Key question:
- How did local climate and environment promote rice agriculture in Holocene?
- Did fire play an important role in early rice agriculture?
The location of the Kuahuqiao site
The unclear environment where rice agriculture started
It is still unclear why early rice agriculture started in some unique regions about 7,000~8,000 years ago, despite its importance regarding that it profoundly influenced both the natural environment and human history. We chose the Kuahuqiao site in eastern China, which is famous for its early rice cultivation and taken to be a typical case of how agriculture emerged in a coastal environment in the early Holocene.
Geochemical records to reconstruct the environment
We report new records of total organic carbon (TOC), black carbon (BC), 𝛿13C(TOC), 𝛿13C(BC), and C/N, alongside published pollen and charcoal data. Our core is 1.5 km from the Kuahuqiao site, at an optimal distance where the core can record both human activities and environmental changesAMS14C dating with calibration from 9 plant fragments provided a precise chronological framework.
New-formed wetlands as cradle of paddy
Plotted 𝛿13C(TOC) and C/N in 25.80~8.40 m (9,200~7,950 aBP) show a transition of sedimentary organic resources from terrestrial plants to algae, confirming the wetland formed due to sea-level rising prior to the Kuahuqiao culture (7,950~7,600 aBP), which offered a favorable landscape for rice paddy.
Hotter but drier climate promoted wild rice
Meanwhile, the positive trends of 𝛿13C(TOC) and 𝛿13C(BC) attribute to improved water-use efficiency of plants driven by the hotter but drier climate, which is confirmed by increased BC representative for more aridity-caused fires. This climatic condition promoted the C4 herbs, as proven by pollen records, and thus left more open land for wild rice.
Human occupation: change in vegetation, no fire, and flood
Human occupation impressed 𝛿13C(TOC) record by a sharp positive trend i.e. decreased C3/C4 ratio by manipulating forest for hunting-gathering or/and by clearing forests for agriculture. As culture developing, rice agriculture advanced but fire never played a vital role in agriculture since our BC and 𝛿13C(BC) remain stable. The increase in charcoal from the site profile shows the fire was limited to domestic use. Hence, we suggest the origin of rice-agriculture in eastern China different from the ‘burning and slashing’ due to the absence of fire use.
Dramatic peaks of TOC, BC, and plummets of 𝛿13CTOC, 𝛿13CBC resulted from the catastrophic flood events, which ruined Kuahuqiao culture, revealing a fragile balance between fertility of estuarine wetland and its vulnerability to hydrological disasters.
In all...
Our study shows in eastern China, wetlands alongside hotter and drier climate favored the origin of the earliest rice agriculture in coastal environment. Human changed the vegetation around their settlements and fire was not involved in rice agriculture management.