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This paper presents new evidence for the widespread occurrence and placement of multiple zones of magnetic bacteria (MB) in Holocene deepsea sediments from the Blake/Bahama Outer Ridge (western North Atlantic Ocean) and the microenvironmental factors (primarily ambient geochemical gradients) that control their distributions. Our data come from reanalysis of rock magnetic data from Schwartz et al. that first identified the presence of MB in this region and new rock magnetic, geochemical, and geobiological measurements on previously studied cores and two new cores. All of our results are consistent with the work of Schwartz et al., but provide a more detailed assessment of the MB and the environmental factors controlling their placement. We find that the MB are distributed in four distinct zones (A-D) within Holocene sediments of the Blake-Bahama Outer Ridge. These four zones are traceable over a region greater than 104 km2 in extent within sediments deposited between 3000-4600 m water depth. The four zones are all relatively narrow (<20 cm thick) with no evidence for significant stratigraphic migration over time. Two zones appear to be causally related to solid-phase peaks in Mn (Zone C) and Fe (Zone D) concentrations that we associate with either current or relict Mn and Fe redox boundaries. Zones A and B lie above these two redox boundaries. None of these zones are synchronous on a regional scale, suggesting that geochemical horizons rather than external time-varying environmental factors control their placement. Different MB morphotypes dominate individual zones; rectangular/cubic forms dominate Zones A and C, while hexagonal/teardrop forms dominate Zones B and D. Both magnetotactic and dissimilatory MB occur in all four zones. Rock magnetic data suggest that magnetosome chains or lysed chains are present in at least Zones A, B, and C. We think the presence of chains is an indicator that the MB were alive when the cores were collected. Although we cannot be sure which of the MB zones represent current environmental conditions (pore water chemistry) versus relict conditions from earlier in the Holocene, the four zones appear to be related to the present and/or past geochemical gradients and thus may serve as paleo-indicators of specific diagenetic regimes.
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