Alkenone Production Research Articles

Temperature-dependent carbon isotope fractionation in coccolithophores

Introduction: The stable carbon isotope ratio of long-chain alkenones produced by marine haptophyte phytoplankton has often been used to estimate past variations in atmospheric CO2 throughout the Cenozoic. However, previous experimental studies and surveys of alkenones from surface sediment and suspended particulate matter document additional environmental and physiological influences on carbon isotopic fractionation in alkenones.Methods: To clarify the non-CO2 effects on the alkenone carbon isotope fractionations, an important alkenone producer, Gephyrocapsa oceanica, was cultured in laboratory. To separate effects of different environment parameters, G. oceanica was grown in continuous cultures under a matrix of environmental conditions in order to explore the influence of temperature independently of CO2(aq). Through careful manipulation of the media carbon system, we can control the variation of the media CO2(aq) independently of temperature solubility. Carbon isotope fractionations from alkenones, coccolith, and particulate organic carbon were measured from this steady state system.Results and Discussion: We find εp in alkenones and particulate organic carbon inversely correlates with temperature, and temperature affects εp more strongly than CO2(aq). The magnitude of the temperature effect can be explained by higher growth rates at warmer temperatures with a similar growth rate dependence as observed in previous cultures in which growth rate was regulated by other factors. Where the past temperature influence on growth rate could be constrained using the UK’37 alkenone index in the same samples, our finding offers an approach to deconvolve an important physiological factor affecting ancient alkenones εp, and may therefore improve past pCO2 estimates.

Spatio-temporal variability of alkenone and calcareous nannofossil abundance records across the southeastern Philippines: Insights on alkenone production and paleoceanographic reconstructions.

Spatio-temporal variability of alkenone and calcareous nannofossil abundance records across the southeastern Philippines: Insights on alkenone production and paleoceanographic reconstructions.

Iberian Margin surface ocean cooling led freshening during Marine Isotope Stage 6 abrupt cooling events

The high-resolution paleoclimate records on the Iberian Margin provide an excellent archive to study the mechanism of abrupt climate events. Previous studies on the Iberian Margin proposed that the surface cooling reconstructed by the alkenone-unsaturation index coincided with surface water freshening inferred from an elevated percentage of tetra-unsaturated alkenones, C37:4%. However, recent data indicate that marine alkenone producers, coccolithophores, do not produce more C37:4 in culture as salinity decreases. Hence, the causes for high C37:4 are still unclear. Here we provide detailed alkenone measurements to trace the producers of alkenones in combination with foraminiferal Mg/Ca and oxygen isotope ratios to trace salinity variations. The results indicate that all alkenones were produced by coccolithophores and the high C37:4% reflects decrease in SST instead of freshening. Furthermore, during the millennial climate changes, a surface freshening did not always trigger a cooling, but sometimes happened in the middle of multiple-stage cooling events and likely amplified the temperature decrease.

Exploring the theoretical upper temperature limit of alkenone unsaturation indices: Implications for paleotemperature reconstructions

The structural specificity, exceptional diagenetic stability, and linear response of unsaturation to temperature have made alkenones an indispensable tool for reconstructing past sea surface temperatures, with the well known U37K′ proxy widely applied in the past 40 years. However, U37K′ approaches unity at around 28 °C in cultures of Emiliania huxleyi (E. huxleyi), the most widely distributed alkenone producer in global oceans. Recent studies using surface sediments suggest U38MeK′ has ∼1.5 °C higher upper temperature limits than U37K′. However, E. huxleyi generally does not grow above 28 °C. Gephyrocapsa oceanica (G. oceanica), on the other hand, is the dominant alkenone producer when sea surface temperature is above 22 °C and thrives in ocean regions such as the Pacific Warm Pool. So far there have been no culture data for G. oceanica above 29 °C to evaluate the temperature response of alkenone distributions and the (theoretical) upper temperature limit of alkenone unsaturation indices. Here we performed the first culture experiments on two strains of G. oceanica isolated from warm ocean sites at up to 32 °C: RCC6484 from the Pacific Warm Pool and RCC3483 from the South China Sea. We show both strains display higher growth rates at higher temperature, with the highest growth rates at 32 °C (the highest temperature we tested), although alkenone production per cell decreases as temperature increases. Importantly, U37K′ and U38MeK′ values of RCC6484 and RCC3483 continue to respond to temperature changes beyond 28 °C, although temperature sensitivity decreases significantly above 28 °C. Above 30 °C, U37K′ and U38MeK′ responses to temperature further diminish, limiting their potential for paleo-SST applications using the currently available analytical technology. The ratio of C38:3 over C38:2 methyl ketone is on average 11 times higher than the ratio of C37:3 over C37:2 from 24 to 32 °C, suggesting a greater potential of using U38MeK′ for paleotemperature reconstruction in high temperature settings.

