Dating the Hemudu Neolithic rice cultivation site, East China, by paleomagnetic chronostratigraphy
Abstract Hemudu is a famous archaeological site with the earliest rice remains unearthed in East China and represents a classic example of how Neolithic people managed coastal farmland. However, appreciation of human adaptation to Holocene sea-level changes has been hampered by low-resolution chronology due to lack of datable materials in coastal sediments. Correlation of paleomagnetic directions and intensity on a regional scale provides independent relative dating of such sediments. We did high-resolution paleomagnetic chronology work on core HMD13 (27 m in length) from Hemudu Neolithic site and discussed relevant early Holocene paleoclimate change of Southern China. Magnetic minerals identified by scanning electron microscopy are magnetite, iron-bearing silicates, and iron sulfide (pyrite or greigite). Rock magnetic measurements indicate that the primary remanence carrier is single-domain magnetite. Stepwise alternating field (AF) demagnetization experiments at 1 cm intervals on U-channel samples show that paleomagnetic inclinations fluctuate around 60°, with five major peaks and four troughs. A relative paleointensity (RPI) was estimated using normalizing natural remanent magnetization intensity to anhysteretic remanent magnetization over 20–40 mT AF range, which has four major peaks. Variations of inclination and RPI in core HMD13 resemble the data from the East China Sea and allowed the development of a refined age-depth model. The new adjusted age-depth model minimized dating errors of 14C ages among different datable materials. From the new age model, inclination and RPI of HMD13 are comparable to paleosecular variation (PSV) stack from East Asia and predicted results from geomagnetic models. Four cold events in the early Holocene recorded by geochemical proxies imply that sedimentary sequence of core HMD13 was influenced by global paleoclimate variation, while its long-term trend points to local paleoenvironmental change. Increases in sedimentation rates and the marine-derived sediment fractions suggest a massive transgression in the early Holocene (9–7.5 ka). Subsequently, ancient people developed and built pile dwellings since ~7 ka and cultivated wild rice from ~6.3 ka. Our study demonstrates that regional stratigraphic correlation with well-dated PSV and RPI records is a practical approach to establishing reliable age-depth models. A high-resolution chronology on continuous sedimentary sequence has excellent potential to elucidate prehistoric human-environment co-evolution.