Molluscan mulching at the margins: investigating the development of a South Island Maori variation on Polynesian hard mulch agronomy.
Hard mineral clast sediments applied to dry archaeological fields in the distant apical islands of the Polynesian triangle are frequently associated with sweet potato / kumara (Ipomoea batatas (L) Lain.) cultivation. In a novel variation on this practice, north-western South Island Maori deposited tuangi (Austrovenus stutchburyi (Wood, 1828)) mollusc beach valves to cap 10-20 cm deep planting holes or pits by or around the sixteenth century AD at Triangle Flat, Golden Bay. Discrete tuangi beach valve sediment had been extended over much of the larger field surface in association with shallow (<10 cm deep) planting depressions. Surface shell deposits would have suppressed weed growth, redirected radiant energy onto young kumara plants, and conserved planting pit soil and moisture against disruptive and desiccating winds, respectively. The temporal extension of Triangle Flat shell sediment could be related to socio-economic and political pressures to improve kumara production from a fixed land unit. However, since climate change beginning in the sixteenth century brought cooler temperatures and eventually, stronger westerly winds to this region, it seems more likely that shell mulch was extended to maintain shallow planting soils and production in a collier, windier period. This local, kumara-focused development may have been encouraged by the opportunity to apply ritually safe and perhaps spiritually potent, uncooked tuangi beach shell to tapu cultivation surfaces.
Keywords: South Island Maori, molluscan mulch, kumara, climate change, tapu.
An ancient cultivation technique involving the addition of hard, predominantly mineral clasts to field surfaces is reported globally, if infrequently. This agronomy is glossed as "lithic mulching" by Lightfoot (1994, 1995, 1997; see also Lightfoot & Eddy 1994). Archaeologists have identified the practice in dry cultivation settings on islands at the margins of the Polynesian triangle that were settled by cognate (originally western Pacific) populations. In archaeological studies, Polynesian hard mulch practices are most often associated with sweet potato / kumara (Ipomoea batatas (L) Lain.) cultivation (Allen 2004: 211-12; Barber 2010; Ladefoged et al. 2010; Louwagie et al. 2006; Stevenson et al. 1999, 2006; Wozniak 1999, 2001; Figure 1).
For subtropical Rapa Nui (Easter Island) in south-eastern Polynesia, the term "lithic mulch" describes volcanic stone applications (grain size >2 mm) to cultivation field surfaces (Figure 1). Researchers have argued that these rocky sediments acted to regulate temperature and conserve cultivation soils and moisture against desiccating winds and erosion, especially following island-wide deforestation (Baer et al. 2008; Bork et al. 2004; Ladefoged et al. 2010; Louwagie et al. 2006; Mieth & Bork 2003; Mieth & Bork 2004: 83-4; Mieth & Bork 2005; Stevenson et al. 1999, 2006; Wozniak 1999, 2001, 2005; Wozniak & Stevenson 2008). Recently, it has been suggested that the addition of fresh basalt clasts may have added important mineral nutrients to old, leached volcanic soils as well (Hunt & Lipo 2011: 46-8, 191-6; Ladefoged et al. 2010: 83). Wallin et al. (2005) associate lithic mulching with the intensive production of kumara surpluses to support the construction of Rapa Nui's monumental landscape (see also Stevenson & Haoa 1998; Stevenson et al. 1999). Mieth and Bork (2005: 62) even argue that "the labor intensity of the stone mulching phase" on Rapa Nui "probably exceeded the labor efforts of the ahu/moai phase by far". It is possible that the practice was intended variously to ameliorate environmental effects, especially increased wind impacts over time, and to improve production results from a fixed land unit in other cases ("intensifcation", after Brookfield 1984; see also Kirch 1994, and discussion in H.M. Leach 1999).
Applications of stone (generally gravel/pebble to small cobble class, 2-100 mm grain size) or coarse sand in Maori crop production are documented from temperate climate, central to south-eastern New Zealand locations as far south as Banks Peninsula (Figure 1; Barber 2004, 2010; McFadgen 1980; Walton 1983; Walton 2000: 20). These Maori field remains have been compared broadly to Rapa Nui lithic mulch agronomy (Bork et al. 2004: 10; Stevenson et al. 1999: 809; Wozniak 2005: 141; see also Lightfoot 1994:181; Lightfoot 1997:217-18). The most extensive, prepared or "made" alluvial gravel and sand soils are recorded from the Hamilton region of the Waikato River Basin and several alluvial South Island environments (Barber 2004: 185-8; Barber 2010; Challis 1991: 102-5; Gumbley et al. 2004). Mollusc shell valves are documented among components of a few Maori cultivation soils in central and northern New Zealand as well, albeit with some general uncertainty over association and intentionality (Barber 2004:186, 188).
