Enhanced olivine weathering in permeable sandy sediments from the North Sea – a laboratory study using flow-through reactors

The Earth’s climate is increasingly warming due to ongoing anthropogenic carbon dioxide (CO2) emissions. In order to mitigate the human-made climate change and to meet the Paris Agreement goals of limiting the warming below 2°C, active carbon dioxide removal (CDR) from the atmosphere is of great importance in addition to massive CO2 emission reductions. A possible CDR method is rock weathering and the associated dissolution of minerals in the ocean, which leads to marine alkalinity enhancement and, thus, an enhanced flux of CO2 from the atmosphere into the ocean. In the framework of the project RETAKE, a consortium of the German Marine Research Alliance (DAM) research mission CDRmare, we investigate the potential, feasibility and side effects of olivine dissolution in high-energy coastal environments where strong currents and advection of seawater through permeable sediments have been proposed to accelerate weathering of silicate rocks. Here, we present data from laboratory experiments with flow-through reactors that are filled with permeable sandy sediments from the North Sea amended with different amounts and grain sizes of olivine. Permeable sediments are generally characterized by advective pore-water flow. Under advective conditions, higher weathering rates than those found in diffusion-controlled depositional settings are expected since the reaction products are rapidly removed and the formation of authigenic mineral coatings on olivine grains is prevented. The flow-through experiments are conducted under oxic conditions whereby air-saturated natural seawater is continuously pumped through the reactors. In addition to the permanent measurement of oxygen, pH and temperature, the circulating water is regularly sampled and alkalinity, dissolved inorganic carbon, major cation and trace metal (e.g., nickel) concentrations are analyzed. Preliminary results indicate an increase in alkalinity up to 3.2 mM in the reactor with the largest amount of olivine while the alkalinity in the control reactor (without olivine addition) is close to background seawater concentrations of 2.3 mM. Similarly, highest dissolved nickel concentrations were found in the reactor with highest olivine contents added. In order to detect and characterize secondary minerals that possibly formed, the sediment/olivine mixtures are sampled after completion of the experiments and analyzed with respect to the mineralogical and chemical composition

Postdepositional Behavior of Molybdenum in Deep Sediments and Implications for Paleoredox Reconstruction

Molybdenum (Mo) is a trace element sensitive to oceanic redox conditions. The fidelity of sedimentary Mo as a paleoredox proxy of coeval seawater depends on the extent of Mo remobilization during postdepositional processes. Here we present the Mo content and isotope profiles for deep sediments from the Nankai Trough, Japan. The Mo signature suggests that these sediments have experienced extensive early diagenesis and hydrothermal alteration at depth. Iron (Fe)‐manganese (Mn) (oxyhydr)oxide alteration combined with Mo thiolation leads to a more than twenty‐fold enrichment of Mo within the sulfate reduction zone. Hydrothermal fluids and Mo adsorption onto Fe‐Mn (oxyhydr)oxides cause extremely negative Mo‐isotope values at the underthrust zone. These postdepositional Mo signals might be misinterpreted as expanded anoxia in the water column. Our findings highlight the importance of constraining postdepositional effects on Mo‐based proxies during paleoredox reconstruction

Influence of Early Low-Temperature and Later High-Temperature Diagenesis on Magnetic Mineral Assemblages in Marine Sediments From the Nankai Trough

Funding Information: This research used samples and data provided by the International Ocean Discovery Program (IODP). The authors thank the Marine Works Japan staff at the Kochi Core Center for support during sampling. This work was supported by the Japan Society for the Promotion of Science Grant-in-Aid for Science Research (grant 17K05681 to Myriam Kars), the German Research Foundation (DFG grants 388260220 to Male Koster and Susann Henkel, and 408178672 to Florence Schubotz), and the Australian Research Council (grant DP200100765 to Andrew P. Roberts). The authors also thank two anonymous reviewers for their constructive comments and Editor Joshua Feinberg for handling the manuscript.Peer reviewedPublisher PD

Post-depositional manganese mobilization during the last glacial period in sediments of the eastern Pacific Ocean

