Recent measurements of pore-water oxygen profiles in ridge flank sediments of the East Pacific Rise revealed an upward-directed diffusive oxygen flux from the hydrothermally active crust into the overlying sediment. This double-sided oxygenation from above and below results in a dual redox transition from an oxic sedimentary environment near the seabed through suboxic conditions at sediment mid-depth back to oxic conditions in the deeper basal sediment. The potential impact of this redox reversal on the paleo- and rock magnetic record was analyzed for three sediment cores from the Clarion-Clipperton-Zone (low-latitude eastern North Pacific). We found that the upward-directed crustal oxygen flux does not impede high quality reversal-based and relative paleointensity-refined magnetostratigraphic dating. Despite low and variable sedimentation rates of 0.1–0.8 cm/kyr, robust magnetostratigraphic core chronologies comprising the past 3.4 resp. 5.2 million years could be established. These age-models support previous findings of significant local sedimentation rate variations that are probably related to the bottom current interactions with the topographic roughness of the young ridge flanks. However, we observed some obvious paleomagnetic irregularities localized at the lower oxic/suboxic redox boundaries of the investigated sediments. When analyzing these apparently remagnetized sections in detail, we found no evidence of physical disturbance or chemical alteration. A sharp increase in single-domain magnetite concentration just below the present lower oxic/suboxic redox boundary suggests secondary magnetite biomineralization by microaerophilic magnetotactic bacteria living as a separate community in the lower, upward oxygenated part of the sediment column. We therefore postulate a two-phased post-depositional remanent magnetization of ridge flank sediments, first by a shallow and later by a deep-living community of magnetotactic bacteria. These findings are the first evidence of a second, deep population of probably inversely oriented magnetotactic bacteria residing in the inverse oxygen gradient zone of ridge flank sediments.
<p>The complex mineralogy and dynamic geochemical processes make the sediments of the Clarion-Clipperton Zone in the eastern Pacific an important research target. Although the sediments and depositional conditions in the study area have been investigated recently, there are still unresolved issues due to the heterogeneity of the sediments. By combining rock magnetic properties and elemental composition data of sediment core SO240-69SL (12&#176; 39.855' N, 119&#176; 13.374' W, water depth of 4275 m) with endmember modeling we aim at contributing to a better understanding of the origin and genesis of the sediments.</p> <p>Based on magnetostratigraphy, the sediments retrieved at this site are backdated to 3.59 Ma and show pronounced changes in sedimentary input through time. The multi-proxy approach reveals four major sediment components: (1) a bottom current-transported lithogenic source derived from weathering of crustal rocks, (2) a low-temperature hydrothermal component of iron-rich clay minerals, (3) a calcareous fraction restricted to the lower part of the core, and (4) authigenic iron-manganese precipitates found at present and past redox boundaries.</p> <p>The sedimentary input from 3.59 to 2.82 Ma is dominated by the low-temperature hydrothermal component and preservation of biogenic carbonates. Endmember modeling shows that the hydrothermal component decreases upward, which is indicative of an increasing distance of the site from the hydrothermal source. From 2.41 to 1.14 Ma, a pronounced change in sediment color and magnetic properties (ARM, ARM/IRM) as well as geochemical properties (Fe/Ti, Si, Al) marks a pause in sedimentation. Thereafter, the sedimentary environment shifts towards lithogenic-dominated sediments with constant content of biogenic magnetite, accompanied by a decrease in sedimentation rate from 1.02 to 0.25 cm ka<sup>-1</sup>.</p> <p>This study demonstrates the potential of using combined rock magnetic and geochemical approaches to reconstruct the complex depositional and early diagenetic history of abyssal sediments. It provides new insights into the sediment formation, mineralogy, and highlights the importance of low-temperature hydrothermal and authigenic minerals, in addition to lithogenic and biogenic sources, on (magnetic) sediment properties.&#160;</p>
<p><span>Shipborne ex-situ oxygen measurements in mid-ocean ridge flank sediment cores from the eastern low-latitude North Pacific (Clarion-Clipperton Zone) revealed a downward increase of pore-water oxygen above the sediment-crust interface (Mewes et al., 2016, Kuhn et al., 2017). This inverse redox zonation is caused by an upward diffusion of oxygen (and other solutes) from fluids circulating through the underlying 20 Mio. Year old and still cooling ocean crust. In consequence, these sediments experience a cyclic change in redox-conditions from oxic seafloor conditions at the top through mostly suboxic conditions throughout the sediment column back to oxygen-rich pore water in the last few sediment meters above the rock basement. </span></p><p><span>We studied paleomagnetic records and bulk magnetic properties of three gravity cores from such settings that were collected during </span><span><em>RV Sonne</em></span><span> expedition SO-240 in 2015 and obtained high-quality magnetostratigraphic records covering the past 3.2 Ma. The generally very good preservation and interpretability of our reversal and RPI records, however, conflicts with a well-defined, but irregular &#8216;ghost event&#8217; of normal polarity within the upper Gilbert reversed C2Ar section. This magnetic polarity and intensity artifact cannot be explained by sediment tectonics, but coincides with the present depth of the lower suboxic-to-oxic redox boundary. Although chemical overprinting could be considered as an obvious explanation of such findings, bulk magnetic analyses (FORCs, thermomagnetics) infer no diagenetic alteration of the magnetic minerals. Over the entire paleomagnetic record, bacterial magnetite appears to be the predominant NRM carrier. We therefore introduce a novel conceptual model of secondary biogenic magnetite formation at crustal depth, hypothesizing that microaerophilic magnetotactic bacteria live and biomineralize not only in the shallow subsurface, but also near the deep oxygen above the sediment-crust interface.</span></p><p>&#160;</p><p><span>References </span></p><p><span>Mewes, K., Mogoll&#243;n, J.M., Picard, A., R&#252;hlemann, C., Eisenhauer, A., Kuhn, T., Ziebis, W., Kasten, S., 2016. Diffusive transfer of oxygen from seamount basaltic crust into overlying sediments: An example from the Clarion-Clipperton Fracture Zone. Earth and Planetary Science Letters 433, 215-225.</span></p><p><span>Kuhn, T., Versteegh, G.J.M., Villinger, H., Dohrmann, I., Heller, C., Koschinsky, A., Kaul, N., Ritter, S., Wegorzewski, A.V., Kasten, S., 2017. Widespread seawater circulation in 18-22 Ma oceanic crust: Impact on heat flow and sediment geochemistry. Geology 45, 799-802.</span></p><p>&#160;</p><p>&#160;</p><p>&#160;</p>
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