Controls on alkenone unsaturation ratios along the salinity gradient between the open ocean and the Baltic Sea

Alkenone unsaturation ratios of sedimentary lipids are used as a geochemical proxy for sea surface temperatures, and interest is growing in their potential as indicators of different water masses and possibly of salinity. We analyzed the abundance of unsaturated C₃₇ to C₃₈ ketones in lipid extracts of 57 surface sediment (0-1 cm) samples along a salinity gradient from 8 to 33 psu in the transition from the Skagerrak to the Baltic Sea (NW Europe). In addition to surface sediments, we analyzed alkenones in suspended particulate matter at 13 stations—over a gradient in salinity from 25 to 33 psu—during a bloom of the coccolithophore Emiliania huxleyi. Alkenones were detected in all samples (suspended matter and sediment) with variable contributions of the tetra-unsaturated C₃₇ alkenone compound (%C_37:4; range from 2 to 10% of total C₃₇ alkenone content). Comparing the alkenone unsaturation index (U₃₇^K′) and %C_37:4 data to climatological sea surface temperature and sea surface salinity data sets revealed that SST estimated from U₃₇^K′ of saline end members (samples from the Skagerrak) is in the general range of modern SST during bloom periods of haptophytes. At salinities below ∼30 psu %C_37:4 increases to above 5% and the unsaturation ratios cease to be related to climatological annual or seasonal sea surface temperatures. On the other hand, the %C_37:4 appears to be inversely and significantly correlated to salinity: Highest C_37:4 proportions in the inner Baltic Sea are caused by an unidentified organism, but in the transition area at salinities down to 10 psu, the producer apparently is E. huxleyi. The suspended matter data together with those from the water column support the hypothesis of changing biosynthesis of alkenones under salt stress by the coccolithophore E. huxleyi, but constrain the maximum of %C_37:4 attributable to salt stress to 10% of all C₃₇ alkenones.     

Temperature and Salinity Effects on Alkenone Ratios Measured in Surface Sediments from the Indian Ocean

We compare alkenone unsaturation ratios measured on recent sediments from the Indian Ocean (20°N–45°S) with modern sea oceanographic parameters. For each of the core sites we estimated average seasonal cycles of sea surface temperature (SST) and salinity, which we then weighted with the seasonal productivity cycle derived from chlorophyll satellite imagery. The unsaturation index (U₃₇^K′) ranges from 0.2 to 1 and correlates with water temperature but not with salinity. TheU₃₇^K′versus SST relationship for Indian Ocean sediments (U₃₇^K′= 0.033 SST + 0.05) is similar to what has been observed for core tops from the Pacific and Atlantic oceans and the Black Sea. A global compilation for core tops givesU₃₇^K′= 0.031 T + 0.084 (R= 0.98), which is close to a previously reported calibration based on particulate organic matter from the water column. For temperatures between 24° and 29°C, however, the slope seems to decrease to about 0.02U₃₇^K′unit/°C. For Indian Ocean core tops, the ratios of total C₃₇alkenones/total C₃₈alkenones and the slope of theU₃₇^K′-SST relationship are similar to those previously observed for cultures ofEmiliania huxleyibut different from those previously published forGephyrocapsa oceanica.EitherE. huxleyiis a major producer of alkenones in the Indian Ocean or strains ofG. oceanicaliving in the northern Indian Ocean behave differently from the one cultured. In contrast with coccolithophorid assemblages, the ratios of C₃₇alkenones to total C₃₈alkenones lack clear geographic pattern in the Indian Ocean.     

Evidence for differential degradation of alkenones under contrasting bottom water oxygen conditions: implication for paleotemperature reconstruction

The successful reconstruction of sea surface temperatures using alkenone paleothermometry (U₃₇^k′) has relied on the premise that there is no significant differential degradation of alkenones with different states of unsaturation during diagenetic processes. To test this assumption, we conducted a comparative study of contemporary sediments in oxic and anoxic bottom waters from the Santa Monica Basin, offshore California. Long-chain alkenones were quantified and sea surface temperature were calculated using the calibrated U₃₇^k′–T relationship of Prahl et al. (1988). Our results show that temperature record from the oxic sediments is higher by as much as 4°C compared to those from time-equivalent anoxic sediments as a result of differential degradation of long-chain unsaturated alkenones and bioturbation mixing in the oxic sediments. The differential degradation of C_37:3 vs. C_37:2 alone could account for up to 2.5°C difference between these two records. This finding has significant implication in the interpretation of paleo–sea surface temperature data using alkenone paleothermometry.     

