Tracking environmental changes of the Baltic Sea coastal zone since mid-Holocene
Sammanfattning: In this thesis, I used two coastal sediment sequences from SE Sweden to study how Baltic Sea coastal environment has changed since the mid-Holocene. The results show the coastal environment has been influenced by multi-stressors, such as climate change, shoreline regression and anthropogenic activities.
Using the sediment sequence from Karlskrona Bay, past coastal environment between 7300 and 3500 cal. yr BP was reconstructed, with a focus on sea surface salinity (SSS). To quantitatively determine the SSS, two methods were employed: measurements of the 87Sr/86Sr ratios in carbonate shells (SSSSr) and the process length variations of dinoflagellate cysts Operculodinium centrocarpum (SSSpl). The SSSSr was ~6–7 between 6800 and 6400 cal. yr BP, similar to modern conditions. Between 6000 and 3900 cal. yr BP, SSSSr was consistently higher, with a range between 9 and 13. Microfossils sensitive to salinity variations, such as Radiosperma corbiforum and Spiniferites spp., supported the SSSSr estimate. The recorded salinity variations were most likely caused by climate changes in the Baltic Sea region. In comparison with the SSSSr, the SSSpl values were not as reliable and the process length-based method overestimated the salinity.
Environmental conditions for the past 5,400 years at Gåsfjärden were reconstructed, with a focus on bottom water oxygen condition. Between 5400 and 4100 cal. yr BP, hypoxic bottom water (O2<2 mg L-1) prevailed in Gåsfjärden, despite its large connection with the open sea. This extended hypoxic interval was most likely caused by a warm and dry climate during the Holocene Thermal Maximum. The most intense hypoxic interval was recorded between 4400 and 4100 cal. yr BP, and coincided with a similar hypoxic interval in the Baltic Sea deep basin. As regional climate became wetter and colder between 4100 and 2700 cal. yr BP, bottom water oxygen conditions improved and salinity decreased. The environment in Gåsfjärden changed to close to modern conditions after 2700 cal. yr BP, which was characterized by less hypoxia and lower salinity. The study shows that climate condition and hydrographic configuration have been important drivers to the hypoxia formation.
I further explored the drivers for the sediment grain-size variations in Gåsfjärden. Over the last 5,400 years, relative sea level decreased 17 m in the region, caused by isostatic land uplift. As a consequence, Gåsfjärden changed gradually from a highly open coastal site into a semi-enclosed inlet surrounded with numerous islands. To examine whether the long-term coastal morphology changes in Gåsfjärden have had important impact on sediment grain-size, a digital elevation model-based openness index was calculated to quantitatively estimate the morphological changes. Between 5400 and 4400 cal. yr BP, Gåsfjärden was characterized with the highest openness index. During this interval, the highest sand content and silt/clay ratios were recorded, indicating relatively high hydrodynamic energy. The average sand content decreased to about 0.2% between 4400 and 200 cal. yr BP, caused by large openness decline. The maximum sand content has been less than 1% in the Gåsfjärden sediments, implying generally low hydrodynamic energy compared with other open coastal areas.
Lastly the thesis studies the linkage between land use and coastal ecosystem over the last millennium. Pollen-based land cover for the Gåsfjärden catchment was reconstructed and I further compared it with paleoenvironmental variables from Gåsfjärden. Prior to 1800s, the minor changes in sedimentary diatom assemblages, C and N isotopes from the Gåsfjärden record were possibly caused by natural climate variations and small-scale anthropogenic activities. The onset of eutrophication in Gåsfjärden can be traced to the early 1800s and intensified land use is the main driver. Anthropogenic activities in the 20th century, mostly associated with agricultural expansion, have caused unprecedented ecosystem changes in the coastal inlet, as reflected in increased pelagic diatoms and the geochemistry proxies.
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