Marine silicate alteration is tightly linked to carbon burial over geological time scales, especially in continental margins known as ‘hotspot’ due to active organic carbon-driven diagenetic processes. Volcanic materials are one of the most reactive silicate groups, which plays an important role in sedimentary Si processes in continental margins. Since the Last Glacial Maximum, continental margins have undergone strong sediment regime shift, including sediment source changes, due to sea level rise. However, how marine silicate alterations respond to such sediment regime shifts remain largely unexplored. This study uses a 155-meter-long sediment core (IODP Exp. 375 U1518) from the Hikurangi margin, eastern New Zealand, one of the most representative sites in the subduction zone that are enriched with volcanic materials and meanwhile suffers from drastic sea level rise over a hundred of meters in the past 20,000 years. A tailored Si isotope-based sequential chemical leaching protocol in combination with porewater chemistry and geochemical analysis enable us to interrogate the sediment core in great details and uncover sedimentary Si mass balance by constraining various Si endmembers and major Si geochemical processes. Our results show that the δ³⁰Si values of Si endmembers can vary by 1‰ within 1.5–155.7 mbsf due to sediment source changes and early diagenetic modifications. This suggests that conventionally fixed δ³⁰Si value of a single Si endmember through one sediment core may result in large uncertainty in constraining sedimentary Si processes. A potential organic-bounded Si phase with a δ³⁰Si value of −0.19 ± 0.21‰ was also observed in this study. Simulations of a reactive-transport model further reveal enhanced clay mineral dissolution in the shallow Holocene sediments, leading to rapid porewater dissolved Si (DSi) increase and δ³⁰Si_DSi value decrease. Compared with the Pleistocene sediments, these changes directly elevate DSi diffusive flux by an order of magnitude with a lower δ³⁰Si value toward seafloor. Meanwhile, slow incongruent volcanic material dissolution in the deep Pleistocene sediments drives the gentle increase of DSi concentrations and δ³⁰Si_DSi values, from 541 μmol/L and + 0.24 ± 0.16‰ at 37.10 mbsf to 686 μmol/L and +0.94 ± 0.23‰ at 155.70 mbsf, respectively. High vs. low silicate alteration rates between shallow and deep sediments generates a large DSi concentration gradient and downward diffusion. We propose that it is necessary to include Si phase-based isotope analysis for better constraining marine silicate alterations and its mass balance, particularly for those environments subjecting to sediment source changes. Our findings also highlight that simply attributing decrease of DSi concentration to authigenic clay precipitation (reverse weathering) may cause an overestimation of its role in regulating marine carbon cycle during glacial-interglacial transgressions.

Elevated methane concentrations in seawater have been reported over extensive areas of the East Siberian Arctic Seas, overlying thawing subsea permafrost. However, observed methane concentrations of the ephemeral seawater are highly variable across both space and time, compromised by both the timing of rare measurements and storm-driven exchanges to the atmosphere. Here, we applied time-integrated signals of the δ¹³C-composition of specific C₃₀ hopanoids (diploptene, hop-17(21)-ene, neohop-13(18)-ene and diplopterol) in surface sediments to trace aerobic methane oxidation as a proxy for enhanced methane cycling. Interpretations of hopanoids and possible sources were further assessed by 16S-rRNA analyses in the surface sediments. The consistently low δ¹³C-C₃₀ hopenes signals, ranging between −57.5 ‰ to −37.1 ‰ (n=23) across the Laptev Sea shelf indicated aerobic methane oxidation. This suggests ubiquitous methane cycling with the most pronounced intensities in the outer shelf region, broadly consistent with the observed methane concentrations. Notably, depleted δ¹³C-C₃₀ hopenes were also found in the mid-shelf region of the Laptev Sea, earlier thought to be an area of comparatively low methane cycling. High methane concentrations were also observed in the vicinity of the Lena River delta, yet the isotopically heavier δ¹³C-C₃₀ hopenes may here reflect a combination of lower aerobic methane oxidation, a greater relative abundance of type II methanotrophs (lower isotope fractionation during hopanoid production) and isotope dilution from non-methanotrophic sources. While this complicates the biomarker interpretation in the unique setting near the Lena River delta, the δ¹³C-C₃₀ hopenes were still much lower than δ¹³C-organic carbon, indicating aerobic methane oxidation and a clear methane cycling signal also in this regime. Taken together, the results unravel the wider cross-shelf patterns of enhanced methane cycling in the Laptev Sea through probing of methane fossilised in membrane lipids of aerobic methanotrophs, with the molecular-isotopic pattern being preserved in the sedimentary archive.

