Beyond rice farming: Evidence from central Korea reveals wide resource utilization in the Songgukri culture during the late-Holocene

Abstract

This study investigates prehistoric subsistence of the middle Bronze (Mumun) period in the central part of the Korean Peninsula during the late-Holocene. Rice farming has often been regarded as a harbinger of complex society with an economic foundation based on rice yields. We test this common hypothesis on intensive rice farming as a primary means of subsistence and political economy through an integrative approach with archaeobotanical and isotopic data. We focus on the sites from the Songgukri culture (2900–2400 cal. BP) which are scattered along the middle and lower reaches of the Geum River. Our study indicates settlers along the Geum River utilized a wide range of crops and wild animals, a picture far removed from the popular view of a rice-dominated diet in the Songgukri culture.

Keywords

archaeobotany diet isotope Korean peninsula Mumun or Bronze period rice farming social complexity subsistence

Introduction

Changes in human diet and subsistence, particularly the emergence of agriculture, have been understood in relation to evolutionary consequences (e.g. Bovin et al., 2016; Gremillion et al., 2014; Smith, 2015; Zeder, 2016) and/or sociopolitical dynamics (e.g. Rowley-Conwy, 2011) by means of various analytical methods worldwide (Zeder et al., 2006). The emergence of agriculture in Korean archaeology has often been portrayed as a consequence of shifting sociopolitical dynamics at the onset of the Bronze or Mumun period (3500–2400 cal. BP), caused by human migrations and emerging social complexity (Kim, 2003, 2006c; Lee, 2001; Norton, 2000, 2007). Particularly, the middle Bronze phase (2900–2400 cal. BP) was commonly seen as a period of drastic changes in socioeconomic realms, including irrigated rice farming, bronze metallurgy, prominent mortuary customs, and communal investment in public structures, all contributing to emerging social complexity (Nelson, 1993). For example, Kim (2005, 2006a, 2006b) suggested that the emergence of social hierarchy was driven by the rapid spread of intensive rice agriculture into existing foraging contexts.

The Songgukri site became a focal point of such claims early on with the discovery of abundant charred grains of rice (Oryza sativa; Buyeo National Museum, 1975). The Songgukri culture, named after this site, has been closely studied as a distinctive social and cultural entity that emerged along the Geum River in central Korea (Kim, 2015b; Figure 1). Findings of irrigation fields and a reservoir at Majeonri, a middle Mumun settlement near Songgukri (Research Institute for Archaeological Resource of Korea University, 2004), further fuel the assumption that rice farming was responsible for the social and economic expansion of the Songgukri culture.

Figure 1.

The locations of archaeological sites mentioned in this study.

What is missing in this explanation is evidence pertinent to rice farming production and other subsistence activities. Early findings of charred rice grains were more or less a result of casual recovery, and systematic collection of flotation samples was sporadically applied in only 3 of 19 field seasons at Songgukri. A high soil acidity in Korea inhibits long-term preservation of organic remains (Rural Development Administration of Republic of Korea, 1988). Faunal remains are extremely rare in Korea except for shell midden sites as alkali conditions preserve organic remains. Shell middens are the most common sites through the long duration of the Neolithic but disappeared abruptly at the beginning of the Bronze period. In contrast, large-scale inland settlements appeared on low hills with gentle slopes, overlooking rivers and alluvial flats. Such landscape preference continued throughout the Bronze period until coastlines were reoccupied during the subsequent early Iron period (2400–2000 cal. BP). The lack of shell middens limits faunal remains and thus isotope analysis of animal remains dating to the Bronze period. The near absence of animal remains and the rare systematic recovery of plant data put the popular claim (Songgukri culture was mostly based on rice farming) on hold. Were any other resources procured along with rice? Did procurement of wild resources no longer play any role by the middle Bronze period? That is, is an absence of evidence (of faunal remains) really an evidence of absence (of animal procurement)?

Isotope data can compensate for the dearth of direct data on animal use. Stable isotope analysis has been effective to trace diet patterns of animals, as the isotopic composition of animals depends on the feed (Malainey, 2011). Isotope analysis on human remains has been widely welcomed as a means to distinguish a C₃ diet from a C₄ diet or a millet-based diet from a rice-based diet (e.g. Barton et al., 2009; Bentley, 2007). In Korean archaeology, isotope analysis has been mainly applied to coastal shell midden sites, dating to either pre-Bronze or post-Bronze periods (e.g. Choy et al., 2012; Choy and Richards, 2010; Lee, 2011c, 2011d; Shin et al., 2013).

