Magnetotactic bacteria (MTB), sensitive to redox status, leave fossil bacterial magnetite (magnetofossils) in sediments. The relative contents of cuboctahedral, elongated prismatic, and bullet‐shaped magnetofossils archive changes in redox conditions, which indicate paleoenvironmental variations. The Eocene‐Oligocene transition (EOT) is a turning point in the Cenozoic climate evolution from greenhouse to icehouse. Global cooling, Antarctic glaciation, and/or a new tectonic structure modified the global ocean and atmosphere circulation mode. In the eastern equatorial Pacific Ocean (EEPO), the organic matter and aeolian supply, which are important for the proliferation of MTB, did not vary synchronously. Here, we study the magnetic particles and environmental magnetism characteristics of samples from EEPO to test the hypothesis that magnetofossil assemblages respond to the dramatic paleoclimatic changes across the EOT. Results show that the abundances of all kinds of magnetic particles are significantly decreased with the reduction of aeolian supply after the EOT. However, the relative abundance of magnetofossils and the proportion of bullet‐shaped ones both increased. Simultaneously, organic matter input was enhanced as indicated by (a) similar productivity but decreased organic carbon consumption and (b) the increased mass accumulation rates of total organic carbon after the EOT. The enhanced organic matter flux increased redox gradient in pelagic sediment, which together with sufficient iron from aeolian particles, supported the metabolism of MTB, especially those synthesizing bullet‐shaped magnetofossils. Therefore, the magnetofossils in the EEPO vary synchronously with the evolution of paleoenvironment before and after the EOT, which thus supports the use of magnetofossil as an effective proxy archiving paleoenvironment.

The anhysteretic remanent magnetization (ARM) is a laboratory-imparted artificial remanence that is used widely in mineral magnetic studies (Dunlop & Özdemir, 1997). An ARM is usually imparted by exposing a sample to an alternating field (AF; e.g., ∼100 mT) with a superimposed small direct current (DC; e.g., ∼50 μT) bias field. The bulk ARM is given as the sum of the ARM of each component (Egli, 2004a(Egli, , 2004bFabian & Leonhardt, 2009):where M ar is the bulk ARM imparted with a DC field H dc for a sample containing N magnetic components with saturation remanence M rs and component-specific ARM ratios, 𝐴𝐴 𝜘𝜘𝑖𝑖 = 𝜒𝜒𝑎𝑎𝑖𝑖∕𝑀𝑀𝑟𝑟𝑟𝑟𝑖𝑖 , where 𝐴𝐴 𝜘𝜘𝑖𝑖 is the ratio of the ARM

Stimulation of the biological pump by iron-bearing dust in the eastern equatorial Pacific Ocean plays an important role in long-term carbon sequestration, yet past dust fertilization and its impact on CO2 perturbations over major climate transitions remain debated. Here, we integrate proxies of dust input, source-region weathering, and biological pump activity from late Eocene to early Miocene sediments of Integrated Ocean Discovery Program Hole U1333, which includes the Eocene-Oligocene Transition (~34 million years ago) when a major ice sheet was first established on Antarctica. We find that intensified chemical weathering in the large central Asian dust source region enhanced atmospheric CO2 removal at ~34 Ma. Superimposed dust fertilization and biological pump action amplified this CO2 removal before ~34 Ma, while weakening of this amplification process helped to moderate the CO2 decline after that time. The observed inter-linked, counteracting processes with different timescales illustrate the complexity of carbon cycle feedbacks associated with major climate changes.

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