Offshore freshening of groundwater has been observed over 100 km off Norway’s coast, sometimes beyond the continental shelf. Yet, no previous studies have modeled freshened groundwater emplacement along the Norwegian continental margin. This study employs a hydrogeological model to simulate groundwater flow along a two-dimensional transect driven by mechanical loading during the Late Pleistocene and Holocene, encompassing three glaciation periods when the ice sheet extended onto the shelf. During glacial advances, glacial meltwater infiltrated the offshore groundwater system at 486 mm year⁻¹, facilitated by intensified hydraulic head gradients from glacial loading. By 23 ka, fully freshened groundwater occupied 21.5 km², while groundwater with chloride concentrations below 400 mM occupied 47.4 km². These areas are 25 and 6.4 times greater, respectively, than in a simulation considering only eustatic sea-level changes. Conversely, glacial retreat and unloading led to groundwater discharge at maximum rates of 246 mm year⁻¹, which may have inhibited seawater recharge after deglaciation in formerly glacially loaded regions. Near the glacial front, lateral groundwater advection remained below 2 mm year⁻¹ as a result of weak horizontal hydraulic head gradients, characteristic of floating ice shelves. The findings challenge previous speculations of a lateral land–ocean connection as the primary mechanism for offshore freshening in the Norwegian continental margin. Instead, it was demonstrated that freshened groundwater was predominantly emplaced vertically when the Fennoscandian ice sheet advanced onto the shelf. Because of the lower hydraulic head potentials post-glaciation, significant portions of this freshened groundwater can persist well into subsequent interglacial or even glacial periods.

Un renouvellement offshore des eaux souterraines par des eaux douces a été observé à plus de 100 km au large des côtes norvégiennes, parfois au-delà du plateau continental. Cependant, aucune étude n’a modélisé l’emplacement des eaux souterraines adoucies le long de la marge continentale norvégienne. Cette étude utilise un modèle hydrogéologique pour simuler l’écoulement des eaux souterraines le long d’un transect 2D sous l’effet d’une charge mécanique pendant le Pléistocène supérieur et l’Holocène, couvrant trois périodes de glaciation au cours desquelles la calotte glaciaire s’est étendue jusqu’au plateau continental. Au cours des avancées glaciaires, l’eau de fonte glaciaire s’est infiltrée dans le système aquifère offshore à un rythme de 486 mm par an, facilité par l’intensification des gradients de charge hydraulique dus à la charge glaciaire. Il y a 23,000 ans, les eaux souterraines entièrement renouvelées par des eaux douces occupaient une superficie de 21,5 km², tandis que les eaux souterraines dont la concentration en chlorure était inférieure à 400 mM occupaient une superficie de 47.4 km². Ces zones sont respectivement 25 et 6.4 fois plus étendues que dans une simulation tenant uniquement compte des changements eustatiques du niveau de la mer. À l’inverse, le recul des glaciers et le déchargement ont entraîné un débit maximal de 246 mm par an dans les eaux souterraines, ce qui a pu empêcher la recharge en eau de mer après la déglaciation dans les régions anciennement recouvertes de glaciers. Près du front glaciaire, l’advection latérale des eaux souterraines est restée inférieure à 2 mm par an en raison des faibles gradients de charge hydraulique horizontale, caractéristiques des plateaux glaciaires flottants. Ces résultats remettent en question les hypothèses précédentes selon lesquelles une connexion latérale terre-océan serait le principal mécanisme à l’origine du renouvellement des eaux au large de la marge continentale norvégienne. Au contraire, il a été démontré que les eaux souterraines adoucies se sont principalement mises en place verticalement lorsque la calotte glaciaire fennoscandienne a avancé sur le plateau continental. En raison de la baisse du potentiel hydraulique après la glaciation, une partie importante de ces eaux souterraines renouvelées par des eaux douces peut persister pendant une grande partie des périodes interglaciaires suivantes, voire pendant les périodes glaciaires.

