C37 Alkenones Research Articles

Biomarker records of phytoplankton productivity and community structure changes in the Central Yellow Sea mud area during the Mid-late Holocene

The Yellow Sea (YS) environmental and ecological changes during the Holocene are driven by the interactions between the Yellow Sea Warm Current (YSWC), the East Asian Winter Monsoon (EAWM) and the Kuroshio Current (KC). We report marine biomarker records of brassicasterol, dinosterol and C37 alkenones in core ZY1 and core ZY2 from the South Yellow Sea (SYS) to reconstruct the spatial/temporal variations and possible mechanisms of phytoplankton primary productivity and community structure changes during the Mid-late Holocene. The contents of the corresponding biomarkers in the two cores are similar, and they also reveal broadly similar temporal trends. From 6 kyr to 3 kyr, the biomarker contents in the two cores were relatively low with small oscillations, followed by a distinct increase at about 3 kyr indicating productivity increases caused by a stronger EAWM. The alkenone/brassicasterol ratio (A/B) is used as a community structure proxy, which also showed higher values in both cores since 3 kyr, indicating increased haptophyte contribution to total productivity. It is proposed that the YS community structure has been mainly influenced by the YSWC, with stronger YSWC influences causing an increase in haptophyte contribution since 3 kyr. Some differences of the biomarker records between ZY2 and ZY1 suggest spatial variations in response to YSWC and KC forcing. When the KC was intensified during the periods of 6–4.2 kyr and 1.7–0 kyr, the YSWC extended eastward, exerting more influence on core ZY1. On the other hand, when the KC weakened during 4.2–1.7 kyr, the YSWC extended westward, exerting more influence on the ZY2.

Alkenones as tracers of surface ocean temperature and biological pump processes on the Northwest Atlantic margin

We have examined alkenone distributions, specifically the temperature proxy U37K′, in sinking particulate organic matter (POM) intercepted at three depths by time-series sediment traps deployed between 2004 and 2007 on the Northwest Atlantic margin. The goal was to assess physical and biogeochemical processes acting upon alkenones during passage through the water column. U37K′ did not exhibit any systematic trend with increasing depth despite several-fold attenuation in alkenone flux. Because of the extensive reduction in C37 alkenone flux in the water column and more efficient alkenone degradation during the period of high alkenone flux, the temperature bias toward that of more productive seasons was reduced with increasing trap depth. The temporal variation of U37K′ and alkenone-derived temperature compared best with the satellite-derived SST at an upstream region approximately 160km east of the mooring site with a time lag of about 30 days, suggesting this region as the dominant source of alkenone-bearing POM. The alkenone-derived temperature of core-top sediments (15°C) at the study site was lower than the flux-weighted average alkenone-derived temperature of sinking POM at 50m above the seafloor. This discrepancy may reflect additional supply of resuspended sediment carrying alkenones produced in cooler waters to the northeast, and transported in bottom nepheloid layers.

Changes in phytoplankton productivity and community structure in the northern South China Sea during the past 260 ka

A high-resolution molecular organic geochemical study of core MD05-2904 reveals changes in the productivity and community structure of paleo-phytoplankton during the last 260ka in the northern South China Sea (SCS). Multiple biomarkers, including C37 alkenones, dinosterol, brassicasterol and C30 diols, are used as representatives of haptophytes, dinoflagellates, diatoms and eustigmatophytes, respectively. The abundances and mass accumulation rates of biomarkers indicate that phytoplankton productivity was relatively higher during glacial periods than during interglacial periods. However, the phytoplankton community structure, inferred from the relative abundance of biomarkers, does not display any evident glacial/interglacial cycles. In agreement with previous studies, our results also suggest a linkage between the increased paleo-productivity and the enhanced East Asian winter monsoon in the glacial northern SCS. The relatively stable PCS may reflect a lack of sensitivity of the phytoplanktonic groups to variations of nutrient composition on glacial/interglacial and shorter time scales. A long-term decrease in the C37 alkenone abundance since MIS 5 might have been related to the increasing influence of the northward intrusion of Antarctic Intermediate Water and/or sub-Antarctic Mode Water on the SCS thermocline waters. Moreover, the evolution of alkenone producers and changes in the coccolithophore genera on C37 alkenones might also have contributed to the long-term decrease in the C37 alkenone abundance.

