Alkenone Unsaturation Research Articles

Thermal response of the western tropical Atlantic to slowdown of the Atlantic Meridional Overturning Circulation

The western tropical Atlantic plays an important role in the interhemispheric redistribution of heat during millennial-scale changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). The proper evaluation of this role depends on a clear understanding of sea surface temperature (SST) variations during AMOC slowdown periods like Heinrich Stadials (HS) in the western tropical Atlantic. However, published SST records from the western tropical Atlantic between ca. 4°S and 7°N show inconsistencies that are apparently related to the employed temperature proxy (i.e., Mg/Ca versus alkenone unsaturation index U37k′). In general, while Mg/Ca values indicate warming during Heinrich Stadials, U37k′ values show cooling. To assess this issue, we sampled core GeoB16224-1 retrieved off French Guiana (i.e., 6°39.38′N) and reconstructed water temperatures at high resolution using Mg/Ca on the foraminifera species Globigerinoides ruber, U37k′, TEX86 and modern analogue technique (MAT) transfer functions using planktonic foraminifera assemblages calibrated for 50 m water depth. Our results show that Mg/Ca and TEX86 values recorded an increase in SST related to AMOC slowdown. Conversely, U37k′ and MAT values registered a decrease in temperatures during HS3 and HS1. Our U37k′ and Mg/Ca results thus confirm the previously reported inconsistency for the period between 48–13 cal ka BP. We suggest that several non-thermal physiological effects probably imparted a negative temperature bias on the U37k′ temperatures during Heinrich Stadials. However, MAT-based temperatures show similar variability with U37k′-based temperatures. Hence, we also suggest that during severe slowdown periods of the AMOC, a steeper meridional temperature gradient together with a southward shift of the Intertropical Convergent Zone produced not only an increase in SST but also a stronger upper water column stratification and a shoaling of the thermocline, decreasing subsurface temperatures. Our new high resolution temperature records allow a better characterization of the thermal response of the upper water column in the tropical western Atlantic to slowdown events of the AMOC, reconciling previously discrepant records.

Evidence of solar insolation and internal forcing of sea surface temperature changes in the eastern tropical Indian Ocean during the Holocene

The forcing and mechanisms governing sea surface temperature (SST) variations in the Indo-Pacific Warm Pool (IPWP) are complex. Insight into the full spectrum of IPWP climate dynamics, however, is limited by the spatial and temporal coverage of the climate data. In particular, the relationships among the changes in the SSTs, the precipitation patterns mainly associated with rainfalls from Asian Monsoons (AM), and the Indian Ocean Dipole (IOD) are poorly understood. To help us further assess the climate linkages, we have reconstructed a more spatial SST pattern during the Holocene by using multiple SST proxies (alkenone unsaturation index U37k' and Mg/Ca of planktic foraminifer) in the Eastern Tropical Indian Ocean (ETIO), the western margin of the IPWP based on three sedimentary cores from NW offshore of Sumatra, and offshore of Sumatra and Java (BS24, SO139-74KL, and SO184-100430). Stable hydrogen and carbon isotope records of terrestrial plant waxes from a nearby marine sediment core SO189-144KL and geochemical tracers measured from the coral reefs within the Mentawai Islands in the ETIO are used here as AM driven precipitation and IOD records in our data synthesis. Not surprisingly, our synthesis suggests that insolation plays a major role that has been responsible for the increased SSTs in ETIO since the early Holocene, while other mechanisms remain effective in determining the timing of our reconstructed SST variations. In particular, our SST pattern shares less similarity with that of coral Sr/Ca SST and is decoupled from the coral IOD events in the mid-Holocene. We interpret that our reconstructed ETIO SSTs are driven dominantly by the solar forcing, but are also affected by other internal climate mechanisms such as the local shifts in AM-controlled upwelling and precipitation, episodic reductions in the flow of warm western Pacific surface water into the Indian Ocean due to increased precipitation over the Indonesian archipelago, and long-term ENSO or IOD-like climate change.