Long-chain alkenones in the lake sediments of North-Minusinsk Valley (southern Siberia): implications for paleoclimate reconstructions

Long-chain alkenones (LCAs) are polyunsaturated methyl or ethyl ketones exclusively produced by Groups 1–3 Isochrysidales, an order of haptophyte algae. LCAs have great potential as temperature and salinity proxies for paleo-reconstructions. Alkenone producers occur extensively in lacustrine and in open marine systems. However, factors influencing LCAs occurrence, abundance, compositional pattern, and unsaturation indices are insufficiently understood in lakes of southern Siberia. Here we investigated alkenone distributions in freshwater to saline lakes of a semi-arid region of southern Siberia (n = 22). We find that total alkenone concentrations increase dramatically above a salinity of about 20 g L−1. Analysis of 18S rRNA and the alkenone composition suggest that all alkenone-containing lakes are inhabited by Group 2 haptophytes except one low-saline lake, where Group 1 representatives prevail. The C37/C38 ratio is negatively correlated with salinity, and, hence, this parameter can be used as a salinity indicator. At the same time, %C37:4 is not correlated with salinity, whereas the average chain length of alkenones increases with salinity. The taxonomic composition of haptophytes and the alkenone composition were comparable to those in the lakes in the Canadian Prairies, potentially because of the similarities in climatic factors and ionic composition of lake water in the two regions. This study shows for the first time that unsaturation indexes U40K and U40K′ are positively correlated with salinity and, thus, can be used as a salinity proxy for regional climate reconstruction.

Paleoclimatic and palaeoceanographic changes coupled to the Panama Isthmus closing (13–4 Ma) using organic proxies

This study reconstructs the climatic and oceanographic changes from 13 to 4 Ma at Ocean Drilling Program (ODP) Sites 999 (Caribbean) and 1241 (East Pacific) during Panama Isthmus closure. We used lipid biomarkers as marine production indicators and organic proxies (U37K´ and TEX86H) to estimate ocean surface and thermocline temperatures. Enhanced terrestrial input during the first 3 Ma of the record, possibly coupled to tectonic processes, may have triggered higher surface productivity at both sides of the Isthmus. Even though marine production is generally higher in the tropical Pacific, a peak in alkenone production at 12.4–10.6 Ma indicates unrestricted inflow of nutrient-rich waters into the Caribbean. Higher production is observed during the global late Miocene biogenic bloom (LMBB 8–4.5 Ma) coupled to enhanced nutrients from terrestrial sources and/or upwelling. We find a good correspondence between U37K´ and modern sea surface temperature (SST). The TEX86H is a good tracer for thermocline values, however, the deeper Thaumarchaeota distribution in the Caribbean may result in an underestimation of absolute temperatures at ODP Site 999. None the less, temperature records at both Sites show similar trends coupled to global climate dynamics, therefore Isthmus build up did not significantly affect surface or thermocline temperatures at a regional level. Globally reported decreasing temperatures during the late Miocene and early Pliocene coupled to decreasing CO2 levels are registered at both Sites down to at least 200 m. The late Miocene warm period (LMWP 9.5–7 Ma), late Miocene cold period (LMC 7–5.4 Ma) and the Pliocene warm period (PWP 4–3.5 Ma) are reflected by our temperature records confirming global climate is the main driver of temperature evolution in the study area. The reconstructed temperatures are mainly coupled to atmospheric forcing in the Caribbean, with a deviation possibly due to inflow of colder Pacific intermediate waters. While at ODP Site 1241 temperatures are mainly controlled by global climate, the Site also reflects changes in the water column as it migrates interacting with the complex current structure of the tropical Pacific. Decoupled temperature patterns at both Sites are related with differential changes in water column structure partially masking the climatic signal. Therefore, we could establish that while tectonic processes and related terrestrial input increases as well as upwelling control production and water column structure, climate in the study area remained tuned to global patterns regardless of Isthmus closure stage.

