Chemicals used to assess past environment change from sediments

Potential biomarkers

• Carbohydrates (sugars)
• Proteins (amino acids)
• Nucleic acids
• Lipids (fatty acids, sterols)
• Lignins
  Most important are lipids and lignins (for this lecture)

Compared to fossils

• Biomarkers cannot help you identify to species, unlike microfossils
  • Not good for estimating community structure
• Instead, they can help you directly proxy temperature or another variable
• Best used in conjunction with biomarkers

Chemicals

Highly Branched Isoprenoid (HBI) Alkenes

• Lipids made by a few diatom genera
• Widespread
• Numerous alkene structures (isomers) which differ on position and number of double bonds, but have the same number and type of atoms

Most widely used HBIs for sea ice reconstruction

• IP25 (Ice Proxy, 25 Carbon atoms)
  • First one that was discovered
  • Produced by diatoms living within the sea ice, so it is a direct proxy of sea ice since it is not found in phytoplankton
    • Only in Arctic
  • Not quite as straightforward as originally thought, since there are other variables which affect its production as well. Its concentration is not 100% correlated with sea ice
• IPSO25 (Ice Proxy Southern Ocean 25)
  • Antarctic ice proxy, also found in arctic in small concentrations
• HBI III and HBI IV
  • Tri-unsaturated isomers produced by open-water phytoplankton (protists) abundant in marginal ice zone and drift ice
  • Recent studies have shown that these can also be produced in sea ice
• Unknown why these are found only in sea ice
  A map showing where each HBI sea ice proxy is produced relative to ice sheets:
  
  From Belt et al 2018
• Since IP25 and IPSO25 are not found under permanent ice cover or in sediments under open water they can be used to reconstruct seasonal sea ice
• HBIII used as a proxy for the Marginal Ice Zone since it is mostly produced on the ice edges
• Ratio between them can give estimates of sea ice concentrations

Alkenones and GDGTs

• Used mainly for temperature reconstruction
• Restricted to haptophyte algae from a certain order (Isochrysidales)
• Biological function is not well known

Alkenone use as proxy

• Species producing these respond to water temp changes by altering relative proportions of different alkenones produced - at higher temps more saturated alkenones are produced
• Moden data shows a clear correlation between sea surface temperature and unsaturation index of di- vs tri- unsaturated C37 alkenones
  • However, doesn't work so well in polar regions and lacustrine settings (requires calibration)

GDGTs as a proxy

• Membrane-spanning lipids produced by Archaea domain prokaryotes
• Used as a temperature proxy in marine settings and large lakes
• Similar to Alkenones, they can be calibrated to show a linear relation between surface water temp and different concentrations, but needs to be recalibrated for polar regions
  • More compex than Alkenones, but its a relationship between a few different GDGTs

Sterols

• A kind of lipid steroid used in cell signaling. Cholesterol is one of them
• Used to determine sources of organic matter in sediments
• Highest concentrations in the open arctic ocean

Plant pigments

• Used in Environment Archives - Lakes but also in Environmental Archives - Oceans
  • Chlorophylls (primary)
  • carotenoids, phycocyanin, phycoerythrin (accessory)
• Acccessory pigments can be used to estimate abundances of different algal classes and community composition changes, since certain accessory pigments are specific to cetain taxa (but not to species level)

Ancient sediementary DNA

• An emerging method for prokaryates
• Recently is becoming less expensive
• Care must be taken to avoid contamination
• "Shotgun metagenomic sequencing" can be used to identify all genes in all organisms from a sample

Time scales

• Lipids can preserve well in sediments, the oldest from 600 Ma
• Intact DNA found in samples > 100 ka

Challenges

• Important to have specific knowledge about which species produces them and why
• Which other variables (other than the variable of interest) prouce them
• Where in the environment are they produced and preserved
• Preservation in the sediment, but also during storage and handling (light and oxygen particularly)
• Consider the price of the analysis method, sometimes expensive equipment is required