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Volcanic rocks erupted among Pitcairn seamounts sample a mantle plume that exhibits an extreme Enriched Mantle-1 signature. The origin of this peculiar mantle endmember remains contentious, and could involve the recycling of marine sediments of Archean or Proterozoic ages, delaminated units from the lower continental crust, or metasomatized peridotites from a lithospheric mantle. Here, we report the sulfur multi-isotopic signature (32S, 33S, 34S, 36S) of 15 fresh submarine basaltic glasses from three Pitcairn seamounts. We observe evidence for magmatic degassing of sulfur from melts erupted ∼2,000 meters below seawater level (mbsl). Sulfur concentrations are correlated with eruption depth, and range between 1300 ppm S (collected ∼ 2,500 mbsl) and 600 ppm S (∼2,000 mbsl). The δ34S values can be accounted for under equilibrium isotope fractionation during degassing, with αgas-melt between 1.0020 and 1.0001 and starting δ34S values between −0.9‰ and +0.6‰. The δ34S estimates are similar or higher than MORB signatures, suggesting the contribution of recycled sulfur with a ∼ 1‰ 34S enrichment compared to the Pacific upper mantle. The Δ33S and Δ36S signatures average at +0.024±0.007‰ and +0.02±0.07‰ vs. CDT, respectively (all 1σ). Only Δ33S is statistically different from MORB, by +0.02‰. The Δ33S enrichment is invariant across degassing and sulfide segregation. We suggest it reflects a mantle source enrichment rather than a high-temperature fractionation of S in the basalts. Despite the small magnitude of the 33S-36S variations, our data require a substantial amount of recycled sulfur overwhelm the Pitcairn mantle source. We show that models involving metasomatized peridotites, lower crust units, or Archean sediments, may be viable, but are restricted to narrow sets of circumstances. Instead, scenarios involving the contribution of Proterozoic marine sediments appear to be the most parsimonious explanation for the EM-1 signature at Pitcairn. 

Abstract Basalts from the Samoan volcanoes sample contributions from all of the classical mantle endmembers, including extreme EM II and high³He/⁴He components, as well as dilute contributions from the HIMU, EM I, and DM components. Here, we present multiple sulfur isotope data on sulfide extracted from subaerial and submarine whole rocks (N = 16) associated with several Samoan volcanoes—Vailulu‘u, Malumalu, Malutut, Upolu, Savai‘i, and Tutuila—that sample the full range of geochemical heterogeneity at Samoa and upon exhaustive compilation of S‐isotope data for Samoan lavas, allow for an assessment of the S‐isotope compositions associated with the different mantle components sampled by the Samoan hotspot. We observe variable S concentrations (10–1,000 ppm) and δ³⁴S values (−0.29‰ ± 0.30 to +4.84‰ ± 0.30, 2σ). The observed variable S concentrations are likely due to sulfide segregation and degassing processes. The range in δ³⁴S reflects mixing between the mantle origin and recycled components, and isotope fractionations associated with degassing. The majority of samples reveal Δ³³S within uncertainty of Δ³³S = 0‰ ± 0.008. Important exceptions to this observation include: (a) a negative Δ³³S (−0.018‰ ± 0.008, 2σ) from a rejuvenated basalt on Upolu island (associated with a diluted EM I component) and (b) previously documented small (but resolvable) Δ³³S values (up to +0.027 ± 0.016) associated with the Vai Trend (associated with a diluted HIMU component). The variability we observed in Δ³³S is interpreted to reflect contributions of sulfur of different origins and likely multiple crustal protoliths. Δ³⁶S versus Δ³³S relationships suggest all recycled S is of post‐Archean origin. 

Abstract. In the current era of rapid climate change, accuratecharacterization of climate-relevant gas dynamics – namely production,consumption, and net emissions – is required for all biomes, especially thoseecosystems most susceptible to the impact of change. Marine environmentsinclude regions that act as net sources or sinks for numerous climate-activetrace gases including methane (CH4) and nitrous oxide (N2O). Thetemporal and spatial distributions of CH4 and N2O are controlledby the interaction of complex biogeochemical and physical processes. Toevaluate and quantify how these mechanisms affect marine CH4 andN2O cycling requires a combination of traditional scientificdisciplines including oceanography, microbiology, and numerical modeling.Fundamental to these efforts is ensuring that the datasets produced byindependent scientists are comparable and interoperable. Equally critical istransparent communication within the research community about the technicalimprovements required to increase our collective understanding of marineCH4 and N2O. A workshop sponsored by Ocean Carbon and Biogeochemistry (OCB)was organized to enhance dialogue and collaborations pertaining tomarine CH4 and N2O. Here, we summarize the outcomes from theworkshop to describe the challenges and opportunities for near-futureCH4 and N2O research in the marine environment.