Lake criteria

1. Water fills a depression fully or partly

• Fully filled - outflow lake or exorheic basin
• Parly filled - endorheic basin like Caspian Sea or Great Salt Lake
• Ephemeral lakes are a special case

2. Equal amount of water enters the lake and is removed by evaporation and outflow

• Usually there is only one outflow and many inflows, expect in so called bifurcation lakes

3. Water level is more or less equal in different parts

• Except for wind and ice

4. Incoming flow is low enough for suspended material to be deposited as sediment

• Except during spring floods

5. Surface - area exceeds the agreed minimum value

• Something like 500m2 or 1 ha

Lake vs pond is difficult to define

• In northern Canada, a pond is something that freezes all the way to the bottom in winter

6. Water level does not regularly follow water level of oceans

Origins of lakes

1. Tectonic lakes

• Caused by earth's crust movements - like bedrock faults, folding, rift valleys, depressions, block movements, schistose (metamorphic) structures, or in old fold structures
• Usually deep and long lakes (especially around metamorphic rocks), right-angle cuts at the shoreline

2. Glacial lakes

• Basins grooved by ice - orientation often follows glaciation
• In the Finnish lake district, about 7m of material was generally deposited, which is the mean depth of lakes at this area
• Kettle lakes formed by melting blocks of ice
  • Often groundwater fed
  • Bolt basins at terminal moraines
• Terminal moraine lakes (Rogen)
  • Moraine dam

3. Coastal lakes (due to land uplift)

• Isostatic uplift results in abayments in the shoreline being closed off and moved inland
  !lakes_sedi_2026, p.20

Supra-aquatic vs sub-aquatic areas in Finland

• In Finland, lakes closer to the Baltic Sea used to be under it (sub-aquatic), if you drill down you'll reach oceanic sediment before glacial clay

Aeolian lakes (deflation basin)

• Formed by wind action 4000-5000 years ago
• Similar to dune formation in the sands and light sediment left after glaciacion

Fluvial lakes (oxbow)

• In Finnish makkarajärvi because they're sausage shaped :)

Thermokarst (Cryogenic) lakes

• thermokarst
• Land sinking with permafrost melting

Meteorite lakes (rare)

• Formed in meteorite crater

Volcanic crater lakes

Anthropogenic lakes

• Created by dams

Lake sedimentation

• Average input rate about 1mm/y but slower in Arctic (0.1)
• Internal currents move sediment from shallow areas to deep areas

Sediment inputs

Atmospheric inputs

• Dust, charcoal, pollutants - can reveal history of air pollution and wildfires and date sediment cores

Catchment area

• Flowing through overland flow or small rivulets
• Mineral particles, nutrients, plant remains
• Forest clearing, draining mires and farming increase sedimentation rates significantly

Internal lake inputs

• Remains of plants, animals and bacteria, especially plankton (diatoms)
• Remains of organisms are sedimented chronologically in sediment
• Some organisms leave no trace
• Proxies for ecosystem structure and process

Groundwater inputs

• Solutes (salts, Ca, Na, Mg)
• Nutrients like N and P
• toxins (pesticides)

Lake morphology and sedimentation

Zones

Erosion zone

• furthest from lake center
• Water content of surface sediemtn < 50%
• Coarse material

transport zone

• Unstable sedimentation

Accumulation zone

• continuous sedimentation
• fine sediment material
• Usually this is the best place to take samples where most consistent sedimentation has occured
• Sometimes its hard to actually identify the deepest part without radar scanning
  • There could be a part of the lake with a deeper sediment before reaching bedrock, even if the top of the sediment is less deep than somewhere else

Factors affecting formation of zones

• Surface area of the lake
• Wind and fetch (the amount of space the wind has to go - interrupted by tree cover or topography)
  • This causes wave action which disturbs sediments if the wave base is big enough to reach the sediment level
  • For example - a lake between Vienna and Hungarian border - cannot get a sediment series from the deepest part because of high wind and wave action
  • Also effects internal currents (circulation)
• Flow
• Slope
  • If you have a steep part the sediment will not deposit on the steep slope
• Shape
  !lakes_sedi_2026, p.45
• Flow rate - higher flow rate results in erosion and transportation of larger particles, put also very small particles (clay) due to electrostatic binding

Bioturbation

• Insects, molluscs and crustaceans will tunnel around and mix the sediment
• If you find lots of little guys crawling around in your sample it's probably not worth analyzing
  • Carbon dating will tell you if your core is in the right order, but this is expensive

Stratigraphy

• Contains abiotic, biotic and chemical portions (litostratigraphy, biostratigraphy, chemostratigraphy)
• Finnish lakes usually contain 2-5m of sediment
• Usually the surface layer is 90% water
• 60% of organic material is organic Carbon
• Sediments in all lakes in finland is about 700 - 900 million tons of carbon - lake sediments store more C than terrestrial forests
• Average carbon storage rate in Finnish lakes 88 000 tons / year
• Relatively more Carbon storage in small lakes than bigger lakes

Carbon budget of lakes

• 2x more terrestrial carbon is sent to lakes than oceans
• 4x more carbon sedimented in lakes than oceas
• equal C emissions with oceans
• Global climate models have not taken C budgets and emissions of lakes into acccount

Elemental chemistry of Finnish lakes

• Usually missing Phosphorous out of the big 3 nutrients (C,P,N)
• Mostly made of major elements (Si, Al, K, Na, Mg)
• Carbonates (Ca, Mg, CO_3 - C) usually 15% of dry sediment

Sediment Classes

• Clastic (sand, silt, clay)
• Chemical (mud)
• Biogenic (Ooze, peat)
  !lakes_sedi_2026, p.55

Loss-on-ignition (LOI) analysis

• Burn the dry sediment and figure out how much burns - this tells us what percentage is organic material
• Helps to determine sediement class
• Also try mouth texturing to determine origin

Holocene LOI variation in Lake Toskaljärvi - NW Finland

• Usually filled by underground karst channels
• There was a sudden drop in LOI in the sediment core 5000 years ago
  • Correlated with increase in silt %
• Might have been the result of a landslide or slush avalanche depositing lots of inorganic silt?