Alkenone Temperature Research Articles

Sclerochronological evidence of pronounced seasonality from the late Pliocene of the southern North Sea basin and its implications

Abstract. Oxygen isotope (δ18O) sclerochronology of benthic marine molluscs provides a means of reconstructing the seasonal range in seafloor temperature, subject to use of an appropriate equation relating shell δ18O to temperature and water δ18O, a reasonably accurate estimation of water δ18O, and due consideration of growth-rate effects. Taking these factors into account, δ18O data from late Pliocene bivalves of the southern North Sea basin (Belgium and the Netherlands) indicate a seasonal seafloor range a little smaller than now in the area. Microgrowth-increment data from Aequipecten opercularis, together with the species composition of the bivalve assemblage and aspects of preservation, suggest a setting below the summer thermocline for all but the latest material investigated. This implies a higher summer temperature at the surface than on the seafloor and consequently a greater seasonal range. A reasonable (3 ∘C) estimate of the difference between maximum seafloor and surface temperature under circumstances of summer stratification points to seasonal surface ranges in excess of the present value (12.4 ∘C nearby). Using a model-derived estimate of water δ18O (0.0 ‰), summer surface temperature was initially in the cool temperate range (<20 ∘C) and then (during the Mid-Piacenzian Warm Period; MPWP) increased into the warm temperate range (>20 ∘C) before reverting to cool temperate values (in conjunction with shallowing and a loss of summer stratification). This pattern is in agreement with biotic-assemblage evidence. Winter temperature was firmly in the cool temperate range (<10 ∘C) throughout, contrary to previous interpretations. Averaging of summer and winter surface temperatures for the MPWP provides a figure for annual sea surface temperature that is 2–3 ∘C higher than the present value (10.9 ∘C nearby) and in close agreement with a figure obtained by averaging alkenone and TEX86 temperatures for the MPWP from the Netherlands. These proxies, however, respectively, underestimate summer temperature and overestimate winter temperature, giving an incomplete picture of seasonality. A higher annual temperature than now is consistent with the notion of global warmth in the MPWP, but a low winter temperature in the southern North Sea basin suggests regional reduction in oceanic heat supply, contrasting with other interpretations of North Atlantic oceanography during the interval. Carbonate clumped isotope (Δ47) and biomineral unit thermometry offer means of checking the δ18O-based temperatures.

Earth-Moon Barycentre Excursions and Anomalous Quaternary Sea Level Highstands

Plate tectonics is driven by Earth-Moon barycentre shifts in the lower mantle. The eastern Canary Islands have geographic and geological conditions derived from the movements of the Central American plates. Some features of these islands are influenced by the rotation of the Earth from west to east in the evolution of the marine currents that surround them and the opening of the North Atlantic to the North Pole with little dependence of the glacial isostatic adjustment (GIA). In addition, their position with respect to the Tropic of Cancer and the African continent affect the north-south and east-west climatic change dynamics and their tectonic stability respectively. Dated lavas contain marine and aeolian deposits and some of the Pleistocene marine deposits indicate higher sea level in cooler circumstances, which is anomalous. Relating those marine deposits produced during the warmest interglacial, the last interglacial and the Holocene with their equivalents in the Southern Hemisphere, they reflect shifts in the barycentre. Thanks to Holocene radiocarbon, topographic and day length data and alkenone temperature, we describe a mechanism by which the oscillation of the Moon’s inclination (and declination) reaches extreme values (14º and 34º; about 4.9º more than current values) approximately every 1450 years. These values occur when there is a harmonic distortion in surface areas of the Earth’s crust as response associated with oscillations in the displacements of the barycentre of the Earth-Moon system. As the declination influences the movement of oceanic waters, there is also a relationship with the Bond Events of the North Atlantic, of unknown cause until now.

