Description: Sulfate-driven anaerobic oxidation of methane (SO4-AOM) in marine sediments commonly leads to the precipitation of pyrite. It is, however, frequently challenging to unequivocally unravel the entire history of pyritization, because of the common coexistence of SO4-AOM derived pyrite with pyrite resulting from organiclastic sulfate reduction (OSR). To better understand how SO4-AOM affects pyritization in methane-bearing sediments and how this can be identified, we applied secondary ion mass spectroscopy (SIMS) to analyze the sulfur isotope composition (d34S) of authigenic pyrite in addition to sulfur isotope measurements of bulk sulfide and hand-picked pyrite aggregates from the two seafloor sites, HS148 and HS217, in the Shenhu seepage area, South China Sea. Authigenic, mostly tubular pyrite aggregates from these sites consist of three types of pyrite: framboids, zoned aggregates with radial overgrowths surrounding a framboidal core, and euhedral pyrite crystals. Framboids with low SIMS d34S values (as low as - 41.6 per mil at HS148, and - 38.8 per mil at HS217) are dispersed throughout the cores, but are especially abundant in the shallow part of the sedimentary column (i.e. above 483 cmbsf in HS148; above 670 cmbsf in HS217). These patterns are interpreted to reflect the dominance of OSR during early diagenetic processes in the shallow sediments. With increasing depth, both d34S values of bulk sulfide minerals and hand-picked pyrite aggregates increase sharply at 483 cmbsf in core HS148, and at 700 cmbsf in core HS217, respectively. Radial pyrite overgrowths and euhedral crystals become abundant at depth typified by high d34S values for hand-picked pyrite. Moreover, SIMS analysis reveals an extreme variability of d34S values for the three pyrite types on a small scale in these zones. Besides some moderately 34S enriched framboids, most of the overgrowths and euhedral crystals display extremely high SIMS d34S values (as high as + 114.8 per mil at HS148, and + 74.3 per mil at HS217), representing the heaviest stable sulfur isotope composition of pyrite ever reported to the best of our knowledge. Such an abrupt and extreme increase in d34Spyrite values with depth is best explained by an enrichment of 34S in the pool of dissolved sulfide caused by SO4-AOM in the sulfate methane transition zone (SMTZ). The increase in d34S values from framboidal cores to overgrowth layers and euhedral crystals indicates continuous, and finally near to complete exhaustion of dissolved sulfate at the SMTZ following a Rayleigh distillation process. SO4-AOM allowed for subsequent growth of later stage pyrite over the initial framboids, part of which formed earlier and at shallower depth by OSR. The combination of a detailed petrographic study of authigenic pyrite with SIMS analysis of stable sulfur isotopes in organic-rich strata proves to be a powerful tool for reconstructing the dynamics of sulfur cycling in modern and, potentially, ancient sedimentary sequences.

Description: © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 208 (2017): 102-118, doi:10.1016/j.gca.2017.03.021.

Description: We measured the concentrations and isotopic compositions of solid phase extracted (SPE) dissolved organic carbon (DOC) and high molecular weight (HMW) DOC and their constituent organic components in order to better constrain the sources and cycling of DOC in a large oligotrophic lacustrine system (Lake Superior, North America). SPE DOC constituted a significant proportion (41-71 %) of the lake DOC relative to HMW DOC (10-13%). Substantial contribution of 14C-depleted components to both SPE DOC (Δ14C = 25 to 43‰) and HMW DOC (Δ14C = 22 to 32‰) was evident during spring mixing, and depressed their radiocarbon values relative to the lake dissolved inorganic carbon (DIC; Δ14C ~ 59‰). There was preferential removal of 14C-depleted (older) and thermally recalcitrant components from HMW DOC and SPE DOC in the summer. Contemporary photoautotrophic addition to HMW DOC was observed during summer stratification in contrast to SPE DOC, which decreased in concentration during stratification. Serial thermal oxidation radiocarbon analysis revealed a diversity of sources (both contemporary and older) within the SPE DOC, and also showed distinct components within the HMW DOC. The thermally labile components of HMW DOC were 14C-enriched and are attributed to heteropolysaccharides (HPS), peptides/amide and amino sugars (AMS) relative to the thermally recalcitrant components reflecting the presence of older material, perhaps carboxylic-rich alicyclic molecules (CRAM). The solvent extractable lipid-like fraction of HMW DOC was very 14C-depleted (as old as 1270-2320 14C years) relative to the carbohydrate-like and protein-like substances isolated by acid hydrolysis of HMW DOC. Our data constrain relative influences of contemporary DOC and old DOC, and DOC cycling in a modern freshwater ecosystem.