This study attempts to overcome the absence of faunal data by applying isotope analysis on cooking vessels. Considering that pots are often used for culinary practices (Crown and Wills, 1995; Stahl, 1989; Wandsnider, 1997), organic residues on vessel surfaces have been subject to isotopic analysis. In one successful case study, carbon and nitrogen isotope ratios were measured within soot on potsherds (Hastorf and DeNiro, 1985). Experimental studies show a certain type of pottery at Songgukri cultural sites functioned as cooking vessels (Shoda, 2008), and our study uses this type of pottery. By doing so, this study aims to evaluate the current idea regarding rice as a staple of the Songgukri culture. We will present results from isotopic and archaeobotanical analyses on the Songgukri, Dosamri, Wolgiri, and Majeonri sites. If food residue in pottery can place the middle Bronze diet in a broad category of C₃ or C₄ plants, macroscopic plant remains can be identified at a species level and provide data on plant resource management strategy. If rice agriculture was a key element in rising social complexity in the Songgukri culture, we would expect a dominance of C₃ diet in isotope analysis and a prevalence of rice from archaeological plant remains.

Archaeological setting

The main focus of this study, Songgukri (127.031E, 36.2642N) in Buyeo City, South Chungcheong Province, is probably one of the most well-known and thoroughly studied sites in Korea (Kim, 2015a), subject of 19 seasons of fieldwork since the late 1970s (Buyeo National Museum, 2000, 2013; Kim et al., 2011, 2013a, 2014, 2016; National Museum of Korea, 1979, 1986, 1987). Songgukri became a type site, representing the middle Bronze period in southwestern and south-central Korea. Similar types of structures and artifacts were found along and south of the Geum River, providing a basis to coin a collective term, Songgukri culture. A total of 26 radiocarbon dates at Songgukri range from 2900 to 2400 cal. BP, indicating a continuous occupation throughout the middle Bronze period (Figure 2, also see Supplementary Table 1, available online). During the first field season in 1975, Songgukri attracted keen interest in Asian archaeological circles, providing the first evidence of bronze artifacts, tubular jade beads, Songgukri-type plain pottery, megalithic burials called dolmen, and rounded pit houses with two postholes in Korea (Supplementary Figure 1, available online).

Figure 2.

The density distribution of radiocarbon dates from the Songgukri, Dosamri, Wolgiri, and Majeonri sites, using the R package BChron (the dates were calibrated using the ‘IntCal13’ calibration curve; cf. Reimer et al., 2013).

At Songgukri, roughly 100 subterranean pit houses were found in hilltops and slopes, covering several square kilometers. The structural remains of wooden fences and ditches around the residential area were often regarded as an indicator of conflict and competition among local communities (National Research Institute of Cultural Heritage, 2002). The site also revealed structures and objects of prestige, including stone-cist burials with a Liaoning-style bronze dagger, large tubular-shaped greenstone ornaments, and a polished ground stone dagger (Supplementary Figure 1, available online). High status materials, elaborate burial offerings, and a cluster of apparent satellite settlements around Songgukri led to a hypothesis that the Songgukri culture represented the earliest form of social hierarchy in Korea (Kim, 2005, 2006a, 2006b).

Several smaller settlements of similar cultural settings were found within a radius of several kilometers from Songgukri. One of them is Majeonri (127.081E 36.1701N) in Nonsan city, 15 km south of Songgukri (Research Institute for Archaeological Resource of Korea University, 2004). This site became key to confirming the irrigated farming practice of the Songgukri culture, with remains of rice paddies and several segments of irrigation canals connected to a reservoir (Supplementary Figure 1, available online). Much smaller settlements were identified along the lower reach of the Geum River, including the Dosamri (126.727E 36.044N) and Wolgiri sites (126.774E 36.0676N) (Research Institute for Archaeological Resource of Korea University, 2005a, 2005b). Excavations at these sites yielded 30 and 14 pit houses, respectively, along with several outdoor pits. Radiocarbon dates from all three sites overlap with those from Songgukri, indicating contemporaneous occupations (Figure 2).

Materials and methods

Archaeobotanical analysis

Our main archaeobotanical data were retrieved from the Songgukri site during the 11th, 14th, and 19th field seasons (Kim et al., 2012, 2013b; Lee, 2000). We also incorporate archaeobotanical data from three settlements contemporaneous and adjacent to Songgukri, including Dosamri (Lee and Cho, 2005a), Wolgiri (Lee and Cho, 2005b), and Majeonri (Lee, 2004; Figure 2). Irrigation canals for rice paddy fields at Majeonri provide a waterlogged condition where uncharred plants can be preserved, in contrast to all other sites. Soil samples were collected from houses and outdoor pits at Songgukri, Dosamri, and Wolgiri, ranging in volume from each feature from 1.0 to 134.5 L. Samples from houses represent floor fills, fills inside pits and pottery, and hearths.

