Chain Alkenones Research Articles

Holocene pyrolytic nitrogen compounds by using pyrolysis-GC/MS and its paleoclimatic implication from Jinchuan peatbog, NE China

We present a Holocene record of pyrolytic nitrogen (N) composition from the Jinchuan peat bog within the Longgang Volcanic Field (LVF), NE China. Based on pyrolysis gas chromatography and mass spectrometry (Py-GC/MS) analysis, N-compounds including acetamide, NHetero and NAro have been identified and quantified. Aromatic N-compounds (NAro, benzonitrile and benzamide) are mainly derived from phytoplankton and higher plants, and are resistant to microbial degradation. We propose ‘NHetero/NAro’ as a proxy based on microbial degradation, and has lower values between 10 and 5.8 ka BP, and higher values during the late Holocene. We suggest that temperature is main factor for NHetero/NAro, and sedimentary facies changed from floodplain to peat deposition, variation of effective precipitaion and pH value have limited influence on the performance of the proxy in Jinchuan. Low NHetero/NAro levels, combined with high levels of non-protein derived acetamide, indicate warmer conditions, and vice versa. NHetero/NAro is sensitive to temperature change, and follows a similar pattern to those of previous paleo-temperature time series derived from br-GDGT and long chain alkenones in this region. This Spectral analysis of the NHetero/NAro time series reveals significant periodicities of 103–105, 145 and 482–506 years, suggesting strong influences of solar radiation and monsoon intensity on temperature variations in NE China during the Holocene.

Systematic chemotaxonomic profiling and novel paleotemperature indices based on alkenones and alkenoates: Potential for disentangling mixed species input

The unsaturation indices (U37K, U37K') of long chain alkenones are powerful paleotemperature proxies and have been widely applied for sea surface temperature (SST) reconstructions in the past three decades. However, these indices encounter major difficulties in systems harboring different alkenone-producing haptophyte species, such as saline lakes and marginal ocean environments. All haptophytes produce C37 alkenones, but different species often display large differences in temperature calibrations and may bloom in different seasons, hindering the use of U37K and U37K' indices for reliable paleotemperature reconstructions in mixed systems. To overcome these problems, we have recently reported a new analytical method that allows comprehensive separation of up to 32 alkenones, alkenoates and their double bond positional isomers in culture and sediment samples. Here we report a systematic analysis of alkenones and alkenoates from six haptophyte cultures growing at a wide range of temperatures (4–25 °C). Together with a compilation of 230 previously published culture data sets, we present here systematic calibrations of temperature-sensitive indices based on all alkenone and alkenoate homologues (including isomers). Using this dataset, we extract systematic chemotaxonomic criteria for differentiating individual haptophyte species and demonstrate such chemotaxonomic features can be encoded into a machine learning model for reliable species identifications. Specifically, we show that temperature calibrations based on C38 methyl ketones and C39 ethyl ketones are potentially useful for disentangling mixed inputs in estuarine systems where Group III (E. huxleyi) and Group II alkenones mix, and that C36 ethyl alkenoate isomeric ratios display minimal species heterogeneity and are potentially more suited for reconstructing temperatures in mixed systems with different Group II haptophytes. Using the culture data as base profiles, we construct a mathematical model for estimating percentage inputs from alkenones of different Isochrysidales groups in mixed systems, with potential implications for inferring past salinity changes. Overall, the results from this study demonstrate important new applications of alkenone and alkenoate biomarkers in paleoclimate and paleoenvironmental research.

Changes in long chain alkenone distributions and Isochrysidales groups along the Baltic Sea salinity gradient

