Shuhui Cai

Archaeointensity results spanning the past 6 kiloyears from eastern China and implications for extreme behaviors of the geomagnetic field

Significance The geomagnetic field is an intriguing fundamental physical property of the Earth. Its evolution has significant implications for issues such as geodynamics, evolution of the life on the Earth, and archaeomagnetic dating. Here, we present 21 archaeointensity data points from China and establish the first archaeointensity reference curve for eastern Asia. Our results record rarely captured extreme behaviors of the geomagnetic field, with an exceptionally low intensity around ∼2200 BCE (hitherto the lowest value observed for the Holocene) and a “spike” intensity value dated at ∼1300 ± 300 BCE (either a precursor to or the same event as the Levantine spikes). These anomalous features of the geomagnetic field revealed by our data will shed light on understanding geomagnetic field during the Holocene. Variations of the Earth’s geomagnetic field during the Holocene are important for understanding centennial to millennial-scale processes of the Earth’s deep interior and have enormous potential implications for chronological correlations (e.g., comparisons between different sedimentary recording sequences, archaeomagnetic dating). Here, we present 21 robust archaeointensity data points from eastern China spanning the past ∼6 kyr. These results add significantly to the published data both regionally and globally. Taking together, we establish an archaeointensity reference curve for Eastern Asia, which can be used for archaeomagnetic dating in this region. Virtual axial dipole moments (VADMs) of the data range from a Holocene-wide low of ∼27 to “spike” values of ∼166 ZAm2 (Z: 1021). The results, in conjunction with our recently published data, confirm the existence of a decrease in paleointensity (DIP) in China around ∼2200 BCE. These low intensities are the lowest ever found for the Holocene and have not been reported outside of China. We also report a spike intensity of 165.8 ± 6.0 ZAm2 at ∼1300 BCE (±300 y), which is either a prelude to or the same event (within age uncertainties) as spikes first reported in the Levant.

The effects of secondary mineral formation on Coe‐type paleointensity determinations: Theory and simulation

Thellier-type experiments are the most widely applied approaches for determining the absolute paleointensities of Earths magnetic field. One major problem, however, is that specimens are prone to thermal alteration due to the intensive thermal treatment during experiments. Linear Arai plots with acceptable partial thermal remanent magnetization (pTRM) checks have been considered as evidence for the absence of or negligible effects of thermal alteration and as reliable indicators of high-quality paleointensity estimates. However, by simulating the Coe variant of the Thellier method on assemblages of single domain (SD) magnetite particles, it is demonstrated that new magnetic minerals, which form during thermal treatments, can result in linear, concave-up, or concave-down Arai plots depending on the magnetic properties of both the primary and secondary magnetic phases. Among this range of behavior, pseudoideal Arai plots, which are linear with acceptable pTRM check statistics, would lead to paleointensity underestimates. It is further demonstrated that pTRM checks are proportional to the degree of underestimate with a magnetic granulometry dependency for SD particles. Due to the complexity of this dependency, pTRM check statistics are only comparable when specimens have similar magnetic properties. This suggests that a universal threshold for pTRM check statistics is not likely to be effective. Since the criteria of linearity and low pTRM check statistics are insufficient to guarantee the fidelity of the estimates auxiliary rock magnetic methods such as temperature-dependent hysteresis parameters and anhysteretic remanent magnetization are highly recommended to identify the presence of alteration.

New constraints on the variation of the geomagnetic field during the late Neolithic period: Archaeointensity results from Sichuan, southwestern China

We have carried out an archaeomagnetic study on a late Neolithic locality (Liujiazhai) in Sichuan, southwestern China. We pull together various dating techniques, including radiocarbon analysis, optically stimulated luminescence dating, stratigraphic information as well as archaeological and archaeomagnetic estimations, to constrain the age of the studied samples. Rock magnetic results indicate thermally stable fine-grained magnetite or titanomagnetite as the dominant magnetic carriers. More than half of the specimens (141/246) in the paleointensity experiment pass the selection criteria and are considered to record robust intensity values. The virtual axial dipole moments range from approximately (2.8 to 7.8)u2009×u20091022u2009Am2 with an average of 5.9u2009×u20091022u2009Am2, indicating that the geomagnetic intensity around 3000 before the Common Era (B.C.E.) is overall lower than the present field intensity (9.8u2009×u20091022u2009Am2) of this area. The new results from Liujiazhai are generally consistent with the published data of similar age but deviate from the only available model of CALS10k.1b at certain time periods, making them important for future improvements of the model. Those data are significant for constraining the variation of geomagnetic field intensity between ~3100 and 2600 B.C.E. and improving the regional model of eastern Asia.

Paleointensity From Subaerial Basaltic Glasses From the Second Hawaii Scientific Drilling Project (HSDP2) Core and Implications for Possible Bias in Data From Lava Flow Interiors

In this study, we collected samples from subaerial basaltic glassy margins from the second Hawaii Scientific Drilling Project (HSDP2) core. We employed the rigorous ‘IZZI’ method during the paleointensity experiment combined with the stringent ‘CCRIT’ criteria for data selection to obtain 21 robust paleointensity estimates recorded by glassy margins from 20 lava flows. We compared our new results to published paleointensities from the interiors of the lava flows from HSDP2 and found that our data are systematically lower than those from the interiors of the same lava flows. The reasons for the discrepancy in intensity are still unclear, but one possibility that could not be absolutely excluded is the effect of cooling rate on the more slowly cooled lava flow interiors. Although our new data from the glassy margins are lower than those from the lava flow interiors, they are still overall higher than the expected field of the study site calculated from a geocentric axial dipole model with an ancient average field of 42 ZAm2; either because of a long-term local anomaly of the field in Hawaii or an insufficient age distribution of our new data (e.g., missing the time period with low field intensities).

Recent Advances in Chinese Archeomagnetism

The geomagnetic field is one of Earth’s fundamental properties with a history of ~3.5 Gyr. The field, generated in Earth’s core is a window to the deep interior of Earth and may have played a key role in evolution of life on our planet. Materials on Earth’s surface that contain magnetic minerals can record information about the geomagnetic field in which they formed. Fired archeological materials (e.g., pottery, brick, and burnt clay) are favorable recorders of the field, and have been widely employed to recover geomagnetic variations over periods of hundreds to thousands of years. The longevity of Chinese civilization and the abundant nature of archeological artifacts make Chinese archeomagnetism a promising source of data. The main work of Chinese archeomagnetism was carried out in the 1980s and 90s, followed by a break of more than a decade; in the 2010s activity resumed. In this paper, we review the development of Chinese archeomagnetism, including a summary of previous work, recent progress, remaining issues and future studies with the aim of promoting an understanding of archeomagnetic work in China and to guide the way for future studies. Here, we compile published data, including some data discovered in old publications that have not yet been included in paleomagnetic databases. We also establish the first, albeit preliminary, archeomagnetic reference curves (with 42 declination / inclination pairs and 76 / 192 archeointensities) for the geomagnetic field in China (ArchInt_China.1a / ArchInt_China.1b, ArchDec_China.1, ArchInc_China.1), which can be used for global comparison of the field and regional archeomagnetic dating.