One of the authors (Lee) applied a manual decanting flotation method (in sensu Crawford, 1983) to separate plant remains from soil matrix at Songgukri (the 11th and 19th field seasons), Dosamri, Wolgiri, and Majeonri. A standard geological sieve of 0.212 mm mesh was used to collect floating materials, while a screen with 1.0 mm mesh was used to catch heavy charcoal fragments at the bottom of the buckets. Kim et al. (2012) used a modified Shell Mound Archaeological Project (SMAP) apparatus, a water tank that was used to separate charred remains from soil matrix (in sensu Watson, 1976) during the 14th field season at Songgukri. They used a coarser mesh (0.5 mm) to catch floating samples.

After drying, flotation samples were observed with low-powered binocular microscopes (10–80×). All the seeds and other vegetative parts were counted and recorded in Lee’s analysis (Supplementary Table 2, available online). Instead of counting individual grains, other researchers estimated the numbers of rice and foxtail millet (Setaria italica ssp. italica) grains from Songgukri samples (14th field season) on the basis of their weights (Kim et al., 2012, 2013b; Kim and Ryu, 2013; see SGR2 listed in Supplementary Table 2, available online). In their analysis, non-crop seeds were rare in houses and absent in pits. We cannot conclude whether the lack of non-crop seeds in their analysis reflects a cultural pattern or whether the use of a coarser sieve (0.5 mm) caused the near absence of usually smaller, non-crop seeds.

Isotope analysis

According to experimental studies (Evershed, 2008a), organic residues are most concentrated in the rims and upper body parts of the pots after cooking. Ethnographic observations also showed high-temperature boiling is a particularly effective way to cook animal and plant resources in pots (Crown and Wills, 1995; Stahl, 1989; Wandsnider, 1997). During cooking, convection currents of boiling water push extracted lipids from foodstuffs to the vessel walls. Since fats float on water, they tend to accumulate and penetrate into the wall of the upper body and rim of the pot. Following such findings, a total of 27 pottery samples collected from pit houses at Songgukri (the 14th field season) were rim or upper body sherds.

The high acidity of Korean soils can destroy even surface residues on pottery during the course of the post-depositional process. Organic residues are found either as visible remains on the pottery surface or absorbed within the fabric of its matrix (Evershed, 2008b; Evershed et al., 1991). Absorbed residues, unlike the visible ones, are more likely to survive against post-depositional processes, as fired clay acts as a ‘trap’ or ‘net’, protecting and preserving lipids (Reber and Evershed, 2004). These absorbed residues are often called lipids, a mixture of naturally occurring compounds such as fatty acids, waxes, and sterols that exist in organisms. With the introduction of compound-specific isotope analysis, the stable carbon isotope values of individual compounds in a lipid mixture can be measured with high precision, providing a unique opportunity to conduct carbon isotopic analysis on the principal fatty acids (C16:0 and C18:0) that are ubiquitous in archaeological ceramics (Mottram et al., 1999). In this study, stable carbon isotope ratio (hereafter δ¹³C) values of C16:0 and C18:0 fatty acids extracted from the potsherds were measured.

Absorbed residues were extracted using an established protocol outlined by Correa-Ascencio and Evershed (2014). An approximately 5-g portion of each potsherd was sampled, and the surface was cleaned using a drill (Dremel 3000) to remove any external contamination. Cleaned samples were ground to a fine powder in a glass mortar and pestle, and accurately weighed. Samples were transferred into a culture tube (I) by adding 5 mL of H₂SO₄ (sulfuric acid):MeOH (methanol) and heating (2% v/v, 70°C, 1 h, vortex mixing every 5 min). We monitored the pH after extraction as carbonate-rich ceramic fabrics might neutralize the acid. If the pH was ⩾3, then more H₂SO₄:MeOH was added.

The H₂SO₄:MeOH solutions containing the extract were transferred to test tubes and centrifuged for 10 min (2500 r/min). The clear solution was transferred to clean culture tubes (II) and 2 mL of nano-purified water was added. Then, 4 mL of hexane was added to the extracted potsherd powder in the culture tube (I) and vortex-mixed to recover any lipids that were not fully extracted by the methanol solution. The hexane portion was transferred to the H₂SO₄:MeOH solution in culture tube (II) and vortex-mixed to extract the lipid. The washing with hexane and vortex mixing in culture tube (II) was repeated twice. Then, the hexane portion was transferred to a clean vial. Following this, 2 mL of hexane were added directly to the H₂SO₄:MeOH solution in culture tube (II) and vortex-mixed to extract remaining lipid residues. The hexane extracts were gathered in a clean vial and evaporated under a gentle nitrogen blow. A procedural blank (no sample) was prepared and analyzed alongside every batch of samples.