Studies have provided compelling evidence that the Pacific Ocean has experienced substantial glacial/interglacial changes in bottom-water oxygenation. While the deep Pacific Ocean is currently well oxygenated, bottom-water oxygen concentrations (O2bw) were most likely lower during the last glacial period (LGP), which must have caused a much more compressed redox zonation in the sediments than at present. We have sequentially leached mobilizable MnO2 and various Fe (oxyhydr)oxides and used transport-reaction modelling in order to reconstruct past redox changes in sediments of the NE Pacific. We have investigated six sites situated in various contract areas for the exploration of polymetallic nodules within the Clarion-Clipperton Zone (CCZ) and one site located in a protected area (APEI3) north of the CCZ. We found bulk sediment Mn maxima of up to 1 wt% in the upper 10 cm of the sediments at all sites except for the APEI3 site. Mobilizable Mn(IV) was the dominant Mn phase representing more than 70% of bulk Mn. As oxygen penetration depths of more than 0.5 m currently do not allow for the formation of authigenic Mn(IV) in the surface sediments of the CCZ, we postulate that lower O2bw during the LGP caused a compressed redox zonation where authigenic Mn(IV) precipitated at a shallow oxic-suboxic redox boundary. Transport-reaction modelling reveals that at O2bw of 35 µM, which were suggested to have prevailed during the LGP, the oxic-suboxic redox boundary is located in the upper 5 cm of the sediments. A distinct mobilizable Mn(IV) maximum was not found in the surface sediments of the APEI3 site indicating that the redox zonation was not as condensed during the LGP at this site due to two- to threefold lower organic carbon burial rates. Our results suggest that oxygen-deprived bottom water conditions prevailed on a basin-wide scale during the LGP and were associated with significantly different rates of biogeochemical processes and element fluxes in sediments of the NE Pacific than today

Post-depositional manganese mobilization during the last glacial period in sediments of the eastern Clarion-Clipperton Zone, Pacific Ocean

Numerous studies have provided compelling evidence that the Pacific Ocean has experienced substantial glacial/interglacial changes in bottom-water oxygenation associated with enhanced carbon dioxide storage in the glacial deep ocean. Under postulated low glacial bottom-water oxygen concentrations (O), redox zonation, biogeochemical processes and element fluxes in the sediments must have been distinctively different during the last glacial period (LGP) compared to current well-oxygenated conditions. In this study, we have investigated six sites situated in various European contract areas for the exploration of polymetallic nodules within the Clarion-Clipperton Zone (CCZ) in the NE Pacific and one site located in a protected Area of Particular Environmental Interest (APEI3) north of the CCZ. We found bulk sediment Mn maxima of up to 1 wt% in the upper oxic 10 cm of the sediments at all sites except for the APEI3 site. The application of a combined leaching protocol for the extraction of sedimentary Mn and Fe minerals revealed that mobilizable Mn(IV) represents the dominant Mn(oxyhydr)oxide phase with more than 70% of bulk solid-phase Mn. Steady state transport-reaction modeling showed that at postulated glacial O of 35 μM, the oxic zone in the sediments was much more compressed than today where upward diffusing pore-water Mn2+ was oxidized and precipitated as authigenic Mn(IV) at the oxic-suboxic redox boundary in the upper 5 cm of the sediments. Transient transport-reaction modeling demonstrated that with increasing O during the last glacial termination to current levels of ∼ 150 μM, (1) the oxic-suboxic redox boundary migrated deeper into the sediments and (2) the authigenic Mn(IV) peak was continuously mixed into subsequently deposited sediments by bioturbation causing the observed mobilizable Mn(IV) enrichment in the surface sediments. Such a distinct mobilizable Mn(IV) maximum was not found in the surface sediments of the APEI3 site, which indicates that the oxic zone was not as condensed during the LGP at this site due to two- to threefold lower organic carbon burial rates. Leaching data for sedimentary Fe minerals suggest that Fe(III) has not been diagenetically redistributed during the LGP at any of the investigated sites. Our results demonstrate that the basin-wide deoxygenation in the NE Pacific during the LGP was associated with (1) a much more compressed oxic zone at sites with carbon burial fluxes higher than 1.5 mg Corg m−2 d−1, (2) the authigenic formation of a sub-surface mobilizable Mn(IV) maximum in the upper 5 cm of the sediments and (3) a possibly intensified suboxic-diagenetic growth of polymetallic nodules. As our study provides evidence that authigenic Mn(IV) precipitated in the surface sediments under postulated low glacial O, it contributes to resolving a long-standing controversy concerning the origin of widely observed Mn-rich layers in glacial/deglacial deep-sea sediments