Isochrysidales species of the phylum Haptophyta are the exclusive producers of C37 to C42 long chain alkyl ketones, also called long chain alkenones (LCAs). While LCA distributions are known to vary with temperature and salinity, it is difficult to tease apart the direct effects of environmental parameters vs changes in the LCA-producing organisms. The Baltic Sea surface salinity gradient, which ranges from oligohaline (0.5–5 g/kg) to polyhaline (18–30 g/kg), represents a unique opportunity to study the relationships between salinity changes, species distribution and LCA biomarkers in a single ecosystem. LCA biomarkers revealed the presence of the three known Isochrysidales groups (Groups I, II and III) in Baltic Sea surface sediments, and the presence of Groups I and II were further confirmed with DNA sequencing. Group III Isochrysidales were present in the mixoeuhaline Skagerrak based on LCA signature alone. Groups I and II Isochrysidales were found for the first time in the Baltic Sea using a combination of LCAs and DNA biomarkers, solving an eighteen-year long mystery of Baltic Sea LCA-producing haptophyte identity. Group II Isochrysidales, which have a large salinity tolerance range, were spread over the Skagerrak and the complete Baltic Sea, but were characteristic for the central Baltic Sea. Oligohaline Group I Isochrysidales were representative for the northern Baltic Sea. However, evidence of Group I Isochrysidales in the central and southern Baltic Sea suggests a possible transport by surface currents since this group is typically confined to oligohaline conditions. Testing the recently developed ratio of isomeric C37 ketones (RIK37) against the Baltic Sea surface salinity gradient revealed a significant positive correlation. This may represent a salinity proxy reflecting the amount of Group I Isochrysidales relative to Group II Isochrysidales in oligohaline environments. The present study elucidates for the first time the identity and the spatial distribution of LCA producers thriving in a large and stable brackish environment.

Towards multiproxy, ultra-high resolution molecular stratigraphy: Enabling laser-induced mass spectrometry imaging of diverse molecular biomarkers in sediments

Mass spectrometry imaging (MSI) collects mass spectra of organic compounds from individual micrometer-sized spots and yields high-resolution images of the spatial distribution of target analytes on sample surfaces. MSI can potentially open a new avenue to ultra-high resolution molecular stratigraphy by resolving the fine-scale distribution of molecular biomarkers in geological records. However, ionization of organic molecules within their sediment matrix remains a critical challenge. Building on the recent introduction of MSI of archaeal tetraether lipids, we have extended the analytical spectrum to additional biomarkers and provide guidelines for the generation of multiproxy, ultra-high resolution paleoenvironmental records. We evaluated the addition of artificial matrices to promote ionization, defined the most suitable MS settings, and increased analytical complexity from pure compounds to their investigation in sediment. Most compounds relevant to molecular stratigraphy are not properly ionized in the presence of conventional matrices, but require atypical ones, such as those based on carbon or silver. Sediments serve as a natural matrix, directly allowing detection of many of these compounds. However, the sediment matrix also inhibits some reactions that otherwise promote detection, such as derivatization of alkenones or silver-mediated ionization of n-alkanes. The robustness of MSI-based molecular stratigraphy is enhanced by analysis of target compounds in narrow m/z ranges, and by summation of mass spectra from several coeval measurement spots. We present an initial inventory of compounds readily detected in the sediment tested, including long chain alkenones and diols, sterols, and pigments, and provide an outlook into the use of MSI in multiproxy studies.

Development and comparison of chromatographic methods for the analysis of long chain diols and alkenones in biological materials and sediment

We have compared and assessed the suitability of several chromatographic methods for the analysis of long chain alkenones and long chain diols and the associated paleotemperature proxies (UK’37 and LDI). We evaluated the traditional methods for the analysis of the UK’37 and the LDI, gas chromatography (GC) − flame ionization detection (FID) and GC mass spectrometry (MS) using selected ion monitoring (SIM), respectively, and developed a new method using GC–MS/MS in multiple reaction monitoring mode (MRM) for the analysis of long chain diols as well as a method for automatic silylation of diols using a robot autosampler. Finally, we evaluated liquid chromatography (LC) methods to simultaneously measure the UK’37 and the LDI, using ultra high performance LC (UHPLC) with low (nominal mass) resolution MS in SIM mode, and UHPLC with high resolution MS (HRMS). Detection and quantification limits and reproducibility were assessed by means of serial dilutions of culture extracts.Automated silylation by a robot autosampler showed similar reproducibility as off-line silylation while substantially decreasing sample preparation time. The novel MRM method had a slightly lower limit of quantification (LOQ; i.e. 0.3pgC28 1,13-diol injected on-column) than the traditional method (0.5pg) and improved reproducibility while allowing more unambiguous identification of LCDs in complex matrices. For diols, UHPLC–MS using SIM had the highest LOQ (i.e. 15pg) and a comparable reproducibility as GC–MS. UHPLC–HRMS had a LOQ of ca. 1.5pg, and an improved reproducibility for diol analysis. For alkenone analysis, both UHPLC–HRMS and UHPLC–MS using SIM were 2–3 orders of magnitude more sensitive (LOQ ca. 20 and 2pgC37:2 alkenone injected on-column, respectively) than GC-FID (LOD ca. 3ng), with a similar reproducibility of the UK’37 index. Hence, UHPLC–HRMS allows simultaneous analysis of the UK’37 and LDI at an increased sensitivity. In addition, it allows simultaneous measurement of TEX86, a temperature proxy based on the isoprenoid glycerol dialkyl glycerol tetraethers. This reduces the preparation time by excluding the need of derivatization and separation of the ketone (containing the long chain alkenones) and polar fractions (containing the long chain diols and GDGTs). However, synthetic standards are required to fully assess the accuracy of the new methods for determination of the LDI and UK’37.

