Archaeointensity study of Vadnagar artefacts: insights into geomagnetic field variation in India during the past 4250 yr

SUMMARY Understanding the Earth's magnetic field through regional records of secular variation is essential for deciphering its short-term behaviour. This study presents an archaeomagnetic and rock-magnetic investigation of archaeological artefacts from Vadnagar, Gujarat, and introduces India's first continuous palaeosecular variation (PSV) curve for the last four millennia. Detailed rock magnetic analyses were applied to investigate the suitability of the artefacts for intensity measurements. The geomagnetic field intensity was calculated using the Thellier–Thellier method modified by Coe, with cooling rate and anisotropy of the thermoremanent magnetization corrections. A total of 80 independent fragments were analysed, from which 66 gave positive responses, resulting in a success rate of 83% after the application of corrections and quality selection criteria. Seven new archaeointensities were calculated, with values ranging from 33.58 ± 2.0 to 43.37 ± 1.9 µT. The new intensities were integrated with previously published data in order to construct India's first PSV curve from 2250 BCE to 2000 CE at the geographical Centre of India (20.5937° N, 78.9629° E) using two different modelling approaches. The first approach employs a bootstrap algorithm, yielding relatively smooth intensity variations, while the second utilizes a transdimensional Bayesian framework, producing sharper variations with occasionally greater amplitudes. The PSV curve developed using the bootstrap algorithm was compared with global models, showing precise temporal alignment only from 400 BCE to 200 CE. This indicates the necessity of new reliable archaeointensity data from dated artefacts in order to acquire a rigorous explanation of geomagnetic field intensity change during the past and gain a deeper understanding of local geomagnetic field variations in India.