Late Holocene swamp transition in the Torres Strait, northern tropical Australia

Quaternary International 385 (2015) 56e68

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Late Holocene swamp transition in the Torres Strait, northern tropical Australia Cassandra Rowe School of Geography & Environmental Science, Building 11 (Arts Faculty) Clayton Campus, Monash University, Wellington Rd, Clayton 3800, Victoria, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 22 July 2014

Pollen and charcoal analyses are presented from the islands of Mua and Badu, western Torres Strait (northern Australia). Sediment core collections from island interior Melaleuca swamps provide a record of hydrological and vegetation change through the period c.2700 BP to present. Seasonally moist-dry open herbaceous habitats are recorded prior to extensive stable boundary swamp and swamp-forest establishment. This island swamp development is important in supporting vegetation differentiated from eucalypt woodland growth. The swamps also constitute an important dry season resource and refugia. Eucalypt-dominated woodland is evident throughout the records, but is increasingly influenced by fire (in structure and composition). This palaeoecological study provides the unique opportunity to explore long term inter-island and island-mainland environmental connection in the Torres Strait. It also facilitates an examination of regional late Holocene humaneenvironment interaction, including discussions of islander colonisation, occupation and identity as taking place within archaeological research. © 2014 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Torres Strait Holocene Island Swamp Woodland Archaeology

1. Introduction The islands of the Torres Strait are located between the mainlands of Australia and Papua New Guinea. Culturally it is a region juxtaposed between Australian Aboriginal societies and those of wider Melanesia (Barham et al., 2004). Environmentally, Torres Strait also exhibits a Melanesian affinity (Hoogland, 1972; Webb and Tracey, 1972). It is however the Strait's resemblance to the lowland monsoonal ecologies of tropical mainland Australia which are of interest in this paper, and in particular the spatio-temporal similarity in change across these environments. The interplay between Torres Strait settlement-occupation and environmental change is also explored. In the context of colonisation, environment and insularity, Rowland (2008, 97) states ‘we all seek to avoid single-factor explanations for events in the past and at a minimum must consider the synergies between history, geography and environment’. The Torres Strait region lends itself toward diverse-disciplinary, coordinated methodologies and discourse; natural history, geography, archaeology, sociology and ethnography are all taking place within a spatially fine-grained working space under a multistudy-site philosophy. Increasingly (and significantly) these

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academic pursuits are conducted with Indigenous Islander collaboration (e.g. Brady et al., 2003; David and Mura Badulgal Committee, 2006; McNiven and Wright, 2008; Ash et al., 2010). This paper presents a late Holocene reconstruction of vegetation and landscape change on two islands in western Torres Strait. McNiven (2008) and McNiven and Hitchcock (2004) discuss contemporary island environments as ‘constructed landscapes’. In these analyses, biotic and/or physical features and forces are descriptively observed across Torres Strait, but within a format incorporating inclusion, exclusions and transitional floral and faunal processes as humaneenvironment interactions and legacies. Herein, the development, operation and maintenance of Torres Strait as a constructed landscape ‘can only be successfully investigated from an integrated archaeological, ethnographic and palaeoenvironmental perspective’ (McNiven, 2008: 451). In the spirit of both Rowland (2008) and McNiven (2008) an integrated, reciprocal methodology adopting archaeological and palaeoenvironmental perspectives has already begun in the Torres Strait (see Rowe, 2006a, 2006b; see also; Crouch et al., 2007) and can only be increasingly addressed and improved upon. In presenting a baseline palaeoecological investigation into island terrestrial Holocene change greater scope exists for documenting all mechanisms behind the long-term emergence of Torres Strait Islander society. Greater understanding of Australian tropical monsoonal biogeographic patterns is also facilitated.

C. Rowe / Quaternary International 385 (2015) 56e68

1.1. Environmental setting and study sites The Torres Strait consists of some 50,000 km2 of shallow open seas and encompasses over 100 islands. These islands are divided into four main geographical groups: western to southern continental, central coral, eastern volcanic and the northern mud islands. The islands of Mua and Badu form part of the westernsouthern group, centred 70 km north of the Australian mainland (Fig. 1). Spanning 275 km2, Mua is separated from Badu to the west by a narrow c. 2 km wide passage. Badu spans a smaller 180 km2 in size (Fig. 2). Each island incorporates hilly outcrops of late Palaeozoic igneous geology (granitoids with interbedded volcanics

57

identified as the Badu Granite Suite and Torres Strait Volcanic Ridge). These hills rise to 372 m above sea level (m.a.s.l.) over undulating but low relief Quaternary sand dune and coastal deposits (Bain and Draper, 1997). Torres Strait is incorporated within the narrow equatorial savanna of northern Australia (Stern et al., 2000), with high temperatures and relative humidity that show no significant annual cycle. Regional rainfall is however heavily seasonal, and driven by the Australian Monsoonal system (Nix and Kalma, 1972; Suppiah, 1992; Sturman and Tapper, 2001). The nearest complete weather station to Mua and Badu is positioned on Horn Island, approximately 30 km south. Mean annual precipitation is 1786 mm, and

Fig. 1. Torres Strait study area and mainland surrounds.

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C. Rowe / Quaternary International 385 (2015) 56e68 Study site

Bar20

BG1

BG2

Fig. 2. Islands of Mua and Badu showing major place names and study site locations.

