Late Holocene multi-proxy records of environmental change on the South Atlantic island Tristan da Cunha

Palaeogeography, Palaeoclimatology, Palaeoecology 241 (2006) 539 – 560 www.elsevier.com/locate/palaeo

Late Holocene multi-proxy records of environmental change on the South Atlantic island Tristan da Cunha Karl Ljung ⁎, Svante Björck, Dan Hammarlund, Lena Barnekow GeoBiosphere Science Centre, Quaternary Sciences, Lund University, Sölveg. 12, SE-223 62 Lund, Sweden Received 20 November 2005; received in revised form 19 April 2006; accepted 7 May 2006

Abstract Sediment stratigraphies from three sites, one lake, one overgrown lake, and an exposed section, on the island Tristan da Cunha in the temperate South Atlantic were analysed by means of pollen analysis, total carbon, nitrogen and sulphur content determination, magnetic susceptibility measurements and detailed radiocarbon dating. The aim of these studies was to reconstruct the late Holocene vegetation and climatic variations. The oldest sediment sequence extends back to 2300 cal. years BP. The vegetation was relatively stable up to the arrival of humans in the 17th century. The appearance of the introduced taxon Rumex acetosa/acetosella at c. 300 cal. years BP and a subsequent decline in forest cover on the lowland plain provide evidence of substantial human influence on the vegetation well before the establishment of the first permanent settlement in the 19th century. Before the first anthropogenic influence centennialscale fluctuations in the proxy records are interpreted as reflections of local hydrological changes, probably caused by variations in precipitation. As inferred mainly from changing proportions of pollen derived from telmatic and terrestrial taxa and corresponding changes in the deposition of mineral matter by fluvial erosion, lake levels were low between c. 1450 and 1050 cal. years BP, and high between c. 1050 and 300 cal. years BP. These variations coincide with known climatic changes in Southern Africa and in the North Atlantic, suggesting that the inferred hydrological changes on Tristan da Cunha were related to large-scale variations in the general oceanic and atmospheric circulation in the Atlantic region. © 2006 Elsevier B.V. All rights reserved. Keywords: Late Holocene; Pollen analysis; Vegetation history; Hydrological changes; Tristan da Cunha; South Atlantic Ocean

1. Introduction The South Atlantic circulation plays an important role in the global climate system and is coupled to the climate of the bordering continents and the North Atlantic region (e.g. Wefer et al., 2003). However, studies of Holocene environmental change in the South Atlantic are hampered by the lack of suitable terrestrial archives. Islands are few and many are situated in areas ⁎ Corresponding author. Tel.: +46 46 2227888; fax: +46 46 2224830. E-mail address: [email protected] (K. Ljung). 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.05.007

dominated by permanent high-pressure cells with low precipitation, and few archives such as peat or lacustrine sequences occur. The stratigraphic resolution of most marine sediment cores obtained from the South Atlantic Ocean outside the diatom ooze belt is often too low to portray detailed information about Holocene changes below the centennial scale (e.g. Nielsen et al., 2004). In this study results from three sites, two sediment cores from crater lakes and one sediment section exposed by fluvial erosion, on the island Tristan da Cunha in the central South Atlantic are presented. The aim is to reconstruct the local environmental history of

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the island by investigating well-dated peat and lake sediment sequences with a multi-proxy approach. Pollen analysis, mineral magnetic analyses, and measurements of total carbon, nitrogen and sulphur content have been performed. The use of a several proxies and several sites makes it possible to distinguish between environmental changes caused by climatic variation from other processes, e.g. volcanic activity. Tristan da Cunha is situated in a strategic position in the South Atlantic Ocean, and the results are of importance for the understanding of the late Holocene climate history of the South Atlantic region. 1.1. General environmental setting and previous studies The Tristan da Cunha island group is situated approximately 2800 km west of Cape Town, South Africa (37° 05′ S, 12° 17′ W) (Fig. 1). It consists of the main island Tristan da Cunha, the two smaller islands Inaccessible and Nightingale approximately 40 km to the southwest, and several smaller islands and islets (Fig. 1). The islands are composite volcanoes formed on a hot-spot situated close to the Walvis Ridge east of the Mid Atlantic Ridge (Smith, 1993). Tristan da Cunha is almost circular with a diameter of approximately 12 km. The island is surrounded by steep precipices known as The Cliffs. The interior of the island is dominated by a symmetric volcanic cone, The Peak at 2067 m above sea level (m a.s.l.), which is surrounded by gently sloping terrain, The Base. Deep ravines radiate out from the

volcano across The Base. In the north and south of the island lowland plains occur. Potassium–argon dating has established that the island is young. The dates range from 0.01 ± 0.02 Ma to 1.1 ± 0.15 Ma (Gass, 1967; Mc Dougall and Ollier, 1982; Dunkley, 2002). Lava from the top of The Peak is dated to 0.05 Ma, which probably reflects the last time of activity in the summit crater (Mc Dougall and Ollier, 1982). Scattered on the island are numerous smaller parasitic volcanic cones, many of which are of Holocene age (Baker et al., 1964). The islands were discovered by, and named after the Portuguese explorer Tristão d'Acunha in 1506 (Wace, 1969). The Portuguese never established a settlement on the islands and they remained uninhabited until the end of the 18th century when Dutch and English sealers and whalers seasonally occupied the islands to harvest the rich populations of elephant seals and fur seals. In 1816 the British Navy established a garrison on Tristan da Cunha to prevent attempts to liberate Napoleon from his imprisonment on Saint Helena (Wace, 1969). The garrison was closed in 1817, after which a few of the men stayed on the island and founded the permanent settlement on a gently undulating fringe of the island called The Settlement Plain (Fig. 1). Since then the settlement has persisted to the present with the exception of 1961–1962 when the complete population was evacuated to Great Britain during a volcanic eruption threatening the village. Today the settlement holds a population of approximately 285 persons.

Fig. 1. Map showing the location of Tristan da Cunha (a), the island group (b), and the location of the studied sites (c).

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Tristan da Cunha has a moist temperate climate, with very small annual variations in temperature and rainfall (Höflich, 1984). The mean annual temperature at sea level is 14.3 °C, with daily variations of 4–5 °C, based on measurements from 1973 to 1983. The extreme daily maximum is 23 °C and the extreme daily minimum 3.0 °C. Snow lies on the peak between May and October. Occasionally snow reaches down to around 600 m a.s.l., but at sea level the temperature never falls below freezing (Dickson, 1965). The mean annual precipitation at The Settlement Plain is 1468 mm. Data from a Norwegian expedition in 1938 indicate that the precipitation varies considerably on the island due to orographic effects and that higher areas receive up to three times as much precipitation as The Settlement Plain (Christophersen and Schou, 1942).

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The vegetation shows a clear altitudinal zonation, roughly corresponding to the main topographical features (Wace and Holdgate, 1958). The coastal plains are at present covered by heavily grazed grasslands (Fig. 2). At the time of settlement the lowland plains were covered by island-tree (Phylica arborea) and tussock grass (Spartina arundinaceae and Poa flabellata) (Wace and Holdgate, 1958). Carmichael (1819) noted that in 1816 when the British settlers arrived there were already clearings on the northern plains of the island, presumably made by seasonal occupants in the late 18th and early 19th century. The Cliffs and the lower parts of The Base are mostly covered by dense P. arborea scrub, which reaches 3– 5 m in height. Associated with the Phylica forest is the tree-forming fern Blechnum palmiforme. The ground

Fig. 2. View from the top of the Hillpiece volcanic cone over Hillpiece Bog and the heavily grazed grass-land of The Settlement Plain. Phylica arborea scrub grows on the cliffs in the background. Hillpiece Bog is the lighter area in the lower left corner.

