The Environmental Importance of Microbial Sulfate Reduction and Disproportionation: Insights from SIMS-based δ34s Measurements

Transcription

1 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

2 SUPPORT: Acknowledgments

3 Geobiology time space

4 Geobiology Metabolism time Metabolism Environment Environment space

5 Geobiology time Metabolism space Environment Depositional Conditions Depositional Conditions

6 time Geobiology Lithification & Diagenesis Metabolism Lithification & Diagenesis Environment space Depositional Conditions

7 Motivation 1) understand modern biogeochemical cycling 2) understand how geochemical signals are preserved 3) paleoenvironmental reconstruction 2 2 Metabolism 1 3 Environment

8 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: 32 S: 95.02%; 33 S: 0.76%; 34 S: 4.20%; 36 S: 0.02% Canonically measure 34 S/ 32 S ratio: δ 34 S = [( 34 S/ 32 S) sample /( 34 S/ 32 S) std -1]*10 3, (V-CDT)

9 What is it? Sulfate reduction (S 6+ S 2- ) Microbial reduction of sulfate to sulfide coupled to oxidation of H 2 or organic C SO H 2 + H + HS - + 4H 2 O or SO CH 2 O HS - + 2HCO 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

10 Fractionation during sulfate reduction Controlled by: sulfate reduction rate - sulfate concentrations - electron donor concentrations Additional factors: electron donor H 2 vs. C org temperature microbial species Sim et al. 2011

11 ~25-50 Sulfur cycling: Not just sulfate reduction Sulfate Reduction SO H 2 + H + HS - + 4H 2 O Sulfide Oxidation HS - + 2O 2 SO H + ~0? Sulfur Disproportionation* 4S 0 + 4H 2 O SO HS - + 5H + *SO 3 2- or S 2 O 3 2- can also be used

12 Environmental δ 34 S fractionation: impact of disproportionation Canfield & Teske, 1996

13 Sulfur Isotopes Over Earth History ) Increase in depletion of δ 34 S pyrite w/ time 2) Occasional δ 34 S pyrite > δ 34 S sulfate 40 δ 34 S ( ) 20 0 Sulfate Pyrite Time (Ga)

14 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

15 Modern Microbial Mats Simple community structure lack macrofauna lack bioturbation lack detrital input essentially 1D systems mm Guerrero Negro, Baja California, Mexico VIS Complex spatial organization Phylogenetically diverse Metabolically diverse Intense metabolic coupling: photosynthesis/respiration sulfate reduction/sulfide oxidation Sulfide Image: N. Pace

16 Sulfate Reducers Amidst Cyanobacteria Fike et al. GCA 2009

17 A New Approach Spatial complexity of microbial mats: metabolism phylogeny substrate availability 7f-GEO instrument Need means to capture spatial diversity in isotopes: Secondary Ion Mass Spectrometry (SIMS)

18 Instrument Schematic Cs+ source secondary ions collectors sample

19 1: Modern microbial mats: Guerrero Negro

20 Microbial Mat Cross Section

21 Quantifying active sulfate reduction: 35 SO 4 2- radiolabel Cross section of active stromatolite Radiograph of 35 S-labeled Ag 2 S Visscher et al. 2000

22 Incubation of silver disk in a microbial mat

23 2D Analysis

24 Improved Spatial Resolution Depth (mm) Max. Vertical sampling resolution cm -8 Typical lateral sample size mm

25 Photosynthetic Redox Forcing

26 Photosynthetic Impact on 34 δ S?!Absolute value and profile of δ34s indep. of photosynthesis!enrichment in δ34s toward chemocline -where disproportionation would be expected to occur

27 Sippewissett Salt Marsh Pink Berry consortia: purple S bacteria & sulfate reducing bacteria Wilbanks et al. 2014

28 Sippewissett Salt Marsh Pink Berry consortia: purple S bacteria & sulfate reducing bacteria wire sulfide deposition! Enrichment in δ 34 S toward chemocline -where disproportionation would be expected to occur Wilbanks et al. 2014

29 Environmental δ 34 S fractionation: disproportionation? cssrr? Canfield & Teske, 1996

30 2. Modern Depositional Environments & δ 34 S pyrite 144E Kikori River 145E 146E Papua New Guinea Turama River Purari River Australia 8S Bamu River GH-8 GH-14 GS-48 Fly River T8-18 GH-50 H G NW Monsoon SE Trades 25 km 9S Depth (m) G H Distance offshore (km)

31 Depositional Environment & δ 34 S pyrite 0 sediment depth (m) shallowing seawater sulfate δ 34 Spyr ( ) [V-CDT] Water Depth 8 m 14 m 18 m 48 m 50 m 40

32 Depositional Environment & δ 34 S pyrite sediment depth (m) shallowing seawater sulfate δ 34 Spyr ( ) [V-CDT] Water Depth 8 m 14 m 18 m 48 m 50 m 40 Deeper water Highly negative δ 34 S pyr Large fractionations Low stratigraphic variability Shallow water Generally positive δ 34 S pyr Small (even negative!) fractionations High stratigraphic variability

33 Oxidative Reworking Mechanism(?) sea level anoxic oxic O 2 /H 2 O NO 3- /N 2 suboxic Mn 4+ /Mn 2+ Fe 3+ /Fe 2+ sulfidic 2- SO 4 /H 2 S reworking event partial H 2 S oxidation increased δ 34 S H2S remobilized Fe-sulfide formation increased δ 34 S pyr remobilized sediment surface remobilized sediment depth methanic CO 2 /CH 4 t 0 time Aller et al. 2010

34 Imagined Impact across a Depositional Gradient sea level sediment surface O 2 /H 2 O 0 depth below sediment surface (m) shallowing seawater sulfate LEGEND magnitude of recycling flux water depth 14 m 50 m CO 2 /CH 4 NO 3- /N 2 Mn 4+ /Mn 2+ Fe 3+ /Fe 2+ SO 4 2- /H 2 S δ 34 Spyr ( )

35 Wild Extrapolation t3 δ 34 S isotopic gradient LEGEND magnitude of recycling flux Facies topset deposits foreset deposits bottomset deposits δ 34 S isotopic range δ 34 S t3 Highstand δ 34 S ( ) TST MFS t2 Lowstand SB LST t1 Highstand sea level HST

36 Preliminary Observations Spatial δ 34 S 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 role in generating elevated δ 34 S pyrite records Modern studies provide key insights into interpreting ancient isotopic records. Essential to incorporate depositional environment and stratigraphic context into interpretations

37 Looking forward: Much to be done to understand the long-term S isotopic record Thank you very much for your attention! 40 δ 34 S ( ) 20 0 Sulfate Pyrite Time (Ga)