A sea-level plateau preceding the Marine Isotope Stage 2 minima revealed by Australian sediments
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Abstract:
Further understanding of past climate requires a robust estimate of global ice volume fluctuations that in turn rely on accurate global sea-level reconstructions. An advantage of Marine Isotope Stage 2 (MIS 2) is the availability of suitable material for radiocarbon dating to allow comparison of sea-level data with other paleoclimatic proxies. However, the number and accuracy of sea-level records during MIS 2 is currently lacking. Here we present the history of MIS 2 eustatic sea-level change as recorded in the Bonaparte Gulf, northwestern Australia by reconstructing relative sea level and then modeling glacial isostatic adjustment. The isostatically-corrected global sea-level history indicates that sea-level plateaued from 25.9 to 20.4 cal kyr BP (modeled median probability) prior reaching its minimum (19.7 to 19.1 cal kyr BP). Following the plateau, we detect a 10-m global sea-level fall over ~1,000 years and a short duration of the Last Glacial Maximum (global sea-level minimum; 19.7 to 19.1 cal kyr BP). These large changes in ice volume over such a short time indicates that the continental ice sheets never reached their isostatic equilibrium during the Last Glacial Maximum.Keywords:
Post-glacial rebound
Last Glacial Maximum
Marine isotope stage
This thesis deals with the incorporation of isostatic processes into realistic models
of ice sheet dynamics. A viscoelastic half-space model of isostatic adjustment
is developed, and as an initial exercise is coupled to a model of the Antarctic
ice sheet simulating the last glacial cycle. The ice sheet model is a three dimensional,
time-dependent model originally formulated by Jenssen (1977)
where the driving input data are net accumulation of snow and eustatic sea
level change. This allows examination of the sensitivity of the ice sheet simulation
to changes in the parameters of the isostatic model. In general, the
maximum ice volume generated over a glacial cycle decreases with increasing
mantle viscosity and increasing lithospheric rigidity.
To obtain realistic values for the isostatic parameters of mantle viscosity and
lithospheric rigidity the retreat of the Northern Hemisphere ice sheets and the
subsequent isostatic adjustment since the last ice age is simulated. The isostatic
parameters are adjusted until the overall model provides the best match to
relative sea level data, with the eustatic component of the relative sea level
change prescribed. (The maximum value of the amplitude of the prescribed sea
level change is 130 m as determined from the Huon Peninsula in Papua New
Guinea). Initially the simulation and matching procedure is performed using a
simple ice sheet model whose time dependent extent is set by the ICE4G dataset
(Peltier, 1994) and whose thickness and volume is set on the assumption of a
parabolic profile of thickness. From these trials the model parameters that most
realistically reproduce the observed isostatic adjustment associated with the
retreat of the Laurentide ice sheet are 3 x 1021 Pa s for lower mantle viscosity,
2 x 1021 Pa s for upper mantle viscosity and 1 x 1025 N m for lithospheric
rigidity. For the Fennoscandian ice sheet the corresponding parameter values
are 6 x 1021 Pa s, 4 x 1021 Pa s and 6 x 1024 N m. The trials are then repeated
with the parabolic profile ice sheet assumption replaced by generation of ice
sheet thickness using the Jenssen ice sheet model. For the Laurentide ice sheet
the same earth model parameters are recovered. For the Fennoscandian ice sheet
the use of the Jenssen model to simulate ice thickness produces earth model
parameters of 1.3 x 1021 Pas for both the lower and upper mantle viscosity and
2 x 1025 N m for the lithospheric rigidity. A problem with the analysis is that
the maximum volume of the combined ice sheets corresponds only to 50 m of
eustatic sea level change in the case of the parabolic profile simulation and to
40 m when using the Jenssen model.
The sensitivity of the Antarctic ice sheet to regional variations in lithospheric
rigidity is examined. Using a range of simple relations between crustal
thickness (for which there exists data on geographic distribution) and lithospheric
thickness, it is determined that the main effect of non-uniform lithospheric
thickness is on the extent of the Ronne and Amery ice shelves.
The constraint of prescribed eustatic sea level change since the last ice age
is removed by linking the Laurentide, Fennoscandian and Antarctic ice sheet
models via the common sea level change determined by the deglaciation of the
combined ice sheets. The constraint on Northern Hemisphere ice sheet extent is
also removed by allowing the ice sheet model (the Jenssen model) t determine
its own extent when driven by climatology and the Milankovitch cycles of solar
input. This overall model produces a realistic eustatic sea level change since
the last ice age (130 m), but unrealistic changes in relative sea level. In some
locations the calculated relative sea level changes are too large by 200 m.
The problem of obtaining a consistent simulation of both eustatic and relative
sea level change is not resolved. There are three possible explanations.
