Alkenone Unsaturation Research Articles

Alkenones are common in prairie lakes of interior Canada

Long chain alkenone (LCA) lipids produced by haptophyte algae are ubiquitous in marine environments and are increasingly reported from enhanced conductivity, solute rich lakes worldwide. Following our survey of lakes in the United States (Toney et al., 10.1016/j.gca.2009.11.021), we examined surface sediments from 19 lakes in central Canada for alkenones and compared our data with lake characteristics. We found that 68% of surveyed lakes contained sedimentary LCAs, all have C37:4me dominated profiles. Lakes containing LCAs had salinities >1.5g/l and SO4/CO3 ratios >3.5. LCA concentrations were highly elevated (∑C37>500μg/g sediment) in sediments of meromictic lakes with permanently anoxic deep waters. Overall, the distribution of LCAs was consistent with a previous survey of lakes in the interior of the United States demonstrating that tetra-unsaturated lipids were most common in sulfate rich, saline lakes. Alkenone unsaturation indices (U37K) from the upper 10cm of the sediment column are weakly correlated with point sampled average spring water temperature (r2=0.30, p=0.067) and the range of inferred temperatures is in agreement with the average, long term potential evapotranspiration weighted temperature.

Abrupt Holocene climate change as an important factor for human migration in West Greenland

West Greenland has had multiple episodes of human colonization and cultural transitions over the past 4,500 y. However, the explanations for these large-scale human migrations are varied, including climatic factors, resistance to adaptation, economic marginalization, mercantile exploration, and hostile neighborhood interactions. Evaluating the potential role of climate change is complicated by the lack of quantitative paleoclimate reconstructions near settlement areas and by the relative stability of Holocene temperature derived from ice cores atop the Greenland ice sheet. Here we present high-resolution records of temperature over the past 5,600 y based on alkenone unsaturation in sediments of two lakes in West Greenland. We find that major temperature changes in the past 4,500 y occurred abruptly (within decades), and were coeval in timing with the archaeological records of settlement and abandonment of the Saqqaq, Dorset, and Norse cultures, which suggests that abrupt temperature changes profoundly impacted human civilization in the region. Temperature variations in West Greenland display an antiphased relationship to temperature changes in Ireland over centennial to millennial timescales, resembling the interannual to multidecadal temperature seesaw associated with the North Atlantic Oscillation.

Mid-Piacenzian mean annual sea surface temperature: an analysis for data-model comparisons

Numerical models of the global climate system are the primary tools used to understand and project climate disruptions in the form of future global warming. The Pliocene has been identified as the closest, albeit imperfect, analog to climate conditions expected for the end of this century, making an independent data set of Pliocene conditions necessary for ground truthing model results. Because most climate model output is produced in the form ofmean annual conditions, we present a derivative of the USGS PRISM3 Global Climate Reconstruction which integrates multiple proxies of sea surface temperature (SST) into single surface temperature anomalies. We analyze temperature estimates from faunal and floral assemblage data,Mg/Ca values and alkenone unsaturation indices to arrive at a single mean annual SST anomaly (Pliocene minus modern) best describing each PRISM site, understanding that multiple proxies should not necessarily show concordance. The power of the multiple proxy approach lies within its diversity, as no two proxies measure the same environmental variable. This data set can be used to verify climate model output, to serve as a starting point for model inter-comparisons, and for quantifying uncertainty in Pliocene model prediction in perturbed physics ensembles.

Mid-Piacenzian sea surface temperature record from ODP Site 1115 in the western equatorial Pacific

Planktic foraminifer assemblages and alkenone unsaturation ratios have been analyzed for the mid-Piacenzian (3.3 to 2.9 Ma) section of Ocean Drilling Program (ODP) Site 1115B, located in the western equatorial Pacific off the coast of New Guinea. Cold and warm season sea surface temperature (SST) estimates were determined using a modern analog technique. ODP Site 1115 is located just south of the transition between the planktic foraminifer tropical and subtropical faunal provinces and approximates the southern boundary of the western equatorial Pacific (WEP) warm pool. Comparison of the faunal and alkenone SST estimates (presented here) with an existing nannofossil climate proxy shows similar trends. Results of this analysis show increased seasonal variability during the middle of the sampled section (3.22 to 3.10 Ma), suggesting a possible northward migration of both the subtropical faunal province and the southern boundary of the WEP warm pool.

