Alkenone Production Research Articles

Expression profiling of genes coding for abundant proteins in the alkenone body of marine haptophyte alga Tisochrysis lutea

BackgroundSeveral abundant proteins have been identified in lipid body of an alkenone-producing marine haptophyte alga Tisochrysis lutea. The gene expression patterns of these proteins were investigated to better understand their roles in alkenone biosynthesis. For this purpose, T. lutea was first cultured in nitrogen-sufficient medium for biomass production and then shifted to nitrogen-deprived medium to induce lipid body formation.ResultsThere were remarkable increases in the volume of alkenone body (AB) and alkenone content in the alga after they were exposed to nitrogen depletion medium. Relative mRNA levels of the genes coding for the identified proteins V-ATPase subunit VA, V-ATPase subunit Vd, hypothetical protein EMIHUDRAFT_465,517, coccolith scale associated protein-1, cycloartenol-c-24-methyltransferase 1-like and SPFH domain-containing protein were investigated over the culture period. RT-PCR data showed that the expression of all these genes except the gene coding for SPFH domain-containing protein was up-regulated during the transition period from nitrogen-sufficient to nitrogen-deficient medium. Among them, the expression of the coccolith scale associated protein-1 gene was up-regulated 50–650 folds. These up-regulations were consistent with the increased alkenone production in nitrogen-deprived medium, suggesting that these proteins are involved in alkenone biosynthesis in T. lutea.ConclusionsExpression analysis of the lipoprotein genes suggests that five out of the six genes are up-regulated and are therefore likely to code for the identified lipoproteins associated with alkenone biosynthesis in T. lutea. These data would help better understand alkenone metabolism and engineer for improved biofuel production in T. lutea.

Phylogenetic diversity in freshwater-dwelling Isochrysidales haptophytes with implications for alkenone production.

Members of the order Isochrysidales are unique among haptophyte lineages in being the exclusive producers of alkenones, long‐chain ketones that are commonly used for paleotemperature reconstructions. Alkenone‐producing haptophytes are divided into three major groups based largely on molecular ecological data: Group I is found in freshwater lakes, Group II commonly occurs in brackish and coastal marine environments, and Group III consists of open ocean species. Each group has distinct alkenone distributions; however, only Groups II and III Isochrysidales currently have cultured representatives. The uncultured Group I Isochrysidales are distinguished geochemically by the presence of tri‐unsaturated alkenone isomers (C37:3b Me, C38:3b Et, C38:3b Me, C39:3b Et) present in water column and sediment samples, yet their genetic diversity, morphology, and environmental controls are largely unknown. Using small‐subunit (SSU) ribosomal RNA (rRNA) marker gene amplicon high‐throughput sequencing of environmental water column and sediment samples, we show that Group I is monophyletic with high phylogenetic diversity and contains a well‐supported clade separating the previously described “EV” clade from the “Greenland” clade. We infer the first partial large‐subunit (LSU) rRNA gene Group I sequence phylogeny, which uncovered additional well‐supported clades embedded within Group I. Relative to Group II, Group I revealed higher levels of genetic diversity despite conservation of alkenone signatures and a closer evolutionary relationship with Group III. In Group I, the presence of the tri‐unsaturated alkenone isomers appears to be conserved, which is not the case for Group II. This suggests differing environmental influences on Group I and II and perhaps uncovers evolutionary constraints on alkenone biosynthesis.

Alkenone Paleothermometry in Coastal Settings: Evaluating the Potential for Highly Resolved Time Series of Sea Surface Temperature

AbstractThe unsaturation ratios of alkenones have been applied widely to reconstruct pelagic sea surface temperatures (SSTs). However, applications to costal settings have been hampered by the effects of salinity and nutrient dynamics. Here we present a 4.5‐year‐long record of alkenones in water column particulate organic matter from sites in Narragansett Bay (RI, USA) spanning wide ranges in salinity and nutrients, and a 300‐year alkenone record from two Narragansett Bay sediment cores. Particulate organic matter results suggest that there are two distinct alkenone producing populations. One is a widespread population that blooms in summer (July–September) and displays alkenones typical of Group III producers—the basis for the widely applied marine Uk’37 temperature index. The other population is confined to brackish waters in the Providence River during spring salinity minima and displays alkenone profiles consistent with Group I production—the first detection of Group I in a brackish nonlacustrine environment. The sedimentary alkenone profiles consist of only Group III alkenone distributions, allowing for the reconstruction of a 300‐year‐long record of Narragansett Bay SST. Comparison of the most recent 100 years of this reconstruction with local and regional instrumental SST records shows excellent agreement, confirming a regional pattern of SST rise in the coastal northeastern United States that exceeds global or hemispheric rates. Group III alkenone production in coastal settings may be common and may open the way to high resolution local and regional SST reconstruction.