Se ha observado un proceso de renovación del agua subterránea en alta mar a más de 100 km de la costa de Noruega, en ocasiones más allá de la plataforma continental. Sin embargo, ningún estudio anterior ha modelado la ubicación del agua subterránea en la costa continental de Noruega. Este estudio emplea un modelo hidrogeológico para simular el flujo de agua subterránea a lo largo de un corte transversal en 2D impulsado por la carga mecánica durante el Pleistoceno tardío y el Holoceno, que abarca tres períodos de glaciación en los que la capa de hielo se extendió hasta la plataforma continental. Durante los avances glaciales, el agua de deshielo glacial se infiltró en el sistema de aguas subterráneas costeras a un ritmo de 486 mm al año, facilitado por la intensificación de los gradientes de carga hidráulica debidos a la carga glacial. En 23 ka, el agua subterránea totalmente dulce ocupaba 21.5 km², mientras que el agua subterránea con concentraciones de cloruro inferiores a 400 mM ocupaba 47.4 km². Estas áreas son 25 y 6.4 veces mayores, respectivamente, que en una simulación que solo tiene en cuenta los cambios eustáticos del nivel del mar. Por el contrario, el retroceso y la descarga glacial provocaron una descarga de agua subterránea a tasas máximas de 246 mm al año, lo que pudo haber inhibido la recarga de agua de mar tras la desglaciación en regiones anteriormente cargadas por glaciares. Cerca del frente glacial, la advección lateral del agua subterránea se mantuvo por debajo de los 2 mm al año debido a los débiles gradientes de carga hidráulica horizontal, característicos de las plataformas de hielo flotantes. Los resultados cuestionan las especulaciones anteriores sobre una conexión lateral entre la tierra y el océano como mecanismo principal del aporte de agua dulce en alta mar en el margen continental noruego. En cambio, se demostró que el agua subterránea dulce se depositó predominantemente en vertical cuando la capa de hielo fennoscandiana avanzó hacia la plataforma continental. Debido a los menores potenciales de carga hidráulica tras la glaciación, una parte significativa de esta agua subterránea dulce puede persistir hasta bien entrados los períodos interglaciales o incluso glaciales posteriores.

离岸地下水淡化现象已在挪威海岸线外约100公里处观测到，甚至有时超出大陆架范围。然而, 迄今尚无研究对挪威大陆边缘的淡化地下水的成位过程进行建模。本研究采用水文地质模型，对 沿一条二维横截面的地下水流动进行模拟，受晚更新世与全新世的机械荷载驱动，涵盖冰盖曾扩 展至陆架的三次冰期。在冰期推进期间，融水以486 mm yr-1速率渗入近海地下水系统，这一过程 受到由冰川加载增强的水头梯度的促进。至约2.3万年前，完全淡化的地下水覆盖面积为21.5 km²，而氯离子浓度低于400 mM的地下水覆盖面积为47.4 km²。这些面积分别比仅考虑海平面等 静变化情景的模拟大约扩大了25倍和6.4倍。相反，冰川退缩与荷载解除导致地下水排泄的最大 速率为246 mm yr^-1，这可能在脱冰后抑制曾经被冰川负荷区域的海水再充灌。在靠近冰川前沿的 区域，由于水平水头梯度较弱，沿向水平方向的地下水平流仍低于2 mm yr^-1，这与浮冰架的特征 相符。研究结果挑战了将陆-海侧向连通视为挪威大陆边缘近岸淡化主要机制的先前推测。相反，研究表明地下淡水主要在Fennoscandian冰盖推进至陆架时沿竖直方向被成位。由于冰后水头势能 下降，后续间冰期甚至下一次冰期中，这些淡化地下水的显著部分仍可长期存在。

A dessalinização das águas subterrâneas em áreas no mar foi observada a mais de 100 km da costa da Noruega, por vezes para além da plataforma continental. No entanto, nenhum estudo anterior modelou a distribuição de águas subterrâneas dessalinizadas ao longo da margem continental norueguesa. Este estudo utiliza um modelo hidrogeológico para simular o fluxo de água subterrânea ao longo de uma seção transversal 2D, impulsionado pelo carregamento mecânico durante o Pleistoceno Superior e o Holoceno, abrangendo três períodos de glaciação em que a camada de gelo se estendeu sobre a plataforma continental. Durante os avanços glaciais, a água de degelo glacial infiltrou-se no sistema de águas subterrâneas no mar a uma taxa de 486 mm ano^–1, facilitada pela intensificação dos gradientes de carga hidráulica devido ao carregamento glacial. Por volta de 23 ka, águas subterrâneas completamente dessalinizadas ocupavam 21.5 km², enquanto águas subterrâneas com concentrações de cloreto abaixo de 400 mM ocupavam 47.4 km². Essas áreas são 25 e 6.4 vezes maiores, respectivamente, do que em uma simulação que considera apenas as mudanças eustáticas do nível do mar. Por outro lado, o recuo e o descarregamento glacial levaram à descarga de água subterrânea a taxas máximas de 246 mm ano^–1, o que pode ter inibido a recarga de água do mar após a deglaciação em regiões anteriormente carregadas por gelo. Perto da frente glacial, a advecção lateral de água subterrânea permaneceu abaixo de 2 mm ano^–1 devido aos fracos gradientes de carga hidráulica horizontais, característicos de plataformas de gelo flutuantes. Os resultados desafiam as especulações anteriores sobre uma conexão lateral terra-oceano como o principal mecanismo para a dessalinização no mar na margem continental norueguesa. Em vez disso, demonstrou-se que a água subterrânea dessalinizada foi predominantemente distribuída verticalmente quando a camada de gelo da Fennoscândia avançou sobre a plataforma continental. Devido aos menores potenciais de carga hidráulica pós-glaciação, porções significativas dessa água subterrânea dessalinizada podem persistir até períodos interglaciais ou mesmo glaciais subsequentes.