Concurrent purification of sterols, triterpenols and alkenones from sediments for hydrogen isotope analysis using high performance liquid chromatography

Three methods are presented on how to purify acetylated sterols, acetylated triterpenols and individual alkenones for hydrogen isotope analysis from marine and lacustrine sediments using reverse-phase high performance liquid chromatography (RP-HPLC). The main advantages over previous HPLC methods are reduced operator time, increased automation and the ability to simultaneously purify multiple target compounds from a sample. These gains are achieved primarily by acetylating compounds prior to purification rather than after, and also by using a fraction collector with semi-preparatory rather than analytic configuration. The effectiveness of the method is demonstrated for (i) dinosterol and taraxerol in sediment from the brackish pond Poza del Diablo, Galápagos, (ii) for di- and tri-unsaturated C37 and C38 alkenones in cultured Emiliania huxleyi, (iii) for brassicasterol, and di-, tri- and tetra-unsaturated C37 alkenones in sediment from Manito Lake, Saskatchewan, Canada, and (iv) for brassicasterol, dinosterol and di-, tri- and tetra-unsaturated C37 alkenones in sediment from the Great Salt Lake, Utah. The purification process yields 80–90% recoveries and results in no measurable hydrogen isotope alteration.

Late quaternary paleoceanographic changes in the northeastern Ulleung Basin, East Sea, inferred from variations in biogenic components

Two piston cores (DD09-ST21, DD09-ST39B) from the northeastern Ulleung Basin in the East Sea were obtained to investigate variations in the biogenic components (calcium carbonate, organic carbon) and biogeochemical processes (δ13C and δ15N). The two cores had distinctive characteristics in terms of surface production, preservation and dissolution capacity of carbonate, and redox conditions of bottom-water. Core DD09-ST21 was characterized by an oxygen-depleted condition from 15 ka (MIS 2) to 60 ka (MIS 3). Core DD09-ST39B, on the other hand, showed oxic bottom-water conditions, possibly due to shallow water depth. These two cores with different redox condition showed opposite trends in terms of CaCO3, TOC, and C37 alkenone concentrations. CaCO3 and C37 concentrations were higher during the LGM in DD09-ST21 while lower contents were observed in DD09-ST39B in the same period. Moreover, consistently low TOC in DD09-ST39B and higher fluctuation of organic matters in DD09-ST21 may suggest difference in primary productivity, preservation capacity, or a potential dissolution effect. During the Holocene, the surface productivity of both cores increased, probably due to renewed ventilation and vertical mixing in the East Sea. Therefore, this study suggests spatial variation in production and preservation of biogenic components in the two cores since last 50 ka for DD09-ST39B and 80 ka for DD09-ST21 due to difference in environmental conditions such as water depth, bottom-water conditions, surface productivity and preservation.

Deglacial–Holocene environmental changes at the Pacific entrance of the Strait of Magellan

We show environmental and nitrogen cycle changes between 13 and 2.5kyr BP (from the latest deglaciation to the Holocene) based on biogeochemical records in a sediment core from the Pacific entrance of the Strait of Magellan. Organic carbon, total nitrogen, and C37 alkenone contents were low during 13–9kyr BP but increased rapidly at 8kyr BP. The relative contribution of tetra-unsaturated C37 alkenone, used as a salinity proxy, suggests low salinity during 13–9kyr BP changing to high salinity at 8kyr BP. Planktic foraminifer data showed that Globigerina bulloides was more abundant during the deglacial–early Holocene than in the mid–late Holocene, whereas abundances of Neogloboquadrina pachyderma showed the opposite pattern. This evidence plus biogenic opal data suggest that sea level rose, surface waters were relatively stratified, and in situ productivity was low except for organisms with opal tests from 13 to 9kyr BP. Bulk δ15N values were high (9–11‰) relative to present-day values during the deglacial–early Holocene, with consistently lower values (~7‰) after ~8kyr BP. These relatively high δ15N values most likely reflect increased nutrient utilization in the Southern Ocean, which resulted in high-δ15N water being advected to the Patagonian fjords, although local denitrification caused by the delivery of copious terrestrial materials and organic matter to the ocean cannot be ruled out. By ~8kyr BP continental influences and surface stratification were reduced and characteristic Holocene conditions were established, with increased influence of Southern Ocean water in the western Strait of Magellan. Sea surface temperatures showed millennial-scale changes during the deglaciation and early Holocene, with relatively warm temperatures (11–12°C) corresponding to the Antarctic counterpart of the Younger Dryas cooling event of the Northern Hemisphere. The longer term pattern (~10kyr time scale) in alkenone-derived sea surface temperatures generally follows orbital changes in insolation during austral spring, suggesting a controlling influence of austral spring insolation on southern Patagonian climate.

