TY - JOUR
T1 - The enigmatic ichnofossil Tisoa siphonalis and widespread authigenic seep carbonate formation during the Late Pliensbachian in southern France
AU - van de Schootbrugge, B.
AU - Harazim, D.
AU - Sorichter, K.
AU - Oschmann, W.
AU - Fiebig, J.
AU - Püttmann, W.
AU - Peinl, M.
AU - Zanella, F.
AU - Teichert, B. M. A.
AU - Hoffmann, J.
AU - Stadnitskaia, A.
AU - Rosenthal, Y.
PY - 2010
Y1 - 2010
N2 - Tubular carbonate concretions of up to 1 m in length and perpendicular
to bedding, occur abundantly in the Upper Pliensbachian (upper Amaltheus
margaritatus Zone, Gibbosus Subzone) in outcrops (Fontaneilles section)
in the vicinity of Rivière-sûr-Tarn, southern France.
Stable isotope analyses of these concretions show negative
δ13C values that decrease from the rim to the center
from -18.8‰ to -25.7‰ (V-PDB), but normal
marine δ18O values (-1.8‰). Carbon isotope
analyses of Late Pliensbachian bulk carbonate (matrix) samples from the
Fontaneilles section show clearly decreasing C-isotope values across the
A. margaritatus Zone, from +1‰ to -3‰ (V-PDB).
Isotope analyses of coeval belemnite rostra do not document such a
negative C-isotope trend with values remaining stable around +2‰
(V-PDB). Computer tomographic (CT) scanning of the tubular concretions
show multiple canals that are lined or filled entirely with pyrite.
Previously, the formation of these concretions with one, two, or more
central tubes, has been ascribed to the activity of an enigmatic
organism, possibly with annelid or arthropod affinities, known asTisoa
siphonalis. Our results suggest tisoan structures are abiogenic. Based
on our geochemical analyses and sedimentological observations we suggest
that these concretions formed as a combination of the anaerobic
oxidation of methane (AOM) and sulfate reduction within the sediment.
Fluids rich in methane and/or hydrocarbons likely altered local bulk
rock carbon isotope records, but did not affect the global carbon cycle.
Interestingly, Tisoa siphonalis has been described from many locations
in the Grands Causses Basin in southern France, and from northern France
and Luxemburg, always occurring at the same stratigraphic level. Upper
Pliensbachian authigenic carbonates thus possibly cover an area of many
thousand square kilometers. Greatly reduced sedimentation rates are
needed to explain the stabilization of the sulfate-methane transition
zone in the sedimentary column in order for the tubular concretions to
form. Late Pliensbachian cooling, reducing run-off, and/or the influx of
colder water and more vigorous circulation could be responsible for a
halt in sedimentation. At the same time (thermogenic) methane may have
destabilized during a major phase of Late Pliensbachian sea level fall.
As such Tisoa siphonalis is more than a geological curiosity, and its
further study could prove pivotal in understanding Early Jurassic
paleoenvironmental change.
AB - Tubular carbonate concretions of up to 1 m in length and perpendicular
to bedding, occur abundantly in the Upper Pliensbachian (upper Amaltheus
margaritatus Zone, Gibbosus Subzone) in outcrops (Fontaneilles section)
in the vicinity of Rivière-sûr-Tarn, southern France.
Stable isotope analyses of these concretions show negative
δ13C values that decrease from the rim to the center
from -18.8‰ to -25.7‰ (V-PDB), but normal
marine δ18O values (-1.8‰). Carbon isotope
analyses of Late Pliensbachian bulk carbonate (matrix) samples from the
Fontaneilles section show clearly decreasing C-isotope values across the
A. margaritatus Zone, from +1‰ to -3‰ (V-PDB).
Isotope analyses of coeval belemnite rostra do not document such a
negative C-isotope trend with values remaining stable around +2‰
(V-PDB). Computer tomographic (CT) scanning of the tubular concretions
show multiple canals that are lined or filled entirely with pyrite.
Previously, the formation of these concretions with one, two, or more
central tubes, has been ascribed to the activity of an enigmatic
organism, possibly with annelid or arthropod affinities, known asTisoa
siphonalis. Our results suggest tisoan structures are abiogenic. Based
on our geochemical analyses and sedimentological observations we suggest
that these concretions formed as a combination of the anaerobic
oxidation of methane (AOM) and sulfate reduction within the sediment.
Fluids rich in methane and/or hydrocarbons likely altered local bulk
rock carbon isotope records, but did not affect the global carbon cycle.
Interestingly, Tisoa siphonalis has been described from many locations
in the Grands Causses Basin in southern France, and from northern France
and Luxemburg, always occurring at the same stratigraphic level. Upper
Pliensbachian authigenic carbonates thus possibly cover an area of many
thousand square kilometers. Greatly reduced sedimentation rates are
needed to explain the stabilization of the sulfate-methane transition
zone in the sedimentary column in order for the tubular concretions to
form. Late Pliensbachian cooling, reducing run-off, and/or the influx of
colder water and more vigorous circulation could be responsible for a
halt in sedimentation. At the same time (thermogenic) methane may have
destabilized during a major phase of Late Pliensbachian sea level fall.
As such Tisoa siphonalis is more than a geological curiosity, and its
further study could prove pivotal in understanding Early Jurassic
paleoenvironmental change.
U2 - 10.5194/bg-7-3123-2010
DO - 10.5194/bg-7-3123-2010
M3 - Article
SN - 1726-4170
VL - 7
SP - 3123
EP - 3138
JO - Biogeosciences
JF - Biogeosciences
IS - 10
ER -