Major increase in winter and spring precipitation during the Little Ice Age in the westerly dominated northern Qinghai-Tibetan Plateau

There have been numerous attempts to use long-chain alkenones (LCAs) in saline lakes for paleotemperature and paleosalinity reconstructions. However, LCAs in saline lakes are often produced by multiple haptophyte species, which may confound data interpretations. Here we analyzed LCAs in a finely laminated, high sedimentation rate core from the hypersaline Lake Gahai in the northern Qinghai-Tibetan Plateau and compared our results with regional instrumental records. We find that LCA unsaturation ratios display a stepwise jump during the instrumental period, most likely originating from a sudden shift in the dominant alkenone producers. In contrast, the percentage of the C37:4 alkenone (%C37:4) displays strong correlations with spring and combined winter-spring precipitation amount (R2 = 0.83 and R2 = 0.81, respectively). We hypothesize that high winter-spring precipitation leads to greater freshening of lake surface water immediately after spring melting, promoting greater production of LCAs with relatively high %C37:4 values by the early blooming haptophyte species. Extending the instrumental calibration downcore allows us to quantitatively reconstruct regional spring and winter-spring precipitation for the past millennium. We find a major increase in winter-spring precipitation during the Little Ice Age (LIA), especially during the phases of solar minima. Our finding provides novel quantitative support to the previous studies suggesting relatively wet conditions during the LIA in the westerly dominated regions of central Asia and northern Tibetan Plateau and infers a substantial increase in regional winter-spring precipitation should the predicted grand solar minimum in the forthcoming decades become a reality.

Genomic identification of the long-chain alkenone producer in freshwater Lake Toyoni, Japan: implications for temperature reconstructions

Identifying the lacustrine haptophyte species that produce long-chain alkenones (LCAs) is essential prior to down-core temperature reconstructions. Here, we investigated the identity of LCA-producing species from Lake Toyoni, Japan using 18S ribosomal DNA (rDNA) and organic geochemical analyses. The rDNA analyses identified eighteen operational taxonomic units (OTUs) of which only one fell within the haptophyte phylotype. This haptophyte belongs to the Group I phylotype, as supported by the LCA distribution found in surface and down-core sediments, and is closely related to a haptophyte found in Lake BrayaSø (Greenland). The inferred temperature using the Greenland calibration is very close to the Lake Toyoni surface temperature recorded during the spring/early summer season, when the LCA-producing haptophyte is likely to bloom. We therefore suggest that the temperature calibration from the Lake BrayaSø, Greenland is a suitable calibration for down-core temperature reconstructions at Lake Toyoni.

Overexpression of Tisochrysis lutea Akd1 identifies a key cold-induced alkenone desaturase enzyme

Alkenones are unusual long-chain neutral lipids that were first identified in oceanic sediments. Currently they are regarded as reliable palaeothermometers, since their unsaturation status changes depending on temperature. These molecules are synthesised by specific haptophyte algae and are stored in the lipid body as the main energy storage molecules. However, the molecular mechanisms that regulate the alkenone biosynthetic pathway, especially the low temperature-dependent desaturation reaction, have not been elucidated. Here, using an alkenone-producing haptophyte alga, Tisochrysis lutea, we show that the alkenone desaturation reaction is catalysed by a newly identified desaturase. We first isolated two candidate desaturase genes and found that one of these genes was drastically upregulated in response to cold stress. Gas chromatographic analysis revealed that the overexpression of this gene, named as Akd1 finally, increased the conversion of di-unsaturated C37-alkenone to tri-unsaturated molecule by alkenone desaturation, even at a high temperature when endogenous desaturation is efficiently suppressed. We anticipate that the Akd1 gene will be of great help for elucidating more detailed mechanisms of temperature response of alkenone desaturation, and identification of active species contributing alkenone production in metagenomic and/or metatranscriptomic studies in the field of oceanic biogeochemistry.