There are indications of intra-regional agronomic diversity behind the particle size homogeneity of extensive Maori gravel applications. For example, in Tasman Bay (northern South Island) fieldwork, 5-12 cm thick alluvial gravel sediments are associated with A-Horizon Maori soils of the lower Waimea River catchment in and around Appleby, and at site N26/80 near Motueka River mouth (Figure 2; Barber 2010; Challis 1976). These mineral deposits served as cultivation surfaces above a growing root crop mass, or lithic mulch proper (after Lightfoot 1997: 207). It is likely that surface gravels helped to warm alluvial cultivation soils and crops (cf. Barber 2004:189), and to suppress weeds. In the Appleby area (especially near sites N27/118, 122) and elsewhere on the Waimea River floodplain, made alluvial gravel soils 14-40 cm thick are also reported in comparable, A-Horizon contexts. It has been proposed that these made, Maori gravel soils were created to provide a warm, free-draining cultivation matrix on the floodplain (Barber 2010; Rigg & Bruce 1923). These distinctions within the same area highlight the potential for variation in the broader practice of Polynesian lithic field applications in any region, consistent with variance in the grain size and soil incorporation of stony applications on Rapa Nui (Baer et al. 2008; Stevenson et al. 2006).
In this paper, I examine further evidence for regional variation and innovation in the application of hard mulch materials at the Polynesian margins. I report on the investigation of mollusc valve beach sediment applied to Maori cultivation features and soils in a dated archaeological sequence that incorporates evidence of agronomic change over time at Triangle Flat in Golden Bay, north-western South Island. In this research, I address the following questions:
* What is the chronology and purpose of beach shell applications at Triangle Flat?
* How does the Triangle Flat evidence inform our understanding of local variation and intentionality in hard mulch Polynesian agronomy?
SOUTH ISLAND CONTEXT AND BACKGROUND
The acclimatisation of tropical Polynesian cultigens to temperate, seasonal New Zealand at Polynesia's coldest south-western corner was especially challenging for the first c. thirteenth century AD settlers of large but cool South Island (Figure 1; Higham & Jones 2004). Pacific yam (Dioscorea spp.) and taro (Colocasia esculenta (L.) Schott) were restricted climatically in New Zealand by the requirement for a warm, nine-month growing season (or longer) in relatively deep, fertile soils that also had to be moist if not wet for taro. The preferred, hardy and versatile kumara with its moderate fertility needs required a growing period of six months or even less in a well-drained soil (Barber 2012a). On archaeological, microbotanical and historical evidence, kumara was the dominant if not sole stem crop grown by South Island Maori in warmer, northern to east coast locations. Evidence for pre-European taro is currently reported from one South Island archaeological site only (Triangle Flat, discussed subsequently). South Island is too cold for yam cultivation (Barber 2004, 2012a; H.M. Leach 1984).
The north-western South Island Nelson region, with high annual sunshine hours and a warm, frost-free half year between spring and autumn (Chittenden et al. 1966: 9-10), is notable for diverse archaeological evidence of Maori cultivation anthrosols (Figure 2). These include the gravel sediment and modified floodplain gravel soils of the lower Waimea River catchment, southern Tasman Bay, and the gravel sediment at Motueka, western Tasman Bay, discussed above. Mixed, coarse sandy soils with a relatively high carbon component are also reported in Maori archaeological horizons from coastal beach ridges, dunes and slopes of north-western Tasman Bay (e.g. Anchorage N26/22, Awaroa Inlet N26/122) and Golden Bay (e.g. multiple sites at Takapou, Ligar Bay, Tata Beach, Triangle Flat). A small (<1 ha), discrete area of coastal sandy soil modified by beach gravel additions probably dates to or beyond the seventeenth century AD at Tata Beach (Barber 1999a: 138, 143-4). Deliberate mollusc valve applications to archaeological cultivation fields in this region are at present reported only from Triangle Flat, western Golden Bay (Figure 2).
THE ARCHAEOLOGICAL LANDSCAPE AND SITES OF TRIANGLE FLAT
Triangle Flat, at the base of Farewell Spit, is a terrestrial complex of low consolidated dunes, relict beach ridges and swales, currently under pasture. It extends and narrows back from the prograding shoreline of Golden Bay as a triangle-shaped wedge of land. Mobile quartz sand dunes continue approximately 25 km to the east beyond Triangle Flat as Farewell Spit (Figures 2 and 3).
In general, valves of the filter feeding, mid- or intertidal bivalve genus Paphies Lesson, 1830 (Mesodesmatidae; Powell 1979: 415-16) are dominant physically in archaeological Maori midden sites found across Triangle Flat (Figure 3; Brooks 2002; Walton & Bagley 2000). Several discrete middens are incorporated within a continuous archaeological complex (site M24/11) that extends hundreds of metres along the Golden Bay shoreline over a gently undulating beach ridge (Figures 3 and 4). M24/11 appears to represent a primary location for early Maori settlement and economic activity at Triangle Flat. The archaeological features and sediments of M24/11 are largely found within or derive from a shallow (30-50 cm) black sand topsoil (A-Horizon) that has developed on a natural shelly sand substrate. The substrate incorporates broken, occasionally intact and frequently water-rolled shell of the tidal soft shore bivalve Austrovenus stutchburyi (Wood, 1828) (Veneridae; Powell 1979: 426), referenced hereafter as tuangi. Consistently, filter-feeding tuangi is the primary mollusc species on the adjacent, shallow tidal flats and margins of Golden Bay.
