Deep-sea rare-earth mineral resources formed in the early Eocene
Hothouse ocean: Insights from deep learning-based microfossil
observations
Abstract
Deep-sea mud enriched in rare-earth elements (REEs), termed REE-rich
mud, is a promising seafloor mineral resource. A decade of surveys has
revealed that the mud with the highest REE concentration occurs in the
pelagic realm of the western North Pacific Ocean, with two layers of
elevated REE concentration. Previous analyses of sediments have revealed
multiple periods of significant REE enrichment, with the first
(youngest) REE enrichment triggered by global cooling during the
Eocene–Oligocene climate transition. However, the depositional
mechanism of older REE peaks remains unclear. Fish debris is the major
host of REE in deep-sea sediments. In this study, the microfossils of
fish teeth and denticles, called ichthyoliths, were observed to
constrain the depositional ages of REE-enriched layers with unknown
genesis. Empowered by deep learning, more than 40,000 ichthyoliths were
observed, and the second (older) REE enrichment was revealed to have
occurred in the early Eocene, when the Earth’s climate was exceedingly
warm. The warm ocean may have enhanced the efficiency of trophic
transfer, leading to an increased supply of fish debris and thus REE, to
the seafloor. Therefore, the Paleogene Hothouse might have been
advantageous for producing valuable seafloor mineral resources.