Widespread occurrence of distinct alkenones from Group I haptophytes in freshwater lakes: Implications for paleotemperature and paleoenvironmental reconstructions

Alkenones are C35–C42 polyunsaturated ketone lipids that are commonly employed to reconstruct changes in sea surface temperature. However, their use in coastal seas and saline lakes can be hindered by species-mixing effects. We recently hypothesized that freshwater lakes are immune to species-mixing effects because they appear to exclusively host Group I haptophyte algae, which produce a distinct distribution of alkenones with a relatively consistent response of alkenone unsaturation to temperature. To evaluate this hypothesis and explore the geographic extent of Group I haptophytes, we analyzed alkenones in sediment and suspended particulate matter samples from lakes distributed throughout the mid- and high latitudes of the Northern Hemisphere (n=30). Our results indicate that Group I-type alkenone distributions are widespread in freshwater lakes from a range of different climates (mean annual air temperature range: −17.3–10.9 °C; mean annual precipitation range: 125–1657 mm yr−1; latitude range: 40–81°N), and are commonly found in neutral to basic lakes (pH > 7.0), including volcanic lakes and lakes with mafic bedrock. We show that these freshwater lakes do not feature alkenone distributions characteristic of Group II lacustrine haptophytes, providing support for the hypothesis that freshwater lakes are immune to species-mixing effects. In lakes that underwent temporal shifts in salinity, we observed mixed Group I/II alkenone distributions and the alkenone contributions from each group could be quantified with the RIK37 index. Additionally, we observed significant correlations of alkenone unsaturation (U37K) with seasonal and mean annual air temperature with this expanded freshwater lakes dataset, with the strongest correlation occurring during the spring transitional season (U37K=0.029⁎T−0.49; r2=0.60; p<0.0001). We present new sediment trap data from two lakes in northern Alaska (Toolik Lake, 68.632°N, 149.602°W; Lake E5, 68.643°N, 149.458°W) that demonstrate the highest sedimentary fluxes of alkenones in the spring transitional season, concurrent with the period of lake ice melt and isothermal mixing. Together, these data provide a framework for evaluating lacustrine alkenone distributions and utilizing alkenone unsaturation as a lake temperature proxy.

Novel alkenone-producing strains of genus Isochrysis (Haptophyta) isolated from Canadian saline lakes show temperature sensitivity of alkenones and alkenoates

Alkenone-producing species have been recently found in diverse lacustrine environments, albeit with taxonomic information derived indirectly from environmental genomic techniques. In this study, we isolated alkenone-producing algal species from Canadian saline lakes and established unialgal cultures of individual strains to identify their taxonomical and molecular biological characteristics. Water and sediments collected from the lakes were first enriched in artificial seawater medium over a range of salinities (5–40 g/L) to cultivate taxa in vitro. Unialgal cultures of seven haptophyte strains were isolated and categorized in the Isochrysis clade using SSU and LSU rRNA gene analysis. The alkenone distributions within isolated strains were determined to be novel compared with other previously reported alkenone-producing haptophytes. While all strains produced the typical C37 and C38 range of isomers, one strain isolated from Canadian salt lakes also produced novel C41 and C42 alkenones that are temperature sensitive. In addition, we showed that all alkenone unsaturation indices (e.g., U37K and U37K′) are temperature-dependent in culture experiments, and that alkenoate indices (e.g., U37A, U38A, RIA38 and A37/A38) provide alternative options for temperature calibration based on these new lacustrine algal strains. Importantly, these indices show temperature dependence in culture experiments at temperatures below 10 °C, where traditional alkenone proxies were not as sensitive. We hypothesize that this suite of calibrations may be used for reconstructions of past water temperature in a broad range of lakes in the Canadian prairies.

