Relative sealevel changes

A wide range of sediments and landforms may be used to reconstruct sea-level histories, such as deltas, beaches, shore ridges and erosional platforms marking former shorelines. More accurate sea-level data can be obtained from sediment cores retrieved from small lakes and bogs situated at different elevations in a small area or along the same isobase (e.g. Svendsen and Mangerud, 1987).

In northwest Europe, the sea-level rise since deglaciation has been reconstructed at numerous sites along the coastline, mainly from estuarine sediments and coastal basins. Coastal lake basins may provide the most accurate history of sea-level changes. As soon as these basins, preferably with a bedrock threshold, have been raised above sea-level, they start to accumulate brackish and freshwater sediments (Fig. 6.3). Such basins are referred to as isolation basins (e.g. Svendsen and Mangerud, 1987). A marine transgression may lead to the resubmergence of these basins, and brackish or marine deposits will accumulate above freshwater sediments. It must be emphasized that it is the position of sea-level relative to the outlet of these basins that determines whether the sediments are marine, brackish or lacustrine. The timing of sea-level being level with the outlet of each lake is determined by identifying (using biostratigraphical methods) and dating the boundary between the brackish and lacustrine sediments in the cores. This method, however, requires that there is no hiatus between the brackish and lacustrine sequences. Pollen and diatom biostratigraphy can normally be used to test whether the sedimentation is continuous across this boundary. The elevation and date of former sea-levels as determined for

Figure 6.3 A bedrock basin at three different stages. Top: when the sea-level (1) was well above the threshold; centre: when the sea-level (2) was at the threshold; and bottom: when the sea-level (3) was below the threshold. A typical core sequence, and the level from which radiocarbon samples are collected, is shown to the right. To construct a sea-level curve, basins from different elevations are cored. (Adapted from Svendsen and Mangerud, 1987)

Figure 6.3 A bedrock basin at three different stages. Top: when the sea-level (1) was well above the threshold; centre: when the sea-level (2) was at the threshold; and bottom: when the sea-level (3) was below the threshold. A typical core sequence, and the level from which radiocarbon samples are collected, is shown to the right. To construct a sea-level curve, basins from different elevations are cored. (Adapted from Svendsen and Mangerud, 1987)

each locality are plotted in an age-elevation diagram, and the relative sea-level curve is the regression line between the points for each lake. The resolution of the curve depends on how many localities from different elevations are included, and therefore the resolution varies both within each diagram and between different diagrams. To avoid problems with reservoir age and resedimentation, radiocarbon dates should preferentially be taken from the lacustrine/brackish boundary. During a regression phase the radiocarbon age may be slightly younger than the boundary, and during a transgression slightly older.

Along the margins of the Quaternary ice sheets, shorelines formed. As deglaciation and uplift proceeded, lower shorelines were formed. Younger shorelines are tilted less steeply than older shorelines due to a decreasing amount of differential uplift through time. This is clearly seen in an equidistant shoreline diagram. Because the outer fjord areas were deglaciated earlier than the inner parts, the lower shorelines can normally be traced further inland than the higher ones. This can be observed in Scotland, Scandinavia, the Canadian Arctic, and the eastern USA. Subsequent to deglaciation, shorelines developed as a result of the complex interplay between glacio-isostatic uplift and glacio-eustatic sea-level rise. Where prominent shorelines of the same age are found in different areas, isobases can be constructed for these shorelines. Isobases do not show absolute uplift since shoreline formation, but the total amount of uplift minus the glacio-eustatic component of sea-level change. As an example, if a shoreline at an altitude of 40 m is dated to 9000 yr bp, when the global eustatic sea-level was ca. 35 m lower than at present, it indicates a total uplift of 75 m. It is important to remember that isobases join points of equal altitude or uplift of the same age. The isobase pattern can either be reconstructed manually or by means of trend surfaces, and gives a 3D

picture of the crustal deformation from ice loading. Recently, models of glacio-isostatic rebound and sea-level fluctuations have been developed (Lambeck, 1991a,b, 1993a,b). These models combine glaciological data, empirical data related to sea level variations, and geophysical parameters to produce numerical models that are in good agreement with the actual sea-level history.

Clark et al (1978) and Clark (1980) divided the surface of the Earth into six sea-level

Isostatic Sea Level Change Canada MapIsostatic Rebound
Figure 6.4 Distribution of sea-level zones and typical relative sea-level curves. (Adapted from Clark et al, 1978)
Eustatic Sea Level Change Earth
Figure 6.5 Regional isobase maps, (a) Shoreline emergence in eastern Canada since approximately 6000 yr bp. (b) Absolute uplift in Scandinavia during the Holocene. (c) Isobases for the Main Postglacial Shoreline (ca. 7000-6000 yr bp) in Scotland. (Modified from Benn and Evans, 1998)

zones (Fig. 6.4) based on typical postglacial relative sea-level curves. Zone 1 is within the limits of the large Pleistocene ice sheets, characterized by continuous land uplift (regression). Zone 2 occurs beyond the limits of the Pleistocene ice sheets, where the sea-level history is influenced by eustatic submergence modified by forebulge collapse. Zone 3 is further away from the former ice sheets. This zone is characterized by initial eustatic submergence, followed by emergence several thousand years after d├ęglaciation. Zone 4, located in the tropics and subtropics, is characterized by continuous eustatic submergence. In zone 5, in the southern oceans, the sea level is initially controlled by eustatic submergence; however, when glacial meltwater stops draining to the oceans, slight emergence takes place. Zone 6 includes all continental margins except those lying in zone 2, and is characterized by slight emergence after meltwater stopped flowing into the oceans.

Within the regions covered by the Pleistocene ice sheets (zone 1), the change in sea-level has been characterized by isostatic uplift and regression. The pattern of uplift can be studied from isobase maps. Isobase maps provide information on the isostatic loading of the crust and therefore dispersal centres of ice sheets. Generalized isobase maps for eastern Canada, Scandinavia and Scotland are shown in Fig. 6.5. The uplift pattern in Scandinavia and Scotland indicates one main loading centre, while the isobases over eastern Canada show three loading centres, indicating multiple domes on the Laurentide ice sheet. In areas with detailed investigations, isobase maps are in general more complex, exhibiting abrupt discontinuities, reflecting possible fault or fracture zones.

0 0

Post a comment