Change of the unsaturation degree of alkenone and alkenoate during acclimation to salinity change in Emiliania huxleyi and Gephyrocapsa oceanica with reference to palaeosalinity indicator

Laboratory cultured strains of Emiliania huxleyi EH2 (identical to NIES-837) and Gephyrocapsa oceanica GO1 (NIES-838) were grown at 20oC under various salinity conditions ranging from 15‰ to 34‰, and were analyzed for long-chain (C37 C39) alkenones and (C37 C38) alkyl alkenoates. In both E. huxleyi EH2 and G. oceanica GO1, there were no tetra-unsaturated (C37:4) alkenones, which were frequently identifi ed at low salinity waters in literatures. The alkenone unsaturation index (U37) in E. huxleyi clearly changed under lower salinities such as 27‰ and 32‰, while the values in G. oceanica were almost constant in various salinities. From these results, the U37 variations affected by salinity should be paid attention in paleotemperature reconstruction from natural samples that E. huxleyi predominates, especially in lower saline environments. Interestingly, our culture experiments also showed that the alkenone chain-length ratio (K37/K38) for E. huxleyi and G. oceanica increased with decreasing salinities, although the ranges of these variations were small. These results suggest that the K37/K38 values were affected by the cellular and physiological factors in a single haptophyte cell, although this value varies mainly depending on the changes of haptophyte species and/or strains in natural environments. Hence, we suggest that the K37/K38 ratio can be more reliable as a paleosalinity indicator. * Department of Natural History Sciences, Faculty of Science, Hokkaido University, N10W8, Kita-ku, Sapporo 060-0810, Japan a corresponding author; Tel: +81-11-706-2733 Fax: +81-11-746-0394 e-mail: sawadak@mail.sci.hokudai.ac.jp ** Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan 1-1-1 Tennoudai, Tsukuba 305-8572, Japan

Systematic pattern in [formula omitted] – Temperature residuals for surface sediments from high latitude and other oceanographic settings

Twenty-seven multicores, collected in late summer 2004 from coastal and inland marine sites throughout SE Alaska (∼55 to 61°N), were analyzed for C 37 to C 39 alkenones of specific haptophyte origin. This biomarker signal, detected in core-top sediments from 20 of the sites, was quantitatively most significant in organic carbon and biogenic silica-rich sediments deposited south of Icy Strait (∼58°N), along the outer coastline of Baranof and Prince of Wales islands. U 37 K ′ -based water temperature estimates derived using a standard calibration equation ( U 37 K ′ = 0.034T + 0.039) were uniform throughout the study area, averaging 11.9 ± 1.0 °C, a value ∼4 °C warmer than mean annual SST (maSST) for the region. Epoxides, indicative of selective alkenone degradation by an aerobic bacterial process, were also detected and quantified in all sediments containing alkenones. Semi-quantitative accounting of the impact that compound-selective degradation can impose on alkenone unsaturation patterns indicates the standard U 37 K ′ -based temperature estimates could be too warm by a variable amount (0.7–2.4 °C) depending upon the site, with an overall average of ∼1.4 °C. Even after adjustment for this potential diagenetic ‘warming’ effect, estimates still best match summer and not maSST in this subarctic region. Re-examination of alkenone signatures preserved in sediment from the North Atlantic and on the continental margin located south of ∼45°S along the Chilean coast indicates the apparent summer signal in U 37 K ′ -derived SST estimates is not unique to SE Alaskan sediments but rather a common finding when alkenone thermometry is applied to sediments from high latitude environments. Examination of model residuals from analysis of surface sediment samples collected at lower latitude settings along the west coast of the Americas also identifies other sites where significant biogeographic patterns seem apparent and viable oceanographic explanations can be offered.

