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1. High‐Latitude Controls on the Silicon Isotope Composition of Silicic Acid in the South Pacific

   https://doi.org/10.1029/2025GB009008  

   Middleton, Julien T; Holzer, Mark; Closset, Ivia; Jones, Janice; Brzezinski, Mark A (May 2026, Global Biogeochemical Cycles)

   Abstract We present measurements of silicon isotopes in silicic acid, δ³⁰Si, from GEOTRACES GP17_OCEbetween 20°S and 60°S along 152°W. The section sampled key water masses, allowing assessment of controls on large‐scale δ³⁰Si features. Observational data paralleled results of a data‐constrained Si isotope model and are consistent with nutrient trapping and partial nutrient consumption in polar surface waters leading to the accumulation of light silicon isotopes in the deep Southern Ocean (SO) and transport of the residually heavy fraction northward within Sub‐Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). Deep‐water δ³⁰Si values were nearly constant along isopycnals in the South Pacific, with vertical gradients driven by an increasing fractional contribution of isotopically light regenerated silicic acid with depth. Isotope values in deep waters from the SO to Tahiti exhibited a greater range than previously observed in the South Pacific or predicted by models, with δ³⁰Si falling between 1.19‰ and 1.45‰. SAMW and AAIW exhibited relatively heavy, uniform δ³⁰Si. Model results suggest that these mode waters are composed of preformed silicic acid subducted in the outcropping region and regenerated silicic acid sourced in roughly equal measure from polar waters and northern distal sources. Distal input is mainly through entrainment of underlying Upper Circumpolar Deep Water into AAIW. 

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2. On the Use of Dissolved Oxygen Isotopologues as Biogeochemical Tracers in the Northeast Pacific Ocean

   https://doi.org/10.1029/2022JC018617  

   Li, Boda; Hu, Huanting; Berelson, William M.; Adkins, Jess F.; Yeung, Laurence Y. (December 2022, Journal of Geophysical Research: Oceans)

   Abstract The isotopic composition of dissolved oxygen offers a family of potentially unique tracers of respiration and transport in the subsurface ocean. Uncertainties in transport parameters and isotopic fractionation factors, however, have limited the strength of the constraints offered by¹⁸O/¹⁶O and¹⁷O/¹⁶O ratios in dissolved oxygen. To improve our understanding of oxygen cycling in the ocean's interior, we investigated the systematics of oxygen isotopologues in the subsurface Pacific using new data and a 2‐D isotopologue‐enabled isopycnal reaction‐transport model. We measured¹⁸O/¹⁶O and¹⁷O/¹⁶O ratios, as well as the “clumped”¹⁸O¹⁸O isotopologue in the northeast Pacific, and compared the results to previously published data. We find evidence that oxygen consumption in the northeast Pacific follows different mass‐dependent fractionation exponents from those typically used in oceanographic studies. These fractionation factors imply that an elevated proportion of¹⁷O compared to¹⁸O in dissolved oxygen—that is, its triple‐oxygen isotope composition—may not uniquely reflect only gross primary productivity and mixing. For all oxygen isotopologues, transport, respiration, and photosynthesis comprise important parts of their respective budgets. Mechanisms of oxygen removal in the subsurface ocean are discussed. 

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3. Meridional Central Pacific Ocean Depth Section for Pb and Pb Isotopes (GEOTRACES GP15, 152°W, 56°N to 20°S) Including Shipboard Aerosols

   https://doi.org/10.1029/2024JC021674  

   Jiang, Shuo; Lanning, Nathan; Boyle, Edward; Fitzsimmons, Jessica; Ramezani, Jahandar; Wang, Avery G; Zhang, Jing (January 2025, Journal of Geophysical Research: Oceans)

   Goodkin, Nathalie (Ed.)

   Abstract Most oceanic lead (Pb) is from anthropogenic emissions into the atmosphere deposited into surface waters, mostly during the past two centuries. The space‐ and time‐dependent emission patterns of anthropogenic Pb (and its isotope ratios) constitute a global geochemical experiment providing information on advective, mixing, chemical, and particle flux processes redistributing Pb within the ocean. Pb shares aspects of its behavior with other elements, for example, atmospheric input, dust solubilization, biological uptake, and reversible exchange between dissolved and adsorbed Pb on sinking particles. The evolving distributions allow us to see signals hidden in steady‐state tracer distributions. The global anthropogenic Pb emission experiment serves as a tool to understand oceanic trace element dynamics. We obtained a high‐resolution (5° station spacing) depth transect of dissolved Pb concentrations and Pb isotopes from Alaska (55°N) to just north of Tahiti (20°S) near 152°W longitude. The sections reveal distinct sources of Pb (American, Australian, and Chinese), transport of Australian style Pb to the water mass formation region of Sub‐Antarctic Mode Water which is advected northward, columnar Pb isotope contours due to reversible particle exchange on sinking particles from high‐productivity particle veils, and a gradient of high northern deep water [Pb] to low southern deep water [Pb] that is created by reversible exchange release of Pb from sinking particles carrying predominantly northern hemisphere Pb.²⁰⁸Pb/²⁰⁶Pb versus²⁰⁶Pb/²⁰⁷Pb isotope relationships show that most oceanic Pb in the North Pacific is from Chinese and American sources, whereas Pb in the South Pacific is from Australian and American sources. 