rainfall maxima occur in the summer months between December and March (1392 mm or 78%). Wet season mean temperature maxima approaches 32
C while dry season maxima averages 28
C. Mean temperature minima for the wet and dry seasons are 25
C and 22
C respectively (BoM, 2013). Mean monthly 9am relative humidity ranges 69e83%. Mua and Badu are currently inhabited. Stone artefact sequences reveal people have been in Torres Strait for at least 9000 years (David et al., 2004). Herein the period 4000e3800 BP marks the beginning of an intensified occupation period, one which included increased use of existing sites (David et al., 2004) and establishment of new sites (McNiven et al., 2006; Crouch et al., 2007). In Torres Strait, 2600 years ago marks further rises in occupation, with an escalation in the diversity and intensity of marine resource exploitation. Widespread increases in coastal midden deposition begin at this time, and such is the frequency of sites that 2600e2500 BP has become known as an ‘event horizon’ (also termed the Torres Strait Cultural Complex - Barham, 2000). McNiven et al. (2006) present a detailed model associating this 2600e2500 BP time frame with an introduction of pottery and influx of Papuan peoples from the Trans-Fly-Papuan Gulf region. After c.2600 BP new social arrangements were established (Crouch et al., 2007). Further settlement expansion and cultural synchronicity occurred 800e600 years ago and is associated with the beginnings of ethnographically-documented social arrangements and ritual activities. An expansion of marine faunal ritual sites took

place 500e400 years ago (McNiven and Feldman, 2003; David and Mura Badulgal Committee, 2006). Regional plant biogeography, incorporating southern PNG provinces and northern mainland Australia has been addressed by Walker (1972) and Webb and Tracey (1972), also outlined by Mackey et al. (2001), (discussions are dominated by explorations of Torres Strait's regional role as an Australasian north-south ecological boundary or filter. See also Turner et al. (2001) for further discussions on the ‘bridge or barrier’ debate). Stanton et al. (2008) provide description and mapping of 27 broad Torres Strait vegetation groups, classified within Queensland's regional ecological framework. At a scale of 1:100,000 Neldner and Clarkson (1995) also map and describe Australia's Cape York Peninsula including Torres Strait (20 different vegetation communities are noted for Mua and Badu). Wannan (2008) provides an outline of island vegetation on Mua, while commenting that (1) the westernsouthern continental, including larger islands Mua and Badu, have the broadest array of plant communities and species diversity in Torres Strait, but that (2) Torres Strait remains poorly known botanically. Stanton et al. (2008) agree that island biodiversity values are largely unrecognised. The author observed island vegetation as heterogeneous in structure and composition. The canopy genera of Corymbia, Eucalyptus and Melaleuca are common, with the latter more widespread on the low relief plains. Lowland woodlands incorporate a more significant graminoid-herbaceous matrix than island hillslopes,

C. Rowe / Quaternary International 385 (2015) 56e68

and are more likely to display mixed (sub)-canopy components (non sclerophyll Myrtaceae, Arecaceae and/or malacophyll semideciduous taxa). Tree height and density increase with moisture availability, but decline on approaching the coast. Interspersed with woodland, occupying drainage channels, rocky outcrops, older sand ridges and/or hill-swales, vine-thicket or monsoonal closed-forest is found. Casuarina and Cocos grow above the coastal high water mark and mangroves (Rhizophoraceae-dominated) occupy sheltered, shallow embayments and estuarine floodplains. Communities adjacent to mangrove stands differ according to local variation in elevation, drainage and salinity; coastal Pandanus swamp conditions, sedgeland or salt-flats may or may not be present. On Mua and Badu economic expansion has been minimal. The primary impact of house, small business, and air-strip or road construction has been the loss of lowland woodland and mangrove forest. The Islander practice of landscape firing was ethnographically recorded in the 19th century (see Haddon, 1935) and is common today. Whether a modern coordinated-managementsystem, verses a randomly-implemented (independent) burning regime, is in place was not clear to the author. Three sites are included within this study (see also Rowe, 2006a, 2007). Boigu Gawat 1 (BG1) and Boigu Gawat 2 (BG2) are situated toward the west-central interior of Mua (centred on 10
080 S, 142
140 E), approximately 1.3 km apart and 3.0 km from the nearest coast. Swamps BG1 and BG2 lay within an expansive sand plain and degraded dune system, positioned less than 15 m.a.s.l. The swamps occupy dune depressions, contained within a zone isolated from two creek systems which currently drain the area. Double Creek and Kai Creek lie to the north and south (respectively) of the swamps and drain westward to a common outflow. On Badu, swamp Bar20 is situated 2.0 km inland of the southeast coastline, north of the Badu township (10
080 S, 142
090 E, 10 m.a.s.l). Bar20 similarly occupies a topographic hollow within a relict system of sand dunes (Figs. 2 and 3). Each swamp is dominated by Melaleuca leucodendron and Melaleuca viridiflora, the latter forming a near-continuous canopy at BG1. On the northern BG2 perimeter Pandanus species (spp.) and unidentified leguminous shrubs intermix with Melaleuca. Restionaceae and general Cyperaceous species discontinuously fringe each swamp and extend into shallow water zones. Water depth is seasonally variable and where receded, ground surfaces consist of Melaleuca leaves and fibrous root-matting. At the time of sampling BG2 and Bar20 demonstrated an open water diameter area greater than 50 m, less than 1.5 m deep and with no obvious outlet. The marginal zone of Melaleuca forest varied in area between sites, with BG1 the smallest site at a total area 70 m in diameter. Standing water was absent from BG1 at the time of sampling, although the swamp and associated sediments were not dry. Open Corymbia and Eucalypt woodland with well-established grassland occupy the surrounding sand plain and dune slopes. Pockets of mixed forest occur occasionally in swales but predominately on elevated ground and incorporating species of Acacia, Bombax, Terminalia, Ficus, unidentified lianes and Cycas media. The open woodland also incorporates sedgeland patches, where sediments appear poorly drained. No recent fire activity was evident.

59

Fig. 3. Photographs of sites Boigu Gawat 1 (a), Boigu Gawat 2 (b) and Bar20 (c).

2. Field and laboratory methods Sediment cores were recovered using a sidewall d-section corer (see Moore et al., 1991). Multiple cores were collected from alternate positions within each swamp, only to reveal similar stratigraphic profiles. Each core from the approximate swamp centre was therefore selected for analysis. Sedimentary characteristics

were recorded in the field, and observed again on return to the laboratory. Pollen and charcoal subsamples were collected at 4 cm intervals for BG1 and BG2 and 8 cm for Bar20. Dating material was collected down-core based on changes in stratigraphy; bulk sediment samples were submitted for Accelerator Mass Spectrometry (AMS) dating at the Australian Nuclear Science and Technology

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C. Rowe / Quaternary International 385 (2015) 56e68

