Deglacial palaeoclimate at Puerto del Hambre, subantarctic Patagonia, Chile

JOURNAL OF QUATERNARY SCIENCE (2000) 15 (2) 101–114 Copyright  2000 John Wiley & Sons, Ltd.

Deglacial palaeoclimate at Puerto del Hambre, subantarctic Patagonia, Chile ´ N MOREIRA M.4 CALVIN J. HEUSSER1,*, LINDA E. HEUSSER2, THOMAS V. LOWELL3, ANDRE´S MOREIRA M.4 and SIMO 100 Clinton Road, Tuxedo, New York 10987 USA 2 Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964 USA 3 Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221 USA 4 Montana 7516, Vitacura, Santiago, Chile

1

Heusser, C. J., Heusser, L. E., Lowell, T. V., Moreira M., A. and Moreira M., S. 2000. Deglacial palaeoclimate at Puerto del Hambre, subantarctic Patagonia, Chile. J. Quaternary Sci., Vol. 15, pp. 101–114. ISSN 0267–8179. Received 26 April 1999; Revised 8 July 1999; Accepted 12 July 1999

ABSTRACT: The primary objective of this study is to further substantiate multistep climatic forcing of late-glacial vegetation in southern South America. A secondary objective is to establish the age of deglaciation in Estrecho de Magallanes–Bahι´a Inu´til. Pollen assemblages at 2-cm intervals in a core of the mire at Puerto del Hambre (53°36⬘21⬙S, 70°55⬘53⬙W) provide the basis for reconstructing the vegetation and a detailed account of palaeoclimate in subantarctic Patagonia. Chronology over the 262-cm length of core is regulated by 20 AMS radiocarbon dates between 14 455 and 10 089 14C yr BP. Of 13 pollen assemblage zones, the earliest representing the Oldest Dryas chronozone (14 455–13 000 14C yr BP) records impoverished steppe with decreasing frequencies and loss of southern beech (Nothofagus). Successive 100-yrlong episodes of grass/herbs and of heath (Empetrum/Ericaceae) before 14 000 14C yr BP infer deglacial successional communities under a climate of increased continentality prior to the establishment of grass-dominated steppe. The Bølling–Allerød (13 000–11 000 14C yr BP) is characterised by mesic grassland under moderating climate that with abrupt change to heath dominance after 12 000 14C yr BP was warmer and not as humid. At the time of the Younger Dryas (11 000–10 000 14C yr BP), grass steppe expanded with a return of colder, more humid climate. Later, with gradual warming, communities were invaded by southern beech. The Puerto del Hambre record parallels multistep, deglacial palaeoclimatic sequences reported elsewhere in the Southern Andes and at Taylor Dome in Antarctica. Deglaciation of Estrecho de Magallanes– Bahı´a Inu´til is dated close to 14 455 14C yr BP, invalidating earlier dates of between 15 800 and 16 590 14C yr BP. Copyright  2000 John Wiley & Sons, Ltd. KEYWORDS:

deglacial vegetation; palaeoclimate; chronology; hemispheric correlation.

Introduction Deglacial palaeoclimate in southern South America, embracing the cold climate reversal of Younger Dryas age, has been subject to long-standing controversy (Peteet, 1995). Controversy originated when it was alleged that whereas pollen data implied an interruption of late-glacial warming (Auer, 1958; Heusser, 1966), glaciers in the Andes Mountains did not advance but continued to retreat at the time of the Younger Dryas (Mercer, 1970, 1976). Fossil beetle

* Correspondence to: C. J. Heusser, 100 Clinton Road, Tuxedo, New York 10987, USA Contract tration Contract Contract Contract

grant sponsor: U.S. National Oceanic and Atmospheric Adminisgrant number: NA 77 RJ 0453 grant sponsor: U.S. National Science Foundation grant sponsor: Office of Naval Research

assemblage data likewise implied uninterrupted warming during deglaciation (Ashworth and Hoganson, 1984; Hoganson and Ashworth, 1992). Interpretation of a late-glacial climatic reversal from the pollen data also drew criticism: fluctuations of indicator taxa were considered to be the result of unrelated local disturbance, edaphic influence and fire (Markgraf, 1991, 1993). Non-uniformity in the data was later explained by weak and variable climatic forcing (Heusser et al., 1996). Widely spaced samples in sediment cores collected for analysis and dating in some of the early studies contributed to the failure to recognise the Younger Dryas. Recognition of palaeoclimate events requires greater detail on a submillennial scale. Replicate, chronologically closely controlled, high-resolution pollen records constructed in the absence of fire at mid-latitude in Chile (Heusser et al., 1999; Moreno, 1997; Moreno et al., 1999) and independently generated observations of glacier behaviour in the Southern Andes (Ariztegui et al., 1997; Marden, 1997; McCulloch and Bentley, 1998) have since made a strong case for climatic variability.

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That there have been multistep, late-glacial changes in the vegetation as a consequence of climate forcing requires further substantiation, especially at higher latitudes of Chile– Argentina. Subantarctic Patagonia – Tierra del Fuego, thus far, has not benefited from palaeoecological studies exercising a combination of tight stratigraphical and chronological control (Markgraf, 1983; Heusser and Rabassa, 1987; Heusser, 1989, 1993a, 1995, 1998; Clapperton et al., 1995). Stratigraphical data are spaced mostly at 10 cm, and radiocarbon dates, generally at intervals of ⱖ1000 radiocarbon years, average less than two at 12 late-glacial sites. For close-interval pollen analysis and chronology from subantarctic Patagonia reported in this paper, late-glacial sediments in the mire at Puerto del Hambre were cored in 1998. The site was chosen for detailed study from preliminary results obtained after reconnaissance coring in 1980 and recoring in 1988 (Heusser, 1984, 1995). The main objective was to test the hypothesis that vegetation has responded to climate variability and, if so, how, to what extent, and when. Palaeoecological data from Puerto del Hambre are of significance in relation to controversial climatic evidence, not only from southern Chile and Argentina but also from elsewhere in the Southern Hemisphere. This is particularly so in Antarctica, where on-going debate stemming from stable isotope records in ice cores concerns synchrony versus asynchrony of climate variability in the polar hemispheres (Sowers and Bender, 1995; Broecker, 1997, 1998; Steig et al., 1998). A secondary objective of our study was to attempt to resolve the problem having to do with age of the site, which reflects the timing of deglaciation in Estrecho de Magallanes – Bahι´a Inu´til. Dates of 15 800 ± 200 (Heusser, 1984), 16 590 ± 320 (Porter et al., 1992), and 16 290 ± 140 14C yr BP (McCulloch and Bentley, 1998) are older than expected. Deglaciation from within the glaciated area occupied by Puerto del Hambre is otherwise dated between 14 260 and 13 280 14C yr BP (McCulloch and Bentley, 1998).

