Diatom-rich sediment formation in lakes

Sammanfattning: Unicellular photosynthetic golden algae called diatoms are one of the most abundant silicifying organisms. Diatoms take up silicon and build their frustules, in the form of biogenic silica (BSi), which have high preservation potential, and thus are found in sediments. Around 27 % of the annual dissolved silicon (DSi) delivery from the land to the ocean is retained in lakes and reservoirs in the form of BSi. Therefore, diatom production in lakes creates silicon sinks and influences the Si cycle. However, the processes driving and regulating lacustrine diatom-rich sedimentation are poorly constrained.This dissertation investigates two lakes in very different settings to evaluate the major factors governing diatomrichsediment accumulation. The first study site is a small subarctic, high-latitude lake, Lake 850 in Northern Sweden, and the other study site is in the hydrothermally active and DSi-rich Yellowstone Lake in Yellowstone National Park, Wyoming, U.S.A.The study of the recent Si cycle in Lake 850 revealed the importance of groundwater input to the lake’s Si budget. Groundwaterbrings 3 times more DSi compared to the stream inlet and thus is the main source of DSi for diatom production. Low sedimentation rates in the last 150 years are responsible for BSi accumulation as high as 20 dry weight%. The Holocene sedimentary record shows that the lake had low detrital input throughout the last 7400 years, likely due to low-reliefgeomorphology and a stable environment in the lake’s watershed. The stable Si isotopes of fossil diatoms suggest a stable DSi supply for the lake, where only the relative proportion of stream influx and groundwater influx of DSi are driving isotopic changes. The BSi accumulation in the sediment of Lake 850, as high as 46 dry weight% throughout the Holocene, is driven by a combination of sufficient DSi supply from groundwater and the stream inlet, low detrital input and good preservation of diatoms in the sediment.In Yellowstone Lake the importance of hydrothermal vents bringing DSi into the lake was shown. The impact of hydrothermalDSi supply is observed in the sedimentary record over the last 9800 years as elevated Ge/Si ratios (up to 37 μmol/mol). Holocene hydrothermal input is responsible for a long-term stable lake DSi concentration and its Si isotopic signature, reflected in the fossil diatoms. The high sustained input of DSi masks the effects of the sublacustrine Elliott’s Crater hydrothermal explosion ~8500 years ago. The relative proportion of the DSi sources from hydrothermal fluids and stream inlets, diatom production and dissolution are concurrent processes responsible for the variation of the Si isotopic signal. The BSi accumulation is accounted for by low accumulation rates with limited amounts of detrital input and high diatom preservation, which results in BSi concentrations in the sediment of up to 52 dry weight%.On Holocene timescales, neither lake was DSi limited, which resulted from presence of the additional sources of DSi –groundwater or hydrothermal fluids. The unlimited DSi is partly responsible for BSi accumulation. Additionally, low sediment accumulation rates due to low detrital input, often driven by changes in climate and hydrology, are factors contributing to high BSi concentration. Finally, a fine balance between diatom production and dissolution may influence BSi accumulation. These studies suggest that diatom-rich sediments are likely to accumulate in lakes situated on silicon-rich bedrock and regions influenced by volcanic and hydrothermal activity if accompanied by low clastic sediment inputs. Highlatitude lakes, which may have high groundwater input also are candidates for high BSi accumulation. Lakes with low-relief watershed morphology and with limited stream input bringing only finegrained clastic input have the potential to accumulate BSi.