Younger Dryas in southernmost south America — An update

Quaternary ScienceReviews,Vol. 12, pp. 351-355, 1993.

0277-3791/93$24.00 © 1993PergamonPressLtd.

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YOUNGER

DRYAS

IN SOUTHERNMOST

SOUTH

AMERICA

-- AN UPDATE

Vera Markgraf

Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309-0450, U.S.A.

Highresolutionpollendatafroma peat sectionin southernTierradel Fuegodocumentshighpaleoenvironmentalvariabilitybetween 11 ka and 9 ka. Detailedanalysisof plant macroremainsin the samesectionshowedthat the repeatedhigh-amplitudechangesin pollentaxa were relatedto repeatedfire disturbanceburning at least once even the surfaceof the mire. In manypeat sectionsfrom southernmost PatagoniaandTierradel Fuegocharcoalparticleshavebeenfoundintermittentlysinceabout 13ka and in greatabundanceespeciallyafter 11 ka. This suggeststhat the environmentalchangespreviouslyinterpretedto implya returnto colderclimatescomparableto the North Atlantic YoungerDryas event instead are in responseto local and regionaldisturbancesby fires.

~TRODUCTION Pollen analysis of a closely dated peat sequence from the eastern Beagle Channel in Tierra del Fuego (Harberto0, lat. 54°53'S, long. 67°10'W) (Fig. 1) shows repeated high frequency and high amplitude paleoenvironmental changes throughout the steppe dominated lateglacial portion of the record (Markgraf, 1991). High amplitude and asynchronous changes of herbaceous taxa are more likely to represent the local component in the pollen rain than the regional component. For this reason I suggested that the cause of the variability was not global, but local or regional (Markgraf, 1991). The environmental variability became ex~en more pronounced analysing pollen percentage changes at an average resolution of 45 years between samples for the interval between 11.5 and 9 ka. However, only after also studying the sediment components in detail was I able to document that fires were the possible cause of the environmental variability. In focusing exclusively on pollen influx changes during the 11 to 10 ka interval in the Caleta Robalo record (Isla Navarino, Beagle Channel, southern Tierra del Fuego) (Fig. 1) a different interpretation was presented by Heusser and Rabassa (1987) and Heusser (1989). They proposed that the decrease in pollen influx of all herb and shrub pollen types dated between 11.8 and 10.5 ka reflected a climatic cooling comparable to the amphi-North Atlantic Younger Dryas cold event and concluded that the event was therefore global. Three aspects render their interpretation doubtful. (1) The dates of the purportedly 'cooler' interval predate the type Younger Dryas interval. (2) Changes in pollen influx data may reflect changes in climate (by affecting pollen production) but also changes in sedimentation rate. In the case of the Caleta Robalo record, radiocarbon age/depth relation (Fig. 2) indicates a substantial change in sedimentation rate at that time. This suggests that the pollen influx changes in this case are unlikely to reflect climatic changes. (3) Interpretation of the Gramineae dominated pollen influx changes in terms of temperature contradicts the modern pollen/climate information. Following Heusser (1989), 351

Gramineae are essentially absent from the supposedly temperature sensitive Andean treeless vegetation but characterise the lowland steppe vegetation instead, which is moisture sensitive. Thus, the first order paleoenvironmental change in Gramineae proportions during that interval should indicate changes in moisture availability rather than temperature. To address this discrepancy in interpretation of past climate change and its cause, a different approach was needed. Foremost, because of the brevity of the interval temporal resolution between the sampling intervals needed to be increased from the previous resolution of 200 to 500 years. Dating control needed to be enhanced to better constrain the age of environmental changes. Finally, significance of environmental changes needed to be better understood, by searching for causes of the high variability.