Organisation (ANSTO; see Tuniz (2001) for technique descriptions). Bulk sediment sampling was preferred given the overall low organic nature of the core material. Low pollen concentrations further excluded direct dating of pollen residues. Laboratory processing of pollen samples followed standard Quaternary techniques as outlined by Brown (2008); selected to at first disperse then progressively remove humic-acids, calcium carbonates, bulk organics and cellulose, silicates, and to ensure pollen wall feature visibility (including Na4P2O7, KOH, HCL, Acetolysis, HF and C2H5OH washes). Lycopodium spike additions provided relative concentration calculations of both pollen and charcoal. Microscopic charcoal was counted within pollen preparations and defined as fragments of angular form, black, opaque and between 10 and 125 mm in size. Macroscopic charcoal particle (>125 mm diameter) counts were obtained from dispersed, sieved and bleached sediment samples (10% Na4P2O7 125 mm scale-sieve, 8% NaClO respectively). The microscopic charcoal record is deemed representative of regional (island catchment) fire activity; macroscopic particles are viewed as capable of only short distance travel, reflective of local fire (Whitlock and Larsen, 2001; Whitlock et al., 2010). Pollen/spore identification was carried out at Monash University, based on regionally appropriate reference collections provided by the School of Geography and Environmental Science. Flowering material collected while in the field also served as reference. Notably, collection of modern island flora assisted in the identification of Myrtaceae and the isolation of Melaleuca and Eucalytpus grain types (Rowe, 2012). Chalson (1989) and Churchill (1957 quoted in Chalson, 1989) also facilitated Myrtaceae pollen differentiation. For data presentation the Tilia suite of programs has been used (spreadsheet application Tilia-1.7.4 and graphing counterpart TGView-2.0.2 e Grimm, 1991; CONISS provided a stratigraphicallyconstrained cluster analysis for numerical zonation e Grimm, 1987). Pollen counts are expressed as a total pollen percentage, with a sum value of 200 grains minimum. 3. Results 3.1. Stratigraphy and chronology Coring at Boigu Gawat 1, 2 and Bar20 revealed firm brown-grey (5Y 4/1 to 5 YR 4/1) clay underlying black (7.5 YR 1.7/1 to 10 YR 1.7/ 1) peaty deposits. The clay units incorporated a coarse sand component with minor, sporadic decomposed plant remains contributing to localise staining of the clay. Heavy compact clay prevented hand-action sediment coring beyond the 50e75 cm collected depth. Additional auguring to 300 cm depth was undertaken during previous geomorphological surveys of the interior Mua plains. Here, Barham and Harris (1985, 1987) describe a deeper grading of the grey clay into leeched white gritty sands and thin bands of gravel. From BG1, 2 and Bar20 the upper peaty unit contained minimal sand and included weak to strongly decomposed leaf and bark fragments. Fine rootlets were common at the surface. A detailed stratigraphic, diagrammatic description for all sites is presented in Rowe (2006a, 2007). Seven out of eight radiocarbon AMS dates provide chronological control. The lower Bar20 sample OZG-585 has been eliminated from discussions; the dating reversal likely incorporating younger carbon either before or during core extraction. Organic material may have carried down in solution as part of the long-term site drainage, or transferred through ground-water, resulting in temporal contamination. Given the inorganic nature of the clay little contaminative material was likely required to affect dating analysis. All sample depth, age and calibration results are presented in Table 1.

Table 1 Boigu Gawat and Bar-20 core sample 14C AMS results (calibration methodology, Fairbanks et al., 2005, Ramsey, 2009; ). Site

Depth (cm)

Sample ID

14 C age, yr BP

BG1 BG1 BG2 BG2 B20 B20 B20

20 38 14 24 16 46 60

OZG-587 OZG-588 WK-14684 OZG-071 OZG-584 OZG-069 OZG-585

1080 2500 726 2560 1490 2610 720

± ± ± ± ± ± ±

50 50 38 90 50 80 40

Calibrated range, cal yr BP (95.4% probability)

Calibrated single/median age, cal yr BP

1058e900 2622e2357 779e621 2753e2357 1411e1258 2784e2359 728e634

981 2612 671 2674 1370 2729 692

± ± ± ± ± ± ±

47 111 19 120 46 79 24*

*Not recorded in analysis due to dating reversal (see text for explanation).

3.2. Pollen and charcoal The palynological diagrams of BG1, BG2 and Bar20 are presented in Figs. 4e6 respectively. Each diagram has been divided into two data zones. Pollen and spores were subdivided into Melaleuca, sclerophyll and mixed woodland taxa (sclerophyll and monsoon forest-associated taxa), herbaceous taxa, sedges and Pteridophyta. The pollen was divided in such a way to help distinguish woody and canopy taxa, notably dryland woody-grassland matrices from wetter adapted growth in a mimic of local to catchment scale vegetation. Poaceae pollen abundance is linked to ‘openness’ in regional woodland cover, although the warnings of Bush (2002), that it is important to recognise the importance of Poaceae pollen contributions derived from surrounding wetlands in the interpretation are heeded, as this may overstate the importance of dry episodes if Poaceae is regarded as a regional indicator. The macroand microscopic charcoal results demonstrated the same trends with depth at each site (Rowe, 2006a, 2007) and only the latter are included. 3.2.1. Zones 1 Insufficient pollen concentrations for standard percentage analysis were encountered in zones 1; only one sample per zone achieved the desired pollen sum. Each of these samples is used as a guide to the remainder of zones 1, presented in a presence/absence format. Boigu Gawat 1: 50e20 cm, >2612e981 cal BP. At 28 cm herbaceous taxa dominate, comprising 77% of the pollen sum. Leptocarpus characterises the assemblage, accompanied by Poaceae and Cyperaceae (including both Cyperus and Schoenus). Melaleuca, Eucalyptus and Myrtaceae exhibit relatively low values. Pandanus and Dodonaea are also represented in the woodland spectra. In the record of presence/absence, Leptocarpus is consistently present down core with the remaining taxa scattered in their presentation. Melaleuca features toward the top of this unit with Myrtaceae and Pandanus. Micro-charcoal is present in almost all samples. Boigu Gawat 2: 50e26 cm, >2674 cal BP. Herbaceous taxa are the dominant pollen types. Leptocarpus comprises 43% of the pollen spectrum with lower values of Poaceae and Cyperaceae types. Myrtaceae, and secondly Eucalyptus, are the major woodland taxa. Pandanus, Sapindaceae and Banksia have only minor representation. Melaleuca achieves a value of 13%. Re-enforcing the record observed at 44 cm depth, Leptocarpus is present in all other samples of zone 1. Cyperus also maintains a consistency and is recorded with Schoenus toward the upper part of the zone. Poaceae is present mid-zone. Myrtaceae is absent from the basal sample. Eucalyptus and Pandanus are present but interspersed, and Melaleuca is recorded through the lower part of this zone. Charcoal particles are found throughout.

