Holocene landscape evolution in northern Henan Province and its implications for archaeological surveys

The heartlands of many of the world's civilizations are situated within alluvial plains, where thick alluvial sediments obscure much of the archaeological record. However, the use of alluvial geoarchaeology remains patchy, particularly in the world's largest alluvial basins. We present results from our geoarchaeological survey at Neihuang County, Henan Province, China, as an example for alluvial geoarchaeological research in the North China Plain and to develop a generalized framework for landscape evolution in the area during the Holocene. We reconstruct the alluvial history of the area around Neihuang County by synthesizing stratigraphic data from seven outcrops into distinct depositional units. Our findings suggest that much of the archaeological record in the North China Plain is buried by meters of sediment or eroded away by the ancient channels of the Yellow River and other tributary streams. Therefore, the presence of buried archaeological sites and river scour in recorded outcrops suggests that the nonsystematic archaeological surveys that are commonly used to interpret cultural changes are not accurate reflections of archaeological site distributions. From the results of this case study, we recommend that archaeologists and paleoclimatologists should exercise more caution when using settlement distribution data gathered through nonsystematic pedestrian surveys to make inferences about ancient processes of cultural change or social dynamics in the North China Plain.


| INTRODUCTION
The heartlands of many of the world's civilizations are situated within alluvial plains, where thick deposits of alluvial sediment bury large portions of the archaeological record (Brown, 1997;Macklin & Lewin, 2015;Stafford & Creasman, 2002;Stafford, 1995). Geoarchaeologists have suggested that water-lain sediments and interbedded paleosols in alluvial plains are some of the most geographically continuous and well-preserved records of ancient landscapes, and yet, archaeological site distributions are frequently displayed on twodimensional maps that avoid the multidimensional complexities of site formation processes in alluvial plains (Jotheri et al., 2016). Since archaeological data are collected around the world using a wide variety of methods, it is essential to critically examine and discuss the archaeological record of alluvial plains on a case-by-case basis to provide a degree of cross-regional comparability.
The North China Plain is one of the largest alluvial plains in the world and has an exceptionally long history of human modification, making it an ideal case study for exploring the relationship between landscape evolution and archaeological site distribution. The North China Plain contains a complex mixture of archaeological deposits interbedded with alluvium that can be systematically reconstructed using an alluvial geoarchaeological approach, an approach that bridges the geosciences and archaeology to accurately reconstruct the interactions between climatic forces, society, and the changing fluvial environment. The vast majority of the North China Plain's alluvium comes from the Yellow River, which flows through the easily eroded Loess Plateau, where it picks up massive quantities of sediment; once the Yellow River enters the North China Plain, the carrying capacity of the river suddenly drops, leading to rapid deposition within the channel bed (C. Liu & Liang, 1989). Before widespread dam construction, the Yellow River's channel was wide and choked with sediments, leading to the formation of multiple branching and reuniting channels that were present nearly as far north as Beijing and as far south as Shanghai (Kidder, Liu, Xu, et al., 2012). As a result, the fluvial processes of the Yellow River buried, scoured, and otherwise obscured significant portions of the North China Plain's archaeological record.

Archaeologists have long acknowledged that the North China
Plain is a complex alluvial landscape that has likely obscured much of the archaeological record in this area. Nevertheless, recently, data from the Atlas of Chinese Cultural Relics, a nationally standardized data set of archaeological sites categorized by time period and site type, have been frequently used to interpret changes in archaeological cultures based on alternating settlement distributions in the North China Plain (Hosner et al., 2016b;Wagner et al., 2013). These settlement distribution patterns observed in the Atlas data are biased in large part because the data were collected through nonsystematic pedestrian surveys and without consideration of long-term depositional processes, particularly in the North China Plain, making them largely unsuitable for these kinds of analyses Jaffe & Hein, 2021). Therefore, our team of geoarchaeologists has organized field work in Neihuang County, Henan Province, China, to explore how landscape evolution in this area intersects with the data seen in the Atlas of Chinese Cultural Relics for the North China Plain ( Figure 1). We have opportunistically targeted on-going archaeological excavations and other available outcrops for detailed sedimentary and geological descriptive analyses. We present our findings from seven outcrops that span the time frame from the early Holocene to the recent historic period. Using geological, archaeological, and chronological data, we reconstruct a schematic chronology for landscape evolution and archaeological features in the region. This case study aims to provide insight into the discrepancy between the real paleo-landscape and site distribution revealed by the Atlas of Chinese Cultural Relics.
F I G U R E 1 Archaeological sites in North China from the Atlas of Chinese Cultural Relics. Neihuang County, Henan Province, is in the yellow outline in the center of the North China Plain. Note the "gaps" in the ancient Yellow River flood area.
Beyond the North China Plain, our study highlights the dynamic nature of rivers and demonstrates that extensive pedestrian survey is no substitute for integrating geoarchaeological knowledge into archaeological research designs in alluvial environments.

