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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.

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.

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.

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.

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.

Increasing interest in high‐tech metals (e.g., rare earth elements and yttrium, REY) has triggered extensive research on the enrichment of these metals in pelagic sediments. In the sediments collected worldwide, Ca‐phosphate and Fe‐Mn (oxyhydr)oxides are targeted as the two most important REY carriers based on multiple geochemical and mineralogical analyses. However, the formation of the REY‐rich stratum remains enigmatic as the influences of diagenesis and the local restrictions of different oceanic basins are complex, which require further investigation. In our study, we analyzed four sediment cores from the Western, Central North, and Eastern South Pacific (WP, CNP, and ESP), which are three of the most promising REY‐rich regions in global oceans. Sequential leaching quantifies the dominance of Ca‐phosphate by controlling ∼73.9%–93.3% of ΣREY and the secondary role of Fe‐Mn (oxyhydr)oxides by holding up to ∼97.8% of Ce and ∼4.82%–13.9% of ΣREY. The shallowly and deeply buried sediments show similar features regarding the REY proportions hosted by different phases, which indicates that the REY accumulation by Ca‐phosphate might majorly occur on the seawater‐sediment interface. The geochemistry of the studied sediment cores reveals that (a) REY enrichment in ESP is affected by hydrothermal Fe‐Mn (oxyhydr)oxides which can promote adsorptions of P and REY. (b) The Fe and Mn contents in WP sediments show an inverse relationship against REY enrichment which might be explained by the dust input from Central Asia. (c) The slow sedimentation rate of CNP enables the extraordinary REY‐uptake by Ca‐phosphate, but the low flux of phosphorus constrains the formation of high‐grade REY deposits. Plain Language Summary Rare earth elements and yttrium (REY) are useful for geological studies and valuable materials for cutting‐edge technologies. Recently, scientists found high REY contents in deep‐sea sediments, which could be future resources. However, its enriching mechanism is not fully understood. Here, we analyze the geochemistry of four sediment cores across the Pacific. The samples were sequentially leached with chemical solutions to separate different mineral phases. The results reveal that Ca‐phosphate (e.g., fish fossils) is the main carrier holding ∼73.9%–93.3% of REY, and Fe‐Mn (oxyhydr)oxides (e.g., small Fe‐Mn nodules) hold ∼4.82%–13.9%. The proportions of REY hosted by the carriers are similar among the layers ranging from the surface to 8‐m depth. This indicates that the REY accumulation mostly happens on the seafloor surface. Additionally, this study reveals that the REY‐rich sediments in the Eastern South Pacific are likely related to underwater hot vents that release more Fe and Mn to accumulate P and REY. In the Western Pacific, the levels of REY seem to be affected by dust coming from Central Asia. In the Central North Pacific with slow sedimentation rates, fish fossils can take up high REY, but their low amount limits the formation of high‐quality REY deposits. Key Points The rare earth elements and yttrium (REY) held by Ca‐phosphate and Fe‐Mn (oxyhydr)oxides in pelagic sediments are quantified by sequential extraction Ca‐phosphate mainly scavenges REY at the seawater‐sediment interface on the seafloor Local constraints on REY‐rich sediments from different pelagic basins relate to their material sources (e.g., eolian dust and hydrothermal vent)

The expansion of oxygen deficient zones (ODZs) within the ocean's interior is anticipated to be a major consequence of anthropogenic climate change, but past changes in ODZs are poorly defined. Recent mapping efforts have revealed plumes of the redox‐active metal cobalt within ODZs, driving a basin‐scale correlation between high cobalt and low O2. Here, we investigate the cobalt flux to Equatorial Pacific sediments along the Line Islands Ridge as a novel record of basin‐scale fluctuations in ODZ extent. After accounting for remobilization by diagenesis, we document a ∼40% increase in cobalt accumulation over the last glacial period, with a more pronounced peak during the Last Glacial Maximum, indicative of larger ODZs compared to the Holocene. Our results link ODZ expansion with colder climates and lend support to model‐based assertions that ongoing deoxygenation may reflect a transient response to warming. Plain Language Summary Climate change is linked to a decline in ocean oxygen levels, impacting fish and other organisms that need oxygen to breathe. Knowledge of past changes in ocean oxygen would help put ongoing deoxygenation trends into context. In this study, we investigated changes in oxygen in the Pacific Ocean over the past 145,000 years. Because low‐oxygen waters are enriched in the metal cobalt, we reconstructed the cobalt abundance of the past oceans as a proxy for oxygen. During the past two Ice Ages, when Earth was colder than today, we find evidence for higher cobalt, and therefore an expansion of oxygen‐poor waters. Key Points The cobalt flux to pelagic sediments reflects heightened sources associated with Oxygen Deficient Zones In Equatorial Pacific sediments, cobalt flux increased by ∼40% during the Last Glacial Period, compared to the Holocene Oxygen Deficient Zones likely expanded during the previous two glacial periods

