The nitrogen isotopic composition of diatom frustule‐bound organic matter (δ 15 N DB ) is often used to study changes in high latitude biological pump efficiency across glacial‐interglacial cycles, but the proxy may be biased by species‐specific effects. The genus Chaetoceros is of particular interest because of its abundance throughout ocean basins, its shifting biogeography during glacial periods, and the ability of many species to form heavily silicified resting spores. Here we investigate how Chaetoceros resting spores (CRS) record surface nitrate conditions in their nitrogen isotopic composition, and thus impact δ 15 N DB records, using assemblage‐specific sedimentary δ 15 N DB measurements and laboratory culture experiments. We find that fossil CRS from ODP Site 1098 record δ 15 N DB values 1.1–7.8‰ lower than non‐CRS diatoms in sediment. CRS grown in culture yield consistent results, recording δ 15 N DB values 2.6–8.2‰ lower than vegetative Chaetoceros in the same cultures. Low values are attributed to assimilation of isotopically light ammonium, heavy silicification, and/or internal nitrogen allocation processes during sporulation. Applying these findings to published δ 15 N DB records, variable CRS relative abundance in open ocean glacial sediments does not significantly bias δ 15 N DB records across glacial‐interglacial cycles, despite the large δ 15 N DB difference observed in CRS versus non‐CRS diatoms, due to the spores' small size.

Although valuable information of North Atlantic circulation paleo-reconstructions by the measurement of oxygen isotopes of benthic and planktonic foraminifera exists, it is still not well-understood how deep-water currents changed over the last ~800,000 years. Moreover, recent studies have shown that some species of microfossils can adapt to low oxygen concentrations, which consequently can impact the reliability of the paleo-reconstructions that are based on these fossils. Marine sediments off the Portuguese Margin have been shown to play a pivotal role in paleoclimate research, and studies have suggested that climate shifts at Mediterranean latitudes are interconnected to changes in deep-water circulation patterns. Changes in bottom-water oxygenation (ventilation) can provide information about changes in deep-water circulation patterns, which can be measured by the enrichment versus depletion of redox-sensitive trace metals. Here we provide the results of a low-resolution geochemical analysis of redox-sensitive trace metals (for example, molybdenum (Mo), vanadium (V), and uranium (U)) to investigate deep-water ventilation changes in the North Atlantic over the last ~800,000 years at Hole U1586A drilled during IODP Expedition 397. Sediment samples underwent a multi-acid digestion technique and were analyzed via an Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for redox-sensitive trace metals. Additionally, sequential iron (Fe) extractions were carried out to differentiate between labile versus mineral Fe phases. Preliminary results suggest minor changes in deep-water ventilation that correspond to glacial-interglacial cycles since the mid-Pleistocene. Future work will involve high-resolution geochemical analyses to better understand the interconnection of deep-water circulation and climate change.

Microbial processes in marine sediments drive changes in redox conditions, ultimately controlling the cycling of elements between the dissolved and solid phases. The microbial community driving these cycles depends on trace metals, but it can also be inhibited at elevated metal concentrations. During diagenesis, many trace elements are released from iron (Fe) and manganese (Mn) (oxyhydr)oxides, potentially affecting microbial metabolisms. Here we present results from geochemical and microbiological analyses of samples collected during R/V Polarstern Expedition PS119 to the East Scotia Ridge. The sediments are dominantly diatomaceous ooze with high contents of reactive Fe and Mn (oxyhydr)oxides and increased trace metal contents from nearby hydrothermal vents. Two multi-corer cores were sampled immediately after collection at five specific sediment depths (three splits each), sealed anaerobically in incubation bags, and analyzed in 4-month intervals post collection for major, minor, and trace metals and 16S rRNA gene sequencing. By isolating the sediment from overlying seawater during the incubation process, we simulated the in situ diagenetic processes of Fe and Mn oxide reduction. Our data show that Mn and trace metals, especially Mo, Ni, Tl, and Cu, are mobilized during early diagenesis. Analysis of 16S rRNA genes revealed shifts in the microbial community from Nitrososphaera and Nanoarchaeia to Bacteroidia and Bacilli alongside a marked decrease in richness, Pielou's evenness, and Shannon alpha diversity during the eight-month incubations. We statistically correlate the microbial community shift with the changes in porewater trace metal concentrations, revealing that Mn, Co, Ag, and Tl are driving the microbial compositions in these samples. In this organic matter limited but Fe and Mn (oxyhydr)oxide rich system, we simulate deeper diagenesis to peer into the role of changing Fe, Mn, and trace metal cycles and highlight the role of Fe and Mn (oxyhdyr)oxides as shuttles for trace metals to the deep biosphere. By identifying key metals that are diagenetically cycled and affect the in situ microbial community, we reveal feedbacks between metals and microbial communities that play important roles in biogeochemical cycles on Earth, provide insight into the origin and potential evolution of metabolic pathways in the deep biosphere, and offer clues that may aid in our understanding of Earth's history and potentially beyond.