| REGIONAL BACKGROUND
The North China Plain is a Cenozoic fault-depression area on the North China Craton. In the late Tertiary and early Quaternary, the fault block mountains on the plain's margin were elevated, forming the outline of the great plain. During the Pleistocene, the Yellow River gradually evolved into a thoroughly connected river from its origin to estuary (Ye et al., 1985). The river went through the Loess Plateau in the middle reaches, transported the easily eroded loess, and deposited alluvium onto the lower reaches, eventually shaping the major body of the present North China Plain.
From the early to mid-Holocene, landscape conditions were relatively stable in the North China Plain (Jing et al., 1995). Climate conditions played a major role in this lengthy era of stability. Climatic proxy data, including pollen, paleosols, and aeolian sand activity, show that the East Asian monsoon was strongest during the middle Holocene, between 8600 and 4200 B.P. (Z. Liu et al., 2014). The Yellow River floodplain was actively and steadily aggrading because of the warm and rainy conditions. This geomorphologically stable period ended with increasingly frequent catastrophic overbank flooding of the Yellow River, roughly contemporaneous with the 4200 B.P. climatic event (Huang et al., 2010(Huang et al., , 2011. Following the 4200 B.P. event, climatic conditions became colder and drier, and human activity such as settlement expansion and agricultural intensification aggravated erosion from the Loess Plateau in the middle reaches and in turn increased the flood intensity and frequency in the lower reaches. The Yellow River is estimated to have flooded over 1000 times and changed its courses over 26 times in the last 2500 years (Y. Chen, 2019;Mostern, 2021). Neihuang County is located at the lower reaches of the Yellow River (Kidder & Liu, 2014;Kidder, Liu, Xu, et al., 2012;. This area is the Yellow River's nodal avulsion zone (Y. Chen et al., 2012;Shi et al., 2010). The fluvial hydrology of the Yellow River makes Neihuang County an epicenter for channel switching events and large avulsions throughout most of the Holocene (W. Chen et al., 1996;Slingerland & Smith, 2004). As a result, the subsurface stratigraphy of Neihuang County is a complex combination of paleo land surfaces overlain by flood deposits and bisected by channel scouring that has often led archaeologists to consider the area void of archaeological remains. Nevertheless, near the gap are large settlement sites, such as Xishuipo, Qicheng, Gaocheng, and Sanyangzhuang, suggesting that this gap in archaeological sites may be more apparent than real (Archaeology & Protection, 2004;Relics et al., 1988). The discovery of these sites revealed that many important archaeological findings may be deeply buried in this sedimentary depression.

| Field survey and lithostratigraphy
From 2011 to 2015, we investigated seven outcrops to identify the continuity and variability of subsurface deposits in Neihuang County (Figures 2 and 3). Specifically, we targeted locations that are near, or on top of, the crevasse splay seen in Figure 2, as we have hypothesized that this area is related to the Han dynasty floods of 15 C.E. . At this location, we opportunistically profiled on-going construction areas and archaeological excavations. The early Holocene deposits are located approximately 10 m beneath the ground surface, making traditional archaeological excavation extremely difficult. Lastly, when possible, we used a Luoyang spade (a hand-powered auger) to collect data on the subsurface stratigraphy. For all cores and profiles, we recorded the Munsell color, texture, and soil horizon to tentatively link together major depositional units with one another (Birkeland, 1999;Holliday, 2004).