Puerto del Hambre Location and environment The mire at Puerto del Hambre (53°36⬘21⬙S, 70°55⬘53⬙W; 6.25 ± 0.5 m altitude) is located on Penı´nsula Brunswick, some 100 m inland from the Estrecho de Magallanes, about 50 km south of Punta Arenas (Fig. 1). Elongate in outline between low ridges trending in a northwest–southeast direction, the mire covers an area estimated at 0.7 ha. Mounds of the moss Sphagnum magellanicum, their drier crowns overgrown by crowberry heath (Empetrum rubrum), dominate its surface. Sphagnous and fibrous peat in the upper part of the mire are underlain by lacustrine sediments. Disturbed southern beech woodland in the vicinity is a mixture of Nothofagus pumilio/N. antarctica, the principal members of deciduous forest rising to 500 m in altitude north of Puerto del Hambre, and of N. betuloides, the dominant of hyperhumid evergreen forest reaching 350 m to the south (Pisano, 1973, 1977; Moore, 1983). Above the treeline, where on Penı´nsula Brunswick altitudes are mostly ⬍ 1000 m, vegetation according to Moore (1975) is made up of cushion heath (Bolax gummifera), dwarf shrub heath (Empetrum rubrum–Pernettya pumila), feldmark (Nassauvia lagascae), and alpine meadow (Abrotanella linearifolia, Caltha appendiculata, Plantago barbata). At low altitudes, Copyright  2000 John Wiley & Sons, Ltd.

deciduous forest passes into steppe north of Punta Arenas and in northern Tierra del Fuego (Moore, 1983). Steppe grassland of Festuca gracillima is extensive among communities consisting principally of Empetrum heath and scrub (the composites Lepidophyllum cupressiforme and Chiliotrichum diffusum). At Puerto del Hambre, mean temperature in summer (January) is approximately 10°C and in winter (July) 2°C, annual precipitation 650 mm, and cloudiness between 5.6 and 6.6 octaves of sky cover (Zamora and Santana, 1979). Puerto del Hambre is located along a climatic gradient constrained by airflow of the westerlies aligned with the Estrecho de Magallanes between the Cordillera Darwin and the Segunda Angostura (Fig. 1). Greater continentality, as becomes evident toward the Atlantic, is responsible for the change from evergreen to deciduous forest and, at length, formation of the Patagonian steppe. Mean summer temperature along the gradient increases from 9°C to 12°C coupled with a decrease in annual precipitation from 2000 to 300 mm (Tuhkanen, 1992).

Glaciation Ice flowing from its principal source in the Cordillera Darwin (2400 m) was directed northward in the Estrecho de Magallanes, with a branch penetrating Bahı´a Inu´til (Fig. 1). At its maximum, glaciation, as originally mapped by Caldenius (1932), reached the Segunda Angostura midway in the strait, ⬎ 200 km distant from the cordillera, and extended beyond the eastern shore of the bay. Successive limits of ice-fronts inside the maximum at distances of 170 and 100 km were tied to the formation of a series of ice-dammed lakes. Except for the need to establish a modern chronology for the events, this basic mapping has since undergone little modification (Raedecke, 1978; Uribe, 1982; Porter et al., 1984, 1992; Porter, 1990; Prieto and Winslow, 1994; Anderson and Archer, 1999). The current status of the chronology is summarised by Clapperton et al. (1995) and updated most recently by McCulloch and Bentley (1998). Both the outermost and proximal stands of the glacier at Segunda Angostura (Fig. 1) appear to date after about 27 800 and before about 23 600 14 C yr BP. The successively younger advance to a position just north of Punta Arenas took place before 14 260 14C yr BP, whereas the advance of ice fronts in the Estrecho de Magallanes and Canal Whiteside to the northern end of Isla Dawson is constrained by dates between 12 010 and 10 050 14 C yr BP.

Methods Multiple cores of the late-glacial body of the mire at Puerto del Hambre were taken with a 5-cm diameter, square-rod piston sampler (Wright, 1967). Coring at depth began below a horizon of grey estuarine silt deposited during a marine incursion at between 7980 and 3970 14C yr BP (Heusser, 1984). Cores were air freighted directly to the core repository at Lamont-Doherty Earth Observatory for archiving and stored under refrigeration prior to sampling. Using 1-cmdiameter plastic tubes, the working half of core HE98–1C was sampled every 2 cm for pollen extraction and loss-onignition and carbonate measurements. Total length of core J. Quaternary Sci., Vol. 15(2) 101–114 (2000)

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Figure 1 Location of the mire at Puerto del Hambre, Penι´nsula Brunswick, subantarctic Patagonia. During the last glaciation, ice fronts (indicated by solid lines at the outer border of the small-circle pattern) were positioned as far as Segunda Angostura in the Estrecho de Magallanes and beyond and near the eastern shore of Bahι´a Inu´til; following recession and readvance, the ice front stood in the vicinity of Punta Arenas and at midpoint in Bahι´a Inu´til (Clapperton et al., 1995). Termini during the late-glacial ultimately were located at the northern end of Isla Dawson before retreating to the Cordillera Darwin (McCulloch and Bentley, 1998). Outer part of the lowland reached by glaciers during successive advances is marked by the small-circle pattern; existing glaciers in the cordillera are shown heavily stippled. Based on Mapa Geolo´gica de Chile (Servicio Nacional de Geologι´a y Minerι´a, 1982). Copyright  2000 John Wiley & Sons, Ltd.

