Alkenone Unsaturation Research Articles

Isolation and characterization of an alkenone composition mutant in the haptophyte Tisochrysis lutea

While long-chain alkenone (defined as alkenone) is known to have a potential for industrial application, its biosynthetic pathway is still largely unknown. In the present study, we obtained a novel mutant named C8–14 strain altering alkenone composition with heavy-ion beam irradiation to the haptophyte Tisochrysis lutea. This mutant showed higher levels of tri-unsaturated alkenones and significantly lower amounts of alkenones and alkenoates than the wild-type cells. In addition, the total amounts of fatty acids in the neural lipid fraction of the C8–14 strain were higher than those in the wild-type cells. The primary sequence of the genes related to alkenone desaturases in the C8–14 strain, including the paralogous genes found in this study, were identical to those in the wild-type cells. However, the expression level of the putative alkenone desaturase gene was altered. The expression of the TOD-2 gene was slightly higher in the C8–14 cells than in the wild-type cells. This is the first report, to the best of our knowledge, to create the mutant with the altered unsaturation of alkenones.

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.

The Relative Stability of Planktic Foraminifer Thermal Preferences over the Past 3 Million Years

Stationarity of species’ ecological tolerances is a first-order assumption of paleoenvironmental reconstruction based upon analog methods. To test this and other assumptions used in quantitative analysis of foraminiferal faunas for paleoceanographic reconstruction, we analyzed paired alkenone unsaturation ratio (U37K′) sea surface temperature (SST) estimates and relative abundances of planktic foraminifera within Late Pliocene assemblages. We established Pliocene temperature preferences for nine species in the North Atlantic: Dentoglobigerina altispira, Globorotalia menardii, Globoconella puncticulata, Neogloboquadrina atlantica, Neogloboquadrina incompta, Neogloboquadrina pachyderma, Trilobatus sacculifer, Globigerinita glutinata, and Globigerina bulloides. We compared these to the temperature preferences of the same extant species, and in the three cases where the species are now extinct (Dentoglobigerina altispira, Neogloboquadrina atlantica, and Globoconella puncticulata), comparisons were made to either the descendant species or other modern species commonly used as analogs. In general, the taxa tested show similar temperature responses in both Late Pliocene and present-day (core-top) distributions. The data from these comparisons are mostly encouraging, supporting past paleoceanographic conclusions, and are otherwise valuable for testing previous taxonomic grouping decisions that are often necessary for interpreting the paleoenvironment based upon Pliocene foraminiferal assemblages.

Holocene centennial variability in sea surface temperature and linkage with solar irradiance

The climate periodically fluctuates on various time scales, however, there remains a lack of consensus on the centennial-scale variabilities and associated driving force. A continuous high-resolution sea surface temperature (SST) record allows for the detection of centennial-scale fluctuations. This study presents a high-resolution SST record covering the last 10,000 years based on the analysis of the alkenone unsaturation index in marine sediment cores off the southwest coast of the Korean Peninsula. Alkenone SST's spectral and wavelet analysis revealed significant periodicities of 414, 190, 135, 102, and 89 years at a > 90% confidence level. These cycles exhibit extreme proximity to the solar activity cycles of 353, 206 (Suess/de Vries cycles), 130, and 104–87 years (Gleissberg cycles), suggesting that the multidecadal to centennial variations in SST are linked to solar forcing. To the best of our knowledge, this is the first high-resolution Holocene SST record that all solar activity cycles on centennial scale match, suggesting centennial-scale variability in the climate system and illustrating the role of solar activity on SST change in the mid-latitude region of the Northern Hemisphere.