tio n en tra C ha rc oa lC on c

el al eu ca Sc (M le ro yr ph ta ce yl H l& ae er bs ) m ix & Se ed gr as dg w oo se es dl s an d M el al eu ca (M yr ta Eu ce ca ae ly ) pt M us yr ta c e (M y ae rt (u ace Pa nd a nd iff e) an er D en od us tia ( o C na Pa te n as e d) ua a ( dan M ri S a el na ap c e as a ( in Ba tom Ca da e) nk a sua cea Ac sia (M rin e) ac (P ela ac Le ia rot sto eae pt (M ea ma ) o c i m ce t ar os ae ace pu ac ) ae s ea ) (R e es ) tio na ce ae ) Po ac e Eu a e ph As orb te ia C rac cea yp ea e e e Sc rac ho eae en (u C yp us nd er (C iffe Po as ype ren lle (Cy ra tia c n t C per eae ed) on a ce cea ) nt e) ra tio n

M

(c al ep yr B th P) (c m ) D

Ag e

Zone

0

5

C. Rowe / Quaternary International 385 (2015) 56e68

2

10

15

981±47

20

25

30

1

35 2612±111 40

45

50

20

40

60

80 100

20

10

20

10 5

5

10 5

5

20

40

60

80

10 5

5

10

10

10

I --- Sclerophyll & mixed woodland --- I I --------- Herbs & grasses -------- I I -- Sedges -- I

dark organic mud

sand

10 20 30 40 50

100 200 300

x10 3 grains/cm3

x103 particles/cm3

0.08 0.16 0.24 0.32 Total sum of squares

brown-grey clay

Fig. 4. Boigu Gawat 1 pollen (percentage and presence) and microcharcoal (concentration per cm3) assemblage plotted against depth and calibrated age. Percentages derived from total pollen sum inclusion (see also Rowe, 2006a, 2007).

61

671±19

2674±120 5

10

15

2

20

25

30

35

40

1

20 40 60 80 100 20 40 60 I ---- Sclerophyll & mixed woodland ---- I I ---------- Herbs & grasses ----------- I I ---- Sedges --- I 20 20 10 10 5 5 5 20 40 20 5 5 5 10 10 10 10 20 40 60 80 20 40 60 80 100

x103 grains/cm3 x104 particles/cm3

C. Rowe / Quaternary International 385 (2015) 56e68

50

(c al ep yr B P) th (c m )

us

(M yr ta

oa l

C ha rc

C on ce

nt ra

tio

n

ce ta ae ce ae ) Pa (u nd nd iff D anu er od en o s( tia C n a Pa te as ea nd d) ua (S an Ba rin a a c p nk a i n ea ( Sa sia( Ca dac e) pi Pr sua ea o n e Le da tea rina ) c pt ce c oc ae ea ea ar ( u e) e) pu n s dif (R fe es ren tio tia na te Po ce d) ae ac ) ea e Eu ph As orb te iac r C ace eae he a no e (un C di yp p o d ffe e i re Sc rac ace nt ia ho ea ae te e e d) C n ( yp us un er (C dif El us y fe eo (C pe re c y ra nt Po har pe cea iate lle is ( rac e) d) C ea n y C on pe e) ce rac e nt ra ae tio ) n

M yr

Eu ca ly pt

M el al eu ca Sc (M le yr ro ta ph ce H yl ae er l & bs ) m Se & i x ed dg gr es as w se oo s dl M an el al d eu ca (M yr ta ce ae )

D

Ag e

62

0

Zone

45

Total sum of squares

0.2 0.4 0.6

dark organic mud sand brown-grey clay

Fig. 5. Boigu Gawat 2 pollen (percentage and presence) and microcharcoal (concentration per cm3) assemblage plotted against depth and calibrated age. Percentages derived from total pollen sum inclusion (see also Rowe, 2006a).

10 5

1370±46

2729±79 15

20

75

dark organic mud 2

25

30

35

40

45

50

55 1

20 40 60 80 100

sand

20 40 I ----------------- Sclerophyll & mixed woodland ----------------- I I--Herbs & grasses---I I-- Sedges -- I I-- Pterid.-- I 20 20 40 10 5 5 5 10 5 5 5 5 10 20 10 5 5 20 10 10 10 10 10 x103 grains/cm3 x104 particles/cm 3

10 20 30 40 40 80

C. Rowe / Quaternary International 385 (2015) 56e68

0 P)

)

yr B

(c m

el al eu ca Sc (M le yr r H op ta er hy ce ae Se bs l l & & ) dg g m Pt es ra ixe ss d er id es w oo M oph dl el an al yta eu d ca (M yr ta ce ae Eu ) ca ly pt us (M M yr yr ta ta ce ce ae ae ) (u nd i f fe Pa re nd nt ia D an te od u s d) o C na (Pa as ea n ua ( da Ba ri Sa na nk na pin ce Ac sia (Ca da ae ac (P su ce ) Sa ia ( rote arin ae) pi Mi ac ac H nda mo ea ea i b c sa e ) e ) is e c Lo cus ae/ ea ra (M Me e) n R th alv liac ub ac e e ac a i Le ace ae eae e pt ae ) o Po car ac pu ea s ( R e es Eu tio ph na As orb ce te ia ae C rac cea ) as ea e si e C a( yp C er ae as sa (C lpin Sc yp ia ho er ce El e n ac ae eo us ea ) c ( h C e) D av aru yp e a s As llia (C rac pi ce yp ea Tr dia ae/ era e) ile ce Po ce t a Po e (p e/P lypo ae) lle sil oly di a n p a C te) od cea on ia e ce (m ce ae o nt ra (m nol tio on ete n ol -o et rn e- #) ps C il a ha te rc ) oa lC on ce nt ra tio n

h

(c al

ep t

M

D

Ag e

Zone

60

65

70

Total sum of squares

0.1 0.2 0.3 0.4

brown-grey clay

Fig. 6. Bar20 pollen (percentage and presence) and microcharcoal (concentration per cm3) assemblage plotted against depth and calibrated age. Percentages derived from total pollen sum inclusion (see also Rowe, 2006a, 2007).

