DNA and lipid molecular stratigraphic records of haptophyte succession in the Black Sea during the Holocene

Coolen, M. J. L.; Sáenz, J. P.; Giosan, L.; Trowbridge, N. Y.; Dimitrov, P.; Dimitrov, D.; Eglinton, T. I. (2009) Earth and Planetary Science Letters, 284(3), 610–621. DOI: 10.1016/j.epsl.2009.05.029

Summary

This study combined ancient DNA analysis with lipid molecular stratigraphy to reconstruct the Holocene succession of haptophyte algae as sources of alkenones in the Black Sea. The work revealed that Emiliania huxleyi colonized the Black Sea approximately 4000 years earlier than previously recognized from coccolith preservation alone, and that brackish Isochrysis-related haptophytes were the initial alkenone producers after sapropel deposition began around 7550 years BP. Strong species- and strain-specific effects on alkenone unsaturation patterns rendered sea surface temperature estimates unreliable before 5250 years BP.

Key Findings

• Isochrysis-related haptophytes were the first alkenone producers in the Black Sea, appearing at the onset of sapropel deposition (~7550 a BP)
• Ancient DNA evidence showed E. huxleyi entered the Black Sea ~7500 a BP, approximately 4000 years earlier than the coccolith record indicates
• E. huxleyi strains were the most likely source of the unusual C36 di-unsaturated “Black Sea alkenone”
• Haptophyte species succession caused spurious alkenone-derived sea surface temperature estimates before 5250 a BP
• From ~5250 a BP onward, a stable E. huxleyi assemblage yielded robust SST values showing gradual cooling from 19 degrees C to ~15 degrees C

Our Contribution

Saenz contributed lipid analytical work at the Woods Hole Oceanographic Institution, including alkenone extraction and characterization from sediment cores as part of his early graduate training in organic geochemistry.

Significance

This work demonstrated the power of combining paleogenetic and lipid biomarker approaches to resolve source organism ambiguity in the sedimentary record. The finding that coccolith preservation dramatically underestimates the timing of haptophyte colonization has broad implications for paleoceanographic reconstructions that rely on alkenone proxies.