PP 16: Paleolimnology

Paleolimnology is an inter-disciplinary science 

• The study of water life and sediments from preexisting geological time periods 

• Uses physical, chemical, and biological information stored in lake sediments 

• Long-term data are essential for understanding environmental and ecological problems 

Lakes as reservoirs of history  

-Study of lake sediments  

-Implications: compares historic conditions with more modern changes  

-Study of the impacts of:  

• Local pollution  

• Worldwide pollution

Why are lake sediments important?

-Preserve information about lake history, specifically:  

• Land-use changes in watershed 

• Ecological changes in lake and watershed  

Sedimentary history of lakes

• Provides additional understanding of how lakes work 

• Contributes information on the geological and biological history of the watershed region: sedimentary cores provide a continuous archive of environmental information  

Basic dynamics of sedimentation

-As material reaches the lake from the watershed  

• Coarse material is deposited near shore 

• Fine material reaches pelagic region, joined by planktonic organic matter 

• Sedimentation rates vary withing a few to several mm per year  

• Sediment traps  

• Cores 

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• From yearly cycles of deposition of varying composition (e.g. ”varves” in calcareous sediments- altering light and dark bands) 

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Sediment chronology

-Fundamental to paleolimnology  

• Determine rates of processes/fluxes  

• Link disturbance to sediment archive  

• Determine synchronicity of events  

-²¹⁰ Pb 

-¹⁴ C 

-Extrapolate ²¹⁰ Pb dates, use ¹⁴C to constrain oldest core dates 

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Watershed development

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Paleolimnology techniques: list

• 1. pollen analysis of lake sediments

• 2. Algae: diatoms, some greens, chrysophytes, dinoflagellates, bluegreen heterocysts

• 3. Macro Fossils 

• 4. ¹⁴C dating

• 5. Zooplankton fossils  

• 6. Known atomic events (bombs and accidents) 

• 7. Geologic

• 8. ²¹⁰Pb dating 

• 9. Chlorophyll and carotenoid degradation products 

• 10. Varves 

• 11. Inorganic chemistry

•  12. Previous lake levels, wave cut terrace  

• 13. Organic content  

Paleolimnology techniques: 1. pollen analysis of lake sediments

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-Most useful is Ambrosia pollen (ragweed), associated with clearing and cultivation 

• Non-natice species that began to flourish after the settlement of N. America 

-Combination of ¹⁴C dating and Ambrosia pollen aids both chronology and sedimentation estimates 

-Range of error can be extreme. Some pollens preverse really well, and others don’t. Presnece or absence of a particular pollen doesn't tell the full story because not everything is equal. Same for the algae 

Paleolimnology techniques: 2. Algae: diatoms, some greens, chrysophytes, dinoflagellates, bluegreen heterocysts

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Paleolimnology techniques: 2. Algae: diatoms, some greens, chrysophytes, dinoflagellates, bluegreen heterocysts cont.

-Diatoms:  

• Indicators of fertility, water temperature, eutrophication  

• Most often used is the ratio of centrics to pennates  

• High= oligotrophy 

• Low= eutrophic 

• eg pic. lake washington sediment core 

• Ratio gives you a better idea, because species can not be there for any number of reasons 

-Diatoms as indicators of environmental change  

• Well preserved in lake sediments 

• Remain stable in sedimentary sequences 

• Once they’re down, they stay pretty put 

• Taxonomically specific ornamentation 

• Unique shell 

• Many have narrow optima and tolerances 

• Allows you to assign them to presence or absence of conditions 

• Respond rapidly to environmental change  

• the best for this method 

• super specific to what species is there with conductivity. If you have presence and then suddenly have absence, you know that something probably happened with conductivity.  

Paleolimnology techniques: 2. Algae: diatoms, some greens, chrysophytes, dinoflagellates, bluegreen heterocysts cont.

-Biogenic silica  

• Diatoms, chrysophytes 

• Indicator of diatom biomass 

-Top layer of sediments- After 

• Diatoms deposited in recent sediments 

• Open water (planktonic) diatoms abundant  

-Bottom- before (in reference to the diagram below) 

• Diatoms deposited in pre-1850 sediments  

• Shallow water (benthic) diatoms abundant 

Paleolimnology techniques: 3. macro fossils

• fish, vertebrae and scales, wood fragments 

• Many groups have some identifiable remains 

Paleolimnology techniques: 4. ¹⁴C dating  

-Indicates age of sediments and organic fragments 

• to do ¹⁴C, you must have organic debris 

-Also other isotopes 

-natural abundance of stable carbon isotopes  

• ¹²C 98.9% 

• ¹³C 1.1% 

-Amount of fractionation based on:  

• Photosynthetic pathway 

• Carbon availability  

-Stable isotopes 

• Are naturally occurring  

• Do not radioactively decay 

• Reported using the ‘d notion’ (delta notion) 

-delta ‰= [(R sample/R standard) - 1] x 1000, where ‘R’ is the ratio of heavy to light isotopes (e.g. ¹³C/¹²C) heavier isotope goes on top  

Paleolimnology techniques: 4. ¹⁴C dating

-Stable Carbon isotopes and productivity change 

• high productivity  

  • less available DIC 

  • Less fractionation 

  • Algae/OM less negative 

• Low productivity 

  • More available DIC 

  • More fractionation 

  • Algae/OM more negative 

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Paleolimnology techniques: 5. Zooplankton fossils  

