Burial,erosion,and transformation of archaeological landscapes

Abstract

A high proportion of archaeological sites are located on the world’s shorelines and recent research has documented the vulnerability of these sites to coastal processes and climate change. However, archaeological landscapes on many temperate coasts have already been degraded as a result of changes in dune dynamics related to changes in dune vegetation. These changes have produced marked spatial and temporal variations in patterns of burial and erosion in transgressive dune systems. This paper examines the modification and conservation of archaeological landscapes from a biogeomorphic perspective, using the example of marram grass (Ammophila arenaria) invasion of dune systems in southern New Zealand. The impact of marram grass on dune system dynamics and the underlying archaeological landscape are complex. Full invasion may result in the general burial and protection of these landscapes, but the risk of degradation of sites is high during the invasion process. In southern New Zealand, marram invasion has resulted in the formation of stable foredunes, often associated with coastal progradation. Archaeological sites located close to the shoreline can be subject to either burial or erosion, or both, as marram grass establishes in the foredune zone. The spatial relationship between cultural sites and the shoreline may be lost as the coast progrades. The impact of marram invasion can extend throughout the hinterland dune system as a result of (i) dune mobility triggered by marram grass invasion and (ii) the development of a negative sand budget, which prevents or reduces beach-foredune-dune system sand exchange. The risk of degradation of the archaeological landscape can be significantly heighted by marram invasion, which can have profound implications for the preservation and interpretation of archaeological sites and materials. Paradoxically, dune system restoration may lead to the re-exposure of these sites, but the principal outcome of dune system restoration is expected to be a decline erosion (manifest as in deflation surfaces) and reburial of the archaeological landscape.

Keywords

Archaeology dunes marram invasion restoration

I Introduction

The coastlines of the world contain a disproportionate concentration of archaeological sites (Branigan, 2005) and a high proportion of these sites are vulnerable to coastal erosion (Erlandson, 2008, 2012). This vulnerability has been examined in relation to eustatic sea level (Kraft et al., 1985; Robinson et al., 2010), episodic erosion during storms (Bird, 1995; Westley et al., 2011), and anthropogenic activities, such as coastal sand mining, that modify coasts to the detriment of archaeological heritage (Fitzpatrick et al., 2006). Clearly, the conservation of archaeological sites or the prioritisation of archaeological effort depends on understanding coastal processes and there has been awareness for some time of the need to better integrate archaeology and geomorphology (e.g. Hassan, 1979). To-date, emphasis has been placed on coastal erosion, however, the erosion and burial of archaeological sites as a result of changes in early-successional plant species on sandy coasts has already occurred on many temperate coastlines. The potential for aeolian processes and dune dynamics to bury or expose archaeological sites has been noted (Dawson, 2013), but not closely examined from a biogeomorphic perspective. Moreover, recent efforts to develop archaeological vulnerability indices do not explicitly recognise vegetation or vegetation change as a risk factor (e.g. Reeder et al., 2012), although linear measures of shoreline flux do, inevitably, incorporate this attribute. The current paper, a collaboration between aeolian geomorphologists and coastal archaeologists, seeks to address this omission by examining case studies of vegetation change, dune dynamics, and archaeology in southern New Zealand.

Archaeological sites are likewise concentrated around the New Zealand coastline, which reflects the strong maritime focus of both Māori and early European economies. New Zealand has a relatively short history of human occupation by world standards. The first Polynesian colonists arrived at the turn of the 14th century and the first European settlements, in the form of sealing and shore-whaling sites, appeared some 400 years later. Yet, the archaeological record in New Zealand is rich and still relatively well-preserved. These individual sites are only a small part of the wider world in which people in the past moved, lived, worked, and made decisions. They can be conceived as being part of an archaeological landscape, with ideational, ritual, spiritual, and economic dimensions (Ashmore and Knapp, 1999). Archaeological sites, and the landscapes of which they are a physical component, occur in a range of dynamic geomorphic environments that are subject to coastal processes, so that few are immune to episodic erosion, accretion, and burial or shoreline dynamics. To some degree this dynamism resulted from occupation and phases of coastal change can be associated with Polynesian settlement.

The exact relationship between Māori and coastal dune systems has not been resolved. The morphostratigraphy of coastal deposits around the New Zealand coast records phases of dune system development during the late Holocene (McFadgen, 1989). The earliest of these phases, characterised by the development of extensive transgressive dune systems, precede human occupation of New Zealand. They have been attributed to variations in climate and sediment supply to the coast (McFadgen, 1989, 1994). But, there is widespread archaeological evidence that Māori occupied coastal dunes, including stable and forested dune systems, (McGlone, 1983, 1989; McWethy et al., 2010) and mobilised dune systems by firing vegetation. A strong association between vegetation change, dune mobility, and Māori is recorded in Northland (Brook, 1999; Brook and Goulstone, 1999; Coster, 1989; Elliot et al., 1995) and the Manawatu (Clement et al., 2010; Hawke and McConchie, 2006; McGlone and Wilmshurst, 1999). Four phases of dune activity and expansion are recognised in the Manawatu Region (Cowie, 1963; Muckersie and Shephard, 1995) and the penultimate phase, the Waiterere, is attributed to Māori occupation and vegetation loss. At this site, and indeed throughout New Zealand, the nature of occupation and the processes of vegetation disturbance by Māori have not been closely examined.