Seep ecosystems connected to subseafloor methane reservoirs are widespread in the Arctic due to factors such as organic material accumulation and an extensive gas hydrate stability zone. Recently, submarine groundwater discharge (SGD) stemming from past glaciation has been identified as an additional factor that can generate Arctic seep systems. Despite seeps having an extended ‘sphere of influence’, links to the surrounding seafloor have not been studied in the Arctic, which is why we investigated the epifaunal megabenthos at the edges of an SGD-derived Arctic seep off northern Norway. Due to local, chemosynthesis-based food production and substrate heterogeneity, Arctic seeps are often local biodiversity hotspots. However, our study revealed that communities at the peripheries of the seep (transition zone/ecotone or ‘chemotone’) are an order of magnitude more diverse and species rich than inside the seep itself. Neither seep nor chemotone contain specialist fauna, and benthic species appear to aggregate differently at Arctic seeps, their peripheries and background, despite not being specifically adapted for any one of the habitats. Through stable isotope analyses, we detected chemosynthetically derived carbon in peripheral animals, suggesting that export beyond the seep is possible. Suspension feeders were numerous and diverse in the chemotone (e.g. thousands of individuals of colonial anemones) as well as in non-seep canyons in the region which brings forth the question of whether high latitude seeps benefit or promote these feeding styles overall as opposed to specific organisms such as corals. The cryosphere of both the past and present can generate seeps which can impact trophic dynamics and species composition well beyond their own boundaries.

Sedimentary biogenic silica (bSi) is a primary sink of oceanic silicon (Si), and its quantification is essential for constraining the marine Si budget. The traditional alkaline leaching method estimates bSi contents by extrapolating a regression of Si leachates at 2, 3, and 5 h, but this approach is biased by lithogenic Si (LSi) dissolution. We analyzed 59 marine sediment samples (0.2 to 57.0 wt% bSi) and used Si isotopes to re-assess this method. Samples with moderate bSi contents (5–20 wt%) show a marked decline in dissolution rates after 3–5 h of leaching. Si isotopes of the leachates reveal that the decline reflects either a shift from dominant bSi to LSi phase or changes in bSi species through time. Sediments with <5 wt% or > 20 wt% bSi display a stable Si dissolution rate throughout 8 h, likely due to unchanged dominant Si phases of lithogenic or biogenic Si. Additionally, we re-assessed the brucite co-precipitation method (MAGIC) for porewater dissolved Si (dSi) recovery. Mg/Si molar ratios <100 yield dSi recovery <90%, resulting in large isotope fractionation. Hence, an Mg/Si ratio of ≥300 is recommended to guarantee full recovery, and H₂O₂ pretreatment is necessary to eliminate matrix effects induced by high Fe, Mn and DOC concentrations. Our findings imply that the traditional leaching method likely underestimates current bSi contents, especially in clay or/and organic-rich sediments with moderate bSi contents.

Glacial meltwater has been increasingly recognised as a potential source of atmospheric methane, yet its origin and variability in the High Arctic remain poorly constrained. In this study, we present measurements of methane concentration and carbon‑isotope composition in meltwater draining the northern Greenland Ice Sheet. Here we show that methane concentrations (12-20 nM) are significantly lower than those reported from other Greenland catchments, despite clear evidence of subglacial input. Isotopic signatures and regional geological context indicate that this methane is predominantly thermogenic, reflecting a geological source that is largely independent of subglacial microbial activity. These findings advance current understanding of methane sources beneath the Greenland Ice Sheet, revealing a thermogenic contribution alongside microbial methane in Arctic methane cycling. In this work we highlight the need to account for geological methane reservoirs when assessing present and future cryosphere–carbon feedbacks.