BAYSPLINE: A New Calibration for the Alkenone Paleothermometer

AbstractThe alkenone‐based U proxy is a cornerstone of paleoclimatology, providing insight into the temperature history of the Earth's surface ocean. Although the relationship between U and sea surface temperatures (SSTs) is robust and well supported by experimental data, there remain outstanding issues regarding the seasonality of production of alkenones and the response of U at very warm and cold SSTs. Using a data set of over 1,300 core‐top U measurements, we find compelling evidence of seasonal production in the North Atlantic, North Pacific, and Mediterranean Oceans. We also find significant attenuation of the U response to SST at warm temperatures (>24°C), with the slope reduced by nearly 50% as U approaches unity. To account for these observations in a calibration, we develop a new Bayesian B‐spline regression model, BAYSPLINE, for the U paleothermometer. BAYSPLINE produces similar estimates as previous calibrations below ∼24°, but above this point it predicts larger SST changes, in accordance with the attenuation of the U response. Example applications of BAYSPLINE demonstrate that its treatment of seasonality and slope attenuation improves paleoclimatic interpretations, with important consequences for the inference of SSTs in the tropical oceans. BAYSPLINE facilitates a probabilistic approach to paleoclimate, building upon growing efforts to develop more formalized statistical frameworks for paleoceanographic reconstruction.

Cold-induced metabolic conversion of haptophyte di- to tri-unsaturated C37 alkenones used as palaeothermometer molecules

The cosmopolitan marine haptophyte alga Emiliania huxleyi accumulates very long-chain (C37-C40) alkyl ketones with two to four trans-type carbon-carbon double bonds (alkenones). These compounds are used as biomarkers of haptophytes and as palaeothermometers for estimating sea-surface temperatures in biogeochemistry. However, the biosynthetic pathway of alkenones in algal cells remains enigmatic, although it is well known that the C37 tri-unsaturated alkenone (K37:3) becomes dominant at low temperatures, either by desaturation of K37:2 or by a separate pathway involving the elongation of tri-unsaturated alkenone precursors. Here, we present experimental evidence regarding K37:3 synthesis. Using the well-known cosmopolitan alkenone producer E. huxleyi, we labelled K37:2 with 13C by incubating cells with 13C-bicarbonate in the light at 25 °C under conditions of little if any K37:3 production. After stabilisation of the 13C-K37:2 level by depleting 13C-bicarbonate from the medium, the temperature was suddenly reduced to 15 °C. The 13C-K37:2 level rapidly decreased, and the 13C-K37:3 level increased, whereas the total 13C-K37 level—namely [K37:2 + K37:3]—remained constant. These 13C-pulse-chase-like experimental results indicate that 13C-K37:2 is converted directly to 13C-K37:3 by a desaturation reaction that is promoted by a cold signal. This clear-cut experimental evidence is indicative of the existence of a cold-signal-triggered desaturation reaction in alkenone biosynthesis.

Spatial and temporal distribution of C37 alkenones in suspended materials in the northern East China Sea

The temporal and spatial distributions of long-chain (C37) alkenones in suspended materials in the northern East China Sea (ECS) were investigated to examine the applicability of alkenone unsaturation index as a proxy for sea surface temperature (SST) and that of total C37 alkenone concentration as a proxy for marine primary productivity. Surface and subsurface seawater samples were collected at three-month intervals (February, May, August, and November) during 2007–2010. The results showed that alkenones were synthesized throughout the year, with peaks in May. Alkenone temperatures corresponded to in situ temperatures. The vertical distribution of alkenone concentration indicated that alkenone production depth is the surface mixed layer (0–20m). These results indicate that alkenone temperatures from marine sediments on the continental shelf most likely reflect the annual average SST. No clear inferences could be made on the influence of nutrients and salinity on the alkenone temperature in this area. Seasonal changes in alkenone concentration were compared with those in chlorophyll-a concentration to investigate the suitability of using total C37 alkenone concentration as a proxy for productivity. Where the chlorophyll-a concentration was high, the alkenone concentration was low, and vice versa, because the main primary producer was not the coccolithophores. Therefore, it is not appropriate to use alkenone concentration as a primary productivity proxy in the ECS. However, comparisons to coccolithophore abundance show that total C37 alkenone concentration can be used as a haptophyte productivity proxy in this area.