Paleoenvironmental and paleoclimatic conditions in the Tagus River Estuary during the last 14 kyr

Estuaries constitute the frontier between terrestrial, fluvial and marine environments, representing areas of high biological productivity that are particularly sensitive to global, regional and local environmental and climatic changes. The Tagus River is the longest river of the Iberian Peninsula and has a catchment area of 80 629 km 2 . In order to understand the interactions between the Tagus system (land-river) and the ocean we have studied a 52,76 m long core collected at the upper Tagus estuary off Vila Franca de Xira (VFX 38°56 24 N; 8°56 19 W, 2 meters elevation). The age model for this core was based on 6 AMS 14 C dates and covers the last 14 calendar (cal) kyr BP (1 kyr = 1000 yr). Sea surface temperature (SST), marine/estuarine productivity and river input has been reconstructed with a high resolution, multi-proxy study that includes: diatoms, phytoliths, C37 alkenones, higher plant C23 – C33 n-alkanes and C20 – C30 n-alkan-1-ols, Fe, Ti and Ca content derived from XRF analysis, Organic Carbon (Corg) and CaCO3 content and grainsize analysis. Although discontinuous, the SST record shows a decreasing trend from Early to Late Holocene, as in the Tagus prodelta SST record of Rodrigues et al. (2009). The lower SST values indicate that the river waters are colder than the coastal ocean. Important terrigenous input (revealed by higher content of terrestrial lipid biomarkers, Fe and Ti content, fresh water diatoms and phytoliths) is recorded during the Younger Dryas/Holocene transition (~11.5 – 10.5 cal kyr BP) as sea level rise allows sediment accumulation inside the estuary. Diatom (marine and brackish genera) and C37 alkenones appear between ca. 11.5 – 4 cal kyr BP supporting the existence of a marine to brackish water environment, as suggested by the previously suggested by Vis, Kasse & Vandenberghe (2008) and Vis & Kasse (2009). During this time interval, relative maxima in diatoms, C37 alkenones and Corg show a cyclic increase in local productivity that can be explained as being a response to coastal water inflow and/or fluvial supply of nutrients. SST minima are recorded at ~ 11.3 – 11.1, 10.5 – 10, 7.8, 6.6, 5.8, 5, 4.5 and 1.4 – 0.8 cal ka BP. The estimated age for these episodes coincide with the equivalent events recorded in the North Atlantic (Bond et al., 1997) and in the Tagus prodelta (Rodrigues et al., 2009). Higher terrigenous input episodes are recorded at ~10.5, 8.3, 7.7, 6.7, 6.4, 5.9, 5.4 and 4.9 cal ka BP, and could be related to flood events and/or periods of major fluvial activity. When comparing the SST minima with the markers of enhanced river discharge recorded at this site, five of these events are II Seminario Iberico IGBP – Mudanca Global Mudanca Global na Peninsula Iberica. Uma Visao Integrada. 4 e 5 | Novembro | 2010 Lisboa – Portugal synchronous: 10.5, 7.8 7.7, 6.7 6.6, 5.9 5.8 and 5 4.5 cal ka BP, corresponding to cold, wet periods and coinciding with the flood events and/or increased fluvial activity detected in the Tagus river and in the Iberian Peninsula (Vis et al., 2010, Benito et al., 2003, Benito et al., 2008). Although higher resolution and a more accurate age model are needed to improve the understanding of this estuarine record, the hydro-sedimentary dynamics, particularly during the Holocene transgression, appears to be controlling the terrigenous input and the local productivity recorded at this site. Still, this sedimentary record provides information on past climate changes occurred in the Iberian Peninsula and contributes to a better understanding of the interactions between the North Atlantic and the Mediterranean.

