Terrestrial geology and geophysics

temperature—was responsible for this environment, which Late Pleistocene/Holocene probably resembled the modern steppe-scrub where annual paleoclimates from precipitation is about 200 millimeters. During this interval, several proportional shifts among the steppe components sug- subantarctic latitudes gest minor shifts in moisture conditions, from initially very dry, to somewhat wetter between 12,500 and 11,000 years ago, and back to dry until 9,000 years ago. VERA MARKGRAF Between 9,000 and 5,000 years ago, Nothofagus pollen in- creased, but steppe elements continued, although much re- Institute of Arctic and Alpine Research duced. The environment might have resembled the modern University of Colorado forest-steppe transition, where precipitation is about 400 mil- Boulder, Colorado 80309 limeters annually. The modern environment of dense Not hofagus forest became A palynological record from the eastern reaches of the Beagle established between 5,000 and 3,000 years ago following a Channel in Tierra del Fuego (Harberton, 54°53S 67°10W, 50 short-term phase with high Empetrum, that was probably re- meters elevation) contains a detailed regional history of pa- lated to local drying conditions in the bog. leoenvironmental and paleoclimatic changes for the last 14,000 Paleoclimatic implications. Late Pleistocene climates in the sub- years. Because the climates in these subantarctic latitudes are antarctic latitudes were characterized by over 50 percent re- greatly affected by conditions in Antarctica, specifically the duction in precipitation, compared to today, suggesting that drift-ice patterns that in turn alter the latitudinal position and the storm tracks of the southern westerlies did not affect these strength of the moisture-bringing southern westerlies (Mark- latitudes in the same intensity as today. Hence, the southern graf 1989), paleoclimatic changes in Tierra del Fuego allow edge of the southern westerlies must have been displaced inferences on antarctic paleoclimates. Regional setting. The Harberton site (figure 1) is a raised Sphagnum peatbog with abundant Empetrum heath growing on the drier areas. It is located beyond (east) of a prominent group of endmoraines, considered to represent the last glacial max- 51° imum ice extent (Porter, Stuiver, and Heusser 1984). Dense deciduous Nothofagus putnilio forest is the surrounding vege- tation, with Nothofagus antarctica, Chiliotrichium diffusion (Corn- 52° positae), Berheris buxifolia, and other shrub taxa at the forest margins. Annual precipitation averages about 500 millimeters, evenly distributed throughout the year, and the mean annual 530 temperature is about 7°C. In regions of higher precipitation (1,200 to 5,000 millimeters), along the western reaches of Tierra del Fuego, a fern-rich evergreen Nothofagus betuloicles forest 540 grows. It is replaced by treeless Magellanic moorland with its characteristic cushion bog plants (Astelia, Donatia, Gaimardia) Empetrum and heath, along the wind-swept westernmost is- 550 lands. In regions of less than 400 millimeters precipitation, east of the Andes, forest is replaced by herbaceous steppe and steppe-scrub represented by Gramineae, Compositae, a di- verse herbaceous flora and, in extremely dry conditions, Em- I I I 75 0 730 710 69° 670 650 petrum (Moore 1983).

Paleoenviron mental history. Paleoenvironmental conditions were Magellanic Moorland Deciduous Forest E interpreted from changes in pollen assemblages in the Har- Andean Vegetation (Nothofagus pumilio, N.antarctica) berton record (figure 2), using the modern environmental and 6A Evergreen Forest climatic parameters described. (Nothofagus betuloides) EJIJ Patagonian Steppe From 14,000 years ago to shortly after 9,000 years ago, tree- less assemblages dominated, with Gramineae, Compositae, Empetrum, and herbaceous taxa. Presence of herbaceous taxa Figure 1. Map of southern South America with major vegetation of steppe affinity (Acaena, Artemisia, Plan tago, Chenopodiaceae, units and sites discussed in text. Star: Harberton. 1: La Mision. 2: etc.,) and absence of either Andean grassland or Magellanic Yehuin. 3: Isla Clarence. 4: Cueva Milodon (Markgraf 1983). 5: Cal- moorland taxa suggest that lower precipitation—and not lower eta Robalo (Heusser 1989).

1989 REVIEW equatorward from its present position as a consequence of late Holocene and modern conditions, however, effective greater extension of antarctic drift ice at that time (Markgraf moisture during the early Holocene was lower. Judging from 1989). All pollen records from the subantarctic latitudes, east a record at latitude 50°S (Puerto Eden; Ashworth, Markgraf, and west of the Andes, show and late-glacial paleoclimates and Villagran unpublished data) that indicates that the wes- (Markgraf 1983) lasting until after 9,000 years ago [not 10,000 terlies had returned to their modern location by 9,000 years years ago as Heusser (1989) states]. This suggests that the ago, the relatively "drier" climates in the early Holocene in subantarctic latitudes emerged substantially later from late- Tierra del Fuego are probably not due to lower precipitation glacial conditions than did the mid-latitudes of southern South but to warmer temperatures instead. This interpretation could America (Markgraf 1983). relate to the results from antarctic ice cores that suggest tem- Early Holocene climates were characterized by higher effec- perature maximum was reached in the early Holocene (Jouzel tive moisture than the late-glacial climates. Compared to the et al. 1987). This study was supported by National Science Foundation grant DPP 86-13980.

HARBERTON (54°53S, 67010W)

e References s00 o C Heusser, C.J. 1989. Late Quaternary vegetation and climate of south- ern Tierra del Fuego. Quaternary Research, 31, 396-406. Jouzel, J., C. Lorius, J.R. Petit, C. Genthon, N.I. Barkov, V.M. Kotly- akov, and V.M. Petrov. 1987. Vostock ice core: A continuous isotope

400 temperature record over the last climatic cycle (160,000 years). Na- ti ture, 329, 403-408.

00 Markgraf, V. 1983. Late and postglacial vegetational and paleoclimatic changes in subantarctic, temperate, and and environments of Ar- - 8,670±130 gentina. Palynology, 7, 00) 43-74. - 11,3000200 Markgraf, V. 1989. Reply to C.J. Heussers "Southern Westerlies dur- ing the Last Glacial Maximum." Quaternary Research, 1000 - 3,3601280 31, 426-432. Moore, D.M. 1983. Flora of Tierra del Fuego. England: A. Nelson.

0 40 60 0 2 4 6 0 4 ± 12 16 0 2 4 0 20 40 60 Porter, S.C., M. Stuiver, and C.J. Heusser. 1984. Holocene sea-level perce 111 changes along the Straits of Magellan and Beagle Channel, south- ernmost South America. Quaternary Research, 22, 59-67.

HA±RBERTON (54 0 53 1 s 670I01W)

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Figure 2. Pollen diagram of Harberton peat section with major taxa in percentage of total pollen. Cyperaceae, Lycopodium, fern spores, and Potamogeton are excluded from pollen sum. (cm denotes centimeter. yr B.P. denotes years before present.)

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