RESULTS AND DISCUSSION The Harberton core with its fast sedimentation rate of 15 years cm 1, its well constrained chronology and its excellent preservation of peat components, lent itself for this study. The 1035 cm long section dating back to 14 ka (Fig. 2) is primarily composed of moss peat, with Sphagnum magellanicum dominant in the upper 600 cm, and different genera and species of brown mosses (Amblystegiaceae) below 850 cm; most of them showing aquatic growth modification. Between 850 cm and 600 cm, i.e. between 11 and 6 ka, the sediment is heterogenic, partially highly decomposed and generally containing only few macro-remains, primarily leaves and roots of Cyperaceae and rare mosses; fungal spores and fungal hyphae are abundant. The most highly decomposed intervals are between 850 to 835 cm, 812 and 784 cm, and 765 and 700 cm. In these levels microscopic charcoal fragments are abundant reaching between 100 and 200% of the total pollen sum. At one level between 812 and 807 cm microscopic charcoal reaches over 1000% of the pollen sum and macroscopic carbonised fragments are abundant including burnt mosses and sedge leaves. This implies that the surface of the mire itself must have burnt. The changes in pollen proportions during the 11.5 to 9 ka

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FIG.4. Charcoalrecordsfromseveralsitesin southernPatagoniaand Tierra del Fuego, after Heusser(1987) and Heusseret al. (1989). time interval (Fig. 3) affected essentially all taxa represented during the lateglacial. Gramineae, dominant in the record between 14 and 9 ka, showed the largest changes, from over 60% to 25% and back to 50% between 10.9 and 10.6 ka; Ericaceae (primarily Empetrum) shifted throughout the whole interval between 30 and 5% with several marked irregular peaks; herbaceous taxa such as Acaena (a speciesrich genus with most species characteristic of open ground and coarse substrate) that had essentially disappeared from its pre-11.5 ka high levels returned briefly at about 10.5 ka to values between 40 and 20%. Umbelliferae fluctuated between 2 and 15%, Gunnera and Plantago (open ground and wetland colonising taxa) showed single asynchronous peaks up to 15 %, as did Nanodea (a parasite on Ericaceae, primarily abundant on top of hummocks), Cyperaceae, fern spores, reaching in one level up to 80% and total pollen influx. The high amplitude changes essentially disappear with the increase of Nothofagus at 9 ka, which is taken to indicate the onset of substantially moister conditions. Apparently the clue to understanding the high paleoenvironmental variability between 11.5 and 9 ka can be found in the sediment. Dominance of brown mosses (cf. Calliergon/CalliergoneUa/Scorpidium) from about 14 ka onwards implies highly minerotrophic conditions with shallow pools or slowly running water. Appearance of fungal remains after about 11.5 ka, replacement of mosses by Cyperaceae and increase in decomposition of the organic materials suggests lower water levels and drying of the mire surface. At 10.8 ka, shortly after this change in peat composition high amounts of charcoal are repeatedly found in the sediment culminating at about 10 ka when the surface of the mire itself burnt. The local burning in turn must have had a direct impact on sediment composition an~growth, as well as on the local environments. Ashes and charcoal ~produced by burning result in a substantial increase in nutrients on the mire, increased alkalinity and increased microbial activity which in turn leads to greater decomposition of organic matter. Greater decomposition decreases sedimentation rates and hence increases pollen influx, shown by the irregular

