solutes are subject to variable degrees of 14C decay during diffusive transport (Table 3) •  diffusive time scale is comparable to

14C half-life over a depth scale of 10s of meters

•  we expect 14C decay during diffusive transport to be most evident in DOC, and least in CH4

•  SO42--CH4 transition (SMT) is evident at ~1.25 m (gray bar)

•  positive gradients below the SMT indicate net production of DIC and DOC at depth (a,b) •  linear DIC and DOC profiles below the SMT suggests this zone is diffusion-dominated (a,b) •  particulate organic carbon (POC) and DOC have similar δ13C values (c) •  highest δ13C values are seen in DIC; lowest in CH4 (c) •  Δ14C values of all carbon pools decrease with depth (d) •  CH4 and DIC have lower Δ14C values than DOC and POC at any given depth below the SMT (d) •  CH4 data (concentration and isotopic values) agree closely with literature values4

Radiocarbon Values of Methane (CH4), Dissolved Inorganic Carbon (DIC), and Dissolved Organic Carbon (DOC) in Santa Barbara Basin Sediments: Implications for Carbon Cycling Below the Sulfate-Reducing Zone

Komada T.1 (tkomada@sfsu.edu), Burdige D. J.2, Li H. L.1, Cada A. K.1, Chanton J.3, Magen C.4 1. Romberg Tiburon Center, San Francisco State University, Tiburon CA, USA

2. Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk VA, USA 3. Department of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee FL, USA

4. Chesapeake Biological Laboratory, University of Maryland, Solomons M, USA

2. Specific questions Q1: Is the CH4 observed at this site

(Fig. 1) biogenic or thermogenic?

Q2: Is the CH4 produced by in situ methanogenesis, or transportedfrom an external source (e.g., decomposing hydrates)?

Q3: What is the depth scale of remineralization at this site?

5. CH4 appears to be mostly from in situ methanogenesis, but input from external source cannot be ruled out (Q2)

7. Summary Q1: CH4 observed at this site appears to be largely of biogenic origin

Q2: the source of the CH4 appears to be in situ methanogenesis, possibly with some input from external source

Q3: Δ14C values of DIC, DOC diffusing into the base of these cores suggest that remineralization occurs over a depth scale of at least 10s of meters

1. Overview Deeply-buried sediments below the sulfate-reducing zone play a major role in organic matter remineralization1-3. However, the nature of the processes occurring in these deep sediments are still unclear. Here, we aim to learn more about these deep processes occurring in the central part of the Santa Barbara Basin (SBB; Fig. 1) by examining Δ14C and δ13C values of pore-water methane (CH4), dissolved inorganic carbon (DIC), and dissolved organic carbon (DOC).

Fig. 1. Map of study site. 34° 13.47 N, 119° 59.00 W, 590 m water depth. Sediment cores were collected by multi-coring and gravity coring. Cores were processed within 3 hours of recovery.

Santa Barbara Basin

Los Angeles

3. Pore-water and solid-phase profiles

8. Next steps and more questions •  complete the dataset (including a similar dataset for

neighboring Santa Monica Basin) •  conduct a more comprehensive data analysis through

construction of a reaction-transport model •  does any of the ~14C-dead DOC reach the water

column? •  what is the composition of this DOC produced at depth?

•  CH4 slope is consistent with a biogenic source7 (Q1) •  DIC slope varied from -27 to -20‰ above the SMT to 46‰ below the SMT ➔ consistent with marine organic matter oxidation near core top, and methanogenesis below SMT

•  DOC slope is intermediate of DIC slopes above the SMT

solute diffusive flux (mmol m-2 d-1)#

DIC JDIC = -0.52±0.01

SO42- JSO4 = 0.473±0.009

Ca2+ JCa = 0.106±0.006

Mg2+ JMg = 0.058

DIC flux corrected for carbonate precipitation

JDIC - JCa - JMg = -0.68±0.02

•  DIC:SO4 flux ratio after correcting for assumed carbonate precipitation is 1.4±0.1 (=0.68/0.473), which is lower than the expected value of ~1.75 (ref 8) à is there some input of external

CH4? •  CH4 from a local seep (a possible

external source) has δ13C values that are much higher than the value observed here (-40 to -50‰)4

6. Estimating the depth scale of remineralization from Δ14C values

POC remineralization should introduce 14C to the pore water to ~50 m depth (Fig. 5) •  Δ14C-POC profile looks linear below 0.5 m (black circles) •  14C decay curve using a sedimentation rate of 0.12 cm yr-1