Paleoproductivity proxies and alkenone precursors in the Western Mediterranean during the Early-Middle Pleistocene transition

Multidisciplinary analyses (taxonomic analysis of coccolithophore assemblages, stable oxygen isotopes, marine and terrestrial biomarkers) have been carried out on sediments from Ocean Drilling Program Site 975 in the Algero-Balearic basin, through late marine isotope stage (MIS) 20-19 (800–756 ka). The aim is to compare coccolithophore paleoproductivity proxies, such as C37 alkenone concentration and nannofossil accumulation rate (NAR), and understand their relationship with paleoceanographic condition and paleoenvironmental changes, alkenone-producing precursors and unsaturated C37 alkenone compounds. The patterns of C37 alkenones and NAR provide reliable information on past paleoproductivity changes since coccolith dissolution and organic matter preservation were excluded as relevant processes at the site. This is testified by the high values of Nannofossil Dissolution Index and relation between C37 alkenone concentration and Alcohol Preservation Index, the latter used as a proxy of sea bottom ventilation in the basin. A weak mismatching between NAR and C37 alkenone concentration records has been observed and related to paleoenvironmental factors and ecological preferences of alkenone-producing species. Temperature variations mostly controlled the alternating interspecific abundance variations of these taxa through glacial-interglacial and stadial-interstadial climate phases. The percentage abundances of alkenone-producing species, Gephyrocapsa caribbeanica and Gephyrocapsa with open central area (mainly G. margerelii-G. muellerae) strongly co-varied with the percentages of C37:2 and C37:3, respectively during warm and cool-cold periods, suggesting their prominent role in producing these unsaturated C37 alkenone compounds. Moreover, Gephyrocapsa spp. with open central area > 3 μm were likely the main C37:4 producers during the colder late MIS 20 stadial and stadial phases. Other factors in addition to temperature influenced the paleoproductivity proxy patterns. The oceanographic condition established during MIS 20-MIS 19 deglaciation and the more nutrient-rich surface waters during the orbitally-controlled organic-rich layer deposition in the early MIS 19 enhanced primary productivity leading to higher production and preservation of total C37 alkenones.

Alkenones in Pleistocene Upper Bed I (1.803–1.900 Ma) sediments from Paleolake Olduvai, Tanzania

The Olduvai Gorge Coring Project (OGCP) recovered sequences of Pleistocene lake sediments that correlate with those known from outcrops directly associated with hominin fossil horizons. The sedimentary succession from Core 2A (02° 58′ 43′' S, 035° 19′ 25.5′' E), which targeted the ancient lake-basinal depocenter, includes stratigraphic intervals (∼1.86 Ma) sandwiched between the Bed I basalt and Tuff IF dated horizons, where remains of two hominin holotypes have been recovered. The lower part of this sequence (72.3–86.9 mbs) consists of lacustrine claystones interbedded with sandy claystones characterized by high Corg (av. 2.5%) and includes several laminated intervals. It displays shifts in terrestrial vegetation recorded by values for δ13CTOC and δ2HnC31 that reflect changes in the proportions of C3 vs C4 plants and precipitation, respectively. These temporal profiles represent higher-resolution signatures of the precession-scale wet/dry cycles previously recognized in the outcropping sedimentary succession of Olduvai Gorge. Several samples from this interval contain alkenones comprising C37 and C39 alkadien-2-ones and alkatrien-2-ones, and C38 and C40 alkadien-3-ones and alkatrien-3-ones. They are distinguished from the vast majority of alkenone distributions reported in lake sediments from polar and temperate latitudes (e.g., Greenland, N. and S. America, Europe, Russia, China, S. Africa, Antarctica) by the absence of tetraunsaturated alkenones (e.g., C37:4). The alkenone distributions vary stratigraphically with higher concentrations in the drier intervals of the precession-scale cycles that exhibit characteristics resembling those of Group II haptophytes found in contemporary saline/alkaline environments rather than those of Group I haptophytes from freshwater ecosystems. Paleolake Olduvai represents the first reported occurrence of alkenones in a low-latitude Pleistocene lake, where the absence of alkatetraenones may reflect contributions from strains of Group II haptophytes that are favored by warmer temperatures or saline conditions. Temporal variations in unsaturation indices (U37K', U38K') and the proportions of alken-2-ones and alken-3-ones likely reflect influences on alkenone production by haptophytes that are associated with changes in the lake environment primarily driven by hydrologic cycles (e.g., salinity and alkalinity) rather than lake temperatures.

