Woods Hole Oceanographic Institution

Environmental DNA (eDNA) metabarcoding can rapidly characterise biodiversity, yet its accuracy and effectiveness are limited by incomplete DNA barcode reference databases. We evaluated how comprehensive reference databases that include sequence variation within genomes (intragenomic) and across individuals and species (intergenomic) improve eDNA-based biodiversity assessments. We collected coral tissue and water samples at deep sites offshore Puerto Rico for reference barcoding and eDNA metabarcoding. Genome skimming coral specimens yielded 28S barcodes for 314 of 346 samples (90.8%) and revealed divergent intragenomic 28S lineages in multiple octocoral families. Incorporating local reference barcodes substantially changed ASV taxonomic classifications: 22 ASVs (8.9%) gained genus-level resolution, 19 ASVs (7.7%) were reassigned to different genera, and 14 ASVs (5.7%) lost incorrect genus-level classifications. Thus, incomplete reference databases produce not only unclassified ASVs but also false positive detections and ecologically meaningful misclassifications. When intragenomic 28S lineages were excluded from the reference database, 18 ASVs (7.4%) could not be classified to family or genus, demonstrating that unrecognised intragenomic variation can be mistaken for unsampled taxa. Integrating reference genome skimming and eDNA metabarcoding expanded known coral family richness by 36% at depths shallower than 1000 m and by 181% at depths greater than 1000 m. eDNA also detected two coral families previously unknown off Puerto Rico and nearby islands, underscoring its potential for biodiversity discovery.

The marine labile dissolved organic carbon (DOC) pool is a dynamic reservoir of thousands of molecules that cycles approximately one-quarter of Earth's primary production within days to weeks. After excretion by phytoplankton and other microbes, metabolites are rapidly consumed, resulting in low standing concentrations (picomolar to low nanomolar). Despite the decades-long search for labile DOC sources and molecular identities, marine phytoplankton exometabolomes are not well characterized, largely due to difficulties in measuring small polar molecules in saline water. Here, we profiled the exometabolomes of six axenic phytoplankton species representing key functional groups including a diatom (Thalassiosira pseudonana CCMP1335), a picoeukaryote (Micromonas commoda RCC299), a coccolithophore (Gephyrocapsa huxleyi CCMP371), a diazotrophic cyanobacterium (Crocosphaera watsonii WH8501), and two picocyanobacteria (Prochlorococcus marinus MIT9301 and Synechococcus WH8102). From these cultures, we quantified 56 amine- and alcohol-containing exometabolites representing 11 compound classes which in sum comprised up to 23.4% of phytoplankton-excreted DOC. We estimated that these phytoplankton-derived exometabolites could supply up to 5% of the daily carbon quota of the dominant heterotrophic bacterium SAR11 in the surface ocean. Substantial variations in exometabolite identity and concentration across phytoplankton taxa underscore taxonomic diversity as a key driver in the supply and composition of labile DOC. This taxonomic variation predicts geographic and seasonal differences in the distribution of marine dissolved metabolites that underpin the cycling of labile DOC back to carbon dioxide (CO2). Overall, our work suggests that phytoplankton exometabolites are key chemical currencies that mediate significant carbon fluxes within the ocean's carbon cycle.

Abstract Neodymium (Nd) isotopes ratios are regarded as a key proxy for past ocean circulation despite unsettled debate on marine Nd cycle controls, which opposes a “top‐down” hypothesis (surface sources dominate) and a “bottom‐up” hypothesis (sedimentary sources dominate), each with important implications for Nd isotope proxy interpretation. We suggest a new, basin‐specific framework for interpreting Nd isotope records. Specifically, while Nd isotope records are reliable water‐mass‐mixing proxies in the Atlantic, they are likely better used as proxies of residence time in the Pacific. Here, we test these hypotheses using a computationally efficient, data‐constrained model of the marine Nd cycle. Our results show Atlantic Nd isotopes are dominated by surface sources, while Pacific Nd isotope ratios are strongly influenced by sedimentary sources. We find that shallow, reactive sediments—especially those with extremely radiogenic or unradiogenic isotopic compositions—are crucial to reproduce observed εNd profiles in the Pacific.