63

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C. Rowe / Quaternary International 385 (2015) 56e68

Bar20: 72e40 cm, >2729 cal BP. The sample at 48 cm is dominated by sclerophyll and mixed woodland taxa, notably Myrtaceae and Eucalyptus. From the mid to upper parts of the zone the presence of woodland taxa increases, where Acacia, Pandanus, Sapindaceae and Hibiscus are recorded. Swamp taxa such as Melaleuca comprise 11% of the pollen sum in sample 7, slightly higher than Cyperus and Leptocarpus. In the record of presence/absence, Myrtaceae, Eucalyptus, Melaleuca and Cyperus are consistently recorded down core with the remaining taxa scattered in their presentation. Charcoal is present in all samples.

4. Discussion

3.2.2. Zones 2 The zones 1e2 transitions are marked by improved preservation, higher taxon diversity and a significant rise in pollen and charcoal concentrations. Here, zones 2 show near correspondence to the dark, organic stratigraphies. The dating results place the zones 1e2 transition at c.2700 cal BP for BG2 and Bar20, and c.1000 cal BP at BG1. Boigu Gawat 1: 20e0 cm, 981 cal BP to modern. A prominent feature of the zone is the rise to peak values in pollen concentration at the surface. Charcoal concentrations rise substantially half way through the zone with a similar rise in charcoal/pollen ratios. From zone 1 the herbaceous component remains high. The most strongly represented pollen type is Leptocarpus which contributes up to 75% of the pollen sum. Myrtaceae is the best represented of the woodland taxa; highest core values are achieved in the basal sample of this zone, and decrease toward the top. Eucalyptus also decreases in abundance through this zone, though less dramatically than other Myrtaceae. The few other woodland taxa show only minor representation (Dodonaea, Acacia, Melastoma, Banksia and Pandanus). Melaleuca increases to attain its highest percentage of the core in surface sample 1. Poaceae also increases toward the top of the core. Boigu Gawat 2: 26e0 cm, 2674 cal BP to modern. At the base of zone 2 herbaceous percentages are high, only to decline mid-way through the zone. Melaleuca and woodland taxa increase in abundance, the former more erratically. Sedge taxa (Cyperaceae, Cyperus, Schoenus and Eleocharis) are best represented at the base of zone 2 and Chenopodiaceae have their only representation here. A sharp mid-zone peak in Melaleuca pollen occurs, largely at the expense of herb and sedge taxa. Toward the top of zone 2 more uniform representation of each pollen group occurs. Sclerophyll woodland taxa and Melaleuca dominate over herbs and sedges (Melaleuca increases slightly). An increase in the contribution from secondary tree or sub-canopy taxa also occurs; higher values of Pandanus and Dodonaea are recorded with small numbers of Casuarina and Banksia. Leptocarpus values dominate over Poaceae and Euphorbiaceae, and Cyperus is the principle sedge. Given the pollen consistency, the upper samples of zone 2 are best characterised by an initial jump in charcoal concentration, with further increase at the surface. Likewise, pollen concentrations show a marked rise to the surface. Bar20: 40e0 cm, 2729 cal BP to modern. An increase in charcoal concentration is accompanied by a zone 2 decrease in sclerophyllmixed woodland taxa. Myrtaceae shows a clearly defined decrease in pollen abundance. Eucalyptus also has slightly lower values midzone only to increase toward the upper zone. All other woodland taxa form minor pollen components, but trend toward increased diversity toward the core surface (Pandanus, Acacia, Dodonaea, Casuarina, Hibiscus, Loranthaceae and Banksia). Melaleuca steadily increases and attains its highest representation of the core in sample 1. An increase in herbaceous taxa is accounted for by the dominant rise in Poaceae and Leptocarpus, accompanied by lowpercentage Euphorbiaceae and Asteraceae, Rubiaceae and Cassia. Sedge taxa and spore types show consistent representation.

4.1. Wetland ecology

The consistency observed in this study (between larger sites, between fossil pollen samples and radiocarbon analyses) suggests the records presented are representative of vegetation and fire trends over a reasonably large inter-island area. The Boigu Gawat swamps on Mua and Swamp Bar20 on Badu are used therefore as a measure of general Holocene Torres Strait environments, and in an explorative interpretation of the late Quaternary northern Australian region more widely.

A two-phased late Holocene transition in wetland site ecology is evident on Mua and Badu. It is argued that the Figs. 4e6 sparse pollen and charcoal records are not prohibitive toward environmental interpretation. Together, changes in sedimentary character and microfossil presence/concentration are indicative of a shift in site hydrology. An earlier phase incorporating fluctuating moist-dry habitats has been recorded, and as a pre-cursor to more extensive stable-boundary swamp and associated swamp-forest establishment. This transition in island site hydrology dates close to 2700 cal BP at BG2 and Bar20. Swamp forest conditions prevailed at BG1 from approximately 1000 cal BP. In zones 1, low pollen concentrations have been derived from largely inorganic clay-based sediments coarse in texture. Prior to 2700e1000 cal BP site moisture regularly varied, but not to the extent of drying out completely or for periods long enough to remove all pollen completely (see also Head and Fullager, 1992). Wet season water accumulation (and/or flooding) combined with sub-aerial exposure during dry seasons, and low-to-ground herbaceous plant growth, have provided the conditions resulting in the observed degradation of pollen, through oxidation and microbial activity. The basal clay sediments would have impeded drainage at the core sites with higher precipitation, but are just as likely to have developed into evaporative cracked-clay surfaces during dry periods. Through zones 1 all sites were land areas subject to inundation, but where water was not present long enough to support wetland/aquatic or swamp-forest vegetation. This resulted in environments akin to modern sedgelands forming in dune swales; tracts of ill-drained ground with seasonal patches of pooled water in which herbs (notably Leptocarpus), sedges and fringing grasses form the dominating vegetation component. Pollen composition prior to 2700e1000 cal BP suggests Melaleuca in the vicinity of the herbaceous site cover, but with substantial woody growth likely to have initially struggled to develop on the coarse, inorganic sediments with only periodic (transient) moisture availability. In zones 2, greater dark organic deposition and pollen preservation implies a change from oxic to anoxic conditions (Schulmeister, 1992). From 2700 to 1000 cal BP site expansion and permanent water body development, incorporating an increasing degree of free-standing open water, has led to the accumulation of organic muds and promoted local expansion of woody plant growth. The zones 2 core site encroachment by woody plant taxa has been dominated by Melaleuca, but includes taxa such as Pandanus, Fabaceae and possibly Asteraceae and Euphorbiaceae. The process here may be somewhat of a positive feedback loop: Melaleuca expansion (and the like) has been encouraged by organic sedimentation and moisture permanency, but in itself also facilitates organic accumulation and moisture retention (Rowe, 2006a, 2007). In zones 2, Leptocarpus records remain dominant, but an intermingling with Cyperaceae (Cyperus and Schoenus) and fern types suggests diverse herbaceous strata in the shallower waters and damp grounds at the swamp edges. Changing patterns in