DNA and lipid molecular stratigraphic records of haptophyte succession in the Black Sea during the Holocene

Previous studies suggest that the coccolithophorid haptophyte Emiliania huxleyi entered the Black Sea ~ 3400 yrs ago and since then a coccolith ooze defined as Unit I has developed. Unit I sediments contain long-chain alkenones derived from E. huxleyi whereas the alkenone distribution of the deeper coccolith-free sapropel (Unit II) is rather unusual. Alkenone-derived past sea surface temperature (SST) estimates suggest a large difference between Unit II and Unit I, which is likely a result of unusual biological precursors of the alkenones in Unit II. Here, we report a high-resolution stratigraphic analysis of ancient haptophyte DNA to establish the Holocene succession of haptophytes as sources of the alkenones in the Black Sea. Haptophytes related to brackish Isochrysis spp. were the initial sources of alkenones, and appeared immediately after the onset of sapropel deposition (~ 7550 yrs before present [a BP]). As salinity increased, Isochrysis-related haptophytes were slowly replaced by a complex suite of E. huxleyi strains as sources of alkenones. Our paleogenetic data showed that E. huxleyi colonized the Black Sea shortly after the onset of sapropel deposition, ~ 4000 yrs earlier than previously recognized based on their preserved coccoliths. E. huxleyi strains were the most likely source of the previously reported abundant and unusual C 36 di-unsaturated “Black Sea alkenone”. Strong haptophyte species and strain-specific effects were observed on the level of unsaturation of alkenones which resulted in spurious alkenone-derived SST estimates before 5250 a BP. In contrast, from ~ 5250 a BP onwards a relatively stable haptophyte assemblage dominated by a different suite of E. huxleyi strains yielded robust alkenone-SST values and indicated a gradual cooling from 19 °C to ~ 15 °C at the top of the record (~ 450 a BP).

High‐amplitude variations in North Atlantic sea surface temperature during the early Pliocene warm period

We provide the first continuous, orbital‐resolution sea surface temperature (SST) record from the high‐latitude North Atlantic, a region critical to understanding the origin of the Plio‐Pleistocene ice ages and proximal to regions that became frequently glaciated after ∼2.7 Ma. We analyzed sediments from Ocean Drilling Program Site 982 over the last 4 Ma for their alkenone unsaturation index and compared this surface water signal to a benthic δ18O record obtained from the same section. We find that while ocean surface temperatures were significantly warmer (∼6°C) than modern temperatures during the early Pliocene, they were also as variable as those during the late Pleistocene, a surprising result in light of the subdued variance of oxygen isotopic time series during the interval of 3–5 Ma. We propose two possible explanations for the high orbital‐scale SST variability observed: either that a strong, high‐latitude feedback mechanism not involving large continental ice sheets alternately cooled and warmed a broad region of the northern high latitudes or that by virtue of its location near the northern margin of the North Atlantic Drift, the site was unusually sensitive to obliquity‐driven climate shifts. On supraorbital time scales, a strong, sustained cooling of North Atlantic SSTs (∼4.5°C) occurred from 3.5 to 2.5 Ma and was followed by an interval of more modest cooling (an additional 1.5°C) from 2.5 Ma to the present. Evolutionary orbital‐scale phase relationships between North Atlantic SST and benthic δ18O show that SST began to lead δ18O significantly coincident with the onset of strong cooling at Site 982 (∼3.5 Ma). We speculate that these changes were related to the growth and subsequent persistence of a Greenland ice sheet of approximately modern size through interglacial states.