Archaeological features excavated at the south-western end of M24/11 include oven pits, Paphies-dominant shell midden deposits, planting holes and sediments, and a rectangular crop storage pit (Figure 4). Midden sediments are between 30 and 40 cm thick in places. The largest, thickest midden identified and excavated at Triangle Flat extends somewhere between 130 and 150 square metres in the south-western woolshed paddock ("primary midden" at Figure 4; see also Brooks 2002). Fish, whale, dog and occasional bird bones were recovered from excavated M24/11 midden deposits. Whale bone elements are generally comparable to the size of pilot whales (Globicephala spp.). These include vertebrae centrum and associated epiphyseal plates, indicating that juvenile whales were processed at this location. Whale pods strand regularly on north-western Golden Bay beaches (Hutching 2009; Jane 1989: 17).
Diagnostic nineteenth century or later historical materials, including sheep bone and discarded farm equipment, are recorded within the upper black sand topsoil layer (generally 10 cm or less below ground surface (bgs)). These historical materials are never located within or below midden deposits or the redeposited (A-Horizon) beach shell sediment described below, other than in disturbed situations.
ARCHAEOLOGICAL MAORI CULTIVATION EVIDENCE AND THE SEQUENCE AT M24/11 (SOUTH-WESTERN END)
A Maori cultivation complex at the south-western end of M24/11 extends 60 m back from the shoreline over a gently undulating beach ridge surface. The cultivation complex and beach ridge both terminate at a swale that carries water occasionally (Figure 4). Two primary field components are identified.
Planting holes (pits)
Dry soil planting holes created by digging sticks are reported in a number of archaeological cultivation contexts at the Polynesian margins, including both of the main New Zealand islands. These are sometimes glossed as "planting pits" in the literature. The terms are interchangeable in this paper, unless qualified otherwise. Depending on size, environment and location, such holes may have accommodated dry field taro, yam or kumara crops (Allen 2004: 198, 216; Barber 2004: 189-90, fig. 8.5; Barber 2012a; Gumbley et al. 2004; Kirch et al. 2005; Stevenson et al. 2006).
At south-western M24/11, excavated planting holes of variable size are circular, oval or irregular-curvilinear in plan (Figure 5). They are sometimes rounded and often asymmetric in section, and up to 60 cm in diameter, or greatest horizontal measurement. In general, these planting pits intrude anywhere from 10-20 cm into shelly, sandy substrate (Figures 6 and 7).
Smaller, upper planting depressions that generally extend less than 10 cm into the shelly substrate are also reported. They tend to be narrow and elliptical in plan (e.g. Figure 6 with reference to Figure 8), difficult to resolve and are almost certainly under-reported. These upper depressions are less convincing as purpose-dug pits, although some may be, albeit of a more shallow type. Otherwise, they may represent planting and harvest disturbance or tuber moulds extending into the shell substrate (e.g. Figure 8).
In analysis, planting pit fill is a black, carbon-enriched, medium-grain sand or loamy sand that extends from the black sand topsoil (generally 10YR 2/1 moist). The pits appear to be holes dug to accommodate production soil matrix and perhaps organic additives for crop propagation on the shell ridge (cf. Barber 2012a,b). Starch grains consistent with kumara morphology have been identified from several planting pits (e.g. Figure 6 B-B', referenced as the "Set A" pit in Horrocks et al. 2004: 149-51, 155; see also Barber 2004:189-90). Starch grains consistent with taro are identified only from soils associated with an unusually deep pit (50 cm) that presents with a basin-like top filled with Paphies-dominant midden ("taro planting pit" at Figure 4, referenced as the "Set B" pit in Horrocks et al. 2004: 151, 155). In some places tuangi beach valve lenses occur immediately above these pits, forming discrete shell caps (e.g. Figures 6 and 7).
Several planting pit clusters are identified in excavation. In one place, a close and largely linear pit alignment is recorded (along El 5 at Figure 5), conforming to historical descriptions of Maori kumara cultivation fields (Best 1976: 148-53; H.M. Leach 1984: 64-5).
