•The range for δD linear mixing calculation should be δD < 10,000‰, or D/H < 0.1713.•Disagreement between slope and intercept in spite of high R2 is due to kinetic fractionation.•Paleoprecipitation reconstruction by adding a Δ would cause error and alpha should be used.•Larger D/H fractionation at higher growth temperature is against the classic isotope theory.•Hydrogen tunneling may account for the large D/H fractionation at higher temperature.Hydrogen isotope measurements of lipid biomarkers preserved in sediments are most commonly interpreted as qualitative, rather than quantitative indicators of paleoprecipitation owing to an imperfect knowledge of all factors controlling the isotopic fractionation occurring during biosynthesis. Here, we first offer a brief review of appropriate procedures for preparing enriched isotope substrates for use in tracer studies and outline the approximate δD threshold at which this transition occurs. We then present new interpretations to explain deviations from common stable isotope effects observed in our previous culture experiments and other studies.We draw particular attention to the disagreement between intercept and slope for product–substrate relationships from those predicted for isotope systems, even when R2 values are high, and attribute it to kinetic isotope fractionation. We demonstrate that reconstructing paleoenvironmental water δD values by simply adding a Δ to measured biomarkers δD values will result in a bias toward deuterium enriched values. This applies even to implicit reconstructions in the form of qualitative interpretations of measured lipid δD values as indicators of past hydroclimate. We therefore recommend reconstructing water δD values from lipid δD values using fractionation factor (α).We also discuss the apparently contradictory increase in D/H fractionation observed at elevated temperature and suggest that this may be the result of the unique wave-particle duality of hydrogen isotopes, which permits isotopologues to avoid surmounting the activation energy barrier that is necessary in traditional kinetic reactions.

•El Junco sediment residues were sequentially hydrolyzed with base and acid.•α-Hydroxy acids, β-hydroxy acids, methyl β-hydroxy acids were not detected in free extract.•Bound alkanols, ω- and (ω-1)-hydroxy acids differed in their distributions from free lipids.•Biomarkers with m/z 341 in the H+ extract are probably C40 cyclic biomarkers derived from B. braunii.•Free lipids are sufficient for the purpose of paleoenvironmental reconstructions.Solvent extracted sediments from El Junco Lake, Galápagos were sequentially hydrolyzed with base and acid. Some of the “bound” lipids thus generated were not detected among the “free” lipids obtained from the initial solvent extract of the freeze dried sediment. These include α-hydroxy acids, β-hydroxy acids, methyl β-hydroxy acids and cutin acids. Other bound lipids differed from the free lipids in their chain length, such as alkanols, sterols, ω-hydroxy acids and (ω-1)-hydroxy acids. A suite of biomarkers with unique ions of m/z 341 were identified in the acid hydrolysates that are most likely C40 cyclic biomarkers derived from the L race of Botryococcus braunii.Free lipids and their isotopic composition are likely to be sufficient for the purpose of paleoenvironmental reconstructions, and therefore base or acid hydrolysis of solvent extracted sediment is not necessary. Nevertheless, the additional lipid biomarkers liberated by base and acid hydrolysis can provide information on microbial inputs to the sediment.

The occurrence of long chain chloroalkenes and chloroalkanes has not been previously reported in natural sediments. Here we report the presence of long chain (C30–C36) n-chloroalkanes and chloroalkenes in the recent sediments of the only permanent freshwater lake in the Galápagos Islands, El Junco. Both chloroalkanes and chloroalkenes have a chlorine atom at the terminal position. The cis and trans alkenyl chlorides have double bonds near the middle of the hydrocarbon chain (ω15, 16, 17). Alkenes in the lake sediments are typically C23:1 to C27:1, substantially shorter than the chloroalkenes and chloroalkanes. The lipid chain lengths and the positions of functional groups imply a structural relationship between chloroalkenes and some alkenols, diols and (or) hydroxy acids that most likely derive from algae.Highlights► We report long chain n-chloroalkanes and chloroalkenes in the recent sediments. ► Chloroalkanes and chloroalkenes have a chlorine atom at the terminal position. ► The cis and trans alkenyl chlorides have double bonds near mid-chain. ► Chloroalkenes and some alkenols, diols and/or hydroxy acids likely from algal origin.

Lipid biomarkers in lacustrine sediments provide valuable information about lake history and environmental change. Here we report the occurrence of a unique set of lipid biomarkers from a freshwater crater lake, El Junco, in the Galápagos. In addition to previously reported alkadienes, botryococcenes and lycopadienes indicative of Botryococcus braunii A, B and L races, we find highly branched C25 isoprenoids (HBIs) from diatoms, monomethyl alkanes likely from insect epicuticular waxes, long chain alkenols, diols and a triol, keto-ols, hydroxy acids and keto acids. Saturated and mono-unsaturated long chain diols from C30–C36 had terminal hydroxyl groups and hydroxyl groups between the ω16 and ω20 positions. Vicinal diols with hydroxyl groups at ω9 and ω10 were likely from the floating fern Azolla. C30–C36 keto-ols, mid-chain hydroxy and keto acids had mid-chain functional groups at similar positions to the diols, suggesting common origins. The predominance of ω20-hydroxy acids and diols, together with 20,21-dihydroxy-nonacosanoic acid is indicative of an Azolla source, while ω16 and ω18 hydroxy acids and diols imply a microalgal source.

Coastal wetlands and estuarine sediments are characteristics of tide-dominated environments and retain a record of seawater intrusion and possibly sea-level changes. A variety of methods including the C/N ratio and δ13C of bulk organic matter in the sediment have been employed in estuarine studies, but they are generally non-specific indicators. Here we report using lipid biomarkers to evaluate the extent of seawater intrusion based on an estuary in eastern China, Xinyanggang. Along the river from the estuary mouth upstream to the freshwater head, the vegetation shifts quickly from salt-tolerant Spartina (C4) to the less tolerant reed Phragmites (C3), both of which have distinguished δ13C values and biomarker distribution. The δ13C values of particulate organic matter (POM) and surface sediment decreased from the estuary mouth upstream, indicating the reduced contributions from Spartina and marine phytoplankton to the POM and surface sediment and increased inputs from Phragmites. The C32/C30 alkanol and cholesterol/sitosterol ratio decreased in the surface sediments, faithfully recording the variations in the contributions from Spartina and Phragmites. The combination of biomarker distribution and organic matter δ13C in the sediments can be used as indicators for sea water intrusion into the estuary/river.