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The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn Island basalts have been labeled enriched mantle 1 (EM1), characterizing them as one of the isotopic mantle end members. The EM1 origin has been vigorously debated for over 25 years, with interpretations ranging from delaminated subcontinental lithosphere, to recycled lower continental crust, to recycled oceanic crust carrying ancient pelagic sediments, all of which may potentially generate the requisite radiogenic isotopic composition. Here we find that δ26Mg ratios in Pitcairn EM1 basalts are significantly lower than in normal mantle and are the lowest values so far recorded in oceanic basalts. A global survey of Mg isotopic compositions of potentially recycled components shows that marine carbonates constitute the most common and typical reservoir invariably characterized by extremely low δ26Mg values. We therefore infer that the subnormal δ26Mg of the Pitcairn EM1 component originates from subducted marine carbonates. This, combined with previously published evidence showing exceptionally unradiogenic Pb as well as sulfur isotopes affected by mass-independent fractionation, suggests that the Pitcairn EM1 component is most likely derived from late Archean subducted carbonate-bearing sediments. However, the low Ca/Al ratios of Pitcairn lavas are inconsistent with experimental evidence showing high Ca/Al ratios in melts derived from carbonate-bearing mantle sources. We suggest that carbonate–silicate reactions in the late Archean subducted sediments exhausted the carbonates, but the isotopically light magnesium of the carbonate was incorporated in the silicates, which then entered the lower mantle and ultimately became the Pitcairn plume source.

A new amphipod species and genus, Chevreuxiopsisfranki , found in a pelagic sediment trap southwest of Tasmania is described. The new species can be recognized by its unique antenna 2, which consists of a narrow peduncle, and a 4-articulate flagellum, which has a massively developed, article 1, large, posteriorly drawn out articles 2 and 3, and an elongate lanceolate 4 th article. The pereopod 1 basis surrounds large maxillipedal plates. Pereopod 3 to 6 are equipped with subchelate propodus dactylus arrangements. The bases of pereopods 5–7 are narrow.

Nearly half of the global seafloor is overlain by sediment oxygenated to the basement. Yet, despite the availability of oxygen to fuel aerobic respiration, organic carbon persists over million-year timescales. Identifying the controls on organic carbon preservation requires an improved understanding of the composition and distribution of organic carbon within deep oligotrophic marine sediments. Here we show that organic carbon in sediment from the oligotrophic North Atlantic and South Pacific is low (<0.1%), yet stable to depths of 25 m and ages of 24 million years. This organic carbon is not bound in biomass and has a low carbon/nitrogen ratio. X-ray imaging and spectroscopic analyses reveal that the chemical composition of this old, deep organic carbon is dominated (40–60%) by amide and carboxylic carbon with a proteinaceous nature. We posit that organic carbon persists in oxic oligotrophic sediment through a combination of protective processes that involve adsorption to mineral surfaces and physical inaccessibility to the heterotrophic community. We estimate that up to 1.6 × 1019 g of organic carbon are sequestered on million-year timescales in oxic pelagic sediment, which constitutes an important, previously overlooked carbon reservoir.A large reservoir of organic carbon persists in oxic pelagic sediments for millions of years as demonstrated by samples from the North Atlantic and South Pacific. This predominantly proteinaceous carbon persists due to physical protection and adsorption to mineral surfaces.

Molybdenum isotopes (δ98Mo) have been used for more than two decades to investigate past‐ocean redox conditions due to the distinct behavior of this element in oxic and anoxic environments. The oxic sink is generally accepted to have a near‐constant δ98Mo value of −0.7‰, a value derived primarily from the δ98Mo signatures of Fe–Mn crusts and nodules. However, little information is available on the downcore evolution of δ98Mo in oxic pelagic sediments, which constitute the largest fraction of the oxic Mo sink. Here, we present δ98Mo authigenic data for two deep‐sea sites with oxygenated pore waters covering 90 Myr of sedimentation, one from the North Pacific Gyre and another from the South Pacific Gyre. In both records, sedimentary δ98Moauth values systematically increase downcore from −0.5‰ at the sediment surface to a maximum of ∼+0.3‰. This shift cannot be explained by changes in the ocean's redox state but is likely related to the diagenetic conversion of Mn‐oxides into different mineral phases. Our findings indicate that the δ98Mo signal generated by adsorption at the sediment‐water interface is not preserved during burial, suggesting that the canonical −0.7‰ value of the oxic sink requires re‐evaluation. In the lower section of the South Pacific Site, we also identify two positive excursions reaching δ98Moauth values as high as +1.7‰. This portion of the site was influenced by near‐field hydrothermal processes, and we propose that these large excursions reflect Mo adsorption onto Fe (oxyhydr)oxides.