Long-chain alkenones in the Shimosa Group reveal palaeotemperatures of the Pleistocene interglacial Palaeo-Tokyo Bays

The Shimosa Group, a Middle- to Late-Pleistocene sedimentary succession, has been the focus of stratigraphic attention because it lies beneath the Tokyo metropolitan area of central Japan. It is also of palaeoclimatic significance because it contains important interglacial marine strata of the past 450,000 years. Because the marine strata of the Shimosa Group were formed in the shallow inner bay known as the Palaeo-Tokyo Bay, rare occurrences of planktonic foraminifera make it difficult to quantitatively reconstruct the palaeo-sea surface temperatures (SSTs). Here, we extracted long-chain alkenones (LCAs) from the core GS-UR-1 penetrating the Shimosa Group to Marine Isotope Stage (MIS) 11. We found that the alkenone unsaturation ratio appears to reflect the SST of the Palaeo-Tokyo Bay formed during the peaks of MISs 5e, 7e, 9, and 11, which was consistent with the inflowing water mass changes inferred from the benthic foraminiferal assemblages. The palaeo-SSTs during each interglacial period were 2–3 °C higher than the pre-industrial levels of Tokyo Bay and seemed to reach a level similar to that of the Holocene thermal maximum. The findings of this study demonstrate that the LCA-based proxy, which has not before been utilised in studies on the Shimosa Group, has strong potential to provide palaeoceanic and stratigraphic information.

Coccolith Sr/Ca is a robust temperature and growth rate indicator that withstands dynamic microbial interactions.

Coccolithophores are a diverse group of calcifying microalgae that have left a prominent fossil record on Earth. Various coccolithophore relics, both organic and inorganic, serve as proxies for reconstruction of past oceanic conditions. Emiliania huxleyi is the most widely distributed representative of the coccolithophores in modern oceans and is known to engage in dynamic interactions with bacteria. Algal–bacterial interactions influence various aspects of algal physiology and alter algal alkenone unsaturation (UK’ 37), a frequently used organic coccolithophore‐derived paleo‐temperature proxy. Whether algal–bacterial interactions influence inorganic coccolithophore‐derived paleo‐proxies is yet unknown. A commonly used inorganic proxy for past productivity and sea surface temperature is the Sr/Ca ratio of the coccolith calcite. Interestingly, during interactions between bacteria and a population of calcifying algae, bacteria were shown to physically attach only to non‐calcified algal cells, suggesting an influence on algal calcification. In this study, we explore the effects of algal–bacterial interactions on calcification and coccolith Sr/Ca ratios. We find that while bacteria attach only to non‐calcified algal cells, coccolith cell coverage and overall calcite production in algal populations with and without bacteria is similar. Furthermore, we find that Sr/Ca values are impacted only by water temperature and algal growth rate, regardless of bacterial influences on algal physiology. Our observations reinforce the robustness of coccolith Sr/Ca ratios as a paleo‐proxy independent of microbial interactions and highlight a fundamental difference between organic and inorganic paleo‐proxies.

Phylogeny, alkenone profiles and ecology of Isochrysidales subclades in saline lakes: Implications for paleosalinity and paleotemperature reconstructions

