Alkenone Paleothermometry

Core Idea

Alkenones are long-chain ketones produced by certain coccolithophore algae whose unsaturation degree (UK'37 index) correlates with growth temperature. The ratio of C37:2 to C37:3 alkenones provides a paleothermometer with ~1-2°C accuracy, independent of carbonate and oxygen isotope systematics. This proxy is especially valuable for tropical and warm-water paleoceanography.

How It's Best Learned

Extract lipids from sediment samples, separate alkenones via chromatography, and measure the C37:2 and C37:3 ratios using gas chromatography. Apply published calibrations to convert UK'37 to SST and compare results with δ18O-derived temperatures from the same sample.

Common Misconceptions

• Alkenones record growth temperature but may be reworked or transported; assuming in-situ formation without independent age constraints can bias interpretations. - Assuming the UK'37-temperature relationship is global; regional calibrations often improve accuracy.

Explainer

From your study of paleoclimate proxies, you know that past climates must be reconstructed from indirect indicators preserved in geological archives. Most ocean temperature proxies rely on the chemistry of calcium carbonate shells — oxygen isotope ratios in foraminifera, for example. Alkenone paleothermometry offers something different: a temperature record encoded not in mineral shells but in organic molecules produced by photosynthetic algae. This independence from carbonate chemistry makes alkenones a powerful cross-check and, in some settings, a superior alternative.

Alkenones are long-chain (C₃₇–C₃₉) unsaturated ketones synthesized by certain species of coccolithophore algae, primarily *Emiliania huxleyi* and *Gephyrocapsa oceanica*. These molecules serve as membrane lipids, and here is the key insight: the organisms adjust the degree of unsaturation in their alkenones in response to growth temperature. At warmer temperatures, they produce more fully saturated (fewer double bonds) alkenones; at cooler temperatures, they produce more unsaturated (more double bonds) forms. The Uᴷ'₃₇ index quantifies this by calculating the ratio of di-unsaturated (C₃₇:₂) to the sum of di- and tri-unsaturated (C₃₇:₂ + C₃₇:₃) alkenones. Higher Uᴷ'₃₇ values correspond to warmer sea surface temperatures.

The practical workflow involves extracting lipids from marine sediment cores using organic solvents, separating the alkenone fraction via gas chromatography, and measuring the relative abundance of the C₃₇:₂ and C₃₇:₃ peaks. Published calibrations — derived from global core-top datasets where modern SST is known — convert the measured Uᴷ'₃₇ to temperature, typically with an accuracy of ±1–2°C. The relationship is approximately linear over the 5–28°C range, making it straightforward to apply. However, at very cold temperatures (below ~5°C), the calibration loses sensitivity because the C₃₇:₃ alkenone dominates almost entirely, and at very warm temperatures (above ~28°C), the C₃₇:₂ form dominates, similarly compressing the signal.

One of the greatest strengths of alkenone paleothermometry is that it is chemically independent of the carbonate system. Oxygen isotope proxies from foraminifera are affected by both temperature and the isotopic composition of seawater (which changes with ice volume), requiring corrections that introduce uncertainty. Alkenones bypass this entirely — they record temperature through organic molecular structure, not mineral chemistry. This makes them especially valuable in tropical and subtropical ocean settings where the carbonate proxies may be complicated by dissolution or diagenesis. The tradeoff is that alkenones can be physically reworked — transported by currents or bioturbation from one sediment layer to another — so independent chronological control (radiocarbon dating, biostratigraphy) is essential to ensure the alkenones in a given sediment horizon actually represent the time period of interest.