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vegetation is dominated by the ferns Drypoteris aquilina and Blechnum penna-marina. Large areas of The Base are completely covered by peat-forming B. palmiforme, which reaches about 1 m in height with a very dense canopy that limits the ground vegetation (Fig. 3). On The Base, P. arborea only grows in sheltered positions such as ravines and depressions. Peat deposits with Sphagnum sp. and sedges are abundant on The Base, especially in the southeastern parts. The upper part of The Base is dominated by heath vegetation with Empetrum rubrum, grasses and the introduced herb Rumex acetosella. The higher exposed scree slopes of The Peak are almost devoid of vegetation. In total, Tristan da Cunha holds 114 angiosperm taxa, of which 82 are introduced and 29 pteridophyta taxa (Wace and Dickson, 1965).

The first scientific expedition to visit Tristan da Cunha was a French expedition that landed on the island in 1793. Their work mostly concerned the biology of the islands (Groves, 1981). When the British garrison was established in 1816 a short scientific note on the flora and fauna of Tristan da Cunha was published (Carmichael, 1819). In 1937–1938 a very ambitious scientific exploration of the islands was made by a Norwegian inter-disciplinary expedition including natural scientists, who performed thorough studies of the vegetation and fauna (Christophersen, 1946). They also sampled peat cores from mires on Tristan da Cunha and Nightingale Island, which were brought back to Norway for pollen analysis. Based on these data, Ulf Hafsten published papers on the vegetation history of the island group (Hafsten, 1951, 1960a,b). After the eruption in 1961 a major British expedition landed on Tristan da Cunha

Fig. 3. View of Bottom Pond from the north-eastern side of the crater. Dense Blechnum palmiforme and Phylica arborea scrub covers the crater walls.

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and performed extensive biological and volcanological fieldwork (Baker et al., 1964; Dickson, 1965; Wace and Dickson, 1965). Later studies of the vegetation history have involved both pollen analysis (Preece et al., 1986; Bennett et al., 1989) and molecular biology (Richardson et al., 2003). These studies show that the species composition of the Tristan flora is old and has been stable for a considerable time. It is clear that it is not a Holocene immigration flora and it is suggested that it was established earlier than 30,000 years ago (Preece et al., 1986). In the case of the Phylica-tree, molecular studies suggest that it arrived more than 2 million years ago (Richardson et al., 2003). Studies of past diatom assemblages have also been performed on Inaccessible Island (Preece et al., 1986). Based mainly on pollen records the Holocene climate of the Tristan da Cunha island group was interpreted as fairly stable (Hafsten, 1960a; Preece et al., 1986). 2. Methods 2.1. Fieldwork Fieldwork was carried out in February and March 2003. Corings were performed with 5 and 7.5 cm Russian corers with 1 m long chambers. At the lake Bottom Pond coring was done from a platform on a rubber dinghy. Surface sediment samples were retrieved using a simple gravity corer. 2.2. Radiocarbon dating The chronologies of the sediment sequences are based on accelerator mass spectrometry (AMS) radiocarbon dates obtained on macroscopic plant remains and bulk sediment samples. Radiocarbon dates were converted to calibrated ages using the program OxCal v3.10 (Bronk Ramsey, 1995, 2001) and the SHCal04 calibration data-set (McCormac et al., 2004). Radiocarbon dates yielding post-1950 ages were related to the atmospheric 14C excess record for age estimates (Hua et al., 2000, 2003). The chronologies of the sites were created individually and correlations between sites were made based on the applied chronologies. Age–depth models were constructed by visually fitting lines through the calibrated ages. Owing to the highly dynamic volcanic environment with reworking processes 14 C ages are more likely to be older than expected, than the opposite. Lithological characteristics and changes were considered when the age–depth models were constructed.

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2.3. Magnetic susceptibility Magnetic susceptibility was measured at 4 mm increments using a Bartington Instruments MS2EI magnetic susceptibility high-resolution surface scanning sensor coupled to a TAMISCAN automatic logging conveyor. Magnetic susceptibility reflects the minerogenic content of the sediment and is used as a proxy for in-wash and tephra deposition (Thompson and Oldfield, 1986). 2.4. Measurements of total carbon, nitrogen and sulphur content Total carbon, total nitrogen, and total sulphur content were measured on dried and homogenized samples at 2– 3 cm intervals with a Costech Instruments ECS 4010 elemental analyser. The accuracy of the measurements is better than ± 5% of the reported values based on replicated standard samples. Total carbon is used as a proxy for the organic content. C/N atomic ratios were obtained by multiplying by 1.167 and are used to discriminate between terrestrial and aquatic organic matter sources (Meyers and Teranes, 2001). Sulphur content in freshwater lake sediments is generally low (< 1%). At coastal sites without bedrock sources high values can indicate increased sulphur deposition by sea spray (Berner and Raiswell, 1984). 2.5. Pollen analysis Pollen samples of between 1 and 2 cm3 were processed following standard method A as described by Berglund and Ralska-Jasiewiczowa (1986), and Lycopodium spores were added for determination of pollen concentration values. The counting was made under a light microscope at magnifications of ×400 and × 1000. Pollen grains were identified by the help of published photos (Hafsten, 1960a), standard pollen keys (Moore et al., 1991), and a small collection of type slides prepared by Hafsten and borrowed from the Botanical Museum in Bergen, Norway. Pollen diagrams were plotted in C2 (Juggins, 2003) and divided into local pollen zones based on the variation of the major taxa. 3. Results and interpretations 3.1. Hillpiece Bog 3.1.1. Site description and stratigraphy Hillpiece is, as the name reveals, the remnants of a once much larger parasitic cone situated on The

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Table 1 Lithostratigraphy of the Hillpiece Bog sediment sequence Unit Depth # (cm) 10 9

0–10 10–36

8

36–97

7

97–127

6

127–136

5

136–146

4

146–175

3

175–277

2

212–213 277–333

1

333–354.5

Tephra Description Surface vegetation, living Sphagnum sp. Low- to medium-humified Sphagnumpeat. UB = gradual Brown homogenous coarse–detritus gyttja. UB = gradual Dark brown silty coarse detritus gyttja with abundant plant macrofossils. UB = rather sharp Dark brown coarse detritus gyttja with abundant plant macrofossils. UB = sharp Dark brown sandy coarse detritus gyttja. Gravel-sized particles at 136–137, 141, and 144–143. UB = sharp Dark brown silty coarse detritus gyttja. Higher minerogenic content from 155 cm. UB = sharp Dark brown coarse detritus gyttja. Low minerogenic content. Higher content of plant macrofossils at 254–213 cm. UB = sharp Tephra Brown faintly laminated coarse detritus gyttja with a high content of sedge macrofossils, in particular at 282 cm depth. UB = rather sharp Dark brown to black coarse detritus gyttja. UB = sharp

UB = upper boundary.

Settlement Plain southwest of The Settlement (Fig. 1). The northern part of the cone is constantly eroded by the ocean and terminates with a 50 m high precipice down to the sea. Today the vegetation at Hillpiece Bog and the surrounding lowlands consists of heavily grazed grasslands. The volcano has several craters in which small marshes have formed. Hillpiece Bog is an overgrown lake, approximately 10 by 22 m in size, which is situated at 62 m a.s.l. on the southern slope of the cone in a small crater, and covered by Sphagnum sp. and sedges (Fig. 2). The basin has no inlet or outlet. A 354 cm long sediment profile was retrieved from the centre of the crater. The corer stopped in minerogenic sediments. The stratigraphy of the core is presented in Table 1 and Fig. 4a. Most of the sequence is composed of a highly organic gyttja, showing that the site was a small pond throughout most of its existence. Apart from a tephra layer at 213–212 cm, the sediments below 175 cm (units 1–3) are homogenous with a very low content of minerogenic matter and a high and slightly variable content of coarse plant remains. Unit 2 (333– 277 cm) has a lighter colour and is rich in large macrofossils, mainly sedges. At 175 cm the minerogenic