First there may have been an extensive ice sheet over Siberia, which has not
been accounted for in this or any other analysis. Second the calculations here
assume linearity between isostatic ·disequilibrium and rate of adjustment. This
may not be the case. Third, significant changes in ice volume may have occurred
before the relative sea level record was laid down in the geological record.
Post-glacial rebound
Isostasy
Ice-sheet model
Asthenosphere
Last Glacial Maximum
Deglaciation
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Since publication of the paper by Bard et al. (1990) it has been known that GCM studies of the climate of the last glacial maximum (LGM), employed lower boundary conditions appropriate to this time but astronomical parameters of an era 3000 years later. The LGM boundary conditions from CLIMAP were for 18 kyr BP on the 14 C timescale. The GCM simulations employed the insolation regime appropriate to 18 kyr BP on the sidereal timescale whereas the appropriate LGM insolation regime is that of 21 kyr BP. These studies also used the CLIMAP ice sheet reconstruction. However, on the basis of most recent analyses the reconstruction by Tushingham and Peltier (1991) is to be preferred. Hyde et al. (1989) showed that a simple EBM compared favourably with the NCAR CCM, when both were used to simulate the temperature distribution of the LGM. Here we shall employ the same EBM to study the effect on LGM climate of the timing mismatch, and of the different horizontal extents of the different ice sheet reconstructions. In each case the climatic effect is found to be significant. Thus we cannot claim an accurate LGM simulation unless the orbital and terrestrial inputs match to within 1,000 years and unless we employ the best possible ice sheet reconstruction in the analysis.
Last Glacial Maximum
Paleoclimatology
Ice-sheet model
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Post-glacial rebound
Ice-sheet model
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Deglaciation
Post-glacial rebound
Ice-sheet model
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Last Glacial Maximum
Ice-sheet model
Proxy (statistics)
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Tide gauge
Post-glacial rebound
Secular Variation
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Further understanding of past climate requires a robust estimate of global ice volume fluctuations that in turn rely on accurate global sea-level reconstructions. An advantage of Marine Isotope Stage 2 (MIS 2) is the availability of suitable material for radiocarbon dating to allow comparison of sea-level data with other paleoclimatic proxies. However, the number and accuracy of sea-level records during MIS 2 is currently lacking. Here we present the history of MIS 2 eustatic sea-level change as recorded in the Bonaparte Gulf, northwestern Australia by reconstructing relative sea level and then modeling glacial isostatic adjustment. The isostatically-corrected global sea-level history indicates that sea-level plateaued from 25.9 to 20.4 cal kyr BP (modeled median probability) prior reaching its minimum (19.7 to 19.1 cal kyr BP). Following the plateau, we detect a 10-m global sea-level fall over ~1,000 years and a short duration of the Last Glacial Maximum (global sea-level minimum; 19.7 to 19.1 cal kyr BP). These large changes in ice volume over such a short time indicates that the continental ice sheets never reached their isostatic equilibrium during the Last Glacial Maximum.
Post-glacial rebound
Last Glacial Maximum
Marine isotope stage
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Citations (46)
We present a new method for determining the rates from a network of regional tide gauge data. The method involves using a model for the sea level observations (annual averages of relative sea level) that explicitly includes a term for the interannual sealevel variations, which are assumed to be constant for all sites in the network. The resulting simultaneous analysis of all the sea level observations yields sea level rates which are biased by the unknown temporal slope of the interannual variations. By differencing sea level rate estimates with respect to one site, “site‐referenced” sea level rates are calculated which are free from this bias. The differencing procedure also removes any signal due to a common sea level rate, such as might be associated with eustatic sea level rise. The resulting site‐referenced sea level rates therefore in principle reflect only the relative vertical crustal motions of the tide gauge sites with respect to the geoid. These site‐referenced sea level rates represent a potentially more accurate data set than have previously been used for sea level studies. We have used these rates to investigate glacial isostatic adjustment (GIA) in Fennoscandia. Specifically, we have used the site‐referenced sea level rates within the Baltic Sea to estimate adjustments to the ice history and Earth model which are used to calculate vertical rates of adjustment due to GIA, a method never before used for Fennoscandia. The parameters selected were lithospheric thickness, the viscosities of (assumed isoviscous) upper and lower mantles, and a Fennoscandian ice thickness scaling parameter. We present the results of this estimation procedure for a specific test ice model. We also find evidence that the sea level record for the site nearest the center of uplift, Furuögrund, may be contaminated by systematic errors of unknown origin. Data from this site have previously been used to determine a rate for the maximum uplift that we find is perhaps 1 mm yr −1 , or ∼10%, too large.
Post-glacial rebound
Tide gauge
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Post-glacial rebound
Tide gauge
Sea-Level Change
Ocean tide
Solid earth
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Post-glacial rebound
Marine isotope stage
Isostasy
Tectonic uplift
Elevation (ballistics)
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