Beach shell sediment (mulch)
There is consistent evidence within the south-western M24/11 A-Horizon that beach shell sediment was redeposited and spread extensively over many square metres. These deposits are generally 2-6 cm thick at depths in the range of 6-8 cm and 20-25 cm bgs. Visible shell valve components are usually broken or water-rolled, and fall mainly within a maximum length range of 5-50 mm. A brown (10YR 4/3 moist) or yellow (2.5Y 7/6 moist) sand matrix presents in some thicker beach shell deposits. As identified in test pits, this shelly sediment is recorded over much of the fenced woolshed paddock, extending on the Golden Bay side into the adjacent, south-south-east lot (Figure 4). The shelly deposits do not extend into or over any substantial midden site. The overwhelmingly dominant mollusc species in this beach sediment is tuangi, in contrast to the Paphies-dominant middens identified generally at M24/11. As noted above, tuangi is the primary component of the natural shelly substrate and adjacent tidal beach deposits.
The upper shell sediment bifurcates into discrete shelly deposits in at least one area of the south-eastern lot beyond the woolshed paddock fence. In an excavation profile of this lot, two shelly deposits are recorded in vertical sequence 6-8 cm and 12-14 cm bgs, above a buried pit 25-35 cm bgs.
These extensive, upper A-Horizon beach deposits may be distinguished from the small, isolated tuangi beach shell lenses that cap the 10-20 cm deep planting pits reported above (Figures 6 and 7). The irregular, discontinuous distribution and isolation behind the shoreline of upper tuangi shell deposits (Figure 4) are not consistent with a pattern of tidal deposition. Furthermore, Maori archaeological materials (e.g. lithic artefacts, rat, bird and fish bones) are very occasionally--and it seems, incidentally--incorporated into the extensive shelly deposits. Sediment modification is also identified in places, including shell mounding above structurally associated black sand depressions and apparent tuber intrusions (<10 cm deep) into shelly substrate, discussed above. Collectively, these things suggest a cultural origin for the upper shell deposits. This sediment is interpreted as a cultivation mulch treatment sourced from the ridge substrate or the open beach.
Cultivation sequence and chronology
A sequence of cultural events is identified from the shallow black sand topsoil of south-western M24/11. Early in the sequence, as described above, discrete tuangi beach shell deposits capped many of the earliest planting pits. In some places, midden lenses were eventually deposited above filled and effectively buried planting pits, or in the case of the "taro planting pit" (Figure 4), in the cavity formed by a large hole. It appears that midden shell was never deposited above these planting pits with any direct cultivation intention, however, unlike tuangi beach shell. Later in time, tuangi beach shell sediment was extended to cover larger cultivation surfaces and more shallow planting depressions in the shelly substrate.
Seven calibrated radiocarbon results date these events. Two of these are on tuangi shell samples from the beach substrate at the topsoil base. Of these, Wk-11696, on the valves of a tuangi shellfish in position of articulation, is preferred as a more primary, tidal sample (Table 1 (1, 2) and Figure 5 for location). This dates the isolation of the ridge at the prograding beach edge above regular tides by about the sixth century AD. There are no radiocarbon determinations from planting pits cut into the shelly substrate. Four dates on Paphies spp. shellfish above four separate planting pits overlap at 2-sigma around the mid-fifteenth to seventeenth centuries (Table 1 (3-6); cf. evidence for radiocarbon integrity of north-western South Island tuangi and Paphies spp., as cited in Barber 2003: 440). At 1-sigma, one of these dates ranges between the mid-fifteenth and mid-sixteenth centuries (3), while three others bracket all or most of the sixteenth century (4-6). These ages suggest a sixteenth century AD terminus ante quem for the excavation of basal planting pits, especially since midden deposition postdates the abandonment and infilling of the holes. A fifteenth century AD chronology for the excavation of the first planting pits at Triangle Flat is entirely possible.
NZA 14734 dates a carbonised, angular kanuka (Kunzea ericoides (A. Rich) J. Thompson) stem fragment that may derive from a young, near contemporary (and potentially colonising) plant, or a shrub to small tree that might have been several decades old at the time of death (cf. Allen et al. 1992). This topsoil charcoal sample was nested 13 cm bgs between an extensive tuangi beach shell deposit and a shallow, upper planting depression. Beach shell is mounded above the charcoal. An apparent tuber mould intrudes into the substrate from the base of the depression (Table 1 (7) and Figures 5 and 8).
In its relative isolation, it is plausible that the dated kanuka charcoal fragment was added and mixed with other fertilizing materials shortly before shell was mounded above the root crop. Since the local Maori archaeological sequence ended in about 1830 (cf. Peart 1937: 31-53), the calibrated (1- and 2-sigma) intercepts from the late seventeenth century to the early eighteenth century and the beginning of the nineteenth century are most relevant to the actual calendar age of this charcoal and, by association, to the chronology of upper shell deposition. Consistently, historical nineteenth and twentieth century materials (fauna and artefacts) are recorded in the topsoil above and on, but never within or below, upper shell mulch sediment. The incorporation of Maori archaeological materials only within upper shell deposits is important to note as well. Given these associations, the c.1830 terminus ante quem for Maori occupation at Triangle Flat, and the possibility of some inbuilt age for NZA14734, a late seventeenth century to early nineteenth century age range is indicated for shell mulch deposition in at least one area of south-western M24/11.