Alkenone distribution impacts the hydrogen isotopic composition of the C37:2 and C37:3 alkan-2-ones in Emiliania huxleyi

The hydrogen isotopic composition (δD) of long-chain alkenones derived from haptophyte algae has recently been applied as paleo sea surface salinity proxy, but this application has been complicated by reported differences in δD of individual C37 alkenones of up to 45‰. Here we show that this difference in D/H ratios is the result of the desaturation of the C37:2 alkenone to form the C37:3 alkenone and related to the relative abundance of the individual alkenones, and, therefore, the UK′37. For the purpose of reconstructing paleo sea surface salinities it is recommended to determine the δD of combined rather than individual C37 alkenones.

Geochemical characteristics of saturate hydrocarbons in crude oils and source rocks of the Qaidam, Tarim, Tupran basins, NW China

Based on the systematic analyses of fifteen typical crude oils and ten typical potential source rocks collected from the Qaidam, Tarim and Turpan basins, Northwest China, the geochemical characteristics of the oils and source rocks were investigated and oil-source rock correlations undertaken. The oils and source rocks deposited in saline lacustrine environment from the western Qaidam Basin were characterized by n-alkanes with even carbon-number preference in the C20-C28 range, low pristane/phytane (Pr/Ph) ratios (less than 0.5), and high abundances of C27 steranes, gammacerane and C35 hopanes. The oils and source rocks deposited in marine environment from the Tarim Basin were characterized by n-alkanes with even carbon-number preference in the C14-C18 range, relatively low Pr/Ph ratios (near to 1), high abundance of C28 steranes, and relatively high gammacerane. In contrast, the oils and source rocks deposited in terrigenous bog environment from the Turpan Basin were characterized by relatively high Pr/Ph ratios (oil samples greater than 6) high abundance of C29 steranes, and relatively low gammacerane and C31–35 hopanes. The higher amounts of C37 and C38n-alkanes of source rocks from the western Qaidam Basin and the Tarim Basin suggest an origin of these alkanes from functionalized C37 and C38n-alkadienes and alkenones in prymnesiophytes living in lacustrine and marine environments. Oil-source rock correlations suggest oils in the western Qaidam Basin were derived from the Oligocene Lower Ganchaigou Formation (E3), oils in the Tabei and Tazhong uplifts from the Tarim Basin have a genetic relationship with the Middle-Upper Ordovician source beds. Oils in the Turpan Basin generally fall into two genetic types. Most oils in the Taibei depression from the Turpan Basin were derived from the Lower-Middle Jurassic coal measures, but the fewer oils in this region are a mixed source derived from the Lower-Middle Jurassic coal measure and the Upper Permian source rocks.

Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency

We present new nitrogen isotope data from the water column and surface sediments for paleo–proxy validation collected along the Peruvian and Ecuadorian margins between 1°N and 18°S. Productivity proxies in the bulk sediment (organic carbon, total nitrogen, biogenic opal, C37 alkenone concentrations) and 15N/14N ratios were measured at more than 80 locations within and outside the present-day Peruvian oxygen minimum zone (OMZ). Microbial N-loss to N2 in subsurface waters under O2 deficient conditions leaves a characteristic 15N-enriched signal in underlying sediments. We find that phytoplankton nutrient uptake in surface waters within the high nutrient, low chlorophyll (HNLC) regions of the Peruvian upwelling system influences the sedimentary signal as well. How the δ15Nsed signal is linked to these processes is studied by comparing core-top values to the 15N/14N of nitrate and nitrite (δ15NNOx) in the upper 200 m of the water column. Between 1°N and 10°S, subsurface O2 is still high enough to suppress N-loss keeping δ15NNOx values relatively low in the subsurface waters. However δ15NNOx values increase toward the surface due to partial nitrate utilization in the photic zone in this HNLC portion of the system. δ15Nsed is consistently lower than the isotopic signature of upwelled NO3−, likely due to the corresponding production of 15N depleted organic matter. Between 10°S and 15°S, the current position of perennial upwelling cells, HNLC conditions are relaxed and biological production and near-surface phytoplankton uptake of upwelled NO3− are most intense. In addition, subsurface O2 concentration decreases to levels sufficient for N-loss by denitrification and/or anammox, resulting in elevated subsurface δ15NNOx values in the source waters for coastal upwelling. Increasingly higher production southward is reflected by various productivity proxies in the sediments, while the north–south gradient towards stronger surface NO3− utilization and subsurface N-loss is reflected in the surface sediment 15N/14N ratios. South of 10°S, δ15Nsed is lower than maximum water column δ15NNOx values most likely because only a portion of the upwelled water originates from the depths where highest δ15NNOx values prevail. Though the enrichment of δ15NNOx in the subsurface waters is unambiguously reflected in δ15Nsed values, the magnitude of δ15Nsed enrichment depends on both the depth of upwelled waters and high subsurface δ15NNOx values produce by N-loss. Overall, the degree of N-loss influencing subsurface δ15NNOx values, the depth origin of upwelled waters, and the degree of near-surface nitrate utilization under HNLC conditions should be considered for the interpretation of paleo δ15Nsed records from the Peruvian oxygen minimum zone.

Beyond Fatty Acid Methyl Esters: Expanding the Renewable Carbon Profile with Alkenones from Isochrysis sp.

In addition to characteristic fatty acid methyl esters (FAMEs), biodiesel produced from Isochrysis sp. contains a significant amount (14%, w/w) of predominantly C37 and C38 long-chain alkenones. These compounds are members of a class of lipids known collectively as polyunsaturated long-chain alkenones (PULCAs) that are produced by a range of other prymnesiophyte taxa. The physical properties of alkenones, such as high melting points (∼60 °C), renders the direct product unsuitable for use as a diesel fuel but, nonetheless, represents an important and as yet unexplored renewable carbon feedstock.

Changes in alkenone and alkenoate distributions during acclimatization to salinity change in Isochrysis galbana: Implication for alkenone-based paleosalinity and paleothermometry

A cultured strain of Isochrysis galbana UTEX LB 2307 was grown at 20°C and 15°C under salinity of 35‰, 32‰, 27‰, 20‰, and 15‰, and analyzed for long chain (C37–C39) alkenones and (C37–C38) alkyl alkenoates. It was characterized by abundant C37 alkenones and C38 ethyl alkenoates (fatty acid ethyl ester: FAEEs) and a lack of C38 methyl alkenones. There were no tetra-unsaturated (C37:4) alkenones, which are frequently found in natural samples from low salinity waters. The alkenone unsaturation index (U37) did not vary in response to change in salinity at 15°C. The alkenone unsaturation index (U37) clearly changed in response to change in salinity at 20°C, but not at 15°C, where algal growth was and was not limited by temperature at 15°C and 20°C, respectively. The U37-temperature calibration for I. galbana UTEX LB 2307 is quite different from those of E. huxleyi and another strain of I. galbana (CCMP1323), while is resemble to that reported for C. lamellosa isolated from a Chinese lake. Also, variation in the alkenone chain length ratio values (K37/K38) did not correlate with salinity. These results implied that a high abundance of tetra-unsaturated alkenone and high K37/K38 values might be attributed to a taxonomic factor rather than a physiological response to salinity change. Interestingly, our culture experiments showed that the ethyl alkenoates/alkenones ratio (EE/K37) correlates with salinity. Hence, it is suggested that the EE/K37 ratios are affected by the cellular and physiological factors against salinity condition in single haptophyte cells.