Extended chain length alkenoates differentiate three Isochrysidales groups

Alkenoates and alkenones are polyunsaturated long-chain esters and ketones produced by members of the Isochrysidales, an order of haptophyte microalgae. Based on phylogenetic data, members of Isochrysidales have been classified into three groups, with each group showing major differences in preferred growth environments (e.g., salinity). Systematic identification of chemotaxonomic differences among the three groups of Isochrysidales is thus important for paleoenvironmental reconstructions and disentangling mixed alkenone temperature signals. Here, we systematically examined alkenone/alkenoate profiles in culture samples of Group 2 and Group 3 Isochrysidales and environmental sample of Group1 Isochrysidales. Using GC columns with stationary phases of contrasting polarities, we discovered for the first time C37 and C38 ethyl alkenoates (C37OEt and C38OEt) in Group 2 and/or 3 Isochrysidales, extending the longest alkenoate chain length of C36OEt in previous publications. We show that Group 3 Isochrysidales only produce C37OEt, whereas Group 2 Isochrysidales produce both C37OEt and C38OEt alkenoates. Because Group 1 Isochrysidales do not produce alkenoates, C38OEt are specific biomarkers for Group 2 species. The identification of C37OEt and C38OEt in sediments from saline lakes and Chesapeake Bay where sequences of Group 2 DNA have been recovered further validates the use of C38OEt as a Group 2-specific biomarker.

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.

Asian Winter Monsoon Imprint on the Water Column Structure at the Northern South China Sea Coast

Coastal regions of the northern South China Sea (SCS) strongly interact with the Asian monsoon circulation (AMC). Thus, variations of sea surface temperature (SST) here are newly suggested to document AMC changes in an effective manner, but additional physical parameters of oceanic conditions, probably also in relation to the AMC system, remain poorly understood. In this study, we analyzed glycerol dialkyl glycerol tetraethers (GDGTs) from a well-dated sediment core YJ, retrieved at the northern SCS coast, to further scrutinize the intrinsic response of water column to winter AMC strength. It shows that within the time frame of past ∼1,000 years, the tetraether index of lipids with 86 carbon atoms (TEX86) and published alkenone (U37K′) temperature records together confirm a reduced thermal gradient during the Little Ice Age (LIA), in comparison to that during the Medieval Climate Anomaly (MCA). Considering concurrent variations of the branched and isoprenoid tetraether (BIT) and the ratio of archaeol to caldarchaeol (ACE), for example, with decreased values (<∼0.3) for the former and relatively high values for the latter at the LIA, indicative of stratification and salinity changes, respectively, these multiple lines of evidence thereby call for well mixing of onsite water at site YJ correspondingly. Our results suggest that winter AMC strength is a critical factor for mixing subsurface waters and modifying thermal/saline conditions at the northern SCS coasts through the last millennium and also, perhaps, on longer timescales.

Impacts of Norse settlement on terrestrial and aquatic ecosystems in Southwest Iceland

Norse colonization of North Atlantic islands in the 1st millennium of the Common Era led to drastic prehistoric environmental changes in these previously “pristine” landscapes. In Iceland, Norse settlement is associated with a rapid decline in birch trees and heightened soil erosion, yet the timing of Norse exploration in the North Atlantic coincided with large climate changes that also influenced Icelandic environments. To date, there are few records that disentangle climatic and human impacts on terrestrial ecosystems, and there has been very little work on the impacts of Norse arrival on Iceland’s aquatic ecosystems. Here we use a high-resolution lake-sediment record from Vestra Gislholtsvatn (VGHV), southwest Iceland to investigate these processes during the last 2,000 years. Norse arrival (c. 870 CE) in Iceland is followed by a rapid increase in sedimentation rate and a transition in leaf wax n-alkanes indicating a decrease in trees and expansion of grasses. This transition coincides with limnological changes, including increased primary productivity (i.e. C17 n-alkane and biogenic opal fluxes) and shifts in the haptophyte algal community. Many of these changes are still apparent today. Comparisons with a new winter-spring alkenone temperature reconstruction from VGHV and marine sea surface temperature records show little to no correlation between terrestrial and aquatic ecological changes and climate at this time. Similarly, volcanic eruptions recorded in VGHV are not associated with any long-term environmental changes. Rather, the VGHV record suggests that human settlement had a lasting impact on the catchment area of VGHV and changes within the lake ecosystem.