Before initiating isotope analysis, we examined lipid samples extracted from vessels with gas chromatography–mass spectrometry (GC-MS) to separate and identify specific organic compounds within the mixture. Such biomarkers are often used to reconstruct the diet of prehistoric people (Evershed, 2008a, 2008b; Heron and Evershed, 1993). The lipids extracted, or fatty acid methyl esters (FAMEs), were analyzed by a 6890N Network GC system with a 5979 mass selective detector from Agilent Technologies (Santa Clara, CA, USA). The GC was equipped with a fused silica capillary column (J&W; DB5-MS; 60 m × 0.32 mm; 0.25 µm film thickness), and the interface was maintained at 110°C. The mass spectrometer was operated in the full-scan mode. Helium was the carrier gas, and the GC oven was programmed as follows: 2-min isothermal process at 50°C is followed by an increase to 350°C at a rate of 10°C/min, and following this, the temperature is held at 350°C for 10 min. The peaks are identified based on their mass spectral characteristics and GC retention times and also by comparison with the NIST mass spectral library.

Compound-specific isotope analysis was performed using a Thermo GC/C-isotope ratio mass spectrometer (IRMS) system composed of a Trace GC ultra gas chromatograph (Thermo Electron Corp., Milan, Italy) coupled to a Delta V advantage IRMS through a GC/C-III interface (Thermo Electron Corp., Bremen, Germany). Compound identification support for the CSIA laboratory was provided by a Varian CP3800 gas chromatograph coupled to a Saturn 2200 ion trap MS/MS (Varian, Inc., Walnut Creek, CA). FAMEs dissolved in hexane were injected in splitless mode and separated on a Varian FactorFour VF-5 ms column (30 m × 0.25 mm ID, 0.25 µm film thickness). Once separated, FAMEs were quantitatively converted to CO₂ in an oxidation reactor at 950°C. Following water removal through a Nafion dryer, CO₂ entered the IRMS. Each sample was measured 10 times. The precision of measured δ¹³C values was determined and corrected using external standards composed of several FAMEs calibrated against NIST standard reference materials (cf. Colonese et al., 2015).

Results and discussion

Crop diversity and distribution

Four sites (Songgukri, Dosamri, Wolgiri, and Majeonri) yielded 29 taxa and several unknown charred plant remains from over 827.2 L of sediments floated (Table 1). A majority of plant remains are seeds or fruits (10,457) along with a small quantity of nutmeat or shells, pine catkins, and tuber fragments. ‘Crops’ here loosely refers to both domesticated species and cultivated or managed ones. Foxtail and broomcorn millets (Panicum miliaceum), rice, wheat (Triticum aestivum), barley (Hordeum vulgare), and bottle gourd (Lagenaria siceraria) are all domesticated taxa, while azuki (Vigna angularis ssp.), soybean (Glycine max ssp.), beefsteak plant (Perilla frutescens), and melon (Cucumis sp.) are in question regarding their domestication status (Kim et al., 2012). Both genetic and morphological studies on legumes indicate size alone cannot be used as a criterion for discerning domesticates from wild counterparts (Lee, 2013; Lee et al., 2011). Often domestication syndrome, particularly morphological changes, such as seed size increase, appeared long after domestication process was initiated (Zeder, 2016). By the middle Bronze period, seed sizes of both taxa increased tremendously, although they look still smaller than modern domesticated varieties (Lee, 2013; Lee et al., 2011). Considering their frequent presence from at least the middle Neolithic period (Lee, 2011a), we regard soybean and azuki were managed species undergoing the domestication process. Seeds of genus Cucumis cannot be further identified to species level, raising the question of whether the remains represent domesticated melon species. The presence of Cucumis within irrigation canals along with bottle gourd may indicate they were a part of managed resource. Beefsteak plant has been used in the diet early on in Northeast Asia, as seen in the Jomon Torihama site (Kasahara, 1984; Matsutani, 1983), and is present as early as the middle Neolithic sites and several Bronze period sites in Korea (Lee, 2011a).

Table 1.

Common and scientific names of macroscopic plant remains in study.

