“One thing that you can’t fake is chemistry” – Blake Shelton
In this installment of the Core Ideas podcast, our introduction to paleolimnology continues with a discussion about some physical and chemical indicators that can preserve in lake sediments. As this is a very broad topic, we attempted to categorize the indicators by analytical approach.
Complete sediment cores:
Many analyses require a sediment core to be sectioned into discrete intervals; however, the time to perform visual inspection for obvious colour and texture changes (e.g. varves) is immediately after collection. Furthermore, if a core can be brought to the laboratory intact (i.e. before sectioning) specialized core scanning equipment can measure:
- X-ray fluorescence spectroscopy (XRF) [metals analysis]
- magnetic susceptibility [erosion events, core correlation, tephra layers]
Inorganic geochemical methods:
- grain size [erosion, catchment dynamics]
- loss on ignition (LOI) [% organic matter, %carbonate, %siliciclastic]
- individual elements, metals and metalloids*
*When measuring individual components of bulk sediment, attention must be paid to whether the chosen indicator is mobile under certain conditions (i.e. pH, anoxia)
In contrast with the microscope approaches used for the bioindicators we have previously discussed, some biomarkers require chemical analyses (although keep in mind this an arbitrary distinction):
- charcoal [fire history]
- fossil pigments [algal composition]
Concentrations of fossil pigments are typically measured using high-performance liquid chromatography (HPLC); however, general trends in the specific pigment chlorophyll a can also be characterized using visible reflectance spectroscopy (VRS). A similar approach has recently been developed to infer long-term trends in total organic carbon (TOC).
In addition to the value radioisotopes have in determining the age of sediments, differences in the concentrations of stable isotopes can be used to provide information on a wide variety of variables. Both from both analyses of bulk sediments as well as the isotopic content of other indicators preserved within the sediments (e.g. cellulose and ostracods).
Organic chemical indicators:
Finally a wide variety of organic chemicals can be used to track various environmental issues of concern:
- polycyclic aromatic compounds (PACs) [fossil fuels]
- polychlorinated biphenyls (PCBs ) [anthropogenic influence]
- organochlorines [pesticides, agriculture]
- brominated flame retardants
Next time we will look at how long-term changes in paleolimnological indicators can be put to use in environmental reconstructions.