Sedimentary sequences from the Holocene can be used to reconstruct the environmental conditions during their deposition in the recent coastal area. The value distribution of lithological, geochemical and paleobotanical parameters in space and time provides evidences how the coastline has changed from the past to today, how salinity, bioproductivity of the coastal waters have altered, how the sea-level has moved up and down and much more. Identification of causes and effects in the past may lead in some cases to possibilities to predict effects in the future caused by recent environmental alterations. Reconstructing the past is therefore a valuable tool to understand the natural evolution.
In this case study the main focus is on the changing environmental conditions in the Oder Estuary, as has become evident from sediment cores from the Oder Lagoon and from often flooded salt meadows in the surrounding low lying areas. The detailed analysis of the sedimentary sequences shows that the evolution of the lagoons has started around 8.000 years before present. In that time former freshwater bodies were flooded by the fast rising Littorina Sea. Initially, the salinity was much higher than today; even in the inner Oder Estuary the salt content reached around 6 PSU.
The benthic productivity was immense due to high nutrient availability from river load and the eroded soils but dropped down later. During the Subboreal the sedimentation rate decreased markedly, therefore a lower sea-level and dominant sediment transport to and deposition in the Baltic has to be assumed. Some 2.000 years ago the sea-level rose slowly again leading to an increasing water depth and sedimentation, but it decreased again around 750 years ago.
The often assumed enhancement of the sedimentation rate in the past 150 years caused by anthropogenic eutrophication is not clearly visible in the sedimentary record due to sediment distortion. The youngest evolution can be reconstructed in more detail from sequences from the surrounding salt marshes. Two degraded peat types intercalated between silt layers point to two lower sea-level phases which are connected to the Bronce Age dry phase (3200 to 2600 years BP) and the Little Ice Age between 1450 and 1690 AD. A slowly rising sea-level leads to silt layer deposition that is poor in organic carbon, whereas a fast rising level causes growth of peat, which in turn may be degraded during a subsequent sea-level fall.
These investigations were related to today's coastal zone problems and their results may highlight that the natural evolution is characterized and forced by ever changing circumstances. Considering that climate change and sea-level change are closely connected and sea-level changes in turn cause alterations in salinity, sedimentation, coastal retreat and biotope distribution should influence long-term decisions in coastal zone management