Primary productivity variability on the Atlantic Iberian Margin over the last 70,000 years: Evidence from coccolithophores and fossil organic compounds

[1] This study analyzes coccolithophore abundance fluctuations (e.g., Emiliania huxleyi, Gephyrocapsa specimens, and Florisphaera profunda) in core MD01-2444 sediment strata retrieved at the Iberian Margin, northeastern Atlantic Ocean. Coccolithophores are calcareous nannofossils, a major component of the oceanic phytoplankton, which provide information about past ecological and climatological variability. Results are supported by data on fossil organic compounds (sea surface temperatures, alkenones, and n-hexacosan-1-ol index) and geochemical analyses (benthic δ13Ccc and planktonic δ18Occ isotopes). Three scenarios are taken into account for this location at centennial-scale resolution over the last 70,000 years: the Holocene and the stadial and interstadial modes. The different alternatives are described by means of elements such as nutrients; upwelling phenomena; temperatures at surface and subsurface level; or the arrival of surface turbid, fresh, and cold waters due to icebergs, low sea level, increased aridity, and dust. During the Holocene, moderate primary productivity was observed (mainly concentrated in E. huxleyi specimens); surface temperatures were at maxima while the water column was highly ventilated by northern-sourced polar deep waters and warmer subsurface, nutrient-poor subtropical waters. Over most of the last glacial stadials, surface productivity weakened (higher F. profunda and reworked specimen percentages and lower diunsaturated and triunsaturated C37 alkenones); the arrival of cold Arctic surface waters traced by tetraunsaturated C37 peaks and large E. huxleyi, together with powerful ventilated southern-sourced polar deep waters, disturbed, in all likelihood, the delicate vertical equilibrium while preventing significant upwelling mixing. Finally, during the last glacial interstadials (lower F. profunda percentages, nonreworked material, and higher diunsaturated and triunsaturated C37 alkenones) a combined signal is observed: warm surface temperatures were concurrent with generally low oxygenation of the deep-sea floor, moderate arrival of northern-sourced deep waters, and subsurface cold, nutrient-rich, recently upwelled waters, probably of polar origin; these particular conditions may have promoted vertical mixing while enhancing surface primary productivity (mainly of Gephyrocapsa specimens).

Short scale (6 h) temporal variation of sinking fluxes of planktonic and terrigeneous lipids at 200 m in the NW Mediterranean Sea

Abstract. Drifting sediment trap measurements were carried out at high temporal frequency in the northwestern Mediterranean Sea in the course of the DYNAPROC 2 campaign, during the transition between late summer to autumn conditions. Molecular biomarkers were analyzed in selected subsets of consecutive samples collected for 6 h at 200 m depth. Fluxes of n-alkanes, long-chain alkenones, sterols and steroid ketones show high variability between consecutive 6-h samples, comparable in range to seasonal variability. n-Alkane export ranges from 1.4 to 29.7μg m−2 d−1, fluxes of C37 alkenones varies from 0 to 14.2 μg m−2 d−1. Fluxes of sterols, steroid ketones and C30 alkane diol respectively range from 31 to 377, 2.2 to 46 and 0.3 to 9.3 μg m−2 d−1. Biomarker flux ranges are coherent with the relatively low primary production and the low export ratio encountered during the study. Molecular characteristics of biomarker composition is consistent with reworked algal and zooplanktonic organic matter. Lipid biomarker composition points to the dominance of haptophytes over dinoflagellates in the exported material, and to a minor contribution of diatoms and eustigmatophytes. The intrusion of coastal water at the study site was recorded by an enhanced imprint of higher plant n-alkanes. Two processes can be responsible for the pulses in biomarker flux: changes in the magnitude of particle flux (dry weight mass flux) and changes in the concentration of biomarkers in the particles. Concurrent variability in fluxes, significant Pearson correlations between fluxes of biomarkers of haptophytes, dinoflagellates, eustigmatophytes, zooplankton activity and higher plants (alkenones, dinosterol, alkyl diols, dehydrocholesterol, steroid ketones, p<0.01, n=31, suggest that the short term temporal variability of biomarker export fluxes depends primarily on the intensity of particle dry weight flux. Biomarker concentrations vary on a relatively narrower range than biomarker fluxes, indicating that changes in particle composition (due to degradation or change in source apportionment) has a weaker influence on flux variability. Thus, particle aggregation and sinking are key processes to explain the dynamic of biomarker export at a temporal scale of 6 h. None the less, abrupt changes of OC-normalized concentrations of biomarkers, in the time scale of 6 h, indicate that the exported particles composition also show short-time variability. Highest biomarker flux values recorded by the trap samples from 27 to 29 September correspond to highest values of zooplankton biomass integrated between the surface and 200 m. The decrease in primary production after the October wind events is echoed by minima in biomarker flux values. Despite these correspondences, the coupling between primary production and biomarker export is complex.