Long-chain alkenones (LCAs) produced by phylogenetically classified Groups 1 and 3 Isochrysidales are increasingly used for paleotemperature and/or paleosalinity reconstructions in oligohaline lakes and marine environments. However, there are considerable difficulties in the paleoenvironmental interpretation of LCAs from Group 2 Isochrysidales thriving in saline lakes. The biggest challenge lies in our poor understanding of the complexity and ecological niches of individual Group 2 subclades in saline lakes. Here, we perform comprehensive analysis of haptophyte-specific 18S rRNA sequences and distributions of LCAs, and long-chain alkenoates (LCEs) in surface sediments and suspended particulate matter (SPM) from 37 saline lakes in northern China. These lakes span a large salinity gradient from 0.5 to 308‰. Combined with published genomic data of Group 2 Isochrysidales, our phylogenetic analysis reveals three Group 2 subclades occupying distinct ecological niches: one ice-related bloomer Group 2i and two warm-season bloomers Groups 2w1 and 2w2. Group 2i, the earliest seasonal bloomer, frequently co-occurs with Group 2w1 in sediments from saline lakes with relatively low to intermediate salinity waters, whereas Group 2w2 blooms in hypersaline waters. Based on existing data, C39:4 methyl alkenone is a chemotaxonomic biomarker for Group 2i. %C37:4 (relative abundance of C37:4 to the total C37 alkenones) values in the three Group subclades follow the order: Group 2i > Group 2w2 > Group 2w1. The %C37:4 in sediment cores of saline lakes does not directly record past salinity changes, but instead reflects variable contributions in production by these three subclades. This could indirectly and partially reflect overall salinity changes in some lakes dominated by Groups 2i and 2w1, but can be more complicated in lakes dominated by other assemblages. For our sites, we also demonstrate that direct use of C37 alkenone unsaturation indices (U37K, U37K', and U37K'') for paleotemperature reconstructions in saline lakes is generally not feasible, except for cases where alkenone-producing Isochrysidales are dominated by one single species/subclade and seasonal production effects can be circumvented. We propose two possible alternative proxies for paleotemperature reconstructions in saline lakes: 1) unsaturation ratios of C41 and C42 alkenones, as these compounds are predominantly produced by a limited number of Group 2 species, such as Isochrysis nuda (Liao et al., 2020); 2) C38Et/C36OEt ratio (ratio of C38 ethyl alkenones and C36 ethyl alkenoates), which appears to have similar temperature sensitivity for Groups 2w1 and 2w2, in lakes with no Group 2i inputs. Our study provides new insights into the phylogenetic classifications of Group 2 Isochrysidales and their ecological/environmental niches, which are fundamental for more quantitative and rigorous applications of LCAs and LCEs in saline lakes as paleosalinity and paleotemperature proxies.

Enhanced climate variability during the last millennium recorded in alkenone sea surface temperatures of the northwest Pacific margin

Previous studies on surface temperature reconstructions for the last 2000 years (2 k) revealed a long-term cooling trend for the last millennium in comparison to the previous millennium. However, knowledge on the decadal- to centennial-scale variability in sea surface temperature and the underlying governing mechanisms throughout the period is limited. We reconstructed high-resolution continuous sea surface temperature changes over the last 2 k in the northwest Pacific margin based on the alkenone unsaturation index. Our alkenone temperature record revealed enhanced and more rapidly changing climate variability during the last millennium (approximately 1200–1850 Common Era) than during the previous millennium. Cold and hot extremes also occurred more frequently during the last millennium. The enhanced and rapidly changing climate variability appears to be associated with frequent volcanic eruptions and grand solar minima. The reconstructed surface temperature variability tends to be associated with variations in the East Asia summer monsoon and the Pacific Decadal Oscillation, implying that these variations are also enhanced in the last millennium than in the previous millennium.

Biomarkers in the rock outcrop of the Kazusa Group reveal palaeoenvironments of the Kuroshio region

Long-chain alkenones and n-alkanes preserved in marine and lake sediment cores are widely used to reconstruct palaeoenvironments. However, applying this technique to exposed sedimentary rock sequences is relatively challenging due to the potential for the diagenetic alteration of organic biomarkers. Here, we extract long-chain alkenones and n-alkanes from an exposed outcrop of the Kazusa Group in central Japan, one of the most continuous sedimentary successions in the world, covering almost the entire Pleistocene. We find that the alkenone unsaturation ratio and average chain length of n-alkanes appears to reflect the glacial-interglacial changes in sea surface temperature and terrestrial climate, respectively. Alkenone-based sea surface temperatures between 1.1 and 1.0 million years ago concur with foraminiferal Mg/Ca-based temperature estimates and may reflect an intrusion of the Kuroshio Current. We suggest that the preservation of these biomarkers in the Kazusa Group demonstrates its potential to provide a detailed palaeoenvironmental record.