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(not including section breaks) is between 78 and 340 cm at depths below surface of 534–816 cm. Pollen and spore identifications were made by use of a modern reference collection and of published descriptions and keys (Heusser, 1971; Villagra´n, 1980). Pollen frequencies (%) of tree and shrub/herb taxa are from sums mostly of ⱖ 300 grains; frequencies of aquatic seed plants and vascular cryptogams derive from sums of ⬎ 300 total pollen and spores. Pollen zonation adheres to the late-glacial numbering system (Zones PH-5–PH-9) used previously (Heusser, 1995) to categorize assemblages of ecologically significant taxa, namely, Nothofagus, Gramineae, Empetrum/Ericaceae, Acaena, Caltha, Gunnera, Euphrasia, Nanodea, Plantago, and Liguliflorae and Tubuliflorae (Compositae). Pollen densities (grains g−1 dry weight) of the principals (Nothofagus, Gramineae, Empetrum/Ericaceae) and total pollen (non-aquatic) were calculated following oven drying at 105°C for 24 h. Charcoal, where present, was recorded as ␮m2 g−1. Methods used follow standard procedures (Berglund, 1986; Faegri et al., 1989). Plant nomenclature is according to Moore (1983). After completion of pollen frequency and density diagrams of the core, samples were selected from 20 key levels for AMS radiocarbon dating. With seeds and fruits in short supply, fibrous plant remains, leaf fragments, indeterminate plant detritus and organic silt served as datable material. To remove black amorphous particulates ⱕ 90 ␮m, suspected of having been eroded from nearby outcropping coal beds of Tertiary formations, datable organics were concentrated on 120-␮m nylon screens after washing. Although particulates were observed under the microscope only in samples from the lowermost 40 cm of the core, all samples for dating were similarly processed. The particulates possibly account for the ambiguous ages of basal sediments at Puerto del Hambre. Dates reported as 14C yr BP by the NSF-Arizona AMS Facility were calibrated (cal yr BP) using INTCAL98 (Stuiver et al., 1998).

Puerto del Hambre core HE98-1C Lithology and chronology Core HE98-1C at depth consists of olive-grey, finely laminated fibrous silt, which rests at 343 cm on grey, gritty, massive silt overlying diamicton (Fig. 2). Stratigraphically higher, the laminated silt changes to light green and shows evidence of disturbance before becoming olive brown and terminating at a depth of 270 cm. The remaining upper part of the core is formed by fibrous silt and dark brown fibrous peat. Interbedded are olive-brown silt at 128–148 cm and a white, 2-mm-thick tephra layer at 230 cm. The tephra layer, dated to 12 840 14C yr BP from an underlying sample, is assigned by way of electron probe analysis to an eruption of Volca´n Reclu´s, located about 350 km northwest of Puerto del Hambre (McCulloch and Bentley, 1998). The layer, as a regional stratigraphical marker, is exposed along the northwestern shore of Bahı´a Inu´til (Fig. 1), samples from below and above the exposure date, respectively, to 12 060 and 12 010 14C yr BP (Heusser et al., 1989–1990; Stern, 1992). Loss-on-ignition at ⬍ 10% in the lowermost laminated silt increases at upper levels in the lacustrine unit to about 25%, before rising to 50% in the fibrous silt. Above the tephra layer, values in the fibrous peat increase steadily to maxima between 85 and 90%, except where a decrease occurs in Copyright  2000 John Wiley & Sons, Ltd.

the silt horizon at 200 cm depth. No comparable trends are recognisable in the carbonate measurements (Fig. 2), all of which are ⬍ 10%. Maximum carbonate values appear to cluster in association with the laminated silt lowermost in the core. The age model for core HE98-1C derives from radiocarbon dates (Table 1) that range between 14 455 and 10 089 14C yr BP. A plot of the dates (Fig. 3) shows a close linear relationship of increasing age with depth to 12 975 14C yr BP at 240 cm (average sedimentation rate, 17.2 yr cm−1). Dates thereafter are at variance, except for those of 14 251, 14 204 and 14 455 14C yr BP, respectively, at 306, 310 and 332 cm. Measurements of ␦13C at dated lower levels contrast with those from the upper part of the core (Table 1). Low amounts of organic matter, shown by loss-on-ignition, and sources of sediments possibly explain the differences.

Pollen frequency Pollen zones PH-9 through to PH-5 displayed in the frequency diagram of core HE98-1C (Fig. 4, Table 2) record assemblages of trees, shrubs and non-aquatic herbs, levels of zonal boundaries and radiocarbon dates. Contributing to the remains of aquatic seed plants in the core, Myriophyllum quitense is featured early in the record during the lacustrine phase in Zones PH-9 through to PH-7a; beginning in Zone PH-7b, variable amounts of Cyperaceae are consistent with the fibrous peat phase. A transient presence of Nothofagus (Zone PH-9) followed the onset of deglaciation at an estimated 14 700 14C yr BP. Initially with a frequency at 48%, Nothofagus later underwent rapid decline, giving way to Acaena at ⬎ 50% in association with Gramineae–Tubuliflorae. Not until four millennia later (Zones PH-5b and PH-5a) are comparable frequencies of Nothofagus again recorded; intervening levels, at no more than 1–2% Nothofagus, are sporadic. The contribution of Empetrum/Ericaceae (Zone PH-8b), with as much as 84% of the pollen sum, was of short duration, lasting for 100–200 radiocarbon years until about 14 250 14C yr BP. Subsequently, Gunnera–Acaena– Empetrum/Ericaceae–Liguliflorae (Zone PH-8a) bridges an additional short-term interval leading up to assemblages dominated by the Gramineae (Zones PH-7c–PH-7a). The dating, as it stands, is not secure, as the upper boundary set at 12 247 14C yr BP is much too young. The plot of age versus depth (Fig. 3) implies a boundary date closer to 14 000 14C yr BP. Spanning an estimated 2000 radiocarbon years (14 000– 12 000 14C yr BP), Zones PH-7c–PH-7a cover the dominance of Gramineae at as much as ⱖ 50%. Mixed with successive assemblages consisting of Acaena, Tubuliflorae, Liguliflorae, Caltha, Euphrasia and Plantago, the Gramineae are most prominent at 70% in Zone PH-7a. Later (12 000–11 000 14C yr BP), an Empetrum/Ericaceae assemblage (Zone PH-6a) with frequencies of ⬎ 90% lasted some 800 radiocarbon years, preceded by a transitional interval of approximately 200 radiocarbon years (Zone PH-6b) containing peak Nanodea (61%) and Plantago (19%). After 11 000 14C yr BP during the last millennium of record, Gramineae (Zone PH-5e) initially gain a maximum 75%, with Acaena (48%), in sharp contrast to high frequencies of Empetrum/Ericaceae (Zone PH-6a). Peak Gramineae (62%) together with Caltha (43%) and Nanodea (17%) in Zone PH 5d are prevalent until the abrupt shift to Empetrum/Ericaceae (78%) in Zone PH-5c at 10 665 14C yr J. Quaternary Sci., Vol. 15(2) 101–114 (2000)

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Figure 2 Lithology, loss-on-ignition and carbonate measured in core HE98-1C.