Sea surface temperatures and environmental conditions during the “warm Pliocene” interval (~ 4.1–3.2 Ma) in the Eastern Mediterranean (Cyprus)

Organic geochemical (alkenones) and micropaleontological (nannofossil) data from the Pissouri South section (PPS) in the island of Cyprus provided a detailed description of the paleoclimatic (sea surface temperature-SST) and paleoenvironmental conditions during the “warm Pliocene” (c. 4.1–3.25Ma) in the Eastern Mediterranean. We found that the suite of sapropel events recorded in the studied interval took place under conditions of increased SST, enhanced water column stratification and development of a productive deep chlorophyll maximum (DCM), as witnessed by the dominance of Florisphaera profunda species. Such conditions are similar to those prevailing during Quaternary sapropel formation, triggered by freshwater discharges from the N. African margin due to insolation-driven intensification of the African monsoon. The absence of F. profunda in Pliocene sapropels from central Mediterranean records highlights the sensitive response of the eastern basin to freshwater perturbations. Comparisons between alkenone and calcareous nannofossil assemblage patterns infer Pseudoemiliania lacunosa as the main alkenone producer in sapropel layers; yet Reticulofenestra spp. contribution cannot be ruled out.The first Pliocene alkenone-SST record in the E. Mediterranean presented here documents the “warm Pliocene” period (~4.1–3.25Ma) characterized by mean SST of c. 26°C. Distinct SST minima at ~3.9Ma, 3.58Ma and between 3.34 and 3.31Ma, correspond to the MIS Gi16, MIS MG12 and MIS M2 global cooling episodes, before the onset of the Northern Hemisphere glaciation. Our findings imply that the peak of the MIS M2 cooling in the Eastern Mediterranean may be up to ~40kyrs older than the age attributed before to benthic stable oxygen isotopes records of this event.

Discovery of alkenones with variable methylene‐interrupted double bonds: implications for the biosynthetic pathway

Alkenones (C37 -C40 ) are highly specific biomarkers produced by certain haptophyte algae in ocean and lacustrine environments and have been widely used for paleoclimate studies. Unusual shorter-chain alkenones (SCA; e.g., C35 and C36 ) have been found in environmental and culture samples, but the origin and structure of these compounds are much less understood. The marine alkenone producer, Emiliania huxleyi CCMP2758 strain, was reported with abundant C35:2 Me (∆12, 19 ) alkenones when cultured at 15°C (Prahl etal. 2006). Here we show, when this strain is cultured at 4°C-10°C, that CCMP2758 produces abundant C35:3 Me, C36:3 Me, and small amounts of C36:3 Et alkenones with unusual double-bond positions of ∆7, 12, 19 . We determine the double-bond positions of the C35:3 Me and C36:3 Me alkenones by GC-MS analysis of the dimethyl disulfide and cyclobutylamine derivatives, and we provide the first temperature calibrations based on the unsaturation ratios of the C35 and C36 alkenones. Previous studies have found C35:2 Me (∆14, 19 ) and C36:2 Et (∆14, 19 ) alkenones with three-methylene interruption in the Black Sea sediments, but this is the first reported instance of alkenones with a mixed three- and five-methylene interruption configuration in the double-bond positions. Thediscovery of these alkenones allows us to propose a novel biosynthetic scheme, termed the SCA biosynthesis pathway, that simultaneously rationalizes the formation of both the C35:3 Me (∆7, 12, 19 ) alkenone in our culture and the ∆14, 19 Black Sea type alkenones without invoking new desaturases for the unusual double-bond positions.

Long-chain alkenones and related distinctive compounds in the late Miocene and Pliocene sediments from the Gulf of Cadiz, eastern North Atlantic

Long-chain alkenones and related compounds were analyzed in Miocene and Pliocene sediments from the Gulf of Cadiz (GoC) in the eastern North Atlantic, which were obtained by Integrated Ocean Drilling Project (IODP) Expedition 339. Both GC and GC–MS analyses using non- and mid-polar columns were performed on the sediments. Isomeric cis-C37 alkenones were identified along with common trans-C37–C39 alkenones. The use of a mid-polar column resulted in the complete separation of the trans-C37 tri-unsaturated alkenone and the cis-C37 di-unsaturated alkenone and thus contributed to a more precise estimation of the alkenone-based paleotemperature. The reconstructed SSTs range from 26 to 28°C during ca. 6–3Ma in the GoC. These variations are concordant with global climate change during the period, particularly the Pliocene Climatic Optimum and the onset of sustained ice sheet growth in the northern hemisphere. The concentrations of C37–C39 alkenones and the C37/C38 alkenone ratios are lower during the late Miocene to early Pliocene and increase after ca. 4.2Ma, which corresponds to the taxonomic change of the plausible alkenone producers from the genus Reticulofenestra to genera Gephyrocapsa and Reticulofenestra. Moreover, a C38 di-unsaturated alkene-2,36-dione was tentatively identified. Variations in the concentrations of a tentatively identified alkenedione are almost synchronous with those of other alkenones, and their relative abundances decrease after ca. 4.2Ma. These results suggest that the alkenedione originated from the ancient alkenone producers such as the genus Reticulofenestra.