content increases and remains high until 97 cm. The transition from gyttja to Sphagnum peat (unit 9) takes place at 36 cm, indicating that the overgrowing was completed very recently. 3.1.2. Radiocarbon dates and chronology The chronology of the Hillpiece Bog sediment sequence is based on 12 AMS radiocarbon dates, of which nine were obtained on plant macrofossils and three on bulk sediment samples (Table 2). The dates are plotted against depth in Fig. 5. The lowermost date is 600 years older than the sample 5 cm above, which either indicates a hiatus, or that the date is too old. An erroneous date may have been caused by re-deposition of older organic matter or incorporation of volcanic CO2 devoid of 14C, derived from seepage following the eruption that created the crater (Pasquier-Cardin et al., 1999). The seven radiocarbon dates in the interval of 345– 175 cm give evidence of a constant sedimentation rate, which is supported by the relatively homogenous lithology. The subsequent date (LuS-6302) at 101.5 cm yielded a calibrated age span similar to the one at 181.5 cm (LuS-6303), which indicates a very high sedimentation rate. The minerogenic content is high between 175 and 97 cm (Unit 4 to 7) and this probably caused the rapid sediment accumulation. The start of the increase in sedimentation rate at 175 cm is estimated to c. 300 cal. years BP, whereas the upper boundary is difficult to determine precisely, given the wide error margins of the date at 101.5 cm and the reversed age relation of the two dates within Unit 8. The uppermost radiocarbon date has an activity that exceeds the modern atmospheric radiocarbon concentration, and has been matched to the atmospheric 14C excess record for the southern hemisphere, yielding an age of either around 1959 AD or later than 1980 AD (Hua et al., 2003). The latter alternative is rejected as accumulation of 45 cm gyttja and peat in less than 20 years is unlikely. Linear segments were visually fitted to the dates (Fig. 5), yielding an age model for the proxy records. 3.1.3. Magnetic susceptibility From the onset of sedimentation up to 175 cm (c. 300 cal. years BP) the magnetic susceptibility is close to 0, which indicates very low contents of minerogenic matter (Fig. 6). The tephra at 213–212 cm (c. 620 cal. years BP) is clearly visible as a sharp peak. The actual tephra layer is only a few mm thick but the magnetic susceptibility signal is broadened, possibly due to bioturbation and penetration of minerogenic particles into the water–sediment interface. At 175 cm (c. 300 cal. years BP) the magnetic susceptibility starts to increase and between 147 and 98 cm it

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Fig. 4. Sediment logs of the Hillpiece Bog sediment sequence (a), the Potato Patches section (b), and the Bottom Pond sediment sequence (c). The facies codes for the different sediment types are as follows: Gy = gyttja; Si= silt; S = sand; Gr = gravel; St= stone; Bl= block; d = detritus; cd = coarse detritus; a = algae; Sph p =Sphagnum peat. Clastic grain size is indicated on the x-axis of the logs. High content of macroscopic plant remains is indicated by a leaf.

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Table 2 Radiocarbon dates from Hillpiece Bog Lab no.

Depth

14

C years BP

Poz-3467 Poz-3420 Poz-3421

43.0–43.5 70.0–70.5 92.0–92.5

LuS-6302

101–102

70 ± 60

LuS-6303

181–182

240 ± 50

Poz-3422 Poz-3423 Poz-3224

194.0–194.5 222–223 253.0–253.5

480 ± 30 820 ± 30 1085 ± 30

Poz-3425

293–294

1295 ± 35

Poz-3426 Poz-3427

308.0–308.5 345–344

1475 ± 25 1790 ± 30

Poz-3429

350–351

2455 ± 30

a

118.2 ± 0.2 pmC 510 ± 35 390 ± 30

Cal years BP, 1σ

Cal years BP, 2σ

Material

550BP (95.4%) 485BP 500BP (95.4%) 320BP

Bulk sediment Bulk sediment Twigs

a

c. 1960 AD 530BP (68.2%) 495BP 490BP (31.0%) 440BP 400BP (0.9%) 390BP 380BP (36.3%) 320BP 250BP (7.6%) 220BP 150BP (58.6%) 20BP − 1BP (2.0%) − 11BP 310BP (24.6%) 260BP 230BP (43.6%) 140BP 520BP (68.2%) 485BP 725BP (68.2%) 675BP 965BP (68.2%) 925BP

1260BP (32.5%) 1200BP 1190BP (28.0%) 1130BP 1110BP (7.7%) 1090BP 1340BP (68.2%) 1295BP 1700BP (65.5%) 1600BP 1580BP (2.7%) 1570BP 2470BP (68.2%) 2350BP

270BP (14.9%) 220BP 150BP (80.5%) − 11BP

Twigs, seeds

440BP (2.5%) 400BP 330BP (92.9%) − 11BP 535BP (95.4%) 450BP 740BP (95.4%) 665BP 1060BP (4.0%) 1030BP 990BP (89.5%) 900BP 870BP (1.9%) 840BP 1270BP (95.4%) 1070BP

Twigs, seeds, leaves

1370BP (95.4%) 1285BP 1720BP (95.4%) 1540BP

Phylica leaves, twigs Phylica leaves, twigs

2700BP (11.8%) 2630BP 2620BP (2.8%) 2590BP 2510BP (80.8%) 2340BP

Bulk sediment

Phylica leaves, twigs Phylica leaves, twigs Phylica leaves, twigs

Phylica leaves, twigs

Calibrated against the atmospheric 14C excess record (Hua et al., 2000, 2003).

reaches a sequence maximum, reflecting a period of high minerogenic input. Above 98 cm (c. 200 cal. years BP) the magnetic susceptibility falls to low and stable values. 3.1.4. Total carbon, nitrogen, and sulphur content The total carbon (TC), nitrogen (TN), and sulphur (TS) content data are presented in Fig. 6. From the bottom of the sequence to 175 cm TC values vary between 25% and 49% and TN values vary between 1% and 3%, with most samples above 2%. Pure organic matter normally exhibits TC values between 40% and 50% (Meyers, 2003). This shows that the sediments consist of almost pure organic matter. The variability of the TN record is rather large, whereas the TC values are more stable. This is reflected in the C/N ratio record, which varies between 16 and 32. Above 175 cm the TC and TN values drop and reach minimum values at 147– 123 cm and the C/N ratio exceeds 24, which indicates an increased accumulation rate of terrestrial organic matter. Above 100 cm (c 150 cal. years BP) the TC values level out at about 37% and the TN record stabilizes at about 2%. Below 240 cm (c. 850 cal. years BP) the TS content is around 0.9% with occasional peaks. At 240 cm the TS content increases sharply to 1.2%, followed by increased variability. TS remains above 1% up to

175 cm, with exception of the tephra layer at 213– 212 cm (c. 600 cal. years BP). The C/S ratio varies between 100 and 200 below 240 cm, followed by a drop to around 100. The corresponding change in C/S ratio indicates that the increase in TS content was not caused by elevated organic content, and since the sedimentation rate is constant the increase in TS above 240 cm was caused by increased sulphur deposition. Above 175 cm the TS content decreases and the C/S ratio increases as a consequence of increased sedimentation rate and lower TC content. 3.1.5. Pollen analysis The Hillpiece Bog pollen record has been divided into five local pollen assemblage zones (Fig. 7). 3.1.5.1. Hillpiece Bog 1 (HB1), P. arborea–Polypodiaceae 355–323 cm (1700–1450 cal. years BP). The pollen spectra are dominated by P. arborea pollen that exceeds 90% and Polypodiaceae spores exceeding 50% of the combined pollen and spore sum. Other pollen and spore types are very rare. Gramineae and Cyperaceae occur at almost negligible frequencies indicating scattered occurrence of these plants at the site. Such high percentages of P. arborea were not recorded by Hafsten (1960a), who stated that this species is a poor

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Fig. 5. Age–depth model for the Hillpiece Bog sediment sequence. Numbers in the left-hand column refer to lithostratigraphic units (Table 1).

pollen disperser. Substantial populations of P. arborea therefore must have been growing very close to the site to yield the observed percentages and concentrations.

Various kinds of Polypodiaceae are associated with the modern P. arborea forest, both as epiphytes and in the ground vegetation. The high Polypodiaceae spore

Fig. 6. Total carbon, nitrogen, and sulphur content, and magnetic susceptibility records from Hillpiece Bog.

