Paleothermometry is the calculation of past temperatures by analysis of specific chemicals in natural samples, like those left behind by prehistoric algae.
Algae are a diverse group of organisms that have been abundant in Earth's oceans and lakes for millennia. Certain chemical compounds, which are deposited in sediment by ancient algae, act as biomarkers – organic compounds that can provide researchers with valuable insight into Earth’s history. In fact, analysis of algal biomarker content in sediment allows researchers to determine the Earth’s temperature hundreds of millions of years ago.
One such record comes from some species of coccolithophores. These algae produce varying amounts of alkenones, a class of robust biomarkers, based on the temperature of their environment. Alkenone analysis is primarily used to calculate the sea surface temperature of Earth's oceans eons and eons ago.
This video will illustrate the use of alkenones in paleoclimatology and describe the process of isolating, purifying, and analyzing alkenones to calculate past sea surface temperature.
As its name implies, “Alkenone paleothermometry” is based on the analysis of lipids, known as alkenones. Alkenone paleothermometry is based on alkenones; long-chain, unsaturated alkyl ketones that contain 37 carbon atoms and 2 to 4 double bonds. Each double bond is a site of unsaturation. At low sea surface temperatures, alkenone producers generate more unsaturated alkenones than saturated. The ratio of saturation to unsaturation is known as the Alkenone Unsaturation Index.
The alkenones usually evaluated are C37:2 and C37:3, which have 37 carbons and two or three double bonds, respectively. The Unsaturation Index of these alkenones, or the UK'37, is positively related to sea surface temperature. The analytical method know as gas chromatography is generally sensitive enough to separate these alkenones from one another. However, alkenone-producing algae often also generate chemically-similar fatty acid methyl esters, or alkenoates, which cannot be distinguished from alkenones using this technique. Hydrocarbon contamination from pollution may also further muddy chromatographic analysis. To accurately determine relative alkenone concentration, alkenoates and unknown hydrocarbons must be removed before analysis by the methods of saponification and urea adduction.
Now that the relationship of sediment alkenone ratios to sea surface temperature has been reviewed, let's look at the techniques for their purification from a total lipid extract and analysis of the unsaturation ratio.
Once marine sediment has been collected and extracted, the total lipid extract, or TLE, must go through a multistep purification process, and analyzed. First, the extract undergoes saponification to convert alkenoates into carboxylate salts and methanol using a strong base and heat. Other fatty acid esters present in the TLE will be saponified into salts and glycerol.
After cooling the mixture to room temperature, an aqueous salt solution is added to form salts and glycerol. The mixture is then acidified to protonate the carboxylate anions, producing fatty acids. Finally, the alkenones and fatty acids are extracted from the mixture with hexane.
Silica gel chromatography is then performed to remove both apolar compounds and the polar fatty acids produced by saponification. The dried and saponified TLE is dissolved in hexane and then loaded onto a column. Silica retains polar compounds more strongly than apolar ones.
First, apolar compounds are removed with an apolar solvent, like hexane. Next, alkenones are eluted by a moderately polar solvent, such as dichloromethane, leaving the highly polar fatty acids and other unwanted polar compounds on the column.
If the original sediment sample was collected from a highly polluted area, urea adduction is performed to remove any remaining highly branched or cyclic hydrocarbons. The dried mid-polarity fraction is dissolved in a solvent mixture in which the strongly polar urea is minimally soluble, such as DCM and hexane. A concentrated solution of urea in methanol is then added to the TLE, causing urea crystals to precipitate.
Straight-chain molecules such as alkenones fit into the spaces between molecules in the urea crystal lattice, but highly branched and cyclic molecules do not, and are expelled.
Once crystal growth has finished, the urea crystals are dried and then washed with an apolar solvent to remove expelled compounds. Then, the crystals are dissolved in a small amount of water. The alkenones are extracted from the water with an apolar solvent for analysis.
While all previous purification steps did not differentiate between alkenone species, small differences in boiling point and molecular structure are sufficient for separation on a gas chromatography column. When paired with a flame-ionization detector, relative concentrations of the alkenones, can be determined.
Molecules are identified on the chromatogram by their retention time, or the time needed for the compound to be exit the column. The retention times of the desired compounds are ascertained with alkenone standards.
The relative concentrations of the alkenones are determined from analysis of the areas under the peaks of interest. The UK'37 value is then calculated from the concentrations of C37:2and C37:3 in the sample. With the sea surface temperature proxy relationship and the UK'37 value, the analyst can solve for sea surface temperature at the time of the sediment deposition.