Formula omitted] Expands the linear dynamic range of the alkenone sea surface temperature proxy

The alkenone paleothermometer, U37K′, has been used to estimate past sea surface temperatures (SST) for nearly forty years. Despite its popularity, U37K′ is conventionally limited to water temperatures between 5 and 28.5 °C. At both low and high temperature ranges [0–8; >24 °C], U37K′ displays nonlinear behavior and significant scattering, leading to major uncertainties in reconstructed SSTs. With data from 171 surface sediments across the global ocean, we demonstrate that U38MEK′, an unsaturation index based on the ratio of di and tri unsaturated methyl alkenones with 38 carbon atoms (C38ME), extends the linear dynamic range of alkenone SST estimates from 0 to 29.5 °C SST. Accurate measurements of U38MEK′ are made possible by recent advances in chromatographic techniques that provide excellent resolution of C38 alkenones, as well as improved separation of all alkenone homologues. Our newly calibrated U38MEK′ permits more accurate SST reconstruction in warmer ocean waters and climates. U38MEK′ also carries less risk than U37K′ of biased SST estimates in regions (e.g. estuaries, closed seas) where both open ocean and coastal alkenone producers (e.g., Isochrysis galbana) are present because the latter do not produce C38 methyl ketones but do produce C37 methyl ketones. The efficacy of the U38MEK′ paleothermometer is shown by downcore analyses of early Pliocene age sediments from ODP sites 806 and 1143, as well as Holocene age sediments from the Black Sea. The U38ETK′ paleothermometer is also evaluated and found to be unsuitable for paleoceanographic applications.

Influence of environmental variability and Emiliania huxleyi ecotypes on alkenone-derived temperature reconstructions in the subantarctic Southern Ocean

Long-chain unsaturated alkenones produced by haptophyte algae are widely used as paleotemperature indicators. The unsaturation relationship to temperature is linear at mid-latitudes, however, non-linear responses detected in subpolar regions of both hemispheres have suggested complicating factors in these environments. To assess the influence of biotic and abiotic factors in alkenone production and preservation in the Subantarctic Zone, alkenone fluxes were quantified in three vertically-moored sediment traps deployed at the SOTS observatory (140°E, 47°S) during a year. Alkenone fluxes were compared with coccolithophore assemblages, satellite measurements and surface-water properties obtained by sensors at SOTS. Alkenone-based temperature reconstructions generally mirrored the seasonal variations of SSTs, except for late winter when significant deviations were observed (3–10 °C). Annual flux-weighted averages in the 3800 m trap returned alkenone-derived temperatures ~1.5 °C warmer than those derived from the 1000 m trap, a distortion attributed to surface production and signal preservation during its transit through the water column. Notably, changes in the relative abundance of E. huxleyi var. huxleyi were positively correlated with temperature deviations between the alkenone-derived temperatures and in situ SSTs (r = 0.6 and 0.7 at 1000 and 2000 m, respectively), while E. huxleyi var. aurorae, displayed an opposite trend. Our results suggest that E. huxleyi var. aurorae produces a higher proportion of C37:3 relative to C37:2 compared to its counterparts. Therefore, the dominance of var. aurorae south of the Subtropical Front could be at least partially responsible for the less accurate alkenone-based SST reconstructions in the Southern Ocean using global calibrations. However, the observed correlations were largely influenced by the samples collected during winter, a period characterized by low particle fluxes and slow sinking rates. Thus, it is likely that other factors such as selective degradation of the most unsaturated alkenones could also account for the deviations of the alkenone paleothermometer.