Indeed, the natural character of New Zealand’s dune systems prior to the arrival of humans is not well documented. The early 20th century descriptions of dune systems in New Zealand (e.g. Cockayne, 1911) followed over 800 years of Māori (and European) occupation of the coast. The dune systems that remain today are complex landscapes, which combine pre-human attributes, modified first by Māori and then European activity and further modified by contemporary coastal processes. The degree to which any particular dune system has been modified by the Māori probably varied in relation to the intensity of occupation – some isolated sections of coast in the South Island were probably seldom visited (probably seasonally) and by few people. The area and vegetation cover of individual dune systems might have been modified, probably resulting in increased sedimentation and dune system expansion, but the ecology of these dune systems had not been significantly altered. This was to change from the late 1800s, when the introduction of marram grass and certain other exotic dune species initiated the widespread ecological conversion of dunes from relatively open, sparsely vegetated, highly dynamic dune systems to densely vegetated grasslands.

Archaeological sites and landscapes associated with coastal dunes are extremely vulnerable to changes in landform and vegetation cover, driven by variations in natural processes. Problematically, the dune systems of New Zealand, from the far northern coasts of the North Island to the southernmost beaches of Stewart Island in the south, contain a rich record of human activity spanning the full duration of New Zealand’s history. There have long been concerns about the impact of ‘erosion’ on archaeological sites – namely the loss of site fabric through time (Bickler et al., 2013; Walton, 2007). But, natural processes in dune environments are complex, involving long-term interplay between such factors as sand supply and wind regime with the potential for erosion and stratigraphic mixing as well as site loss.

By failing to fully understand the geomorphic nature of dune systems, there is a danger that archaeologists will misunderstand the nature and history of the record in those places. This paper examines a poorly understood source of record loss – the introduction (and removal) of exotic grasses to the temperate coasts of the South Pacific. We will show that the introduction of these species, particularly Ammophila arenaria (marram grass, European beach grass) in the late 1800s, resulted in new processes and patterns of foredune formation and transgressive dune development that had significant implications for archaeological site survival and condition and, hence, for archaeological interpretation. It is not a simple matter of site loss; more complex and nuanced patterns of change have emerged that have altered the whole nature of the archaeological landscape. Importantly, however, the introduction and removal of marram leads to a number of specific and increasingly well-understood patterns of landform transformation, each of which has predictable effects on the archaeological record. This transformation was not restricted to southern New Zealand or, indeed, New Zealand. Marram grass has been introduced to most of the world’s temperate coasts. It is more widespread and assertive south of Auckland, but the geographic range of this species overlaps with a high proportion of recorded archaeological sites (Figure 1).

Figure 1.

The location of (a) recorded archaeological sites (from ArchSite, http://www.archsite.org.nz) and (b) Ammophila arenaria (from herbarium records). The location of Ammophila is indicative of its distribution and we know it is present and usually dominant in almost all beach and dune systems south of Auckland.

We discuss the relationship between exotic grasses, landform transformation, and the archaeological record, illustrated with examples from southern New Zealand – specifically the west coast of Rakiura (Stewart Island) (Figure 2) – where marram grass grows close to its climatic optima. Marram grass was introduced to Mason Bay deliberately in the 1930s and spread from these plantings by rhizome and through the hinterland of the system by seed. The recent establishment of marram grass on this coast has allowed observations of the invasion process and the geomorphic response of the dune system and the ways in which these processes impact on the archaeological record. This coast also provides opportunities to compare modified and unmodified dune systems in a similar coastal environment. Geographic features of the section of Mason Bay examined in detail in this paper are identified in Figure 3.

Figure 2.

Locations on Stewart Island (Rakiura), New Zealand (Aotearoa), referred to in the text.

Figure 3.

Geographic features of the ‘central dune’ section of Mason Bay between Duck and Martins Creeks.

II Conversion of coastal dunes and the impact of marram grass

Despite Cockayne’s (1911) observation that New Zealand dunes were ecologically distinctive, he recommended they be converted to ‘productive’ landscapes using botanical technologies developed in Europe. A process of dune stabilisation and afforestation was industrialised by the New Zealand Forest Service from the 1950s, culminating in the establishment of pine plantations (largely of Pinus radiata) across most of the west coast transgressive dune systems between North Cape and Wellington. Dune systems were then seen as wastelands (Beattie, 2011), which could be converted to productive plantation or farmland. Marram grass and Californian tree lupin (Lupinus arboreus) were systematically planted to stabilise dunes and increase available nitrogen before afforestation with pine. During this period, approximately 70% of the active dune systems of New Zealand were afforested (Hilton et al., 2000) and most of the rest were stabilised by marram grass and tree lupin. A range of weed species then established in the newly stable dune systems, concluding the widespread degradation of this landscape.

Marram grass is a fast-growing rhizomatous grass capable of rapid vertical and horizontal growth. It is a very efficient dune-building species compared with the indigenous foredune species (a sedge, Ficinia spiralis (pīngao) and two grasses, Spinifex sericeus (spinifex) and Poa billardierei (sand tussock)) and generally displaces these species in conditions of high sediment supply (Hilton et al., 2005). Marram grass dispersed naturally from sites where it was planted and now dominates the foredune and hinterland environments of most central and southern New Zealand dune systems (Hilton, 2006). It disperses long distances as sections of rhizome in the marine environment and by wind-blown seed within dune systems (Hilton and Konlechner, 2011; Konlechner and Hilton, 2009; Konlechner et al., 2013). Marram does not appear to be as invasive in northern New Zealand, north of about 35^o S latitude, at least in the foredune environment; however, marram grass has been widely planted in the west coast transgressive dune systems throughout the North Island and is now often abundant in hinterland environments.

