Question 1

What are alkenones and why are they useful for studying past ocean temperatures?

Accepted Answer

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.

Question 2

How does the UK'37 index relate to sea surface temperature?

Accepted Answer

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.

Question 3

What contaminants interfere with alkenone analysis and how are they removed?

Accepted Answer

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.

Question 4

What is the purpose of silica gel chromatography in alkenone purification?

Accepted Answer

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.

Question 5

How does urea adduction remove branched hydrocarbons from alkenone samples?

Accepted Answer

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.

Question 6

How are alkenone concentrations determined using gas chromatography?

Accepted Answer

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.

Question 7

Why is alkenone paleothermometry more reliable than other paleoclimate proxies?

Accepted Answer

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.

An Overview of Alkenone Biomarker Analysis for Paleothermometry - Assessment Quiz