| Radiocarbon dating
Refining the chronology of sedimentation is critical for understanding the development and continuity of ancient landscapes in Neihuang County. We also collected radiometric dates from paleosols and archaeological deposits. Our sampling strategy is largely opportunistic, searching for charcoal within profiles, and when charcoal is absent, we collect clays for soil organic matter (SOM) dating. Charcoal, shell, and soil organic matter were dated at Beta Analytic, the National Marine Science Accelerator Mass Spectrometry Laboratory (NOSAMS) of the United States, the Center for Applied Isotope Studies (CAIS) of the University of Georgia. The calibration was performed using the IntCal20 curve in Oxcal 4.4 Calibration Software (Ramsey, 2013;Reimer et al., 2020). All ages are reported as both "cal. B.P." and "cal. B.C.E./ C.E." The results are listed in Table 1.

| Lithostratigraphy
Although the broad strokes of the lithostratigraphy of the North China Plain are clearly defined, the temporal and stratigraphic resolutions of these geologically oriented studies are often too coarse for archaeological application (Jing et al., 1997;Wu et al., 1996). Therefore, we have compiled these data and generated a schematic diagram of the subsurface stratigraphy of parts of Neihuang County. We searched for and correlated lithologically and chronologically similar units at each site as well as spatially extensive anthropogenic units. Our approach helps generate a schematic understanding of the archaeological deposits in this deeply buried alluvial environment.   Kidder, Liu, Xu, et al., 2012;Storozum et al., 2017; and synthesize the stratigraphic data into six units ( Figure 5). Data for the schematic illustration of each outcrop can be found in Supporting Information:

| Unit V
Unit V dates from the end of early Holocene to middle Holocene.
The unit is a series of lacustrine deposits that are gleyed, organically enriched, and varved. In most areas, these deposits are approximately 10 m beneath the modern ground surface and vary in thickness and composition. Numerous shells are found in these deposits. We interpret this unit as several lakes or backwater swamps located along or near the ancestral course of the Yellow River that formed during the wet and warm Holocene Climatic Optimum (An et al., 2000;Cai et al., 2010;F. Chen et al., 2015; J. Zhang et al., 2020).
During the middle Holocene, these lacustrine deposits are buried by a thick fluvial deposit. We believe that the overlying flood deposit

| Unit IV
An increase in the frequency of flooding buried the stable land surfaces of Unit V. Although there is a great deal of variation within each outcrop investigated, there appears to be a comparatively brief period of landscape stability around 3000 B.P. at Anshang and Sanyangzhuang. Within Unit IV, we have found archaeological evidence of canalization, mortuary contexts, and agricultural fields (Kidder & Liu, 2014;Kidder, Liu, Xu, et al., 2012;Storozum et al., 2017

| Unit II
There is significant archaeological evidence of a Tang  F I G U R E 4 (a) Han dynasty ridge and furrow field system at Sanyangzhuang (SYZ), roughly 5m below modern land surface and (b) canal system at Anshang (ANS) dating to ca. 3000 cal. B.P.
F I G U R E 5 Unit model created by graphically displaying the stratigraphic data from each outcrop investigated in Neihuang County. The first column represents the unit and the second column represents the lithology, flood, or paleosol. The approximate location of radiocarbon dates is also included in this diagram; more detailed information can be found in Supporting Information: Table 1, which includes radiocarbon date depth as well as stratigraphic information for each outcrop. archaeological site distributions. Here, we discuss our general model for landscape evolution in Neihuang County, followed by an exploration of the usefulness of the archaeological site distribution data in the Atlas. We argue that while individual archaeological sites point toward general geomorphic processes operating at a basinwide scale, in deeply buried alluvial areas, a regional stratigraphic approach is required to isolate and describe synchronous events (Macklin & Lewin, 2008).