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4. Secular Evolution of Mantle Sulfur Isotopes Since the Archean

   https://doi.org/10.1029/2025GC012858  

   Eguchi, J; Black, B A; Nisbet, E G; Moussallam, Y; Georgeais, G (May 2026, Geochemistry, Geophysics, Geosystems)

   Abstract Sulfur isotopes in mantle plume‐derived magmas show heterogeneities attributed to recycling of material from the Earth's surface. The sedimentary sulfur isotope record exhibits dramatic temporal variations over Earth history, raising the question of whether its secular evolution is echoed in mantle plume‐derived magmas. We present new secondary ion mass spectrometry and X‐ray absorption near‐edge structure measurements of δ³⁴S and Fe³⁺/ΣFe in naturally glassy melt inclusions from the 2.7 Ga Belingwe komatiites. We find that δ³⁴S of Belingwe melt inclusions is relatively homogeneous (+3.2 ± 0.9) and elevated relative to the depleted upper mantle. We evaluate explanations for elevated magmatic δ³⁴S including degassing, sulfide fractionation, assimilation, subduction‐like processes, and mantle recycling. We find no evidence for significant sulfur isotope fractionation via degassing or sulfide saturation. The elevated δ³⁴S in the Belingwe komatiites could reflect late‐stage assimilation of sediments or seawater. Alternatively, Belingwe komatiites may have formed in a subduction‐like setting, as modern arcs display a bias toward positive δ³⁴S. However, we find these scenarios less favorable. Instead, our preferred interpretation is that elevated δ³⁴S was supplied to the komatiite mantle source via recycled lithologies such as sediments, altered oceanic crust, and/or pyroxenite. Combined with δ³⁴S data from mantle plume‐derived magmas spanning a wide age range, our results resemble the secular evolution of δ³⁴S in surface reservoirs. We suggest that the δ³⁴S record in mantle plume‐derived magmas may echo secular evolution in the surficial sulfur reservoir with a delay of several hundred million years, linking Earth's surface and interior sulfur cycles. 

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5. A High‐Precision Analytical Technique for Dissolved N ₂ Isotopes in Aquatic Systems: Biogeochemical Applications and Determination of Solubility Equilibrium Isotope Effects

   https://doi.org/10.1002/rcm.10094  

   McPaul, Katelyn; Wankel, Scott D; Seltzer, Alan M (October 2025, Rapid Communications in Mass Spectrometry)

   ABSTRACT RationaleThe isotopic composition of dissolved dinitrogen gas (δ¹⁵N‐N₂) in water can offer a powerful constraint on the sources and pathways of nitrogen cycling in aquatic systems. However, because of the large presence of atmosphere‐derived dissolved N₂in these systems, high‐precision (on the order of 0.001‰) measurements of N₂isotopes paired with inert gas measurements are required to disentangle atmospheric and biogeochemical signals. Additionally, the solubility equilibrium isotope fractionation of N₂and its temperature and salinity dependence are underconstrained at this level of precision. MethodsWe introduce a new technique for sample collection, processing, and dynamic dual‐inlet mass spectrometry allowing for high‐precision measurement of δ¹⁵N‐N₂and δ(N₂/Ar) with simultaneous measurement of δ(⁴⁰Ar/³⁶Ar) and δ(Kr/N₂) in water. We evaluate the reproducibility of this technique and employ it to redetermine the solubility equilibrium isotope effects for dissolved N₂across a range of temperatures and salinities. ResultsOur technique achieves measurement reproducibility (1σ) for δ¹⁵N‐N₂(0.006‰) and δ(N₂/Ar) (0.41‰) suitable for tracing biogeochemical nitrogen cycling in aquatic environments. Through a series of air–water equilibration experiments, we find a N₂solubility equilibrium isotope effect (ε = α/1000 − 1, where α = (²⁹N₂/²⁸N₂)_dissolved/(²⁹N₂/²⁸N₂)_gas) in water of ε(‰) = 0.753 − 0.004•TwhereTis the temperature (°C), with uncertainties on the order of 0.001‰ over the temperature range of ~2°C–23°C and salinity range of ~0–30 psu. We find no apparent dependence of ε on salinity. ConclusionsOur new method allows for high‐precision measurements of the isotopic composition of dissolved N₂and Ar, and dissolved N₂/Ar and Kr/N₂ratios, within the same sample. Pairing measurements of N₂with inert gases facilitates the quantification of excess N₂from biogeochemical sources and its isotopic composition. This method allows for a wide range of applications in marine, coastal, and freshwater environments to characterize and quantitatively constrain potential nitrogen‐cycling sources and pathways and to differentiate between physical and biological isotope signals in these systems. 

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