Black Sea
8.5–8.0 ka

Isoprenoidal GDGTs.
The TEX₈₆ thermometer.

Four membrane-spanning archaeal tetraethers with 0–3 cyclopentane rings. Ring count rises with growth temperature — the foundation of the most widely used molecular SST proxy.

“A significant linear correlation (r² = 0.92) is found between the number of cyclopentane rings in sedimentary membrane lipids derived from marine crenarchaeota and the annual mean sea surface temperatures.” — Schouten et al. 2002, EPSL 204, 265–274 (TEX₈₆ origin paper)

Crenarchaeol.
The cyclohexane ring that maps the marine signal.

A 66-membered macrocycle with 22 stereocenters and a cyclohexane–cyclopentane motif found nowhere else in nature. The diagnostic biomarker for all ammonia-oxidizing archaea (AOA).

“‘Ca. N. gargensis’ is the first cultivated archaeon to synthesize substantial amounts of the crenarchaeol regioisomer … supports the hypothesis that crenarchaeol is specific to all AOA.” — Pitcher et al. 2010, ISME J 4, 542–552

“We propose a revised structure of crenarchaeol, wherein the stereochemistry of the all-carbon quaternary stereocenter [A15′] is inverted compared to the original proposal.” — Holzheimer et al. 2021, Angew. Chem. Int. Ed. 60, 17504–17513 (first total synthesis)

Branched GDGTs.
Continental temperature, written by Acidobacteria.

Bacterial tetraethers built on a linear C₃₀ backbone with syn-positioned methyl branches and 1,3-trans cyclopentane rings. Methylation rises in cold, acid soils — the MBT′₅ME thermometer.

“Solibacter usitatus makes a large portion of its cellular membrane (24 ± 9% across all experiments) out of a structurally diverse set of tetraethers including the common brGDGTs Ia, IIa, IIIa, Ib, and IIb.” — Halamka et al. 2023, Geobiology 21, 102–118 (first axenic brGDGT culture)

“Herein, we report the first total synthesis of brGDGT Ia, thereby elucidating the relative configuration of the methyl branches as syn.” — Mahapatra et al. 2025, Chem. Eur. J. e202500702

6-methyl primed brGDGTs.
The signal that almost wasn't there.

Until 2013, 5-methyl and 6-methyl brGDGT isomers co-eluted on standard LC. Separation revealed the 6-methyl family preferentially abundant in alkaline, dry, arid soils — and a distinct climate proxy.

“Identification of novel penta- and hexamethylated branched glycerol dialkyl glycerol tetraethers in peat using HPLC-MS², GC-MS and GC-SMB-MS.” — De Jonge et al. 2013, OG 54, 78–82

Hydroxylated isoGDGTs.
A second AOA thermometer, born in the cold.

Marine isoprenoidal GDGTs with a tertiary hydroxyl on the biphytane backbone. Enriched in cold and polar waters — and the producer was a 12-year mystery solved only in 2024.

“Nitrosopumilus adriaticus NF5 shows a substantially higher relative abundance of OH-isoGDGTs (~49%) compared to Nitrosopumilus piranensis D3C (~5%).” — Varma et al. 2024, Biogeosciences 21 (first axenic OH-isoGDGT culture)

Alkenones, leaf waxes, and the C₃₀ monomer.
Where the Black Sea complicates the textbook.

Long-chain ketones from haptophyte algae give the classic UK′₃₇ thermometer. But the Black Sea inverts the standard story — and the manuscript engages with that directly.

“Haptophytes related to brackish Isochrysis spp. were the initial sources of alkenones, and appeared immediately after the onset of sapropel deposition (~7550 yrs before present). E. huxleyi colonized the Black Sea shortly after, ~4000 yrs earlier than previously recognized.” — Coolen et al. 2009, EPSL 284, 610–621