Glacial rebound and sea-level change: solutions for ocean-volume
fluctuations, ice sheets and mantle rheology
Kurt Lambeck. Tony Purcell, Jason Zhao, Andrea Dutton.
As ice sheets melt or grow, the load on the earth's surface
changes and the planet's shape and gravity field is modified. This is seen in
a range of geological, geophysical and geodetic observations. An important
geological observation is the height and elevation of former shorelines above
or below present sea level. Geodetic observations include the radial
displacement of the crust or the acceleration of satellites as the gravity
field changes. Inversion of such observations provides information on the
EarthÕs rheology and on the history of the ice sheets. The rheological results
are robust and the main emphasis of current work is on improving models of the
ice sheets. This is achieved through a combination of field, laboratory, and
numerical modelling. Study areas included in 2005 projects include Scandinavia
and Arctic Russia (see below), The Baltic Sea (through a joint project with
Shiyong Yu and Bjoern Berglund of Lund University, Sweden), Greenland (through
a joint project with Charlotte Sparrenbom of Lund University, Sweden), the Gulf
of Mexico (through a joint project with Alex Simms of Rice University, USA),
the Mediterranean (through joint projects with Marco Anzidei of INGV, Rome, and
Fabrizio Antonioli of ENEA, Rome), and Singapore (through a joint project with
Michael Bird, now at St Andrews, Scotland).
The focus of the glacial rebound work in 2005 has focussed
on Arctic Eurasia from the time of the penultimate glacial maximum (MIS 6 or
the Late Saalian at ~145,000 years ago) up to the present. The aim of this
work is to establish constraints on the ice thickness and ice margins for some
of the major phases of the last glacial cycle that are independent of
glaciological or climate assumptions. The penultimate glacial maximum was one
of the largest ice sheets over northern Europe and the Russian arctic,
extending from the British Isles to the Taymyr Peninsula. This was followed by
the Eemian Interglacial and by a series of alternating stadials and
interstadials over Russia and Scandinavia culminating in the last glacial
maximum at ~ 20,000 years ago.at the end of which the next cycle of glaciation
started. The principal outcomes are: the completion of compilations of field
evidence for the ice margin locations and shoreline elevations and sea levels
across the region; the inversion of this data for ice thickness during the
principal glaciations corresponding to the marine isotope stages 6, 5d, 5c and
4; and the palaeo-geographic reconstructions for these periods. During the
Late Saalian the ice extended across northern Europe and Russia with a broad
dome centred from the Kara Sea to Karelia that reached a maximum thickness of c.
4500 m and ice surface elevation of c.
3500 m above sea level. A secondary dome
occurred over Finland with ice thickness and surface elevation of 4000 m and
3000 m respectively (Figure 1). When ice retreat commenced, and before the
onset of the warm phase of the Early Eemian, extensive marine flooding occurred
from the Atlantic to the Urals and, once the ice retreated from the Urals, to
the Taymyr Peninsula. The Baltic Ð White Sea connection is predicted to have
closed at about 129 kyr BP although large areas of arctic Russia remained
submerged until the end of the Eemian (Figure 1).
Figure 1 (A-E): Palaeogeographical reconstruction for
selected epochs for Eurasia from the time of the penultimate glacial maximum to
the stadial corresponding to MIS 4. The areas covered by grounded ice are shown
by the white translucent areas with ice thickness contours (white lines) at 250
m interval from 0 to 1000 m and at 500 m interval thereafter. The contours of
negative and zero sea level change are in red and positive values in yellow
(e.g. the 200 m contour represents palaeoshorelines that are now 200 m above
sea level). For (A) to (C) the negative contours are at 50 m intervals and the
positive contours are at 100 m interval. For (D) and (E) the negative contours
are at 25 m intervals and the positive contours are at 50 m interval. The
palaeo-shoreline locations are defined by the green-blue boundary and
palaeo-topography above coeval sea level is indicated by the green and brown
colour gradations at 25, 50, 100, 200 m and higher elevations. Palaeo-water
depths are defined by the blue colour gradations at -25, -50, -100, -200 m and
deeper depths. The ocean depths and land elevations are with respect to sea
level for the specified epoch. Water depths of enclosed bodies are with
respect to the sill elevation that defines the enclosed basin.