Table 1 Ages of sediment samples in core HE98-1C, Puerto del Hambre, subantarctic Patagonia, Chile

Depth (cm)

Core depth (cm)

Material dateda

Age 14 C yr BP

Age cal. yr BPb

␦13C PDB (‰)

Laboratory number

540 574 590 594 600 606 610 638 650 671 685 711 751 776 782 786 796 802 808 812

84 104 120 124 130 136 142 168 180 200 214 240 280 300 306 310 320 326 332 336

Fibrous Fibrous Fibrous Fibrous Detritus Detritus Detritus Detritus/fibrous Detritus/fibrous Fibrous Detritus/fibrous Detritus Detritus/silt Detritus/silt Detritus/silt Detritus/silt Organic silt Leaf fragments Organic silt Organic silt

10 089 ± 74 10 400 ± 95 10 440 ± 60 10 665 ± 95 10 796 ± 77 10 993 ± 73 11 070 ± 85 11 844 ± 77 12 017 ± 88 12 325 ± 82 12 532 ± 81 12 975 ± 83 13 106 ± 84 12 247 ± 126 14 251 ± 91 14 204 ± 124 13 865 ± 85 13 625 ± 80 14 455 ± 115 11 834 ± 186

11 690 12 550 12 600 12 820 12 880 13 000 13 100 13 830 14 090 14 260 15 150 15 600 15 750 14 270 17 070 17 040 16 640 16 360 17 320 13 830

−28.013 −28.328 −28.513 −28.128 −28.405 −28.52 −27.631 −27.558 −27.21 −27.706 −26.896 −19.864 −20.65 −20.909 −12.001 −17.604 −15.712 −12.978 −10.582 −22.482

AA30633 AA30634 AA30635 AA30636 AA30637 AA30638 AA30639 AA30640 AA30641 AA30642 AA30643 AA30644 AA30645 AA30646 AA30647 AA30648 AA30649 AA30650 AA30651 AA30652

a

Sample ± 1 cm interval centred at core depth. From intcal98 extended 14C calibration set (Stuiver et al., 1998).

b

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Figure 3 Age–depth plot of AMS radiocarbon dates in core HE98-1C.

BP. The succession of short-term assemblages that follows surrounds the rise of Nothofagus to 60%, with a variable complement of Gramineae and Empetrum/Ericaceae, including Gunnera (33%) in Zone PH-5b and Acaena (19%) in Zone PH-5a.

Pollen density As a means of assessing independent behaviour of important pollen taxa and detecting trends over the length of core HE98–1C, densities of Nothofagus, Gramineae, Empetrum/Ericaceae and total non-aquatic pollen were calculated (Fig. 5). Results at stratigraphical levels subdivided on a millennial scale are according to depth in the core and the chronology established for pollen frequency (Fig. 4 and Table 2). Before 14 000 14C yr BP (Zones PH-9–PH-8a), densities totalling ⱕ 2000 g−1 are quite low early in the record. Contributed mainly by Empetrum/Ericaceae, values later rise to 31 000 g−1. No indication of density change for either Nothofagus or Gramineae is evident, as might be implied by their peak frequencies in Zone PH-9 (Fig. 4). During the interval between 14 000 and 13 000 14C yr BP (Zone PH7c), Gramineae with maxima of 22 000 and 27 000 g−1 at two levels account primarily for corresponding totals of 37 000 and 39 000 g−1. These totals are not much different than amounts reached before 14 000 14C yr BP. At 13 000–12 000 14C yr BP (Zones PH-7c–PH-7a) is the first expression of a trend towards increasingly higher densities. As in Zone PH-7c through to Zone PH-7a in the frequency diagram (Fig. 4), Gramineae gain prominence, achieving a maximum of 107 000 g−1 and contributing to Copyright  2000 John Wiley & Sons, Ltd.

the bulk of the record. Total amounts as high as 140 000 g−1 represent a considerable increase compared with the previous millennium. Continuing the trend of increasing density set by the Gramineae, Empetrum/Ericaceae values at 12 000–11 000 14C yr BP (Zones PH-6b–PH-6a) greatly exceed amounts set earlier. At a maximum of 691 000 g−1, Empetrum/Ericaceae are the major contributors to total pollen density of 760 000 g−1, the largest amount measured over more than four millennia of record. Their peak number during the interval is coincident with frequency maxima in Zone PH-6a (Fig. 4). Total densities after 11 000 until 10 000 14C yr BP (Zones PH-5e–PH-5a) decrease, at first abruptly, to minima of 20 000 and 33 000 g−1, equalling amounts registered before 14 000 14C yr BP. Later, densities rise to a high of 714 000 g−1, contributed primarily by Gramineae and secondarily by Empetrum/Ericaceae, enriched, in addition, by steadily increasing Nothofagus to a high of 180 000 g−1. Density fluctuations and trends are a gross reflection of frequency data over the same interval (Fig. 4).

Discussion Deglacial chronology, vegetation, and palaeoclimate at Puerto del Hambre Radiocarbon dates of between 15 800 and 16 590 14C yr BP ascertained from cores taken at Puerto del Hambre (Heusser, 1984; Porter et al., 1992; McCulloch and Bentley, 1998) are now considered to be unreliable because of conJ. Quaternary Sci., Vol. 15(2) 101–114 (2000)

Figure 4 Pollen frequency diagram of core HE98-1C subdivided by pollen assemblage zones showing AMS radiocarbon dates, charcoal, black particulates and tephra layer. Date of 12 840 beneath the tephra layer is taken from McCulloch and Bentley (1998).