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Fig. 7. Hillpiece Bog pollen percentage (a) and pollen concentration (b) diagrams.

K. Ljung et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 241 (2006) 539–560 Table 3 Lithostratigraphy of the Potato Patches section Unit Depth # (cm) 5

4

3 2 1

Lithological description

0–108 Alluvial and colluvial sediments. The grain size varies from silty sand to gravel. Some visible plant remains and coal particles. 108–275 Lacustrine and fluvial sediments. The lower part of the unit consists of laminated clayey silt of lacustrine origin. Sand with ripples at 172–187 cm. 275–288 Black silty gyttja. The upper boundary is disturbed. 288–349 Stony gravel composed of pumice/scoria particles (tephra deposit). 349–360 Clast-supported stony gravel with high content of boulder sized clasts

percentages probably reflect a dense ground vegetation of ferns and probably the presence of B. palmiforme where the P. arborea canopy was more open. 3.1.5.2. Hillpiece Bog 2 (HB2), Cyperaceae, 323– 265 cm (1450–1050 cal. years BP). This zone is defined by the high Cyperaceae and relatively low Polypodiaceae frequencies. Chenopodiaceae, Nertera sp., and Umbelliferae are more common than in the previous and following zones. P. arborea pollen and Polypodiaceae spores and sporangia decrease dramatically and reach their lowest values between 305 and 275 cm (c. 1330–1100 cal. years BP), which indicates that Polypodiaceae did not occur frequently in the immediate vicinity of the site. The Polypodiaceae percentages are lower than subsequent to the human settlement and the associated clearing of the lowland plains, which indicates that the coverage of Polypodiaceae did not exceed modern conditions. The increase in Cyperaceae was at least partly local, as evidenced by abundant sedge remains in the sediments, which indicates an expansion of sedges in the littoral zone of the pond. The vegetation on The Settlement Plain was probably dominated by Phylica forest with a lowered proportion of ferns. Herbs were abundant in open areas and probably also in the under storey where P. arborea stands were more open. 3.1.5.3. Hillpiece Bog 3 (HB3), P. arborea, Polypodiaceae, 265–175 cm (1050–300 cal. years BP). The pollen spectra of this zone are almost identical to those of HB1, with P. arborea and Polypodiaceae frequencies exceeding 90%. Herb pollen frequencies are very low with occasional Nertera sp., Chenopodiaceae, and Umbellifereae. The high frequencies of fern sporangia indicate that ferns were growing close to the site. The forest cover was dense, probably with a completely

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closed canopy of P. arborea and B. palmiforme. Ferns dominated the ground vegetation. In the uppermost part of the zone Cyperaceae, Umbellifereae, Nertera sp., and Rhumora adiantiformis frequencies increase, which may indicate a slightly more open forest. 3.1.5.4. Hillpiece Bog 4 (HB4), Gramineae, E. rubrum, R. acetosa/acetosella, 175–75 cm (300–100 cal. years BP). The transition to HB4 is characterized by the appearance of the introduced taxon R. acetosa/acetosella, which implies that this vegetation change was caused by human activity and represents the landnam phase on Tristan da Cunha. Gramineae increases and reaches a sequence maximum. P. arborea and Polypodiaceae decrease. Fern sporangia also decrease and disappear in the upper part of the zone. The herbs Nertera sp. and Umbellifereae are present throughout most of the zone. The aquatic species Callitriche christensenii is present at low frequencies. Gymnogramma and Vittaria vittarioides do also attain low values in most of the zone. R. adiantiformis is present throughout the zone with a slight decrease in the middle part. Today R. adiantiformis occurs extensively on dry peat banks and other well-drained areas (Hafsten, 1960a; Groves, 1981). E. rubrum is present continuously throughout the zone. The high values of Gramineae may indicate the presence of tussock grass (P. flabellata or S. arundinaceae). Today tussock grass is more or less absent at Tristan da Cunha, but burning of areas with tussock grass has been reported from the early 19th century (Wace and Holdgate, 1958). The pollen spectra probably represent a more open vegetation with higher species diversity than in previous zones. 3.1.5.5. Hillpiece Bog 5 (HB5), Cyperaceae, Plantago lanceolata, Gramineae, 75–0 cm (100 cal. years BP – present). This zone is defined by an increase in Cyperaceae and the first occurrence of the introduced taxa P. lanceolata and Anthemis-type. P. arborea decreases throughout the zone whereas Polypodiaceae is rather stable. Polypodiaceae sporangia are absent, which indicate that ferns decreased locally and possibly also on The Settlement Plain. The onset of the zone coincides with the time of the establishment of the permanent settlement around 1800 AD. Plantago and Anthemistype were most likely unintentionally introduced by agricultural activity. Most of The Settlement Plain was probably cleared of Phylica forest shortly after the settlement. However, the percentage of P. arborea remains above 50% in all except the uppermost samples, either representing a local background or that scattered Phylica trees were still growing in the crater, which is

550

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Table 4 Radiocarbon dates from the Potato Patches section Lab no.

Depth (cm below the surface)

14

C years BP

Cal years BP, 1σ

Cal years BP, 2σ

Material

300BP (27.2%) 270BP 210BP (41.0%) 150BP 1530BP (28.1%) 1480BP 1470BP (40.1%) 1410BP

310BP (33.4%) 270BP 220BP (62.0%) 150BP 1550BP (95.4%) 1400BP

Fern leaves, twigs

Poz-2903

93–94

245 ± 30

Poz-2902

280–281

1645 ± 25

now devoid of trees. The increase in Cyperaceae in the uppermost samples is probably a consequence of the overgrowing of the pond. 3.2. Potato Patches A small section in a ravine that cuts through the farmland on The Settlement Plain southeast of Hillpiece was examined and sampled (Fig. 1). The section is 3.6 m high and consists mostly of lacustrine sorted sediments (Fig. 4b). The Hillpiece volcano and lava flows north of Potato Patches probably dammed parts of the area in the past, which led to deposition of lacustrine and alluvial sediments. The stratigraphy of the section is shown in Fig. 4b and summarized in Table 3. The bottom unit of the section (360–349 cm) consists of coarse alluvial sediments with angular clasts. On the alluvium rests a 61 cm thick gravelly pyroclastic unit that most likely originates from the eruption of the Hillpiece volcano. This tephra is followed by a 13 cm thick unit of silty gyttja with an organic content of c. 50%, abruptly

Bulk sediment

followed by a sequence of sandy and laminated silty sediments with a low organic content. The fine-grained sediments are interrupted at 182 cm by a 21 cm thick layer of sandy sediments with ripples, indicating shallow water. From 108 cm below the surface the sediments are coarser and contain horizons with abundant macroscopic plant remains, such as leaves of P. arborea. The chronology of the section was established by two radiocarbon dates, one on bulk sediment from unit 3280 cm below the surface, which yielded an age of 1474 ± 75 cal. years BP, and one on plant macrofossils imbedded in sand in unit 593.5 cm below the surface, which yielded an age of 229 ± 80 cal. years BP (Table 4). Five pollen samples with sufficiently high pollen concentrations were analyzed (Fig. 8). The pollen samples below 150 cm have high P. arborea and Polypodiaceae frequencies, which indicates a closed Phylica forest with abundant Polypodiaceae. The two uppermost samples have low P. arborea and Polypodiaceae percentages and high grass and herb percentages. The sharp decline in forest vegetation was probably caused by clearance by the early settlers. In

Fig. 8. Pollen percentage data, loss-on-ignition record, and radiocarbon dates from the Potato Patches section. Numbers in the left-hand column refer to lithostratigraphic units (Table 3).

