detritus preceding the deposition of detrital carbonate-rich
debris from the LIS (Bond et al., 1999; Grousset et al.,
2001) further support the conclusions by Dowdeswell
et al. (1999) that the dynamics of Quaternary ice sheets
surrounding the Nordic Seas and North Atlantic were
asynchronous. This implies that each glaciated margin
may have behaved differently in response to external
ocean–climate forcing.
Iceberg surging mechanisms
Since the early 1990s, several possible mechanisms
responsible for forcing large-scale IRD events have been
proposed (e.g., Alley and Clark, 1999). The binge–purge
hypothesis of MacAyeal (1993) invoked internal ice sheet
dynamics as a driver for H-events. However, this mechanism is not supported by the detailed structure of
H-events and cannot explain North Atlantic records showing millennial and centennial scale IRD fluctuations that
appear to be closely coupled to the ocean–atmosphere climate system (e.g., Bond and Lotti, 1995). Apart from
internal ice sheet dynamics, ocean circulation changes,
sea-level fluctuations, variations in solar parameters, as
well as ice-load-induced earthquakes have been proposed
being responsible for iceberg surging. In addition, Hulbe
et al. (2004) postulate catastrophic ice shelf breakup
induced by climate-controlled meltwater infilling of surface crevasses as recently witnessed along the Antarctic
Peninsula. Observations of recent iceberg discharge processes and ice rafting in an east Greenland fjord reported
by Reeh et al. (1999) demonstrated that the transport and
deposition of iceberg-derived IRD increased in periods
of enhanced (subsurface) advection of “warm” Atlantic
water. These findings were used in a study by Moros
et al. (2002) and Kuijpers et al. (2005) who concluded that
large-scale IRD events in the North Atlantic most likely
were triggered by enhanced northward ocean heat transport which caused bottom melting of floating outlet glaciers and ice shelves, leading to ice sheet destabilization
and iceberg surging. More recently, a sudden acceleration,
thinning, and retreat of the Jakobshavn Isbræ, west Greenland, were observed, which have been attributed to the
warming of the subsurface ocean currents off west Greenland (Holland et al., 2008). Studies of sedimentary records
of the past ca. 100 years from this area and from another
active glacier calving site on the southeast Greenland
coast (Lloyd et al., 2011; Andresen et al., 2012) have
meanwhile confirmed a correlation between enhanced
ocean subsurface warming and associated glacier bottom
melting and increase in glacier acceleration and iceberg
IRD production. It is without doubt that also the other factors referred to above as, for instance, surface (air) temperature warming and ice sheet dynamics, influence iceberg
calving activity. However, increasing evidence has demonstrated the important role of ocean (sub)surface
warming when trying to explain iceberg surging over
larger areas, such as during the Heinrich events (e.g.,
Alvares-Solas et al., 2010), or more locally, as recently
observed in South Greenland fjords. Within this context,
one should keep in mind that ocean heat transport pathways are influencing the temperature regime of various
parts of the North Atlantic not everywhere in the same
way, which may be one of the reasons for reported asynchronous behavior of glacial ice sheets. The occurrence
of iceberg-derived IRD in sedimentary records thus not
only provides a tool to reconstruct former ice sheet
dynamics, glacier calving activity, and iceberg drift but
also yields information on ocean current patterns. In addition, having a different origin and a distribution pattern
more depending on atmospheric circulation, sea
ice-derived IRD can provide additional information for
assessing prevailing wind directions and albedo
conditions.
Summary
IRD has been found to be widely distributed in the entire
subpolar North Atlantic. Glacial sedimentary records from
this region display discrete IRD layers at time intervals of
about 7,000 years named “Heinrich” layers that witness
large-scale iceberg surging of the North American
Laurentide Ice Sheet. This conclusion was made based
on the lithology of the IRD involved, showing, among
others, a large contribution of detrital dolomitic carbonate
derived from the sedimentary rocks around Hudson Bay
and Strait. Mineral studies have provided also evidence
for IRD originating from European and Greenland glaciers. Asynchronous deposition of IRD from these various
sources suggests a different stability regime of ice sheets
west and east of the North Atlantic. Although several
mechanisms may have played a role, increasing evidence
arises which demonstrates that (sub)surface ocean
warming and associated bottom melting of floating glaciers and ice shelves have been an important mechanism
triggering large-scale iceberg calving (“Heinrich”) events,
both under glacial climate and under present-day warming
conditions.
Bibliography
Alley, R. B., and Clark, P. U., 1999. The deglaciation of the Northern Hemisphere: a global perspective. Annual Review of Earth
and Planetary Sciences, 27, 149–182.
Alvares-Solas, J., Charbit, S., Ritz, C., Paillard, D., Ramstein, G.,
and Dumas, C., 2010. Links between ocean temperature and iceberg discharge during Heinrich events. Nature Geoscience, 3,
122–126.
Andresen, C. S., Straneo, F., Ribergaard, M. H., Bjørk, A. A.,
Andersen, T. J., Kuijpers, A., Nørgaard-Pedersen, N., Kjær, K.,
Schjøth, F., Weckström, K., and Ahlstrøm, A. P., 2012. Rapid
response of Helheim Glacier in Greenland to climate variability
over the past century. Nature Geoscience, 5, 37–41.
Andrews, J. T., 2000. Icebergs and iceberg rafted detritus (IRD) in
the North Atlantic: facts and assumptions. Oceanography,
13(3), 100–108.
Andrews, J. T., and Tedesco, K., 1992. Detrital carbonate-rich sediments, northwest Labrador Sea: implications for ice-sheet
dynamics and iceberg rafting (Heinrich) events in the North
Atlantic. Geology, 20, 1087–1090.