Description: This work was funded by the National Science Foundation OCE 0825600 to E.C.M. and J.P.W., a graduate student internship fellowship to P.K.Z by National Ocean Sciences Accelerator Mass Spectrometry Facility (OCE 0753487), and the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution to P.K.Z, with funding provided by the National Ocean Sciences Accelerator Mass Spectrometry Facility (OCE 0753487).

Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 44 (2017): 2407–2415, doi:10.1002/2016GL071348.

Description: We present concentration and isotopic profiles of total, size, and polarity fractionated dissolved organic carbon (DOC) from Station ALOHA (A Long-term Oligotrophic Habitat Assessment), an oligotrophic site in the North Pacific Ocean. The data show that, between the surface and 3500 m, low molecular weight (LMW) hydrophilic DOC, LMW hydrophobic DOC, and high molecular weight (HMW) DOC constitute 22–33%, 45–52%, and 23–35% of DOC, respectively. LMW hydrophilic DOC is more isotopically depleted (δ13C of −23.9‰ to −31.5‰ and Δ14C of −304‰ to −795‰; mean age of 2850 to 15000 years) than the LMW hydrophobic DOC (δ13C of −22‰ to −23‰ and Δ14C of −270‰ to −568‰; 2470 to 6680 years) and HMW DOC (δ13C of ~−21‰ and Δ14C of −24‰ to −294‰; 135–2700 years). Our analyses suggest that a large fraction of DOC may be derived from allochthonous sources such as terrestrial and hydrothermal DOC and cycle on much longer time scales of 〉10000 years or enter the ocean as preaged carbon.

Description: NSF Cooperative Agreement for the Operation of a National Ocean Sciences Accelerator Mass Spectrometry Facility Grant Number: OCE-0753487; Gordon and Betty Moore Foundation Grant Numbers: GBMF3298, GBMF3794; Simons Foundation Grant Number: 329108

Description: 2017-09-07

Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 872–879, doi:10.1002/2017GL076295.

Description: We report four profiles of the radiocarbon content of dissolved organic carbon (DOC) spanning the South Indian Ocean (SIO), ranging from the Polar Front (56°S) to the subtropics (29°S). Surface waters held mean DOC Δ14C values of −426 ± 6‰ (~4,400 14C years) at the Polar Front and DOC Δ14C values of −252 ± 22‰ (~2,000 14C years) in the subtropics. At depth, Circumpolar Deep Waters held DOC Δ14C values of −491 ± 13‰ (~5,400 years), while values in Indian Deep Water were more depleted, holding DOC Δ14C values of −503 ± 8‰ (~5,600 14C years). High-salinity North Atlantic Deep Water intruding into the deep SIO had a distinctly less depleted DOC Δ14C value of −481 ± 8‰ (~5,100 14C years). We use multiple linear regression to assess the dynamics of DOC Δ14C values in the deep Indian Ocean, finding that their distribution is characteristic of water masses in that region.

Description: National Science Foundation (NSF) Grant Numbers: OPP-1142117, OCE-1436748

Description: 2018-07-24

Description: The development of robust sample preparation techniques for ocean science research has been a hallmark of NOSAMS since its inception. Improvements to our standard methods include reducing the minimum size of the samples we can analyze, building modular graphite reactors of different sizes that we can swap in and out depending on our sample stream, and modifying our carbonate acidification methods to improve handling of the smaller samples we now receive. A relatively new instrument, the Ramped PyrOx, which allows the separation of organic matter into thermal fractions, has attracted much interest as a research and development tool. We will also discuss our progress on incorporating a Picarro isotope analyzer into our sample preparation options.