C yr BP

14

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Table 2 Pollen assemblage and chronostratigraphical data for Puerto del Hambre, Chile

Pollen zone

PH-5a PH-5b PH-5c PH-5d PH-5e PH-6a PH-6b PH-7a PH-7b PH-7c PH-8a PH-8b PH-9 a

Pollen assemblage

Depth in core (cm)

Nothofagus–Gramineae–Empetrum/Ericaceae– Acaena–Nanodea Gramineae–Empetrum/Ericaceae–Acaena–Gunnera–Nothofagus Empetrum/Ericaceae Gramineae–Caltha–Nanodea Gramineae–Acaena Empetrum/Ericaceae Nanodea–Plantago–Empetrum/Ericaceae–Gramineae Gramineae–Euphrasia–Plantago Gramineae–Caltha–Acaena–Euphrasia–Plantago Gramineae–Acaena–Tubuliflorae–Liguliflorae Gunnera–Acaena–Empetrum/Ericaceae–Liguliflorae Empetrum/Ericaceae Nothofagus–Acaena–Gramineae–Tubuliflorae

14

C yr BP

78–104

10 000a 10 400

104–120 120–124 124–130 130–142 142–168 168–180 180–200 200–214 214–300 300–306 306–320 320–340

10 500 10 665 10 796 11 070 11 844 12 017 12 325 12 532 14 000a 14 251 14 375 14 700a

Estimated from average age–depth plot (Fig. 3).

tamination by older carbon. Contaminant in the form of black particulates totalling ⬎12 ␮m2 g−1 × 106 represents palynodebris (Boulter, 1994) found in the lowermost 40 cm of core (Fig. 4). The presence of reworked palynomorphs associated with the particulates strengthens the view that local Tertiary beds are the source of the allochthonous contaminant. Tertiary palynomorphs identified (Cyathidites, Phyllocladidites, Podocarpidites, Nothofagidites and Tricolpites, and dinoflagellate cysts) are similar to those figured by Fasola (1969) from the Loreto Formation of Oligocene– Miocene age at nearby Punta Arenas (Fig. 1). Palynomorphs are distinguishable from late-glacial pollen by differential staining, thicker exines and generally modified morphology. Cyathidites and Podocarpidites, moreover, are unrepresented in the existing regional flora, and taxa resembling Phyllocladidites are extinct in Chile–Argentina. Dates as old as 14 455 14C yr BP reported in this paper (Table 1) are allied with the age of deglaciation from within the glaciated area at before 14 260 14C yr BP (McCulloch and Bentley, 1998). The dates are also compatible with other deglacial ages of 14 640 14C yr BP at 54°52⬘S (Heusser, 1998), 14 335 14C yr BP at 46°25⬘S (Lumley and Switsur, 1993), and 14 900 14C yr BP at 42°38⬘S (Heusser et al., 1999). According to Denton et al. (1999), the age of the glacial maximum reached earlier in the Chilean Lake District–Isla Chiloe´ (41°00⬘-42°30⬘S) is between 14 900 and 14 800 14C yr BP. Deglacial vegetation inferred by the initial pollen assemblage at Puerto del Hambre (Zone PH-9, Fig. 4) contained Nothofagus in association with steppe taxa, Gramineae, Acaena and Tubuliflorae. The assemblage on the basis of the chronological control represents a time span apparently of no more than a few hundred radiocarbon years. Immediate presence of Nothofagus in amounts as high as 48% is to be questioned, however, because of the low pollen density (Fig. 5). Quantities could be overrepresented, enhanced by long-distance atmospheric transport under conditions of low pollen production by other taxa making up the assemblage. Nothofagus possibly occupied small enclaves in the steppe under locally adaptive, moderating conditions but its presence immediately following wastage of the Estrecho de Magallanes–Bahı´a Inu´til glacier is interpreted as limited and transient. Communities may compare in their make-up with the southwestern steppe of Tierra del Fuego, which contains Copyright  2000 John Wiley & Sons, Ltd.

scattered Nothofagus pumilio and N. antarctica, with representative species Festuca gracillima, Acaena ovalifolia and Chiliotrichum diffusum (Moore, 1983). Amounts of Nothofagus pollen in the steppe of Fuego–Patagonia at present are comparable to amounts in Zone PH-9 (Heusser, 1995). Considerable reshuffling of plant communities, all under comparatively depauperate conditions implied by low pollen density, evidently took place over a duration of several hundred radiocarbon years through Zone PH-8a at an estimated 14 000 14C yr BP. Treeless communities, characterised successively by Gramineae, Acaena, Empetrum/Ericaceae and Gunnera, became differentiated, invading substrates in a deglacial setting of variable edaphic stability. Disturbed laminations, noted in the lower part of core HE98-1C (Fig. 2), infer an added disturbance factor, although local slumping due to compaction or melting of subsurface ice could be involved. Interpretation of climate from the data is made difficult by the forcing of vegetation by unrelated non-climatic factors, as well as by the presence of an array of taxa exhibiting broad ecological amplitudes. A key to climatic reconstruction during initial deglaciation at Puerto del Hambre is possibly to be found in the presence and distribution of the aquatic, Myriophyllum quitense (Fig. 4), which in Tierra del Fuego, according to Moore (1983), grows in water bodies and slowmoving drainage in the dry steppe and forest-steppe ecotone at altitudes below treeline between sea-level and 300 m. Based on the distribution of Myriophyllum, the inference is that Nothofagus early in the record was locally present. That Nothofagus later (Zone PH-8b) failed to persist may be explained by a change to climate less humid and insufficiently wet to support the presence of arboreal communities. The change in climate is indicated by a corresponding drop in Myriophyllum frequency, coupled in this instance with peak frequency of Empetrum/Ericaceae. Empetrum by its distribution in the steppe of Tierra del Fuego (Moore, 1983) likewise infers less precipitation. Owing to a greater land mass, while late-glacial sea-level was lower than present, climate was apparently drier and more continental with short, cool summers and cold winters. Climate afterwards (Zone PH-8a), dry and cold, remained unfavourable for Nothofagus, while frequencies of Myriophyllum increased. Intervals of summer drought, indicated by intermittent low frequencies of Myriophyllum, may have J. Quaternary Sci., Vol. 15(2) 101–114 (2000)

Figure 5

Pollen density diagram (grains g−1 dry weight) of Nothofagus, Gramineae, Empetrum/Ericaceae and total pollen in core HE98-1C.

