http://www.sciencemag.org/news/2021/02/ancient-kauri-trees-capture-last-collapse-earth-s-magnetic-field

Radiocarbon from a 42,000-year-old kauri tree in New Zealand helped unravel Earth’s last magnetic upheaval.

Several years ago, workers breaking ground for a power plant in New Zealand unearthed a record of a lost time: a 60-ton trunk from a kauri tree, the largest tree species in New Zealand. The tree, which grew 42,000 years ago, was preserved in a bog and its rings spanned 1700 years, capturing a tumultuous time when the world was turned upside down—at least magnetically speaking.