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herbaceous swamp taxa through the lower section of BG2 zone 2 reflect an initial mosaic of swamp-like conditions as water levels increased. A substantial peak in Cyperaceae types occurs at the start of zone 2. These conditions alter further as a decline in the lower herbaceous strata occurs with on-site tree establishment. In this respect, Grindrod (1988) notes that herbaceous swamp taxa vary in luxuriance and composition according to the amount of shading imposed by a Melaleuca canopy. In interpreting similar pollen trends at Bar20, an existence of open water from c.2700 cal BP is inferred from a high proportion of pollen from regional terrestrial (woodland) vegetation, and comparatively low input from local herbaceous pollen. Increased water depth, and therefore an absence of plant growth directly on the core-site floor, is in keeping with a reduced zone 2 representation of Leptocarpus. In a vegetation discussion of Cape York Peninsula, Mackey et al. (2001) describe (semi)permanent surface water as critical for the persistence of important elements of the flora; that such locations support vegetation differentiated from the surrounding landscape matrix, constituting important dry season habitat resources and refugia. The existence and expanding distribution of such ‘wet’ vegetation types dates to 2700 14C cal BP on both Mua (BG2) and Badu (Bar20). The initiation of organic deposition and swamp forest formation at BG1 occurs at least 1700 years later. The reasons for a more recent onset date of c.1000 cal BP are largely unclear. It is possible that BG1 zone 2 sediments are representative of the most recent phase in continual swamp accumulation. Under such a scenario, and as the smaller-shallower topographic depression, the BG1 sedimentary record was regularly removed by disturbance events (e.g. inwash or dry erosion, burning). In holding less water, sediment interruption through repeated desiccation would also have been prolonged. What remains therefore is the record of most recent site stability (of water availability and woody canopy growth). No evidence of such disturbances exists for BG2 and Bar20. As the two larger dune depressions in this study, modest disturbance events are likely indiscernible, with site size and depth acting as a buffer. A significant late-Holocene ecological trend for the Torres Strait therefore is the onset of freshwater swamp forest habitat around 2700 14C cal BP, but more specifically, incorporating a phase of notable site stability to within the last 1000 years. 4.2. Woodland and fire ecology Throughout the late Holocene records sclerophyll woodland dominates the vegetation on the sand plain and dune slopes surrounding Boigu Gawat as well as Bar20. Poor pollen preservation inhibits detailed discussions concerning changes within the sclerophyll communities prior to 2700 14C cal BP. Regional pollen types are less likely to have been preserved and incorporated into the exposed site clay sediments than local pollen. Through zones 1 Myrtaceae, Eucalyptus and Pandanus are present as significant components of the regional woodland. Therefore, since at least 2700 14C cal BP similar taxa dominate the terrestrial vegetation canopy as today. At this same stage in the record secondary tree taxa are not extensively represented, and prior to 2700 14C cal BP the woodland ground cover was dominated by grasses. On Mua and Badu from 2700 14C cal BP (and notably c.700 14 C cal BP) the dryland vegetation undergoes some changes in structure and composition. A shift toward more uniform uppercanopy openness is proposed to have influenced composition. A less continuous cover of Myrtaceae/Eucalytpus is observed, encouraging a gain in mixed forest (arboreal) and/or understorey taxa. Therefore, although Myrtaceous taxa remain dominant, the regional woodland diversifies, possibly becoming a more tiered community. Sapindaceae, Acacia, Dodonaea, Casuarina, Hibiscus,