The deep oceans are environments of complex carbon dynamics that have the potential to significantly impact the global carbon cycle. However, the role of hadal zones, particularly hadal trenches (water depth > 6 km), in the oceanic dissolved organic carbon (DOC) cycle is not thoroughly investigated. Here we report distinct DOC signatures in the Japan Trench bottom water. We find that up to 34

Hadal trenches (>6000 m water depth) have been revealed as hotspots of organic carbon burial and microbial respiration in the ultradeep ocean environment. However, understanding of the anaerobic metabolic pathways and rates, as well as carbon-silicon cycling, in the hadal trench sediments remains very fragmentary because of the shallow nature of traditional coring penetration. Using materials collected during International Ocean Discovery Program (IODP) Expedition 386 in the Japan Trench and a reaction-transport model, we provide a regional quantitative assessment of organic carbon turnover by sulfate reduction, anaerobic oxidation of methane, and methanogenesis in addition to silicate weathering and authigenic carbonate and clay formation. We show that rapid burial of relatively labile organic carbon resulting from subduction earthquakes triggers organic carbon and methane turnover at rates comparable to those in continental margin sediments, thereby stimulating active silicate weathering and authigenic carbonate formation. Despite vigorous organic carbon turnover, the vast majority of organic carbon is buried, implying an important role of tectonic-associated events in translocating and preserving organic carbon in the deepest part of the ocean. These results quantitatively demonstrate, for the first time, active coupled carbon-silicon cycling in hadal trench sediments and have implications for the subduction zone carbon budget.

The early diagenetic interplay between reactive iron, sulfur, and organic matter in the bathymetrically isolated Santa Monica Basin (SMB) sediments are investigated in this study. We explore solid-phase and porewater profiles from the basin, supplemented with a transect from 71 to 907 m water depth that includes oxygenated (>60 lM O2) bottom waters near the coast and oxygen-deficient waters (-4 lM O2) in the basin. The geochemical data of the basin sediments are further scrutinized by means of reactive transport modeling. The results show that the basin sediments do not follow the traditional geochemical signatures of oxygen-deficient settings. A lack of dissolved sulfide accumulation and sulfurized iron persists despite the sediments being deposited under reducing conditions (without bioturbation/bioirrigation), strong organic carbon input (TOC up to 5.0 wt%), and active dissimilatory sulfate reduction. Not only did we find an exceptional enrichment in highly reactive Fe in the surface sediments (-45 % of total Fe), but the enrichment of reactive Fe, including ferrihydrite, persists downcore and coexists with high levels of dis-solved Fe. The enhanced preservation of Fe oxides and lack of iron-sulfide precipitation is in part explained by detection via Mossbauer spectra of iron oxides bounded to organic matter (Fe[III]-OM coprecipitates). The modeled Fe budget shows that most of the Fe oxides in the surface sediments are internally recy-cled by upward diffusion and subsequent oxidation of Fe2+. Sulfide oxidation coupled to Fe reduction effectively precludes sulfide accumulation while enhancing build-up of dissolved Fe, fueling the Fe cycle within the first 5 cm depth. Continuous reoxidation of Fe2+ enhances the formation of Fe(III)-OM copre-cipitates, limiting the amount of reactive organic matter. In the unavailability of labile organic matter, other than within the uppermost layers, the organic-rich sediment profiles are dominated by Fe cycling that limits the production and preservation of sulfides and enhances the preservation of Fe oxides and organic carbon. This study highlights key local controls on Fe availability in marginal basins and describes an intricate biogeochemical C-Fe-S cycling in modern and possibly ancient marine systems with impor-tant implications for Fe availability in the marine realm. (c) 2022 Elsevier Ltd. All rights reserved.

Hadal trenches are unique geological and ecological systems located along subduction zones. Earthquake-triggered turbidites act as efficient transport pathways of organic carbon (OC), yet remineralization and transformation of OC in these systems are not comprehensively understood. Here we measure concentrations and stable- and radiocarbon isotope signatures of dissolved organic and inorganic carbon (DOC, DIC) in the subsurface sediment interstitial water along the Japan Trench axis collected during the IODP Expedition 386. We find accumulation and aging of DOC and DIC in the subsurface sediments, which we interpret as enhanced production of labile dissolved carbon owing to earthquake-triggered turbidites, which supports intensive microbial methanogenesis in the trench sediments. The residual dissolved carbon accumulates in deep subsurface sediments and may continue to fuel the deep biosphere. Tectonic events can therefore enhance carbon accumulation and stimulate carbon transformation in plate convergent trench systems, which may accelerate carbon export into the subduction zones.