Optimizing chromatographic resolution for simultaneous quantification of long chain alkenones, alkenoates and their double bond positional isomers

We report here our latest attempt to optimize gas chromatographic (GC) separation of long chain alkenones and alkenoates from haptophyte cultures and sediments. Building upon the previously reported separation using VF-200ms column (60m×250μm×0.1μm) (Longo et al., 2013), we tested multiple GC columns with different column length, film thickness and variable loadings of poly(trifluoropropylmethylsiloxane) stationary phase from two separate manufacturers (Agilent and Restek). We show that Rtx-200 (105m×250μm×0.25μm) GC column completely eliminates the co-elution between long chain alkenones and alkenoates, fully resolves the previously partially co-eluting alkenones (e.g., C38:2 ethyl and C38:4 methyl alkenones) and improves the resolution of alkenone and alkenoate double bond positional isomers.Since the debut of the alkenone paleothermometer three decades ago, long chain alkenoates have always been considered an annoyance for paleotemperature reconstructions and are routinely removed by saponification. Our new separation method simplifies the sample preparation for alkenone analysis and improves the application of alkenone/alkenoate proxies, which opens the door for paleoclimate application of alkenoate-based paleothermometers alongside alkenones. We also report the structure determination of new alkenoate isomers using dimethyl disulfide derivatization.

Optimizing the yield of transient mono-dimethyl disulfide adducts for elucidating double bond positions of long chain alkenones

Long chain alkenones (LCAs) are among the most successful biomarkers for paleotemperature reconstructions. However, fundamental questions regarding the biosynthesis and cellular functions of alkenones in haptophyte algae remain poorly understood. Recent discoveries of LCAs with double bond positions and chain lengths that differ from common structures further highlight the importance of continued research into structural variations of this important class of lipid biomarkers to improve LCA applications as temperature proxies. Double bond positions on alkyl chains can be effectively determined by preparing mono-double bond adducts with dimethyl disulfide (DMDS-1), and subsequent gas chromatography–mass spectrometry (GC–MS) analysis. However, previously published procedures for adduct preparation were originally designed for mono-unsaturated fatty acids, and generally produce low product yields when applied to alkenones. Here we demonstrate that the problem originates mainly from DMDS and alkenone overreaction at high temperatures for long time periods, and, secondarily, insufficient amount of iodine catalyst. The overreaction results in DMDS reacting with multiple double bonds, and the possible formation of intermolecular linkages, creating non-volatile products. These products are of little use for elucidating alkenone structures. We demonstrate that by reducing the reaction temperature and time, and by using an optimal amount of iodine, we can maximize the yield of transient DMDS-1 adducts for alkenone structure determination.

Composition of long chain alkenones and alkenoates as a function of growth temperature in marine haptophyte Tisochrysis lutea

We investigated the compositions of long chain alkenones and alkenoates in cultured strains of the marine haptophyte Tisochrysis lutea CCMP463 and T. lutea NIES-2590 (formerly classified as Isochrysis galbana). Both T. lutea strains grown at various temperatures of 15–35°C could be characterized by the lack of tetraunsaturated alkenones and alkenoates in comparison with strains in other genera Isochrysis and Ruttnera, which are classified in the same family Isochrysidaceae and order Isochrysidales that contain alkenone-producing haptophytes. We found that T. lutea has a distinct alkenone response to temperature, which is characterized by high U37K-temperature sensitivity at 15–30°C and continuing sensitivity at warm-end temperatures over 30°C, where the other species fail to adapt. Both strains showed similar trends in alkenone compositions and alkenone unsaturation index-temperature calibrations. In addition, both strains CCMP463 and NIES-2590 showed notably close phylogenetic relationships, even those that were collected from remote regions of the Atlantic Ocean, namely the Caribbean Sea and English Channel, where their habitats, particularly the sea surface temperature, are notably different. Considering previous published datasets, three genera, Isochrysis, Ruttnera and Tisochrysis, showed very different trends in the alkenone unsaturation index-temperature calibration. These results suggest that the lack of tetraunsaturated alkenones, warm-water oriented growth and the high sensitivity to growth temperature serve as distinct chemotaxonomic characteristics of T. lutea in the Isochrysidaceae family.