Growth phase dependent hydrogen isotopic fractionation in alkenone-producing haptophytes

Abstract. Recent works have investigated use of the hydrogen isotopic composition of C37 alkenones (δDK37s, lipid biomarkers of certain haptophyte microalgae, as an independent paleosalinity proxy. We discuss herein the factors impeding the success of such an application and identify the potential alternative use of δDK37s measurements as a proxy for non-thermal, physiological stress impacts on the U37K' paleotemperature index. Batch-culture experiments with the haptophyte Emiliania huxleyi (CCMP 1742) were conducted to determine the magnitude and variability of the isotopic contrasts between individual C37 alkenones. Further experiments were conducted with Emiliania huxleyi (CCMP 1742) andGephyrocapsa oceanica (PZ3-1) to determine whether, and to what extent, δDK37s varies between the physiological extremes of nutrient-replete exponential growth and nutrient-depleted senescence. Emiliania huxleyi was observed to exhibit an isotopic contrast between di- and tri-unsaturated C37 alkenones (αK37:3-K37:2≈0.97) that is nearly identical to that reported recently by others for environmental samples. Furthermore, this contrast appears to be constant with growth stage. The consistency of the offset across different growth stages suggests that a single, well-defined value for αK37:3-K37:2 may exist and that its use in an isotope mass-balance will allow accurate determination of δD values for individual alkenones without having to rely on time- and labor-intensive chemical separations. The isotopic fractionation between growth medium and C37 alkenones was observed to increase dramatically upon the onset of nutrient-depletion-induced senescence, suggesting that δDK37s may serve as an objective tool for recognizing and potentially correcting, at least semi-quantitatively, for the effects of nutrient stress on U37K' temperature records.

Holocene interdependences of changes in sea surface temperature, productivity, and fluvial inputs in the Iberian continental shelf (Tagus mud patch)

Sea surface temperature (SST), marine productivity, and fluvial input have been reconstructed for the last 11.5 calendar (cal) ka B.P. using a high‐resolution study of C37 alkenones, coccolithophores, iron content, and higher plant n‐alkanes and n‐alkan‐1‐ols in sedimentary sequences from the inner shelf off the Tagus River Estuary in the Portuguese Margin. The SST record is marked by a continuous decrease from 19°C, at 10.5 and 7 ka, to 15°C at present. This trend is interrupted by a fall from 18°C during the Roman and Medieval Warm Periods to 16°C in the Little Ice Age. River input was very low in the early Holocene but increased in the last 3 cal ka B.P. in association with an intensification of agriculture and deforestation and possibly the onset of the North Atlantic Oscillation/Atlantic Multidecadal Oscillation modes of variability. River influence must have reinforced the marine cooling trend relative to the lower amplitude in similar latitude sites of the eastern Atlantic. The total concentration of alkenones reflects river‐induced productivity, being low in the early Holocene but increasing as river input became more important. Rapid cooling, of 1–2°C occurring in 250 years, is observed at 11.1, 10.6, 8.2, 6.9, and 5.4 cal ka B.P. The estimated age of these events matches the ages of equivalent episodes common in the NE Atlantic–Mediterranean region. This synchronicity reveals a common widespread climate feature, which considering the twentieth century analog between colder SSTs and negative North Atlantic Oscillation (NAO), is likely to reflect periods of strong negative NAO.