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been critical, so as to inhibit the presence of Nothofagus. Through much of Zone PH-7c, limited cover of grassdominated steppe is implied by relatively low pollen densities until about 13 000 14C yr BP (Fig. 5). Communities included scrub (sensu Moore, 1983), consisting of woody composites, both Liguliflorae and Tubuliflorae, Empetrum heath and Acaena, in the present flora of which A. pinnatifida and A. magellanica are important. After 13 000 14C yr BP late in Zone PH-7b, steadily rising pollen densities, principally of Gramineae, reflect greater expansion of steppe. Demise of Myriophyllum (Zones PH7b–PH-6b, Fig. 4) can be equated with a more positive water balance in the basin at Puerto del Hambre, brought on by a less dry climate. Replacing Myriophyllum are semiaquatic Cyperaceae, associated with mire plants, Caltha, Euphrasia, Plantago and Nanodea. The implication from these data is of more mesic steppe. A late stage in the sequence is the Empetrum/Ericaceae assemblage (Zone PH-6a), which for some 800 radiocarbon years until 11 017 14C yr BP gained dominance. Its high frequency (Fig. 4) and pollen density (Fig. 5), unparalleled in the record for so long an interval, are unusual and indicative of factors involving long-term control. It needs to be noted that semi-aquatic Cyperaceae, as well as other mire species, were virtually excluded by Empetrum/Ericaceae. The exceptional abundance of Empetrum/Ericaceae can be interpreted as an expansion of heath communities not only at the mire but also on surrounding upland. Empetrum, ubiquitous on mires characterised by low exchange capacity and pH, is found on upland of coarse-textured glacial drift with low base saturation. Soils in Tierra del Fuego investigated by Collantes et al. (1989) along a gradient from mesic grassland through heath–grass to eroded heathland show a decrease in pH, base exchange, mineralisation, and plant cover accompanied by a increase in C/N ratios. Where Empetrum heath is found today in Fuego–Patagonian steppe (Moore, 1983), summer temperatures are estimated on average 1°–2°C higher and annual precipitation about 400 mm lower compared with present-day conditions at Puerto del Hambre (Tuhkanen, 1992). Climate during the interval sustained extended flowering of Empetrum/Ericaceae with resultant high pollen density in the record. At first viewing, the abrupt increase of Empetrum/Ericaceae in Zone 6a appears to be related to silt interbedded at 128– 148 cm (Fig. 4). Neither the abrupt increase nor equally abrupt termination of Empetrum/Ericaceae, however, is apparently related, as the lower boundary of Zone 6a rests below the silt, and the upper zonal boundary lies within the silt. The lithological unit, in part overlapping the Empetrum/Ericaceae assemblage, is more closely related to the Gramineae-dominant assemblage in Zone PH-5e and rise of Gramineae, which begins in upper Zone PH-6a. The age of the silt corresponds to the timing of glacial readvance adjacent to Puerto del Hambre (Fig. 1), which is bracketed by maximum and minimum dates of 12 010 and 10 050 14C yr BP (McCulloch and Bentley, 1998). Origin of the silt, although speculative, may be tied to the readvance. There is no indication that the silt results from late-glacial marine incursion. Transgression of the mire by the sea, as recorded by Porter et al. (1984), was a Holocene event. Gramineae dominance, in conjunction with semi-aquatic Cyperaceae and Caltha in Zones PH-5e–PH-5d (Fig. 4) and with low pollen density (Fig. 5), implies a return to a cooler, more humid climate not unlike conditions noted in Zone 7a. It is reasonable to assume that increased levels of humidity accompanied by cooler conditions implied by these data account for late-glacial readvance of the ice to a position Copyright  2000 John Wiley & Sons, Ltd.

proximal to Puerto del Hambre. Fluctuation of climate was short-term, until 10 665 14C yr BP at the upper boundary of Zone PH-5d, followed by an overall rise in temperature and with humidity adequate to support open communities of Nothofagus containing light-dependent Filicinae. The fern, Blechnum penna-marina, possibly represented in this instance, is common in understories of present-day, open stands of Nothofagus; also noted is the earliest occurrence of the seed plant, Misodendrum, which as an epiphytic parasite attacks Nothofagus. Assemblage changes, as expressed in hundreds of radiocarbon years or less by peaks of Empetrum/Ericaceae, Gunnera and Acaena (Zones PH5c–PH-5a), are assumed to reflect a variety of successional sequences on the deglacial landscape. Similar assemblages following glacier recession were recorded earlier at Puerto del Hambre (Zones PH-9–PH-8a). Fire, evident from charcoal in sediments of Zone PH-5a, may have been a factor causing reversal in the rising trend of Nothofagus from 60% to 30% at uppermost levels (Fig. 4). Storm frequency apparently intensified in the latitude of Puerto del Hambre when times of Nothofagus expansion and glacier activity indicate a greater supply of moisture. Likewise, protracted intervals of comparative drought associated with steppe expansion infer infrequent storm activity associated with weaker atmospheric circulation. Over the course of oceanic–atmospheric reorganisation (Broecker and Denton, 1990), positions of storm tracks varying during deglaciation ultimately became concentrated at 50°S. Rising temperatures brought about general wastage of the glacier complex in the Southern Andes, as noted by the closely compatible set of dates between 14 900 and 14 260 14C yr BP (Lumley and Switsur, 1993; Heusser, 1998; McCulloch and Bentley, 1998; Heusser et al., 1999). Deglacial climates in subantarctic Chile–Argentina, interpreted from selected pollen data with optimal chronological control, adhere with some modification to the classical northwest European scheme of late-glacial climates set forth in chronozones (Mangerud et al. 1974; Walker, 1995): Oldest Dryas, ⬎ 13 000 14C yr BP, cold, moderating climate; Bølling–Allerød, 13 000–11 000 14C yr BP, climate moderated but variable; and Younger Dryas, 11 000–10 000 14C yr BP, cold climatic reversal. Adherence to the European scheme at 41°38⬘S on Isla Chiloe´ in Chile is shown by deglacial plant communities diversifying at about 13 500 14C yr BP (Oldest Dryas), with optimum warmth indicated by peak frequency of myrtle (Myrtaceae) at 12 500 14C yr BP (Bølling–Allerød), preceding cooling (Younger Dryas) coincident with maximum podocarp (Podocarpus nubigena), which dates between about 11 500 and 10 500 14C yr BP (Heusser et al., 1999). At higher latitudes, besides the data from Puerto del Hambre, there is an apparent parallel with Canal Beagle close to 55°S in Argentina, where fluctuations of both pollen frequency and influx of Nothofagus before 14 000 until 10 000 14C yr BP show climatic trends similar to the European sequence (Heusser, 1993b, 1998). Lumley and Switsur (1993), on the other hand, dismiss a late-glacial climatic reversal at the time of the Younger Dryas at 46°25⬘S on the Penı´nsula de Taitao. Compared with the Chilean records and unlike the glacier record from the Southern Alps (Basher and McSaveney, 1989; Denton and Hendy, 1994; Ivy-Ochs et al., 1999), New Zealand pollen sequences fail to reveal late-glacial climatic reversal (McGlone, 1995; Singer et al., 1998). Deglaciation of the Southern Andes occurred as a multistep process, apparently synchronous with the pattern recorded in the North Atlantic and in other parts of the Northern Hemisphere (Lowell et al., 1995; Denton et al., J. Quaternary Sci., Vol. 15(2) 101–114 (2000)