| Holocene landscape evolution in northern Henan Province
After five years of survey work, our geoarchaeological research has found substantial archaeological deposits at Neihuang County that   Liu, 1996;L. Liu & Chen, 2001). L. Liu (1996, p. 249) makes the case that much of the area around the lower course of the Yellow River is covered with swampy deposits, making human habitation and exploitation of these environments unlikely.
However, data from our survey and elsewhere suggest that humans, although they may not have extensively settled in this landscape, certainly used these diverse ecological conditions to their advantage.
At Puyang, archaeological evidence of the contemporaneous Yangshao site of Xishuipo reveals that these sites are located close to the modern-day land surface (Nan, 2012;Relics et al., 1988). Sites like Xishuipo were located tops of hills; therefore, archaeological interpretations of human behavior are frequently limited to people settling in these upland areas. It is likely that these swamp-like deposits were used by the nearby Yangshao and Longshan villages. In particular, the course of the Yellow River appears to be stable following the 4200 B.P. event, but because the Yellow River's main channel before 4000 B.P. has predominantly been determined from historical records, it is difficult to be certain of the location of the river's main course during this time frame.
Scholars have suggested that the Yellow River switched courses to the south around 4000-2000 B.P., meeting with the Yellow Sea (Wang, 1993(Wang, , 1999. However, our evidence, along with evidence from other studies, suggests that the Yellow River was never located in the south from 4000 to 2000 B.P. (Rapp & Jing, 2011).
We argue that the amount of sediment accumulated throughout found evidence of a multilayered brick wall, several refuse pits, and a buried brick building that date to the end of the Tang dynasty . At Dazhanglongcun, radiocarbon dates from archaeological deposits indicate that the occupation of the town and the flooding event are nearly contemporaneous, revealing that people living at that site most likely abandoned the village quickly.
The stratigraphic sequence at Xidacheng reveals a stratigraphic sequence of rapid sedimentation rates as well as a quick turnover between incipient paleosols and alluvium. At all these sites, the amount of sediment above these recent deposits ranges from 2-3 m.
In some areas, the amount of sedimentation exceeds 4 m. In aggregate, geomorphologists have estimated that Yellow River alluvium in the North China Plain that dates to the Song period is on the order of 6.63 million tons of sediment per year from 1038 to 1127 C.E., representing a substantial increase in the sediment load of the Yellow River, a transformation also seen in the sediment cores taken from the submerged delta lobes of the Yellow River (Milliman et al., 1987;Ren & Zhu, 1994 (Jing et al., 1995(Jing et al., , 1997Rapp & Jing, 2011;Rosen, 2007Rosen, , 2008Rosen et al., 2015).

| An alluvial geoarchaeological perspective on the Atlas of Chinese Cultural Relics
In alluvial environments, hydro-geomorphic processes deeply bury or erode most signs of human activity that occur in depressions, preferentially leaving archaeological sites that are elevated above the floodplain relatively undisturbed. Over thousands of years, the floodplain of the Yellow River gradually filled in with alluvium and connected once topographically elevated areas with the now filled-in sedimentary depression. It is difficult to assess the "original" state of the landscape because the majority of the archaeological record is found in ancient areas of topographic elevation, biasing the data collection of the pedestrian survey-based Atlas of Chinese Cultural Relics toward surficial sites that result in "gaps" on maps with site locations.
As can be seen in Figure 7, the Yellow River's paleochannels and ancestral courses are correlated with these blank spots in the Atlas' data. In the highly elevated areas of Shandong and on the edges of the North China Plain, there is a higher prevalence of archaeological sites included in the Atlas data set. This is in no small part due to the geomorphic processes of these areas; the archaeological sites are less deeply buried and easier to identify in these more mountainous areas.  (Wu et al. n, 1996)  Finally, our study has highlighted the importance of integrating knowledge gained from alluvial geoarchaeological methods into archaeological interpretations of site distributions. Although fluvial processes are specific to each river, surface surveys should be coupled with profiling to determine the extent to which buried landscapes, and river scour, are present in the sub-surface record.
Archaeological surveys ranging from the Mississippi to the Mekong River basins that integrate these methods into their research design will benefit from a greater understanding not only of the geomorphic processes influencing site distributions but also the history of the regional landscape evolution.