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the uppermost sample pollen of the introduced species P. lanceolata are abundant, which indicates intensified land use. Charcoal particles in the two upper samples probably originate from the use of fire for clearance of the Phylica forest. The lowermost organic unit was deposited in a shallow water body, and the following laminated silty sediments indicate increased in-wash from the catchment. The gradual infilling of the basin caused a coarsening upwards of the sequence. The uppermost alluvial sediments were possibly deposited after the human settlement as a consequence of increased erosion after clearance of the Phylica forest. 3.3. Bottom Pond 3.3.1. Site description and stratigraphy At 550–600 m a.s.l. on the northeastern side of the island three crater lakes are situated (Fig. 1). The lakes are positioned on a line radiating from the centre of the

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island and are named Upper, Middle, and Bottom Pond according to their altitudinal relationship. The craters are conical depressions with the rims at about the same altitude as the surrounding surface. The craters were formed by explosive eruptions with no lava flows and very little tephra production (Baker et al., 1964). The uppermost lake is fed by a stream from the peak and the middle is fed by over-flow from Upper Pond. The lowermost lake, Bottom Pond, has no inlets or outlets. pH was measured to 6.5 and conductivity to 60 μS. The crater walls of Bottom Pond are steep and covered by P. arborea and B. palmiforme scrub (Fig. 3). The lake is 150 m in diameter and has a maximum depth of 11 m with a rather flat bottom. While the rim of the crater is situated at c. 625 m a.s.l. the lake surface is at c. 550 m a. s.l. A coring point in the centre of the lake at a water depth of 9.53 m was chosen. We penetrated down to 14.45 m below the lake surface and retrieved a 4.92 m long sediment sequence including 12 cm of surface sediment retrieved with a gravity corer.

Table 5 Lithostratigraphy of the Bottom Pond sediment sequence Unit #

Depth (cm)

26 25

953–978 978–1042 1013–1013.5 1042–1044.5 1044.5–1062 1062–1070.5 1070.5–1077.5 1077.5–1119 1119–1128 1125–1127 1128–1137.5 1137.5–1144 1144–1165.5 1165.5–1270 1203–1205 1221–2122 1266–1268.5 1270–1290.5 1290.5–1297 1297–1309.5 1309.5–1326 1326–1329 1329–1339 1339–1341 1341–1378 1378–1393 1393–1397 1397–1406.5 1406.5–1410.3 1410.3–1417 1417–1445

24 23 22 21 20 19 18 17 16 15

14 13 12 11 10 9 8 7 6 5 4 3 2 1

UB = upper boundary.

Tephra

Tephra

Partly tephra Tephra

Tephra Tephra Tephra

Lithological description Dark brown algal-gyttja. Some macrofossils. Brown algal-gyttja. UB = sharp Black sandy tephra. UB = sharp, LB = sharp Dark brown algal-rich detritus gyttja. Rich in macrofossils. UB = sharp Silty brown–dark brown coarse detritus gyttja. UB = sharp Brown algal-rich coarse detritus gyttja. UB = sharp Dark brown coarse detritus gyttja. UB = sharp Dark brown sandy coarse detritus gyttja. High content of coarse organic matter. UB = sharp Dark brown algal-rich detritus gyttja. Abundant plant macrofossils. UB = sharp Sandy clast-supported tephra. UB = sharp, LB = sharp Brown algal-rich detritus gyttja. Layer with higher minerogenic content at 1127–1128 cm. UB = sharp Dark brown algal-rich detritus gyttja. UB = sharp Dark brown sandy algal-rich coarse detritus gyttja. UB = sharp Brown algal-rich detritus gyttja. UB = sharp Silty tephra, UB = rather gradual, LB = sharp Silty tephra, UB = rather gradual, LB = sharp Gravely clast-supported tephra. UB = sharp, LB = sharp Dark brown sandy detritus gyttja. UB = gradual Brown algal-gyttja. UB = sharp Dark brown algal-rich coarse detritus gyttja. Abundant coarse organic matter. UB = gradual Brown sandy silty algal-rich detritus gyttja. UB = sharp Brown algal-rich detritus gyttja gyttja. UB = rather sharp Brown algal-rich detritus gyttja. UB = gradual Dark brown algal-rich coarse detritus gyttja. UB = gradual Brown coarse detritus gyttja. Abundant coarse plant remains. UB = sharp Dark brown coarse detritus gyttja. UB = gradual Dark brown algal-rich coarse detritus gyttja. Rich in coarse organic material. UB = gradual Brown algal-rich coarse detritus gyttja. UB = rather sharp Brown algal-rich detritus gyttja. UB = sharp Brown algal-gyttja. UB = very sharp Dark brown algal-rich detritus gyttja. UB = gradual

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The sediments consist of gyttja with a highly variable content of minerogenic and coarse organic matter (Table 5, Fig. 4c). The boundaries between units are often sharp but not erosive. The strata with high minerogenic content often also have a high content of coarse organic matter. Five distinct tephra

horizons were identified and unit 20 is also partly composed of tephra (Fig. 4c). 3.3.2. Radiocarbon dates and chronology The chronology of the Bottom Pond sediment sequence is established by 24 radiocarbon dates, 17 of

Table 6 Radiocarbon dates from Bottom Pond Lab no.

Depth

14

C years BP

Cal years BP, 1σ

Cal years BP, 2σ

13

Material

270BP (18.4%) 220BP 150BP (4.5%) 130BP 120BP (29.1%) 60BP 30BP (16.2%) −11BP 450BP (60.5%) 360BP 340BP (7.7%) 320BP 525BP (68.2%) 500BP 655BP (26.6%) 630BP 605BP (41.6%) 560BP 655BP (26.1%) 625BP 610BP (42.1%) 560BP 560BP (68.2%) 520BP

280BP (27.1%) 210BP 160BP (68.3%) − 11BP

− 29.2 ± 1.6

Twigs, Phylica leaves

470BP (95.4%) 310BP

− 23.7 ± 0.7

Bulk sediment

540BP (95.4%) 495BP 670BP (95.4%) 550BP

− 28.6 ± 1.1 − 22.3 ± 0.4

Phylica leaves, twigs Bulk sediment

660BP (95.4%) 550BP

− 26.5 ± 1

Twigs, Phylica leaves

630BP (17.0%) 600BP 570BP (78.4%) 510BP 640BP (22.0%) 590BP 570BP (73.4%) 510BP 730BP (95.4%) 650BP

− 30.2 ± 1.5

Phylica leaves, twigs

− 25.7 ± 0.3

Phylica leaves, twigs

− 26.3 ± 0.5

Phylica leaves, twigs

920BP (95.4%) 740BP

− 14.1 ± 0.5

Fern leaves, twigs, Phylica leaves

1060BP (14.2%) 1020BP 1010BP (81.2%) 920BP 1060BP (95.4%) 930BP

− 22.2 ± 0.8

Bulk sediment

− 30.8 ± 0.5

Phylica leaves, twigs

375BP (95.4%) 1285BP 1880BP (95.4%) 1700BP 1270BP (95.4%) 1060BP

− 28.2 ± 1.7 − 17.3 ± 1.8 − 25.5 ± 0.6

Bulk sediment Bulk sediment Twigs

1260BP (12.7%) 1200BP 1190BP (82.7%) 1060BP 1290BP (95.4%) 1080BP 1330BP (95.4%) 1170BP

− 24.5 ± 0.7

Phylica leaves, twigs

− 25.6 ± 0.3 − 24.8 ± 0.7

Twigs Phylica leaves

1710BP (95.4%) 1540BP

− 25.8 ± 0.7

Bulk sediment sample

1520BP (95.4%) 1330BP

− 27.8 ± 0.8

Phylica leaves, twigs

1900BP (95.4%) 1720BP

− 17.2 ± 1.6

Twigs Phylica leaves

1970BP (94.1%) 1770BP 1760BP (1.3%) 1740BP 2690BP (6.7%) 2640BP 2620BP (1.1%) 2600BP 2500BP (87.6%) 2330BP 2310BP (16.9%) 2240BP 2180BP (78.5%) 1990BP 2340BP (95.4%) 2150BP