Abstract Saltwater is migrating into freshwater aquifers globally with water quality and biogeochemical implications, yet saltwater intrusion in glaciated regions is sparsely investigated. Field observations suggest that groundwater head in glaciated systems is influenced by ice sheet forcings and provides evidence that seawater infiltrated into offshore aquifers during past deglaciation events. To understand links between ice sheet dynamics, groundwater head, and saltwater intrusion, we use numerical models to explore the effects of deglaciation on nearshore head and salinity distributions. We find that ice sheet thinning diminishes groundwater head, and the resulting shift in subsurface pressure gradients drives rapid landward movement of the subsurface freshwater‐saltwater interface up to 4.0 km or 1.3 m per m ice sheet loss. Results highlight an overlooked saltwater intrusion mechanism that aligns with field observations and affects glaciated coastlines undergoing ice sheet retreat, underscoring the need to consider this mechanism in studies of contemporary coastal water quality.

It remains a persistent point of contention among igneous petrologists as to how alkaline magmas generated by small degrees of melting within the upper-most convecting mantle can traverse through relatively cool continental lithosphere without freezing and being trapped at depth. Melting experiments demonstrate that volatile-rich, silica-undersaturated liquid react with peridotite under P-T conditions equivalent to the base of lithosphere and can form hydrous cumulates consisting of clinopyroxene, amphibole and phlogopite1-3. Furthermore, this metasomatic process enriches the mantle in incompatible elements in amounts similar to basalt (i.e. nephelinite, basanite) and may be a melt source for alkaline lavas4. The experiments and theoretical models provide important clues as to the cause and source of alkaline volcanism that occur within plates but demand evidence from natural settings. Here we present major and trace elements and 40Ar/39Ar ages from Pliocene-Pleistocene, olivine-phyric alkaline basalt erupted through extended continental lithosphere within and bordering the Terror Rift, southwestern Ross Sea, Antarctica. Subaerial and submarine basaltic tephra and lava from this region contain mantle xenoliths that include hydrous-phases that also display melt-solid reaction textures5,6. We compare basalt erupted across the central portion of the rift with basalt erupted at the rift shoulder along the base of the Transantarctic Mountains7,8. Our comparison shows that silica-undersaturation (i.e. nepheline-normative content) and highly incompatible trace element concentrations decrease with decreasing degree east longitude within the rift and on average are at their lowest on the rift shoulder. Variable degrees of partial melting of a common mantle source are modelled to match the trace element trends but require an unrealistic range of values: F = 95 km) relative to the central rift (

The western part of the Ross Sea embayment of Antarctica is a showcase of the interaction among Earth systems at various time and spatial scales marked by volcanic and magmatic emergences. We present a comprehensive investigation on the distribution and the vicinity of volcanic constructs within the western Ross Sea seafloor, which likely interacted with multiple advances and retreats of continental icesheets over time, using data acquired during the NBP25-01 Expedition(February-April 2025) on RVIB Nathaniel B. Palmer. Our study area is delimited by Ross Ice Shelf and Ross Island on the south and the Pacific to the north and is bordered by Transantarctic Mountains to the west and the Victoria Land Basin to the east with Terror Rift, currently an active magmatic rift under thick sediments, in between. Our expedition provides a refined view of the seafloor composed of widespread underwater volcanism within the Terror Rift Volcanic Field (TRVF) that include several polygenetic volcanic edifices, some of which appear to be highly eroded by ice sheets. Numerous monogenic volcanic cones were also identified, including a remarkable morphological type of flat-topped seamounts that are found throughout the western Ross Sea. They were mapped, sampled, and imaged, all of which provide evidence of varying amounts of erosion, that we suggest is caused by their interaction with grounded or pinned icesheets/shelves in past, including possible interaction during eruption of submarine volcanoes (i.e. glaciovolcanism). To better understand the lithosphere evolution with widespread volcanism that comprise the TRVF, including within the modern rift itself, we also present new heat flow measurements made during the NBP2501 Expedition via a violin-bow type heat flow probe. We conducted a total of 28 heat flow measurements along and across Terror Rift, from the Drygalski Ice Tongue to offshore Ross Island, which is twice the number of measurements taken by previous expeditions in total. The measured heat flow is ~30 and ~5 mW/m2 higher than that of previously modeled in the northern and southern part of the basin, respectively. Conductive thermal modeling of volcanism along faults cannot fully explain the heat flow pattern of 90-110 mW/m2 across the Terror Rift. Whereas hydrothermal cooling can effectively extract heat from young volcanism, as evidenced by imagery of and recovery of thermally altered materials, fluid circulation alone cannot simulate the heat flow pattern. The seafloor may experience a near-pure conductive heating condition during the Last Glacial Maximum as been suggested by our seafloor morphology characterization above. However,the high heat flow (at average of 100 mW/m2) would melt the base of thick ice at a rate of ~1 mm/yr, creating a nearly equivalent condition as in an open ocean setting. We therefore suggest the observed heat flow pattern is overwhelmingly reflecting a broader tectonic process, likely associated with a steeper geotherm through the lithosphere while minimizing the “icy blanket” effect in the Ross Sea, implying a shallower lithosphere-asthenosphere boundary at 45-55 km below seafloor across the Terror rift. These findings are critical to models for lithospheric rigidity and isostatic response to glacial cyclicity.