During the stadials (MIS-5d, 5b, 4) the maximum ice was
centred over the Kara-Barents Seas with a thickness not exceeding c. 1200 m. Ice dammed lakes and the elevations of sills
are predicted for the major glacial phases and used to test the ice models
(Figure 1). For example, large lakes are predicted for west Siberia at the end
of the Saalian and during MIS-5d, 5b and 4, with the lake levels, margin
locations and outlets depending interalia on ice thickness and isostatic adjustment and comparison with the
field evidence then permits further inferences about the ice margins and ice
thickness. The results are to appear in Boreas. Subsequently the investigation has been continued
from MIS-4 to MIS-2 to include the period of rapid changes in ice cover over
Scandinavia and to provide constraints on the European and Russian ice sheets
for the complete interval from MIS 6 to the present.
Related to this investigation is the study of sea-level
fluctuations in the Mediterranean during the last interglacial (MIS 5e). The
Mediterranean lies on the forebulge of the Late Saalian ice sheets and the
present elevations of 5e shorelines are interalia a function of the Stage 6 and subsequent ice sheets. The predicted
Eemian sea-level signal for these locations is distinctly different from that
at sites further from the former ice sheets and the differences are determined
by the ice and earth model parameters. The amplitudes of the signals at both
sites are determined by these parameters and by the ice volumes during the
interglacial and the comparison of the data from the Mediterranean with
far-field evidence from Western Australia indicates that land-based ice volumes
during the middle and later parts of the Eemian were less than during the
present interglacial by as much as 5 m sea-level equivalent (Figure 2).
2 (A-E): Same as Figure 1 but for the post Eemian period. The rebound contours are
the same as in Figure 1D.
Related work in the Mediterranean includes the study of
speleothems from submerged caves and dating of molluscan shells to refine the
sea level curve for the past and earlier cycles (
see U-series dating of molluscan shells
from the Mediterranean ) and further work on the use of Roman-era
fish tanks as sea-level indicators (see Anzidei, M., Benini, A., Lambeck, K.,
Antonioli, F., Esposito, A., Surce, L. 2005. Siti archeologici costieri di etˆ
romano come indicitori della variazioni del livello del mare: unÕapplicazione
al mare Tirreno (Italia centrale). In Evoluci—n Palaeoambiental de Los Puertos
y Fondeaderos Antiguos en el Mediterr‡neo Occidental. L. de Maria and R.
Turchetti, Eds, Rubbetino, Rome, pp115-126).
A second focus has been on the stress evolution in the crust
during a glacial cycle to answer specific questions about the stability of the
crust during and after major glaciations. What is the evolution of the state
of stress in the crust during a glacial cycle and how does the crust respond to
the superpositioning of these stresses on long-term background stress fields.
If these questions can be resolved it becomes possible to estimate the
likelihood of fault reactivation during future glacial cycles in areas that may
otherwise be appropriate for the long-term storage of nuclear waste. These
questions have been investigated for waste repository sites in Sweden, using
the past glacial cycle as representative of future cycles and it has been
possible to show that fault stability enhancement or reduction is strongly
dependent on location relative to the ice sheets and that the glacial-load
stress is a factor to be considered in any final decisions on repository site
Lambeck, K., Purcell, A. 2005; Sea-level change in the
Mediterranean Sea since the LGM: model predictions for tectonically stable
areas. Quaternary Science Reviews, 24,
Anzidei, M., Benini, A., Lambeck, K., Antonioli, F.,
Esposito, A., Surce, L. 2005. Siti archeologici costieri di etˆ romano come
indicitori della variazioni del livello del mare: unÕapplicazione al mare
Tirreno (Italia centrale). In Evoluci—n Palaeoambiental de Los Puertos y
Fondeaderos Antiguos en el Mediterr‡neo Occidental. L. de Maria and R.
Turchetti, Eds, Rubbetino, Rome, pp115-126.
Lambeck, K. 2005.Glacial load stresses: Can existing
faults or other zones of crustal weakness be reactivated during glacial
cycles? In Expert Panel Elicitation of Seismicity Following Glaciation in
Sweden (Eds S. Hora and M. Jensen).
Statens strŒlskyddinstitut rapport 2005:20 pp 85-106. Stockholm, Sweden.
Sparrenbom, C.J. Bennike, O., Bjorck, S., Lambeck, K.,
2006. Relative sea-level changes since 15000 cal. Yr BP in the Nanortalik
area, southern Greenland. J. Quaternary Science, 21, 29-48.
Sparrenbom, C.J. Bennike, O., Bjorck, S., Lambeck, K.,
2006. Holocene relative sea-level changes in the Qaqortoq area, southern
Greenland. Boreas 35(2) (in press)
Lambeck, K., Purcell, A., Funder, S., Kj¾r, K., Larsen,
E., Mšller, P., 2006. Constraints on the Late Saalian to early Middle
Weichsellian ice sheet of Eurasia from field data and rebound modeling. Boreas (in press).