Assessment of lipid biomarker paleoproxy signatures
Various studies have shown that during the Holocene the Black Sea evolved from a lacustrine, oxic system to a highly stratified anoxic marine basin as a consequence of postglacial sea level rise. The mode of introduction of seawater to the Black Sea remains the subject of debate as has its impact on surface water phytoplankton communities, notably the bloom-forming long-chain alkenone (LCA)-producing coccolithophorid haptophyte alga, Emiliania huxleyi. My lab has shown via combined paleogenetic and lipid biomarker analyses that a complex succession of LCA-biosynthesizing haptophyte algae have populated surface waters of the Black Sea during the Holocene(Coolen et al., 2009). Paleogenetic data showed that E. huxleyi colonized the Black Sea 7.5 ka cal BP, which was 4800 years earlier than previously recognized based on their preserved calcified tests (coccoliths). Alkenone-inferred past sea surface temperature (SST) showed a general cooling during the Subboreal and Subatlantic climate intervals (last 5200 years), whereas a complex mixture of previously unidentified alkenone-producing haptophyte species resulted in spurious alkenone-inferred SST data due to unknown species-specific effects on the level of unsaturation of alkenones during the preceding mid-Holocene climatic optimum (HCO). A species-specific effect in sediments older than 5.2 ka cal BP was also apparent for the level of hydrogen isotopic fractionation of the fossil alkenones resulting in spurious SSS estimates. These examples indicate the important role that the sedimentary paleome can play in the verification of paleoenvironmental information inferred from lipid biomarker proxies (Coolen et al., 2009).
In addition, taxonomical information from lipid biomarkers is often limited by either unknown- or a broad range of potential biological sources. This distinction is most complex for bacteria and archaea. For example, bacteriohopanepolyols are widespread bacterial lipid biomarkers in the sedimentary record, but the numerous potential bacterial sources of these compounds limiting their use as proxies for ecological and environmental change. Using a combined compound-specific stable isotopic and paleogenetic approach, we were able to identify the biological precursors of some of these important geolipids and their role in key-metabolic processes in Ace Lake (Antarctica) (Coolen et al., 2008), which, like the Black Sea, experienced dramatic post-glacial hydrological change.