New quantitative evidence of extreme warmth in the Pliocene Arctic

The most recent geologic interval characterized by warm temperatures similar to those projected for the end of this century occurred about 3.3 to 3.0 Ma, during the mid-Piacenzian Age of the Pliocene Epoch. Climate reconstructions of this warm period are integral to both understanding past warm climate equilibria and to predicting responses to today’s transient climate. The Arctic Ocean is of particular interest because in this region climate proxies are rare, and climate models struggle to predict climate sensitivity and the response of sea ice. In order to provide the first quantitative climate data from this region during this interval, sea surface temperatures (SST) were estimated from Ocean Drilling Program Sites 907 and 909 in the Nordic Seas and from Site 911 in the Arctic Ocean based on Mg/Ca of Neogloboquadrina pachyderma (sin) and alkenone unsaturation indices. Evidence of much warmer than modern conditions in the Arctic Ocean during the mid-Piacenzian with temperatures as high as 18°C is presented. In addition, SST anomalies (mid-Piacenzian minus modern) increase with latitude across the North Atlantic and into the Arctic, extending and confirming a reduced mid-Piacenzian pole-to-equator temperature gradient. The agreement between proxies and with previously documented qualitative assessments of intense warming in this region corroborate a poleward transport of heat and an at least seasonally ice-free Arctic, conditions that may serve as a possible analog to future climate if the current rate of Arctic sea-ice reduction continues.

A 5 million year comparison of Mg/Ca and alkenone paleothermometers

Geochemical sea surface temperature (SST) proxies such as the magnesium to calcium ratio (Mg/Ca) in foraminifera and the alkenone unsaturation index (UK′37) are becoming widely used in pre‐Pleistocene climate records. This study quantitatively compares previously published Mg/Ca and UK′37 data from Ocean Drilling Program (ODP) site 847 in the eastern equatorial Pacific to assess the utility of these proxies to reconstruct tropical SST over the last 5 Ma. Foraminiferal Mg/Ca–SST calibrations that include a dissolution correction are most appropriate at this location because they provide SST estimates for the youngest sample that are close to modern mean annual SST. The long‐term trends in the two records are remarkably similar and confirm a ∼3.5°C cooling trend from the early Pliocene warm period to the late Pleistocene noted in previous work. Absolute temperature estimates are similar for both proxies when errors in the dissolution correction used to estimate SST from Mg/Ca are taken into account. Comparing the two SST records at ODP site 847 to other records in the region shows that the eastern equatorial Pacific was 2–4°C warmer during the early Pliocene compared to today.

Reevaluation of mid‐Pliocene North Atlantic sea surface temperatures

Multiproxy temperature estimation requires careful attention to biological, chemical, physical, temporal, and calibration differences of each proxy and paleothermometry method. We evaluated mid‐Pliocene sea surface temperature (SST) estimates from multiple proxies at Deep Sea Drilling Project Holes 552A, 609B, 607, and 606, transecting the North Atlantic Drift. SST estimates derived from faunal assemblages, foraminifer Mg/Ca, and alkenone unsaturation indices showed strong agreement at Holes 552A, 607, and 606 once differences in calibration, depth, and seasonality were addressed. Abundant extinct species and/or an unrecognized productivity signal in the faunal assemblage at Hole 609B resulted in exaggerated faunal‐based SST estimates but did not affect alkenone‐derived or Mg/Ca–derived estimates. Multiproxy mid‐Pliocene North Atlantic SST estimates corroborate previous studies documenting high‐latitude mid‐Pliocene warmth and refine previous faunal‐based estimates affected by environmental factors other than temperature. Multiproxy investigations will aid SST estimation in high‐latitude areas sensitive to climate change and currently underrepresented in SST reconstructions.

