hydrate zone to lower frequencies in the free gas zone
(Berndt et al., 2004) beneath the BSR may be used.
Dynamics of BSRs
Both the diagenetic- and the hydrate-related BSR may be
used to evaluate the thermal state or reconstruct the thermal development of a sedimentary basin (Grevemeyer
and Villinger, 2001; Nouzé et al., 2009). Particularly, the
hydrate-related BSR is widely used as a proxy for heat
flow in marine sediments. On a small scale,
BSR-derived heat flow changes may often be associated
with structures that focus warm fluids from deep sediments like mud volcanoes (Depreiter et al., 2005) or chimneys (Rajan et al., 2013). In such instance, the BSR may
not mimic the seafloor if lateral variations in heat flow or
changes in gas compositions exist. Generally, an increase
in heat flow causes a shoaling of a BSR, whereas an
increase of higher-order hydrocarbons causes a deepening. Hence, the depth of the BSR may vary greatly as,
for example, in the Barents Sea depending on the contribution and thus the amount of thermogenic gases migrating
into the GHSZ (Chand et al., 2008; Rajan et al., 2013).
More recently, BSR observations and hydrate stability
zone modeling of the upper pinch-out zone on continental
margins have been used to assess past and contemporary
changes in hydrate stability through warming of ocean
bottom water (Vogt and Jung, 2002; Mienert et al., 2005;
Biastoch et al., 2011; Ferré et al., 2012; Phrampus and
Hornbach, 2012).
Inferred former positions of a BSR are often referred to
as paleo-BSR. One of the best examples for a paleo-BSR
can be found on the Blake Ridge, approx. 450 km offshore
Georgia on the East Coast of the United States (Hornbach
et al., 2003). This BSR formed when erosion by strong
contour currents on the eastern flank of the Blake Ridge
removed the top sediments of a hydrated formation causing an adjustment of the hydrate/free gas boundary by
moving it deeper. The free gas layer beneath the former
BSR crystallized into a newly formed concentrated layer
of hydrates causing both a density and velocity increase.
Beneath this paleo-BSR, the new BSR formed with free
gas underneath the base of the gas hydrate stability zone
(BGHSZ). Though the timing of the readjustment is
unknown, it presents one good example for the dynamic
behavior (in this case deepening) of a BSR due to erosion
of sediments and a drop in seafloor temperature.
Summary and conclusions
Bottom-simulating reflectors (BSRs) occur in a wide
range of sediments in the world oceans. Such creation of
BSRs involves the existence of free gas and water in the
pore space of sediments under low temperature and high
pressure, forming hydrates beneath the ocean floor. The
depth of the BSR defines the base of the gas hydrate stability zone (BGHSZ) under which free gas accumulates. Free
gas becomes trapped beneath the hydrate-charged layer
causing a distinct impedance contrast and a seismic
reflection of reversed (negative) polarity if compared to
the seafloor. The GHSZ depends on temperature and pressure and to a lesser degree on salinity and gas composition
(thermogenic, biogenic). The thickness of the GHSZ
decreases toward the upper continental margins (lower
pressure) and sedimented ocean ridges (higher temperature). The second type of BSRs concentrates in regions
of siliceous ocean sediments. Here, increases in temperature with burial depth result in dissolution of siliceous
skeletons, which in turn creates an interface with higher
porosity and permeability above and lower values beneath
the interface. If the contrast becomes large enough, a seismic reflector occurs but with positive polarity (no phase
reversal). As a consequence, diagenetic BSRs occur commonly deeper, show no phase reversal, and exist over large
areas at the opal-A/opal-CT interface in ocean sediments.
Bibliography
Berndt, C., Bünz, S., Clayton, T., Mienert, J., and Saunders, M.,
2004. Seismic character of bottom simulating reflectors: examples from the mid-Norwegian margin. Marine and Petroleum
Geology, 21, 723–733.
Biastoch, A., Treude, T., Rüpke, L. H., Riebesell, U., Roth, C.,
Burwicz, E. B., Park, W., Latif, M., Böning, C. W., Madec, G.,
and Wallmann, K., 2011. Rising Arctic Ocean temperatures
cause gas hydrate destabilization and ocean acidification. Geophysical Research Letters, 38, L08602.
