Glacial Erosion/Sedimentation of the Baltic Region and the Effect on the Postglacial Uplift - The Baltic Sea Basin - page 54

margin of the Fennoscandian (Baltic) shield (Amantov 1992 , 1995 ) , for example.
The Atlantic margin shows increased Late Pliocene and Pleistocene deposition rates
(Riis and Fjeldskaar 1992 ) . Worldwide, erosion of exposed unconsolidated clastic
shelf sediments and consequent isostatic compensation has resulted in large masses
of sediment being offloaded from the continental shelves onto deep-sea fans and
abyssal plains by turbidity currents (Hay 1994 ) . But opinions differ on the intensity
of the glacial erosion. To some the glaciations were crucial in changing the land-
scape. These authors emphasize that glacial erosion can be much greater than fluvial
erosion (White 1972 , 1988 , Bell and Laine 1982 , 1985 , Clague 1986 , Braun 1989 ,
Harbor and Warburton 1992 , 1993 , Clayton 1996 , Hallet et al. 1996 , Montgomery
2002 , James 2003 ) . In mountain glaciers, the erosion rate is greatest near the equi-
librium line altitude (ELA) where ice accumulation changes to melting. Here the
glaciers are often considered “buzz saws” (Brozovic et al. 1997 , Meigs and Sauber
2000 , Montgomery et al. 2001 , Mitchell and Montgomery 2006 ) . Glaciers increase
topographic relief through a combination of focused erosion in valleys and the
regional isostatic rebound the incision induces (Small and Anderson 1998 ) , and this,
in turn, increases erosion.
Other researches point to the moderate transformation of preglacial landscapes
and find evidence for low rates of glacial erosion and little difference between flu-
vial and glacial erosion rates (e.g., Gravenor 1975 , Sugden 1976 , 1978 , Lindström
1988 , Hebdon et al. 1997 ) . In this view, the glaciers merely polished the north-
ern shields, and the erosion they caused (although sometimes highly variable;
Lidmar-Bergström 1997 ) was generally less than tens of meters in magnitude.
Glacial erosion is intriguing because on the local scale it is highly irregular but
at the large scale it is regular. We would like to understand it quantitatively. For
example, we would like to assess whether most of the sediment redistribution took
place during the first or last glacial cycles. The shifts of sediment loading could be
enough to affect subsurface temperature and cause isostatic tilting. But local spatial
variations, the wealth of data that must be assembled and integrated, and the large
spatial scales involved make analysis difficult.
Our approach is to apply computer software adept at creating and manipulat-
ing surfaces to infer glacial erosion and sedimentation rates across Europe in a
locally detailed but regionally coherent way. At every instant of time and across
the Quaternary, our method requires that erosion and sedimentation are balanced,
locally and across all of Europe. Our analysis honors bounds on what erosion and
sedimentation rates are reasonable, and a great many local geological constraints.
The redistribution is process-driven. We develop algorithms that honor the pattern
of glacial flow suggested by geological evidence and the locations of the ice margins
as the glaciers grew and retreated. From this we build erosion and sedimentation
modules that redistribute the sediments. We calibrate these tools to current glaciers
and to the observed present-day sediment pattern, and this assures they are rea-
sonable (but not necessarily correct). In this manner, we infer how the sediments
may have been created and redistributed across Quaternary time and tentatively con-
clude that most of the major bedrock landscape changes were probably produced by
the earliest glaciations. Even so, the erosion and sedimentation that occurred over