ICE-RAFTED DEBRIS (IRD)
361
debris from the LIS (Bond et al., 1999; Grousset et al.,
2001) further support the conclusions by Dowdeswell
et al. (1999) that the dynamics of Quaternary ice sheets
surrounding the Nordic Seas and North Atlantic were
asynchronous. This implies that each glaciated margin
may have behaved differently in response to external
ocean–climate forcing.
Iceberg surging mechanisms
Since the early 1990s, several possible mechanisms
responsible for forcing large-scale IRD events have been
proposed (e.g., Alley and Clark, 1999). The binge–purge
hypothesis of MacAyeal (1993) invoked internal ice sheet
dynamics as a driver for H-events. However, this mechanism is not supported by the detailed structure of
H-events and cannot explain North Atlantic records showing millennial and centennial scale IRD fluctuations that
appear to be closely coupled to the ocean–atmosphere climate system (e.g., Bond and Lotti, 1995). Apart from
internal ice sheet dynamics, ocean circulation changes,
sea-level fluctuations, variations in solar parameters, as
well as ice-load-induced earthquakes have been proposed
being responsible for iceberg surging. In addition, Hulbe
et al. (2004) postulate catastrophic ice shelf breakup
induced by climate-controlled meltwater infilling of surface crevasses as recently witnessed along the Antarctic
Peninsula. Observations of recent iceberg discharge processes and ice rafting in an east Greenland fjord reported
by Reeh et al. (1999) demonstrated that the transport and
deposition of iceberg-derived IRD increased in periods
of enhanced (subsurface) advection of “warm” Atlantic
water. These findings were used in a study by Moros
et al. (2002) and Kuijpers et al. (2005) who concluded that
large-scale IRD events in the North Atlantic most likely
were triggered by enhanced northward ocean heat transport which caused bottom melting of floating outlet glaciers and ice shelves, leading to ice sheet destabilization
and iceberg surging. More recently, a sudden acceleration,
thinning, and retreat of the Jakobshavn Isbræ, west Greenland, were observed, which have been attributed to the
warming of the subsurface ocean currents off west Greenland (Holland et al., 2008). Studies of sedimentary records
of the past ca. 100 years from this area and from another
active glacier calving site on the southeast Greenland
coast (Lloyd et al., 2011; Andresen et al., 2012) have
meanwhile confirmed a correlation between enhanced
ocean subsurface warming and associated glacier bottom
melting and increase in glacier acceleration and iceberg
IRD production. It is without doubt that also the other factors referred to above as, for instance, surface (air) temperature warming and ice sheet dynamics, influence iceberg
calving activity. However, increasing evidence has demonstrated the important role of ocean (sub)surface
warming when trying to explain iceberg surging over
larger areas, such as during the Heinrich events (e.g.,
Alvares-Solas et al., 2010), or more locally, as recently
observed in South Greenland fjords. Within this context,
one should keep in mind that ocean heat transport pathways are influencing the temperature regime of various
parts of the North Atlantic not everywhere in the same
way, which may be one of the reasons for reported asynchronous behavior of glacial ice sheets. The occurrence
of iceberg-derived IRD in sedimentary records thus not
only provides a tool to reconstruct former ice sheet
dynamics, glacier calving activity, and iceberg drift but
also yields information on ocean current patterns. In addition, having a different origin and a distribution pattern
more depending on atmospheric circulation, sea
ice-derived IRD can provide additional information for
assessing prevailing wind directions and albedo
conditions.
Summary
IRD has been found to be widely distributed in the entire
subpolar North Atlantic. Glacial sedimentary records from
this region display discrete IRD layers at time intervals of
about 7,000 years named “Heinrich” layers that witness
large-scale iceberg surging of the North American
Laurentide Ice Sheet. This conclusion was made based
on the lithology of the IRD involved, showing, among
others, a large contribution of detrital dolomitic carbonate
derived from the sedimentary rocks around Hudson Bay
and Strait. Mineral studies have provided also evidence
for IRD originating from European and Greenland glaciers. Asynchronous deposition of IRD from these various
sources suggests a different stability regime of ice sheets
west and east of the North Atlantic. Although several
mechanisms may have played a role, increasing evidence
arises which demonstrates that (sub)surface ocean
warming and associated bottom melting of floating glaciers and ice shelves have been an important mechanism
triggering large-scale iceberg calving (“Heinrich”) events,
both under glacial climate and under present-day warming
conditions.
Bibliography
Alley, R. B., and Clark, P. U., 1999. The deglaciation of the Northern Hemisphere: a global perspective. Annual Review of Earth
and Planetary Sciences, 27, 149–182.
Alvares-Solas, J., Charbit, S., Ritz, C., Paillard, D., Ramstein, G.,
and Dumas, C., 2010. Links between ocean temperature and iceberg discharge during Heinrich events. Nature Geoscience, 3,
122–126.
Andresen, C. S., Straneo, F., Ribergaard, M. H., Bjørk, A. A.,
Andersen, T. J., Kuijpers, A., Nørgaard-Pedersen, N., Kjær, K.,
Schjøth, F., Weckström, K., and Ahlstrøm, A. P., 2012. Rapid
response of Helheim Glacier in Greenland to climate variability
over the past century. Nature Geoscience, 5, 37–41.
Andrews, J. T., 2000. Icebergs and iceberg rafted detritus (IRD) in
the North Atlantic: facts and assumptions. Oceanography,
13(3), 100–108.
Andrews, J. T., and Tedesco, K., 1992. Detrital carbonate-rich sediments, northwest Labrador Sea: implications for ice-sheet
dynamics and iceberg rafting (Heinrich) events in the North
Atlantic. Geology, 20, 1087–1090.
ICE-RAFTED DEBRIS (IRD)
361