Category	Family	Common names	Scientific names
Crops
Crops	Poaceae	Foxtail millet	Setaria italica ssp. italica
		Broomcorn millet	Panicum miliaceum
		Rice	Oryza sativa
		Barley	Hordeum vulgare
		Wheat	Triticum aestivum
	Fabaceae	Azuki	Vitis angularis ssp.
	Fabaceae	Soybean	Glycine max ssp.
	Cucurbitaceae	Bottle gourd	Lagenaria siceraria
	Cucurbitaceae	Cucumber	Cucumis sp.
	Lamiaceae	Beefsteak plant	Perilla frutescens
Non-crops
Wetland species	Cyperaceae	Sedge	Cyperus sp.
Wetland species	Cyperaceae	Club rush	Scirpus sp.
Weedy grass	Poaceae	Barnyard grass	Echnichloa crus-galli
		Foxtail grass	Setaria sp.
		Panic grass	Panicum sp.
		Millet tribe	Paniceae
		Wheat tribe	Triticaceae
Other upland herbaceous species	Brassicaceae	Mustard	Brassica sp.
	Chenopodiaceae	Chenopod	Chenopodium sp.
	Polygonaceae	Knotweed	Polygonum lapathifolium
			Polygonum thunbergii
			Polygonum sp.
		Sorrel	Rumex sp.
	Oxalidaceae	Wood sorrel	Oxalis sp.
	Rosaceae	Bramble	Rubus sp.
	Vitaceae	Grape	Vitis sp.
	Solanaceae	Ground cherry	Physalis sp.
Nut-drupe bearing trees	Fagaceae	Oak	Quercus sp.
	Pinaceae	Pine	Pinus sp.
	Styracaceae	Japanese snowbell	Styrax japonicus

We further investigate whether rice farming was used to provide staple food to Songgukri settlers as a primary agricultural investment, based on both distribution patterns and quantities of plant remains. If rice farming was a dominant political economy, we expect prevalence of rice in all three sites examined.

Species diversity does not necessarily correlate to soil volumes analyzed but to preservation conditions and features (Figure 3). For example, samples from canals at Majeonri show a higher diversity of species (3–15 per sample), compared to non-waterlogged, open sites (1–5 per sample). Crops also show uneven distribution among the sites examined. Again, the difference in preservation condition explains the presence of bottle gourd and Cucumis seeds only in waterlogged canals at Majeonri. As bottle gourd was probably used for containers and Cucumis can be consumed raw, these two taxa were usually not cooked over fire and thus had less likelihood of being charred and preserved in open sites. Among sites with similar preservation conditions, Dosamri and Wolgiri sites show a similar crop composition with lack of broomcorn millet and wheat, while barley was not found at Songgukri (Figure 4, Supplementary Table 2, available online).

Figure 3.

Species diversity in each site.

Figure 4.

Percentages of crops per total number of seeds, including millet (foxtail and broomcorn), barley and wheat, rice, bean (soybean and azuki), and other taxa (bottle gourd, Cucumis sp., beefsteak plant).

Such a difference may result not from a real difference in crop choice but merely from research bias. Although a huge number of seeds were found at Majeonri, soil volumes were not recorded, and Majeonri was excluded from the comparison of seed densities (seed counts per unit volume of soil, 1 L). More features and larger flotation samples were examined at Songgukri (466.2 L) than the Dosamri (294 L) and Wolgiri sites (195 L). Moreover, seed diversity and density fluctuate widely within Songgukri features (Figure 5). For example, house 14-52 and pit 14-2 contributed 98% of all seeds found at Songgukri and have the two highest seed densities, 48.7 and 497.9 each (Supplementary Table 2, available online). Without these two outliers, the overall seed density at Songgukri (0.4) is similar to Dosamri (0.2) and Wolgiri (0.3). That is, we may have simply missed features with abundant remains of diverse taxa from the Dosamri and Wolgiri sites. As all the sites examined are in close proximity and contemporaneous (Figures 1 and 2, Supplementary Table 1, available online), we tentatively conclude that all these 10 taxa of crops were farmed and used in the Songgukri cultural settlements. Overall, our study confirms that multi-crop farming of both dry and wet fields was well established in the Songgukri culture.

Figure 5.

Seed density comparison (seed number per 1 L of soil sample). Each point represents a total seed density in each feature. Three outliers excluded in this graph are samples with the highest densities, including house 11-21 (density, 7.24), house 14-52 (48.78), and pit 14-2 (497.94) all at Songgukri (the 14th field season).

A remaining question is whether rice played a key role in diet and agriculture. Seed counts themselves cannot be a direct proxy for crop preference due to different preservation rates among crops and post-depositional disturbance (Lee, 2012a). With this in mind, we compare both ubiquity and relative quantity of each taxon. At Songgukri, rice and other crops appeared in a similar number of features, 12 and 13, respectively (Supplementary Table 2, available online). At the other three sites, rice and other crops were also found in an equal number of features. In brief, rice is not more prevalent than other crop taxa in all sites examined.

Rice is present in all four sites examined, but its relative abundance varies greatly across the sites, ranging from 0.2% of the total number of seeds at Majeonri to 33.3% at Dosamri (Figure 4). At Songgukri, millets are the most dominant crops overall by far, but feature types show a difference in a crop variety and quantity. Millets are dominant over any other crops in pits outside houses (inner pie chart in Figure 4), while rice is dominant in houses (middle pie chart in Figure 4). Abundance of rice in houses is, however, skewed by remains from one house (14-52). Three vessels found in this house yielded 867 rice grains, 95% of all rice grains found in all houses (Supplementary Table 2, available online). Without this sample, millets and rice are found in house contexts at more or less the same rate (outer pie chart in Figure 4). This pattern indicates that rice was not equally distributed throughout the households. Kim (2015a) argues that rice in the Mumun period was more of a status symbol rather than a community staple. Crop distribution patterns in Songgukri houses demonstrate that rice was stored in a selective household for reasons other than regular consumption, while millets are at least equally important in the general population.