Sedimentation may decline or cease within marram-dominated dunes, resulting in a second phase of invasion by opportunistic indigenous and exotic species that would not usually establish in an active dune system. In sum, marram invasion is associated with a rapid increase in plant biomass; displacement of the indigenous dune flora and fauna; lateral and vertical dune growth; and general dune system stabilisation. This impact has occurred concomitant with a wide range of other activities, including gold mining, sand mining, farming, infrastructure development, and urbanisation. Consequently, less than 50 major dune systems (of around 1000) retain significant conservation values. The conservation of these dune systems now depends on active and ongoing management of marram grass and associated weed species by central and local government agencies. The effects of marram introduction have resulted in at least five geomorphic processes that have implications for archaeological landscapes in coastal dunes in New Zealand. In addition, processes of dune restoration undertaken to re-establish indigenous biodiversity also have repercussions for the integrity of archaeological landscapes. These processes are examined in the following sections.

2.1 Foredune accretion and progradation

The most conspicuous impact of marram grass invasion in southern New Zealand is the development of large and regular foredunes. F. spiralis is the primary indigenous foredune species in southern New Zealand, including Rakiura. F. spiralis typically forms a relatively sparse vegetation cover associated with low (2–6 m) hummocky foredunes and, indeed during a visit in the early 1900s prior to the arrival of marram grass, Cockayne (1911) described the dunes behind Mason Bay beach, between Duck and Martins Creeks, as ‘low hummocks, 2–3 m high’. Cockayne’s account is consistent with photographs of the back-beach taken by Edgar Williams in 1945, at Duck Creek, which show scattered nebkha (a mound or hummock of sand developed around discrete plants), formed with F. spiralis (Figure 4(a)). In contrast, a large Type 1 foredune (after Hesp, 1988) now occupies the central section of Mason Bay, between Duck and Martins Creeks (Figure 4(b)).

Figure 4.

Prior to marram grass invasion, the foredune adjacent to Duck Creek, Mason Bay, comprised scattered nebkha (arrows) developed in association with pīngao (Photo taken in 1945 by Edgar Williams, courtesy Alexander Turnbull Library, Wellington) (a). Subsequent marram grass invasion led to the development of a massive foredune (b).

Aerial photographs record the development of this foredune following the invasion and subsequent spread of marram grass (Figure 5). Marram established in the study area by marine-dispersed rhizome from colonies established by farmers at Kilbride, 8 km to the south of Duck Creek in the 1930s (Hilton et al., 2005). By 1958 (the first aerial photograph), multiple small patches of marram had established between Duck and Martins Creeks within 400 m of the high-tide line (Figure 5(a)). Expansion of these patches and the redistribution of marine-dispersed rhizome meant that by 1978 A. arenaria had virtually replaced F. spiralis, forming a continuous, albeit, irregular foredune (Figure 5(b)). Foredune growth and stability has continued to progress since 1978, as the marram patches increased in size and density and coalesced (Hart et al., 2012; Hilton et al., 2006; Konlechner et al., 2016a) (Figure 5(c) and (d)). Lateral growth of the foredune occurred primarily through seaward progradation; however, the location of the shoreline has not changed significantly since 2003, suggesting the accommodation space for this foredune was by then fully occupied (Konlechner et al., 2013). Surveyed and reconstructed profiles indicate the degree of progradation (seaward advancement) and accretion (vertical growth) of the foredune between Duck and Martins Creeks following marram grass invasion (Figure 6). Since 1958, the toe of the foredune (edge of vegetation) advanced 73 m, on average, across the former beach and accreted by up to 11 m (Hart et al., 2012).

Figure 5.

The Mason Bay foredune developed rapidly between 1958 and 1989. During this period, the foredune prograded (on average, alongshore between Duck and Martins Creeks, 73 m; determined as the seawards advance of the vegetation line). The 1958 shoreline and the location of a surveyed profile (Figures 6, 10, and 12) across ‘Parabolic 6’ are indicated (arrow).

Figure 6.

Comparison of surveyed (2010) and derived (1958) profiles along the profile line indicated in Figure 5. The shoreline (as indicated by the dune vegetation line) has prograded over 70 m since marram invasion. The foredune has formed and accreted at least 12 m above the high-tide line over this time. Archaeological materials are located along the eastern (right) edge of the foredune and an unknown number of sites are probably buried beneath the foredune. The 1958 profile is inferred from the history of progradation derived from aerial photographs, early botanical descriptions, driftwood, whale bone, and other flotsam located along the landward edge of the foredune. The 2010 profile is derived from cross-shore surveys with a Trimble^TM total station.