Monsoon versus ocean circulation controls on paleoenvironmental conditions off southern Sumatra during the past 300,000 years

A multiproxy record has been acquired from a piston core (SO139‐74KL) taken offshore southern Sumatra, an area which is situated in the southwestern sector of the tropical Indo‐Pacific Warm Pool. The high‐resolution data sets (X‐ray fluorescence, total organic carbon, and C37 alkenones) were used to track changes in paleoproductivity, freshwater budget, and sea surface temperature (SST) of the tropical climate system at orbital time scales over the past 300 ka. Our paleoclimatic data show that enhanced marine paleoproductivity was directly related to strengthening of coastal upwelling during periods of increased boreal summer insolation and associated SE monsoon strength with a precessional cyclicity. Changes in freshwater supply were primarily forced by precession‐controlled changes in boreal NW winter monsoon rainfall enclosing an additional sea level component. SST variations of 2°–5°C occurred at eccentricity and precessional cyclicity. We suggest that the sea surface temperature variability off southern Sumatra is predominantly related to three major causes: (1) variations in upwelling intensity; (2) an elevated freshwater input into the southern Makassar Strait leading to reduced supply of warmer surface waters from the western Pacific and increased subsurface water transport via the Indonesian Throughflow into the Indian Ocean; and (3) long‐term changes in the intensity or frequency of low‐latitude climate phenomena, such as El Niño–Southern Oscillation.

The measurement of D/H ratio in alkenones and their isotopic heterogeneity

Abstract Previous paleoenvironmental studies reported the δD values of a mixture of coeluting alkenones. Here, we present a semi-preparative normal-phase high-performance liquid chromatography–mass spectrometry (NP-HPLC–MS) method for purifying long chain (C37 and C38) unsaturated methyl and ethyl ketones (alkenones) on the basis of chain length and degree of unsaturation. The method was applied to purify alkenones in suspended particles and surface sediments from a site in Chesapeake Bay, eastern USA. The hydrogen isotopic composition of di- and triunsaturated C37 and C38 alkenones differed significantly on the basis of chain length and the degree of unsaturation, demonstrating the importance of gas chromatography–isotope ratio-mass spectrometry (GC–irMS) analysis of individual alkenones for accurate paleoenvironmental reconstruction. Constant fractionation factors between alkenones with different chain length but the same degree of unsaturation ( α C 37 : 2 – C 38 : 2 and α C 37 : 3 – C 38 : 3 = 1.01 ) and those with the same chain length but different degree of unsaturation ( α C 37 : 2 – C 37 : 3 and α C 38 : 2 – C 38 : 3 = 0.97 ) in all samples suggest that the values may represent hydrogen isotope fractionation associated with elongation and desaturation during alkenone biosynthesis.

ベーリング海東部陸棚域における円石藻Emiliania huxleyiブルーム-過去50年の記録-

ベーリング海東部陸棚域における円石藻Emiliania huxleyiブルーム-過去50年の記録-

Freshwater impacts recorded in tetraunsaturated alkenones and alkenone sea surface temperatures from the Okhotsk Sea across millennial‐scale cycles

We present records of phytoplankton‐produced alkenones down a long piston core, which reveal changes of sea surface temperature (SST) and sea surface salinity (SSS) in the southwestern Okhotsk Sea over the past 120 ka. Between 20 and 60 ka B.P., alkenone‐derived temperatures typically increased by 6°C–8°C from periods corresponding, within a few hundred years, to stadials to those corresponding to interstadials recorded in Greenland ice cores. The abundance of C37:4 alkenone relative to total C37 alkenones (percent C37:4), a possible proxy for salinity, indicated that during most low SSS was associated with high SST. The warm freshwater events might be related to (1) a decline in the supply of saline water entering the Okhotsk Sea through the Soya Strait; (2) strengthening of the freshwater supply from the Amur River and precipitation over the Okhotsk Sea, associated mainly with increased Asian summer monsoon activity; and (3) the effect of melting sea ice. These findings increase our understanding of the close linkage between high and low latitudes in relation to climate change and the synchronicity of climate changes within a few centuries between the Pacific and the Atlantic sides of the Northern Hemisphere.