTEVEDRA POMBAL, JUAN CARLOS NÓVOA MUÑOZ.

Ushuaia, Argentina

Workshop; IMCG Mires and peatlands Tierra del Fuego. Argentina.; 2005

International Mire Conservation Group

Fuegian peatlands contributes to Quaternary studies since the early and middle XX century. Scientists as Guido Bonarelli from Argentina and Väino Auer from Finland, made the first contributions considering geology, botany, morphology, stratigraphy, charcoal and tephra contents in peatlands all around Tierra del Fuego.  Since middle eighties, these are been thoroughly studied so far for pollen, including spore, total organic matter, volcanic ash and charcoal particles. Also, there are studied for the geomorphological reconstruction of deglaciation processes occurred during the late Pleistocene-Holocene in Tierra del Fuego and Staaten Island.  On going research includes, geochemical and microparticles studies, mostly volcanic ash and dust. Geomorphological and botanical studies of peatlands were included in inventory surveys that allows the characterization of basic components such us floristic diversity, including abundance and dominance, stratigraphy, depth, volume, shape, gradient, drainage, degree of conservation and anthropogenic influences. Detailed mapping of 847 Shpagnum bogs and Graminaceae mires, with size between 0.05 – 949 Ha. were mapped in central Tierra del Fuego. Also, in southern Tierra del Fuego, Sphagnum bogs and Cyperaceae and Bolax mires located in Carbajal-Tierra Mayor valley, and Astelia, Sphagnum bogs and Juncaceae and Bryales mires located in Moat were mapped.  These inventory surveys were done in order to know the distribution, quality and quantity of peat resources as a tool for the design of land planning strategies. Peat-bogs of Tierra del Fuego started their formation several millennia after the late Pleistocene glaciers begun their recession and melt water channels, ponds and shallow lakes were abundant in the landscape. Those peat-bogs developed in closed depressions among terminal morainic arcs in the bottom of the valleys are use to estimate the minimum age of ice recession. The basal peat layers, in contact with glaciolacustrine silts and clays, have radiocarbon ages such us 14,6  ka B.P., the oldest one in Harberton bog. Along the Beagle Channel, the basal age is progressively younger westwards. 14C dating of 12,7- 12 ka B.P. were found between Caleta Róbalo and Ushuaia, and 10 14C  ka B.P. in  Peninsula Ushuaia and Lapataia bogs. Basal ages of ca. 9 14C  ka B.P. in  Andorra and Pipo peat bogs point out a postglacial lacustrine phase occurred in the Andean inner valleys during the early Holocene. Besides glacial landforms sequences, palaeoclimatic aspects are also studied from peat-bogs. Fossil peat, intercalated in ice-marginal lacustrine – diatomite deposits sediments in lake Fagnano´s cliffs and dated at 38 14C  ka  B.P. and > 51 14C  ka B.P. brings the palaeoenvironmental conditions of  previous to the Last Glacial Maximum, established ca. 25 14C ka B.P. Pollen records show the complete absence of Nothofagus  pollen content and a tundra-shrubby steppe association fully established. Late Glacial climatic variations were interpreted based on pollen studies made on several bogs, located along the Beagle Channel coast and in the inner Andean valleys. Along the Beagle Channel, the climate was apparently warmer between 14.6-13 14C  ka B.P. and between 11.7-11.1 14C  ka B.P. and became cooler earlier than 13-12 14C  ka B.P. and from around 11.1 until 10.2-10 14C  ka B.P. The Nothofagus pollen signal shows also a fast forest recovery during the first deglaciation phases, sometimes before 16 14C  ka B.P. The basal peat of Harberton bog shows a large content of tree pollen between 14.6-14 14C  ka B.P. decreasing to two minimum values, the first one between 13-12 14C  ka B.P. and the second one  between 11-10 14C  ka B.P. separated by tree pollen content rise. This data is coincident also with Caleta Róbalo, Ushuaia 1, 2 and 3 and Lapataia bogs along the Beagle Channel and with Andorra and Carbajal bogs in the inner valleys. All these pollen bogs profiles indicate the existence of two cold and dry climatic events in this part of South America during the Late Glacial period. The Holocene environments are represented in peat-bogs records from different areas of Tierra del Fuego. They indicate the progressive occupation of the land firstly by the open forest and later to the present forest conditions. Several climatic oscillations have been record for this period, but the present climatic conditions for forest definitive installation have been established ca. 6 14C ka B.P. A delay of the definitive forest expansion is recorded in the forest-steppe contact zone showing continentality climatic conditions. Neoglacial climatic oscillations are not clearly identifiable from the available pollen records, though detailed studies in the upper valleys bogs are being carrying on. A warm period in the late Holocene was also identified from different proxy sources between AD 960-1020. Numerous tree-trunks in excellent preservation, found in ancient peat layers at the Carbajal and Monte Gallinero peat bogs, dated by 14C  between 260 – 3770 years B.P. allowed the definition of the first floating tree-ring chronologies obtained from buried wood in South America.             Geochemical studies on volcanic ash deposition episodes are being carried on in order to establish both, the influence of Patagonian volcanic processes in Tierra del Fuego landscape and in the peat-bogs development and evolution. The study of these lithogenic elements allow to define possible erosive events over the ancient mire surfaces as well as the origin, intensity and fingerprint geochemistry of the tephra phases, particularly during the Holocene. Moreover, they can serve as indicators of the natural variations in the atmospheric deposition fluxes. It seems to be that at least three major ash accumulation events and four minor ones were deposited in Harberton peat-bog during the Holocene. How did they interfered in the bog dynamics and evolution is a matter of present study. Nowadays, the analysis of a 9 m depth core from the Harberton interdrumlins confined raised bog, an 8 m depth core from the Puerto Rancho-Moat mixed raise-quaking bog, and a 7.5 m depth core from the Andorra valley raised bog is being analyzed for these purposes. In addition, the accumulative patterns of several trace elements during the last 14 ka B.P. are being analyzed in order to identify its natural or anthropogenic source and to establish the possible atmospheric pathways. Its biogeochemical behaviour could also allow the identification of climatic signals.             Fuegian peatlands store very important palaeoenvironmental and palaeoclimatic data for Southernmost South America. Therefore, a great effort is needed concerning peatland preservation, since these ecosystems shall work as continuous data archives in the future for future research.