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.

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.

Changes in latitudinal sea surface temperature gradients along the Southern Chilean margin since the last glacial

The comparison between different climate model simulations of water hosing experiments under glacial conditions points toward diverging responses in Sea Surface Temperature changes (SST) especially in the Southeast Pacific (SEP). This suggests that reconstituting the latitudinal SST gradient in the SEP is a critical parameter for a better understanding of the mechanisms behind the abrupt climatic events since the last glacial period. Here we present, high-resolution records of SST, using planktonic foraminiferal assemblages and alkenone temperature reconstructions and stable oxygen and carbon isotopes from three deep-sea sediment cores along a latitudinal transect off the southern Chilean coast. This allowed us to reconstruct the variations of the latitudinal SST gradient and the Subtropical Front movements in the SEP. The SST results suggest a clear Antarctic timing consistent with the bipolar seesaw control, especially during the late glacial and the deglaciation. Our records do not suggest a complete oceanic heat transfer signal, highlighting the implication of an atmospheric component in the heat transfers between the two hemispheres, controlled by the latitudinal movements of the intertropical convergence zone in the Atlantic and the associated weaker South Pacific westerly split jet. Furthermore, our records indicate variable conditions during the Holocene, and also emphasize the influence of local fresh water inputs from the Patagonian ice sheet and/or precipitation on the SST fresh water input estimates along the Chilean margin (North and South of 49° S) from the onset of the deglaciation until 8 kyr cal. BP.

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.

Cooling Mediterranean Sea surface temperatures during the Late Miocene provide a climate context for evolutionary transitions in Africa and Eurasia

In the Late Miocene, grasslands proliferated, succulent plants diversified in the mid-latitudes, and the desert-like conditions appeared in the Sahara. Despite this major environmental change on land, the coeval deep-sea oxygen isotope record does not provide evidence for significant high latitude cooling or continental ice growth, making it difficult to relate widespread terrestrial environmental change to global climatic changes. A U37K′ –derived sea surface temperature (SST) reconstruction spanning 13 to 6 Ma from uplifted hemipelagic sediments in Northern Italy provides the first continuous mid-latitude temperature record with which to compare the evolution of aridity and biotic events at similar latitudes in Northern Africa and Pakistan. Between 13 and 8.8 Ma, Mediterranean SST lay near the upper limit of the alkenone temperature proxy (∼28 °C), exceeding modern SST at the site by as much as 10 °C. Throughout the record, sapropel layers correspond to local SST maxima, suggesting that Late Miocene hydrological conditions in the Mediterranean responded to insolation forcing via mechanisms similar to those documented for the Plio-Pleistocene. Mediterranean SST cooled rapidly beginning at ∼8 Ma, with an episode of intense cooling to ∼19 °C between 7.2 Ma and 6.6 Ma, followed by a rebound to ∼25 °C preceding the Messinian Salinity Crisis at 5.9 Ma. These observations establish, for the first time, a direct relationship between increasing aridity in the Northern hemisphere mid-latitudes and significant cooling. Evidently, this cooling was not accompanied by significant growth in continental ice volume. The extreme warmth and subsequent cooling of the Mediterranean Sea are not well-represented in current Late Miocene climate models, which our results suggest underestimate regional warmth prior to the Late Miocene cooling. Evidence of secular cooling during the Late Miocene gives new support to the much-debated link between a possible decline in atmospheric CO2 levels and Late Miocene changes in vegetation on land.