Climatic and ecological changes of the past 1900 years inferred from long-chain alkenones in Kusai Lake, northern Qinghai-Tibetan Plateau

We report 1900-year long records of alknone concentration, C38/C37 ratio, and U37k, U37k' and a newly-developed alkenone unsaturation index U37k''-based temperatures in a sediment core retrieved from Kusai Lake in the northern Qinghai-Tibetan Plateau. The predominance of C37 and C38 alkenones with minor presence of C39 and C40 alkenones, and the absence of C38 methyl alkenones are indicative of alkneones produced by Isochrysis, which is consistent with the results of sedimentary haptophyte DNA analyses. The interval from 400 to 1200 AD is characterized by higher alkenone concentrations and C38/C37 ratios than the other intervals, which can be attributed to the predominant production of Isochrysis during the entire phytoplankton blooms. With the assistance of information on gene abundance, we attributed this phenomenon to drastic ecological changes in Kusai Lake around 1200 AD in response to climatic changes from warm and wet to cold and dry conditions, and resultant decreased riverine inflow but increased nutrient concentration. The U37k''-based temperatures with U37k'' substituting U37k' in a equation concluded based on studies on lakes from high latitude areas in Germany are the most reliable reconstructed summer surface water temperatures for Kusai Lake since they are close to the current summer monthly mean temperature of Wudaoliang Meteorological Station, approximately 50 km to the south of Kusai Lake, and exhibit variations that are generally in accordance with those of the 51-year moving average summer temperatures reconstructed from thicknesses of varve light layers in Kusai Lake sediments. Besides, variations showed by this U37k''-based summer surface water temperatures are also generally consistent with those showed by reconstructed temperatures in China.

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.

Assessing environmental controls on the distribution of long-chain alkenones in the Canadian Prairies

Long-chain alkenones (LCAs) have been used for decades to reconstruct quantitative sea-surface temperature records, but they also have a great potential for paleotemperature reconstructions in lacustrine settings. Here, we investigated how the presence and abundance of LCAs in surface sediments from 106 lakes varied with environmental conditions in lakes of the northern Great Plains (Canadian Prairies) in southern Saskatchewan, Canada. Consistent with preliminary research, we found LCAs in 55% of surveyed lakes, with mean concentrations of 143 μg/g dry sediment, but very high concentrations (up to 2.3 mg/g dry sediment) in 7% of lakes. Statistical analyses indicate that salinity and stratification play key roles in determining LCA presence and abundance supporting previous research in Spain and the northern Great Plains, USA. Overall, the alkenone unsaturation index (U37K) was not correlated significantly with average summer water temperature, probably because the timing of maximum LCA production occurs during the spring season. We conclude that improved seasonal sampling is required within the study lakes to better identify the timing and habitat of haptophyte production, and allow development of environmental temperature reconstruction tools.

Low-latitude climate control on sea-surface temperatures recorded in the southern Okinawa Trough during the last 13.3 kyr

Paleotemperature records were reconstructed for the past 13.3kyr using the alkenone unsaturation index (UK′37) and the tetraether index of 86 carbon atoms (TEX86) proxies measured in sediment core OKI-151, which was collected in the southern Okinawa Trough. The UK′37- and TEX86-derived temperatures for the top sample are close to the modern mean annual sea-surface temperature (SST). Changes in SSTs in the southern Okinawa Trough were primarily controlled by the western tropical Pacific Ocean, low-latitude winter insolation, and sea-level rise. Unlike previous studies, we found that the high-latitude North Atlantic played no significant role in the SST variations in this area. Moreover, SSTs in the southern Okinawa Trough decoupled from the East Asian summer monsoon system in the last 13.3kyr. The UK′37- and TEX86-based SSTs dropped significantly at 7.6–6.9kyr BP (centered on 7.3kyr BP). These findings suggest that the cold event at 7.3kyr BP was widely recorded in the Okinawa Trough. However, the TEX86-based SSTs dropped approximately 14°C during this period for an unknown reason.