Over the vast area of present‐day Europe, the Tithonian–Berriasian transition was a time of climate aridization, which was supposedly related to the more general trend of the latest Jurassic–earliest Cretaceous cooling and restrictions in atmospheric circulation. Recent studies suggest that such conditions affected also some other paleoenvironmental processes such as monsoonal upwellings, seafloor ventilation and circulation of nutrients within the water column. In order to test this model, the uppermost Jurassic–lowermost Cretaceous sedimentary succession of the Slovenian Basin was correlated with a reference data from the Bakony Basin (Transdanubian Range, Hungary). Stratigraphic calibration was ensured by integrated stratigraphy, utilizing bio‐ (calpionellids, calcareous dinocysts) and chemostratigraphic tools (δ13C stratigraphy) as well as regional correlations of magnetic susceptibility and terrigenous input. Paleoclimate, paleoredox and paleoproductivity conditions were evaluated based on various geochemical proxies. Both the Slovenian and the Bakony basin sections were found to document late Tithonian–early Berriasian climate aridization as well as related signals of seafloor hypoxia and elevated accumulations of micronutrients. Significant geochemical contrast between the basal (lower Tithonian) radiolarites and overlying upper Tithonian–Berriasian carbonates evidences the inverse relation between the surface productivity and the amount of nutrient‐type trace metals buried in sediments. The rhythm of paleoclimatically controlled environmental changes, with relatively humid early Tithonian, arid late Tithonian–early Berriasian, and again humid late Berriasian, correlates with those estimated for Vocontian Basin (SE France) and the Sub‐Boreal domain of Western and Central Europe. This indicates that climatic stratigraphy is a useful tool for global correlation of the Jurassic/Cretaceous boundary interval. Plain Language Summary During the Mesozoic, much of the present‐day Alpine orogenic belt (Alps–Carpathians–Dinarides) constituted the westernmost part of the so‐called Neotethys Ocean. Recent studies suggest that the gradual cooling during the Jurassic/Cretaceous transition affected this area by weakening of monsoons, what caused significant aridization. In addition, changes in atmospheric circulation are thought to have had an impact also on oceanographic conditions and the intensity of (wind‐induced) upwelling currents among others. In this study, we attempt to test the above model by comparing the geochemical signals obtained from sedimentary successions of western Slovenia (Slovenian Basin) and western Hungary (Transdanubian Range). Our study confirms the hypothesis that late Tithonian–early Berriasian (ca. 145–140 Ma) climate aridization was associated with less intense mixing of the water column. This, in turn, limited the availability of oxygen at the seafloor on one hand, and increased the burial of nutrients on the other. Accordingly, the results of this study not only provide new data on the latest Jurassic–earliest Cretaceous sedimentary trends and events but also contribute to our understanding of a complex relationships between the state of atmosphere (climate) and its impact on marine environments and support the global definition of the Jurassic/Cretaceous boundary. Key Points Geochemical contrast between radiolarites and limestones evidences the inverse relation between surface productivity and nutrient burial Arid climate of the Tithonian–Berriasian transition was associated with seafloor hypoxia and elevated accumulations of micronutrients A minor, but well pronounced peak in δ13C is characteristic for the lower/upper Berriasian boundary interval