Sea surface temperature and salinity changes near the Soya Strait during the last 19 ka

Past sea surface temperature (SST) and salinity (SSS) changes in the northeastern East Sea/Japan Sea since 19 cal ka BP have been reconstructed by using an alkenone unsaturation index and the oxygen isotope ratio of planktonic foraminifera Neogloboquadrina pachyderma(s) in Core MR0604-PC03B. Coretop alkenone temperature and calcification temperature correspond to the temperature of seawater in summer and spring at the depth of 10–30 m, respectively. Based on the records from this core and previous studies, spatio-temporal changes in alkenone temperature of the East Sea/Japan Sea have been reconstructed to investigate changes of surface circulations and the subpolar front (SPF) position. During the transitional period from the glacial to the Holocene, the SST was lower (about 2–3 °C) than that of today and the SSS dramatically increased at that time in the entire East Sea/Japan Sea but it was still lower than that of today. However, the northern SSS was higher than that of the southern part. The source of the cold and saline seawater could have been the Oyashio Current (OY, inflow from the north), not the Tsushima Warm Current (TWC, inflow from the south). Probably, sea level rose rapidly at that time. There might be no strong development of the SPF during the transitional period due to the restricted inflow of the TWC. During the early Holocene, the SST and SSS of the East Sea/Japan Sea increased compared to the transitional period due to increased seawater exchange through straits. The SPF was tilted orienting from southwest to northeast because the inflow of the TWC was not fully developed due to lower sea level. Subsequently, the surface condition of the East Sea/Japan Sea was similar to that of today at 6 cal ka BP. The Forerunner of Soya Warm Water (FSWW) that flowed out through the Soya Strait in early spring did not affect the formation of the Okhotsk Sea Intermediate Water and the North Pacific Intermediate Water during the glacial and interglacial periods.

Variations of alkenone temperature in the Sea of Japan during the last 170 ka and its paleoceanographic implications

Two sediment cores, KCES1 and ODP797, which were recovered from the Sea of Japan (JS), were measured for alkenone-derived sea surface temperatures (\(U^{{\text{K}}^{\prime}}_{37} {\text{-SSTs}}\)). Our results revealed that the SSTs closely follow the glacial-interglacial cycles during the last 170 ka, except in the last glacial maximum (LGM), during which the SST was higher than in the Holocene. The anomalous high temperature in the LGM is considered as an effect of the intrusion of a low salinity water mass into the JS when the sea level was almost below 130 m. On the glacial-interglacial to orbital timescale, the \( U^{{\text{K}}^{\prime }}_{37} {\text{-SSTs}} \) record in the JS correlated well with the benthic foraminiferal δ18O record and solar insolation, which suggests the dominant control of solar insolation and its related sea ice development on the SST in the JS. On the sub-orbital/millennial timescale, reduced SST corresponds to an enhanced east asian winter monsoon (EAWM) during the last glacial period (MIS3 and MIS4), indicating the dominant control of sea ice expansion due to the enhanced EAWM on the SST in the JS. In contrast, during the last interglacial period (MIS5), the SST in the JS was controlled by variations in the east Asian summer monsoon. These results highlight the key role of solar insolation and associated glacial-interglacial conditions in the variations of the SST in the JS since the last 170 ka.

Alkenone production in the East Sea/Japan Sea

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

Spatial and temporal changes in sea surface temperature, circulation and subpolar front of the East Sea (Japan Sea) during the last 130,000 years

In order to reconstruct spatial and temporal changes in sea surface temperature (SST) of the East Sea (Japan Sea) on glacial–interglacial time scale, alkenone temperature records from the both Korean and Japan margins have been investigated for the period over the last 130 kyr. Results show that the temperature gradient between the Korean and Japan margins was relatively small (~ 2 °C) during the warm interval of the interglacial period (MIS 5e). But it was large (~ 10 °C) during the transitional period from the interglacial to the glacial (MIS 5a and b). The temperature of the Korean margin was colder than that of the Japan margin during the transitional period. Based on the reconstructed alkenone temperature, temporal changes in surface circulation and subpolar front (SPF) have been investigated. During the interglacial period when sea level was higher than today (MIS 5e), the influx of the Tsushima Warm Current (TWC) was enhanced and the both Korean and Japan margins were influenced by the warm water. At that time, the SPF appears to be located in the northern part compared to today. During the transitional period when the sea level was lower than today (MIS 5a and b), the TWC volume transport was reduced contributing to the lower SST. Its effect on the surface temperature appears to be larger along the Korean margin than the Japan margin, indicating that the strength of the East Korea Warm Current appears to be strongly correlated with the TWC volume transport. The SPF would displace southward orienting from southwest to northeast at that time. During the glacial period when the sea level was very lower (~ 100 m deep), the influx of warm water was almost completely restricted and the cold water expanded into the southern area. The SPF would disappear at that time. Model simulation results also support the relationship between the TWC volume transport and surface circulation and the SST.