Abnormally warm-saline alkenone signature in Lake Tuosu in centennial cold-wet periods over the late Holocene: An exceptional case for alkenone application

Paleoclimatic reconstructions on the arid/semi-arid northern Tibetan Plateau are important for understanding the complex interactions between the mid-latitude westerly and subtropical Asian monsoon circulations. The development of paleoclimatic reconstructions largely relies on the invention and application of proper quantitative/semi-quantitative temperature and salinity proxies. Over the last few decades, alkenones have shown great potential as indicators of the past temperatures and salinities of lakes on the northern Tibetan Plateau. Several alkenone-based temperature and salinity records have confirmed centennial warm-dry and cold-wet variations during the Holocene on the northern Tibetan Plateau. Herein, we present alkenone-based records from Lake Tuosu in the Qaidam Basin over the last ∼1200 years. The alkenone signals show similar characteristics to published alkenone records in the nearby region during centennial warm periods (the Medieval Warm Period and Current Warm Period). However, the alkenones from Lake Tuosu demonstrate abnormally warm and saline signals during centennial cold periods (mainly during the Little Ice Age), in contrast to the widely recognized cold-wet conditions in the Qaidam Basin. Accordingly, the alkenone records from Lake Tuosu provide an exceptional case for the application of alkenones. The abnormally warm and saline alkenone signals from Lake Tuosu during centennial cold periods cannot be explained by either the presence of the C37:3 alkenone isomer or seasonal bias in alkenone production. Changes in species composition might be an alternative reason for this, which requires a further investigation of the sources of alkenones in lakes in the future.

Carbonate dissolution and environmental parameters govern coccolith vs. alkenone abundances in surface sediments from the northwest North Atlantic

We determined the abundances and concentrations of coccoliths and alkenones in 66 surface sediment samples from the northwest North Atlantic to evaluate the role that surface ocean temperature, salinity, sea-ice cover, and productivity have on the regional distribution of these two biological remains produced by haptophytes in the photic zone. In areas with sea-ice cover of more than 1 month per year, coccolith and alkenone concentrations in sediments are extremely low to nil. Elsewhere, the distribution of coccolith taxa generally displays strong relationships to water temperature, salinity, and productivity. Coccolithus pelagicus is associated with low summer sea-surface temperatures (<8°C) and relatively high summer sea-surface salinities (>33.5), whereas Helicosphaera carteri seems to follow the path of the North Atlantic Drift. The distribution of Emiliania huxleyi, the dominant alkenone producer, is not strongly correlated with that of alkenones. Calcite dissolution in shelf sediments could explain the occurrence of alkenones in the absence of coccoliths but alkenone production by non-calcifying haptophytes seems to also exert some control on alkenone concentrations in surface sediments, thus blurring alkenone abundance links to coccolithophorid production and their relative preservation.

Carbon Isotope Fractionation in Noelaerhabdaceae Algae in Culture and a Critical Evaluation of the Alkenone Paleobarometer

AbstractThe carbon isotope fractionation in algal organic matter (εp), including the long‐chain alkenones produced by the coccolithophorid family Noelaerhabdaceae, is used to reconstruct past atmospheric CO2 levels. The conventional proxy linearly relates εp to changes in cellular carbon demand relative to diffusive CO2 supply, with larger εp values occurring at lower carbon demand relative to supply (i.e., abundant CO2). However, the response of Gephyrocapsa oceanica, one of the dominant alkenone producers of the last few million years, has not been studied closely. Here, we subject G. oceanica to various CO2 levels by increasing pCO2 in the culture headspace, as opposed to increasing dissolved inorganic carbon (DIC) and alkalinity concentrations at constant pH. We note no substantial change in physiology, but observe an increase in εp as carbon demand relative to supply decreases, consistent with DIC manipulations. We compile existing Noelaerhabdaceae εp data and show that the diffusive model poorly describes the data. A meta‐analysis of individual treatments (unique combinations of lab, strain, and light conditions) shows that the slope of the εp response depends on the light conditions and range of carbon demand relative to CO2 supply in the treatment, which is incompatible with the diffusive model. We model εp as a multilinear function of key physiological and environmental variables and find that both photoperiod duration and light intensity are critical parameters, in addition to CO2 and cell size. While alkenone carbon isotope ratios indeed record CO2 information, irradiance and other factors are also necessary to properly describe alkenone εp.