During the penultimate deglaciation, which largely coincided with Heinrich Stadial 11 (HS-11, ~136-129 ka), meltwater pulses cooled the North Atlantic and weakened the Atlantic Meridional Overturning Circulation (AMOC), driving a southward shift of the Intertropical Convergence Zone (ITCZ) and arid conditions in northern South America. Although deglacial effects persisted for several millennia into the subsequent Last Interglacial period (LIG, ~129-115 ka), the response of the ITCZ to this transitional climate state remains poorly constrained. Here, we present paleoenvironmental records from a marine sediment core north of the Orinoco River delta, where runoff-sensitive proxies track northern South American rainfall and Atlantic ITCZ migration, and benthic δ¹³C records indicate AMOC strength. Our records show a gradual increase in precipitation during the early LIG, indicating a progressive northward migration of the ITCZ. Notably, the onset of peak wet conditions at 126.5±1 ka coincides with stabilized benthic δ¹³C values, consistent with the re-establishment of a fully developed interglacial AMOC. This temporal alignment suggests that the lingering effects of the penultimate deglaciation, such as gradual cessation of freshwater influence, subpolar North Atlantic SST warming and AMOC recovery, played an important role in shaping tropical hydroclimate during the first 4 millennia of the LIG, and should be incorporated in climate models.

The South Aegean Volcanic Arc (Greece) is among the most active volcanic systems in Europe and poses an ideal natural laboratory to study the interplay of volcanism and tectonics as drivers of explosive eruptions, earthquakes, submarine landslides and tsunamis. This study focuses on a structurally independent sub-basin in the eastern Christiana Basin, located between the Christiana and Santorini island groups and southeast of the regionally significant Christiana Fault. Although the Christiana-Santorini-Kolumbo volcanic field has been extensively investigated, this basin has not yet been specifically targeted in a comprehensive study.During the MULTI-MAREX research cruise 2 (MSM135), nearly 640 km of hydroacoustic and 2D multi-channel seismic reflection data were acquired across the eastern Christiana Basin. The MSM135 seismic grid provides increased profile density and signal penetration and establishes a connection with the IODP 398 sites U1591 and U1598. Using the prominent Archaeos Tuff (765 ka) as a marker unit, we updated and harmonised the regional seismostratigraphic model. We refine the estimated volume of the Archaeos Tuff, and map deposits of the Poseidon eruption, providing an initial minimum bulk-volume estimate of 9 km³.We discovered a syncline, measuring around 8 km in diameter, beneath the almost flat seafloor. The Archaeos Tuff drapes a pre-existing central cone in a W-shaped geometry and reaches a maximum thickness of almost 200 m near the central cone. The syncline accommodates an additional 500 m of post-Archaeos deposits, primarily the Thera Pyroclastic Formation. The infill transitions quickly from an undulating W-shape to a horizontal stratification, indicating short-lived sag-style subsidence. To the northwest, the syncline is bounded by a major fault system, dubbed Thera Fault System, that strikes parallel to the Christiana Fault exhibiting vertical offsets of up to 160 m. Like the Christiana Fault, the Thera Fault System is likely a continuation of the normal faults northeast of Santorini.The seismostratigraphic model constrains the timing of eruptive and tectonic events, assembled in a comprehensive timeline. We date the activity of at least 10 previously little-considered volcanic cones near the margin of the basin to the Late Pleistocene, based on their relative position between known stratigraphic units. Our findings imply a slow, continuous down-faulting at the Christiana Fault, likely related to the rift extension in the region, whereas the Thera Fault System faulted in two stages of shorter duration. The timing of the subsidence coincides approximately with the first explosive eruption cycle on Santorini.