Multiple‐stage deglacial retreat of the southern Greenland Ice Sheet linked with Irminger Current warm water transport

There is limited knowledge pertaining to the history of the Greenland Ice Sheet (GIS) during the last glacial‐interglacial transition as it retreated from the continental margins to an inland position. Here we use multiproxy data, including ice‐rafted debris (IRD); planktonic isotopes; alkenone temperatures; and tephra geochemistry from the northern Labrador Sea, off southwest Greenland, to investigate the deglacial response of the GIS and evaluate its implications for the North Atlantic deglacial development. The results imply that the southern GIS retreated in three successive stages: (1) early deglaciation of the East Greenland margins, by tephra‐rich IRD that embrace Heinrich Event 1; (2) progressive retreat during Allerød culminating in major meltwater releases (δ18O depletion of 1.2‰) at the Allerød–Younger Dryas transition (12.8–13.0 kyr B.P.); and (3) a final stage of glacial recession during the early Holocene (∼9–11 kyr B.P.). Rather than indicating local temperatures of ambient surface water, the alkenones likely were transported to the core site by the Irminger Current. We attribute the timing of GIS retreat to the incursion of warm intermediate waters along the base of grounded glaciers and below floating ice shelves on the continental margin. The results lend support to the view that GIS meltwater presented a forcing factor for the Younger Dryas cooling.

Sediment trap record of alkenones from the East Sea (Japan Sea)

Long chain (C 37) alkenones in sinking particles from the East Sea were examined using time-series sediment trap records from the eastern Japan Basin (trap depth 1057 m and 3043 m) and the Yamato Basin (1057 m and 2100 m) for the period of October 2000 to August 2001. The 11month records of alkenone flux show that the flux to the upper trap appears to be higher during summer (June to August) and fall (October to November) than during the rest of the sample collection period at both stations. However, the seasonal trends in alkenone flux to the lower trap do not show such a strong seasonality. In general the alkenone sinking flux decreased with increasing depth. Despite the decomposition of the alkenones, there was little difference in U 37 K ′ values and hence alkenone temperatures between traps. A comparison of alkenone-derived temperatures with satellite sea surface temperatures (SSTs), however, reveals that the alkenone temperatures were higher (7–11 °C) than SST during winter time and lower (2–5 °C) during summer time. This indicates that there is a time lag between the SST and trap records (about 3 months in winter and 1 month in summer). The reasons for the time lag could be (i) slow settling velocity and long residence time of alkenones in the upper water column especially during winter time when alkenone flux was low, (ii) biotic and abiotic degradation of alkenones and (iii) subsurface production of alkenones in summer. The flux-weighted alkenone temperatures are close to annual average SST at the sites, indicating that the sedimentary alkenone signal from the seafloor may represent the annual averaged temperature.

Phylogenetic diversity and evolutionary relatedness of alkenone-producing haptophyte algae in lakes: Implications for continental paleotemperature reconstructions