Evaluation of reconstructed sea surface temperatures based on [formula omitted] from sediment surface samples of the North Pacific

The alkenone unsaturation index (U37k′) as proxy for sea surface temperature (SST) is an important tool in paleoclimatology for reconstructing past ocean temperature variability. Typically, U37k′ recorded in marine surface sediments shows a linear correlation with modern mean annual SST. However, in high-latitude oceanic regions, such as the subpolar Pacific, U37k′-based SSTs do overestimate the mean annual temperature by up to 6 °C, potentially leading to obscured paleoclimatic information drawn from stratigraphic U37k′-records. The reason for this “warm bias” is still not well understood. Here, we present a compilation of 97 sediment surface samples from Multicores collected in the Bering Sea, the Okhotsk Sea and the North Pacific to evaluate the alkenone-temperature proxy against observational data from the North Pacific. Sediment surface samples were analysed for alkenones and the derived U37k′-indices converted to water temperatures using different calibration equations established in the literature. U37k′-based SSTs were then compared to instrumental SST data, as well as modern alkenone flux data from sediment traps in the North Pacific. Our results confirm that most U37k′-based SSTs from the subpolar Pacific are 2–6 °C too warm compared to instrumental mean annual SSTs for calibrations applied. However, with an uncertainty at the level of ±1.5 °C or less reconstructed SSTs fit quite well to modern autumn temperatures north of the Subarctic Front (SAF), when maximum export flux of alkenones to the seafloor is indicated by sediment trap data. South of the SAF, reconstructed SSTs largely mimic the modern mean annual SST signal with an uncertainty of ±1.5 °C or less, which is likely due to the attenuation of seasonality and longer growth season of coccolithophorids according to sediment trap data. Our study further demonstrates that U37k′, when seasonality in alkenone production and export are known and considered, is able to provide reasonable estimates of SSTs in modern high-latitude ocean settings. We conduct a case study using available alkenone time-series derived from a sediment core collected from the south-western Okhotsk Sea to better understand the potential effect of seasonality in alkenone production on stratigraphic U37k′-record in the subpolar Pacific. The case study from the Okhotsk Sea indicates that even a small shift in seasonality may lead to strongly biased SSTs with broader regional implications for paleoclimate reconstructions in high-latitude ocean settings.

Appraisal of alkenone- and archaeal ether-based salinity indicators in mid-latitude Asian lakes

Lake water salinity, an important indicator of lake hydrological conditions, is critical to deciphering terrestrial paleoclimatic and paleoenvironmental changes. The proportion of C37:4 alkenone to total C37 alkenones (%C37:4) and the relative abundance of archaeol to caldarchaeol (ACE) are promising salinity indices in lacustrine settings. Developing regional calibrations for both indicators is challenging due to limited datasets and a poor understanding of the factors that influence them. Here we present %C37:4 and ACE data collected from 55 lakes in mid-latitude Asia with a wide salinity range, to evaluate how well the two proxies can be correlated with salinity and to infer potential influencing factors. The %C37:4 values only show a weak negative correlation with salinity from all investigated lakes. The occurrence of C37:3 alkenone isomer and abnormal values in the alkenone C37/C38 ratio might signal species shifts or environmental factors that compromise the %C37:4-salinity relationship in our data set. Seasonal bias in alkenone production could further obscure this %C37:4-salinity relationship. A stronger relationship emerges after removing samples influenced by these factors: %C37:4=−8.56⁎log10 (salinity) + 80.6, r2=0.62, n=37; or %C37:4=−13.46⁎log10 (salinity) + 101.48, with uncertainty in both variables considered. The ACE values show a strong positive correlation with salinity from all investigated lakes: ACE=2.27⁎10−4⁎salinity+25.4, r2=0.75, n=68; or ACE=1.86⁎10−4⁎salinity+38.1, with uncertainty in both variables considered. However, substantial deviations of the ACE values in low salinity range (<60,000 mg/L) and majority of the ACE shift in the range of 60,000–100,000 mg/L suggest that the ACE primarily responds to Euryarchaeota/Archaea community changes when a salinity threshold is crossed. Accordingly, both lake salinity indicators have the potential to reconstruct past salinity changes when their influencing factors could be constrained, and the two independent proxies, when they are used together, could further refine salinity reconstructions.