Many different facets of Earth's history can be investigated by analysis of sediment and sedimentary rock.
Biostratigraphy is the study of determining the ages of layers, or strata, of rock by analysis of the fossils present. As there are many sources of sediment, sedimentary rocks from the same time period may have dramatically different compositions around the world. Certain sets of species throughout Earth's history, such as the ammonites, existed worldwide and underwent rapid evolution. If visually dissimilar rock strata both contain the same species of ammonite, then a temporal correlation between the strata can be drawn. When combined with techniques such as paleothermometry, extensive information about Earth's history can be determined from fossil records in natural samples.
Many species of foraminifera, or forams, are found in marine sediments worldwide. Forams have calcium carbonate shells and have existed throughout Earth's oceans for millions of years. Many species live on the ocean floor, and thus can provide temperature information about deeper parts of the ocean. The magnesium to calcium ratio of forams corresponds to temperature, as they incorporate more magnesium into their shells in warmer climates. The multitude of species and the abundance of forams makes their fossil record useful for tracking changes in ocean currents throughout Earth's history and for biostratigraphy.
As tectonic plates diverge, new rock forms between them. Correspondingly, the properties of the rock surrounding a divergent plate boundary provide information about plate movements over time. For instance, changes in Earth's magnetic field are preserved in some minerals found in fossils, rock, and sediment. The discovery of symmetric changes in magnetism about mid-ocean ridges significantly contributed to the current understanding of seafloor spreading and plate tectonics.
You've just watched JoVE's Overview of Alkenone Paleothermometry. You should now understand the principles of paleothermometry and the relationship of alkenone ratios in marine sediment to sea surface temperature. The following videos in this series will go into more detail about this complex process.
Thanks for watching!
Source: Laboratory of Jeff Salacup- University of Massachusetts Amherst
Throughout this series of videos, natural samples were extracted and purified…
Paleothermometry is the calculation of past temperatures by analysis of specific chemicals in natural samples, like those left behind by prehistoric algae.
Algae are a diverse group of organisms that have been abundant in Earth's oceans and lakes for millennia. Certain chemical compounds, which are deposited in sediment by ancient algae, act as biomarkers – organic compounds that can provide researchers with valuable insight into Earth’s history. In fact, analysis of algal biomarker content in sediment allows researchers to determine the Earth’s temperature hundreds of millions of years ago.
One such record comes from some species of coccolithophores. These algae produce varying amounts of alkenones, a class of robust biomarkers, based on the temperature of their environment. Alkenone analysis is primarily used to calculate the sea surface temperature of Earth's oceans eons and eons ago.
This video will illustrate the use of alkenones in paleoclimatology and describe the process of isolating, purifying, and analyzing alkenones to calculate past sea surface temperature.
As its name implies, “Alkenone paleothermometry” is based on the analysis of lipids, known asalkenones.Alkenone paleothermometry is based on alkenones; long-chain, unsaturated alkyl ketones that contain 37 carbon atoms and 2 to 4 double bonds. Each double bond is a site of unsaturation. At low sea surface temperatures, alkenone producers generate more unsaturated alkenones than saturated. The ratio of saturation to unsaturation is known as the Alkenone Unsaturation Index.
The alkenones usually evaluated are C37:2and C37:3, which have 37 carbons and two or three double bonds, respectively. The Unsaturation Index of these alkenones, or the UK'37, is positively related to sea surface temperature. The analytical method know as gas chromatography is generally sensitive enough to separate these alkenones from one another. However, alkenone-producing algae often also generate chemically-similar fatty acid methyl esters, or alkenoates, which cannot be distinguished from alkenones using this technique.Hydrocarbon contamination from pollution may also further muddy chromatographic analysis. To accurately determine relative alkenone concentration, alkenoates and unknown hydrocarbons must be removed before analysis by the methods of saponification and urea adduction.
Now that the relationship of sediment alkenone ratios to sea surface temperature has been reviewed, let's look at the techniques for their purification from a total lipid extract and analysis of the unsaturation ratio.
Once marine sediment has been collected and extracted, the total lipid extract, or TLE, must go through a multistep purification process, and analyzed. First, the extract undergoes saponification to convert alkenoates into carboxylate salts and methanol using a strong base and heat. Other fatty acid esters present in the TLE will be saponified into salts and glycerol.
