Rock magnetic properties of uncultivated magnetotactic bacteria and paleo-redox changes across the Paleocene-Eocene boundary, New Jersey Coastal Plain

Magnetotactic bacteria (MTB) can biosynthesize ferrimagnetic magnetite (Fe₃O₄) and/or greigite (Fe₃S₄) nanocrystals. Their ubiquitous presence in oxygen-stratified environments makes them useful redox indicators, in addition to being major players in the biogeochemical cycling of iron and other elements. Furthermore, since the MTB magnetite and/or greigite organelles, or magnetosomes, are within the magnetic stable single-domain (SSD) size range, they are candidate carriers of stable remanent magnetization. As such, the detection of fossilized magnetosomes (magnetofossils) is of special interest. Here, at least seven uncultivated MTB morphotypes, including magnetite producers, magnetite and greigite co-producers, and greigite-only producers were collected from the chemically stratified water column of Pettaquamscutt River Estuary, Rhode Island, U.S.A. In stark contrast to putative Neogene greigite magnetofossil records, the anhysteretic remanent magnetization (ARM) coercivity spectra for greigite producing MTB are fundamentally left-skewed with a lower median, and are strikingly similar to magnetite producing MTB. The ferromagnetic resonance (FMR) spectra of uncultivated MTB are characterized by a wider empirical parameter range than cultured single-chain magnetite magnetosomes, which has important implications for magnetofossil detection by FMR. The MTB population turnover rate, once every 28.3 days, is determined for the first time in the investigated estuary. The associated magnetosome iron flux, 15.3 to 18.2 μg Fe cm⁻² yr⁻¹, is nearly half of the reduced Fe flux in the open Black Sea. The propensity of magnetite and greigite magnetosomes to acquire gyroremanent magnetization (both static and at a high rotation rate of 95 r.p.s.) has been assessed. Compared to framboidal greigite particles that can acquire a strong gyroremanence, magnetosomes acquire significantly less. Lastly, the above newly delineated magnetofossil characteristics are used to probe the unusually magnetic Marlboro Clay at the New Jersey Coastal Plain dating to the Paleocene-Eocene boundary. Analyses of ARM coercivity spectra suggest that between 22-37% of the ARM signal can be attributed to magnetofossils, while semi-quantitative analytical microscopy as well as rotational remanent magnetization (RRM) suggest framboidal greigite as a heretofore overlooked magnetic signal carrier. The redox significance of magnetofossils is assessed by independent inorganic geochemical proxies (trace metal and iron species) as well as lipid biomarkers (gammacerane index and pristane/phytane ratio). Overall, these proxies suggest a transition toward a more reducing condition at this continental margin setting at the onset of abrupt warming with a concomitant negative carbon isotopic excursion.