Submarine groundwater discharge—the flow of groundwater into the ocean—plays an important role in shaping coastal biogeochemical cycles. The absence of temporal constraints on offshore groundwater dynamics driven by proximal glacial loading hinders our assessment of how its circulation may vary in conceivable ice-free polar regions. Here we estimate residence times of saline groundwater at an active submarine groundwater discharge and methane seep site off the coast of northern Norway, near the continental shelf break. The subsurface hydrology in this area experienced drastic changes due to Fennoscandian Ice Sheet dynamics, offering insights into the consequences of glacial–interglacial transitions for offshore groundwater. Using radiocarbon dating of dissolved inorganic carbon in the upwards-advected groundwater, we determined saline groundwater residence times of 11.5 to 8.8 kyr and 4.8 to 2.6 kyr at two distinct discharge sites. The presence of a meteoric water component in sediment porewaters confirms offshore groundwater freshening driven by past glacial loading. This indicates that, as the ice sheet retreated and sea levels rose, seawater began to infiltrate the subsurface, replacing freshwater recharge. Our results provide observational evidence pinpointing the onset of seawater infiltration following deglaciation of the margin. These findings suggest that retreating marine-terminating glaciers will profoundly alter offshore groundwater composition and reduce discharge rates.

Chemical weathering of silicate rocks redistributes major, minor and trace elements through coupled dissolution–precipitation reactions. These weathering processes drive shifts in ocean acid–base chemistry, modulating atmospheric carbon dioxide levels and providing a stabilizing feedback in the carbon cycle. Silicate weathering occurs in both terrestrial and marine environments, releasing (‘forward’) or consuming alkalinity (‘reverse’), but these have largely been perceived as independent and studied in isolation. However, weathering products are transported downstream across terrestrial and to marine environments, suggesting a dynamic coupling of these weathering processes across scales. Here we propose that the Earth’s silicate weathering occurs along a continuum linking mountains to the deepest sedimentary environments and forward to reverse weathering. In this framework, the magnitude and direction of a local weathering flux depends on the materials’ origin, weathering–erosion history and environmental conditions. Consequently, global silicate weathering fluxes and the long-term carbon cycle feedback may be governed by the dynamic interplay of various environments along the silicate weathering continuum.

Marine silicate alteration includes the processes of lithogenic silicate (LSi) dissolution, clay formation, and biogenic silica dissolution. LSi dissolution consumes CO₂ and results in marine silicate weathering. Formation of cation-rich clay minerals produces CO₂, which is known as reverse weathering. The net effects on carbon cycling of both processes are poorly constrained as the responsible silicate phases and controlling factors are unclear. We investigate the coupling between LSi dissolution and clay formation by analyzing stable Si isotopic signatures (δ³⁰Si) of porewater and solid Si phases (reactive LSi, biogenic silica, and amorphous secondary Si phases) in two drill cores from the Ulleung Basin, East/Japan Sea. High porewater total alkalinity (up to 131 meq L⁻¹) was measured, indicating net marine silicate weathering. Based on the elemental composition (Si, K, and Al) as well as δ³⁰Si of the reactive LSi phase in sediments, phyllosilicates that are potentially mica group silicates are identified as the primary silicate group that sustains marine silicate weathering in the Ulleung Basin. Our reactive transport modeling supports such an inference and further reveals how early diagenetic reactions could affect the downcore occurrence and rates of LSi dissolution and clay formation. Predominant clay formation/reverse weathering in sulfate-reducing sediments is evident from the high δ³⁰Si values in porewater. In the shallow methanogenesis zone, net marine silicate weathering due to phyllosilicate dissolution explains the observed high total alkalinity and low δ³⁰Si in porewater. Nonetheless, we show that the rates of clay formation primarily control the level of porewater total alkalinity, even in the condition of net marine silicate weathering. Clay formation is strongly suppressed by the low porewater pH as a result of the active fermentation in the site with the highest rate of organic matter degradation. Deep in the methanogenesis zone, enrichment of dissolved aluminum from LSi dissolution counteracts the influence of pH on silicate alteration processes, thereby further limiting the extent of LSi dissolution, as supported by the downcore increasing of porewater δ³⁰Si. Our results demonstrate that δ³⁰Si in porewater reflects downcore variations in marine silicate alteration, with microbial processes and dissolved aluminum accumulation regulating the rates of LSi dissolution and clay formation.