− 23.3 ± 1.3

Bulk sediment

− 20.8 ± 0.8

Twigs

− 29.7 ± 0.6

Twigs, Phylica leaves

− 26.6 ± 0.4

Twigs, mosses

Poz-2798

979–980

160 ± 25

Poz-2799

983–984

360 ± 30

Poz-2800 Poz-2801

1013–1014 1035–1036

510 ± 25 690 ± 30

Poz-2806

1042–1043

680 ± 30

Poz-2802

1079–1080

590 ± 30

Poz-2804

1103–1102

595 ± 30

Poz-2805

1129–1130

790 ± 30

Poz-2808

1169–1170

960 ± 30

Poz-2886

1175–1176

1110 ± 30

Poz-2809

1205–1206

1140 ± 30

Poz-2810 Poz-2887 Poz-2811

1246–1247 1269–1270 1272–1273

1470 ± 30 1900 ± 30 1285 ± 30

Poz-2812

1272–1273

1270 ± 25

Poz-2814 Poz-2815

1308–1309 1319–1320

1315 ± 30 1395 ± 40

Poz-2888

1332–1333

1775 ± 30

Poz-2816

1343–1343

1575 ± 30

Poz-2818

1375–1376

1940 ± 30

Poz-2889

1375–1376

1980 ± 30

Poz-2891

1424–1423

2435 ± 30

Poz-2819

1427–1428

2190 ± 30

Poz-2820

1438–1439

2290 ± 30

625BP (6.5%) 610BP 560BP (61.7%) 525BP 720BP (23.1%) 700BP 690BP (45.1%) 660BP 910BP (32.4%) 860BP 850BP (4.3%) 830BP 820BP (31.6%) 770BP 980BP (68.2%) 925BP 1055BP (22.2%) 1030BP 1005BP (46.0%) 955BP 1345BP (68.2%) 1295BP 1825BP (68.2%) 1730BP 1240BP (15.8%) 1200BP 1190BP (52.4%) 1080BP 1175BP (68.2%) 1080BP 1265BP (68.2%) 1170BP 1310BP (56.3%) 1240BP 1210BP (11.9%) 1180BP 1700BP (28.5%) 1650BP 1640BP (39.7%) 1560BP 1490BP (8.9%) 1460BP 1420BP (59.3%) 1340BP 1890BP (55.0%) 1810BP 1800BP (6.7%) 1780BP 1760BP (6.5%) 1740BP 1920BP (1.8%) 1910BP 1900BP (66.4%) 1825BP 2460BP (68.2%) 2340BP

2300BP (6.8%) 2270BP 2160BP (61.4%) 2050BP 2330BP (16.5%) 2300BP 2250BP (51.7%) 2150BP

C

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Fig. 9. Age–depth model for the Bottom Pond sediment sequence. Numbers in the left-hand column refer to lithostratigraphic units (Table 5).

Fig. 10. Total carbon and nitrogen content, C/N ratios, and magnetic susceptibility records from Bottom Pond.

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which were obtained on plant macrofossils and seven on bulk sediment samples (Table 6). The calibrated ages are plotted against depth in Fig. 9. Dates that are bracketed by younger dates are considered to be too old, probably as a consequence of re-deposition of organic matter, and are thus not included in the age–depth model. Erroneously young ages are less likely to occur because of the generally high sedimentation rate that limits the effect of bioturbation, and the water depth that limits benthic plant growth. The age model was visually fitted within the error envelopes of the accepted radiocarbon dates, and sedimentation rate changes are placed at distinct lithological boundaries.

3.3.5.1. Bottom Pond 1 (BP1), P. arborea, Polypodiaceae, 1145–1320 cm (2300–1250 cal. years BP). This zone is characterised by high and variable P. arborea pollen percentages in the range of 25–58%. E. rubrum is present throughout the zone. Cyperaceae reaches a maximum between 1375 and 1355 cm (c. 1800–1600 cal. years BP) with values up to 25%. The aquatic species C. christensenii is present throughout the zone. Pollen grains of Asteraceae and Chenopodiaceae are found scattered throughout the zone. Umbellifereae exhibits high and variable frequencies. The vegetation was probably similar to present-day conditions with dominance of P. arborea and Polypodiaceae.

3.3.3. Magnetic susceptibility The magnetic susceptibility is generally high throughout the core as a result of volcanically derived particles with strong magnetic properties (Fig. 10). From the bottom of the sediment sequence to 1319 cm (c. 1250 cal. years BP) rather stable values were recorded, interrupted by brief excursions. Subsequently the magnetic susceptibility increases sharply and reaches very high and variable values, followed by an abrupt drop to low and stable values at 1266 cm (c. 1200 cal. years BP). Two peaks at 1225–1234 cm (c. 1050 cal. years BP) and 1205–1211 cm (c. 1000 cal. years BP) represent tephra layers, after which the magnetic susceptibility declines gradually. Additional peaks occur between 1169 and 1042 cm (800–750 cal. years BP), at 1079–1106 cm (c. 700 cal. years BP) and between 1062 and 1042 cm (c. 600 cal. years BP). The latter two peaks probably reflect tephra horizons.

3.3.5.2. Bottom Pond 2 (BP2), P. arborea, Polypodiaceae, 1320–1264 cm (1250–1200 cal. years BP). This zone is dominated by P. arborea with values up to 60%. E. rubrum also increases slightly while the frequencies of most herbs, Cyperaceae and C. christensenii decline. Asteraceae, and Chenopodiaceae are absent. Polypodiaceae reaches high values, whereas Gymnogramma sp., Lycopodium diaphanum/magellanicum, Lycopodium insulare, and Hypolepis rugosula decrease or disappear. The total pollen concentration is high throughout the zone, which is accompanied by elevated contents of minerogenic matter and coarse terrestrial organic matter and an increase in the sedimentation rate. The pollen spectra indicate a closed P. arborea forest with abundant ferns. However, these data may be biased by redeposited pollen associated with increased deposition of terrestrial material.

3.3.4. Total carbon and nitrogen content The carbon and nitrogen content records closely follow the magnetic susceptibility record and reflect variations in the content of organic matter (Fig. 10). The C/N ratio of the Bottom Pond sediment sequence varies between 14 and 60, with most of the samples around 20, which indicates that a substantial proportion of the organic matter is of terrestrial origin (Meyers, 2003). The periods of maximum magnetic susceptibility are associated with high C/N ratios, indicating increased inwash of both terrestrial organic matter and minerogenic matter. The clearly distinguishable tephra horizons at 1225–1234 cm (c. 1050 cal. years BP) and 1128–1123 cm (c. 700 cal. years BP) are not associated with elevated C/N ratios, which imply that these volcanic eruptions did not lead to increased deposition of terrestrial organic matter.

3.3.5.3. Bottom Pond 3 (BP3): Gramineae, Umbellifereae, 1264–1105 cm (1200–600 cal. years BP). This zone is characterized by high P. arborea and Umbelliferae frequencies and rather high Asteraceae and Chenopodiaceae frequencies. The high P. arborea percentages indicate extensive forest growth with herbs limited to more open areas. The vegetation was probably similar to modern conditions, with dominance of P. arborea and Polypodiaceae.

3.3.5. Pollen analysis The Bottom Pond pollen record has been divided into six local pollen assemblages zones (Fig. 11).

3.3.5.4. Bottom Pond 4 (BP4), P. arborea, E. rubrum, 1105–1050 cm (600–575 cal. years BP). This zone is dominated by high frequencies of P. arborea and relatively high values of E. rubrum and Nertera sp., while the frequencies of Gramineae and C. christensenii are low. Asteraceae and Chenopodiaceae are absent throughout the zone. The total pollen concentration is high and the zone corresponds to a period of high minerogenic content, high proportions of terrestrial organic matter, and a high sedimentation rate. This zone is associated with increased

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Fig. 11. Pollen percentage (a) and pollen concentration (b) diagrams from Bottom Pond.

