Magnetostratigraphic effects and artifacts of an inverse redox zonation in bottom-up oxygenated East Pacific mid-ocean ridge flank sediments

<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 ‘ghost event’ 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> </p><p><span>References </span></p><p><span>Mewes, K., Mogollón, J.M., Picard, A., Rühlemann, 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> </p><p> </p><p> </p>

Local natural background levels assessment through a groundwater redox zonation, the case of Lombardy Region.

<p>Discretizing anthropogenic and natural contaminations represents a crucial step in groundwater management and regulation. Natural background levels (NBLs) have a huge impact on groundwater protections and remediation strategies, but it is still an issue on the ground in terms of reliability and accuracy, thus its derivation needs further scientific efforts.</p><p>The derivation of local NBLs (LNBLs) is intended to overcome the limitation of considering a groundwater body (GWB) homogeneous, hence accounting hydrogeochemical heterogeneities within the aquifer system.</p><p>This work presents a statistical approach assessing LNBLs for sensitive redox species (As, Fe, Mn, NH<sub>4</sub>) in 30 GWBs within the Lombardy Region. Under the monitoring network of the Regional Agency for Environmental Protection of Lombardy (ARPA), more than 500 wells were investigated, thus each GWBs were identified within 4 aquifer types: shallow, intermediate, deep Po Plain aquifers and Alpine valley aquifers. The initial dataset underwent preselection and multivariate analyses, appointing at each well a geogenic redox zonation. It leaded to discretize geochemically-homogeneous subgroups and characterize them as function of site-specific natural facies: oxidised (293 wells), reduced (199 wells) and saline (11 wells). Interquartile range criteria, validations’ tests (Mann-Kendall and Shapiro-Wilk), probability density histograms and probability plots inferred temporally and spatially the datasets, one for each target species, discretized for aquifer and natural facies appartenances. This resulted in the identification of the statistical distributions from redox-homogeneous sets of data from which the LNBLs were derived.</p><p>Considering the Po Plain aquifer (shallow, intermediate and deep), NBLs derivation for As revealed three subgroups within the oxidised facies, for which the NBLs values are of 2, 3 and 7 μg/L, four subgroups ascribe to the reduced facies with NBLs of 13, 49, 71 and 291 μg/L, and two subgroups for the saline facies with NBLs of 3 and 12 μg/L. According Fe, two are the subgroups within the oxidised facies, with NBLs of 40 and 94 μg/L, four subgroups fall in the reduced facies with NBLs of 653, 1430, 3200 and 6000 μg/L; within the saline facies, two subgroups are identified with NBLs of 1647 and 6000 μg/L. Two subgroups characterize the oxidised facies for NBLs of Mn with values of 8 and 27 μg/L, and NBLs of 34, 216, 485, 912 and 1514 μg/L refer to five subgroups in reduced facies, while within the saline facies fall two subgroups with NBLs of 381 and 921 μg/L. With regards to NH<sub>4</sub>, NBLs reach values of 49, 110 and 190 μg/L for the three subgroups within the oxidised facies, whereas values of 834, 2600, 3090, 4480 μg/L are derived for the four subgroups in the reduced facies; the two subgroups ascribed to the saline facies reveal NBLs of 1860 and 6620 μg/L.</p><p>Data demonstrate how an in depth understanding of aquifers’ redox-zonations turned out to be functional for assessing LNBLs. Regional Legislation (D.G.R. 23novembre2020 n.3903) has been amended on the basis of the outcomes of this work, revealing site redox-specific LNBLs of practical significance.</p><p> </p><p>Funding: this work was granted and carried out in collaboration with Lombardy Region.</p>

Denitrifying biofilm processes for wastewater treatment: developments and perspectives

Biofilms can retain microorganisms with very different growth kinetics and different electron acceptor preferences, due to their natural redox zonation.

