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 ([email protected]), 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
1. Overview
3. Pore-water and solid-phase profiles
Deeply-buried sediments below the sulfatereducing zone play a major role in organic matter
remineralization1-3. However, the nature of the
processes occurring in these deep sediments are
still unclear.
Ca2+
(a)
POC
SO42-
(b)
5. CH4 appears to be mostly from in situ methanogenesis, but input
from external source cannot be ruled out (Q2)
(c)
Table 2. Flux budget above the SMT
(d)
POC
SMT
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).
SMT
DIC
DIC
CH4
CH4
DOC
DOC
Fig. 2
Santa Barbara
Basin
Los Angeles
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.
2. Specific questions
Q3: What is the depth scale of
remineralization at this site?
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.
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).
we use mixing
DIC
JDIC = -0.52±0.01
SO42-
JSO4 = 0.473±0.009
Ca2+
JCa = 0.106±0.006
Mg2+
JMg = 0.058
curves5,6
to estimate the
δ13C
and Δ14C
DIC flux corrected for JDIC - JCa - JMg = -0.68±0.02
carbonate precipitation
#calculated from concentration gradients (Fig. 2a) as
described in ref 8 with exception of Mg (from ref 9)
-23.6 ± 0.7‰
1.0-4.4 m
-27.2 ± 0.2‰
0.2-1.1 m
Fig. 3
δ13CDOC
δ13CDIC
δ13CCH4
approx.
location
of SMT
[C] (mmol kg-1), where C is CH4 (left), DIC (middle) or DOC (right)
•  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
•  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
POC
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
Table 3. Flux budget above the SMT
46± 2‰
1.7-4.4 m
•  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?
6. Estimating the depth scale of remineralization from Δ14C values
values of C added to a given depth interval
-20.4 ±
0.5‰
-87.2 ± 0.8‰
0.9-2.4 m
d-1)#
Fig. 5
< 0.1 m
[C]*δ13C (mmol kg-1*‰)
Q2: Is the CH4 produced by in situ
methanogenesis, or
transportedfrom an external
source (e.g., decomposing
hydrates)?
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
diffusive flux (mmol
4. Evaluation of carbon sources using δ13C and Δ14C mixing plots
solute Ds (cm2 yr-1) L (m)#
diffusion
diffusive displacement over
coefficient 450 kyr (~8 x 14C half-life)
DOC
50
~20
DIC
122
~33
CH4
180
~40
#calculated using the Einstein-Smoluchowski equation:
L2 = 2Dst , where t = time (ref 12)
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
➔ 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)
7. Summary
[C]*Δ14C (mmol kg-1*‰)
Q1: Is the CH4 observed at this site
(Fig. 1) biogenic or thermogenic?
• 
• 
• 
• 
• 
• 
• 
• 
solute
m-2
-358 ± 5‰
0.85-1.25 m
-390 ± 10‰
0.2-0.9 m
-440 ± 30‰
1.5-1.8 m
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
-57 ± 3‰
0-2 cm
-590 ± 30‰
2.0-2.4 m
Δ14CCH4
>2.4 m?
Δ14CDIC
-1030 ± 80‰
2.3-4.4 m
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
-930 ± 40‰
2.3-4.4 m
Δ14CDOC
Fig. 4
[C] (mmol kg-1), where C is CH4 (left), DIC (middle) or DOC (right)
•  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
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?