Is H Atom Abstraction Important in the Reaction of Cl with 1-Alkenes?

The relative yields of products of the reaction of Cl atoms with 1-alkenes (C4-C9) were determined to see whether H atom abstraction is an important channel and if it is to identify the preferred position of abstraction. The presence of all the possible positional isomers of long chain alkenones and alkenols among the products, along with chloroketones and chloroalcohols, confirms the occurrence of H atom abstraction. A consistent pattern of distribution of abstraction products is observed with oxidation at C4 (next to allyl) being the lowest and that at CH2 groups away from the double bond being the highest. This contradicts with the higher stability of allyl (C3) radical. For a better understanding of the relative reactivity, ab initio calculations at MP2/6-311+G (d,p) level of theory are carried out in the case of 1-heptene. The total rate coefficient, calculated using conventional transition state theory, was found to be in good agreement with the experimental value at room temperature. The preferred position of Cl atom addition is predicted to be the terminal carbon atom, which matches with the experimental observation, whereas the rate coefficients calculated for individual channels of H atom abstraction do not explain the observed pattern of products. The distribution of abstraction products except at C4 is found to be better explained by reported structure activity relationship, developed from experimental rate coefficient data. This implies the reactions to be kinetically dictated and emphasizes the importance of secondary reactions.

Environmental controls on long-chain alkenone occurrence and compositional patterns in lacustrine sediments, northwestern China

Long chain alkenones occur widely in lacustrine settings and their compositions are potential proxies for reconstructing terrestrial paleoclimatological and paleoenvironmental changes. However, factors influencing their occurrence, abundance, compositional pattern and unsaturation indices are not well understood. Here we investigated the alkenones in surface sediments from 19 lakes in northwestern China as well as potential environmental factors. Of the lakes, 12 contain long chain alkenones, 3 have the isomeric C37:3 alkenone and 3 have very low abundance of alkenones. Our results show that salinity has a substantial influence on the occurrence, concentration and compositional pattern. Most of the compositions are dominated by tri-unsaturated compounds, with C38 methyl ketones absent. The predominance of C37:4 and the occurrence of a C37:3 isomer are suggested to be responses to low salinity as both are only found in the three alkenone-containing freshwater lakes. The proportion of C37:4 varies by up to 50% and shows a negative relationship with salinity within the surveyed region. An arched relationship of alkenone content and salinity is apparent, with no or very low contents of alkenones in freshwater (salinity<3g/L) and hypersaline (>100g/L) environments. The U37K′ index is suggested to be a better terrestrial temperature proxy than U37K as U37K′ values from lakes containing elevated alkenone content are all within the ranges of published U37K′-temperature calibrations, and U37K values show a weak positive relationship with salinity.

A new approach for reconstructing Holocene temperatures from a multi-species long chain alkenone record from Lake Qinghai on the northeastern Tibetan Plateau

The extent of unsaturation in long chain alkenones (LCKs) in lakes has potential for quantitatively indicating paleotemperature in terrigenous environments. However, the traditional temperature proxies derived from marine LCKs may not be able to address the problem related to the prevalent species effect, which restricts the application of LCKs in lacustrine studies. In our LCK record from the northeastern Tibetan Plateau, C37/C38 differences revealed that the LCK producer species in Lake Qinghai may have changed during the Holocene. This possible species effect may have caused inconsistency between the UK′37 and UK37 records. Therefore, we formulated a new proxy, UKs37, with a parameter “s” to indicate the different exponential response in LCK unsaturation to temperature change in multiple species. Our UKs37-derived temperature time series resembles the general temperature trends of the Northern Hemisphere on a millennial scale, while exhibiting several apparent cold events, especially at ca. 9ka. The result shows that the Holocene optimum in Lake Qinghai was interrupted by cold events in the early Holocene. This could be a result of a weak Asia summer monsoon and cold air transmission in the North Atlantic.