Distribution of the C37 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 C₃₇ production (%C_37:4) can reflect surface salinity changes in lacustrine systems. Here we present long-chain C₃₇ 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, r² = 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 (r² = 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.     

Long-chain alkenone distributions and temperature dependence in lacustrine surface sediments from China

Long-chain alkenones (LCK) of lacustrine surface sediments were analyzed in 37 lakes from China. The results obtained were complemented by published data from 13 other Chinese lakes. These lakes are located across large temperature and precipitation gradients, therefore allowing for an assessment of the distribution pattern of LCK and their temperature dependency. Different distribution patterns of LCK (C₃₇ predominant pattern and C₃₈ predominant pattern) were detected in the surface sediment samples. The ratio of C_37:4 methyl ketone to the sum of C₃₇ alkenones observed in the different lakes is highly variable (5%-96%, with mean value of 55%), and more than that seen in marine systems. The finding that some of the ocean LCK precursor algae (Gephyrocapsa oceanica, Coccolithus pelagicus) were also present in the limnic systems suggested that both systems might have similar biosynthetic sources. Empirical relationships between the alkenone unsaturation index U₃₇^k′ and different temperature sets (mean annual air temperature, mean annual air temperature in different seasons, and lake surface water temperature of July) were tested. The best correlation between U₃₇^k′ and temperature was obtained using mean annual air temperature. A general linear regression of U₃₇^k′ and MAAT can be expressed as U₃₇^k′ = 0.0328 × T + 0.126 (n = 38, r² = 0.83). Although questions such as species-uncertainty and other unknown factors for U₃₇^k′ temperature dependence still remain, the equation might be representative of the average contribution of LCK to sediments for these data over a wide range of surface temperatures, water chemistry and different alkenones-producer algal populations. The general relationship of U₃₇^k′ and mean annual air temperature is consistent with that in marine systems. It supports the suggestion that the biosynthetic pathway of alkenones and the mechanism of their temperature signal may be similar in both marine and limnic systems. LCK might be used as an important paleotemperature proxy in limnic environment.     

Alkenones and coccoliths in ice‐rafted debris during the Last Glacial Maximum in the North Atlantic: implications for the use of UK37′ as a sea surface temperature proxy

The U^K₃₇′ index has proven to be a robust proxy to estimate past sea surface temperatures (SSTs) over a range of time scales, but like any other proxy, it has uncertainties. For instance, in reconstructions of the Last Glacial Maximum (LGM) in the northern North Atlantic, U^K₃₇′ indicates higher temperatures than those derived from foraminiferal proxies. Here we evaluate whether such warm glacial estimates are caused by the advection of reworked alkenones in ice‐rafted debris (IRD) to deep‐sea sediments. We have quantified both coccolith assemblages and alkenones in sediments from glaciogenic debris flows in the continental margins of the northern North Atlantic, and from a deep‐sea core from the Reykjanes Ridge. Certain debris flow deposits in the North Atlantic were generated by the presence of massive ice‐sheets in the past, and their associated ice streams. Such deposits are composed of the same materials that were present in the IRD at the time they were generated. We conclude that ice rafting from some locations was a transport pathway to the deep sea floor of reworked alkenones and pre‐Quaternary coccolith species during glacial stages, but that not all of the IRD contained alkenones, even when reworked coccoliths were present. We speculate that the ratio of reworked coccoliths to alkenone concentration might be useful to infer whether significant reworked alkenone inputs from IRD did occur at a particular site in the glacial North Atlantic. We also observe that alkenones in some of the debris flows contain a colder signal than estimated for LGM sediments in the northern North Atlantic. This is also clear in the deep‐sea core studied where the warmest intervals do not correspond to the intervals with large inputs of reworked coccoliths or IRD. We conclude that any possible bias to U^K₃₇′ estimates associated with reworked alkenones is not necessarily towards higher values, and that the high SST anomalies for the LGM are unlikely to be the result of a bias caused by IRD inputs. Copyright © 2011 John Wiley & Sons, Ltd.     

