Magnetofossil Gigantism in Modern Andaman Sea Sediments

Abstract “Giant” magnetofossils were first discovered in pelagic Paleogene‐Eocene Thermal Maximum event sections, but have also been recently identified in Late Pleistocene hemipelagic sediments such as the Bay of Bengal (BoB). Their obvious time‐transgressive existence raises questions on the modern niches and habitat conditions of their still unknown producers. Here we report the occurrence of giant magnetofossils in Holocene surface sediments of the Andaman Sea (AS). Rock magnetic analyses indicate mixed (single, vortex, and multidomain state) magnetic mineral assemblages, where a prominent central ridge in first‐order reversal curve diagrams signalizes a dominant biogenic magnetite component. Transmission electron microscopy reveal coexistence of conventional (prismatic, cuboctahedral, bullet) and giant (needle, bullet) magnetofossil morphologies, composed of stoichiometric, non‐oxidized magnetite as confirmed by low‐temperature magnetometry, energy dispersive spectrometry and crystallography. Based on the water column conditions, magnetofossil abundance and morphologies, we make inferences on habitat and environmental factors favored by the respective iron‐biomineralizing organisms. Comparative analyses of water column profiles from AS and BoB revealed that relatively low, but uniform bottom water oxygenation and persistent suboxic conditions maintained by rapid renewal of deep water through circulation accompany the biomineralization of giant magnetic particles in the AS. Our investigations of two deep sediment core sites suggest that the modern AS is an ecological niche and promising investigation area for the yet‐to‐be‐discovered, putative eukaryotic organisms, that biomineralize and release giant needle‐ and bullet‐shaped magnetosomes to the sediment.