The direct effects of marram invasion and associated foredune formation and shoreline progradation are: (i) to change the relationship between archaeological sites and the shoreline and (ii) to bury sites located in the pre-marram foredune environment. Archaeological sites, midden and ovens, have been recorded along the landward edge of the current foredune between Duck and Martins Creeks, most 80–100 m from the current shoreline (Figure 7). The coast of Mason Bay is easily accessed and traversed on land or offshore and it is likely that Māori moved freely along the bay establishing temporary campsites and preparing and cooking food in the sheltered lee of the (pre-marram) F. spiralis nebkha. Such activity would have been seasonal and it is possible that shoreline camps were used in summer to prepare and dry fish for transport elsewhere for winter use (e.g. Anderson, 1982; Leach et al., 1999). All such sites would have been much closer to the sea than are recorded today. The historic (1958 to present) development of a major foredune has, thus, buried an unknown number of sites obscuring the nature of Māori activity in the coastal zone, including all potential evidence of seasonal marine-food processing. Those sites that were not disturbed during the phase of marram invasion and nebkha development are now buried and protected from erosion and stratigraphic mixing.

Figure 7.

Archaeological sites, indicated by the arrows, occur along the eastern margin of the foredune between Duck and Martins Creeks at Mason Bay (UAV image, September 2017, Tom Simons-Smith).

2.2 Foredune erosion and parabolic dune development

The burial of the archaeological landscape is clearly illustrated by the progradation and accretion of the Mason Bay foredune following marram grass invasion. However, the early phases of foredune formation were also marked by erosion and transgressive dune formation, with significant implications for archaeological sites. Marram had formed multiple high (4–7 m), steep-sided nebkha by 1958. These nebkha, were superimposed on the former foredune landscape (Hilton et al., 2005) (Figure 8). The series of aerial photographs from 1958 to 1989 record the formation and growth of the foredune, but also the development of blowouts and parabolic dunes. During this period, onshore winds were steered and accelerated between adjacent nebkha, resulting in some erosion of the inter nebkha substrate. It is likely that at this time erosion reached and reworked the former archaeological landscapes. Over time, these nebkha expanded and coalesced as the cover of marram increased. The narrow corridors connecting the beach with the hinterland closed by 1989 (Konlechner et al., 2016a).

Figure 8.

The initial stages of the formation of a large marram grass foredune involved the establishment of high marram nebkha. Examples of this type have been recorded in Mason Bay north of Duck Creek. These landforms had the duel effects of burying and eroding the former archaeological landscape. Erosion between adjacent nebkha results from topographic steering and acceleration of incident onshore winds.

The blowouts initiated by nebkha development led to the formation of a series of parabolic dunes, which continued to migrate inland after these corridors were closed (Figure 5). Since 1978 the deflation surfaces of these parabolic dunes have continued to lengthen as the depositional lobes of the parabolic dunes migrate downwind. The net effect of these changes was to convert the hinterland of the dune system, within approximately 1000 m of the foredune, from a sandy, undulating, sparsely-vegetated, dune landscape into a highly ‘organised’ dune system, with discrete landforms and a landscape with a mosaic of clearly defined areas of erosion and deposition (Konlechner et al., 2016a). This organisation followed a phase of intense re-working of the surface of the dune field, such that virtually no section of the pre-marram landscape was left undisturbed (Figure 9).

Figure 9.

The proportion of the study area comprising stable and dynamic dunes and deflation surfaces has changed significantly following establishment of the foredune and the resulting halt in beach-dune system sand exchange. The area of deflation surface across the dune system increased between 1978 and the current day, which we attribute to a negative sand budget east of the marram foredune.

The effect of marram grass colonisation of the Mason Bay foredune has extended well inland and led to a fundamental reorganisation of the dune landscape. In 1958, the dune system comprised an irregular mosaic of deflation surfaces and depositional landforms, generally associated with relatively broad and poorly defined parabolic dune forms (Figure 9). This landscape was sparsely vegetated and, because of the (relatively weak) sand-trapping capabilities of the indigenous foredune species, probably characterised by a high degree of substrate mobility. Between 1958 and the present, this undulating, sparsely-vegetated, landscape has been ‘organised’ into parabolic dunes – clearly defined by discrete areas of erosion (parabolic-shaped deflation surfaces), inter-dune ridges of sand (vegetated trailing arms), and areas of deposition (depositional lobes). This ‘organisation’ was made possible by the ability of marram grass to form high (8–12 m) and steep-sided trailing arms capable of funnelling the wind towards the erosional throat (or apex) of the parabolic dune (Hart et al., 2012).

In summary, new landforms developed following marram grass invasion between 1958 and 2013, including nebkha and associated blowouts in the foredune zone. These blowouts, evolved into long-walled parabolic dunes downwind of the foredune. Archaeological sites would have initially been affected by erosion between adjacent marram nebkha (erosion and/or burial) and then by the landward migration of the parabolic dunes and subsequent reworking of the deflation surfaces. It is likely that nearly every section of the pre-marram landscape within and landwards of the pre-marram foredune has been either eroded or buried (or both) as the deflation surfaces, trailing arms and depositional lobes evolved (Figure 10).

Figure 10.

Concomitant foredune and parabolic dune development occurred between 1958 and 2010. The depositional lobes of the parabolic dunes have transgressed through and reorganised the former pre-marram dune landscape, producing clearly defined deflation and depositional surfaces.

