Composition of metabolizable organic matter in anoxic sediments of the Santa Monica Basin inferred from 14 C and 13 C signatures of particulate and dissolved organic carbon Tomoko Komada 1 David J. Burdige 2, Sabrina M. Crispo 1, Ellen R. M. Druffel 3, Sheila Griffin 3, Leah Johnson 1 1. Romberg Tiburon Center, San Francisco State University, 3152 Paradise Drive, Tiburon CA 94925, USA 2. Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk VA 23529, USA 3. Department of Earth System Science, University of California Irvine, Irvine CA 92697, 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? OS41A-1695 ③ PORE-WATER DOC & DIC PROFILES APPROACH We used natural 14 C and 13 C as proxies for the provenance and diagenetic history of OC. We estimated Δ 14 C and δ 13 C of metabolizable POC by: a)analyzing the isotopic compositions of compound classes extracted from bulk POC b)applying a selective degradation model to Δ 14 C and δ 13 C values of pore-water DOC and DIC ① SAMPLING AND ANALYSES Multicores were collected in Jul 08 aboard R/V Pt. Sur from the Santa Monica Basin, 900 m water depth Bottom-water O 2 ~2 μmol kg -1 Cores were sectioned in N 2 atmosphere in refrigerated van To aid pore-water data interpretation, we built and implemented a selective degradation model: Quasi steady-state, reaction-diffusion model with variable porosity (see 2); first-order degradation kinetics 3 pools of metabolizable POC (G mi ) with unique isotopic compositions (Δ i and δ i ) Δ 1 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 ② ORGANIC COMPOUND CLASSES OCorganic carbon POCparticulate OC A cid S oluble fraction T otal L ipid E xtract A cid I nsoluble residue whole sediment pore water solids residue centrifuge solvent extraction 5 2:1(v/v) CH 2 Cl 2 :CH 3 OH 0.2 μm Nylon filter acid hydrolysis 5 6N HCl, 100°C, 19 hrs DIC sample DOC sample all AS fractions were lost; isotopic values were calculated through mass balance CO 2 for 14 C and 13 C analyses UV oxidation 1 or thermal SO 4 2- reduction 3 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 14 C and 13 C Relative to bulk POC, AS was enriched in 14 C and 13 C These data show that POC is isotopically and chemically heterogeneous ⑤ COMPARISON OF ORGANIC COMPOUND-CLASS AND MODEL RESULTS 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 δ 13 C 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 DOCdissolved OC DICdissolved inorganic carbon Δ i (‰)δ i (‰) G m1 +48±25 (pre) - 52±40 (post) -20.6±0.2 modern, marine, labile to semi-labile G m2 -66± ±0.1 G m3 -520±180-27±2 aged, terrestrial? refractory wt%k i (yr -1 )k DOCi (yr -1 ) G m1 6.5± ±0.02DOC 1 33±9 G m2 1.9±0.2(0.7±0.2)e-2DOC ±0.06 G m3 0.18±0.08=k2=k2 DOC 3 (9±5)e-5 k i ranged by an order of magnitude; k DOCi ranged by 5 orders of magnitude DOC 3 was virtually non- reactive in this model Model results and compound-class data appear internally consistent TLE AS AI Δ 14 C: -120 to +40‰ δ 13 C: -20±1‰ Δ 14 C: -500 to -770‰ δ 13 C: ~-24‰ depth (cm) C (wt%)Δ 14 C (‰)δ 13 C (‰)Porosity TLE AS AI bulk POC post bomb turbidite 45 depth (cm) DOC (mM) DIC (mM) Δ 14 C DOC (‰)δ 13 C DOC (‰) Δ 14 C DIC (‰)δ 13 C DIC (‰) Thermal SO 4 2- Reduction model UV ox Δ 14 C POC ΣDOC DOC 1 DOC 2 DOC 3 δ 13 C POC 45 turbidite post bomb strip in acid 4 Significant depth variability was observed in all profiles Both Δ 14 C DOC and Δ 14 C DIC reached maximum values within the post-bomb layer, then decreased with depth δ 13 C DOC values were similar to δ 13 C POC, but Δ 14 C DOC exceeded Δ 14 C POC at depths >10 cm These data strongly suggest selective degradation of a sub- component of bulk POC 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 , OCE ). References: 1. Beaupré et al. (2007) Limnol. Oceanogr. Meth. 5, Gorsline et al. (2000) Sed. Geol. 135, Johnson and Komada (2011) Limnol. Oceaongr. Meth. 9, McCorkle et al. (1985) Earth Planet. Sci. Lett. 74, Wang et al. (1998) Geochim. Cosmochim. Acta 62, ④ SELECTIVE DEGRADATION MODEL model