In summary, the development of the shallow topsoil per se above beach deposits at M24/11 represents about 1300 years. The Maori cultivation sequence identified within this topsoil incorporates a time depth of perhaps 300-400 years, possibly beginning at or soon after the fifteenth century and terminating at about the eighteenth century.
An agronomic interpretation of this archaeological evidence must consider the possible effects of mollusc mulch in relation to the soils, environment and landform of Triangle Flat.
It would seem unlikely that beach shell mulch represented any part of a treatment to improve cultivation soil fertility. Mulch matrix, where distinguished from the black topsoil, is the same texture and colour as local unweathered sand, including sand of the substrate beach ridge deposit (cf. brown 10YR 4/3 moist sand matrix of beach shell deposit ii and brown 10YR 4/3 moist sand matrix of the natural substrate, Figure 6 B-B'). This means that organic materials were not mixed with beach shell mulch in any quantity, if at all. Furthermore, shell mulch layers or lenses are identified as undifferentiated sediments above the cultivation soil matrix (Figures 6-8). Beach shell is recorded in some planting soils and pits, but usually as occasional or discrete intrusions from upper shelly lenses, perhaps as a result of harvest disturbance (e.g. discrete shelly intrusion in a planting pit at Figure 6 B-B'). There is no evidence otherwise that beach shell was ever mixed into soils or features to act as a slow release fertilizer.
It is much more likely that growing plants were helped and even protected by the physical and mechanical properties of shell mulch. Cultivation in loamy sand on an elevated shelly ridge would ensure effective drainage, an important consideration for kumara. However, the natural topsoil of the younger dunes and beach ridges at Triangle Flat (Tahunanui soil unit) is generally thin (about 50 cm deep or less), with a low clay and organic content. Consequently, the water and nutrient holding capacity of the soil is relatively low. Topsoil is also vulnerable to exposure and loss from dominant and often dry westerly winds (Jane 1989: 8). This last factor may have been of particular concern as kumara beds were established in spring at an exposed north-western South Island coastal locality. As Shulmeister et al. (2004: 45) observe, "the greatest near-surface wind speeds in mid-latitude regions" such as central New Zealand are during spring (October-November) "when the Antarctic re-enters the circulation system". Shell caps above planting pits and more extensive shell sediment above upper topsoil may have helped secure planting matrix and even plants against strong winds throughout the growth cycle.
Shell mulch may have helped to regulate field temperatures and even warm plants as well. Modern agricultural trials for grape vines in the Nelson region suggest that there is no overall thermal increase in soils from mollusc shell mulch around grape plants. Rather, shell mulch results in a flattening of soil temperature extremes, with another crop benefit being increased lower canopy temperatures from radiant energy reflected off weathered valves. There is also clear evidence for consistent weed suppression and improved soil moisture retention from shell mulch experiments (Creasy et al. 2007). For Polynesian cultivators at Triangle Flat, reflected energy from weathered shell mulch would benefit juvenile kumara only, since the low, spreading leaves of the kumara plant would eventually cover the shell surface. However, this may have been a critical benefit as kumara beds were established, along with the suppression of weeds competing for nutrients and water. At Triangle Flat, later twentieth century records also indicate that droughts are frequent in the latter part of the kumara growth season in January and February. Furthermore, late summer winds bring little moisture to the region and can be desiccating (Jane 1989: 4 and app. 1). Shell mulch may have countered the effects of soil moisture loss and temperature spikes later in the season.
Collectively, therefore, the physical properties of shell mulch may have ameliorated the varying effects and extremes of changing circumstances and climate throughout the growth cycle at Triangle Flat. Local tuangi mulch, in short, may have been an agronomic treatment for all seasons.
EXPLAINING AGRONOMIC CHANGE
If these environmental observations elucidate possible reasons for shell mulching at south-western M24/11, one must also consider the evidence that beach shell was spread widely over field surfaces in the latter part of the sequence. As reported above, this new management approach involved some changes in practice, and was associated with a planting emphasis on the upper topsoil. What might account for this development?
The chronological indicators for south-western M24/11 allow for the hypothetical possibility that mollusc mulch was extended in the early nineteenth century to accommodate recently introduced Solanum tuberosum L. (white potato) in Maori cultivation (see H.M. Leach 1984: 98-9). Here, however, it is important to consider the evidence that extensive mollusc mulch was targeted towards tuber production in the upper section of a thin sandy topsoil of low natural fertility. This is not consistent with the cultivation of white potato, which has a higher soil fertility requirement than kumara, but can also tolerate a greater variety of colder soils (H.M. Leach 1984). Soil depth and fertility rather than cover would have been limiting factors for white potato cultivation at Triangle Flat. Thus Solanum potato is cultivated widely, and successfully, in soils that are deeper and more fertile than M24/11 topsoil throughout Golden Bay today.