Alteration of alkenone unsaturation ratio with depth in the Black Sea: Potential roles of stereomutation and aerobic biodegradation

Suspended particulate matter and sediment samples collected from the Black Sea were investigated in order to (i) obtain additional information about the haptophytes present in this zone and (ii) try to determine the cause (biological or chemical) of the strong loss of the more unsaturated alkenones with depth previously observed and reflected in U 37 k ′ . After NaBH 4 reduction and silylation, we detected MeC 38:4 and EtC 38:4 alkenones. The presence of these two tetraunsaturated alkenones suggests that the source(s) of alkenones in the Black Sea could be one (or more) unknown strain(s) of Emiliania huxleyi adapted to the low salinity. The detection of an EtC 36:2 alkenone exhibiting the same unusual ω15,20 double bond positions as the EtC 36:2 ethyl alkenone previously detected in Holocene (Unit II) Black Sea sediments shows that the unidentified source of this alkenone is still present today in the Black Sea water column. As previously observed, the MeC 37:2 concentration declines slowly with depth, while MeC 37:3 and MeC 37:4 decrease dramatically, resulting in a strong increase in U 37 k and U 37 k ′ values. The detection of stereomutated alkenones and epoxyalkenones in some samples suggests that the strong increase in U 37 k ′ with depth in the oxic zone could be due to a combination of selective aerobic bacterial degradation and stereomutation, but with biodegradation apparently predominating. However, the presence of two populations of haptophytes exhibiting distinct responses toward growth temperature could also play a role in this strong increase in U 37 k ′ .

Analysis of Alkenone Unsaturation Indices with Fast Gas Chromatography/Time-of-Flight Mass Spectrometry

Extensively purified C37 alkenone references and mixtures thereof were analyzed by gas chromatography/flame ionization detection (GC/FID) and fast gas chromatography/time-of-flight mass spectrometry (GC/TOF-MS), to establish the latter as an alternative, fast, and reliable analysis method for alkenone unsaturation indices (U(k')(37)). This index is a tool for past sea surface temperature reconstructions with extensive use in paleoclimate and paleoceanographic research. TOF-MS was chosen because of its unique capability to acquire full-range spectra at high data rates (up to 500 spectra s(-1)) and to produce homogeneous spectra across a gaschromatographic peak, allowing faster separations than conventional GC/MS and the employment of enhanced peak deconvolution algorithms. Analysis time per sample could be reduced to run times of <10 min, i.e., by a factor of approximately 10 compared to conventional GC/FID (90-100 min) methods. However, %@mt;sys@%%@ital@%%@bold@%U%@reset@%%@rsf@%%@sx@%37%@be@%%@ital@%k%@rsf@%'%@sxx@%%@mx@% values from GC/TOF-MS showed deviations from those obtained by GC/FID, resulting from sensitivity differences between the C37:2 and C37:3 alkenone when analyzed by GC/TOF-MS. A solution to this bias is presented by determining compound-specific linear response factor equations to derive sensitivity ratios (SR) that allow conversion of GC/TOF-MS values into calibrated GC/FID data. Using alkenone mixtures of known composition and a variety of samples from natural environments, the applicability of this approach is demonstrated.

Distribution of the C 37 tetra-unsaturated alkenone in Lake Qinghai, China: A potential lake salinity indicator

The alkenone unsaturation index U K′ 37 has been applied to reconstruct past temperature changes in both marine and lacustrine systems. However, few studies have addressed whether the relative abundance of the C 37:4 alkenone to the total C 37 production (%C 37:4) can reflect surface salinity changes in lacustrine systems. Here we present long-chain C 37 alkenone distribution patterns in surface sediments from Lake Qinghai, China. Surface sediments were sampled over a large range of surface salinity changes (1.7–25 g/l) within Lake Qinghai and its surrounding lakes, while temperature differences at these sampling locations should be relatively small. We have found that %C 37:4 varies from 15% to 49% as surface salinity decreases. We tentatively describe this %C 37:4–salinity link with a general linear regression: %C 37:4 = 53.4 (±7.8) − 1.73 (±0.45) × S ( n = 28, r 2 = 0.62), although step-wise %C 37:4 changes in response to salinity variation may exist. U K′ 37 values vary between 0.10 and 0.16 at these sites and the inferred range of lake water temperature changes is ∼2–3 °C, suggesting that U K′ 37 largely reflects temperature signal across a large salinity range, consistent with previous findings that U K′ 37 can indicate temperature changes over a large diversity of environmental settings. We have also found that U K′ 37 values are correlated with salinity changes ( r 2 = 0.4), and thus cannot exclude potential temperature effect on %C 37:4 and salinity effect on U K′ 37 in this study. However, even extreme estimates of temperature differences within the lake are still unable to explain the observed %C 37:4 changes. We therefore suggest that %C 37:4 could be used to infer past lake salinity changes at a regional scale.