Brekke, H., 2000. The tectonic evolution of the Norwegian Sea continental margin with emphasis on the Vøring and More basins. In
Nottvedt, A. (ed.), Dynamics of the Norwegian Margin. Geological Society of London Special Publication 167. London: Geological Society, pp. 327–378.
Bünz, S., Mienert, J., and Berndt, C., 2003. Geological controls on
the Storegga gas-hydrate system of the mid-Norwegian continental margin. Earth and Planetary Science Letters, 209(3–4),
291–307.
Cartwright, J. A., and Dewhurst, D. N., 1998. Layer-bound compaction faults in fine-grained sediments. Bulletin of the Geological
Society of America, 110(10), 1242–1257.
Chand, S., Mienert, J., Andreassem, K., Knies, J., Plassen, L., and
Fotland, B., 2008. Gas hydrate stability zone modelling in areas
of salt tectonics and pockmarks of the Barents Sea suggest an
active hydrocarbon venting system. Marine and Petroleum
Geology, 25, 625–636.
Davies, R. J., and Cartwright, J. A., 2002. A fossilized opal-A to
opal C/T transformation on the northeast Atlantic margin: support for a significantly elevated paleogeothermal gradient during
the Neogene? Basin Research, 14, 467–486.
Depreiter, D., Poort, J., Van Rensbergen, P., and Henriet, J. P., 2005.
Geophysical evidence of gas hydrates in shallow submarine mud
volcanoes on the Moroccan margin. Journal of Geophysical
Research, 110, B10103, doi:10.1029/2005JB003622.
Dickens, G. R., and Quinby-Hunt, M. S., 1997. Methane hydrate
stability in pore water: a simple theoretical approach for geophysical applications. Journal of Geophysical Research, 102,
773–783.
Ferré, B., Mienert, J., and Feseker, T., 2012. Ocean temperature variability for the past 60 years on the Norwegian-Svalbard margin
influences gas hydrate stability on human time scales. Journal of
Geophysical Research, Oceans, 117, C10017.
Grevemeyer, I., and Villinger, H., 2001. Gas hydrate stability and
the assessment of heat flow through continental margins. Geophysical Journal International, 145, 647–660.
66
BOTTOM SIMULATING SEISMIC REFLECTORS (BSR)
(Berndt et al., 2004) beneath the BSR may be used.
Dynamics of BSRs
Both the diagenetic- and the hydrate-related BSR may be
used to evaluate the thermal state or reconstruct the thermal development of a sedimentary basin (Grevemeyer
and Villinger, 2001; Nouzé et al., 2009). Particularly, the
hydrate-related BSR is widely used as a proxy for heat
flow in marine sediments. On a small scale,
BSR-derived heat flow changes may often be associated
with structures that focus warm fluids from deep sediments like mud volcanoes (Depreiter et al., 2005) or chimneys (Rajan et al., 2013). In such instance, the BSR may
not mimic the seafloor if lateral variations in heat flow or
changes in gas compositions exist. Generally, an increase
in heat flow causes a shoaling of a BSR, whereas an
increase of higher-order hydrocarbons causes a deepening. Hence, the depth of the BSR may vary greatly as,
for example, in the Barents Sea depending on the contribution and thus the amount of thermogenic gases migrating
into the GHSZ (Chand et al., 2008; Rajan et al., 2013).
More recently, BSR observations and hydrate stability
zone modeling of the upper pinch-out zone on continental
margins have been used to assess past and contemporary
changes in hydrate stability through warming of ocean
bottom water (Vogt and Jung, 2002; Mienert et al., 2005;
Biastoch et al., 2011; Ferré et al., 2012; Phrampus and
Hornbach, 2012).
Inferred former positions of a BSR are often referred to
as paleo-BSR. One of the best examples for a paleo-BSR
can be found on the Blake Ridge, approx. 450 km offshore
Georgia on the East Coast of the United States (Hornbach
et al., 2003). This BSR formed when erosion by strong
contour currents on the eastern flank of the Blake Ridge
removed the top sediments of a hydrated formation causing an adjustment of the hydrate/free gas boundary by
moving it deeper. The free gas layer beneath the former
BSR crystallized into a newly formed concentrated layer
of hydrates causing both a density and velocity increase.