Iron isotope signals: Use and limitations in natural marine sediments

Oral presentatio

Evolution of (bio-)geochemical processes in deep subseafloor sediments from the Nankai Trough along the tectonic migration of ocean floor and related changes in depositional and thermal conditions

(Bio-)geochemical processes in deep subseafloor sediments can notably change over geological timescales due to variations in oceanographic, climatic or depositional conditions. These changes, in turn, can have significant impacts on global element cycles of carbon, sulfur and iron and the carbon sequestration in marine sediments. An ideal setting to study (bio-)geochemical processes under the influence of strongly changing environmental and depositional conditions is the Nankai Trough subduction zone offshore Japan in the northwestern Pacific Ocean. The sediment cores that were investigated in the framework of this cumulative doctoral thesis were taken from a 1,180 m deep hole (Site C0023) in the Nankai Trough off Cape Muroto during International Ocean Discovery Program Expedition 370, which aimed at exploring the temperature limit of microbial life in the deep subseafloor biosphere. At present, the temperature increases from 2°C at the seafloor up to 120°C at the sediment-basement interface. Over the past 15 million years, the sediments at Site C0023 have moved several hundreds of kilometers relative to its present-day position due to tectonic plate motion. During this tectonically induced migration, the sediments at Site C0023 have experienced significant changes in depositional, geochemical and thermal conditions. By combining comprehensive geochemical and rock magnetic analyses, the potential succession of (bio-)geochemical processes, in particular the cycling of iron, in the deep subseafloor sediments at Site C0023 was reconstructed along the tectonic migration of ocean floor. In a next step, the resulting evolution of (bio-)geochemical processes was quantitatively tested by reactive transport modeling. In addition, stable iron isotope analyses were performed on dissolved and sequentially extracted iron to disentangle microbially mediated and abiotic drivers of the iron cycling in deep and hot subseafloor sediments at Site C0023. The presented study contributes to an improved understanding of long-term variations in (bio-)geochemical processes in the deep subseafloor biosphere and provides important aspects for the interpretation of stable iron isotope data

Evolution of (bio-)geochemical processes in deep subseafloor sediments from the Nankai Trough along the tectonic migration of ocean floor and related changes in depositional and thermal conditions

(Bio-)geochemical processes in deep subseafloor sediments can notably change over geological timescales due to variations in oceanographic, climatic or depositional conditions. These changes, in turn, can have significant impacts on global element cycles of carbon, sulfur and iron and the carbon sequestration in marine sediments. An ideal setting to study (bio-)geochemical processes under the influence of strongly changing environmental and depositional conditions is the Nankai Trough subduction zone offshore Japan in the northwestern Pacific Ocean. The sediment cores that were investigated in the framework of this cumulative doctoral thesis were taken from a 1,180 m deep hole (Site C0023) in the Nankai Trough off Cape Muroto during International Ocean Discovery Program Expedition 370, which aimed at exploring the temperature limit of microbial life in the deep subseafloor biosphere. At present, the temperature increases from 2°C at the seafloor up to 120°C at the sediment-basement interface. Over the past 15 million years, the sediments at Site C0023 have moved several hundreds of kilometers relative to its present-day position due to tectonic plate motion. During this tectonically induced migration, the sediments at Site C0023 have experienced significant changes in depositional, geochemical and thermal conditions. By combining comprehensive geochemical and rock magnetic analyses, the potential succession of (bio-)geochemical processes, in particular the cycling of iron, in the deep subseafloor sediments at Site C0023 was reconstructed along the tectonic migration of ocean floor. In a next step, the resulting evolution of (bio-)geochemical processes was quantitatively tested by reactive transport modeling. In addition, stable iron isotope analyses were performed on dissolved and sequentially extracted iron to disentangle microbially mediated and abiotic drivers of the iron cycling in deep and hot subseafloor sediments at Site C0023. The presented study contributes to an improved understanding of long-term variations in (bio-)geochemical processes in the deep subseafloor biosphere and provides important aspects for the interpretation of stable iron isotope data