Rapid and simultaneous analysis of three molecular sea surface temperature proxies and application to sediments from the Sea of Marmara

Reconstructing ocean temperature values is a major target in paleoceanography and climate research. However, most temperature proxies are organism-based and thus suffer from an “ecological bias”. Multiproxy approaches can potentially overcome this bias but typically require more investment in time and resources, while being susceptible to errors induced by sample preparation steps necessary before analysis. Three lipid-based temperature proxies are widely used: UK′37 (based on long chain alkenones from phytoplanktonic haptophytes), TEX86 [based on glycerol dialkyl glycerol tetraethers (GDGTs) from pelagic archaea] and LDI (based on long chain diols from phytoplanktonic eustigmatophytes). So far, separate analytical methods, including gas chromatography (GC) and liquid chromatography (LC), have been used to determine these proxies. Here we present a sensitive method for determining all three in a single normal phase high performance LC–atmospheric pressure chemical ionization mass spectrometry (NP-HPLC–APCI-MS) analysis. Each of the long chain alkenones and long chain diols was separated and unambiguously identified from the accurate masses and characteristic fragmentation during multiple stage MS analysis (MS2). Comparison of conventional GC and HPLC–MS methods showed similar results for UK′37 and LDI, respectively, using diverse environmental samples and an Emiliania huxleyi culture. Including the three sea surface temperature (SST) proxies; the NP-HPLC–APCI-MS method in fact allows simultaneous determination of nine paleoenvironmental proxies. The extent to which the ecology of the source organisms (ecological bias) influences lipid composition and thereby the reconstructed temperature values was demonstrated by applying the new method to a sediment core from the Sea of Marmara, covering the last 21kyr BP. Reconstructed SST values differed considerably between the proxies for the Last Glacial Maximum (LGM) and the period of Sapropel S1 formation at ca. 10kyr BP, whereas the trends during the late Holocene were similar. Changes in the composition of alkenone-producing species at the transition from the LGM to the Bølling/Allerød (B/A) were inferred from unreasonably high UK′37-derived SST values (ca. 20°C) during the LGM. We ascribe discrepancies between the reconstructed temperature records during S1 deposition to habitat change, e.g. a different depth due to changes in nutrient availability.

Large effect of irradiance on hydrogen isotope fractionation of alkenones in Emiliania huxleyi

The hydrogen isotopic (δD) composition of long-chain alkenones produced by certain haptophyte algae has been suggested as a potential proxy for reconstructing paleo sea surface salinity. However, environmental parameters other than salinity may also affect the δD of alkenones. We investigated the impact of the level of irradiance on hydrogen isotopic fractionation of alkenones versus growth water by cultivating two strains of the cosmopolitan haptophyte Emiliania huxleyi at different light intensities. The hydrogen isotope fractionation decreased by approximately 40‰ when irradiance was increased from 15 to 200μmolphotonsm−2s−1 above which it was relatively constant. The response is likely a direct effect of photosystem I and II activity as the relationship of the fractionation factor α versus light intensity can be described by an Eilers–Peeters photosynthesis model. This irradiance effect is in agreement with published δD data of alkenones derived from suspended particulate matter collected from different depths in the photic zone of the Gulf of California and the eastern tropical North Pacific. However, haptophyte algae tend to bloom at relatively high light intensities (>500μmolphotonsm−2s−1) occurring at the sea surface, at which hydrogen isotope fractionation is relatively constant and not affected by changes in light intensity. Alkenones accumulating in the sediment are likely mostly derived from these surface water haptophyte blooms, when the largest amount of biomass is produced. Therefore, the observed irradiance effect is unlikely to affect the applicability of the hydrogen isotopic composition of sedimentary long chain alkenones as a proxy for paleosalinity.

Testing the alkenone D/H ratio as a paleo indicator of sea surface salinity in a coastal ocean margin (Mozambique Channel)

