Paleomagnetic imprints of sulfate reduction pathways in continental
shelf sediments: organoclastic versus anaerobic oxidation of methane
Abstract
Marine continental shelf sediments with high deposition rates may
provide useful archives of rapid geomagnetic secular variation as long
as the primary magnetization is not altered substantially by diagenesis.
To quantify the effects of sulfate (SO42-) reduction, which is a
dominant early diagenetic processes in such sediments, on paleomagnetic
recording, we analyzed four ~6-m long sediment cores
from the Mediterranean shelf. Two cores did not reach the methanogenic
zone and are characterized by continuous organoclastic sulfate reduction
(OSR), while the other two have a distinctive shallow sulfate-methane
transition zone (SMTZ). Depth-age models based on 28 radiocarbon ages
show that deposition was mostly non-synchronous, suggesting that
different parts of the shelf stopped accumulating sediments at different
times during the Holocene. The upper sediment column in all cores is
dominated by detrital titanomagnetite and biogenic magnetite.
OSR-affected sediments record continuous dissolution of the
(titano)magnetites, resulting in a steady decrease in magnetic
susceptibility and remanent magnetic properties. For cores that reach
the methanogenic zone, similar behavior is observed at or above the
STMZ, but the magnetic properties stabilize at greater depths.
Paleomagnetic directions in these sediments are more coherent, with
better agreement with geomagnetic models than sediments affected by OSR.
We suggest that methane-rich sediments with a shallow SMTZ and high
sedimentation rates can better preserve primary paleomagnetic signals
than OSR-dominated sediments due to a lack of dissolved sulfide in the
main methanogenic zone, and that a susceptibility decline with depth
should be a warning sign for paleomagnetic studies.