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Banksia and Loranthaceae become established, generally increasing within the woodland community (and/or of a higher frequency in localised areas). The rise in Pandanus toward the modern period is used to suggest that such localised areas include disturbance zones (Prebble et al., 2005). Pandanus is common today in marginal habitats, located in disturbed and open areas not only in swamp forests but extending into dryland woodlands. Pandanus is also tolerant of frequent burning. (Dodonaea is also observed through disturbed Indo-Pacific habitats, including environments with widespread fire regimes e Prebble et al., 2005). With elevated Poaceae, raised percentages of Asteraceae and Eurphorbiaceae further indicate steadily diversifying woodland, incorporating more prominent shrub and broad-leaf ground cover. Prior to 2700e1000 14C cal BP the continued presence of charcoal particles attests to fire as on ongoing component of island woodland environments. In the approximate period 2700e700 14 C cal BP at BG2 and Bar20, and beginning 1000 14C cal BP at BG1, fire activity is comparatively less to that of the past c.700 years, as recorded by both micro and macro size charcoal particles. Burning increases both regionally and locally (Rowe, 2006a, 2007) to each core site, coinciding with observed changes in island woodland and potentially extending into the margins of each swamp forest habitat. In this case, burning appears to facilitate an opening of the woodland (canopy and subcanopy) and encourage a diverse woodland flora, creating a mosaic of patches of different plant components and likely at different successional stages. 4.2.1. Regional comparisons and climatic interpretations The interpretation of swamp vegetation history on Mua and Badu is that island water reliability within the past 2700 years is generally good. An overall increase and reduced fluctuation in water accumulation has promoted swamp development and forests with a relatively dense Melaleuca canopy, particularly around lowlying depressions (Rowe, 2007). This is in accord with wetland peat establishment and swamp forest expansion in central Cape York Peninsula after 3000 BP (Stephens and Head, 1995, Butler, 1998, Stevenson et al., submitted for publication), of hydrological swamp changes in the past 3000e2000 years at Weipa western Cape York Peninsula (Stevenson et al., submitted for publication), initiation and inundation of late Holocene herbaceous swamp communities at Waigani in southern New Guinea (2500 BP e Osborne et al., 1993) and for the onset of organic swamp deposition as far reaching as the Kimberley (2100 BP e Head and Fullager, 1992). Fluvial sedimentation is also observed to have increased in the Kimberley from around 3000 BP, corresponding to greater presence of ferns and aquatics (McGowan et al., 2012). A change in lake character, from a fluctuating brackish water body to one of permanent fresh water, is recorded at Three-Quarter Mile Lake (<5000 yr B.P, northeastern Cape York e Luly et al., 2006). Schulmeister (1992) similarly describes a more stable, expansive lake phase on Groote Eylandt (Gulf of Carpentaria) within the past 3500e1000 years than previously (see also Prebble et al., 2005). This late Holocene synchronous phase of wetland deposition and/ or expansion in lowland Greater Australia suggests some broader regional climatic control. Swamp establishment in the Torres Strait can be linked to the climatic regimes of Holocene tropical Australia as discussed by Schulmeister (1992, 1999), Schulmeister and Lees (1995) and Prebble et al. (2005). A 10,000 to 4000e3700 BP phase is marked by increasing effective precipitation (EP) (with a maximum toward the end of the period), and a gradually enhanced monsoon within the context of an otherwise stable climate. The period after c.4000e3700 BP incorporates increased variability in EP and an unstable climatic system (Schulmeister, 1992; Schulmeister and Lees, 1995; also; McGowan et al., 2012). Prebble et al.

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(2005) quote an increase in EP within the past 3000e2000 years. Schulmeister and Lees (1995) state an EP recovery in the last 2000 years. A resulting model of Holocene palaeoclimate combines changing intensity across the Walker Circulation and adjustments in the position of the Intertropical Convergence Zone (ITCZ). From the mid- to late-Holocene a strengthening Walker ~ o-Southern Oscillation (ENSO) is outlined Circulation and El Nin for the region (after 4000 BP). This climatic shift is then related to fluctuation in the state of the Australian Monsoon (McGowan et al., 2012). Moss et al. (2012) vary in their description of key Holocene climate alterations; in north-eastern Queensland an onset of drier more variable conditions following a mid-Holocene climatic optimum is described, with greatest variability occurring around 2500e1700 years ago. Climate variability is linked to enhanced ENSO activity beginning mid Holocene (5000 BP onwards). Conversely, Moss et al. (2012) provide no evidence of a late Holocene EP recovery which suggests some difference in north Australian rainfall patterns (e.g. the extent of EP decline, extent of monsoonal penetration or phases of recovery) according to geography. Study site sensitivity may also be questioned. A comentioning of ENSO triggers and/or ENSO activity however does suggest this is a significant climatic variable with widespread impact. Following Head and Fullager (1992), climatically variable periods may include not only increased dryness but periods of increased precipitation capable of maintaining water levels (including water tables) and thus sedimentation at sites such as in Torres Strait. Any increased EP would further encourage vegetation transition. McGowan et al. (2012) discuss the Holocene reemergences of intense monsoon systems in the Kimberley, and interestingly observe a transition to stable monsoonal climate with reliable surface water supply after 1300 BP, timing similar to the continuing presence of site BG1. Further, Sturman and Tapper (2001) observe in modern regional climate that early and longer monsoon onset tend to precede ENSO events. Such monsoon extent may have been enough in maintaining wetland sites through drier periods. From the north Australian Holocene geomorphological record, periods of dune emplacement between 2600 and 1800 BP have been observed (see studies quoted in Schulmeister, 1992; Stevenson et al., submitted for publication, including sites at Cape Flattery, Cape Arnhem and Groote Eylandt). These are interpreted to reflect the stabilisation of previous wide-spread dune activation events, and that given this occurs across much of northern Australia, may be climatically driven (Schulmeister, 1992). Dune stabilisation would in turn encourage swamp stability at Boigu Gawat and Bar20, and at sites more widely as listed above. On Mua and Badu, swamp presence occurs along-side an opening of woodland structure, increased mixed composition as well as disturbance (marked by indicator taxa and charcoal). Net positive island water balances would encourage a biomass structure and diversity capable of hosting (and utilising) increased burning, whether the result of climatic variability or human intervention. Disturbance may also have been at a level beneficial to diversity, rather than opposing it. 4.3. Archaeological interactions Island inland swamp and swamp-forest establishment coincides with the Torres Strait ‘event horizon’ settlement and demographic expansions 2600e2500 years ago. Both McNiven and Hitchcock (2004) and Crouch et al. (2007) declare that the significance of freshwater swamp development should not be underestimated in prehistory; that, dramatic increase in regional human activity