Past ocean conditions are often reconstructed using the elemental composition of foraminiferal calcite. However, foraminiferal remains are often impacted by post‐depositional contaminants; thus, they require cleaning prior to element/Ca (El/Ca) analysis. To explore the impact of sample pretreatment on foraminiferal El/Ca ratios (Li, Na, Mg, Al, Mn, Fe, Zn, Sr, I, Ba, and U) we performed six cleaning procedures on four foraminifera populations from distinct depositional environments: Two from South Pacific carbonate ooze (ELT25‐11) and two from the hemi‐pelagic sediments of the California Margin (ODP1017E, SPR0901‐04BC). Despite differences in regional oceanography, sample type (i.e., surface‐ or deep‐dwelling planktic, benthic), and cleaning procedure, the main driver of El/Ca variability in the data set is the sedimentary depositional environment, suggesting site‐specific differences in element concentrations and contaminants. This finding challenges the notion that sample cleaning procedures should be informed by the El/Ca of paleoclimate interest, as elements may be found in different contaminants and/or elemental abundances in unique environments. Our data also show that traditional cleaning methods which use a combination of rinsing, sonication, oxidation/reduction, and complexation reactions effectively remove contaminants found on foraminifera in either depositional environment. However, even after contaminant removal, some elements (i.e., U and Fe) remain higher in California Margin foraminifera relative to South Pacific foraminifera. This suggests that the range of acceptable El/Ca values in the literature must be expanded when working with foraminifera from unusual depositional environments (i.e., hemipelagic, siliciclastic sites with high sedimentation regimes) versus values associated with more typical sites (i.e., a pelagic carbonate ooze). Key Points Differences in the depositional environment contribute to the greatest variability in the elemental composition of foraminifera samples The cleaning procedure steps developed for contaminants in pelagic carbonate oozes are effective in dissimilar depositional environments Traditional El/Ca indicators of diagenetic contamination (i.e., Fe, U) may be a primary calcite signal in some depositional environments

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.

The northern Hikurangi subduction margin hosts slow slip events (SSEs), which are exceptionally shallow (<15 km). The sedimentary sequence on the incoming plate is therefore representative of the shallow fault material where the SSEs will take place once they enter the subduction zone. Knowledge about the frictional behavior of these sediments is required to know which lithologies are capable of hosting SSEs, and what mechanisms are causing them. Frictional behavior is material specific and depends on sliding velocity, but it is unknown how these natural sediments behave at plate‐rate velocities. We performed laboratory shearing experiments testing the major lithologies sampled during International Ocean Discovery Program (IODP) Expedition 375, at velocities ranging from the plate convergence rate at the Hikurangi margin (5 cm/year), up to those characteristics of the shallow SSEs (160 and 530 cm/year), under simulated in‐situ as well as standardized conditions. We find that the calcite‐rich pelagic sediments are relatively strong and display the velocity‐weakening frictional behavior required for slip events, whereas other lithologies are weaker and show velocity‐neutral to velocity‐strengthening friction. We observe spontaneous laboratory SSEs in the calcareous pelagic sediments, which show partial locking in between sliding events, consistent with the interpretation of SSEs within the spectrum of slow to fast earthquakes. For the Hikurangi margin, our results suggest that SSE occurrence requires the stronger carbonate‐rich unit to be incorporated into the plate‐boundary fault zone, which we suggest occurs because the rough incoming plate introduces geometrical complexity into the fault zone. Plain Language Summary In the Hikurangi subduction zone, located offshore the east coast of the North Island of New Zealand, the movement between the downgoing and overriding tectonic plates can occur as slow slip events (SSEs). During SSEs the two plates move relative to each other, a process in many aspects similar to ordinary earthquakes, except SSEs take weeks instead of seconds and no ground‐shaking can be felt on the surface. SSEs occur in many subduction zones worldwide, but at the Hikurangi margin they occur shallow, relatively often and close to many mostly land‐based GPS stations that track the movement of the two plates, which makes them ideal to study. A recent research expedition (IODP Expedition 375) drilled the stack of sediments going into the subduction zone to learn more about the circumstances that control where and when SSEs occur. In this paper, we test all the major different sediment types going into the subduction fault zone to find out which one of them is responsible for the SSEs. Using a laboratory device that mimics the subduction zone, we find that sediments containing the mineral calcite cause slip events that are similar to the SSEs observed in the Hikurangi margin. Key Points Spontaneous laboratory slow slip events (SSEs) suggest the shallow Hikurangi SSEs are favored in carbonate‐rich sediments Rate‐and‐state friction and critical stiffness theory can partly explain SSEs, but they can occur under velocity‐strengthening conditions SSE generation in Hikurangi is probably related to heterogeneity in the plate‐boundary fault zone