Erosion of the pre-marram dune layer would have exposed sections of the archaeological landscape (in the deflation surfaces of the parabolic dunes) to strong onshore winds. Winds remove archaeological features differentially, first affecting the midden shell and bone, which becomes scattered downwind, along with the lighter weight artefacts in bone and shell. This body of material comprises the sum information on subsistence economics and diet. The remaining archaeological evidence typically comprises burned oven stones and larger stone artefacts, but sand loss introduces vertical mobility and the structural evidence of these features is also lost. As substrate mobility and erosion increase, areas such as parabolic deflation surfaces develop with lag deposits of oven stones and stone artefacts. The lighter weight midden and the bone and shell tools are removed from any ‘site’ context and are smeared over the landscape as individual artefact ‘finds’ or, in the case of the midden, as part of the background substrate. In archaeological terms, deflation surfaces become ‘palimpsests’, where material from all time periods exists on a single archaeological surface (Bailey, 2007). Scattered midden shell is characteristic of the deflation surfaces in the study area.

2.3 Disruption of beach-hinterland sedimentation

Foredune accretion has created a significant obstacle to sediment exchange between the beach and hinterland sections of the dune system. In addition, a large amount of sand has been trapped in the foredune. Between 1958 and 1999 (the first foredune survey), approximately 9.84 × 10⁵ m³ of sand accumulated in the foredune between Duck and Martins Creeks. These estimates are derived from the difference between the (surmised) 1958 beach-foredune profile, before the foredune developed, and the first surveyed profiles in 1999 (undertaken by the first author). The sequence of aerial photographs shows that the foredune did not advance significantly after 1999, although it did continue to accrete. Between 1999 and 2010 (when marram eradication operations commenced as part of a strategy of dune restoration), the elevation of the foredune plateau increased by up to 2.0 m, primarily across the plateau and rear slopes of the foredune – a process driven by topographic acceleration and steering and sand transport across the stoss face of the foredune and deposition across the plateau of the foredune (Hesp et al., 2017). Less than 3% of the sand transported from the beach to the foredune during strong onshore winds passes over the foredune (Petersen et al., 2011). The foredune is effectively a sand sink and an effective barrier to the supply of sand to the hinterland.

Foredune development has contributed to a negative sand budget across the hinterland of the dune system, particularly in the vicinity of the ‘Great Stonefield’. This feature did not exist in 1958. Prior to marram invasion, the sparsely vegetated F. spiralis foredune (Figure 3) allowed beach-hinterland sand exchange during strong onshore incident wind conditions. Some sand was probably returned by northeast winds. As discussed above, by 1978 the dune system was showing signs of significant ‘reorganisation’ and the development of the above sequence of landscape elements (Figures 9 and 10). The ‘Great Stonefield’ had formed and has since expanded in area, as the dunes along the eastern margin of the stonefield have eroded. This is a dynamic landscape, where a fine balance exists between vegetation and dune formation and devegetation and erosion. The indigenous primary sand colonisers (F. spiralis and P. billardierei) are associated with dynamic dune areas inland of the marram foredune – including the sand plateau and depositional lobes of parabolic dunes. These species thrive in conditions of moderate sand accumulation, where sand deposition provides nutrients for growth and new substrate (free from pathogens). Decline and necrosis occurs where sand deposition in the vicinity of the plant declines or ceases and the dune erodes. In part, this is a natural process in an active dune system, where there is rapid (10⁰–10² y⁻¹) landform development and decay resulting from variations in sand pathways and local sand budgets throughout the dune system. However, we suspect that plant necrosis and dune erosion is currently occurring over a wider area and occurring more rapidly than before marram established a stable foredune in Mason Bay.

The contemporary dune environment at Mason Bay is, therefore, largely comprised of landform elements formed in the late 1970s. Archaeological sites inland of the foredune may have survived; however, a significant proportion of the hinterland between the foredune and the Great Stonefield has been eroded and redeposited as parabolic dunes have advanced inland. Further inland there is increased likelihood of localised erosion because of shifts in sediment transport pathways across the dune system forced by a net reduction in sand supply following foredune and parabolic dune formation. The contemporary dune landscape retains the dynamism of the pre-marram landscape, but it would be unrecognisable to pre-European (Māori) inhabitants.

This history of dune system evolution affects sites throughout the system. In the hinterland of Mason Bay, the main effects on archaeological sites are due to erosion. Sand that might previously have been transported through the permeable foredune zone, and through the system to bury and protect sites, is now trapped in the foredune. This results in negative sand budgets, dune erosion, and site exposure. Hinterland sites, often visible today as isolated cooking features, are scattered through the hinterland on thin paleosols marking older dune formations. These sites are up to 2 km inland of the coast and represent different sets of human activity to those closer to the coastline. Most are probably campsites occupied for longer time periods than the coastal sites and some were probably linked to inland activities, such as bird hunting and foraging in the forests behind the dune complex. Seasonal Māori travellers would have approached Mason Bay through inland routes, from Paterson Inlet, as well as from the sea and the hinterland dune sites may be located on or close to trails from the east. We have recorded intact oven sites that have been exposed, eroded, and scattered (within a few months of exposure) within the deflation surfaces of parabolic dunes, advancing landward well inland of the modern foredune. Although the sites may have gone through multiple exposure and reburial cycles, regardless of the local or system-wide changes in sand budget following marram invasion, we suspect that the erosion of hinterland sites has intensified since foredune formation.