One must also consider whether extensive molluscan mulching represents a strategy of "landesque intensification" by way of field improvements to increase kumara production yields from a fixed land unit (after Brookfield 1984, 2001; see also Kirch 1994). A terraced, headland earthwork fortification or pa with transverse ditches (M24/1) is recorded at Puponga Point, about 1 kilometre away from south-western M24/11 (Figure 3; Brailsford 1981: 90-1). It is possible that this pa was associated with socio-economic and political developments that may have encouraged a more intensive cultivation strategy to increase crop returns. However, it is also important to consider that widespread late-sequence shell mulch at south-western M24/11 covers generally shallow planting depressions only (where these can be resolved and recorded). It is not at all evident that this extension of shell sediment would have improved total crop productivity from the same fixed land unit, especially since planting pits are also located to the west of the shelly mulch area (Figure 4, western residential lot).
Instead, it is plausible that late-sequence shell applications were extended above shallow kumara planting soils in response to a western South Island convergence of cooler temperatures and enhanced westerly winds. These regional conditions are documented over several centuries after the fifteenth century AD (F. Leach 2006: 169-81; Lorrey et al. 2007: tables 8, 422, 425-6; Lorrey et al. 2008: 67-71; Shulmeister et al. 2004; Winkler 2004). Several researchers interpret this record of cooler conditions as the temperate zone, Southern Hemisphere onset of the global "Little Ice Age" ("LIA") that began something around the seventeenth century AD (e.g. F. Leach 2006: 169-81; Newnham et al. 1998: 452; Shulmeister et al. 2004; Winkler 2004). While the inter-hemispherical correlation of glacier chronologies as global climate (including LIA) proxies is debated (Brook et al. 2011 ; Schaefer et al. 2009; Winkler & Matthews 2010), Shulmeister et al. (2004: 44), note that there is now "widespread" New Zealand evidence for a period of "enhanced westerly flow during the LIA ... preceded by a period of reduced westerly flow" (Shulmeister et al. 2004: 44). For western South Island, 'Lorrey et al." also observe that speleothem data "indicate very cold temperatures" over the sixteenth century AD (2008: 71). "Cool" temperatures and "wet" precipitation conditions follow between about 1650 and the late nineteenth century, coincident with a "widespread advance" of South Island glaciers (Lorrey et al. 2008: tables 5 and 70). Lorrey et al. (2008: 71) also identify this length of time (i.e. approximately 1650 "to the late 19th century AD") as "probably" one of "the strongest periods of westerly flow in the last 4000 years".
Kumara beds at exposed Triangle Flat may have been affected adversely by the impacts of lower sixteenth century temperatures and thereafter, stronger, westerly spring winds that prevailed over north-western South Island. Accordingly, the new planting focus on the warmer, dryer upper ground soil may represent a practical strategy to sustain temperature sensitive crop production in a colder and wetter climate. At the same time, these shallow cultigens were more vulnerable to enhanced, westerly wind erosion, especially after AD 1650. As discussed above, there is no experimental reason to believe that shell mulch would have raised soil temperatures per se. However, extensive shell surfaces that were mounded above young plants would have helped secure seedlings and stabilise shallow, upper A-Horizon planting matrix against stronger and persistent eighteenth century winds in the first instance. Also, this broad treatment would have extended the shell mulch benefits of weed control and, as needed, late-summer temperature regulation and topsoil moisture retention.
COMPARISON AND CULTURAL CONTEXT
It is instructive to compare evidence of Maori agricultural change and abandonment elsewhere in central New Zealand. There are indications that the alluvial Waimea Maori gravel soils of southern Tasman Bay were not gardened in at least one core locality (associated with Appleby sites N27/118 and N27/122 at Figure 2) after the seventeenth century. This inland floodplain locality had a much higher risk of ground frosts than Triangle Flat at the beginning and end of the growth season, with fewer if any options to mitigate adverse climate change (Barber 2010; Challis 1991: 101-3; also cf. average days of ground frost for Appleby and Farewell Spit respectively in New Zealand Meteorological Service 1983: 93, 104). It has been argued that the LIA onset may have been a factor in the abandonment of Maori cultivation fields on coastal fans at Palliser Bay on the south-eastern end of the North Island (H.M. Leach 1984: 61-4). Late eighteenth century Maori of the north-eastern South Island Marlborough Sounds had apparently abandoned agriculture in an area with archaeological crop storage pits and field systems as well (Barber 1999b).
Farewell Spit offered the climate advantage of a north-western South Island coastal locality with no more than a few annual frost days, all of which fell in months outside of the kumara growth season in twentieth century records (Challis 1991: fig. 1; Garr & Fitzharris 1991: app. 1 [Climate region 6]; Jane 1989: 4; New Zealand Meteorological Service 1983: 93). Furthermore, coarse beach mulch materials were readily available at Triangle Flat along the Golden Bay shoreline or from shallow beach ridge substrate. As referenced earlier, late-sequence Maori agriculture is also identified in a relatively warm coastal location at Tata Beach in eastern Golden Bay, where it appears that beach gravel was added to the garden soil (Barber 1999a: 138, 143-4).