-Zooplankton are useful, esp. Cladocerans, midges, ostracods; also rhizopods (protists), but they’re really small so they’re not as useful as diatoms 

-Cladoceran mouth parts, abdominal claws 

• Very specialized, have to be able to identify by just a mouthpiece  

• Indication of community structure 

• Ex. Size regulation of Mysis stomach contents 

-Chironomids as indicators of environmental change  

• Blood worms 

• Hypolimnetic oxygen 

  • Some species can upregulate their hemoglobin, so if you have a lot of those you know it was an oxygen poor system  

• Climate 

• Salinity 

• Lake productivity  

-Chaoborus as indicators of environmental change 

• Chaborus americanus- presence of this taxon is a good indication of fishless conditions  

-Best work has involved species diversity of chydorid cladocerans as a function of lake productivity 

-Resurrection ecology  

• Hatch the eggs that were really old, look at the DNA and who’s coming out of the sediment 

-Cladocerans used to track:  

• Climatic changes 

• Trophic oscillations 

• Acidification 

• Water-level changes 

-Ostracodes as indicators of environmental change  

• Nutrient status  

• Salinity 

• Temperature 

• Chemical composition of their host water 

• Trace element (especially Mg and Sr) content and stable isotope ratios of their shells reflect 

  • Water temperature 

  • Water chemistry 

  • Productivity  

Paleolimnology techniques:6. Known atomic events (bombs and accidents) 

Increase in radioactive isotopes  

Paleolimnology techniques: 7. geologic events 

• Dating of core sections  

• Ex. Dating of Katmi and St. Helens eruption 

Paleolimnology techniques: 8. ²¹⁰Pb dating 

From earliest human use to tetra-ethyl-lead gasoline additives 

Paleolimnology techniques: 8. ²¹⁰Pb dating cont

• ! wont test on this specifically

Paleolimnology techniques: 8. ²¹⁰Pb dating cont

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Paleolimnology techniques: 9. Chlorophyll and carotenoid degradation products 

• Pigments are community indicators 

• Chlorophyll has been found in samples as old as 11,000 years 

• Chlorophyll and phosphorous both can indicate nutrient loading  

Paleolimnology techniques: 10. Varves 

-Alternating light and dark layers corresponding to a year  

• Dark= reducing conditions, indicates oxygen, oxygen has been given up 

• Light= spring inflows, more oxygen coming in 

-Indicators of annual deposition  

Paleopigments 

-Indicative of  

• Total algal abundance 

• Specific algal types 

• Paleoproductivity 

Phosphorus

-Increases due to  

• Cultural inputs 

• Upward migration in the core 

• Biological uptake  

Paleolimnology techniques: 11. Inorganic chemistry 

Ex. Greenland ice studies 

Paleolimnology techniques: 12. Previous lake levels, wave cut terrace  

Ex. Lake Tahoe has a 100m high terrace and also dramatically lower lake levels 

Paleolimnology techniques: 13. Organic content  

• peat layers

Things to know for the test

• Coring  

• Porosity 

• Chlorophyll 

• Organic matter 

• Percent CaCO3 

• Sedimentation 

• Phosphours 

• Diatoms 

• dating  

Principal methodology

-The core sample is the traditional and still primary technique for obtaining a vertical profile of sediment 

• Used to use metal, but now we have strong plastics that we can use 

• for very long cores, a piston apparatus helps pull sediment into corer; also, a freefall mechanism may be employed to maximize depth penetration 

• a full liner is removed to lab, frozen if appropriate 

• in lab, core is extruded or liner is slices lengthwise, can also be done on site 

• Sediment profile is excised and analyzed in appropriate increments  

  • Figure out the sedimentation in your lake roughly and then you can see how big/small you need to cut your core 

Core sediment analyses: Inorganic matter

-1. inorganic matter 

• A. Reflects the chronology of mineral leaching rates from the watershed; conservative minerals are the primary focus 

  • CaCO₃ is commonly used 

  • bands of unusual minerals are markers of known geological events, esp. Volcanic eruptions and impacts producing episodes of atmospheric fallout and deposition  

• B. radioistope dating provides time sequence with sediment depth  

  • for >150 years: ¹⁴C 

  • for <150 years: ²¹⁰Pb 

Core sediment analyses: organic compounds

-2. Organic compounds 

• A. especially important for 14C dating  

• B. Amino acids, carbohydrates and pigments generally greatest near sediment surface were they are being deposited  

• C. reflect general quantity of biotic productivity; but difficult to distinguish terrestrial from aquatic; most biopolymers are not stable  

• D. chlorophyll derivative pigments  

• Don't have to know the equation  

• These pigments can be quantified photometrically by their unique absorption spectra, and by gas/liquid chromatography  

• other pigments can be informative, e.g. myxoxanthin, a bluegreen carotenoid, can indicate eutrophic conditions (at time of deposition) 

• Commonly, lakes in post-glacial period show early increase in mineral deposition and biotic productivity (as pigments), followed by lower stable rates, and finally recent eutrophication  

• In marl lake cores, Wetzel showed an inverse correlation of productivity and extreme calcareous conditions 

Ancient DNA

PCR can now amplify traces of DNA to identify species in sediments 

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Field methods- winter

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Different cores

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Some important lake management questions

• What is the “natural" or baseline condition of the lake?  

• Has the water quality changes since pre-development (or pre-industrial) times? if so, when did these changes occur? 

• What is the direction and magnitude of this change? 

• What are the possible reasons for this change?