Degradation of alkenones by aerobic heterotrophic bacteria: Selective or not?

Four bacterial communities were isolated from Emiliania huxleyi strain TWP1 cultures before and after the algal cells had been treated with different antibiotics. Incubation of E. huxleyi with these bacterial communities resulted in dramatically different extents of alkenone degradation, ranging from effectively none to extensive. Selective degradation of the more unsaturated alkenones was observed in experiments using the total bacterial community and one of the communities isolated from antibiotic-treated algal cells. The observed increases in U 37 K ′ are equivalent to a +2 °C and +3.3 °C change in the inferred temperature. Our results clearly show that intense aerobic microbial degradative processes have the potential to introduce a significant ‘warm’ bias in palaeotemperature reconstruction and could explain apparent anomalies in palaeotemperatures inferred from alkenone distributions in strongly oxidizing sedimentary environments. The results show that aerobic bacteria capable of selectively degrading alkenones are not limited to particular environments such as microbial mats and can be actually associated with living E. huxleyi cells. The detection of epoxyketones in some cultures indicates that metabolic pathways involving attack at the terminal groups of the molecule are essentially non-selective, while those acting on alkenone double bonds are selective. The epoxyketones resulting from bacterial epoxidation of alkenone double bonds could be useful indicators of aerobic bacterial alteration of the alkenone unsaturation ratio in situ. The production of alkenols during incubation with one of the bacterial communities demonstrated for the first time that bacterial reduction of alkenones can be a potential source of these compounds in the environment. The intriguing production of small amounts of monounsaturated alkenones by one of the bacterial communities also raises the possibility of a bacterial reduction of alkenone double bonds.

No-analog planktonic foraminiferal faunas in the glacial southern South China Sea: Implications for the magnitude of glacial cooling in the western Pacific warm pool

Sea-surface temperature (SST) estimates in the sediment core MD01-2390 based on planktonic foraminiferal species abundances using five different transfer function techniques suggest nearly unchanged or unusually higher temperatures in the tropical southern South China Sea (SCS) during the Last Glacial Maximum (LGM) relative to modern temperatures. These results are in contrast to substantial cooling of 2–5 °C inferred by geochemical (U k' 37, Mg/Ca ratios) and terrestrial proxies from the western tropical Pacific region. Using multivariate statistics we show that the glacial southern SCS harboured unique planktonic foraminiferal assemblages that have no modern analogs. Analyses of faunal variation through the core reveal that planktonic foraminiferal assemblages responded to temperature changes inferred from Mg/Ca data but that this signal is subdued by superimposed variations in the relative abundance of Pulleniatina obliquiloculata and Neogloboquadrina pachyderma (dextral). These species occur in glacial samples at proportions that are not observed in the calibration data set. The glacial high abundance of N. pachyderma (dextral) are interpreted to reflect a seasonal (winter) inflow of cold surface water from the northeast via the Bashi Strait due to the combined effects of an intensified winter monsoon, a southward shift of the polar front and the eastward migration of the Kuroshio Current. In contrast, processes controlling the high relative abundances of P. obliquiloculata during the LGM may be unique to the southern SCS. We propose a scenario involving a stronger (winter) mixing or enhanced upwelling due to an intensified winter monsoon that prevented shallow-dwelling, warm indicators to establish larger populations during the LGM. Our results indicate that a no-analog behaviour of planktonic foraminifera faunas is responsible for the warm glacial conditions in this part of the western Pacific warm pool as implied by foraminiferal transfer functions and that a more significant surface cooling in the region as implied by terrestrial and geochemical (Mg/Ca ratios; alkenone unsaturation index) marine proxies is a more likely scenario.