Description: Author Posting. © American Chemical Society, 2019. This is an open access article published under an ACS AuthorChoice License. The definitive version was published in Environmental Science and Technology 53(14), (2019):8244-8251, doi:10.1021/acs.est.9b02344.

Description: Perylene is a frequently abundant, and sometimes the only polycyclic aromatic hydrocarbon (PAH) in aquatic sediments, but its origin has been subject of a longstanding debate in geochemical research and pollutant forensics because its historical record differs markedly from typical anthropogenic PAHs. Here we investigate whether perylene serves as a source-specific molecular marker of fungal activity in forest soils. We use a well-characterized sedimentary record (1735 to 1999) from the anoxic-bottom waters of the Pettaquamscutt River basin, RI, USA to examine mass accumulation rates and isotope records of perylene, and compare them with total organic carbon and the anthropogenic PAH fluoranthene. We support our arguments with radiocarbon (14C) data of higher plant leaf-wax n-alkanoic acids. Isotope-mass balance calculations of perylene and n-alkanoic acids indicate that ~40 % of sedimentary organic matter is of terrestrial origin. Further, both terrestrial markers are pre-aged on millennial time-scales prior to burial in sediments and insensitive to elevated 14C concentrations following nuclear weapons testing in the mid-20th Century. Instead, changes coincide with enhanced erosional flux during urban sprawl. These findings suggest that perylene is definitely a product of soil derived fungi, and a powerful chemical tracer to study spatial and temporal connectivity between terrestrial and aquatic environments.

Description: We thank John King, Sean Sylva, Brad Hubeny, Peter Sauer, and Jim Broda for their help in sampling; Carl Johnson and Daniel Montluçon for their incessant help with analyses; as well as Mark Yunker for critical discussion on the perils of perylene. Professor Phil Meyers and two anonymous reviewers provided comments that improved the quality of the manuscript. U.M.H. acknowledges the Swiss National Science Foundation for his postdoctoral fellowship and T.I.E. and K.A.H. acknowledges the NSF for research grants CHE-0089172 and OCE-9708478.

Description: 2020-06-19

Description: Elemental sulfur is commonly regarded as the product of oxidative sulfur cycling in the sediment. However, reports on the occurrence of elemental sulfur in seepage areas are few and thus its origin and mechanisms controlling its distribution are insufficiently understood. Here, we analyzed the multiple sulfur isotopic compositions for elemental sulfur and pyrite from an iron-dominated gas hydrate-bearing sedimentary environment of the South China Sea to unravel the impact of sulfate-driven anaerobic oxidation of methane (SO4-AOM) on the formation of elemental sulfur. The multiple sulfur isotopes reveal variable ranges for both elemental sulfur and pyrite (δ34S: between −15.7 and +23.3‰ for elemental sulfur and between −35.3 and +34.4‰ for pyrite; Δ33S: between −0.08 and +0.06‰ for elemental sulfur and between −0.03 and +0.15‰ for pyrite). The enrichment of 34S in pyrite throughout the sediment core suggests pronounced SO4-AOM in paleo-sulfate-methane transition zones (SMTZ). In addition, the occurrence of seep carbonates with very negative δ13C values (as low as −57‰, V-PDB) coincides with the inferred paleo-SMTZs and agrees with formerly locally pronounced SO4-AOM. Interestingly, the multiple sulfur isotopic composition of elemental sulfur reveals a different pattern from that of pyrite derived from organoclastic sulfate reduction (i.e., with low δ34S and high Δ33S values for the latter). In comparison to coexisting pyrite, most of the elemental sulfur reveals higher δ34S values (as much as +28.9‰), which is best explained by an enrichment of 34S in the residual pool of dissolved sulfide generated by SO4-AOM. As an intermediate sulfur phase, elemental sulfur can form via sulfide oxidation coupled to iron reduction, but it can only persist in the absence of free sulfide. Therefore, the occurrence of 34S enriched elemental sulfur is likely to represent an oxidative product after hydrogen sulfide had vanished due to vertical displacement of the SMTZ. Our observations suggest that elemental sulfur may serve as a useful recorder for reconstructing the dynamics of sulfur cycling in modern and possibly ancient seepage areas.