Bihemispheric Warming in the Miocene Climatic Optimum as Seen From the Danish North Sea

AbstractThe Miocene epoch witnessed major changes in climate. The marine oxygen isotope record, our best single continuous representation of the time interval, contains large shifts indicating substantial changes in the glaciation of Antarctica and/or deep ocean temperatures, with the interval of the most depleted isotopic composition occurring between ~18 and 13.9 Ma (mid‐Miocene Climatic Optimum [MCO]). We present here a record of alkenone unsaturation estimates from a borehole penetrating marine sediments spanning most of the Miocene at ~56°N. Marine temperatures reconstructed for the Miocene are up to 20°C warmer than present day. A maximum occurs at the time of the MCO. The record is strikingly similar to discontinuous alkenone results published from other North Atlantic sites. As in the southern hemisphere, ocean temperature gradients were extraordinarily low in the North Atlantic in comparison to today. Our findings suggest that the MCO was a global temperature maximum and link the partial deglaciation of the Antarctic and subsequent “refrigeration” to a global forcing process. However, the small magnitude of the ocean cooling from ~14 to 10 Ma suggests a highly nonlinear (threshold) relationship between East Antarctic ice volume and global temperature.

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.

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.

Simultaneous extraction and chromatographic separation of n-alkanes and alkenones from glycerol dialkyl glycerol tetraethers via selective Accelerated Solvent Extraction

A method is described for simultaneous extraction and chromatographic separation of saturated hydrocarbons (n-alkanes) and unsaturated long-chain ketones (alkenones) from glycerol dialkyl glycerol tetraethers (GDGTs) using selective accelerated solvent extraction (ASE). We show that the method can be used to successfully separate n-alkanes and alkenones from GDGTs in marine sediment samples and n-alkanes from GDGTs in lake sediments and soil samples. A comparison of our method with analysis of samples purified using classical silica column chromatography demonstrates that the new method does not affect the alkenone unsaturation index (U37K'), the TetraEther indeX (TEX86), the Methane Index (MI), the Ring Index (RI), the branched-GDGT-based paleotemperature and soil pH indices, the Branched and Isoprenoid Tetraether (BIT) index, the n-alkane average chain length (ACL) or the carbon preference index (CPI). The new procedure results in higher sample recoveries for n-alkanes and more precise U37K' estimates at low concentrations, and contributes to reduced analyst laboratory time.

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.

High time-resolution alkenone paleotemperature variations in Tokyo Bay during the Meghalayan: Implications for cold climates and social unrest in Japan

The regional paleoenvironmental record of Tokyo, central Japan, is required to enrich our understanding of climate change in East Asia and its impact on Japanese civilizations. Previous studies have generated paleotemperature records based on the analysis of shell mounds along the coast of Tokyo Bay and pollen from terrestrial borehole cores. However, a continuous, high time-resolution, quantitative paleotemperature record does not exist. A piston core was recovered from Tokyo Bay from which 22 mollusk shells were extracted. An age model for the core, determined via accelerator mass spectroscopy (AMS) 14C dating of the shells and a scoria layer from the Hoei eruption (1707 CE), showed that the core contains continuous record from ca. 2400 BCE. We analyzed the alkenone unsaturation ratios (UK’37) to reconstruct the variations in the paleotemperature. The temperature during the Meghalayan was generally warmer than that at present exhibiting a declining trend, which roughly matches the changes in the orbital-forcing summer insolation and the millennial-scale latitudinal shift of the subarctic front in the northeastern Pacific Ocean. The largest cold period, which occurred ca. 2300 BCE and had a minimum temperature of 19.5 °C, interrupted the warm conditions. This cold period may correspond to the global cooling/drying “4.2 ka BP event,” which marks the boundary between the Northgrippian and Meghalayan. Our results also revealed that several cold periods occurred, which may have been caused by decreases in solar activity or large volcanic eruptions. Particularly, large decadal to centennial-scale cold periods were observed ca. 440 BCE and 100 CE during the Yayoi era (ca. 900 BCE–300 CE). The coldest period after the establishment of a centralized system of government (ca. 300 CE–present) was recorded ca. 1050 CE, which may have been caused by the combination of a strong El-Niño mode and reduced solar activity. These climate change events can all be linked and, therefore, may be the potential causes of significant periods of famine and civil war in Japanese history, which eventually led to major shifts in Japanese social systems.