Here, it is enlightening to compare the situation on Rapa Nui. Louwagie et al. argue that lithic mulch "has proven to be most effective in case of i) extreme drought and ii) crops that are grown under less favorable conditions". In this view, lithic mulch management was "an effective strategy to limit the risks linked to climate variability" (Louwagie et al. 2006: 307, 308). Accordingly, one of the "clearest examples of lithic mulch management" on Rapa Nui "appears to be an adaptive response" in a territory with low precipitation and the threat of "conditions of extreme drought" (Louwagie et al. 2006: 310). These observations suggest that Rapa Nui lithic mulch intentions were more comparable to the mitigation of seasonal extremes and climate change proposed for Triangle Flat shell mulch than to the improvement of soil drainage and temperatures proposed for modified gravel floodplain soils in western Tasman Bay (Barber 2010; Challis 1976).
Finally, it is salutary to consider the ritual context of eighteenth to nineteenth century Maori kumara cultivation. Evidence of this context is consistent and documented widely in the historical record for Maori iwi (tribes) of North and South Islands, especially around requirements associated with the tapu (sacred, forbidden, spiritually dangerous condition) of kumara fields under the care of the deity Rongo (Barber 2012b). In the imposition and maintenance of the tapu domain, it was critical to remove and keep separate noa (common, tapu-removing) cooking activities and materials. The last could include non-ritual food and general waste food remains (Best 1976; Firth 1959: 264-81; Johansen 1958: 112-88; H.M. Leach 1984: 71-2, 108-9; Smith 1974: 28-9, 31-3).
In contrast, live or uncooked animals from the marine domain, including the stranded whales delivered regularly to local beaches, were associated with the natural processes and rituals of Tangaroa, god of the ocean (Barber 2003). Conceivably, tuangi shellfish that had expired naturally may have been linked conceptually to the raw foods of the gods. As noted above, it is clear from the archaeology of M24/11 that tuangi was not a target subsistence species, while the tuangi "family" in particular is critical to an important, aetiological myth (Smith 1974: 28, 34). Natural tuangi valves, in short, may not have interfered with the tapu of kumara fields. They may even have had some potency in reinforcing the separation of cooked food waste, such as contemporary or exposed midden materials, and non-cooked food sediments. Consistently, there is no evidence that the very discrete, Paphies-dominant, M24/11 midden deposits incorporated within the larger field system (Figure 4) and extending to the north-east (Walton & Bagley 2000) were ever considered to be potential mulch resources. Very occasional and clearly incidental midden components only are identified within the extensive upper beach shell deposits.
This cultural interpretation cautions against the conception of extensive mollusc beach mulch as an entirely pragmatic development. The ethnohistorical record indicates that a breach of the kumara ritual and tapu was of as much concern as any environmental effect, and punishable by death (Best 1976: 184-9; Johansen 1958: 136-7). Conceivably, agronomic change and persistence were encouraged by the local opportunity to extend a hard mulch strategy that satisfied the needs of spiritual and practical protection. This possibility is resonant of highly selective Maori fishing behaviour referenced to the northern South Island archaeological record that may have been ritually regulated as well (Barber 2003).
In brief, late-sequence beach mulch management at Triangle Flat may represent a local agronomic development that sustained kumara cultivation through a colder and windier climate period. It is also possible that this development was encouraged by, and extended, a ritual engagement and worldview in the choice and use of tuangi mulch materials.
DISCUSSION AND CONCLUSION
The evidence that a local, predominantly mineral (albeit biogenic) clast equivalent was adopted as a hard mulch application at Triangle Flat reinforces the pattern of variance emerging in hard mulch studies within and between the archaeological landscapes of Rapa Nui and South Island, in spite of superficial similarities. Differences in hard mulch practices and outcomes are consistent with a measure of local agronomic innovation associated with the Polynesian diaspora of dry field, kumara cultivation (Barber 2004, 2010, 2012a).
For the first Triangle Flat cultivators, an enriched sandy topsoil matrix from burnt, cleared vegetation dug into holes would have satisfied both the moderate nutritional and critical soil drainage requirements of kumara production. The further application of a moisture- and soil-retaining shelly surface would have helped protect growing plants and a nutritionally fortified soil matrix against strong, wetter spring winds and eventually, hot, dry and windy summer conditions. As well, shell surfaces would have suppressed early weed growth, and may have helped to warm young growing plants. It is possible that the later extension of shell mulch treatments over larger garden surfaces and shallower planting soils at Triangle Flat relates to new socio-economic or political pressures to intensify production and improve crop outputs. However, since late-sequence agronomy targeted cultivation in the warmer, upper topsoil matrix, it seems more likely that this change represents a local, ongoing development to sustain kumara production in a colder and windier climate in the first instance. Here, I propose that shell mulch was extended to help secure shallow garden soils and plants. This treatment would also have suppressed weeds over a wider area, and assisted in the regulation of late summer soil temperatures and moisture. Extensive late-sequence shell applications also represented a ritually safe and perhaps spiritually potent response to the problem of climate variability.