Alkenone unsaturation during virus infection of Emiliania huxleyi

Alkenones are found in diverse prymnesiophytes including Emiliania huxleyi, which ranges throughout the world ocean. The number of double bonds in alkenones corresponds with growth temperature, and calibrations between sea surface temperature and alkenone unsaturation have been used to establish alkenone unsaturation as a temperature proxy in ancient sediments. Here we report virus infection causes a relative increase in the proportion of diunsaturated alkenones in E. huxleyi strain CCMP 374. As virus infection routinely terminates E. huxleyi blooms in the ocean, it is possible that this contributes to the variance in alkenone-based temperature estimates for core-top sediments.

Investigation of the UK37’ vs. SST relationship for Atlantic Ocean suspended particulate alkenones: An alternative regression model and discussion of possible sampling bias

Alkenone unsaturation, expressed as the UK37’ index, is closely related to growth temperature of prymnesiophytes, thus providing a reliable proxy to infer past sea surface temperatures (SSTs). Here we address two lingering uncertainties related to this SST proxy. First, calibration models developed for core-top sediments and those developed for surface suspended particulates organic material (SPOM) show systematic offsets, raising concerns regarding the transfer of the primary signal into the sedimentary record. Second, questions remain regarding changes in slope of the UK37’ vs. growth temperature relationship at the temperature extremes. Based on (re)analysis of 31 new and 394 previously published SPOM UK37’ data from the Atlantic Ocean, a new regression model to relate UK37’ to SST is introduced; the Richards curve (Richards, 1959). This non-linear regression model provides a robust calibration of the UK37’ vs. SST relationship for Atlantic SPOM samples and uniquely accounts for both the fact that the UK37’ index is a proportion, and so must lie between 0 and 1, as well as for the observed reduction in slope at the warm and cold ends of the temperature range. As with prior fits of SPOM UK37’ vs. SST, the Richards model is offset from traditional regression models of sedimentary UK37’ vs. SST. We posit that (some of) this offset can be attributed to the seasonally and depth biased sampling of SPOM material.

Holocene hydrological and sea surface temperature changes in the northern coast of the South China Sea

In order to reconstruct the Holocene environmental history of a coastal site in the northern South China Sea, this study analysed the organic carbon isotope ratios (δ13Corg) and alkenone unsaturation ratios (UK′37) from a 36.5m-long sediment core drilled at seabed in the mouth region of the Pearl River estuary and generated a coupled hydrological and temperature record. This record reveals changes of monsoon-induced sediment discharge and sea surface temperature of the Holocene in four stages. In Stage I, the site was under fluvial conditions prior to postglacial marine transgression. Stage II saw an increase of sea surface temperature from c. 23.0°C to 27.0°C, associated with a strengthened summer monsoon from c. 10,350 to 8900cal. years BP. This was also a period of rapid sea-level rise and marine transgression, during which the sea inundated the palaeo-incised channel, i.e. the lower part of the T-shape accommodation space created by the rising sea. In these 1500years, fluvial discharge was strong and concentrated within the channel, and the high sedimentation rate (11.8mm/year) was very close to the rate of sea-level rise. In the subsequent 2000years (Stage III) sea level continued to rise and the sea flooded the broad seabed above the palaeo-incised channel, resulted in fluvial discharge spreading thinly across the wide accommodation space and a much reduced sedimentation rate (1.8mm/year). Sea surface temperature in this stage reached 27.3°C initially, but dropped sharply to 26.1°C towards c. 8200cal. years BP. The final stage covers the last 7000years, and the site was under a stable sea level. Sedimentation in this stage varied a little, but averaged at 1.8mm/year. Whilst fluvial discharge and sea surface temperature didn’t change much, two short periods of hydrological and temperature change were observed, which are related to the climatic cooling events of c. 4200cal. years ago and the Little Ice Age.

