Tracing marine hypoxic conditions during warm periods using a microanalytical approach

• Deoxygenation, i.e. loss of oxygen from the oceans, often considerably influences the aquatic organisms and the wholeecosystem and changes biogeochemical cycles. It results in increasing bottom areas of hypoxia (<2 mg/l dissolved oxygen),which has been primarily attributed to global warming and increased eutrophication. It is vital to study the present-dayanthropogenically-induced environmental changes in coastal settings such as hypoxia and their outcomes. The Baltic Sea ishighly sensitive to hypoxia, which has occurred during several warm periods in the past. The studies of comparable hypoxicevents during the warm periods in the past can help us better understand the cause, severity, and potential outcomes ofenvironmental changes in the present day.In this thesis, I reconstructed the past environmental conditions, i.e. water temperature, salinity, and oxygen concentrationsfrom eight sites in the Baltic Sea using a multi-method approach including synchrotron X-ray spectroscopy and plasmaanalytical methods. I used trace elements and stable isotopes analyses on benthic foraminifera from two warm periods in thepast, the Eemian (130 – 115 thousand years before present AD 1950, ka BP) and the Holocene (11.5 ka BP to present) to studyhow the extent and severity of hypoxia and other environmental factors have varied in the Baltic Sea over time.During the Eemian period, the bottom water in the southern and western Baltic Sea show larger seasonal variations. There wasa rapid salinity increase in the early Eemian due to a wider and deeper passage from the North Sea to the Baltic Sea. Thetemperature differences between cold and warm seasons were increasing in the first half of the Eemian period. During the mid- andlate-Eemian, the bottom water became more stagnant with lower oxygen content. The trends agree with the simulationresults, indicating influences from North Atlantic Oscillation and precipitation-evaporation balance. During the Holoceneperiod, the bottom water salinity increased dramatically ~7,700–7,500 years ago and decreased ~4,100–2,500 years ago,coinciding with the variations in bottom water oxygen content and temperature. The reconstructions were based on species-specificcalibration and the geochemistry of ‘clean’ foraminiferal calcite without contamination from authigenic minerals. Thediagenetic coatings on foraminifera formed under extremely low oxygen conditions in the deepest basin in the Baltic Sea werehighly enriched in multiple elements, which could significantly alter the foraminiferal geochemistry. The study can be used asguidance for interpreting foraminiferal trace element analyses from extreme environmental conditions. The calibration studyfrom the low oxygen basin, the Santa Barbara Basin, shows the importance of species-specific calibrations under a restrictedoxygen environment and improved the application of oxygen proxy using trace elements, i.e. manganese incorporation inforaminiferal calcite.

Subject headings

NATURVETENSKAP  -- Geovetenskap och miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences (hsv//eng)

Keyword

environmental changes

hypoxia

foraminifera

Eemian

Holocene

trace elements

stable isotopes

LA-ICP-MS

synchrotronbased μXRF

Publication and Content Type

dok (subject category)

vet (subject category)