(solid line; ref 10,11) fits the data well •  this decay curve approaches -1000‰ at ~ 50 m depth

solute Ds (cm2 yr-1) diffusion coefficient

L (m)# diffusive displacement over 450 kyr (~8 x 14C half-life)

DOC 50 ~20

DIC 122 ~33

CH4 180 ~40

•  all slopes of the Δ14C mixing plots decrease with increasing depth •  DIC and DOC added to the deeper sections of these cores appear to be ~14C-dead •  14C-depleted C likely originates from deeper sediments, because POC has relatively high Δ14C values at the base

of these cores (Fig. 2d) à linear DIC and DOC profiles below the SMT are consistent with this suggestion (Fig. 2a,b)

•  CH4 data from depths >2.4 m are not yet available, but pore water profiles (Fig. 2d) predict similar trend as DIC

Acknowledgments We thank Dale Hubbard and the OSU Coring Facility, Meghan Donohue, captain and crew of R/V New Horizon, Adrian Gerretson, Ashley Grose, Jeremy Bleakney, Patrick Tennis, Bryce Riegel, and Sheila Griffin. This work was funded by NSF (OCE-1155764, 1155562, 1155320).

CH4

SO42-

Ca2+

DIC

DOC

POC

DOC

DIC

CH4

POC

SMT SMT

(a) (b) (c) (d)

Fig. 2

4. Evaluation of carbon sources using δ13C and Δ14C mixing plots we use mixing curves5,6 to estimate the δ13C and Δ14C values of C added to a given depth interval

#calculated from concentration gradients (Fig. 2a) as described in ref 8 with exception of Mg (from ref 9)

Table 2. Flux budget above the SMT

POC

Fig. 5

Table 3. Flux budget above the SMT

#calculated using the Einstein-Smoluchowski equation: L2 = 2Dst , where t = time (ref 12) References

1.  D’Hondt et al. (2002) Science 295, 2067-2070. 2.  D’Hondt et al. (2004) Science 306, 2216-2221. 3.  Berelson et al (2005) Geochim. Cosmochim.

Acta 69, 4611-4629. 4.  Kessler J. D. et al. (2008) J. Geophys. Res.

113, C12021, doi:10.1029/2008JC004822. 5.  Aller R. C. et al. (2008) J. Geophys. Res. 113,

F01S09, doi:10.1029/2006JF000689. Cosmochim. Acta 60, 4037-4057.

6.  Hu X. and Burdige D. J. (2007) Geochim. Cosmochim Acta 71, 129-144.

7.  Whiticar M. J. (1999) Chem. Geol. 161, 291-314.

8.  Burdige D. J. and Komada T. (2011) Limnol. Oceanogr. 56, 1781-1796.

9.  Reimers C. E. et al. (1996) Geochim. Cosmochim. Acta 60,4037-4057.

10. Hill T. M. et al. (20004) Earth Planet. Sci. Lett. 223, 127-140.

11.  Ingram B. L. and Kennett J. P. (1995) Proc. ODP. Sci. Results 146 (Pt. 2).

12. Boudreau B. P. (1997) Diagenetic Models and Their Implementation. Springer-Verlag.

[C] (mmol kg-1), where C is CH4 (left), DIC (middle) or DOC (right)

[C]*Δ1

4 C (m

mol

kg-1

*‰) -440 ± 30‰

1.5-1.8 m

-590 ± 30‰ 2.0-2.4 m

Δ14CCH4 >2.4 m?

-358 ± 5‰ 0.85-1.25 m

-930 ± 40‰ 2.3-4.4 m

Δ14CDOC Fig. 4

➔  input of uniformly ~14C-dead DIC and DOC into the base of our cores (~4.5 m; Fig. 4) despite differences in Ds suggests that the remineralization occurs over a depth scale sufficiently large to overshadow any effects of differential 14C decay during diffusive transport (Q3)

➔ see related discussions in Burdige et al. (poster# 525, session 117)

-390 ± 10‰ 0.2-0.9 m

-1030 ± 80‰ 2.3-4.4 m

Δ14CDIC

-57 ± 3‰ 0-2 cm

[C] (mmol kg-1), where C is CH4 (left), DIC (middle) or DOC (right)

[C]*δ1

3 C (m

mol

kg-1

*‰)

-20.4 ± 0.5‰

-27.2 ± 0.2‰ 0.2-1.1 m

46± 2‰ 1.7-4.4 m

< 0.1 m

δ13CDIC

-87.2 ± 0.8‰ 0.9-2.4 m

δ13CCH4

-23.6 ± 0.7‰ 1.0-4.4 m

δ13CDOC

approx. location of SMT

Fig. 3