Reaction kinetics of alkenone and n‐alkane thermal alteration at seismic timescales

AbstractRecent experiments and field observations have indicated that biomarker molecules can react over short timescales relevant to seismic slip, thereby making these compounds a useful tool in studying temperature rise in fault zones. However, short‐timescale biomarker reaction kinetics studies have previously focused on compounds that have already experienced burial heating. Here, we present a set of hydrous pyrolysis experiments on Pleistocene‐aged shallow marine sediment to develop the reaction kinetics of long‐chain alkenone destruction, change in the alkenone unsaturation ratio ( ), and change in the n‐alkane chain length distribution. Our results show that biomarker thermal maturity provides a useful method for detecting temperature rise in the shallow reaches of faults, such as subduction zone trench environments. Through the course of our work, we also noted the alteration of total alkenone concentrations and values in crushed sediments stored dry at room temperature for durations of months to years but not in the solvent extracts of these materials. This result, though parenthetical for our work in fault zones, has important implications for proper storage of sedimentary samples to be used for alkenone paleotemperature and productivity analysis.

Svalbard ice-sheet decay after the Last Glacial Maximum: New insights from micropalaeontological and organic biomarker paleoceanographical reconstructions

A marine sediment core retrieved from the middle continental slope of the northwestern Barents Sea was analyzed for its geochemical (alkenones) and micropalaeontological (diatoms, coccolithophores and dinocyst) content in order to reconstruct the evolution of upper ocean conditions and ice-sheet dynamics during the last 25kyr. Additionally, quantitative reconstructions of sea surface conditions (temperature, salinity and sea-ice cover extent) were conducted based on the best analogue technique applied to dinocyst assemblages and on the alkenone unsaturation index. The sediment core contains a post Last Glacial Maximum depositional sequence unaffected by stratigraphic discontinuities. Low salinity and laminated sediments after 20calkyr BP, indicate a massive settling of meltwater sediment-laden plumes from the initial melting of the Svalbard-Barents Sea Ice Sheet on Western Svalbard. First record of measurable alkenones, together with a drop of the number of months of sea-ice cover and increase in SSTs suggests an intensification of the influx of Atlantic waters into the study area at ~15calkyr BP representing the termination of the last glacial period and onset of the Bølling interstadial. The first occurrence of diatoms and increase in the abundance of all microfossils marked the onset of the Holocene at 11.2calkyr BP when modern-type sea surface conditions were rapidly established in Western Svalbard. Reconstructions based on dinocyst data and alkenone unsaturation index suggest relatively warm and stable temperatures between 9.9 and 8.9kyr BP and a decrease of SSTs from 4.2calkyr BP to present coinciding with the Holocene Thermal Maximum and the decrease of summer insolation in the high latitude northern hemisphere, respectively.

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.

Alkenone paleothermometry in the North Atlantic: A review and synthesis of surface sediment data and calibrations

AbstractDespite a clear correlation of alkenone unsaturation and sea surface temperatures (SST) throughout most parts of the ocean, scatter of the regression for various calibration equations has been shown to increase significantly at low SSTs. In this study, we combine previously published (n = 101) and new (n = 51) surface sediment data from the northern North Atlantic to constrain uncertainties of alkenone paleothermometry at low SSTs and to discuss possible sources of the increased scatter in the regression. The correlation between alkenone unsaturation and SSTs is strongest, in particular at the cold end (SSTs &lt; 10°C), when the tetra‐unsaturated alkenones (C37:4) are included in the unsaturation index (expressed as ) and regressed against spring‐summer temperature. Surface ocean salinity and sea ice cover are not correlated with per se. However, samples located in regions of permanent winter sea ice cover exhibit a significant warm bias. Deviation from the linear regression is posited to be related to a number of additional non‐exclusive factors, such as advection of allochthonous material, local temperature stratification, and uncertainty in the absolute age of surface sediment samples assumed to be equivalent to modern conditions. We conclude that alkenone unsaturation allows accurate reconstruction of SST records from many regions of the North Atlantic if the factors confounding alkenone paleothermometry detailed here can be excluded.