Paleoecological and recent data show a steady temporal evolution of carbon dioxide and temperature

There is much debate today about the role of carbon dioxide in global warming. In this study we examine the intrinsic dynamics of the temporal evolution of the carbon dioxide amount and temperature by analyzing instrumental measurements data and paleo-reconstructed data to see if this dynamics remains stable in terms of its internal properties. The paleoecological data were based on over 20,000 sea surface temperature point reconstructions obtained from ocean sediment cores using alkenone unsaturation indices, applying ratios of Mg/Ca in planktonic foraminifera, and based upon microfossil abundances. The results of the analysis of the instrumental data fluctuations in land-ocean temperature and carbon dioxide in the period March 1958–April 2017, showed that the long-term dynamics of the fluctuations of these two parameters exhibit persistent power-law type behavior. This type of behavior also emerges from the same analysis of the reconstructed global mean sea temperature and carbon dioxide amount fluctuations over the past 805 thousand years. In addition, the time series of both parameters are characterized by multifractality for time scales of less than 4 years. Consequently, the intrinsic properties of carbon dioxide and global temperature have not changed over the past nearly one million years.

Two long and pronounced cold periods 3,000–5,000 and 6,600–8,400 years B.P. in East Asia and the southward migration of the westerly jet

To reconstruct the variations in sea surface temperature (SST) during the Holocene, the alkenone unsaturation index of marine sediments from two deep-drilled cores and 81 different core-top surface sediments recovered from the Heuksan Mud Belt, located off the southwestern coast of the Korean Peninsula, was measured. First, comparison of the alkenone temperature estimates of 81 core-top sediments with in situ temperatures indicated that the alkenone temperatures corresponded to the average SST in April to October. The spatial distribution pattern of the core-top alkenone SST showed a north–south temperature gradient, which represented the in situ temperatures well. This indicated that the effects of the resuspension and lateral transport of sediments on the past temperature estimation might be insignificant. Based on the two deep-drilled cores, variations in the alkenone SST during the Holocene were reconstructed. In general, the alkenone SST decreased by less than 1 °C from the early to late Holocene. An interesting feature was the presence of two long and pronounced cold (approximately 2 °C) periods, which occurred at 3–5 kyr and 6.6–8.4 kyr B.P. These cold periods were also observed in other mid-latitude regions in East Asia, indicating that the cooling pattern was regional. The cold periods appeared to be associated with the southward migration of the westerly jet at that time. Investigations into the effects of local sea level changes and resultant changes in the tidal regime on changes in the SST during the Holocene period suggested that they were insignificant in the study area.

Sea-surface temperature, productivity and hydrological changes in the Northern Indian Ocean (Maldives) during the interval ~575-175 ka (MIS 14 to 7)