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detrital input and thus may not represent a vegetation change. 3.3.5.5. Bottom Pond 5 (BP5), P. arborea, Gramineae, Umbellifereae, Polypodiaceae, 1050–965 cm (575– 150 cal. years BP). This pollen zone is dominated by high frequencies of P. arborea¸ Gramineae, Cyperaceae, and Umbellifereae with limited variations, and the vegetation was dominated by P. arborea and probably B. palmiforme. Umbellifereae, probably mostly Apium australe, occurred frequently in the under storey together with L. diaphanum/magellanicum, and H. rugosula. Asteraceae was growing together with grasses in more open areas. The between-sample variation is relatively low in this zone compared to the earlier periods, indicating a more stable environment and a vegetation similar to the present with high abundance of ferns and P. arborea. 3.3.5.6. Bottom Pond 6 (BP6), R. acetosa/acetosella, Gramineae, 965–953 cm (150 cal. years BP–present). The lower boundary of this zone is defined by the first occurrence of pollen grains from the introduced taxon R. acetosa/acetosella, which appears at 965 cm (c. 150 cal. years BP). P. arborea reaches a peak in the first half of the zone, followed by a distinct minimum. In the uppermost sample P. arborea rises. R. acetosa/acetosella was introduced with the first settlers on the island and became established in the interior of the island at about the time of the formation of the settlement in the early 19th century. The minimum in P. arborea corresponds to the population increase in the 20th century when the utilisation of The Base as grazing land was most intense. The cover of P. arborea was probably lower than today during this period, and the rise in the uppermost sample is probably a result of lowered grazing intensity in the late 20th century. 4. Discussion The palaeoecological, geochemical, and sediment stratigraphic data obtained from the three sites allow evaluation of various aspects of the vegetation and climate history of Tristan da Cunha during the last 2300 years. The general characteristics of the datasets from Hillpiece Bog and Bottom Pond show some notable dissimilarities, and this is undoubtedly related to differences in altitude and topographical settings of the two sites. Bottom Pond is situated at 550 m a.s.l., with the crater rim at 625 m a.s.l. (Fig. 3) whereas Hillpiece Bog and the Potato Patches section are situated on the restricted lowland plains in the north (Fig. 2). Bottom Pond receives more precipitation, the crater walls are higher and steeper, and the forest cover is less dense compared to the pre-landman phase at Hillpiece

Bog. These topographical and vegetational factors explain the much higher content and variability of minerogenic matter at Bottom Pond. The higher areas are also affected by eruptions to a larger extent, which can be seen in the thicker, coarser and more numerous tephra layers at Bottom Pond. The latter site is also closer to the tree line in an area more sensitive to environmental changes and may therefore register short-term variations of lesser magnitude. The Hillpiece Bog and Potato Patches records more directly reflect the history of human colonization of the island. The following discussion deals with the vegetation history, hydrological changes, and regional climatic variations inferred from the stratigraphic data. 4.1. Environmental history, and human impact Generally, the vegetation on Tristan da Cunha has been fairly stable during the last 2300 years, and there is no evidence of significant altitudinal shifts in the vegetation zones or major changes in the species composition. The most significant vegetational change was caused by the first arrival of humans in the 17th century and the establishment of the settlement in the 19th century. However, some distinct changes also occurred before the first anthropogenic influence. The first pollen zone at Hillpiece Bog (BP1) contains the same species at similar frequencies as the zone immediately before the first occurrence of introduced species, which indicates that the dense Phylica forest that the first visitors encountered on the lowland plains was present already at c. 1700 cal. years BP. Between 323 and 265 cm (c. 1450–1050 cal. years BP) a significant change in the pollen assemblage at Hillpiece Bog indicates a local expansion of Cyperaceae and decreased dominance of ferns and Phylica trees (Fig. 7). This pollen-stratigraphic change is accompanied by increased content of macroscopic sedge remains in the sediments, which implies an expansion of the shallow littoral zone with high abundance of sedges. The most likely interpretation is a lake-level lowering. The Hillpiece Bog pollen record represents mostly local conditions and the lowland plain was probably still covered by Phylica forest. However, the frequency and concentration of Polypodiaceae spores are lower in zone HB2 than after 1800 AD when large areas of The Settlement Plain had been cleared, and Polypodiaceae sporangia are absent, which may indicate a general decline in the abundance of ferns on the lowland plain in response to drier conditions. Above 265 cm (1050 cal. years BP) the Cyperaceae pollen frequency decreases and P. arborea and ferns increase, which indicates a closed Phylica forest close to the shore and a more restricted shallow littoral zone. We interpret this as a consequence of more humid conditions.

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Chronologically pollen zone HB2 at Hillpiece Bog partly corresponds to pollen zone BP2 at Bottom Pond (1250–1200 cal. years BP), which is characterized by increased P. arborea and Polypodiaceae frequencies (Fig. 11a) and increased total pollen concentration (Fig. 11b). The reason for this could be lowered lake level and erosion of littoral sediments, which may have caused redeposition of minerogenic matter and pollen. Subsequent sediment focusing in the basin caused concentration of the pollen to the central deeper parts and resulted in both high pollen concentrations and higher minerogenic content. In a crater lake on the Pico Island of the Azores similar processes have been suggested as the explanation of increased pollen concentrations and rapid sedimentation rates during lake-level lowerings (Björck et al., 2006). The increase in pollen concentration may, however, seem somewhat contradictory considering that the sedimentation rate increased as a consequence of increased deposition of minerogenic matter. Therefore an alternative explanation is increased in-wash from the catchment bringing more pollen together with minerogenic and terrestrial organic matter, as indicated by high susceptibility and high C/N ratios, suggesting increased precipitation. The pollen zones HB2 and BP2 are partly correlated and probably represent a hydrologic change centred around 1200 cal. years BP that affected both the lowland plains and the higher areas. The most likely common explanation is less precipitation and lower lake levels, but we have also presented an alternative explanation above. However, increased precipitation can only be inferred from the Bottom Pond record. In the Potato Patches sequence deposition of gyttja (unit 3) during calm conditions was followed by deposition of silt and sand (unit 4), which may be a result of increased catchment erosion (Fig. 8). The dates from the Potato Patches sequence suggest that this sequence of events also took place in the Hillpiece Bog and perhaps also in Bottom Pond during pollen zones HB2/BP2. The geochemical records give further indications of hydrological changes. The sulphur content at Hillpiece Bog increases at 240 cm (c. 800 cal. years BP) and remains high up to 175 cm (c. 300 cal. years BP), with the exception of the tephra horizon at 213–212 cm (c. 600 cal. years BP; Fig. 6). Since the TC content is stable and the sedimentation rate constant the increase in sulphur is not a consequence of changing organic content or lowered sedimentation rate. Deposition of sulphur by volcanic eruptions is also unlikely since the S/C ratio remains stable within the tephra horizon, which suggests equal decreases in organic matter and sulphur

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in response to elevated minerogenic input. Thus, the increase in TS content can be interpreted as an increase in sulphur deposition and the most likely source of sulphur at coastal sites is sea-spray, which is often related to wind action and could be an indication of increased storminess between c. 800 and 300 cal. years BP. The increase in sulphur deposition at Hillpiece Bog corresponds to a period of increased magnetic susceptibility, elevated minerogenic content and higher C/ N ratios at Bottom Pond between 1170 and 1145 cm (800–750 cal. years BP), which indicates higher inwash of minerogenic and organic matter from the catchment. This could have been caused by increased precipitation associated with more frequent or stronger storms as indicated by the increase in sulphur deposition. However, there is no clear change in the vegetation that supports this development. The following part of the Bottom Pond pollen record, BP3–BP5, until the uppermost zone when introduced species appear, is very similar to BP1 and probably represents similar vegetational conditions. It is interrupted by pollen zone BP4 1105–1050 cm (c. 600–575 cal. years BP) which exhibits higher frequencies of P. arboreae and Polypodiaceae, similar to BP2. This pollen-stratigraphic change is accompanied by an increase in sediment accumulation rate (Fig. 9) and a general increase in minerogenic matter and coarse organic matter (Figs. 4c and 10), which indicates catchment erosion and re-deposition rather than vegetation change. Correlation based on the age model suggests that these anomalies coincide with a tephra layer in unit 3 at Hillpiece Bog. The radiocarbon dates below unit 20 and above unit 23 of the Bottom Pond record are within the same age ranges, which indicates very rapid sedimentation. Possibly the sediments from 1119 to 1044 cm at Bottom Pond were deposited more or less instantaneously by slumps caused by earthquakes during a major eruption on the island around 600 cal. years BP. The first occurrence of pollen from the introduced taxon R. acetosa/acetosella at 170 cm in the Hillpiece Bog record is tentatively dated to c. 300 cal. years BP which corresponds to c. 1650 AD (Fig. 5), and the following marked decline in P. arboreae pollen frequencies reflects the presence of humans before the establishment of the settlement in 1816 AD. The influence of human activity on the landscape was significant and increased erosion of The Settlement Plain is indicated by increased magnetic susceptibility, higher C/N ratio and higher total pollen concentration at Hillpiece Bog (Figs. 6 and 7b). The first written report of extended visits on Tristan is from 1790 when hunters stayed on the island during the southern summer to