Reconstructing past ocean salinity is important for assessing paleoceanographic change and therefore past climatic dynamics. Commonly, sea water salinity reconstruction is based on planktonic foraminifera oxygen isotope values combined with sea surface temperature reconstruction. However, the approach relies on multiple proxies, resulting in rather large uncertainty and, consequently, relatively low accuracy of salinity estimates. An alternative tool for past ocean salinity reconstruction is the hydrogen isotope composition of long chain (C37) alkenones (δDalkenone). Here, we have applied δDalkenone to a 39ka sedimentary record from the Eastern South African continental shelf in the Mozambique Channel, close to the Zambezi River mouth. Despite changes in global seawater δD related to glacial – interglacial ice volume effects, no clear changes were observed in the δDalkenone record throughout the entire 39ka. The BIT index record from the same core, which provides information on relative contributions of soil organic matter (OM) vs. marine input, indicates high soil OM input during the glacial and low input during the Holocene. This suggests a more pronounced freshwater influence at the core location during the glacial, resulting in alkenones depleted in D during that time, thereby explaining the lack of a clear glacial-interglacial alkenone δD shift. The correlation between the BIT index and δDalkenone during the glacial period suggests that increased continental runoff potentially changed the growth conditions of the alkenone-producing haptophytes, promoting coastal haptophyte species with generally more enriched δDalkenone values. We therefore suggest that the application of δDalkenone for reconstructing past salinity in coastal settings may be complicated by changes in the alkenone-producing haptophyte community.

Occurrence of extended chain length C41 and C42 alkenones in hypersaline lakes

The chain length of long chain alkenones (LCAs) in ocean and lake sediments commonly ranges from C37 to C39. Alkenones >C40 are extremely rare: C41 and C42 alkenones have only been reported in a Cretaceous black shale (Cenomanian, ca. 95Ma) from the Blake Bahama Basin, western North Atlantic (DSDP Leg 76, site 534). The significance of these exceptionally long chain alkenones is, however, unclear. We have conducted a systematic survey of LCAs in surface sediments from 12 lakes with a wide range of salinity (0.7–146.3g/l) from arid northwestern China. They showed a general trend of increasing maximum chain length with increasing salinity. In particular, C41 and C42 LCAs occurred only in the surface sediments of hypersaline lakes (113.6–146.3g/l), but were absent from lakes with lower salinity, indicating that these compounds may be produced by an unknown haptophyte species living in hypersaline environments.

Unprecedented separation of long chain alkenones from gas chromatography with a poly(trifluoropropylmethylsiloxane) stationary phase

Long chain alkenones (LCAs) have played a central role in reconstructing paleo-sea surface temperature throughout much of the world oceans. The standard method for LCA separation and quantification utilizes gas chromatography (GC) with a non-polar column and flame ionization detection (FID). The method has remained nearly unchanged since the debut of alkenone paleothermometry over three decades ago. The conventional stationary phase used [poly(dimethylsiloxane)] is adequate for calculation of the U37K′ index for most marine samples, but performs poorly in the separation of methyl and ethyl C38 LCAs and fails to separate certain lacustrine LCAs, including structural isomers featuring differing double bond and carbonyl positions. Here, we demonstrate major improvements in the selectivity and resolution of numerous LCAs using a mid-polarity, dipole selective poly(trifluoropropylmethylsiloxane) GC stationary phase with FID. Various columns featuring this phase achieved baseline resolution of all C38 LCAs in a marine sediment and Emiliania huxleyi culture samples, improving peak resolution of C38 LCAs by>350%. Such dipole selective stationary phases also reveal novel tri-unsaturated isomeric LCAs in lacustrine samples, for which we provide the first report and preliminary characterization. We conduct a thorough evaluation of LCA retention behavior on poly(trifluoropropylmethylsiloxane) and poly(dimethylsiloxane) columns and report an optimal range of operating conditions for both phases.

Production and temperature sensitivity of long chain alkenones in the cultured haptophyte Pseudoisochrysis paradoxa

The alkenone unsaturation index (U37K or U37K′) serves as a critical tool for reconstructing temperature in marine environments. Lacustrine haptophyte algae are genetically distinct from their ubiquitous and well studied marine counterparts, and the unknown species-specific genetic imprints on long chain alkenone production by lacustrine species have hindered the widespread application of the U37K temperature proxy to lake sediment records. The haptophyte Pseudoisochrysis paradoxa produces alkenones but its U37K calibration has never been determined. It has an alkenone fingerprint abundant in tetraunsaturated alkenones, a hallmark of lacustrine environments. We present here the first calibration of the U37K index to temperature for a culture of P. paradoxa. We found that the U37K index accurately captured the alkenone response to temperature whereas the U37K′ index failed to do so, with U37K′ values below 0.08 projecting to two different temperature values. Our results add a fifth species-specific U37K calibration and provide another line of evidence that different haptophyte species require different U37K calibrations. The findings also highlight the necessary inclusion of the C37:4 alkenone when reconstructing temperatures from P. paradoxa-derived alkenone records.

