Description: The manganese nodule belt within the Clarion and Clipperton Fracture Zones (CCZ) in the abyssal NE Pacific Ocean is characterized by numerous seamounts, low organic matter (OM) depositional fluxes and meter-scale oxygen penetration depths (OPD) into the sediment. The region hosts contract areas for the exploration of polymetallic nodules and Areas of Particular Environmental Interest (APEI) as protected areas. In order to assess the impact of potential mining on these deep-sea sediments and ecosystems, a thorough determination of the natural spatial variability of depositional and geochemical conditions as well as biogeochemical processes and element fluxes in the different exploration areas is required. Here, we present a comparative study on (1) sedimentation rates and bioturbation depths, (2) redox zonation of the sediments and element fluxes as well as (3) rates and pathways of biogeochemical reactions at six sites in the eastern CCZ. The sites are located in four European contract areas and in the APEI3. Our results demonstrate that the natural spatial variability of depositional and (bio)geochemical conditions in this deep-sea sedimentary environment is much larger than previously thought. We found that the OPD varies between 1 and 4.5 m, while the sediments at two sites are oxic throughout the sampled interval (7.5 m depth). Below the OPD, manganese and nitrate reduction occur concurrently in the suboxic zone with pore-water Mn2+ concentrations of up to 25 µM. The thickness of the suboxic zone extends over depth intervals of less than 3 m to more than 8 m. Our data and the applied transport-reaction model suggest that the extension of the oxic and suboxic zones is ultimately determined by the (1) low flux of particulate organic carbon (POC) of 1–2 mg Corg m−2 d−1 to the seafloor, (2) low sedimentation rates between 0.2 and 1.15 cm kyr−1 and (3) oxidation of pore-water Mn2+ at depth. The diagenetic model reveals that aerobic respiration is the main biogeochemical process driving OM degradation. Due to very low POC fluxes of 1 mg Corg m−2 d−1 to the seafloor at the site investigated in the protected APEI3 area, respiration rates are twofold lower than at the other study sites. Thus, the APEI3 site does not represent the (bio)geochemical conditions that prevail in the other investigated sites located in the European contract areas. Lateral variations in surface water productivity are generally reflected in the POC fluxes to the seafloor across the various areas but deviate from this trend at two of the study sites. We suggest that the observed spatial variations in depositional and (bio)geochemical conditions result from differences in the degree of degradation of OM in the water column and heterogeneous sedimentation patterns caused by the interaction of bottom water currents with seafloor topography.

Description: High-resolution sedimentary records of major and minor elements (Al, Ba, Ca, Sr, Ti), total organic carbon (TOC), and profiles of pore water constituents (View the MathML sourceSO42-, CH4, Ca2+, Ba2+, Mg2+, alkalinity) were obtained for two gravity cores (core 755, 501 m water depth and core 214, 1686 m water depth) from the northwestern Black Sea. The records were examined in order to gain insight into the cycling of Ba in anoxic marine sediments characterized by a shallow sulfate–methane transition (SMT) as well as the applicability of barite as a primary productivity proxy in such a setting. The Ba records are strongly overprinted by diagenetic barite (BaSO4) precipitation and remobilization; authigenic Ba enrichments were found at both sites at and slightly above the current SMT. Transport reaction modeling was applied to simulate the migration of the SMT during the changing geochemical conditions after the Holocene seawater intrusion into the Black Sea. Based on this, sediment intervals affected by diagenetic Ba redistribution were identified. Results reveal that the intense overprint of Ba and Baxs (Ba excess above detrital average) strongly limits its correlation to primary productivity. These findings have implications for other modern and ancient anoxic basins, such as sections covering the Oceanic Anoxic Events which Ba is frequently used as a primary productivity indicator. Our study also demonstrates the limitations concerning the use of Baxs as a tracer for downward migrations of the SMT: due to high sedimentation rates at the investigated sites, diagenetic barite fronts are buried below the SMT within a relatively short period. Thus, ‘relict’ barite fronts would only be preserved for a few thousands of years, if at all.