Alkenones have been found in an increasing number of lakes around the world, making them a promising new tool for continental paleoclimate reconstruction. However, individual lakes may harbor different species of haptophyte algae with different sensitivities to temperature variations, thus presenting a significant challenge to the use of lacustrine alkenones for paleotemperature reconstructions. To explore the extent of lacustrine haptophyte diversity, we conducted the first comprehensive phylogenetic and geochemical survey of lacustrine alkenone producers. We sampled 15 alkenone-containing lake surface sediments from a variety of geographic locales and inferred identities of environmental sequences using 18S ribosomal RNA (rRNA) gene-based phylogenies. For two lakes, BrayaSø in southwest Greenland and Tso Ur on the Tibetan Plateau, we also analyzed both surface and downcore sediments to characterize haptophyte populations through time. In parallel with phylogenetic analyses, we determined the alkenone distributions (including C 37/C 38 ratios, and the presence/absence of C 38 methyl ketones and C 40 compounds) in all the samples. The resulting alkenone profiles from this study do not all align with traditional “marine” versus “coastal/lacustrine” alkenone profiles. Additionally, our genetic data indicate the presence of multiple haptophyte species from a single lake sediment sample; these distinct haptophyte populations could not be discerned from the alkenone profiles alone. These results show that alkenone profiles are not a reliable way to assess the haptophyte algae in lakes and that DNA fingerprinting is a preferred approach for species identification. Although closely related haptophyte species or subspecies may not warrant different temperature calibrations, our results emphasize the importance of genetic data for inferring haptophyte identities and eventually selecting alkenone–temperature calibrations for paleoclimate reconstructions.

Late Pleistocene surface and thermocline conditions of the eastern tropical Indian Ocean

Surface and thermocline conditions of the eastern tropical Indian Ocean were reconstructed through the past glacial–interglacial cycle by using Mg/Ca and alkenone-paleothermometry, stable oxygen isotopes of calcite and seawater, and terrigenous fraction performed on sediment core GeoB 10038-4 off SW Sumatra (∼6°S, 103°E, 1819 m water depth). Results show that annual mean surface and thermocline temperatures varied differently and independently, and suggest that surface temperatures have been responding to southern high-latitude climate, whereas the more variable thermocline temperatures were remotely controlled by changes in the thermocline temperatures of the North Indian Ocean. Except for glacial terminations, salinity proxies indicate that changing intensities of the boreal summer monsoon did not considerably affect annual mean conditions off Sumatra during the past 133,000 years. Our results do not show a glacial–interglacial pattern in the thermocline conditions and reject a linear response of the tropical Indian Ocean thermocline to mid- and high-latitude climate change. Alkenone-based surface temperature estimates varied in line with the terrigenous fraction of the sediment and the East Asian winter monsoon proxy records at the precession band suggestive of monsoon (sea level) to be the dominant control on alkenone temperatures in the eastern tropical Indian Ocean on sub-orbital (glacial–interglacial) timescales.

Paleotemperature response to monsoon activity in the Japan Sea during the last 160 kyr

Paleo-sea-surface temperatures in the northeastern- and southeastern-parts of the Japan Sea were reconstructed for the last 160 kyr using alkenone temperatures (U K ′ 37-temperatures). U K ′ 37-temperatures at two sites show distinct glacial–interglacial changes during the last 160 kyr except for the interval corresponding to middle MIS 3 to MIS 2. On orbital-timescales, U K ′ 37-temperature tends to be high during MIS 5e, MIS 5c, and MIS 5a, which coincides with the intervals of stronger East Asian summer monsoon activity. The amplitude of temperature fluctuations in the Japan Sea is significantly higher than those in the neighboring seas. We suggest that the SST variation was amplified by the increasing source water (Kuroshio water) temperature and the changes in the volume transport of the Tsushima Warm Current (TWC) and/or the north–south oscillation of the sub-polar front position within the Japan Sea. Millennial-scale temperature fluctuations in the Japan Sea show that the temperature at the northern site was higher than that at the southern site during warmer periods of MIS 5, which is called “temperature reversal.” By analogy with modern oceanography, the temperature reversal could reflect the enhanced volume transport of the TWC and the spatial relationship between the studied site and the branches of the TWC, which is an essential factor in north–south temperature reversal around the eastern Japan Sea. Temperature drops were found at 114 ka, 111 ka, 93 ka, 87 ka, and 77 ka in MIS 5. Those events were associated with an increase in organic carbon and alkenone contents and can be correlated with the abundance peaks of ice-rafted debris (IRD) at Site GH05-1208 in the northern Japan Sea, suggesting that the surface water was cooled by enhanced mixing and consequent upwelling in a stronger winter monsoon regime.