Variability in upwelling intensity and nutrient regime in the coastal upwelling system offshore Namibia: results from sediment archives

The coastal upwelling system offshore Namibia is ideally suited to address a focal question of the Integrated Marine Biogeochemistry and Ecosystem Research Programme: what are the mechanisms that drive long-term changes in ecosystems? Considerable interannual variability in climatic forcing is indicated by long time series of meteorological and remote sensing observation; these accompany considerable interannual to interdecadal changes in the upwelling intensity over the last 100 years, as well as a centennial trend. On longer time scales, the only archives available are sediment records spanning the late Holocene. To decipher the sediment record, we mapped surface-sediment patterns of proxies for physical (sea surface temperature/SST from alkenone unsaturation indexes) and nutrient (δ15N on bulk sedimentary N) variables. Their present-day surface-sediment patterns outline the coastal upwelling cells and filaments and associated high productivity area. Analysed in an array of dated sediment cores, the spatial patterns of SST suggest long-term (>100 years) variability in the location and intensity of individual upwelling cells. The patterns of δ15N outline an area of intense denitrification near the coast, and advection of water with low-oxygen concentrations in the undercurrent from the North. δ15N exhibits considerable downcore variability, in particular over the last 50 years. The variability appears to be governed by differences in extent of denitrification and thus of the shelf oxygen balance, which appears to have deteriorated in the last 50 years. Together, the data suggest that SST and denitrification conditions have remained in the narrow bounds outlined by today’s patterns in surface sediments, but that spatially small variability in upwelling intensity and make-up of upwelling feed waters induced considerable changes in the lower trophic levels of the coastal upwelling ecosystem over the last 6,000 years. Attempts to correlate proxy records from sediments with observational time series and regional climate reconstructions were not successful, possibly because annual to interannual environmental signals are erased in the process of sediment formation.

A multiproxy assessment of the western equatorial Pacific hydrography during the last 30 kyr

Sea surface temperature (SST) and salinity of the Western Pacific Warm Pool (WPWP) reflect global climate effects such as the El Niño–Southern Oscillation phenomenon. However, reconstructions of past changes in the WPWP from the geologic record vary depending on the specific proxy record used. Here we develop a multiproxy record of the last deglaciation from a radiocarbon‐dated sediment core (MD97‐2138) retrieved in the heart of the WPWP. SST reconstructions for the past 30,000 years based on planktonic foraminiferal Mg/Ca (Globigerinoides ruber and Globigerinoides sacculifer), alkenone unsaturation index, and foraminiferal transfer functions differ notably. Mg/Ca‐based SST estimates from the surface dwelling species G. ruber in MD97‐2138 indicate a larger surface cooling (3° ± 0.6°C) during the Last Glacial Maximum (LGM) than inferred from Mg/Ca ratios in G. sacculifer (2.3° ± 0.7°C), statistical transfer functions based on planktonic foraminiferal species assemblages, and Uk37′ (1°–2°C). These estimates are consistent with estimates from other WPWP cores, thereby suggesting that the discrepancy is due to proxy compatibility rather than differences in cores qualitity. Postdepositional dissolution above the lysocline might have altered the Mg/Ca‐based temperature estimates in our site, but this effect is insufficient to resolve discrepancies between Mg/Ca in G. ruber and the other proxies. We suggest that the lower estimates obtained from Mg/Ca in G. sacculifer, faunal transfer functions, and Uk37′ might reflect subsurface temperature changes rather than strict surface estimates. Accounting for potential artefacts, including dissolution and bioturbation, we suggest that the glacial WPWP SST was about 2.5° ± 0.7°C cooler than during the Holocene, whereas the subsurface/upper thermocline temperature change was only about 1.8° ± 0.7°C. Interpreting variations in δ18OSW in terms of salinity changes suggests a possibly slight decrease in surface salinity at the site of MD97‐2138 during the LGM. Though LGM freshening in MD97‐2138 is not robust to postdeposition dissolution effects, this inferred freshening appears to be a general feature of the western equatorial Pacific.