radiocarbon dates between 1t and 10 ka and several incidences of strong increases in pollen influx (Fig. 3). The environmental changes (Fig. 3) apparently most directly related to fires are represented by decreases in Cyperaceae, permanent following the fire that burnt the mire surface, temporary decreases in Ericaceae and its parasite Nanodea, and irregular high-amplitude changes in open ground and disturbance indicators, such as ferns, Acaena, and Gunnera. From 9 ka onwards, when Nothofagus pollen increased and brown mosses (Limprichtia (Drepanocladus) revolvens) reappeared in the sediment jointly with increasing amounts of Sphagnum (cf. Sphagnum magellanicum) the direct impact of fires seems to have decreased. Continuing high amounts of Gramineae and repeated levels with charcoal until 6 ka suggest, however, that at least seasonally fires continued, although at a reduced level compared to earlier times. Presence of arboreal vegetation during the early Holocene, however, might exaggerate the actual frequency and/or intensity of fires compared to earlier times when the vegetation was treeless and thus the fuel availability was far lower. Analysis of charcoal in numerous peat sections from the high southern latitudes (Fig. 4) suggests that these fires were common and regionally extensive, primarily during the lateglacial and early Holocene and again during the late Holocene (after 2 ka)(Heusser, 1987). The oldest radiocarbon date for a major charcoal event comparable in amount to the local burn in the Harberton record is 13.3 ka from a peat layer in a section at Bahia Inutil (Heusser et al., 1989). Another major charcoal layer was dated 10.8 ka in a peat section from Torres del Paine (Heusser, 1987). Substantial differences in magnitude and timing of charcoal events in these peat records must be related to our finding that high charcoal amounts reflect primarily incidences when the bog itself was burnt. This, however, depends in large measure to the bog's local setting and topography and therefore is probably not occurring synchronously. This aspect makes the search for a cause of the fires difficult. Fires during the lateglacial steppe period could be due to natural causes, the result of greater frontal activity and lightning storms at a time when conditions appeared overall drier and markedly different from today. Volcanic eruptions were proposed as well as natural cause for fires (Heusser, 1987). However, there is little correlation between the timing of volcanic eruptions and fire events; especially during the intervals of high fire frequency such as between 11 ka and 10 ka no volcanic ash layers have been dated (Stern, 1990, 1992). Finally, fires could also be related to human activities. Archaeological information documents the presence of paleoindians in Tierra del Fuego and southern Patagonia from about 13 ka onwards (Markgraf, 1985; Bird and Bird, 1988; Massone, 1987). Reports from the time of European contact indicate that fire was used by the Patagonian indians as hunting strategy. However, without conducive climatic conditions man-set fires would probably not have been regionally as extensive as documented by the pollen and charcoal records. After 9 ka, when Nothofagus woodland expanded suggesting regional moisture increase, the continuing occurrence of substantial amounts of charcoal in peat records is more difficult to explain in terms of climate.

Younger Dryas in Southernmost South America

355

There are two possible explanations for this problem: (1) man REFERENCES may have been a factor in continuing to disrupt the natural environmental sequence; or (2) the continuing amounts of Bird, J.B. and Bird M. (1988). Travels and Archaeology in South Chile. (J. Hyslop, ed.), University of Iowa Press, 246 pp. charcoal exaggerate the fire frequency because of the greater Heusser, C.J. (1987). Fire history of Fuego-Patagonia. Quaternary of South fuel availability and ~fires actually declined. A clue to this America and Antarctic Peninsula, 5, 93-110. question of cause of fires may come from the study of fire Heusser~ C.J. (1989). Late Quaternary vegetation and climate of southern Tierra del Fuego. Quaternary Research, 31, 396-406. history from records from different latitudes in southern Heusser, C.J., Heusser, L.E. and Hauser, Y. (1989). A 12,000 yr B.P. tephra South America, where the climate history is different in layer at Bahia Inutil (Tierra del Fuego, Chile). Anales del lnstituto de la terms of timing of climate change and type of climate signal, Patagonia, 19, 39--49. while the history of human activities is thought to be Heusser, J.C. and Rabassa J. (1987). Cold climatic episode of Younger Dryas age in Tierra del Fuego. Nature, 328, 609-611. comparable. Markgraf, V. (1985). Late Pleistocene faunal extinctions in southern In conclusion, the detailed and high resolution study of Patagonia. Science, 228, 1110-1112. paleoenvironmental changes in the Harberton record Markgraf, V. (1991). Younger Dryas in southern South America. Boreas, 20, 63-69. suggests that the high environmental variability, especially in Massone, M. (1987). Los cazadores paleoindios de Tres Arroyos (Tierra del terms of pollen influx between 12.5 and 9 ka is apparently Fuego). Anales del Instituto de la Patagonia, 17, 47-60. due to local and regional disturbance by fires. Successively Rabassa, J., Heusser, C.J. and Rutter, N. (1991). Late-Glacial and Holocene of Argentine Tierra del Fuego. Quaternary of South America and more add conditions after 12.5 ka documented by changes in Antarctic Peninsula, 9, 327-351. peat components suggest climates markedly different from Stem, C.R. (1990). Tephrochronology of southernmost Patagonia. National today that could have been conducive to convective storms. Geographic Research, 6, 110-126. But other causes, volcanic activity and man cannot be Stem, C.R. (1992). Implications of tephra chronology for the age of human occupation in Tierra del Fuego. National Geographic Research and excluded as cause for the fires at this time. Exploration, 8, 239-240.

ACKNOWLEDGEMENTS Support for this research is acknowledged from the National Science Foundation, Climate Dynamics Program ATM-90-03839.