2.4 Barrier progradation and river mouth dynamics

Archaeological sites are commonly associated with river mouths and estuaries – situations that afforded ready access to marine resources, shelter from wind (compared to the open coast), campsites, fuel, and freshwater. Such sites are also vulnerable to natural and human-related changes in the position and morphology of the entrance channels. At Doughboy Bay, 8 km south of Mason Bay, marram grass invasion transformed the morphology of two late Holocene barriers (Hilton et al., 2009), which caused changes in the position of the river mouth and the loss of an adjacent archaeological site (midden). Prior to marram grass invasion (in the early 1950s), these barriers comprised transgressive dune elements associated with indigenous dune species. Following marram grass invasion, the barriers evolved as prograded bay-head foredune barriers. The southern barrier was comprised of a series of five distinct foredune ridges. By 1998, this barrier had advanced seawards, but also prograded to the north, deflecting the Doughboy River in the same direction. This change in channel position caused the semi-continuous erosion of the dunes on the northern margins of the river. In this case, marram grass formed dunes and contributed to barrier growth where indigenous species could not. A small shell midden was subsequently exposed and eroded (Figure 11). Loss or damage to rare site classes like this significantly affects the ability to interpret archaeological landscapes.

Figure 11.

Erosion of midden on the northern side of the Doughboy River occurs during phases when the river enters the sea well north of its usual path. Progradation of the southern barrier following marram invasion has tended to displace the river to the north. The inset locates a small midden – shell, possibly the result of a single meal – (midway up the scarp to the left of the figure), located on the map, which was exposed as the river eroded the true right bank in August 2007.

2.5 Dune system stability

The archaeological landscape in the study area was largely protected from erosion by a mantle of dunes until the introduction of marram grass. Changes in the steepness and relief of dunes associated with marram grass have generated a more dynamic dune system with higher rates of sand flux. New dune landscapes have resulted, characterised by larger deflation surfaces. Extensive areas of the hinterland of the dune system, the area east of Great Stonefield, contain dunes and still have a positive sand budget. In time, had the dune restoration programme not commenced, internal recirculation of sand within the dune system would have declined with marram grass colonisation, and the system would have stabilised. The archaeological landscape, or the fragments that have survived marram invasion, would then be buried. In some circumstances, allowing marram to colonise a dune system and accelerate burial might be considered a positive outcome. In this case, marram posed an overwhelming threat to one of New Zealand’s most valuable (in biodiversity terms) dune systems.

2.6 Dune system restoration

In recent years, management agencies in several countries (notably New Zealand, Australia, and the Netherlands) have sought to re-establish the indigenous biodiversity of dune systems by reducing or removing marram grass and other exotic species (Hesp and Hilton, 2013). These initiatives aim to restore the natural dynamism of dune systems by re-establishing the indigenous vegetation cover, which is invariably less dense and more vulnerable to aeolian processes of sedimentation. A more dynamic surface tends to favour those plant (and animal) species adapted to these conditions so that the natural biodiversity and dynamic character of the dune system is restored. The process and outcome of restoration is an ideal that is challenging in practice. In the case of marram grass, the morphology of contemporary dune systems may reflect decades of sand accumulation in the foredune or other environments and (conversely) downwind erosion of large areas of the dune system. Indigenous plant species, including early successional species, may have been depleted or entirely displaced during the invasion process. Such species may require strategic and timely reintroduction to a site – but, agencies must be aware that the ‘restored’ dune system will be largely determined by how such introductions are managed. The modified nutrient conditions of soils associated with some exotic species – nitrogen-fixing species, as for example L. arboreus – may present significant obstacles to restoration (Konlechner et al., 2015). Finally, the method of restoration must be acceptable to affected communities of interest. Marram grass is controlled with a grass-specific herbicide in New Zealand, particularly in isolated settings, on local or central government lands, and where there is no other practical method of control (Hilton and Konlechner, 2010). In contrast, herbicide is generally avoided in California, where hand-weeding or mechanical excavation is preferred. This method has been effective over relatively small areas of dune and where there is strong community support (Pickart, 2013). Large-scale mechanical excavation is employed in the Netherlands, where there is a history of coastal engineering (Arens et al., 2004).

The Department of Conservation in New Zealand has invested in marram grass eradication on Stewart Island since the early 1980s, with an intensified program since 1998. At Mason Bay, restoration initially targeted marram grass nebkha through the hinterland of the dune system. In 2010, the Department commenced marram grass eradication along and across a 400-m length of foredune. The area treated was expanded in 2016 to include a further 600 m of foredune. Annual treatment (in the southern hemisphere autumn, near the end of the annual marram growth cycle) results in necrosis of above-ground marram grass culms by the spring of the same year. Vigorous regrowth occurs from dormant buds on the rhizome system by the end of the following summer (Woodley and Hilton, 2003). Foredune marram has to be resprayed (annually), over three or four successive years, before the bud bank is depleted and regeneration is reduced to a level when ground operations are feasible. Regrowth from rhizome may continue for 3–4 years (Konlechner et al., 2016b). Consequently, significant erosion of treated foredunes may not commence until the third or fourth year of treatment, depending on the setting. Erosion may be further slowed by regeneration from buried seed, which germinates as the upper layers of the foredune erode – particularly in more sheltered situations, where seedlings have the opportunity to establish after germination (within the mass of exposed marram rhizome). In sum, marram grass necrosis occurs incrementally, over 4–7 years; and the process of eradicating marram grass does not result in rapid erosion of a foredune or rapid onset of downwind sediment transport. At Mason Bay, a depositional sand sheet started to develop in 2015 and is only now (January 2018) encroaching on the deflation surface associated with the parabolic dunes described above.