Seasonal and depth variations in molecular and isotopic alkenone composition of sinking particles from the western North Pacific

Seasonal and depth variations in alkenone flux and molecular and isotopic composition of sinking particles were examined using a 21-month time-series sediment trap experiment at a mooring station WCT-2 (39°N, 147°E) in the mid-latitude NW Pacific to assess the influences of seasonality, production depth, and degradation in the water column on the alkenone unsaturation index U K′ 37. Analysis of the underlying sediments was also conducted to evaluate the effects of alkenone degradation at the water–sediment interface on U K′ 37. Alkenone sinking flux and U K′ 37-based temperature showed strong seasonal variability. Alkenone fluxes were higher from spring to fall than they were from fall to spring. During periods of high alkenone flux, the U K′ 37-based temperatures were lower than the contemporary sea-surface temperatures (SSTs), suggesting alkenone production in a well-developed thermocline (shallower than 30 m). During low alkenone flux periods, the U K′ 37-based temperatures were nearly constant and were higher than the contemporary SSTs. The nearly constant carbon isotopic ratios of C 37:2 and C 38:2 alkenones suggest that alkenones produced in early fall were suspended in the surface water until sinking. The alkenone sinking flux decreased exponentially with increasing depth. The decreasing trend was enhanced during the periods of high alkenone flux, suggesting that fresh and labile particles sank from spring to fall, while old and stable particles sank from fall to spring. The U K′ 37-based temperature usually increased with increasing depth. The preservation efficiency of alkenones was ∼2.7–5.2% at the water–sediment interface. Despite the significant degradation of the alkenones, there was little difference in U K′ 37 levels between sinking particles and the surface sediment.

Calibration of alkenone unsaturation index with growth temperature for a lacustrine species, Chrysotila lamellosa (Haptophyceae)

Although long chain alkenones (LCKs) occur widely in lacustrine sediments, their origin is not clear. Here, we report a lacustrine source, the non-calcifying species Chrysotila lamellosa Anand (Haptophyceae), collected and isolated from an inland saline water body, Lake Xiarinur (Inner Mongolia, China). Its alkenone pattern is similar to those of coastal marine strains of C. lamellosa,but the relationship between U 37 K ′ index and culture temperature for the lacustrine species is quite different from that of the coastal species. A significant feature of the alkenones in this strain of C. lamellosa is a lack of C 38 methyl alkenones, which might be used to distinguish the species from the marine haptophyte species Emiliania huxleyi and Gephyrocapsa oceanica. The higher C 38 tetraunsaturated compound abundance might be another important feature for distinguishing the C. lamellosa alkenone producer from the coastal species Isochrysis galbana. This alkenone distribution pattern has been detected in many lakes, which suggests that C. lamellosa or a closely related species might be a very common alkenone precursor in lacustrine systems. We examined U 37 K ′ and U 37 K values for C. lamellosa as a function of culture temperature in a batch culture experiment. The calibration for U 37 K ′ vs. culture temperature ( T) was U 37 K ′ = 0.0011 × T 2 - 0.0157 × T + 0.1057 ( n = 14 , r 2 = 0.99 ) from 10 °C to 22 °C or U 37 K ′ = 0.0257 × T - 0.2608 ( n = 9 , r 2 = 0.97 ) from 14 °C to 22 °C. U 37 K vs. culture temperature was U 37 K = 0.0377 × T - 0.5992 ( n = 14 , r 2 = 0.98 ) from 10 °C to 22 °C. Our experiments show that the alkenone unsaturation index ( U 37 K ′ ) is strongly controlled by culture temperature and can be used for palaeoclimate reconstruction.