Global weathering is the primary control of the Earth's climate over geologic timescales, converting atmospheric pCO2 into dissolved bicarbonate, with carbon sequestration by marine plankton as carbonate and organic carbon on the ocean floor. The accumulation rate of pelagic marine biogenic sediments is thus an indication of weathering history. Previous studies of Cenozoic pelagic sedimentation have yielded contrasting results, though most show a dramatic rise (up to 6 times) in rates over the Cenozoic. This contrasts with model expectations for approximate steady state in weathering, pCO2, and sequestration over time. Here we show that the Cenozoic record of sedimentation recovered by deep-sea drilling has a strong, systematic bias towards lower rates of sedimentation with increasing age. When this bias is removed, accumulation rates are shown to actually decline by ca. 2 times over the Cenozoic. However, when accumulation area is adjusted for changes in available deposition area, global sediment flux to the deep sea is shown to have nearly doubled at the Eocene–Oligocene boundary but was otherwise essentially constant. Compilations of other metrics correlated to sedimentation rate (e.g. productivity, biotic composition) also must have a strong age bias, which will need to be considered in future paleoceanographic studies.

The Eurasian Basin in the Arctic Ocean, comprising the Amundsen and Nansen Basins separated by the Gakkel Ridge, has sediment deposits up to 4–5 km thick. However, its sedimentation history and processes remain poorly understood. Using 31 seismic profiles, we have estimated deposition rates for 54 Ma. From 54 to 45 Ma, the Nansen Basin averaged ∼15 cm/kyr, while the Amundsen Basin exhibited higher but variable rates (15–50 cm/kyr). From 45 to 20 Ma, the Amundsen Basin's rates decreased significantly, dropping to 6–7 cm/kyr (34–45 Ma) and ∼3.5 cm/kyr (20–34 Ma). Meanwhile, the Nansen Basin maintained higher rates (∼12 cm/kyr to ∼5 cm/kyr). After 20 Ma, sedimentation rates diverged further. The Nansen Basin stabilized at ∼5 cm/kyr and was significantly influenced by glaciation and iceberg rafting, while the Amundsen Basin continued to decline to ∼2 cm/kyr, with pelagic sediments dominated by sea‐ice and iceberg rafting, and debris flows near the Lomonosov Ridge. The Nansen Basin's higher rates are likely due to its proximity to the Barents and Laptev Sea shelves, while the general declined rates across the basin are related to basin expansion, climate cooling, and reduced tectonic activity. Additionally, the Eurasian Basin's sedimentation is shaped by two phases of Siberian river activity. Before 45 Ma, the Lena and Indigirka rivers dominated, particularly near the eastern Laptev Sea Shelf. After 45 Ma, the Pyasina and Yenisey rivers became the main contributors, with significant sediment delivery through the St. Anna Trough. Sediment deposits (0.6–1 km) along the Gakkel Ridge (70°E−100°E) are also caused by these processes.

Six diamond-bearing eclogite xenoliths with oceanic crust protoliths and 370 mineral inclusions in 104 diamonds recovered from the Koidu kimberlite complex in Sierra Leone provide insight into the lithological and compositional diversity of the lithospheric mantle beneath the West African Craton. Diamond formation beneath Koidu is predominantly associated with eclogitic substrates that originated from subduction and high-pressure metamorphism of oceanic crust, as indicated by a dominance of eclogitic (78%) over peridotitic (17%) and mixed paragenesis diamonds (5%). Peridotitic diamonds contain olivine inclusions with very high Mg# (92.2–94.7; median = 94.2), indicative of derivation from dunite or harzburgite protoliths. Moreover, a peridotitic spinel with Cr# = 50.9 suggests that it equilibrated with orthopyroxene-free dunite. 44% of Koidu diamonds contain coesite, of which some coexist with omphacite, eclogitic garnet, and/or kyanite. Most analysed eclogitic garnet inclusions have extremely high δ 18 O values ( ≥ + 9.9‰) and occur with clinopyroxene inclusions that have very high jadeite components (~ 70 mol%). These high jadeite components are a close match to clinopyroxenes in high-pressure metapelites, which have a phase assemblage that includes coesite and kyanite. Our data suggest that the eclogitic mineral inclusions in most Koidu diamonds have oceanic basalt protoliths that were mingled with pelagic sediments, which may have increased δ 18 O values to levels much higher than observed for other eclogites at Koidu and shifted the originally basaltic bulk compositions closer to that of pelites. Most eclogitic mineral inclusions in Koidu diamonds have elemental compositions not observed for Koidu eclogite xenoliths, which have clear oceanic crust protolith (oceanic lavas and cumulates) signatures without significant crustal sediment contamination. These findings suggest the subduction of distinct packages of oceanic crust into the Koidu lithospheric mantle through time.