Rapid detection and quantification of haptophyte alkenones by Fourier transform infrared spectroscopy (FTIR)

Several haptophyte algae produce unique neutral long-chain (C37C40) ketones (alkenones) with two to four trans-carbon-carbon double (CC) bonds and a keto-group. These molecules are of great biological interest and may have substantial commercial relevance as biofuel sources. Unfortunately, their detection and quantification, involving extraction from cells with organic solvents and fractionation of lipids through columns, are rather cumbersome. Hence, we developed a method for the rapid and reliable quantification of alkenones in intact cells using Fourier transform infrared spectroscopy (FTIR). The method is based on the absorption band at 962.5cm−1, which corresponds to a trans-CC bond that is unequivocally attributable to alkenones. This peak was exclusively observed in alkenone-producing haptophytes such as Emiliania huxleyi NIES-837, Emiliania huxleyi CCMP 2090, Tisochrysis lutea (former Isochrysis galbana T-iso), Tisochrysis lutea CCMP 463, and Chrysotila lamellosa CCMP 1307, as well as in appropriate standards. We compared our FTIR method with a typical GC analysis method. The alkenone quantity determined by these two methods showed similar values. Nevertheless, the FTIR method developed in this study does not require complex extraction procedures and is therefore a much easier and rapid quantification method. This FTIR method can also be used for the screening of strains and the optimization of culture conditions for alkenone production.

Testing the D / H ratio of alkenones and palmitic acid as salinity proxies in the Amazon Plume

Abstract. The stable hydrogen isotope composition of lipid biomarkers, such as alkenones, is a promising new tool for the improvement of palaeosalinity reconstructions. Laboratory studies confirmed the correlation between lipid biomarker δD composition (δDLipid), water δD composition (δDH2O) and salinity; yet there is limited insight into the applicability of this proxy in oceanic environments. To fill this gap, we test the use of the δD composition of alkenones (δDC37) and palmitic acid (δDPA) as salinity proxies using samples of surface suspended material along the distinct salinity gradient induced by the Amazon Plume. Our results indicate a positive correlation between salinity and δDH2O, while the relationship between δDH2O and δDLipid is more complex: δDPAM correlates strongly with δDH2O (r2 = 0.81) and shows a salinity-dependent isotopic fractionation factor. δDC37 only correlates with δDH2O in a small number (n = 8) of samples with alkenone concentrations > 10 ng L−1, while there is no correlation if all samples are taken into account. These findings are mirrored by alkenone-based temperature reconstructions, which are inaccurate for samples with low alkenone concentrations. Deviations in δDC37 and temperature are likely to be caused by limited haptophyte algae growth due to low salinity and light limitation imposed by the Amazon Plume. Our study confirms the applicability of δDLipid as a salinity proxy in oceanic environments. But it raises a note of caution concerning regions where low alkenone production can be expected due to low salinity and light limitation, for instance, under strong riverine discharge.

Does phylogeny control [formula omitted]-temperature sensitivity? Implications for lacustrine alkenone paleothermometry

Alkenone paleothermometry (via the U37K and U37K′ indices) has long been used to reconstruct sea surface temperature and has more recently been proven effective in lacustrine settings. Genetic analyses indicate that there is a diversity of different alkenone-producing lacustrine haptophytes, and differences among U37K-temperature calibrations suggest that unique calibrations might be required to quantify past temperature variation from individual lakes. The only term necessary to quantify U37K-inferred temperature relative to a reference period (e.g., modern temperature 20th Century mean) is the slope of the calibration regression, the U37K-temperature sensitivity (i.e., the change in U37K per °C temperature change). Here, we bring together all of the existing U37K-temperature calibrations in order to compare the variability among U37K-temperature sensitivities. We also report a new in situU37K-temperature calibration along with environmental genomic analysis based on the 18S rRNA gene of an alkenone producing haptophyte from lake Vikvatnet in Norway. We propose and test the hypothesis that U37K-temperature sensitivity is controlled by phylogeny and that this term can be used to quantify past temperature variation from lake sediments if the genetic identity of the lake’s alkenone-producer is known. Using the existing calibration data sets, we determine four phylotype-specific U37K-temperature sensitivities for use in cases where in situ calibration is unavailable but the phylogeny of the alkenone producers is known.