Description: Sulfate reduction is a globally important yet poorly quantified redox process in marine sediments. We developed an artificial neural network trained with 199 sulfate profiles, constrained with geomorphological and geochemical maps to estimate global sulfate reduction rate distributions. Globally, 11.3 Tmol sulfate are reduced yearly, ~15% of previous estimates, accounting for the oxidation of 12-29% of the organic carbon flux to the sea floor. Combined with global cell distributions in marine sediments, these results indicate a strong contrast in sub–sea-floor prokaryote habitats: in continental margins global cell numbers in sulfate-depleted sediment exceed those in the overlying sulfate-bearing sediment by an order of magnitude, whereas in the abyss most life occurs in oxic and/or sulfate-reducing sediments.

Description: Subglaziale Seen sind unter dem Antarktischen Eisschild weit verbreitet. Sie bilden eine Quelle von subglazialem Schmelzwasser und modulieren weitgehend die Fließgeschwindigkeiten des überlagernden Eisstromes. Der Ausbruch von subglazialem Schmelzwasser am Rande des Eisschildes kann die globale ozeanographische Zirkulation, den Meeresspiegelanstieg und geochemische Zyklen beeinflussen. Trotz ihrer Bedeutung sind subglaziale Seen eine der am wenigsten erforschten Umgebungen auf unserem Planeten. Ihre Auswirkungen auf die Dynamik der Eisbedeckung und ihr Potential primitives aber autarkes Leben zu beherbergen, sind weitgehend schlecht charakterisiert. Wir präsentieren den ersten direkten Beweis für einen paläo-subglazialen See auf dem antarktischen Kontinentalschelf. Sedimentablagerungen beweisen, dass subglaziales Schmelzwasser während oder kurz nach der letzten Eiszeit unter dem Antarktischen Eisschild vorhanden war und erst bei der Enteisung entleert wurde. Im Pine Island Bay (Amundsen Sea Embayment, ASE) haben wir zum ersten Mal in der Antarktis mehr als drei Meter dieser typischen Sedimentfazies, die auf wenig dynamische Ablagerungsbedingungen in einem subglazialen See schließen lassen, beprobt. Niedrige Chloridkonzentrationen im Porenwasser des Sedimentkernes und diffus-advektive Modellierung der zeitlichen Entwicklung dieser Chloridkonzentrationen im Porenwasser zeigen eindeutig die Genese der Sedimente in einem subglazialen, schmelzwassergefüllten See. Basierend auf bathymetrischen Daten können wir zeigen, dass die Lage des subglazialen Sees mit einer prognostizierten Verteilung von subglazialen Seen in diesem Gebiet übereinstimmt. Dieser Befund ermöglicht Modellierungsstudien der Eisdynamik bei bekannter Geometrie des Eisuntergrundes, des Volumens der subglazialen Seen, der Eigenschaften des unterlagernden Substrates und der Form des Eisstroms. Dies ist besonders wichtig für den Pine Island Gletscher, der als "Weak Underbelly" des Westantarktischen Eisschildes (WAIS) gilt. Bisher wurde nur die Eis/Wasser Grenzfläche im subglazialen Wostoksee und wenige Zentimeter diamiktischen Sedimentes vom Whillans-See beprobt. Ansonsten wurde das antarktische subglaziale hydrologische System durch Fernerkundung und numerische Modelle rekonstruiert und die Anzahl der potenziellen subglazialen Seen auf mehr als 12000 geschätzt. Unsere Studie liefert nicht nur den ersten Nachweis für eiszeitliche subglaziale Seen und Beprobung dieser Sedimente in der Antarktis, sondern auch einen Rahmen für weitere Untersuchungen dieser einzigartigen subglazialen See-Umgebung in einer kostengünstigen Variante durch Schiffsexpeditionen auf dem saisonal eisfreien inneren Kontinentalschelf.