Crop composition patterns in pits also support this hypothesis. Pits can represent either a primary (granary) or a secondary deposit (waste discharge). If pits represent the primary function of granaries, we can expect a dominance of crops. However, if pits represent secondary deposits, we can expect a high quantity of waste materials such as weedy grains, crop by-products (e.g. stem, husks, and immature grains) from threshing or cooking waste. It is not clear whether the absence of such waste in pits results from research bias (a coarser sieve used for flotation) or rather indicates the pit served as a granary. In any case, the abundance of millets in pits can be interpreted as evidence for millets as a main staple. An overall higher crop diversity in houses and an usually high abundance of rice in one household represent both the daily activity of diverse crop consumption and a special occasion using rice as a symbolic food. In brief, rice was neither equally distributed across sites nor households within a settlement. This uneven distribution pattern is similar to rice distribution in the Neolithic and Erlitou cultural sites along the Yiluo River in north China (Lee et al., 2007). Rice in the Yiluo region was regarded as a ritual, occasional crop, compared to millets as staple.

Rice is the most dominant taxa at Wolgiri, but the total number of grains is too low (13) to make any meaningful interpretation on abundance of rice. Unlike other settlements, Dosamri shows a relative abundance of barley and beans. The lack of rice remains at Majeonri, where rice paddy fields were found, was related to the provenience where sampling was done (irrigation canals) rather than an actual absence of rice at the site. If rice were the dominant staple, it should be more abundant and prevalent than other crops, regardless of feature type. Based on our observations, we cannot conclude rice was more a dominant staple than others.

Crop and weed ecology

Not only crops but also wild plants can be a useful source of data to understand plant use patterns in prehistory. Particularly, weedy plants can reflect the degree of anthropogenic disturbance, scheduling of farming activities, and agricultural technologies (e.g. Boggard, 2004; Crawford, 2011; Lee, 2011b; Smith, 2011). Weeds adapt well in disturbed landscapes and become camp followers (Anderson, 1952) and arable taxa in farming fields. As Harlan (1992) succinctly puts, weeds tell something about where people have been and what they have done. Most weedy remains in this study represent millet tribe (Paniceae) and other arable species common in upland cultivation, including herbaceous annuals and bush (Table 1).

At Songgukri, weedy grass comprises almost 80% of all non-crop remains, while both weedy grass and other upland species almost equally constitute all non-crop remains at Wolgiri (Figure 6). We expect that difference in preservation conditions between canals and other features is a prime factor that causes a difference in variety and types of plant remains among them. As canals are not features that represent either consumption or disposal of food, a lack of crops is not a surprise. Instead, abundance of bottle gourd seeds and rinds indicates that bottle gourd was used as containers in canals and/or that gourd floated in canals and were broken. Also, a high quantity of bulrush and sedge in canals, which are absent in other features, reflects local wetland vegetation as well as weeds invading rice paddy fields. Even at the Majeonri site where rice paddy fields were found, however, upland herbaceous species are dominant over wetland weedy species. Our study reflects anthropogenic ecology established in dry farming fields where weedy annuals and other upland species spread and flourished.

Figure 6.

Percentages of non-crop categories per total number of non-crop seeds, including weedy grass, other upland herbaceous species, and wetland species. Non-crop categories are listed in Table 1.

Another way to check the importance of crops in subsistence is to examine architectural investment for farming. A reservoir and irrigation canals at Majeonri reflect a considerable investment in infrastructure for rice paddy farming (Research Institute for Archaeological Resource of Korea University, 2004; Supplementary Figure 1, available online). By the same token, dry field farming also requires substantial efforts in construction and maintenance as seen in the middle Bronze period sites along the Nam River (Yun et al., 2016). Considering their contemporaneity, cultural similarity, and the same list of crops and weeds found among the sites along the Nam and Geum rivers (Lee, 2011a, 2012b), we can anticipate the importance of dry field agriculture in Songgukri culture. In conclusion, our archaeobotanical studies indicate that both rice and dry field crops were a part of diet, contrary to rice being singled out as a staple. Not rice farming alone but multiple cropping of both wet and dry fields were a backbone of the economy at Songgukri cultural settlements.