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1999). In the Chilean Lake District–Isla Chiloe´ sector (41°– 43°S), glacial maxima were reached at 22 800–22 300 14C yr BP and at 14 900–14 800 14C yr BP. Deglaciation followed the latest maximum, with ice fronts receding into the interior Andes until a final late-glacial advance of Younger Dryas age. The advance on Monte Tronador (41°15⬘S) in the Argentine Andes dates to 11 400–10 200 14C yr BP (Ariztegui et al., 1997); an advance of glaciers emanating from the South Patagonian Icefield at Torres del Paine (51°S) in Chile is bracketed by dates of 11 880 and 9180 14C yr BP (Marden, 1997); and in the Estrecho de Magallanes (53°35⬘S) in Chile, the ice front came forward between 12 010 and 10 050 14C yr BP (McCulloch and Bentley, 1998). Of considerable interest in the Chilean Lake District–Isla Chiloe´ sector is the latitudinal variation of the last glacial maximum at 14 900–14 800 14C yr BP, which at 41°S stood within limits of the ice front at 22 500–22 300 14C yr BP, whereas at 43°S, the last was the greater of the two maxima. Storm tracks of the southern westerlies shifting and impacting differential amounts of snow in the Andes may explain, as in this case, the contrast in behaviour of adjacent glacier systems. Because of latitudinal proximity of ice fronts, system temperature regimes need not have varied significantly. Implied from both terrestrial and marine records, zonal atmospheric circulation identified with the southern westerlies occupied both a wider and variable latitudinal berth during the last ice age (Heusser, 1990; Lamy et al., 1998, 1999).

Southern Ocean and Antarctica: polar hemispheric synchrony versus asynchrony At issue, besides the apparent inconsistency of multi-step deglaciation at higher latitudes of the Southern Hemisphere, is the interhemispheric synchrony versus asynchrony of climatic events. Palaeoclimate reconstructions from stable isotopes in ice cores in Antarctica and cores of marine sediments from the Southern Ocean exhibit both in-phase and out-of-phase relationships. Jouzel et al. (1987, 1995) in cores from Antarctica (Byrd, Dome B, Dome C) observed that the course of deglaciation differed from that in Greenland. Cold climate reversal (Antarctic Cold Reversal) occurred an estimated 1000–1500 calendar years earlier than the Younger Dryas in Greenland ice cores. These observations have been corroborated since by other ice-core data from Antarctica (Byrd, Vostok) by Sowers and Bender (1995) and Blunier et al. (1998). Similarly, recorded in marine cores from the Southern Ocean (Labracherie et al., 1989; Charles et al., 1996), the timing of the climate reversal imposes a pattern of asynchrony and an earlier response to changes in atmospheric–thermohaline oceanic circulation by comparison with the Northern Hemisphere. A mechanism is needed to account for what Broecker (1997, 1998) refers to as a ‘polar seesaw’, the antiphasing of deglacial warming and cooling in the polar hemispheres. During deglaciation, in order to effect the series of thermal changes, deep-water formation may have alternated between the Southern Ocean and North Atlantic, but the cause for the possible scenario is not at hand. The issue is further complicated by ice-core isotope records from Taylor Dome in coastal Antarctica (77°48⬘S, 158°43⬘E). Taylor Dome palaeotemperatures in contrast to the interior indicate inphase, interhemispheric deglacial synchrony (Steig et al., 1998). By way of explanation for the disharmony, these authors suggest that climate in coastal Antarctica may be Copyright  2000 John Wiley & Sons, Ltd.