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harvest fur and elephant seals (Munch, 1971). Previously, Dutch ships from South Africa landed during reconnaissance trips and while on route from Europe to the Far East (Wace, 1969). Our pollen data show that the utilization of natural resources on Tristan da Cunha started already in the 17th century. The establishment of the settlement in 1816 AD brought new alien plants to the island and the clearing of its native vegetation accelerated. P. lanceolata and Anthemis-type pollen appear at 71 cm (just over 100 cal. years BP) at Hillpiece Bog, which corresponds to slightly before 1850 AD (Fig. 5). These species are common on pasturelands, and probably became established on The Settlement Plain following the introduction of livestock and cultivated plants. R. acetosa/ acetosella appears at 965 cm (c. 100 cal. years BP) in the Bottom Pond record (around 1850 AD; Fig. 9), which suggests a delay in the spreading of Rumex to higher parts of the island, probably in response to increased grazing on The Base. Increased concentration of P. lanceolata at Hillpiece Bog and of R. acetosa/acetosella at Bottom Pond indicate that the human activity increased in the early 20th century, which is coincident with a population increase (Crawford, 1982). The onset of grazing and clearing of forest on The Settlement Plain is also evident in unit 5 of the Potato Patches section in the form of pollen of P. lanceolata and charcoal, respectively (Fig. 8). These changes occurred after the earliest indications of human presence at Hillpiece Bog, as evidenced by the radiocarbon date at 93–94 cm (1640–1800 AD). Thus the coarse minerogenic sediments in the upper metre of the Potato Patches section were deposited after the settlement and probably as a consequence of increased erosion following clearing of the P. arborea forest. 4.2. Regional climatic correlations and implications The inferred hydrological changes on Tristan da Cunha may be related to large-scale Holocene climate dynamics in the South Atlantic region, and in the following discussion our data are compared to palaeoclimatic proxy records from Southern Africa, the Southern Ocean, Antarctica, and the North Atlantic. The late Holocene climatic development of southern Africa is characterized by changes associated with the Medieval Warm Period from about 1070 to 630 cal years BP, which was generally warm but variable, and the Little Ice Age, which was colder and drier (Cohen and Tyson, 1995). As inferred from speleothem data, the mean annual temperature was approximately 3 °C higher during the medieval warming (950–650 cal.

years BP), and 1 °C lower during the Little Ice Age (650–150 cal. years BP; (Holmgren et al., 1999). Evidence of the Little Ice Age are also found in pollen sequences from the Namibian desert (Scott, 1996), and borehole temperature measurements (Jones et al., 1997). The climate changes associated with the Medieval Warm Period in Southern Africa partly coincide with the period of increased precipitation on Tristan da Cunha at c. 1050–300 cal years BP. Possible correlations to the inferred climatic development on Tristan da Cunha can be found in some records in the Southern Ocean and Antarctica. On South Georgia glacier advances inferred from lake sediments indicate cool conditions at 2500–1600 cal. years BP and after 1000 cal. years BP, with an intervening warm period peaking around 1100 cal. years BP (Rosqvist and Schuber, 2003). The latter warming may correspond to the period of lowered lake levels on Tristan da Cunha. On the Antarctic Peninsula expansion of aquatic systems and lowered lake salinity at 1200 cal. years BP indicate a relatively humid and warm climate (Björck et al., 1996), which partly coincides with the slightly drier period at 1450–1050 cal. years BP on Tristan da Cunha. The correlations between the inferred precipitation changes on Tristan da Cunha and North Atlantic seasurface temperature (SST) records (Andresen and Björck, 2005) and stacked ice-rafted detritus (IRD) events (Bond et al., 2001) are especially good. The decreased humidity at 1450–1050 cal. years BP corresponds to periods of low IRD content and high SST, whereas the period of increased precipitation at 1050–300 cal. years BP corresponds to low SST and high IRD content. There is also a rather good correlation with the winter SST record from off West Africa (deMenocal et al., 2000), where a pronounced low in winter SST coincides with the humid period at 1050– 300 cal. years BP on Tristan da Cunha. The general match between the inferred hydrological changes on Tristan da Cunha and proxy records from areas bordering the South Atlantic indicate that there might be a climatic coupling with these areas. Variations in precipitation on Tristan da Cunha may be caused either by stronger and/or more frequent storms, or by increased SST and higher humidity, giving rise to increased orographic rainfall. Variations in storm tracks of the westerlies are known to be related to the strength of the South Atlantic Anticyclone (SAA) (Tyson and Preston-Whyte, 2000). However, when the SAA is strong it also moves farther south, pushing the westerlies pole-ward. Thus the apparent correlation between the precipitation patterns on Tristan da Cunha and in

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southern Africa may be related to variations in the strength of the SAA. The temperature gradient between the equatorial and the southern Atlantic, which has been shown to affect the strength of the trade winds and hydrology over central Africa may also be a cause of correlation between the African continent and the South Atlantic ocean (Schefusz et al., 2005). Even though the ultimate cause of the hydrological changes on Tristan da Cunha during the last 2300 cal. years BP cannot be determined definitively the apparent correlations to areas bordering the South Atlantic suggests large-scale variations in the oceanic and atmospheric circulation.

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Acknowledgements Without the help from the Tristan da Cunha islanders this work would not have been possible: James Glass is thanked for great logistic support and assistance during the fieldwork, Felicity Glass for the excellent food and accommodation, and all islanders for their hospitality. Ole Bennike (Copenhagen) is acknowledged for assistance during the fieldwork. The study was mainly financed by the Swedish Research Council (VR) with grants to the ATLANTIS project (G 5103-200005076/ 2000). Two anonymous reviewers are thanked for constructive comments.

5. Conclusions In general, the vegetation on Tristan da Cunha was stable during the last 2300 cal. years BP with no major changes in species composition or significant altitudinal shifts in vegetation zones. The most significant vegetational change was caused by the arrival of humans in the 17th century and the establishment of The Settlement in the 19th century. The appearance of R. acetosa/acetosella at c. 300 cal. years BP, followed slightly later by declining forest cover and increased erosion on the lowland plain, provides evidence of substantial human influence on the vegetation. These environmental changes, which took place well before the establishment of the permanent settlement, were probably caused by seasonal seal and whale hunters and ships calling on Tristan to replenish freshwater and fire wood. The establishment of The Settlement is clearly reflected both at the lowland sites by the introduction of P. lanceolata and decreased forest density, and at the upland site by the appearance of R. acetosa/acetosella. Before the first anthropogenic influence changes in effective humidity had some discernible effects on the hydrology and vegetation of the island. From 2300 to 1450 cal. years BP humid conditions favoured the growth of dense forest on the lowland plains. At 1450 cal. years BP the effective humidity decreased and remained low until 1050 cal. years BP, which caused lake levels to fall. After 1050 cal. years BP the lake levels rose. These hydrological changes were probably a consequence of precipitation changes, which may be related to the frequency and/or strength of storms brought to Tristan da Cunha by the westerlies. The position and strength of the westerlies are coupled to the general circulation over the South Atlantic and therefore there may be a coupling between precipitation changes on Tristan da Cunha and the climatic development in other parts of the Atlantic region.

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