Investigation of sedimentation rates and sediment dynamics in Danube Delta lake system (Romania) by ~(210)Pb dating method

摘要

Being a dynamic environment associated with complex costal, fluvial and marine processes, only a few studies regarding the evolution of the Danube Delta and the human impacts on its ecosystem have been carried out. Being a sensible to all processes occurring in its catchment area, information is stored in the deposited sediments, which can be used as tracers for natural and anthropogenic processes. The aim of this study is to determine a detailed reconstruction of the sedimentation rates in the last century by applying the Pb-210 dating method validated by Cs-137 profiles. Additionally, the impacts of the construction of river-regulating structures (mainly the Iron Gates Hydro-Energetic Power Plants) are investigated, along with the assessment of natural phenomena (floods, storms etc.). To achieve this, 26 sediment cores from seven lakes were collected. Pb-210(sup) and Cs-137 were determined using gamma spectrometry, while Pb-210(tot) was measured via alpha spectrometry (Po-210), using the CRS model for age determination. From the assessed lakes, the most affected was the Matita Lake with a maximum sedimentation rate of 10.93 g cm(-2) yr(-1) and the least affected was the Isac Lake. Average sedimentation rates are: 0.95 g cm(-2) yr(-1) for Cruhlig Lake, 0.70 g cm(-2) yr(-1) for Uzlina Lake, 0.44 g cm(-2) yr(-1) for Isac Lake, 0.47 g cm(-2) yr(-1) for Cuibida Lake, 0.51 g cm(-2) yr(-1) for Iacob Lake, 1.00 g cm(-2) yr(-1) for Matita Lake and 0.76 g cm(-2) yr(-1) for Merhei Lake. Physical parameters (water content, porosity and bulk density) and LOI (organic matter and inorganic carbon content) were determined for each core to differentiate organic and non-organic sedimentation. Beside the natural influences, it is difficult to track the effects of the Iron Gates and not all analysed lakes were suitable for this task. The 1940-1970 period and the following ten years were compared in means of sedimentation: a decrease in sedimentation can be observed in four of the lakes: 59% in Cruhlig Lake, 16% in Uzlina Lake, 10% in Iacob Lake and 42% in Isac Lake, leading to an average 32% for the four lakes. The other three lakes show increasing tendencies of 39% in this period: 87% in Malta Lake, 6% in Merhei Lake and 24% in Cuibida Lake. Sedimentation rates show growths of 3 times after 1989, the most affected being the two northern lakes (3 times increase in both Matita Lake and Merhei Lake) and the four central lakes (2 times in case of Cuibida Lake, 3 times In Iacob Lake, 3 times in Isac Lake and 4 times in Uzlina Lake) with an average increase of 3 times, while the southern one (Cruhlig Lake) 2 times.