Response of redox zonation to recharge in a riverbank filtration system: a case study of the Second Songhua river, NE China

Bank filtration induced by groundwater pumping results in redox zonation along the groundwater flow path. Besides the river water, recharge from other sources can change local redox conditions; therefore, redox zonation is likely to be complex within the riverbank filtration (RBF) system. In this study, hydrodynamics, hydrogeochemistry, and environmental stable isotopes were combined together to identify the redox conditions at an RBF site. The recharge characteristics and redox processes were revealed by monitoring the variations of water level, δ2H and δ18O, and redox indexes along shallow and deep flow paths. The results show that local groundwater is recharged from river, regional groundwater, and precipitation. The responses of redox zonation are sensitive to different sources. In the river water recharge zone near shore, O2, , Mn(IV), Fe(III), and are reduced in sequence, the ranges of each reaction are wider in deep groundwater because of the high-velocity deep flow. In the precipitation vertical recharge zone, precipitation intermittently drives O2, , and organic carbon to migrate through vadose zone, thereby decreasing the groundwater reducibility. In the regional groundwater lateral recharge zone in the depression cone, the reductive regional groundwater is continuously recharging local groundwater, leading to the cyclic reduction of Mn(IV) and Fe(III).

Impact of the accuracy of the conceptual geological model on predicting hot spots of redox-zones at the surface water – groundwater interface

<p>Managed aquifer recharge through riverbank filtration is an important method to produce drinking water in densely populated regions. Due to the discharge of wastewater into streams, this type of drinking water production can be affected by organic contaminants originating from surface water inflow. Transport and degradation of anthropogenic contaminants depend on several factors, such as pH, temperature, oxygen content, and redox conditions. One of the key factors that drive the degradation of organic contaminants like x-ray contrast media is the prevailing redox system as many pharmaceuticals and pesticides are transformed under aerobic conditions but are more persistent under anaerobic conditions.</p><p>We conducted a 1-year monitoring campaign at an active riverbank filtration plant at the Rhine river in Düsseldorf, Germany. Samples were taken every two weeks from the Rhine, a production well, and five observation wells with three different depths along a transect perpendicular to the river and parallel to the main flow direction. Samples were analyzed for main cations and anions, redox-species, and microbiological parameters. Water samples were also screened for 100 organic contaminants, pharmaceuticals, and pesticides.</p><p>A 2D reactive transport model was set-up using PFLOTRAN to simulate the redox zonation during a hydrological year. It includes aerobic respiration and denitrification with dissolved organic carbon using Monod kinetics and also accounts for temperature-dependency. Our results show that hot spots for biogeochemical processes develop close to the river, and thus most of the inflowing oxygen is already consumed within the first few decimeters. We also found a substantial seasonal variability of reaction rates due to seasonal temperature variations leading to oxygen depletion and limited denitrification in the warmest period (late summer/early fall).</p><p>Reactive transport is affected by the hydrogeological properties of the aquifer, which are influenced by its geological development. Thus, model results will depend on the reliability and accuracy of the employed conceptual geological model. Based on structural information obtained from grain size sieve analysis, and geophysical investigations such as geoelectric and natural gamma-ray measurements, we created a set of plausible conceptual models with increasing complexity. These models range from a simple homogeneous aquifer, to a multi-layer aquifer system or a cross-bedded aquifer structure. The conceptual models include different representations of the colmation layer at the interface between river and aquifer.</p><p>Numerical analysis of the different conceptual models indicates that a homogeneous aquifer can represent a single flow path over a hydrological year. However, only more complex aquifer structures were able to reproduce the spatial and seasonal temporal variability of temperature and redox species observations (O<sub>2</sub>, NO<sub>3</sub><sup>-</sup>). Additionally, proper integration of the colmation layer is the key factor to simulate heat transport as well as the spatial distribution of redox-species and thus redox-zonation during the entire hydrological year, including droughts and flooding periods. Therefore, an accurate and detailed integration of the geological system into the reactive transport model, especially characteristics (e.g., size, type of material) of the colmation layer, are of highest relevance for enhanced predictions of redox zones in highly transient hydrogeological systems and at hydrodynamic interfaces.</p>