Evidence for variation in hard mulch applications at the southern margins of kumara productivity demonstrates that localised innovation could occur within this agronomy over time and space. The Golden Bay evidence also indicates that at least some central New Zealand Maori persisted in growing kumara in accordance with ritual requirements in spite of the onset of a cooler and windier climate from AD 1650. This argues for a more nuanced understanding of agricultural economies, abandonment and resilience in the later central New Zealand Maori sequence than some earlier archaeological models have allowed (Barber 2004).
Triangle Flat archaeological work (1999-2007) was approved by Golden Bay iwi (tribal) authority Manawhenua ki Mohua (with particular acknowledgement to Trina Mitchell, Chris Hill and John Ward-Holmes), the Department of Conservation as landowner/manager (where Tony Walton and Steve Bagley deserve special mention) and the statutory archaeology authority, the New Zealand Historic Places Trust. University of Otago Research Grants and students supported excavation work and analysis, with special thanks to Emma Brooks and Adrian Taylor. Rod Wallace, University of Auckland (Anthropology), identified charcoal fragments to species from one planting pit. Mark Horrocks, University of Auckland, identified microbotanical remains from several samples. Alan Hogg and Fiona Petchey from Waikato Radiocarbon Dating Laboratory provided calibration data. The observations of the anonymous referees were helpful in the final revision of this paper. Les O'Neill, Department of Anthropology and Archaeology, University of Otago, prepared the illustrations.
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University of Otago
Correspondence: Ian Barber, Department of Anthropology and Archaeology, University of Otago, Dunedin 9054, New Zealand. Email: firstname.lastname@example.org
Table 1. Conventional radiocarbon ages (numbered sequentially) before present (CRA BP) and calibrated age ranges (ca1BC-AD) for dates in the text. In the sample column, the taxonomic abbreviations for marine shellfish species are as follows: Pa, Paphies australis (Gmelin, 1790) (pipi); Ps-Pd, Paphies subtriangulata (Wood, 1828) or P. donacina (Spengler, 1793) (both tuatua); As, Austrovenus stutchburyi (Wood, 1828) (tuangi). Marine calibration data are from Reimer et al. (2009) with NZ delta R of -7 [+ or -] 45. For terrestrial calibrations, Southern Hemisphere atmospheric data are from McCormac et al. (2004). Lab no. ([delta] CRA BP ca1BC/AD [sup.13]C 1-sigma [per thousand] value) (1) Wk-8120 2300 [+ or -] 50 60 BC-AD 120 (+0.4 [+ or -] 0.2) (2) Wk-11696 1943 [+ or -] 48 AD 390-530 (+1.0 [+ or -] 0.2) (3) Wk-17250 795 [+ or -] 31 AD 1465-1535 (+1.4 [+ or -] 0.2) (4) Wk-8052 750 [+ or -] 50 AD 1490-1630 (+1.5 [+ or -] 0.2) (5) Wk-9611 751 [+ or -] 40 AD 1490-1620 (+1.3 [+ or -] 0.2) (6) Wk-11542 698 [+ or -] 39 AD 1530-1660 (+1.4 [+ or -] 0.2) (7) NZ14734 94 [+ or -] 55 AD 1700-1722 (11%) (-18.4 [+ or -] 6.9) AD 1809-1838 (16%) AD 1845-1867 (10%) AD 1878-1930 (03%) Lab no. ([delta] calBC/AD Sample context and [sup.13]C 2-sigma type (M = marine [per thousand] value) T = terrestrial)/species (1) Wk-8120 160 BC-AD 200 Natural shelly ridge, (+0.4 [+ or -] 0.2) M/As (2) Wk-11696 AD 310-590 Natural shelly ridge, (+1.0 [+ or -] 0.2) M/As (3) Wk-17250 AD 1440-1620 Midden above planting (+1.4 [+ or -] 0.2) pit, M/Pa (4) Wk-8052 AD 1440-1680 Midden above planting (+1.5 [+ or -] 0.2) pit, M/Ps/Pd (5) Wk-9611 AD 1450-1670 Midden above planting (+1.3 [+ or -] 0.2) pit, MlPa (6) Wk-11542 AD 1470-1700 Midden above planting (+1.4 [+ or -] 0.2) pit, M/ Ps/Pd (7) NZ14734 AD 1677-1735 (20%) Soil between upper (-18.4 [+ or -] 6.9) AD 1799-1956 (75%) shelly lens and depression, T/stem charcoal Kunzea ericoides (A. Rich.) J. Thomps.
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|Publication:||Archaeology in Oceania|
|Date:||Apr 1, 2013|
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