After cooling the mixture to room temperature, an aqueous salt solution is added to form salts and glycerol. The mixture is then acidified to protonate the carboxylate anions, producing fatty acids. Finally, the alkenones and fatty acids are extracted from the mixture with hexane.
Silica gel chromatography is then performed to remove both apolar compounds and the polar fatty acids produced by saponification. The dried and saponified TLE is dissolved in hexane and then loaded onto a column. Silica retains polar compounds more strongly than apolar ones.
First, apolar compounds are removed with an apolar solvent, like hexane. Next, alkenones are eluted by a moderately polar solvent, such as dichloromethane, leaving the highly polar fatty acids and other unwanted polar compounds on the column.
If the original sediment sample was collected from a highly polluted area, urea adduction is performed to remove any remaining highly branched or cyclic hydrocarbons. The dried mid-polarity fraction is dissolved in a solvent mixture in which the strongly polar urea is minimally soluble, such as DCM and hexane. A concentrated solution of urea in methanol is then added to the TLE, causing urea crystals to precipitate.
Straight-chain molecules such as alkenones fit into the spaces between molecules in the urea crystal lattice, but highly branched and cyclic molecules do not, and are expelled.
Once crystal growth has finished, the urea crystals are dried and then washed with an apolar solvent to remove expelled compounds. Then, the crystals are dissolved in a small amount of water. The alkenones are extracted from the water with an apolar solvent for analysis.
While all previous purification steps did not differentiate between alkenone species, small differences in boiling point and molecular structure are sufficient for separation on a gas chromatography column. When paired with a flame-ionization detector, relative concentrations of thealkenones, can be determined.
Molecules are identified on the chromatogram by their retention time, or the time needed for the compound to be exit the column. The retention times of the desired compounds are ascertained with alkenone standards.
The relative concentrations of the alkenones are determined from analysis of the areas under the peaks of interest. The UK'37value is then calculated from the concentrations of C37:2and C37:3in the sample. With the sea surface temperature proxy relationship and the UK'37value, the analyst can solve for sea surface temperature at the time of the sediment deposition.
Many different facets of Earth's history can be investigated by analysis of sediment and sedimentary rock.
Biostratigraphy is the study of determining the ages of layers, or strata, of rock by analysis of the fossils present. As there are many sources of sediment, sedimentary rocks from the same time period may have dramatically different compositions around the world. Certain sets of species throughout Earth's history, such as the ammonites, existed worldwide and underwent rapid evolution. If visually dissimilar rock strata both contain the same species of ammonite, then a temporal correlation between the strata can be drawn. When combined with techniques such as paleothermometry, extensive information about Earth's history can be determined from fossil records in natural samples.
Many species of foraminifera, or forams, are found in marine sediments worldwide. Forams have calcium carbonate shells and have existed throughout Earth's oceans for millions of years. Many specieslive on the ocean floor, and thus can provide temperature information about deeper parts of the ocean. The magnesium to calcium ratio of foramscorresponds to temperature, as they incorporate more magnesium into their shells in warmer climates. The multitude of species and the abundance of forams makes their fossil record useful for tracking changes in ocean currents throughout Earth's history and for biostratigraphy.
As tectonic plates diverge, new rock forms between them. Correspondingly, the properties of the rock surrounding a divergent plate boundary provide information about plate movements over time. For instance, changes in Earth's magnetic field are preserved in some minerals found in fossils, rock, and sediment. The discovery of symmetric changes in magnetism about mid-ocean ridges significantly contributed to the current understanding of seafloor spreading and plate tectonics.
You've just watched JoVE's Overview of Alkenone Paleothermometry. You should now understand the principles of paleothermometry and the relationship of alkenone ratios in marine sediment to sea surface temperature. The following videos in this series will go into more detail about this complex process.
Thanks for watching!
Paleothermometry is the calculation of past temperatures by analysis of specific chemicals in natural samples, like those left behind by prehistoric algae.
Algae are a diverse group of organisms that have been abundant in Earth's oceans and lakes for millennia. Certain chemical compounds, which are deposited in sediment by ancient algae, act as biomarkers ? organic compounds that can provide researchers with valuable insight into Earth?s history. In fact, analysis of algal biomarker content in sediment allows researchers to determine the Earth?s temperature hundreds of millions of years ago.
One such record comes from some species of coccolithophores. These algae produce varying amounts of alkenones, a class of robust biomarkers, based on the temperature of their environment. Alkenone analysis is primarily used to calculate the sea surface temperature of Earth's oceans eons and eons ago.