Long chain alkenones preserved in Miocene lake sediments

Over the last few decades, the occurrence of long chain alkenones (LCAs) in Quaternary lacustrine settings has been reported, the distributions being remarkably different from those of open ocean settings. We report here the occurrence of LCAs with modern lake-type composition from Middle Miocene sedimentary strata in the Dahonggou Section, Qaidam Basin, northern Tibetan Plateau. This is the first report of LCAs with preserved lake compositions in pre-Quaternary lacustrine sediments. The discovery implies that the producers of modern lacustrine LCAs could originate from similar lake species living at least as far back as the Miocene epoch. It also sheds new light on the potential of “alkenones” as palaeotemperature indicators in lacustrine systems back to at least 10.5Ma.

Changes in alkenone and alkenoate distributions during acclimatization to salinity change in Isochrysis galbana: Implication for alkenone-based paleosalinity and paleothermometry

A cultured strain of Isochrysis galbana UTEX LB 2307 was grown at 20°C and 15°C under salinity of 35‰, 32‰, 27‰, 20‰, and 15‰, and analyzed for long chain (C37–C39) alkenones and (C37–C38) alkyl alkenoates. It was characterized by abundant C37 alkenones and C38 ethyl alkenoates (fatty acid ethyl ester: FAEEs) and a lack of C38 methyl alkenones. There were no tetra-unsaturated (C37:4) alkenones, which are frequently found in natural samples from low salinity waters. The alkenone unsaturation index (U37) did not vary in response to change in salinity at 15°C. The alkenone unsaturation index (U37) clearly changed in response to change in salinity at 20°C, but not at 15°C, where algal growth was and was not limited by temperature at 15°C and 20°C, respectively. The U37-temperature calibration for I. galbana UTEX LB 2307 is quite different from those of E. huxleyi and another strain of I. galbana (CCMP1323), while is resemble to that reported for C. lamellosa isolated from a Chinese lake. Also, variation in the alkenone chain length ratio values (K37/K38) did not correlate with salinity. These results implied that a high abundance of tetra-unsaturated alkenone and high K37/K38 values might be attributed to a taxonomic factor rather than a physiological response to salinity change. Interestingly, our culture experiments showed that the ethyl alkenoates/alkenones ratio (EE/K37) correlates with salinity. Hence, it is suggested that the EE/K37 ratios are affected by the cellular and physiological factors against salinity condition in single haptophyte cells.

Culturing of the first 37:4 predominant lacustrine haptophyte: Geochemical, biochemical, and genetic implications

Long chain alkenones (LCAs) are potential biomarkers for quantitative paleotemperature reconstructions from lacustrine environments. However, progress in this area has been hindered, because the conditions necessary for the growth of haptophytes responsible for alkenone distributions in lake sediments: the predominance of C 37:4 LCA are not known. Here we report the first enrichment culturing of a novel haptophyte phylotype (Hap-A) from Lake George, ND that produces predominantly C 37:4-LCA. Hap-A was enriched from its resting phase collected from deep sediments rather than from water column samples. In contrast, enrichments from near surface water yielded a different haptophyte phylotype (Hap-B), closely related to Chrysotila lamellosa and Pseudoisochrysis paradoxa, which does not display C 37:4-LCA predominance (similar enrichments have been reported previously). The LCA profile in sediments resembles that of enrichments containing Hap-A, suggesting that Hap-A is the dominant alkenone producer of the sedimentary LCAs. In enrichments, increased lighting appeared to be crucial for triggering alkenone production. Both U 37 K and U 38 K indices show a promising, positive relationship with temperature for Hap-A in enrichments, but the offset from the environmental calibration suggests that other factors (e.g., the growth stage or nutrients) may influence the absolute U 37 K value. Based on 18S rRNA gene analyses, several lakes from the Northern Great Plains, as well as Pyramid Lake, NV and Tso Ur, Tibetan Plateau, China contain the same two haptophyte phylotypes. Analysis of surface sediment from the Great Plains lakes show the Hap-A-type LCA distribution, whereas Pyramid and Tso Ur show the Hap-B type distribution. Waters of the Great Plain lakes are dominated by sulfate ions, whereas those Pyramid and Tso Ur are dominated by carbonate ions, suggesting that the sulfate to carbonate ratio may be a determining factor for the dominance of the Hap-A and Hap-B phylotypes in natural settings.