Salinity control on long-chain alkenone distributions in lake surface waters and sediments of the northern Qinghai-Tibetan Plateau, China

Long-chain alkenones in lacustrine settings are potentially excellent biomarkers for the reconstruction of past terrestrial environmental conditions, and have been found in many different types of lakes around the globe. A wider range of factors influence the occurrence and distribution of alkenones in lake sediments and waters when compared to marine systems. Lake environmental conditions, such as temperature (in particular) and salinity, are among the key factors controlling alkenone distributions in lacustrine settings. Here we investigated alkenone distribution patterns in lakes of the northern Qinghai-Tibetan Plateau, China, and their possible relationship with environmental conditions, by analyzing paired samples of suspended particulate matter in surface waters and surface sediments. Salinity of investigated lake waters ranges from almost 0 to ∼100 g/L, while temperature variation among the lakes is minimal, effectively eliminating temperature effects on the alkenone distribution patterns observed here. We show that (1) alkenone concentrations vary substantially between the lakes, yet controlling mechanisms remain elusive; (2) C₃₇/C₃₈ ratios are substantially lower in the lakes of the Qaidam Basin than in the Lake Qinghai region, probably indicating different alkenone producers in the two regions; and (3) large variations in %C_37:4 (the percentage of the C_37:4 alkenone), determined from both surface waters and sediments, are negatively correlated with salinity. We suggest that the %C_37:4 index could be used as a salinity indicator at least on a regional scale, with careful considerations of other potentially complicating factors. However, potential reasons for why salinity could significantly affect %C_37:4 values need further investigation.     

Characteristics of alkenone distributions in suspended and sinking particles in the northwestern North Pacific

We investigated alkenones recorded in suspended particles and a settling particle time series collected at three stations, 40N (40°N, 165°E), KNOT (44°N, 155°E), and 50N (50°N, 165°E), in the northwestern North Pacific from December 1997 to May 1999. Emiliania huxleyi, the most abundant alkenone producer in this area, is present in surface to subsurface (to ∼50 m depth) waters. The alkenone concentrations recorded in the suspended particles indicated that the seasonal alkenone particle distribution differed significantly interannually. Alkenone export fluxes at the three sediment-trap stations ranged from 0.16 to 49.3 μg m⁻² day⁻¹, and the maximum export flux, which occurred in summer to fall (July-November), was associated with a high organic carbon export flux. The amount of alkenone produced during the maximum export season accounted for 60-80% of the total annual amount of alkenone, and the alkenones accumulated in the sediment below the traps had characteristics corresponding to subsurface waters during the summer-autumn season. Alkenone-derived temperatures recorded in suspended particles corresponded to the in situ temperature within ∼2 °C. Although alkenone-derived temperatures corresponded approximately to the temperatures observed in the stratified subsurface waters at the three trap stations during the high-export season, large differences were observed during the low-export (winter-spring) period. For example, the alkenone-derived temperatures observed at stations KNOT and 50N were much higher than the in situ subsurface temperatures reported in the World Ocean Atlas 2001. Relatively large differences between alkenone-derived temperatures and in situ temperatures in the subarctic might be due to (1) a low-light limitation or (2) contributions of allochthonous alkenones in particulate material transported from subtropical areas within a warm-core ring.     

Determination of sediment provenance at drift sites using hydrogen isotopes and unsaturation ratios in alkenones

The large (∼20‰) hydrogen isotopic gradient in surface waters of the northwest Atlantic Ocean is exploited to track changes in the source of alkenones to the Bermuda Rise sediment drift. Cultures of the predominant alkenone-producing coccolithophorid, E. huxleyi, were grown in deuterium-enriched seawater and shown to possess alkenones with a D/H ratio that closely tracked the water D/H ratio (r² = 0.999, n = 5 isotopic enrichments) with a fractionation factor (α) between 0.732 and 0.775. A hydrogen isotopic depletion of -193 ± 3‰ (n = 9) was measured in alkenones from suspended particles relative to seawater in the subpolar and subtropical northwest Atlantic Ocean. This value was used to calculate the water δD values in which alkenones from Bermuda Rise sediment were synthesized, and by extension, the water mass in which they were produced. Applying this technique we find that 60% to 100% of the alkenones in late Holocene Bermuda Rise sediment were produced in deuterium-depleted subpolar water to the northwest of the drift. To reconcile values of the alkenone unsaturation ratio (U^k′₃₇), a widely used proxy for sea surface temperature, with the δD values of alkenones in late Holocene sediments from the Bermuda Rise at least three sources of sediment must be invoked: a cold, very isotopically depleted source, almost certain to be the Scotian Margin; a warm, moderately isotopically-depleted source, likely to be the northwestern edge of the subtropical gyre; and a cold, isotopically enriched source, which we hypothesize to be the subpolar waters overlying the main branch of North Atlantic Deep Water flowing southwest from the Nordic Seas.     