Nitrogen (N) dominates Earth's atmosphere (78% N2) but occurs in trace abundances in silicate minerals, making it a sensitive tracer of recycled surface materials into the mantle. The mechanisms controlling N transfer between terrestrial reservoirs remain uncertain because low N abundances in mineral‐hosted fluid inclusions (FIs) are difficult to measure. Using new techniques, we analyzed N and He isotope compositions and abundances in olivine‐ and pyroxene‐hosted FIs from arc volcanoes in Southern Chile, Cascadia, Central America, and the Southern Marianas. These measurements enable an estimate of the global flux of N outgassing from arcs (4.0 × 1010 mol/yr). This suggests that Earth is currently in a state of net N ingassing, with roughly half of subducted N returned to the mantle. Additionally, the N outgassing flux of individual arcs correlates with the thickness of subducting pelagic sediment, suggesting that N cycling in the modern solid Earth is largely controlled by sediment subduction. Plain Language Summary Nitrogen (N) largely behaves like an inert gas, and so it is substantially more concentrated at Earth's surface than in Earth's deep interior. Over geologic time, N can be transported between the solid Earth and the surface, and its concentration can change in both of these settings. Volcanic gases transport N from the interior to the surface, while some surface N returns into the solid Earth via plate subduction. Here, we present measurements of N and helium (He) gas trapped within crystals in volcanic rocks to determine how much N is transported to the surface through volcanism associated with plate subduction. We find that the amount of N returning to the surface through volcanism is less than estimates of how much N is transported into the solid Earth, suggesting that, overall, N is being returned to the planet's deep interior. Additionally, we observe that the amount of oceanic sediment that is subducted correlates with the amount of N that comes out of volcanoes, making it the primary carrier of N into the solid Earth. Key Points Arc lavas yield fluxes of 4.0 × 1010 mol N/yr, similar to estimates from volcanic arc gases, likely resulting in net mantle ingassing of N Nitrogen isotopes and N‐He mixing models highlight that small contributions of sediment dominate volcanic arc N budgets Subducted sediment thickness correlates with N2/3He ratios, and likely controls arc N fluxes rather than slab parameters or thermal state

Molybdenum isotopes serve as critical proxies for reconstructing ancient ocean oxygenation, yet the modern oceanic Mo isotopic budget remains incompletely understood. Deep-sea pelagic sediments enriched in Fe-Mn (hydro)oxides represent a major oxic sink, but their authigenic Mo isotopic composition is poorly constrained. Here, we show Mo isotope data from Pacific deep-sea sediment cores revealing systematic depth-dependent δ 98 Mo enrichment from ‒0.55‰ to 0.19‰, controlled by Fe-Mn cycling during early diagenesis. Combined with existing datasets, we calculate a revised authigenic oxic Mo flux of 1.52 × 10⁸ mol yr⁻¹ with δ 98 Mo = ‒0.09 ± 0.23‰—more than double previous estimates and ~0.6‰ heavier than Fe-Mn crusts. These findings necessitate recalibration of the global Mo isotope budget and demonstrate that pelagic sediments exert greater influence on oceanic Mo cycling than previously recognized with implications for quantitative paleoceanographic reconstructions. Deep-sea sediments are a key sink for molybdenum (Mo). Here it is found that the their isotopic composition is heavier than typical endmembers; refining the global Mo budget, and improving reconstructions of past ocean oxygen levels.