The Environmental Importance of Microbial Sulfate Reduction and Disproportionation: Insights from SIMS-based δ34S Measurements David A. Fike Washington University, St. Louis, MO 63130, USA Metabolism Environment Acknowledgments SUPPORT: Geobiology time space Geobiology Metabolism time Metabolism Environment Environment space Geobiology time Metabolism Depositional Conditions Depositional EnvironmentConditions space Geobiology time Lithification & Diagenesis Metabolism Lithification & Depositional Conditions Diagenesis Environment space Motivation 1) understand modern biogeochemical cycling 2) understand how geochemical signals are preserved 3) paleoenvironmental reconstruction 2 2 Metabolism 1 3 Environment The Utility of Stable Isotopes 1) metabolic activity generates large isotopic fractionations • which depend upon environmental & ecological conditions 2) isotopic composition of sedimentary phases: best record of ancient biogeochemical cycling over Earth history. S isotopes: 32S: 95.02%; 33S: 0.76%; 34S: 4.20%; 36S: 0.02% Canonically measure 34S/32S ratio: 34 34 32 34 32 3 δ S = [( S/ S)sample/( S/ S)std-1]*10 , ‰ (V-CDT) Sulfate reduction What is it? (S6+→S2-) • Microbial reduction of sulfate to sulfide coupled to oxidation of H2 or organic C 2- + - SO4 + 4H2 + H → HS + 4H2O or 2- - - + SO4 + 2CH2O → HS + 2HCO3 + H Why do we care? • Dominant pathway for organic C remineralization in modern marine sediments • impacts global carbon cycling • Inferred to be among oldest metabolic pathways • Isotopic evidence(?) at ca 3.5 Gyr Fractionation during sulfate reduction Controlled by: • sulfate reduction rate - sulfate concentrations - electron donor concentrations Additional factors: • electron donor • H2 vs. Corg • temperature • microbial species Sim et al. 2011 Sulfur cycling: Not just sulfate reduction Sulfate Reduction 2- + - SO4 + 4H2 + H → HS + 4H2O ~0 Sulfide Oxidation ‰ - 2- + ‰? HS + 2O2 → SO4 + H ~25-50 Sulfur Disproportionation* 0 2- - + 4S + 4H2O → SO4 + 3HS + 5H 2- 2- *SO3 or S2O3 can also be used Environmental δ34S fractionation: impact of disproportionation Canfield & Teske, 1996 Sulfur Isotopes Over Earth History 80 34 1) Increase in depletion of δ Spyrite w/ time 60 34 34 2) Occasional δ Spyrite > δ Ssulfate 40 ) 20 Sulfate ‰ S ( 34 δ 0 -20 Pyrite -40 -60 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Time (Ga) Vignettes 1) Nature of Environment-Microbe Feedbacks at the Chemocline (Oxic-Anoxic Interface) • Modern Microbial Mats 2) How Depositional Environment Impacts Signals Preserved in Sediments • Modern Shallow Marine Sediments Modern Microbial Mats Simple community structure Guerrero Negro, Baja California, Mexico • lack macrofauna • lack bioturbation mm • lack detrital input VIS • essentially 1D systems Complex spatial organization • Phylogenetically diverse • Metabolically diverse • Intense metabolic coupling: Sulfide Image: N. Pace • photosynthesis/respiration • sulfate reduction/sulfide oxidation Sulfate Reducers Amidst Cyanobacteria Fike et al. GCA 2009 A New Approach… Spatial complexity of microbial mats: • metabolism 7f-GEO instrument • phylogeny • substrate availability Need means to capture spatial diversity in isotopes: Secondary Ion Mass Spectrometry (SIMS) Instrument Schematic Cs+ source secondary ions collectors sample 1: Modern microbial mats: Guerrero Negro Microbial Mat Cross Section Quantifying active sulfate reduction: 35 2- SO4 radiolabel Cross section of Radiograph of 35 active stromatolite S-labeled Ag2S Visscher et al. 2000 Incubation of silver disk in a microbial mat 2D Analysis Improved Spatial Resolution 1 10 0 Max. -1 0 Vertical -2 sampling -3 -10 resolution -4 -20 -5 Depth (mm) -6 -30 -7 1 cm -8 -40 Typical lateral sample size -9 0.9 mm Photosynthetic Redox Forcing Photosynthetic Impact on δ34S? ! Absolute value and profile of δ34S indep. of photosynthesis ! Enrichment in δ34S toward chemocline -where disproportionation would be expected to occur Sippewissett Salt Marsh Pink Berry consortia: purple S bacteria & sulfate reducing bacteria Wilbanks et al. 2014 Sippewissett Salt Marsh Pink Berry consortia: purple S bacteria & sulfate reducing bacteria wire sulfide deposition ! Enrichment in δ34S toward chemocline Wilbanks et al. 2014 -where disproportionation would be expected to occur Environmental δ34S fractionation: disproportionation? csSRR? Canfield & Teske, 1996 2 . Modern Depositional 34 Environments & δ Spyrite Papua New Guinea 144E Kikori River 145E 146E Turama River Purari River Australia Bamu River GH-8 GH-14 8S GS-48 GH-50 G SE Trades 25 km H NW Fly T8-18 Monsoon River 0 9S 20 50 40 Depth (m) 80 G H 0 20 40 60 80 Distance ofshore (km) 34 Depositional Environment & δ Spyrite 0 2 shallowing 4 Water Depth 8 m 14 m sediment depth (m) sediment sediment depth (m) sediment 6 18 m 48 m 50 m seawater sulfate seawater 8 sulfate seawater -40 -30 -20 -10 0 10 20 30 40 δ34Spyr (‰) [V-CDT] 34 Depositional Environment & δ Spyrite 0 Deeper water 34 • Highly negative δ Spyr shallowing 2 • Large fractionations • Low stratigraphic variability 4 Water Depth 8 m Shallow water 14 m sediment depth (m) sediment 6 18 m • Generally positive 48 m 34 50 m δ Spyr 8 sulfate seawater • Small (even negative!) -40 -30 -20 -10 0 10 20 30 40 δ34Spyr (‰) [V-CDT] fractionations • High stratigraphic variability Oxidative Reworking Mechanism(?) seaseasea levellevellevel reworkingreworking eventevent sedimentsedimentsediment surfacesurfacesurface oxicoxicoxic OOO222/H/H/H222OOO --- NONONO333/N/N/N222 4+4+4+ 2+2+2+ suboxic depth suboxicsuboxicsuboxic MnMnMn /Mn/Mn/Mn depth depth depth sediment sediment FeFeFe3+3+3+/Fe/Fe/Fe2+2+2+ sediment sediment sediment remobilized remobilized remobilized partialpartial HH2SS remobilized oxidationoxidation22 Fe-sulfde formation anoxic anoxic anoxic anoxic sulsulsulfffdicdicdic 3434 2-2-2- increasedincreased increasedincreased δ S SO /H S 3434 pyrpyr SOSOSO444 /H/H/H222SSS δδ SSH2SH2S methanicmethanicmethanic COCOCO222/CH/CH/CH444 tt 00 timetime Aller et al. 2010 Imagined Impact across a Depositional Gradient sea level sediment surface O2/H2O 0 NO -/N 3 2 4+ Mn /Mn2+ 2 shallowing 3+ Fe /Fe2+ LEGEND SO 2-/H 4 magnitude of 4 2S CO /CH recycling fux 2 4 depth below depth below 6 water depth 14 m sediment surfacesediment (m) seawater sulfate seawater 50 m -40 -30 -20 -10 0 10 20 30 34 δ Spyr (‰) Wild Extrapolation t3 δ34S isotopic gradient LEGEND magnitude of recycling fux Facies topset deposits foreset deposits bottomset deposits 34 δ S isotopic range -30 -20 -10 0 10 20 δ34S t3 Highstand 34 δ S (‰) TST MFS -30 -20 -10 0 10 20 t2 Lowstand SB LST -30 -20 -10 0 10 20 t1 Highstand sea level HST -30 -20 -10 0 10 20 Preliminary Observations Spatial δ34S variability pervasive in modern/ ancient microbial/sedimentary systems: • No isotopic evidence for widespread disproportionation across oxic-anoxic transition • Physical reworking and oxidation likely play key 34 role in generating elevated δ Spyrite records Modern studies provide key insights into interpreting ancient isotopic records. • Essential to incorporate depositional environment and stratigraphic context into interpretations Looking forward: Much to be done to understand the long-term S isotopic record 80 60 Thank you very much for your attention! 40 ) 20 Sulfate ‰ S ( 34 δ 0 -20 Pyrite -40 -60 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Time (Ga).