The role of microorganisms in shaping Earth’s dynamics is becoming increasingly evident; therefore, understanding how they influence and control environmental processes is essential for deciphering the functioning of Earth’s systems and for the effective management of natural resources.Hydrothermal systems offer natural laboratories for investigating the interplay between geological and microbial processes and in this context, Levante Bay on Vulcano Island (Aeolian Archipelago, Italy) represents an ideal setting to explore how these two components interact. This area is a typical hydrothermal system characterized by several CO2-dominated fluid manifestations of varying intensity and temperature. These manifestations exhibit a pronounced H2, CH4 and fluid temperature gradient along a south–north direction, as consistently confirmed by long-term geochemical observations. A distinctive feature of this site is the unusually heavy δ13C values of CH4 (up to -4.8‰ vs. V-PDB), which have led to the hypothesis of an abiotic origin for CH4.In particular, the geochemical observations indicate that elevated fluid temperatures co-occur with higher H2 concentrations, whereas decreasing temperatures are accompanied by a marked increase in CH4 concentrations. This evidence is consistent with cytofluorimetric detection of F420+ autofluorescent cells, providing direct evidence for methanogenic archaea inhabiting the cooler points of the Bay. Our overarching hypothesis is that the observed CH4 gradient is linked not only to geological dynamics but also to microbial activity, particularly methanogenic metabolism. To test this, we designed a microbial incubation experiment to assess whether, and to what extent, the microbial communities inhabiting five distinct points of the aquifer can influence gas and fluid chemistry, with a particular focus on elucidating their contribution to CH4 production. In particular we set three treatments for each hydrothermal fluid sample: (i) BIO_H2, unfiltered fluid incubated under an H2:CO2 (80:20) atmosphere to stimulate hydrogenotrophic methanogenesis; (ii) AB_H2, filtered fluid under H2:CO2 (80:20) conditions serving as abiotic controls; and (iii) BIO_N2, unfiltered fluid incubated under N2:CO2 (80:20) to maintain microbial communities while preventing H2-driven methanogenesis. During the incubation period, we monitored both hydrothermal fluid and headspace gas composition, with particular focus on H2 consumption, CH4 production and stable carbon isotope composition of CH4 (δ13C-CH4). Microbiological characterization was conducted through 16S rRNA gene sequencing and shotgun metagenomics to detect shifts in taxonomic composition and functional potential, with a particular focus on metabolic pathways underpinning methanogenesis and other hydrogenotrophic processes. Preliminary results reveal that BIO_H2 incubations showed increasing alkalinity, pH, and H2S and CH4 production compared to the other treatments. Surprisingly, the AB_H2 condition also produced measurable CH4, occasionally approaching biotic levels, pointing to the need to elucidate this phenomenon. δ13CH4 signatures displayed strong site-specific variability, with high negative values under BIO_H2 treatment and comparatively less negative signatures under BIO_N2 conditions, indicating different CH4 sources or pathways. Overall, these results highlight the complexity of CH4 origin in Levante Bay and indicate that geological and biological controls on methane cycling remain insufficiently resolved.