Sea surface temperature proxies (alkenones, foraminiferal Mg/Ca, and planktonic foraminiferal assemblage) and their implications in the Okinawa Trough

We reviewed three sea surface temperature (SST) proxies in the Okinawa Trough (OT): alkenones, planktonic foraminiferal Mg/Ca, and planktonic foraminiferal assemblages. The seasonal and vertical distribution patterns of each proxy-related organism in the water column were reviewed to confirm the applicability of each proxy. In addition, current SSTs (Japan Oceanographic Data Center dataset from 1906 to 2003) were compared with core-top sediment temperatures reconstructed using the proxies. Temperatures calculated using the alkenone unsaturation index represent annual mean SSTs, and temperatures calculated using Mg/Ca of Globigerinoides ruber capture summer to autumn (June–November) SSTs. Core-top August SSTs calculated from planktonic foraminiferal assemblages corresponded well with the observed SSTs, but core-top February temperatures were ~3.6 °C warmer than the observed SSTs. SST proxy estimates from marine sediments dating back to the late Holocene (0–3 cal ky BP) and the last glacial maximum (18–21 cal ky BP) were compared. Comparisons between proxy SST estimates show that foraminiferal assemblage-based August SSTs were the warmest. Alkenone-based temperature estimates were lower than Mg/Ca-based temperature estimates, probably because the alkenone-based temperature represents the annual mean temperature, whereas the Mg/Ca-based temperature represents the summer–autumn mean temperature. February assemblage SSTs seem to be greatly affected by the statistical technique and/or database used. These results suggest that seasonality should be considered in past SST reconstruction using alkenone and Mg/Ca in the OT. The planktonic foraminiferal assemblage technique does not appear to be promising with respect to accurately reconstructing past SSTs (especially winter) in the OT. Habitat depth may not be an issue because both alkenone producers and G. ruber live at the upper surface mixed layer in the area. Glacial–interglacial changes in the surface hydrography of the OT reconstructed based on the SST and salinity proxies were also reviewed here. The surface hydrography of the OT has been influenced by changes in the Kuroshio Current and the East Asian monsoon system during the late Quaternary. Comparisons of the hydrography records from the OT with records of stalagmites in China, the Tropical Pacific, and the North Atlantic show that there is a teleconnection between them.

Holocene sea surface temperature and sea ice extent in the Okhotsk and Bering Seas

Accurate prediction of future climate requires an understanding of the mechanisms of the Holocene climate; however, the driving forces, mechanisms, and processes of climate change in the Holocene associated with different time scales remain unclear. We investigated the drivers of Holocene sea surface temperature (SST) and sea ice extent in the North Pacific Ocean, and the Okhotsk and Bering Seas, as inferred from sediment core records, by using the alkenone unsaturation index as a biomarker of SST and abundances of sea ice-related diatoms (F. cylindrus and F. oceanica) as an indicator of sea ice extent to explore controlling mechanisms in the high-latitude Pacific. Temporal changes in alkenone content suggest that alkenone production was relatively high during the middle Holocene in the Okhotsk Sea and the western North Pacific, but highest in the late Holocene in the eastern Bering Sea and the eastern North Pacific. The Holocene variations of alkenone-SSTs at sites near Kamchatka in the Northwest Pacific, as well as in the western and eastern regions of the Bering Sea, and in the eastern North Pacific track the changes of Holocene summer insolation at 50°N, but at other sites in the western North Pacific, in the southern Okhotsk Sea, and the eastern Bering Sea they do not. In addition to insolation, other atmosphere and ocean climate drivers, such as sea ice distribution and changes in the position and activity of the Aleutian Low, may have systematically influenced the timing and magnitude of warming and cooling during the Holocene within the subarctic North Pacific. Periods of high sea ice extent in both the Okhotsk and Bering Seas may correspond to some periods of frequent or strong winter–spring dust storms in the Mongolian Gobi Desert, particularly one centered at ∼4–3 thousand years before present (kyr BP). Variation in storm activity in the Mongolian Gobi Desert region may reflect changes in the strength and positions of the Aleutian Low and Siberian High. We suggest that periods of eastward displacement or increased intensity of the Aleutian Low correspond with times of increased extent of sea ice in the western Okhotsk Sea and eastern Bering Sea.