Temperature calibration and phylogenetically distinct distributions for freshwater alkenones: Evidence from northern Alaskan lakes

Alkenones are a class of unsaturated long-chain ketone biomarkers that have been used to reconstruct sea surface temperature and, more recently, continental temperature, by way of alkenone unsaturation indices (e.g. U37K and U37K′). Alkenones are frequently found in brackish and saline lakes, however species effects confound temperature reconstructions when multiple alkenone-producing species with different temperature responses are present. Interestingly, available genetic data indicate that numerous freshwater lakes host a distinct phylotype of alkenone-producing haptophyte algae (the Group I or Greenland phylotype), providing evidence that species effects may be diminished in freshwater lakes. These findings encourage further investigation of alkenone paleotemperature proxies in freshwater systems. Here, we investigated lakes from northern Alaska (n=35) and show that alkenones commonly occurred in freshwater lakes, where they featured distinct distributions, characterized by dominant C37:4 alkenones and a series of tri-unsaturated alkenone isomers. The distributions were characteristic of Group I-type alkenone distributions previously identified in Greenland and North America. Our analysis of suspended particulate matter from Toolik Lake (68°38′N, 149°36′W) yielded the first in situ freshwater U37K calibration (U37K=0.021*T−0.68; r2=0.85; n=52; RMSE=±1.37°C). We explored the environmental significance of the tri-unsaturated isomers using our northern Alaskan lakes dataset in conjunction with new data from haptophyte cultures and Canadian surface sediments. Our results show that these temperature-sensitive isomers are biomarkers for the Group I phylotype and indicators of multiple-species effects. Together, these findings highlight freshwater lakes as valuable targets for continental alkenone-based paleotemperature reconstructions and demonstrate the significance of the recently discovered tri-unsaturated isomers.

Identification of double-bond positions in isomeric alkenones from a lacustrine haptophyte.

Measurements of alkenone unsaturation ratios are widely used for paleotemperature reconstructions in ocean and lake environments. Previously, we reported the discovery of a series of tri-unsaturated alkenone positional isomers (Δ(14, 21, 28) ) from oligosaline and freshwater lakes in Greenland and Alaska. In this work we provide a detailed analysis of the structures and isotopic compositions (δ(13) C and δ(2) H) of the alkenones produced by the "Greenland haptophyte". Alkenones were extracted from sediments of Lake BrayaSø, Greenland. Alkenone double-bond positions were determined by GC/EI-MS analysis of alkenone dimethyl disulfide and cyclobutylimine derivatives. Alkenones were purified by semi-preparative HPLC using a silver(I) thiolate stationary phase. Carbon and hydrogen isotope analysis was performed by gas chromatography/isotope ratio mass spectrometry (GC/IRMS). A series of novel tri-unsaturated alkenone positional isomers were identified among four alkenone homologues (i.e. C37 Me , C38 Me , C38 Et , and C39 Et ) with double-bond positions at Δ(14, 21, 28) . The hydrogen isotope compositions (δ(2) H, VSMOW) of the tri-unsaturated positional isomers from C37 Me and C38 Et were slightly depleted (~ -11 ‰) relative to the common tri-unsaturated alkenone. The carbon isotope composition (δ(13) C, VPDB) of the tri-unsaturated positional isomers from the C37 Me , C38 Me , C38 Et , and C39 Et alkenones were significantly enriched (~ +4 ‰) relative to the common alkenones (di-, tri-, and tetra-unsaturated). The novel tri-unsaturated alkenone positional isomers produced by the Greenland haptophyte possess Δ(14, 21, 28) double-bond positions, instead of the common Δ(7, 14, 21) double-bond positions. The hydrogen isotope values suggest the novel tri-unsaturated positional isomers could be biosynthetic precursors to the tetra-unsaturated alkenones (Δ(7, 14, 21, 28) ). However, the significantly higher carbon isotope values of the tri-unsaturated positional isomers relative to the common di-, tri- and tetra-unsaturated alkenones suggest these positional isomers may have different/additional biosynthetic precursors.