Isotope implication

Among 27 samples prepared, 18 samples (67%) contained lipid residues with concentration up to 59,871 ng/g (a mean of 33,450 ng/g for the sherds containing lipids; Table 2). In the GC-MS results, each of the peaks in the chromatogram with a different retention time (along the horizontal axis) indicates a different compound (cf. Figure 7). The lipid distributions of all samples were dominated by palmitic (C16:0) and stearic (C18:0) fatty acids. This composition implies that most of the organic compounds from the original contents of the vessels have been severely degraded during the several thousand years of post-depositional process (Evershed, 2008b; Figure 7). Degraded fats similar to these have been identified in archaeological ceramics in numerous contexts. Besides C16:0 and C18:0 fatty acids, the results of GC-MS analysis helped us identify both major odd-numbered and even-numbered saturated fatty acids, including C13:0, C14:0, C15:0, C16:0, C17:0, C18:0, and C20:0. Generally, a high C18:0 saturated alkanoic acid content indicates an animal source (Copley et al., 2005; Enser, 1991; Evershed et al., 2002: 664). Most of the lipid residues analyzed in this study showed a high concentration of C18:0 fatty acid (Figure 7a; Supplementary Figure 2, available online). In addition to this pattern, some of the lipid samples displayed higher abundance of odd-numbered saturated fatty acids such as C15:0, C17:0, and C19:0 than others (Figure 7a). According to Evershed et al. (2002), these odd carbon number fatty acids are more abundant in ruminant animals than other monogastric organisms. The blank sample did not produce any specific compounds except for internal standard (IS), 5-α cholestane (Figure 7b).

Table 2.

The concentrations of lipids in each pottery sample analyzed in this study.

Sample ID	Concentration (ng/g)	Sample ID	Concentration (ng/g)
SON001	59,871.38189	SON016	22,493.5607
SON002	45,940.07202	SON017	43,597.94157
SON003	57,875.47586	SON018	32,389.18091
SON004	30,984.09865	SON020	23,822.26938
SON005	43,799.94295	SON022	21,294.71319
SON006	24,328.13837	SON024	34,873.4968
SON012	30,339.12245	SON025	26,917.57207
SON013	29,814.2609	SON026	25,711.27365
SON014	27,026.5773	SON027	21,016.96263

Figure 7.

The result chromatograms of the GC-MS analysis of our samples: (a) sample SON024 and (b) blank sample. 5-α Cholestane was added as an internal standard (1232 ng).

Proximity to the Geum River, 7 km southwest of Songgukri, may have provided easier access to aquatic resources to Songgukri dwellers. The presence of aquatic biomarkers, including phytanic acid (3,7,11,15-tetramethylhexadecanoic acid), 4,8,12-trimethyltridecanoic acid (4,8,12-TMTD), and thermally produced long-chain (ω-o-alkylphenyl) alkanoic acids, may reflect a reliance on riverine resource (cf. Craig et al., 2011; Evershed et al., 2008). Except for phytanic acid from one sample (SON024), no other aquatic biomarker was detected (Figure 7a). Since phytanic acid is also found in the tissues of ruminant animals, we conclude phytanic acid alone from one sample is not a sufficient indicator of aquatic resource on its own (Heron and Craig, 2015). Despite the lack of fish bone remains and a rarity of aquatic biomarkers, we cannot exclude a possibility of fish as a part of diet in Songgukri culture, as it could have been cooked in a direct fire, leaving no mark on pottery.

Further isotopic interpretation was made by applying the method from Salque et al. (2013). The Δ¹³C (δ¹³C18:0 − δ¹³C16:0) proxy was used to remove the exogenous factors related to the local environment. As a result, the metabolic and biosynthetic characteristics of the animal fat source can be more attainable (Copley et al., 2003; Dunne et al., 2012), making distinction between non-ruminant fats, ruminant fats, and ruminant milk fats easier. Among 18 samples with enough residues, 8 indicated that they were used for cooking ruminants, clearly showing the consumption of terrestrial animals on a regular basis (Figure 8). A total of 10 samples showed the presence of non-ruminant fats, but none of the Songgukri samples were used for processing milk fats.

Figure 8.

(a) The results of compound-specific isotope analysis of the samples from the Songgukri site using the approach of Salque et al. (2013). (b) The Δ13C values suggest a C₃ bias across majority of the samples, indicating main foodstuff of terrestrial herbivores were indigenous wild C₃ plants (Ahn, 2006; Choy and Richards, 2010; Kim, 2010; Lee, 2011c).