would have been accommodated by increasing terrestrial attractiveness in the form of fresh water availability. Drinking water was (and remains) a key limiting factor for island occupation (McNiven and Hitchcock, 2004). The Torres Strait (like much of northern Australia and southern PNG) experiences a long dry season, with profound biological implications (Mackey et al., 2001). The development of Biogu Gawat and Bar20 swamps, and by inference a regional expansion of swamps, would have increased the late Holocene options available to occupants of Torres Strait. It would have made the region more favourable for occupation, and reduced stress toward the end of the dry season or across a strong phase of ENSO activity (c.f. Stephens and Head). It is difficult to relate the Boigu Gawat and Bar20 pollen records to changing abundances in plant foods (as listed by Harris, 1976, 1977; as related in central Cape York Peninsula by Stephens and Head, 1995). Nonetheless, it is argued that changes in the extent and permanency of swamps were significant for food and not only water resource exploitation strategies, complimentary to the use of marine ecosystems in the Torres Strait (c.f. Stephens and Head, 1995). Osborne et al. (1993) similarly suggest wetland expansion inland of the southern Papua New Guinea coast would have assisted the establishment of coastal communities. Relationships between increased Holocene climatic variability and mainland Aboriginal tropical subsistence strategies are further discussed in Cosgrove et al. (2007). Where archaeological reference is made to Torres Strait as a constructed landscape, questions are posed as whether or not any island environment is pristine (McNiven and Hitchcock, 2004; McNiven, 2008). With intensive human occupation back at least 2500 years McNiven and Hitchcock (2004) conclude it's highly likely that most marine and terrestrial habitats have been modified to some extent by Islanders to support their culture. Crouch et al. (2007, 62) describe Islander-Island relationships as ‘mutually transformative’. The vegetation changes recorded as occurring on Mua and Badu are believed to have an anthropogenic dimension. This has been discussed elsewhere (Rowe, 2006a, 2006b; Crouch et al., 2007) and is particularly true for the development then maintenance of the open tree canopied inland woodlands. Increased burning can be linked to strategic Indigenous landscape management practices (Clarke, 2007; Crouch et al., 2007). Motivations for maintaining more open woodland may relate to mobility, ease of access to swamp and monsoon forest resources, for gardening, and/or to encourage woodland plant diversity for procurement. The latter is consistent with the lack of archaeological evidence for island hunting of terrestrial mammals such as macropods prior to 2600 within Torres Strait (McNiven and Hitchcock, 2004; Rowe, 2006a; Crouch et al., 2007). 5. Conclusion In a review of Australian tropical Quaternary change (Kershaw and van der Kaars, 2012) a major issue to emerge is the temporal and spatial differences in regional patterns of developing Holocene variability, in the intensity of change (particularly toward dryness) and their causes. The vegetation and landscape changes observed in the Torres Strait refine our understandings of long-term north Australian biogeography. A significant amount of wetland synchronicity in the lowland monsoonal tropics is suggested, also demonstrating degrees of variation when compared more widely, based on site sensitivity and geography, and with respect to the main rain-baring systems as well as ENSO. In the Torres Strait, late Holocene ill-drained sedgelands were replaced by combined swamp-forest and open water as moisture levels and site sedimentation stabilised. This replacement centres

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on 2600 cal BP (but is dependent on dune swale size). Simultaneously, gradual opening of island woodland encouraged an increase in terrestrial floral diversity and potentially more complex vegetation structure. Landscape burning has also increased with time. The Torres Strait has a unique social and material culture. This study has provided an opportunity to integrate palynological with archaeological information. This in turn has extended the view to which island vegetation may have been anthropogenic developed and maintained. Palaeoecological results from Mua and Badu also lend support to existing archaeological interpretations for Torres Strait and provides additional insight into how island landscapes may have played a key role in facilitating permanent late Holocene settlement. Acknowledgements The author would like to thank the Mualgal and Mura Badulgal Corporation Committees and the people of Mua and Badu for their generous support and hospitality. Warms thanks are further extended to the Monash Geography field team for their help and company. The assistance received from Kara Rasmanis in drafting the figures is also gratefully acknowledged. The reviewers are thanked for their time and comments in evaluating this paper. This research was funded by a Monash University combined Arts FacultyeSchool of Geography and Environmental Science postgraduate scholarship. References Ash, J., Manas, L., Boson, D., 2010. Lining the path: a seascape perspective of two Torres Strait missions, northeastern Australia. International Journal of Historical Archaeology 14, 56e85. Bain, J.H.C., Draper, J.J., 1997. North Queensland Geology. AGSO Bulletin 240 Queensland Geology 9. Queensland Department of Mines and Energy, Brisbane. Barham, A.J., Harris, D.R., 1985. Relict field systems in the Torres Strait region. In: Farrington, I.S. (Ed.), Prehistoric Intensive Agriculture in the Tropics, BAR International Series, vol. 232. Oxford, pp. 247e283. Barham, A.J., Harris, D.R., 1987. Archaeological and Palaeoenvironmental Investigations in Western Torres Strait, Northern Australia. Institute of Archaeology, University of London and Department of Geography, University College, London. Final Report to the Research and Exploration Committee of the National Geographic Society on the Torres Strait Research Project Part IIB: JulyOctober 1985. (Unpublished report). Barham, A.J., 2000. Late Holocene maritime societies in the Torres Strait Islands, northern Australia e cultural arrival or cultural emergence? In: O'Connor, S., Veth, P. (Eds.), East of Wallace's Line: Studies of Past and Present Maritime Culutres of the Indo-Pacific Region. Modern Quaternary Research in Southeast Asia. A.A. Balkema, Rotterdam, pp. 223e314. Barham, A.J., Rowland, M.J., Hitchcock, G., 2004. Torres Strait bepotaim: an overview of archaeological and ethnoarcaeological investigations and research. In: McNiven, I.J., Quinnell, M. (Eds.), Torres Strait: Archaeology and Material Culture, Memoirs of the Queensland Museum, Cultural Heritage Series, vol. 3, pp. 1e72. Brisbane. Brady, L., David, B., Manus, L., 2003. Community archaeology and oral tradition: community and teaching cultural awareness on Mua Island, Torres Strait. Australian Journal of Indigenous Education 31, 41e49. Brown, C.A., 2008. Palynological Techniques. American Association of Stratigraphic Palynologists, Dallas, Texas. Bureau of Meterology (BoM), 2013. Climate Data Online: Monthly Statistics. Bureau of Meteorology (92637533532), Commonwealth of Australia. Accessed June 2013. http://www.bom.gov.au/climate/data/. Bush, M.B., 2002. On the interpretation of fossil Poaceae pollen in the lowland humid neotropics. Palaeogeography, Palaeoclimatology, Palaeoecology 177, 5e17. Butler, D., 1998. Environmental change in the Quaternary. In: David, B. (Ed.), Ngarrabullgan: Geographical Investigations In Djungan Country, Cape York Peninsula. Monash Publications in Geography and Environmental Science, Monash University, Clayton. Chalson, J.M., 1989. The Late Quaternary Vegetation and Climatic History of the Blue Mountains, New South Wales, Australia. University of New South Wales (PhD thesis). Clarke, P.A., 2007. Aboriginal People and Their Plants. Rosenberg Publishing, Sydney. Cosgrove, R., Field, J., Ferrier, A., 2007. The archaeology of Australia's tropical rainforests. Palaeogeography, Palaeoclimatology, Palaeoecology 251, 150e173.

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