Magnetotactic Bacterial Activity in the North Pacific Ocean and Its Relationship to Asian Dust Inputs and Primary Productivity Since 8.0 Ma

Abstract

Magnetotactic bacteria (MTB) require iron and organic carbon for biomineralization, which can be supplied by dust inputs that stimulate primary productivity in iron-limited oceans. However, MTB activity and its relationship to Asian dust inputs and primary productivity are not well known in the North Pacific Ocean. Here, we present an eight-million-year record of magnetofossil abundance, Asian dust flux, and primary productivity from North Pacific sediments (Ocean Drilling Program Hole 885A). Our results demonstrate that Asian dust delivered iron to stimulate primary productivity and then boost MTB biomineralization in the late Miocene. Since the late Pliocene, dust inputs and primary productivity appear to have been decoupled, with MTB activity controlled mainly by primary productivity rather than dust inputs. Equant magnetofossil abundances co-vary with eolian flux, and may provide here a proxy for dust inputs. This work provides perspectives on relationship between MTB activity, dust inputs, and primary productivity on million-year timescales.Plain Language Summary Magnetotactic bacteria (MTB) biomineralize chains of characteristic magnetically single domain magnetite (or greigite) crystals in their cells and can thrive in deep-sea ecosystems and absorb iron for biomineralization. Magnetic particles that are preserved in sediments after MTB death are known as magnetofossils. Eolian dust supplied from continents is a major iron source in pelagic environments. Hence, magnetofossils can be important indicators of the impacts of dust-supplied iron on deep-sea ecosystems. A better understanding of the relationship between MTB activity, dust inputs, and primary productivity is needed for reliable magnetofossil interpretation. However, this relationship is not well determined in the North Pacific Ocean. By integrating rock magnetic, transmission electron microscope, and diffuse reflectance spectroscopy observations, we present an eight-million-year record of magnetofossil variations, Asian eolian inputs, and primary productivity from the North Pacific Ocean. Our results demonstrate that Asian dust inputs appear to have promoted MTB activity by stimulating surface primary productivity in the late Miocene. Since the late Pliocene, MTB activity was controlled mainly by primary productivity, with decoupling of the relationship between dust inputs and primary productivity. Moreover, equant magnetofossil abundances vary with eolian flux and provide a proxy for dust inputs in these marine sediments. ZHANG ET AL.

Inverse Magnetic Fabrics Caused by Magnetofossils in the Northwestern South China Sea Since End of the Last Glacial

“…Magnetofossils in ideal magnetic SD sizes have been detected commonly in marine sediments during warm stages (Chang et al., 2012). The abundance of magnetofossils depends mainly on their production and preservation, which are sensitive to the paleoenvironment and can be affected by redox conditions, total organic carbon (TOC), deep‐water ventilation and oxygen solubility (Chang et al., 2018; Kopp et al., 2007; Roberts et al., 2011; Snowball et al., 2002; Suk, 2016; Zhang et al., 2021). The bottom oxygen concentration that is affected by changes in TOC and deep‐water ventilation could influence the preservation of magnetofossils (Mathias et al., 2021).…”

MagNet: Automated Magnetic Mineral Grain Morphometry Using Convolutional Neural Network

“…Furthermore, changes in the morphology distribution of magnetic mineral grain populations within natural samples can also be used to trace paleoenvironmental evolution. For example, it is suggested that the crystal shape of magnetofossils is indicative of total organic carbon flux (Hesse, 1994; Yamazaki, 2012; Yamazaki & Kawahata, 1998; Q. Zhang et al., 2021). Chang et al.…”

A Depth‐Transect of Ocean Deoxygenation During the Paleocene‐Eocene Thermal Maximum: Magnetofossils in Sediment Cores From the Southeast Atlantic

“…Previous studies suggest that environmental factors apart from redox conditions, including iron supply (i.e., aeolian dust; Zhang et al, 2018Zhang et al, , 2021, organic carbon decomposition (e.g., Lu et al, 2021;Yamazaki & Kawahata, 1998), and temperature (Katzmann et al, 2013), potentially affect magnetofossil morphologies. Sites 1263 (∼1,500 m paleodepth) and 1262 (∼3,600 m paleodepth) show a similar pattern of iron concentration (Zachos et al, 2005), whereas reconstructed paleoredox conditions based on magnetofossils are different along the depth-transect (Figure 13), suggesting that iron supply likely had a negligible influence on the morphological features of magnetofossils on Walvis Ridge.…”

“…Magnetofossil concentrations within deep-sea sediments may reflect a balance between their production and preservation, which relate to pore water chemistry, and ultimately the supply of organic carbon and dissolved oxygen (e.g., Larrasoaña et al, 2012;Roberts et al, 2011;Yamazaki & Horiuchi, 2016;Yamazaki & Ikehara, 2012;Zhang et al, 2018Zhang et al, , 2021. Sedimentary redox conditions may be affected by marine productivity (e.g., organic carbon delivery) and overlying seawater chemistry, and thus, the ocean's biogeochemical systems (Keeling et al, 2010;Oschlies et al, 2018).…”