The dominance of two terrestrial animals, sika deer (Cervus nippon) and wild boar (Sus scrofa), in faunal data indicates that they were the main protein source since the Neolithic period (Choy et al., 2012; Lee, 2011c, 2011d; Shin et al., 2013). Considering these data, most of the ruminant adipose fats presented in our samples were probably from hunted sika deer. The reference ellipse of porcine adipose fat by Craig et al. (2011) was employed to check whether non-ruminant fats in our samples came from wild boar. Indeed, several samples seem to reflect wild boar (Figure 8a). Since Craig et al.’s (2011) reference ellipse was created based on the modern wild animals collected from Europe, however, we cannot be free from the exogenous factors related to the local environment. It is open to question whether the δ¹³C values of modern European references are comparable with those of the archaeological samples from Korea. In Europe, wild C₄ plants are rare (Tafuri et al., 2009), and thus wild herbivores mainly consume C₃ plants in general. Similarly, indigenous wild plants in Korea are C₃ plants (cf. Figure 8b). Earlier isotopic research on faunal remains from the Neolithic and Bronze periods revealed that wild animals such as boars mainly consumed C₃ plants (Ahn, 2006; Choy and Richards, 2010; Kim, 2010; Lee, 2011d). Even if we count the overall similarities in C₃ and C₄ plant proportions between Europe and Korea, it is a stretch to generalize that local environments and ecosystems between the two are comparable. For this reason, identification of wild boar fat in our samples remains conjectural.

Finally, our result cannot exclude the possibility that harvested corps were processed or cooked in a pot. Animal (and fish) fats are more concentrated than plant oil in general, and the former often mask the signal of the latter if they are cooked in a same pot. Therefore, we are not removing the possibility that some (or most) of the pots analyzed in this study were used for cooking both animals and grains.

Conclusion

One of the most debated issues on the Korean Bronze period is the role of intensive rice agriculture in emerging social complexity. Often rice agriculture was viewed as a ‘cure-all remedy’, which substituted for other subsistence resources (Lee, 2011b). In addition to that claim, the emergence of social hierarchy and complexity was considered to be driven by the rapid spread of the intensive rice agriculture during the middle Bronze period (Kim, 2006a, 2006b). Earlier, casual findings of rice grains at Songgukri created an impression of rice-based diet during the middle Bronze period, although a few archaeobotanical analyses cast a doubt on the claim (Lee, 2011b; Kim et al., 2012). Poor preservation of faunal remains in most of the Bronze period sites, due to highly acidic soils in Korea, causes a lack of discussion on animal resource procurement.

As a result, a dramatic departure from the Neolithic hunting-gathering economy was further emphasized in discourse on the Bronze economy. Our study offers a way to overcome poor preservation and investigate the relative contribution of various resources to the prehistoric diet. We test a popular belief by integrating evidence from isotope and archaeobotanical analyses. Our research strength lies in capturing daily-level and long-term practice of food procurement, repeated practice of food preparation/consumption, and generational farming activities.

An emerging picture from our work seems to be more nuanced than a portrayal of the Songgukri economy as a rice-focused one in central Korea. The middle Bronze settlements document multiple cropping based on a variety of upland crops, including foxtail and broomcorn millets, wheat, barley, bottle gourd, azuki, soybean, beefsteak plant, and melon species as well as rice. Moreover, consumption of wild terrestrial mammals, particularly deer and possibly wild boar, persisted throughout the middle Bronze period, well after farming was introduced possibly as early as the early Neolithic period by ca. 7500 cal. BP (Aikens and Lee, 2013; Cho et al., 2014).

Irrigation canals and reservoir at Majeonri reflect considerable investment in wet rice cultivation. Maintenance of dry fields for multiple cropping can be as costly as wet field farming, as seen in the Pyeonggeodong site along the Nam River (Supplementary Figure 1, available online; Yun et al., 2016). Although upland farming fields have not been unearthed along the Geum River, the same composition of dry crops and arable weeds between the Geum and Nam River sites indicates dry farming was practiced along the Geum River. We suggest settlers in the Geum River valley carried out a flexible subsistence strategy, combining wet and dry field farming and wild resource procurement to a greater degree than suggested by previous studies. The integrated approach in our study can be applicable to other case studies in Korea and elsewhere suffering from poor conditions of organic preservation.

Footnotes

Acknowledgements

We appreciate all the field staff members at the Korean National University of Cultural Heritage who assisted our data collection. A special thanks to Dr Richard Evershed, Dr Julie Dunne, and Dr Marisol Correa-Ascencio at the University of Bristol for sharing their knowledge on the lipid extraction and the isotope analysis. Rory Walsh and Hyunsoo Lee at the University of Oregon assisted with archaeobotanical analysis of some of the Songgukri samples. Walsh also read an earlier version of this paper. Habeom Kim (University of Oregon) prepared Figures 2 and .

Funding

This work was supported by the Laboratory Program for Korean Studies through the Korean Studies Promotion Service/Academy of Korean Studies, Ministry of Education of Korea (AKS-2015-Lab-2250001).

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