The South Asian Monsoon (SAM) drives seasonal changes in the atmospheric and ocean circulation of the tropical Indian Ocean, affecting precipitation on land and oceanic primary productivity. This work examined sediments from the International Ocean Discover Program (IODP) Site U1467 (IODP Expedition 359) located in the Maldives Inner Sea (Northern Indian Ocean) at a water depth of 487 m. The Maldives Inner Sea is a perfect location to study past changes in tropical climate and ocean circulation related to monsoon dynamics in the Equatorial Indian Ocean. This study focuses on the ~575-175 ka interval, from Marine Isotope Stage (MIS) 14 to 7, an important period for human evolution and dispersal to Eurasia.We reconstructed sea-surface temperature (SST), based on alkenone unsaturation index (U37K'), hydrological changes, based on terrestrial input of n-alkanes, and past surface ocean productivity, based on total C37 alkenones concentration. The U37K' -SST record shows a difference of about 1.5 °C between glacial and interglacial periods, clearly showing all stages and substages from MIS 14 to 7, and revealing a connection between ice sheets extension and SST at the equatorial region. The n-alkanes concentration and average chain length index indicate vegetation changes at the Indian Peninsula with drier conditions generally associated with glacial periods. Precipitation increases abruptly at the end of terminations, lasting for a variable time interval in each interglacial period, except for MIS 13. However, other mechanisms superimposed to the glacial-interglacial forcing, such as the Indian Ocean Dipole (IOD) mode, have been invoked to explain shorter-scale variability in precipitation over India. The total alkenone concentration record indicates that primary productivity at Site U1467 is strongly associated with orbital changes, probably related to the summer inter-tropical insolation gradient (SITIG, 23N-23S on June 21st). High primary productivity occurred during intervals of low SITIG, which resulted in strong inter-monsoon (April-May and October-November) Indian Ocean Equatorial Westerlies (IEW) and reduced precipitation in the equatorial region. This mechanism may also be related to the IOD, which affects the strength of the IEW.

Successional blooms of alkenone‐producing haptophytes in Lake George, North Dakota: Implications for continental paleoclimate reconstructions

AbstractAlkenone‐derived paleotemperature reconstruction holds great promise in lake environments. However, the occurrence of multiple species of alkenone‐producing haptophyte algae in a single lake can complicate the translation of alkenone unsaturation to temperature if each species requires an individual temperature calibration. Here, we present the first systematic monitoring of two alkenone‐producing haptophytes throughout the course of a seasonal cycle in Lake George, North Dakota, using a combined approach of DNA sequencing and alkenone lipid characterization. Field sampling revealed a nonoverlapping haptophyte succession, with both an early and late season haptophyte bloom event. Culturing experiments demonstrated that the two haptophyte species responsible for these blooms had statistically similar alkenone‐temperature responses, although the culture‐based calibrations were distinct from the in situ calibration. Bloom timing of each haptophyte species corresponded to surface‐water temperatures that differed by more than 10°C, revealing that changes in bloom intensities for each species will skew the sediment‐inferred temperatures to a different stage of the growth season. These results highlight the importance of accounting for bloom timing when interpreting alkenone‐derived temperatures in sediment cores, especially in lakes that experience large seasonal fluctuations in water column temperature and salinity.

Midlatitude Temperature Variations in the Oligocene to Early Miocene

AbstractAntarctic ice sheet margin extent and the sensitivity of benthic δ18O to orbital forcing have varied on million‐year timescales during the Oligocene to Early Miocene. However, few sea surface temperature (SST) records for this time interval exist to evaluate links between polar processes and mean temperature outside polar regions. Here, we present a new record of SST for the time interval 30 to 17 Ma derived from the long‐chain alkenone unsaturation ratio ( ) at Integrated Ocean Drilling Program Site 1406A in the midlatitude North Atlantic. Results confirm that warm temperatures from 24°C to over 30°C prevailed in midlatitudes in this time and suggest a transition from colder early‐middle Oligocene to warmer average conditions after 24.5 Ma. The global significance of this transition is highlighted by the coincidence with changes in the dominance from marine‐ to terrestrial‐terminating ice sheets in the Ross Sea around Antarctica. The longest continuous section of the record (20.6 to 26.6 Ma) contains multiple 2 million‐year cycles in SST, potentially paced by long obliquity modulation. Complex and temporally varying relationships are observed between North Atlantic SST and benthic δ18O in paired samples; significant covariation is only observed around the Oligocene‐Miocene transition, coincident with a lower average marine ice extent. These North Atlantic temperature records provide a new context in which to examine the stability of climate and the Antarctic ice sheet during the Oligocene and early Miocene.