The introduction of marram has caused demonstrable impacts on archaeological sites; however, dune restoration involving marram eradication may introduce another set of problems for understanding archaeological landscapes and for archaeological site management. In general, the release of sand stored in the foredune, after marram grass eradication, and the re-establishment of beach-hinterland sand transport pathways will increase the likelihood of burial and conservation of hinterland archaeological sites. Removing marram nebkha in the core of the dune system should have similar positive outcomes. We anticipate that, in the long-term, the dune landscape will return to the pre-marram condition with reduced topographic relief, an increase in the extent of the native sand-colonising species (F. spiralis and P. billardierei; albeit the cover will be generally sparse), and fewer and smaller deflation surfaces. The landscape will become more undulating and sandier because these indigenous species are less able to trap sand and form high and steep-sided dunes.

In the short to medium term (10⁰–10¹ y^-1), however, erosion of the foredune may impact archaeological sites that are buried beneath the lee slopes of the foredune. The stoss face of ‘restored’ sections of the Mason Bay foredune now comprises a series of evolving erosional gullies (not dissimilar to ‘blowouts’, but functionally different), separated by remnant sections of foredune (Figure 12). Erosion of the sides and floor of these gullies occurs during strong (onshore) southwest to northwest incident winds. We know very little about how foredunes of this type erode and whether blowouts of this form will persist and lengthen or deepen as the foredune ‘rolls back’ and decays. Processes of topographic steering and acceleration of incident winds will become less intense as the topography of the foredune is reduced, but there is the potential for the floor of the blowouts to erode to the underlying archaeological landscape, which is probably <1 m above the level of spring high tides (based on the elevation of known sites landward of the foredune).

Figure 12.

Erosion of the foredune in the vicinity of ‘Parabolic 6’ following marram eradication across a section of the foredune by the Department of Conservation. The foredune and hinterland profile lines (Figures 6 and 10) are indicated by the arrows. The restoration works commenced in 2010. Sand deposition downwind of the foredune, within the deflation surface of Parabolic 6, commenced in 2016. Archaeological sites in the lee of the foredune (that survived parabolic dune formation after marram invasion, either intact or disturbed) are likely to be buried over the next decade. Those sites currently buried near the rear of the foredune may be exposed and eroded as the ‘restored’ foredune erodes (UAV image – September 2018 by David Borrie).

Foredunes in southern New Zealand do not erode rapidly during and following marram eradication because of the propagule pressure (buds and seed) and because vertical rhizome exposed by substrate erosion armours the substrate. In the case of the Mason Bay foredune, it may take some years or decades before the stoss face has eroded back to the likely position of pre-marram archaeological sites. It might be possible to slow the rate of foredune decay, and maintain a certain topography, by reintroducing F. spiralis once the overall topography of the foredune has declined. At the very least, the area of pre-marram foredune should be periodically monitored for archaeological sites and a response strategy maintained.

III Discussion and conclusions

In general, the introduction of marram grass has led to the accumulation of sand over former archaeological landscapes throughout New Zealand. In part, this has been accidental and indirect. Naturalised, marram grass has spread to almost all beach and dune systems in New Zealand, where it has formed dunes of a height and (eventual) stability that would not otherwise have occurred. A sparse or patchy indigenous flora associated with active dune systems was rapidly, over just a few decades at most locations, replaced by a dense and continuous cover of grass. The relative relief of the marram-affected landscape was significantly greater than the pre-marram landscape, since marram tends to form high, steep-sided dunes. Once stable, the dunes were then colonised by a wide range of opportunistic plant species, indigenous and exotic, that would not usually establish in active dune systems.

Burial of archaeological sites offers protection from erosion and stratigraphic mixing in a range of situations; however, we have demonstrated that the process of marram grass invasion can have fundamental impacts on the geomorphology and stability of a dune system. There is always some risk of erosion and loss of buried archaeological sites arising from variations in sand supply and dune development within larger transgressive dune systems (Figure 13); but, the risk of loss has substantially increased as a result of marram introduction in the case examined. Rates of sand flux, erosion of dunes, and the expansion of deflation surfaces all increased during and as a result of marram invasion. This risk decreased once the foredune had formed and stabilised; however, the onset of a negative sand budget then increased the risk of erosion (and burial) of the archaeological landscape through the section of hinterland dune system subject to parabolic dune mobility. This risk would have remained high at Mason Bay until marram had achieved near-total colonisation of the dune system. Hilton et al. (2005) estimated this condition would have been reached in 2030 – after approximately 80 years of disturbance related to marram grass invasion. We anticipate that dune restoration will re-expose sections of the archaeological landscape now buried under the current foredune, but conservation managers now have the benefit of understanding the relationship between the current topography and this landscape and are better prepared to monitor and protect archaeological sites as they emerge. Erosion of the hinterland of Mason Bay should decline and the risk to archaeological sites decline in parallel as sand released from the ‘restored’ foredune blows inland. Restoration should, on balance, be beneficial for the conservation of heritage values at Mason Bay, albeit authorities will need to monitor the current foredune closely over the next 10–20 years.

Figure 13.

The risk of loss of archaeological sites in the Mason Bay dune system was not constant through the process of marram grass invasion (and restoration) and was never nil. The risk of burial and erosion is high, as marram establishes in the foredune zone, during the period of maximum parabolic dune activity and, we forecast, during the ongoing phase of foredune restoration.