Alkenone production in the East Sea/Japan Sea

To test the applicability of alkenones as a proxy for past sea surface temperature (SST) in the East Sea (Japan Sea), this study investigated the season and depth of alkenone production in the area. Surface and subsurface seawater samples were collected from the East Sea during cruises carried out by the National Fisheries Research and Development Institute of Korea in 2008–2010. Surface samples were filtered for suspended material at two-month intervals. Subsurface samples were collected at water depths of 20, 50, 70 and 100m by CTD bottle casts at two stations, one a coastal station and the other an offshore station. The results of alkenone analysis show that the concentration of total C37 alkenones was generally high in the surface mixed layer and decreased with depth, indicating that alkenones were most likely produced in or close to the surface mixed layer. Alkenone concentration varied seasonally: high in spring to fall and significantly reduced in winter. Comparisons of alkenone-based temperatures with in situ seawater temperatures show that alkenone temperatures measured from suspended particles in the surface waters were close to in situ SST in summer but were lower in winter. During winter, when alkenone production is significantly reduced, alkenones may be suspended for relatively long times and are likely to be advected from the north by eddies from Subpolar Front meanders. In summer when new production of alkenones increases, the settling velocity of alkenones appears to increase and residence time becomes shorter than in winter, suggesting that particles are less likely to be significantly advected at that time. Importantly, at the offshore station, coretop alkenone temperature corresponds to annual-averaged SST, while at the coastal station it corresponds to summer-to-fall averaged SST.

Nutricline shoaling in the eastern Pacific warm pool during the last two glacial maxima

Isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) and alkenones were analyzed in sediment samples retrieved from Ocean Drilling Program Site 1241 covering the last 150000 years to understand the hydrological evolution of the eastern Pacific warm pool (EPWP). GDGT and alkenone concentrations showed higher values in marine isotope stage (MIS)-2 and MIS-6, which suggests the enhancement of primary production at glacial maxima. \( {\text{TEX}}_{86}^{\text{H}} \)- and \( U_{ 3 7^\prime }^{\text{K}} \)-derived temperature depicted different temperature evolutions. \( U_{ 3 7^\prime }^{\text{K}} \)-derived temperature was marked by small variation during the glacial–interglacial cycles, whereas \( {\text{TEX}}_{86}^{\text{H}} \)-derived temperature showed pronounced glacial–interglacial variation that was similar to Mg/Ca-derived temperature records from nearby cores in the EPWP. Given that enhanced primary production during glacial maxima suggests nutricline shoaling, unchanged \( U_{ 3 7^\prime }^{\text{K}} \) over glacial–interglacial cycles can be interpreted as the shift of alkenone production depth. \( {\text{TEX}}_{86}^{\text{H}} \) seems not to be influenced by glacial–interglacial changes in nutricline depths, recording an integrated temperature in surface and thermocline water. The shallow nutricline in the EPWP during glacial maxima most likely reflected the intense formation of Antarctic intermediate water.

The influence of salinity on D/H fractionation in alkenones from saline and hypersaline lakes in continental North America

We present a preliminary assessment of the hydrogen isotopic composition of individual alkenones (C37–39 di-, tri- and tetra-unsaturated methyl- and ethyl-ketones) purified from surface lake sediment from sites spanning a range of salinities from 20 to 133ppt. Combining these measurements with measurements of the hydrogen isotopic composition of modern lake water, we estimate the magnitude of D/H fractionation across our sample set and observe reduced sensitivity to salinity compared to observations from Emiliania huxleyi and Gephyrocapsa oceanica. This lends support to the hypothesis that D/H fractionation during biosynthesis is less sensitive to changes in salinity in alkenone producers from continental interior sites than in producers from open marine environments. We also observe stronger correlations between the tetra-unsaturated alkenone δD values with water δD values, and associated α values with salinity than we do for the di- and tri-unsaturated compounds, and suggest that this may result from increased number of lacustrine haptophyte species producing the di- and tri-unsaturated as compared to the tetra-unsaturated.