This video will illustrate the use of alkenones in paleoclimatology and describe the process of isolating, purifying, and analyzing alkenones to calculate past sea surface temperature.
As its name implies, ?Alkenone paleothermometry? is based on the analysis of lipids, known as?alkenones.?Alkenone paleothermometry is based on alkenones; long-chain, unsaturated alkyl ketones that contain 37 carbon atoms and 2 to 4 double bonds. Each double bond is a site of unsaturation. At low sea surface temperatures, alkenone producers generate more unsaturated alkenones than saturated. The ratio of saturation to unsaturation is known as the Alkenone Unsaturation Index.
The alkenones usually evaluated are C37:2?and C37:3, which have 37 carbons and two or three double bonds, respectively. The Unsaturation Index of these alkenones, or the UK'37, is positively related to sea surface temperature. The analytical method know as gas chromatography is generally sensitive enough to separate these alkenones from one another. However, alkenone-producing algae often also generate chemically-similar fatty acid methyl esters, or alkenoates, which cannot be distinguished from alkenones using this technique.?Hydrocarbon contamination from pollution may also further muddy chromatographic analysis. To accurately determine relative alkenone concentration, alkenoates and unknown hydrocarbons must be removed before analysis by the methods of saponification and urea adduction.
Now that the relationship of sediment alkenone ratios to sea surface temperature has been reviewed, let's look at the techniques for their purification from a total lipid extract and analysis of the unsaturation ratio.
Once marine sediment has been collected and extracted, the total lipid extract, or TLE, must go through a multistep purification process, and analyzed. First, the extract undergoes saponification to convert alkenoates into carboxylate salts and methanol using a strong base and heat. Other fatty acid esters present in the TLE will be saponified into salts and glycerol.
After cooling the mixture to room temperature, an aqueous salt solution is added to form salts and glycerol. The mixture is then acidified to protonate the carboxylate anions, producing fatty acids. Finally, the alkenones and fatty acids are extracted from the mixture with hexane.
Silica gel chromatography is then performed to remove both apolar compounds and the polar fatty acids produced by saponification. The dried and saponified TLE is dissolved in hexane and then loaded onto a column. Silica retains polar compounds more strongly than apolar ones.
First, apolar compounds are removed with an apolar solvent, like hexane. Next, alkenones are eluted by a moderately polar solvent, such as dichloromethane, leaving the highly polar fatty acids and other unwanted polar compounds on the column.
If the original sediment sample was collected from a highly polluted area, urea adduction is performed to remove any remaining highly branched or cyclic hydrocarbons. The dried mid-polarity fraction is dissolved in a solvent mixture in which the strongly polar urea is minimally soluble, such as DCM and hexane. A concentrated solution of urea in methanol is then added to the TLE, causing urea crystals to precipitate.
Straight-chain molecules such as alkenones fit into the spaces between molecules in the urea crystal lattice, but highly branched and cyclic molecules do not, and are expelled.
Once crystal growth has finished, the urea crystals are dried and then washed with an apolar solvent to remove expelled compounds. Then, the crystals are dissolved in a small amount of water. The alkenones are extracted from the water with an apolar solvent for analysis.
While all previous purification steps did not differentiate between alkenone species, small differences in boiling point and molecular structure are sufficient for separation on a gas chromatography column. When paired with a flame-ionization detector, relative concentrations of the?alkenones, can be determined.
Molecules are identified on the chromatogram by their retention time, or the time needed for the compound to be exit the column. The retention times of the desired compounds are ascertained with alkenone standards.
The relative concentrations of the alkenones are determined from analysis of the areas under the peaks of interest. The UK'37?value is then calculated from the concentrations of C37:2and C37:3?in the sample. With the sea surface temperature proxy relationship and the UK'37?value, the analyst can solve for sea surface temperature at the time of the sediment deposition.
Many different facets of Earth's history can be investigated by analysis of sediment and sedimentary rock.
Biostratigraphy is the study of determining the ages of layers, or strata, of rock by analysis of the fossils present. As there are many sources of sediment, sedimentary rocks from the same time period may have dramatically different compositions around the world. Certain sets of species throughout Earth's history, such as the ammonites, existed worldwide and underwent rapid evolution. If visually dissimilar rock strata both contain the same species of ammonite, then a temporal correlation between the strata can be drawn. When combined with techniques such as paleothermometry, extensive information about Earth's history can be determined from fossil records in natural samples.