Reconstruction of Holocene Optimum paleoclimatic variations using long-chain n-alkanes and alkenones in sediments from Dabusu Lake, northeastern China

Long-chain n-alkanes and alkenones were analyzed in sediments from Dabusu Lake, northeastern China, from ca. 7.2–4.5 kyr BP (the Holocene Optimum) and the present. Long-chain n-alkanes are derived from leaf waxes of terrestrial vascular plants and aquatic macrophytes. Using n-alkane proxies such as Paq and average chain length (ACL) indices, variations in paleovegetation were reconstructed. The Paq values were generally low, indicating that aquatic macrophytes were a minor floral component in Dabusu Lake. The ACL data indicated that forest/grassland mixed vegetation occurred around the lake. From the alkenone distribution patterns (C37/C38, C40/C37, and %C37:4 ratios), the main alkenone producers were inferred to be Group II producers, especially Ruttnera lamellosa. We estimated water temperatures based on alkenone unsaturation indices (UK37 and UK’’37) using the temperature calibrations obtained from culture strains of R. lamellosa. The variation pattern of the UK’’37-based water temperatures was almost similar to that of the UK37-based pattern, although some spikes differed between the temperatures based on the two indices. The UK’’37-based water temperatures were lower from 7.2 to 6.2 kyr BP, and subsequently increased after 6.2 kyr BP. The alkenone-based temperatures were highest from 5.5 to 5.3 kyr BP, indicating that this maximal stage was the Holocene Thermal Maximum in the Dabusu region. The markedly decreasing spikes of UK’’37-based water temperatures in our study likely corresponded to the cooling events during the Holocene Optimum, as previously reported, especially Asian monsoon events (AM) 1 and Bond event 4 (BE4). Also, the increasing spike (7.2 kyr BP) in temperatures was possibly associated with the warming regime just after AM2. Moreover, the rapid increasing spikes of alkenone-based temperatures corresponded to those of Paq at 7.2 kyr BP (just after AM2) and 6.4 kyr BP. These results imply that the rapid warming reconstructed using alkenone-based temperatures might have resulted in increased precipitation around the lake at these ages, especially just after the AM2 cooling.

Genomic and geochemical identification of the long-chain alkenone producers in the estuarine Lake Takahoko, Japan: Implications for temperature reconstructions

Identifying the lacustrine haptophyte species that produce long-chain alkenones (LCAs) is essential for using alkenone unsaturation ratios to create lake water temperature reconstructions. We discovered LCAs in the brackish Lake Takahoko in northern Japan. The identity of LCA-producing species was investigated using 18S ribosomal DNA (rDNA) and organic geochemical analysis. Two distinct genetic groups, termed Tak-A and Tak-B, were identified within the Group II haptophyte phylotype. Tak-A was closely related to Hap-A, which was obtained from Lake George, USA; and Tak-B was identified as Isochrysis galbana. Because Hap-A and Isochrysis galbana have similar temperature calibrations, Tak-A and Tak-B were also expected to share similar calibrations. Therefore, the changes of their relative abundances in the lake should not significantly disturb paleotemperature reconstructions. The alkenone temperature recorded in the surface sediment corresponded to the lake temperature in early to late summer. This is likely related to the haptophyte bloom season in Lake Takahoko suggesting that this lake may be a viable location for creating a lacustrine alkenone paleotemperature record.

