INTRODUCTION

1. Composition of metabolizable organic matter in anoxic sediments of the Santa Monica Basin inferred from 14C and 13C signatures of particulate and dissolved organic carbonTomoko Komada1 (tkomada@sfsu.edu), David J. Burdige2, Sabrina M. Crispo1, Ellen R. M. Druffel3, Sheila Griffin3, Leah Johnson11. Romberg Tiburon Center, San Francisco State University, 3152 Paradise Drive, Tiburon CA 94925, USA2. Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk VA 23529, USA3. Department of Earth System Science, University of California Irvine, Irvine CA 92697, USA OS41A-1695 SELECTIVE DEGRADATION MODEL INTRODUCTION Organic-rich continental margin sediments are important sites for OC degradation and burial. We want to better understand the controls behind OC degradation in these environments. What is the composition of OC that is susceptible to microbial degradation in anoxic sediments of the Santa Monica Basin? ORGANIC COMPOUND CLASSES To aid pore-water data interpretation, we built and implemented a selective degradation model: Porosity C (wt%) Δ14C (‰) δ13C (‰) post bomb model depth (cm) turbidite APPROACH We used natural 14C and 13C as proxies for the provenance and diagenetic history of OC. We estimated Δ14C and δ13C of metabolizablePOC by: analyzing the isotopic compositions of compound classes extracted from bulk POC applying a selective degradation model to Δ14C and δ13C values of pore-water DOC and DIC TLE AS bulk POC AI • A discontinuity was present at ~20 cm due to emplacement of a turbidite 2 • AI was the dominant fraction in all samples; TLE was least abundant • Relative to bulk POC, TLE was depleted in 14C and 13C • Relative to bulk POC, AS was enriched in 14C and 13C • These data show that POC is isotopically and chemically heterogeneous • Quasi steady-state, reaction-diffusion model with variable porosity (see 2); first-order degradation kinetics • 3 pools of metabolizable POC (Gmi) with unique isotopic compositions (Δi and δi) • Δ1 is allowed to vary across the bomb horizon • No isotopic fractionation associated with degradation • Fit to pore-water DOC, DIC and their isotopic values; lower boundary at 45 cm; solved numerically PORE-WATER DOC & DIC PROFILES OCorganic carbonPOC particulate OC DOC dissolved OC DICdissolved inorganic carbon DOC (mM) Δ14CDOC (‰) δ13CDOC (‰) COMPARISON OF ORGANIC COMPOUND-CLASS AND MODEL RESULTS post bomb DOC1 SAMPLING AND ANALYSES UVox • ki ranged by an order of magnitude; kDOCiranged by 5 orders of magnitude • DOC3 was virtually non-reactive in this model • Model results and compound-class data appear internally consistent DOC2 • Multicores were collected in Jul 08 aboard R/V Pt. Sur from the Santa Monica Basin, 900 m water depth • Bottom-water O2 ~2 μmol kg-1 • Cores were sectioned in N2 atmosphere in refrigerated van depth (cm) Δ14CPOC δ13CPOC Thermal SO42- Reduction ΣDOC DOC3 45 DIC (mM) Δ14CDIC (‰) δ13CDIC (‰) solids centrifuge whole sediment solvent extraction 52:1(v/v) CH2Cl2:CH3OH depth (cm) 0.2 μm Nylon filter model Δ14C: -120 to +40‰ AS δ13C: -20±1‰ turbidite pore water TotalLipidExtract residue 45 Δ14C: -500 to -770‰ • Significant depth variability was observed in all profiles • Both Δ14CDOCand Δ14CDIC reached maximum values within the post-bomb layer, then decreased with depth • δ13CDOC valueswere similar to δ13CPOC, but Δ14CDOC exceeded Δ14CPOCat depths >10 cm • These data strongly suggest selective degradation of a sub-component of bulk POC TLE AI δ13C: ~-24‰ acid hydrolysis 56N HCl, 100°C, 19 hrs • SUMMARY • Both organic compound class data and a selective degradation model suggest that relatively modern, marine-like components of bulk POC are more susceptible to degradation than older OC with lower δ13C values • Compound class data and model results appear internally consistent • Carbon isotope measurements may help provide mechanistic insight into the controls behind organic matter degradation and preservation DOC sample DIC sample AcidInsolubleresidue AcidSolublefraction combust @850°C strip in acid 4 all AS fractions were lost; isotopic values were calculated through mass balance combust @850°C References: 1. Beaupré et al. (2007) Limnol. Oceanogr. Meth. 5, 174-184. 2. Gorsline et al. (2000) Sed. Geol. 135, 21-35. 3. Johnson and Komada (2011) Limnol. Oceaongr. Meth. 9, 485-498.4. McCorkle et al. (1985) Earth Planet. Sci. Lett. 74, 13-26.5. Wang et al. (1998) Geochim. Cosmochim. Acta62, 1365-1378. CO2 for 14C and 13C analyses UV oxidation 1orthermal SO42- reduction 3 Acknowledgments: We thank the captain and crew of R/V Point Sur, J. Polly, M. Jinuntuya, and A. Pitts for their assistance. This work was funded by grants from the NSF (OCE-0726819, OCE-0727179).