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responding to the flow of North Atlantic deep water. Penetration of the Southern Ocean by the water mass during the Bølling–Allerød is associated with warming of circumpolar deep water and, in turn, the atmosphere, resulting in conditions that were not possible during the comparatively cold Younger Dryas, when flow of North Atlantic deep water was inactive. Of significance is the fact that climatic trends at Taylor Dome and Puerto del Hambre are not dissimilar (Fig. 6). Steig et al. (1998) establish their chronology at Taylor Dome from the Greenland GISP2 ice core, which according to Alley et al. (1993) places the Bølling–Allerød chronozone between 14 600 and 12 900 calendar yr BP and the Younger Dryas chronozone between 12 900 and 11 600 calendar yr BP. Warming during the Bølling/Allerød followed by Younger Dryas cooling is interpreted at Taylor Dome by the pattern set by the stable isotope D. The ␦D stratigraphical record as a temperature proxy compares well with trends set at Puerto del Hambre by frequencies of the principal pollen types, southern beech, grass and heath, and by total pollen density. Apparent synchrony between Puerto del Hambre and Taylor Dome palaeoclimatic reconstructions (Fig. 6) infers that these locations during deglaciation were subject to the same oceanic–atmospheric forcing. Both sites are coastal and may be responding to warming and cooling as heat transfer from the tropics to the Southern Ocean via North Atlantic deep-water circulation is switched on and off (Steig et al., 1998). Asynchronous interior locations at Vostok and Byrd in Antarctica, evidently under a different set of climatic controls during deglacial millennia, record an earlier timing for both the beginning of warming and climatic reversal (Antarctic Cold Reversal). At present, according to Taljaard (1972), there is marked asymmetry in the atmosphere surrounding the South Pole, which features an anticyclone dominant over East Antarctica (Vostok) and a cyclone over West Antarctica (Byrd Station). Cyclonic centres are coastal and not likely to move into the continental interior. Of possible significance in the context is their highest concentration in the eastern Ross Sea in the vicinity of Taylor Dome. To what extent location and differential atmospheric circulation are effecting the contrast among ice-core records from Antarctica is unclear. Admittedly the problem of synchrony versus asynchrony in and between the polar hemispheres is a complex issue, but as White and Steig (1998) point out, the matter of timing can be resolved only from a greater array of core locations.

Conclusions Deposition at Puerto del Hambre began at approximately 14 455 14C yr BP following an advance of the Estrecho de Magallanes–Bahı´a Inu´til glacier, which independently is dated at around 14 260 14C yr BP. Previous older basal dates for the mire at between 15 800 and 16 590 14C yr BP, apparently caused by the presence of infinitely aged carbon, are no longer acceptable. Invading vegetation before 14 000 14C yr BP during the Oldest Dryas chronozone at Puerto del Hambre possibly contained southern beech. Despite significant frequencies, pollen density is so low as to imply long-distance atmospheric transport. Rapid succession in the course of centuries, first by grass–herbs and later by heath, proved any early presence of beech to be only short-term. The implication J. Quaternary Sci., Vol. 15(2) 101–114 (2000)

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Figure 6 Summary pollen diagram of southern beech, grass–herbs and heath in core HE98-1C subdivided on a millennial 14C scale and in relation to Taylor Dome, Antarctica, stable isotope ␦D ice-core stratigraphy (Steig et al., 1998). Chronozones and chronology in calendar yr BP used by Steig et al. (1998) are from Alley et al. (1993).

from the community series with low pollen density values is of impoverished steppe under continental palaeoclimate with cool short summers, cold lengthy winters and protracted dryness. Heath by its extended distribution in the steppe of Tierra del Fuego is supremely indicative of increased seasonal drought. By its ability to adapt to unstable edaphic conditions, heath was a primary invader of newly deglaciated terrain. Grass–herb steppe became dominant by 13 000 14C yr BP at the close of the Oldest Dryas chronozone. Pollen density remained low under a continuing cold and relatively dry palaeoclimate in the early part of the Bølling–Allerød. By its distribution in the present-day steppe, the aquatic, Myriophyllum, in place since the beginning of sedimentation, serves as an indicator of semi-arid steppe vegetation. At about 12 500 14C yr BP in the Bølling–Allerød, when grass– herb frequencies varied little, replacement of Myriophyllum by semi-aquatic Cyperaceae, together with a conspicuous increase in pollen density, suggest a more positive water balance in the basin at Puerto del Hambre, as well as the spread of more mesic species in the steppe. Dramatic increase in frequency and density of heath over the last millennium of the Bølling–Allerød until 11 000 14C yr BP captures apparent burgeoning heath communities resulting from an apparent shift to warmer, drier continental palaeoclimate. Flowering of heath with resultant high pollen density attained a level unprecedented in the record. Where heath is found today in Fuego–Patagonian steppe, summer temperatures are estimated on average 1°–2°C warmer and annual precipitation about 400 mm lower than at Puerto del Hambre. Signalled initially by high frequency and increased density of grass–herbs, return of grass steppe with colder, more Copyright  2000 John Wiley & Sons, Ltd.

humid climate occurs at the time of the Younger Dryas chronozone. Total pollen density during succeeding centuries, by contrast, is remarkably low and does not again increase until after 10 796 and before 10 665 14 C yr BP. The colder climate with higher but variable levels of humidity apparent at the beginning became sufficiently moderated later to support invasion of the steppe by southern beech. Variability is noted by peaks of Cyperaceae, Acaena, Caltha, Empetrum/Ericaceae and Gunnera. Low pollen density from the early part of the Younger Dryas characterises impoverished steppe communities, corroborating similar evidence of impoverishment in Tierra del Fuego. Deglacial vegetation in subantarctic Puerto del Hambre developed as a series of phases forced in the main by climate changes that bear a parallel to the climatic sequence expressed by European chronozones. The multi-phase process is tied in with multi-step deglaciation, involving recession of the ice during warming of Bølling–Allerød age and readvance during cooling at the time of the Younger Dryas chronozone. Acknowledgements This study, part of the Lamont-Scripps Consortium for Climate Research, was supported by the U.S. National Oceanic and Atmospheric Administration, Office of Global Programs (grant NA 77 RJ 0453). R. Lotti, Curator, and P. Priore archived Puerto del Hambre cores for sampling at the Lamont-Doherty Earth Observatory Deep-Sea Sample Repository, Palisades, NY, the core archive supported by grants from the U.S. National Science Foundation and Office of Naval Research. A. J. T. Jull, NSF-Arizona AMS Facility, reported the 20 AMS radiocarbon dates for core HE98–1C. The study was completed (CJH/LEH) at the Godwin Laboratory, University of Cambridge, through the courtesy of M. A. Hall, Senior Technical Officer, and N. J. Shackleton, Director. Thanks are extended to these persons and organisations for their assistance, J. Quaternary Sci., Vol. 15(2) 101–114 (2000)

DEGLACIAL PALAEOCLIMATE AT PUERTO DEL HAMBRE G. H. Denton and W. S. Broecker for arranging support for this study, and K. D. Bennett and H. Hooghiemstra for their in-depth reviews of the manuscript.

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