Diversity of long-chain alkenone producers—Developments of temperature proxy in inland waters—

Diversity of long-chain alkenone producers—Developments of temperature proxy in inland waters—

Climatic reconstruction at the Sannai-Maruyama site between Bond events 4 and 3\u2014implication for the collapse of the society at 4.2\u2009ka event

The Jōmon period/culture corresponds to the Neolithic period/culture in Japanese prehistory. The Sannai-Maruyama site (5.9–4.2 cal. kyr BP), the most famous, the largest, and the well-studied mid-Holocene (mid-Jōmon) archeological site inhabited by hunter-gatherers with sedentary lifestyle in northern Japan, started at early Bond event 4 and collapsed at late Bond event 3 (4.2 cal. kyr BP at the boundary between mid-Holocene, Northgrippian, and late-Holocene, Meghalayan), synchronous with the decline of the north Mesopotamian civilization and the Yangtze River civilization in China. Alkenone sea surface temperatures (SSTs), a proxy for early-midsummer SSTs, generally suggest that the early-midsummer SSTs (and atmospheric temperatures (ATs)) at 41° 00′ N, 140° 46′ E, about 20 km north to the Sannai-Maruyama site, located in Aomori Prefecture, peaked around 4.8–4.3 cal. kyr BP and showed minima at 5.9 and 4.1 cal. kyr BP. In spite of some discrepancy in short periods, this feature is consistent with that estimated from the assemblages of Ostracodas. δ18O value of benthic foraminifera of Nonionellina labradorica and Nonionella stella, alkenone production flux, and pollen assemblages could reflect annual-based temperature, which generally suggests that the climate was warmer at 6.0–4.2 cal. kyr BP, which could show the warmer environments at 6.0–5.0 cal. than expected from alkenone SST in early-midsummer. Overall, northward shift of the westerly jet, in association with a strengthened East Asian Summer Monsoon, could cause a relatively warm climate around 6.0–4.3 cal. kyr BP, when the Sannai-Maruyama site flourished. High food production density, by effective hansaibai (selective preservation or growth) in Castanea- and Aesculus-dominated forests, up to one sixth of the rice production density, could have supported high population density, resulting large community at the Sannai-Maruyama site. Cooling episode at 4.2 cal. kyr BP could have resulted in the decline of chestnut hansaibai, leading to the collapse of the site. Recent results from a compiled archeological site map suggested no large decline of the population but, instead, a dispersal to the surrounding area at 4.2 cal. kyr BP. It is consistent with ancestral population dynamics for the descendent from Jōmon people, in contrast to those from the immigrants from Far East Asia to the Japanese Archipelago with paddy rice cultivation technology after 2.9 cal. kyr BP, based on modern Japanese molecular sequences.

New insights into environmental controls on the occurrence and abundance of Group I alkenones and their paleoclimate applications: Evidence from volcanic lakes of northeastern China

The distinctive long-chain alkenones (LCAs) produced by Group I Isochrysidales from freshwater and oligohaline lakes have great potential for quantitative paleotemperature reconstructions. The widespread application of sedimentary Group I LCAs, however, is hampered by an incomplete understanding of the environmental controls on the occurrence of Group I LCAs in freshwater lakes. The correspondence between Group I LCA concentrations and pH (6.2–8.5) in northern Alaskan freshwater lakes (Longo et al., 2016) suggests that Group I Isochrysidales may preferentially thrive in freshwater lakes with high pH. Here, we systematically study LCA distributions, haptophyte-specific 18S rDNA sequences, and concentrations of major ions and trace elements in 18 freshwater volcanic lakes in northeastern China with an extended pH range from 7.17 to 9.99. We find that 11 of the 18 lakes examined contain Group I LCAs and the corresponding DNA sequences of their producers. Our DNA results indicate that the dominant alkenone producer in all 11 lakes is closely related to the Group I Greenland OTU 5 genotype, with the exception of two anthropogenically impacted lakes where small numbers of Group II sequences are found. Statistical analyses indicate that the highest concentrations of Group I LCAs are found in oligotrophic freshwater lakes with pH ranging from ∼7.3 to 8.8. We find that elevated concentrations of certain trace elements may lead to the disappearance of Group I LCAs despite lake water falling within the optimal pH range. Together with previously published Group I LCA data from the temperature calibration in Northern Hemisphere freshwater lakes (Longo et al., 2018), we find, for the first time, that Group I R3b (R3b = C37:3b/(C38:3bEt + C37:3b)) values are most sensitive to winter temperature changes when mean winter temperature is higher than ∼−20°C. Our results suggest that the freshwater volcanic lakes in northeastern China are highly valuable targets for paleotemperature reconstructions using Group I LCAs.