Taphonomic factors affecting macrobotanical preservation

In document Agriculture in Tongan Prehistory: An Archaeobotanical Perspective (Page 67-70)

Unlike the analysis of starch granules within archaeological and palaeoenvironmental contexts, the taphonomy of vegetative storage parenchyma is poorly known. This is a direct result of the lack of archaeobotanical projects targeting the charred remains of these relatively soft tissues under the presumption that preservation will be low or non-existent. Conditions for preservation

50 of parenchyma are primarily limited to those that either desiccate, water-log or char these botanical remains (Hather 1991, 1992, 1994, 2000). Another less common form of preservation includes mineralised or semi-mineralised remains that are either impregnated or coated by semi- crystalline minerals that protect the soft tissues from fast decay (Paz 2001:40). While wood charcoal and seeds may change little on preservation (unless compressed), vegetative storage tissues tend to undergo a number of physical transformations (Hather 1991:662). This is because roots and tubers have a higher proportion of parenchyma tissues and a smaller portion of cells with lignified walls than stem wood (Pearsall 2010:161). Experimental charring has emphasised the nature of some of these changes which include expansion or shrinkage of tissues, the deterioration or loss of more fragile regions, and fragmentation (Hather 2000:74, 1991:662-3).

The paucity of root and tuber macro-remains in archaeological sites is often directly related to how these resources are processed and used. As these are generally sources of food the only part of the organ that often remains is the peel (Allen 1983; Kahn and Ragone 2013; Pearsall 2010:154). Some root peelings are tough and fibrous but others have periderms that decay relatively quickly and are very fragile if charred. Other aspects of the biology of roots and tubers also make any preservation unlikely. For example, if these organs are dropped or discarded by humans they are often scavenged by animals or are susceptible to agents of decay due to the high calorific value and moisture content (Holden et al. 1995:777). High moisture content also makes these organs prone to distortion and fragmentation if exposed to heat, especially once charred (Holden et al. 1995:777; Pearsall 2010:157). Those fragments or whole organs of vegetative storage parenchyma that do preserve are most likely roots and tubers that have been discarded into the fire as spoiled goods or accidentally charred during roasting (2010:157). Ethnoarchaeological observation in the Philippines has confirmed these as possible cultural taphonomic processes that enable charred parenchyma to enter the archaeological record (Paz 2001:80). Charcoal is formed in the reducing conditions within the ash in the base of a fire or hearth, rather than the oxidising conditions of the open flame which eventually reduces tissue to ash. Some small dense fragments of tissue can fall through the structure of the fire into the ash and transform into preserved charcoal (Hather 1991:663).

Hather warns that quantification of taxa within preserved parenchyma is unlikely to result in meaningful interpretation because the cultural and taphonomic processes affecting the conditions of preservation are largely unknown (Hather 2000:74). However, it is possible to utilise some of the current data about macrobotanical preservation and post-depositional processes derived from studies of wood charcoal or anthracology. Thery-Parisot and others (2010) describe the range of ‘filters’ that botanical remains will pass through before being incorporated into palaeoenvironmental or archaeobotanical reconstructions. These include societal filters such as selection, hearth maintenance and storage; combustion filters such as anatomical changes and differential fragmentation; depositional and post-depositional filters

51 include anthropogenic factors, mechanical factors and diagenesis; and archaeological or anthrocological filters including sampling, identification and quantification (Thery- Parisot et al. 2010:143). Post-depositional filters can be broken down into specific cultural processes such as trampling, re-working and sweeping, and natural taphonomic agents such as bioturbation, atmospheric factors, mechanical constraints that cause pressure or friction in the sediments and can induce chemical alteration, water and the pH of soils (Thery-Parisot et al. 2010:147-150). Where these process act homogenously within a particular context, they will not affect the palaeoecological signature, but this is rare and differential preservation can often be observed within these records.

These post-depositional processes can lead to vertical and horizontal migration of remains as well as fragmentation and disappearance (Paz 2001, 2005; Oliveira 2008; Hather 1994, 1995). Nelson (1992) and Greenlee (1992) assessed the nature of downward displacement of macrobotanical remains within shell midden contexts. Each argues that the ratio of shell to soil matrix will affect the porosity of the midden, and thus also the amount of movement that is possible (Greenlee 1992:262; Nelson 1992:254). As a general rule, particles smaller than 2mm are most susceptible to movement through mechanical processes, as well as bioturbation by roots, earthworms and burrowing animals (Nelson 1992:254; Thery-Parisot et al. 2010:147). Paz (2001) also explores the role that soil matrix can play in the preservation and movement of macrobotanical remains. He argues that the compaction of clay soils inhibit matrix mixing except where bioturbation has occurred, but agrees with Nelson (1992) and Greenlee (1992) that the porosity of shell midden and also sandy deposits allow greater degrees of turbation and downward displacement (Paz 2001:40).

Questions of context security can also be addressed through assessment of the forms and taxa of preserved macrobotanical remains within a given strata. Purely charred remains can be considered reasonably representative of an in situ deposit, especially where upper layers do not contain any untransformed or intrusive remains that have not undergone charring, desiccation, water-logging or mineralisation (Paz 2001:262). Where downward percolation has occurred, the distribution of seeds and small charcoal should be indicative of this movement, with greater densities of remains in lower levels of particular deposits or facies within shell middens (Nelson 1992:243). Post-depositional homogenisation by chemical processes such as prolonged submersion in brackish groundwater can also occur and mask variability in separate and disparate depositional events (Nelson 1992:251). These studies argue that by assessing the context, sedimentary matrix, and nature of botanical remains, some natural and cultural post- depositional taphonomic processes can be recognised and incorporated into the interpretation of the archaeobotanical record.

52 Macrobotanical preservation within tropical climates like those in the Pacific is generally thought to be poor. However, a number of archaeobotanical projects in the Asia- Pacific region have proven that these conditions do not necessarily inhibit long term preservation of soft tissues (Allen 1983; Barton and Paz 2007; Hather 1991, 1992, 1994a, 1994b, 1995; Paz 2001, 2005). Desiccated remains are rarely found unless in caves or rock- shelters. Water-logged remains are a more common form of soft vegetative tissue recovered in the region (Hather 1992:71). Anaerobic brackish lagoon settings have preserved large assemblages of botanical remains within modern beach deposits formed by clay erosion. For example nuts and other large seeds were preserved within these conditions at the site of Talepakemalai on Eloaua Island in the Mussau Island group (Hather 1992; Kirch 1989). Charred remains of roots, tubers, wood, seeds and nuts form the majority of preserved plant tissue found in archaeological and palaeoenvironmental contexts. Within these remains, fruits, roots and tubers are less common, but have been extracted and identified on a number of occasions within rock shelter deposits (Hather and Kirch 1991; Oliveira 2008, 2012; Paz 2001), shell middens, house floors (Coil and Kirch 2005; Kahn and Ragone 2013; Yen 1974) and dry sandy open sites (Hather 1994).

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