The case studies presented indicate that the relationship between current dune landscapes and the earlier pre-European archaeological landscapes, and the impact of marram grass, cannot be simply represented as the result of burial and stability. Archaeological landscapes in New Zealand are particularly vulnerable to burial because most midden and other sites associated with pre-contact Māori activities are located close to former shorelines. Marram is an efficient coloniser of back-beach and foredune environments and at most sites has formed large, stable foredunes and displaced the indigenous foredune species. Concomitant accretion has resulted in the widespread burial of former cultural surfaces and pre-marram archaeological landscapes during the 20th century. Burial and dune system stability may result, but marram invasion has been shown to (i) force significant coastal progradation that changes the relationship between archaeological sites and the shoreline; (ii) form massive, stable foredunes that significantly disrupt the flow of sand between beach and hinterland environments; (iii) cause localised erosion of archaeological surfaces in the foredune zone, as marram-formed nebkha modify the direction and strength of onshore winds and patterns of sedimentation; (iv) force the formation of large transgressive (mobile) dune forms, that may migrate over and erode archaeological landscapes; and (v) divert rivers by modifying the morphology of coastal barriers. A key concern is that many of these processes result in stratigraphic mixing during the invasion process, which may result in the loss of archaeological information, before burial and the onset of stability.

The effect of marram grass can extend well inland of the foredune in large transgressive dune systems. The period of instability associated with the formation of nebkha can initiate blowouts and sustained downwind parabolic dune movement, a process which may change patterns of erosion or deposition across the dune system. As they migrate downwind, parabolic dunes erode and mix strata associated with archaeological landscapes, resulting in palimpsest or time-averaged landscapes. Archaeological materials may remain as a lag deposit with loss of stratigraphic context and cultural information. Subsequently, the development of a large, stable foredune, in conjunction with marram grass, may disrupt sand supply to the dune system hinterland, resulting in a negative sand budget, surface lowering, and erosion of the archaeological landscape.

Polynesians encountered extreme environmental conditions in southern New Zealand as they settled the South Pacific. The limits of horticulture lay 500 km to the north and the relatively cold southern environment required them to radically adjust the social, technological, and economic systems carried with them from the tropics. This makes understanding southern New Zealand archaeological landscapes crucial for understanding the full range of Polynesian adaptive strategies. Māori occupation of Rakiura and offshore islands from the turn of the 14th century seemed to consist of brief seasonal visitations, presumably associated with targeted resource exploitation (Anderson and O’Regan, 2000; Jacomb et al., 2010). However, the manner in which these activities were linked into wider exchange and mobility networks, resource scheduling, and settlement patterns is poorly understood. Part of the problem is the issue of site preservation and some uncertainties about the representativeness of the sample in key areas like Mason Bay, where there are very active dune systems. In this paper, however, we have shown that, while even a single process like marram grass invasion can have complex and radical impacts on sites and landscape, many of these impacts can be predicted and factored into interpretive models. In respect of Mason Bay, for example, we can make the following observations that will influence an interpretation of pre-European Māori occupation of Stewart Island (Rakiura):

Foredune growth has likely buried a large number of sites that are pertinent to understanding the nature of marine exploitation, including the issue of whether people were simply passing through the Bay (e.g. campsites) or whether they visited to target specific seasonal resources (e.g. processing sites).

Erosion of the hinterland of dunes systems will have reduced the quantity and quality of evidence relating to exploitation of the coastal forests and of connections to the east to Patterson’s Inlet and, thus, to wider social and settlement networks.

Changing river-mouth dynamics may have affected the preservation or burial of potentially rare site types. These are the most likely places to preserve evidence of villages and, if they are present, would require a major reinterpretation of southern settlement patterns.

If there was any sustained activity at Mason Bay, much of it would have been located in sheltered areas behind the pre-marram coastal dunes. This zone, has been subject to significant erosion and burial and archaeological material will have been distributed and conflated into deflation zones and blowouts. These deflation zones, might contain the best record of the full range of activities in the region, but will have little spatial integrity, so will require specific modes of analysis.

In conclusion, patterns of erosion and burial, associated with historic changes to the plant cover of coastal dunes, are having a profound impact on archaeological landscapes. In some cases, these changes are exacerbated by coastal erosion related to climate change forcing in combination with a suite of potential anthropogenic activities. Transgressive dune systems containing archaeological landscapes now bear little similarity to the prehistoric landscape, although fundamental geomorphic processes may have changed little. Interpretation of cultural activity based on the juxtaposition of the archaeological and geomorphic landscapes needs to consider the degree of modification and loss resulting from a fundamental redistribution of sediment and loss and/or burial of cultural surfaces. Recent efforts to eradicate marram grass from sites of high conservation value by management agencies have seen a reversal of this process – as foredune erosion and shoreline retreat to a new equilibrium position. This process can also have significant outcomes for the conservation of archaeological landscapes.

Footnotes

Acknowledgements

We acknowledge Rakiura Māori as tangata whenua of Stewart Island (Rakiura). The authors gratefully acknowledge Les O’Neill and Jessie Hurford (Department or Anthropology and Archaeology) and Chris Garden (Department of Geography) of the University of Otago for their expertise in cartography and GIS. We are also indebted to the Department of Conservation, Rakiura National Park, for their support for our research and practical assistance with fieldwork.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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