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.

Excluding the di-unsaturated alkenone in the UK37 index strengthens temperature correlation for the common lacustrine and brackish-water haptophytes

Alkenone unsaturation indices, represented by either UK37 or UK′37, are important tools for paleoclimate studies. The UK37 index is a reflection of the average number of double bonds from di-, tri-, tetra-unsaturated alkenones, but UK′37 excludes the C37:4 in the calculation. Extensive studies indicate UK′37 provides better regressions against in situ Sea Surface Temperatures (SST) than UK37 and is the most widely used SST proxy. However, recent studies have shown that including C37:4 alkenones for lacustrine or brackish water haptophytes such as Ruttnera (Chrysotila) lamellosa and Isochrysis galbana improves temperature correlations although there are still significant deviations at the extreme high and low temperatures. In this study, we use new culture-based calibration experiments alongside published culture data and in situ water column or surface sediment data, to demonstrate that a further improved temperature calibration for alkenones is, in fact, achieved when the di-unsaturated alkenone is excluded from the computation of the unsaturation index. We propose new indices, termed UK″ [UK″37=C37:3/(C37:3+C37:4) or UK″38=C38:3/(C38:3+C38:4)], for lacustrine, brackish and estuarine waters. Our observation suggests that di-unsaturated alkenones play a less important role than tri- and tetra-unsaturated alkenones in regulating cell functions to temperature variations in lacustrine and brackish waters. We suggest using UK″ indices for paleotemperature reconstructions in the lacustrine and estuarine settings.

Parallelisms between sea surface temperature changes in the western tropical Atlantic (Guiana Basin) and high latitude climate signals over the last 140 000 years

Abstract. Sea surface temperatures (SST) in the Guiana Basin over the last 140 ka were obtained by measuring the C37 alkenone unsaturation index Uk'37 in the sediment core MD03-2616 (7° N, 53° W). The resulting data set is unique in the western tropical Atlantic region for this period. The SSTs range from 25.1 to 28.9 °C, i.e. glacial–interglacial amplitude of 3.8 °C, which is in the range of change of other tropical areas. During the last two interglacial stages (marine isotope stages; MIS1 and MIS5e) and warm long interstadials (MIS5d-a), a rapid transmission of climate variability from Arctic–tropical latitudes is recorded. During these periods, the MD03-2616 SSTs show a conspicuous parallelism with temperature changes observed in Greenland and SST records of North Atlantic mid-latitude cores (Iberian Margin 38° N, Martrat et al., 2007). The last deglaciation in the Guiana Basin is particularly revealing. MIS2 stands out as the coldest period of the interval analysed. The events recorded in Guiana parallel northern latitude events such as the Bølling–Allerød warming and the Younger Dryas cooling which ensued. These oscillations were previously documented in the δ18O of the Sajama tropical ice core (Bolivia) and are present in Guiana, with rates of ca. 6 °C ka−1 and changes of over 2 °C. During the glacial interval, significant abrupt variability is observed, e.g. oscillations of 0.5–1.2 °C during MIS3, which is about 30 % of the maximum glacial–interglacial SST change. In the MD03-2616 record, it is possible to unambiguously identify either the Dansgaard–Oeschger oscillations described in northern latitudes or the SST drops associated with the Heinrich events characteristic of North Atlantic records. Although these events form the background of the climate variability observed, what truly shapes SSTs in the Guiana Basin is a long-term tropical response to precessional changes, which is modulated in the opposite way to Northern Hemisphere variability. This lack of synchrony is consistent with other tropical records in locations to the north or south of the Guiana Basin and evidences an Arctic–tropical decoupling when a substantial reduction in the Atlantic meridional overturning circulation (AMOC) takes place.