Many species of foraminifera, or forams, are found in marine sediments worldwide. Forams have calcium carbonate shells and have existed throughout Earth's oceans for millions of years. Many species?live on the ocean floor, and thus can provide temperature information about deeper parts of the ocean. The magnesium to calcium ratio of forams?corresponds to temperature, as they incorporate more magnesium into their shells in warmer climates. The multitude of species and the abundance of forams makes their fossil record useful for tracking changes in ocean currents throughout Earth's history and for biostratigraphy.
As tectonic plates diverge, new rock forms between them. Correspondingly, the properties of the rock surrounding a divergent plate boundary provide information about plate movements over time. For instance, changes in Earth's magnetic field are preserved in some minerals found in fossils, rock, and sediment. The discovery of symmetric changes in magnetism about mid-ocean ridges significantly contributed to the current understanding of seafloor spreading and plate tectonics.
You've just watched JoVE's Overview of Alkenone Paleothermometry. You should now understand the principles of paleothermometry and the relationship of alkenone ratios in marine sediment to sea surface temperature. The following videos in this series will go into more detail about this complex process.
Thanks for watching!
View the full transcript and gain access to JoVE Science Education videos
Q1: What are alkenones and why are they useful for studying past ocean temperatures?
Alkenones are long-chain, unsaturated alkyl ketones with 37 carbon atoms and 2 to 4 double bonds produced by coccolithophore algae. These robust biomarkers vary in saturation based on water temperature, allowing researchers to reconstruct sea surface temperatures from ancient marine sediments. The ratio of unsaturated to saturated alkenones, called the UK'37 index, directly correlates with past ocean temperatures.
Q2: How does the UK'37 index relate to sea surface temperature?
The UK'37 index is calculated from the ratio of two alkenone types: C37:2 and C37:3. At lower sea surface temperatures, alkenone producers generate more unsaturated alkenones, increasing the UK'37 value. This positive relationship between unsaturation and temperature has been calibrated through laboratory and core-top sediment studies, enabling analysts to convert UK'37 values into quantitative sea surface temperature estimates.
Q3: What contaminants interfere with alkenone analysis and how are they removed?
Alkenoates (fatty acid methyl esters) and hydrocarbon pollution co-elute with alkenones during gas chromatography, complicating quantification. These are removed through conversion of fatty acid methyl esters by saponification for UK'37 paleothermometry, which converts alkenoates into water-soluble salts. Silica gel chromatography then separates remaining polar compounds, and urea adduction removes branched or cyclic hydrocarbons from highly polluted samples.
Q4: What is the purpose of silica gel chromatography in alkenone purification?
Silica gel chromatography separates compounds by polarity. After saponification, the total lipid extract is loaded onto a silica column where apolar compounds elute first with hexane, alkenones elute next with dichloromethane, and highly polar fatty acids remain on the column. This purification of a total lipid extract with column chromatography isolates alkenones from unwanted polar and apolar contaminants before gas chromatography analysis.
Q5: How does urea adduction remove branched hydrocarbons from alkenone samples?
Urea adduction exploits molecular geometry differences. Straight-chain alkenones fit into spaces within urea crystal lattices, while branched and cyclic hydrocarbons cannot and are expelled. The urea crystals are then washed with apolar solvent to remove expelled contaminants, and alkenones are recovered by dissolving crystals in water and extracting with apolar solvent. This removal of branched and cyclic compounds by urea adduction for UK'37 paleothermometry is essential for highly polluted samples.
Q6: How are alkenone concentrations determined using gas chromatography?
Purified alkenones are separated on a gas chromatography column based on boiling point and molecular structure differences. A flame-ionization detector measures peak areas corresponding to C37:2 and C37:3 alkenones. Retention times are confirmed using alkenone standards, and relative concentrations are calculated from peak areas. These values are then used to compute the UK'37 ratio and convert it to sea surface temperature.
Q7: Why is alkenone paleothermometry more reliable than other paleoclimate proxies?
Alkenones are exceptionally stable over geologic time, persisting in marine sediments from the early Eocene to present. The UK'37 proxy correlates well with mean annual sea surface temperature across diverse ocean climates and algal production regimes. Alkenones can document temperature changes at decadal to orbital timescales, making them versatile for paleoclimate reconstruction. Their abundance in open-ocean sediments worldwide provides extensive paleoclimate records.
Chapters in this video
0:00
Overview
1:36
Principles of Alkenone Paleothermometry
3:42
Alkenone Purification
6:32
Analysis of Relative Alkenone Concentrations
7:48
Applications
9:40
Summary
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