Beneath this paleo-BSR, the new BSR formed with free
gas underneath the base of the gas hydrate stability zone
(BGHSZ). Though the timing of the readjustment is
unknown, it presents one good example for the dynamic
behavior (in this case deepening) of a BSR due to erosion
of sediments and a drop in seafloor temperature.
Summary and conclusions
Bottom-simulating reflectors (BSRs) occur in a wide
range of sediments in the world oceans. Such creation of
BSRs involves the existence of free gas and water in the
pore space of sediments under low temperature and high
pressure, forming hydrates beneath the ocean floor. The
depth of the BSR defines the base of the gas hydrate stability zone (BGHSZ) under which free gas accumulates. Free
gas becomes trapped beneath the hydrate-charged layer
causing a distinct impedance contrast and a seismic
reflection of reversed (negative) polarity if compared to
the seafloor. The GHSZ depends on temperature and pressure and to a lesser degree on salinity and gas composition
(thermogenic, biogenic). The thickness of the GHSZ
decreases toward the upper continental margins (lower
pressure) and sedimented ocean ridges (higher temperature). The second type of BSRs concentrates in regions
of siliceous ocean sediments. Here, increases in temperature with burial depth result in dissolution of siliceous
skeletons, which in turn creates an interface with higher
porosity and permeability above and lower values beneath
the interface. If the contrast becomes large enough, a seismic reflector occurs but with positive polarity (no phase
reversal). As a consequence, diagenetic BSRs occur commonly deeper, show no phase reversal, and exist over large
areas at the opal-A/opal-CT interface in ocean sediments.
Bibliography
Berndt, C., Bünz, S., Clayton, T., Mienert, J., and Saunders, M.,
2004. Seismic character of bottom simulating reflectors: examples from the mid-Norwegian margin. Marine and Petroleum
Geology, 21, 723–733.
Biastoch, A., Treude, T., Rüpke, L. H., Riebesell, U., Roth, C.,
Burwicz, E. B., Park, W., Latif, M., Böning, C. W., Madec, G.,
and Wallmann, K., 2011. Rising Arctic Ocean temperatures
cause gas hydrate destabilization and ocean acidification. Geophysical Research Letters, 38, L08602.
Brekke, H., 2000. The tectonic evolution of the Norwegian Sea continental margin with emphasis on the Vøring and More basins. In
Nottvedt, A. (ed.), Dynamics of the Norwegian Margin. Geological Society of London Special Publication 167. London: Geological Society, pp. 327–378.
Bünz, S., Mienert, J., and Berndt, C., 2003. Geological controls on
the Storegga gas-hydrate system of the mid-Norwegian continental margin. Earth and Planetary Science Letters, 209(3–4),
291–307.
Cartwright, J. A., and Dewhurst, D. N., 1998. Layer-bound compaction faults in fine-grained sediments. Bulletin of the Geological
Society of America, 110(10), 1242–1257.
Chand, S., Mienert, J., Andreassem, K., Knies, J., Plassen, L., and
Fotland, B., 2008. Gas hydrate stability zone modelling in areas
of salt tectonics and pockmarks of the Barents Sea suggest an
active hydrocarbon venting system. Marine and Petroleum
Geology, 25, 625–636.
Davies, R. J., and Cartwright, J. A., 2002. A fossilized opal-A to
opal C/T transformation on the northeast Atlantic margin: support for a significantly elevated paleogeothermal gradient during
the Neogene? Basin Research, 14, 467–486.
Depreiter, D., Poort, J., Van Rensbergen, P., and Henriet, J. P., 2005.
Geophysical evidence of gas hydrates in shallow submarine mud
volcanoes on the Moroccan margin. Journal of Geophysical
Research, 110, B10103, doi:10.1029/2005JB003622.
Dickens, G. R., and Quinby-Hunt, M. S., 1997. Methane hydrate
stability in pore water: a simple theoretical approach for geophysical applications. Journal of Geophysical Research, 102,
773–783.
Ferré, B., Mienert, J., and Feseker, T., 2012. Ocean temperature variability for the past 60 years on the Norwegian-Svalbard margin
influences gas hydrate stability on human time scales. Journal of
Geophysical Research, Oceans, 117, C10017.
Grevemeyer, I., and Villinger, H., 2001. Gas hydrate stability and
the assessment of heat flow through continental margins. Geophysical Journal International, 145, 647–660.
66
BOTTOM SIMULATING SEISMIC REFLECTORS (BSR)