Kinetics of organic carbon mineralization and methane formation in marine sediments (Aarhus Bay, Denmark)

TY - JOUR

T1 - Kinetics of organic carbon mineralization and methane formation in marine sediments (Aarhus Bay, Denmark)

AU - Dale, A. W.

AU - Flury, S.

AU - Fossing, H.

AU - Regnier, P.

AU - Røy, H.

AU - Scholze, C.

AU - Jørgensen, B. B.

PY - 2019/5

Y1 - 2019/5

N2 - Sediments were sampled at nine stations on a transect across a 7–10 m thick Holocene mud layer in Aarhus Bay, Denmark, to investigate the linkages between CH 4 dynamics and the rate and depth distribution of organic matter degradation. High-resolution sulfate reduction rates determined by tracer experiments ( 35 S-SRR) decreased by several orders of magnitude down through the mud layer. The rates showed a power law dependency on sediment age: SRR (nmol cm −3 d −1 ) = 10 6.18 × Age −2.17 . The rate data were used to independently quantify enhanced SO 4 2− transport by bioirrigation. Field data (SO 4 2– , TCO 2 , T 13 CO 2 , NH 4 + and CH 4 concentrations) could be simulated with a reaction-transport model using the derived bioirrigation rates and assuming that the power law was continuous into the methanogenic sediments below the sulfate-methane transition zone (SMTZ). The model predicted an increase in anaerobic organic carbon mineralization rates across the transect from 2410 to 3540 nmol C cm −2 d −1 caused by an increase in the sediment accumulation rate. Although methanogenesis accounted for only ∼1% of carbon mineralization, a large relative increase in methanogenesis along the transect led to a considerable shallowing of the SMTZ from 428 to 257 cm. Methane gas bubbles appeared once a threshold in the sedimentation accumulation rate was surpassed. The 35 S-measured SRR data indicated active sulfate reduction throughout the SO 4 2− zone whereas quasi-linear SO 4 2− gradients over the same zone indicated insignificant sulfate reduction. This apparent inconsistency, observed at all stations, was reconciled by considering the transport of SO 4 2− into the sediment by bioirrigation, which accounted for 94 ± 2% of the total SO 4 2− flux across the sediment-water interface. The SRR determined from the quasi-linear SO 4 2− gradients were two orders of magnitude lower than measured rates. We conclude that models solely based on SO 4 2− concentration gradients will not capture high SRRs at the top of the sulfate reduction zone if they do not properly account for (i) SO 4 2− influx by bioirrigation, and/or (ii) the continuity of organic matter reactivity with sediment depth or age.

AB - Sediments were sampled at nine stations on a transect across a 7–10 m thick Holocene mud layer in Aarhus Bay, Denmark, to investigate the linkages between CH 4 dynamics and the rate and depth distribution of organic matter degradation. High-resolution sulfate reduction rates determined by tracer experiments ( 35 S-SRR) decreased by several orders of magnitude down through the mud layer. The rates showed a power law dependency on sediment age: SRR (nmol cm −3 d −1 ) = 10 6.18 × Age −2.17 . The rate data were used to independently quantify enhanced SO 4 2− transport by bioirrigation. Field data (SO 4 2– , TCO 2 , T 13 CO 2 , NH 4 + and CH 4 concentrations) could be simulated with a reaction-transport model using the derived bioirrigation rates and assuming that the power law was continuous into the methanogenic sediments below the sulfate-methane transition zone (SMTZ). The model predicted an increase in anaerobic organic carbon mineralization rates across the transect from 2410 to 3540 nmol C cm −2 d −1 caused by an increase in the sediment accumulation rate. Although methanogenesis accounted for only ∼1% of carbon mineralization, a large relative increase in methanogenesis along the transect led to a considerable shallowing of the SMTZ from 428 to 257 cm. Methane gas bubbles appeared once a threshold in the sedimentation accumulation rate was surpassed. The 35 S-measured SRR data indicated active sulfate reduction throughout the SO 4 2− zone whereas quasi-linear SO 4 2− gradients over the same zone indicated insignificant sulfate reduction. This apparent inconsistency, observed at all stations, was reconciled by considering the transport of SO 4 2− into the sediment by bioirrigation, which accounted for 94 ± 2% of the total SO 4 2− flux across the sediment-water interface. The SRR determined from the quasi-linear SO 4 2− gradients were two orders of magnitude lower than measured rates. We conclude that models solely based on SO 4 2− concentration gradients will not capture high SRRs at the top of the sulfate reduction zone if they do not properly account for (i) SO 4 2− influx by bioirrigation, and/or (ii) the continuity of organic matter reactivity with sediment depth or age.

KW - Bioirrigation

KW - Gas accumulation

KW - Marine

KW - Methanogenesis

KW - Model

KW - Organic matter mineralization kinetics

KW - Seabed

KW - Sulfate reduction

UR - http://www.scopus.com/inward/record.url?scp=85063010282&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2019.02.033

DO - 10.1016/j.gca.2019.02.033

M3 - Journal article

AN - SCOPUS:85063010282

SN - 0016-7037

VL - 252

SP - 159

EP - 178

JO - Geochimica et Cosmochimica Acta

JF - Geochimica et Cosmochimica Acta

IS - May

ER -