This is the source data file of the Nature Communications manuscript 'Earthquake-enhanced dissolved carbon cycles in ultra-deep ocean sediments'.

3. Generate results for comparison with those from all other Expedition 386 sites to explore spatiotemporal distribution of event deposits and the southern extent of sediment transport routed through the Ogawara submarine canyon to eventually develop a long-term record for giant earthquakes. Operations summaryBecause of very strong winds (>15 m/s) on 25 April 2021 at Site M0089, the decision was made to move the R/V Kaimei to Site M0084 in the northern Japan Trench focus area (Sites M0084 and M0085).Kaimei reached the site at 1430 h and multibeam echo sounder/subbottom profiler (MBES/SBP) surveys were carried out at Site M0084 from 1530 to 1645 h.Winds remained strong (>18 m/s) on the morning of 26 April, with wave heights of 3 m and 1.3 kt currents; the decision was made to delay GPC deployment until the winds dropped to <15 m/s.At 1200 h, the captain confirmed it was possible to maintain position under manual mode, and the go ahead was given to deploy the GPC.GPC operations started in Holes M0084A and M0084B with a 20 m GPC barrel string at 1230 h and were completed when the GPC system was recovered on deck at 1840 h.The deck crew and GPC operation team withdrew the core from the GPC assembly and cut it into 5 m segments.The Science Party cut the core into 1 m sections from 1900 to 2100 h.The GPC assembly was prepared for the next run at 2300 h.At 0000 h on 27 April, Kaimei began drifting to Site M0085, standing by at the site at 0200 h.Conditions were cold but calm (winds 8 m/s; wave height 1 m; <1.5 kt current).GPC preparations for Holes M0085A and M0085B with a 20 m barrel string began at 0600 h, and the GPC was recovered on deck at 1500 h.The deck crew and GPC operation team withdrew the core from the GPC assembly and cut it into 5 m segments.The Science Party cut the core into 1 m sections.Cutting was completed at 1610 h, and the GPC was made up for the next run at 2115 h.An MBES/SBP survey around Sites M0084 and M0085 commenced at 2115 h on 27 April and continued until 0700 h on 28 April.Conditions were warmer and calm (winds <3 m/s; <1.3 kt current; wave height <1 m).GPC operations started in Holes M0084C and M0084D with a 40 m GPC barrel string at 0700 h, and the GPC system was recovered on deck at 1500 h.The deck crew and GPC operation team withdrew core from the liner, and the Science Party sampled from section bottoms and cut core into 1 m sections from 1600 to 1930 h.GPC make up and preparation with a 40 m barrel was complete at 2130 h on 28 April, and MBES/SBP surveying around Sites M0084 and M0085 resumed at this time.MBES/SBP surveying around Sites M0084 and M0085 continued until 0445 h on 29 April and was suspended for 40 m GPC operations in Holes M0085C and M0085D.Conditions were overcast but calm (winds <5 m/s; <1.2 kt current; wave height <1 m).GPC operations started in Holes M0085C and M0085D with a 40 m core barrel string at 0600 h, and the GPC was recovered on deck at 1315 h.The GPC core bit was removed at 1400 h and the GPC was secured, but a rough weather forecast meant that withdrawing core was delayed.The ship began transit to a standby point off Miyako at 1400 h and arrived at 2300 h.Core from Holes M0085C and M0085D was withdrawn, and cores were cut into 5 and 1 m sections from 1130 to 1600 h on 30 April.The 40 m GPC assembly was made up at 2300 h, when the ship started the transit to Site M0083 (see Operations in the Sites M0083 and M0089 (Basin C2) chapter [Strasser et al., 2023d]).Kaimei returned to Site M0084 on 12 May at 0445 h.GPC operations started in Holes M0084E and M0084F using the 40 m core barrel string at 0800 h, and the GPC was recovered on deck at 1415 h.Conditions were calm (winds <4 m/s; wave height ~0.2 m) with a relatively strong current (~1.8 kt).The deck crew and GPC operation team withdrew the core from the liner, and the Science Party sampled from the section bottoms and cut the core into 1 m sections from 1430 to 1900 h.GPC make up and preparation with the 40 m barrel was complete at 2100 h on 12 May, and MBES/SBP surveying around Site M0084 commenced at 2100 h and continued until 0215 h on 13 May.