Climatic and environmental controls on the occurrence and distributions of long chain alkenones in lakes of the interior United States

Long chain alkenones (LCA) are temperature-sensitive lipids with great potential for quantitative reconstruction of past continental climate. We conducted the first survey for alkenone biomarkers from 55 different lakes in the Northern Great Plains and Nebraska Sand Hills of the United States. Among those surveyed, we found 13 lakes that contain LCAs in the surface sediments. The highest concentrations of alkenones in sediments are found in cold (mean annual air temperature ∼11 °C versus 17 °C in our warmest sites), brackish to mesosaline (salinity = 8.5-9.7 g/L), and alkaline (pH = 8.4-9.0) lakes with high concentrations of sodium and sulfate. The dynamics of stratification and nutrient availability also appear to play a role in LCA abundance, as early spring mixing promotes a bloom of alkenone-producing haptophytes. Four of the alkenone-containing sites contain the C_37:4 alkenone; however, we discovered an unprecedented lacustrine alkenone distribution in a cluster of lakes, with a total absence of C_37:4 alkenone. We attribute this unusual composition to a different haptophyte species and show that the sulfate:carbonate ratio may control the occurrence of these two distinct populations. We created a new in-situ temperature calibration for lacustrine sites that contain C_37:4 using a water-column calibration from Lake George, ND and show that is linearly correlated to lake water temperature (R² = 0.74), but is not. A number of lakes contain an unidentified compound series that elutes close to the LCAs, highlighting the importance of routine GC-MS examination prior to using lacustrine LCAs for paleotemperature reconstructions.     

Carbon isotopic fractionation during a mesocosm bloom experiment dominated by Emiliania huxleyi: Effects of CO2 concentration and primary production

We investigated the effect of CO₂ and primary production on the carbon isotopic fractionation of alkenones and particulate organic matter (POC) during a natural phytoplankton bloom dominated by the coccolithophore Emiliania huxleyi. In nine semi-closed mesocosms (∼11 m³ each), three different CO₂ partial pressures (pCO₂) in triplicate represented glacial (∼180 ppmv CO₂), present (∼380 ppmv CO₂), and year 2100 (∼710 ppmv CO₂) CO₂ conditions. The largest shift in alkenone isotopic composition (4-5‰) occurred during the exponential growth phase, regardless of the CO₂ concentration in the respective treatment. Despite the difference of ∼500 ppmv, the influence of pCO₂ on isotopic fractionation was marginal (1-2‰). During the stationary phase, E. huxleyi continued to produce alkenones, accumulating cellular concentrations almost four times higher than those of exponentially dividing cells. Our isotope data indicate that, while alkenone production was maintained, the interaction of carbon source and cellular uptake dynamics by E. huxleyi reached a steady state. During stationary phase, we further observed a remarkable increase in the difference between δ¹³C of bulk organic matter and of alkenones spanning 7-12‰. We suggest that this phenomenon is caused mainly by a combination of extracellular release of ¹³C-enriched polysaccharides and subsequent particle aggregation induced by the production of transparent exopolymer particles (TEP).     

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.     

Hydrogen isotopes in individual alkenones from the Chesapeake Bay estuary

Hydrogen isotope ratios of individual alkenones from haptophyte algae were measured in suspended particles and surface sediment from the Chesapeake Bay (CB) estuary, eastern USA, in order to determine their relationship to water δD values and salinity. δD values of four alkenones (MeC_37:2, MeC_37:3, EtC_38:2, EtC_38:3) from particles and sediments were between −165‰ and −221‰ and increased linearly (R² = 0.7-0.9) with water δD values from the head to the mouth of the Bay. Individual alkenones were depleted in deuterium by 156-188‰ relative to water. The MeC₃₇ alkenones were consistently enriched by ∼12‰ relative to the EtC₃₈ alkenones, and the di-unsaturated alkenones of both varieties were consistently enriched by ∼20‰ relative to the tri-unsaturated alkenones. All of the increase in alkenone δD values could be accounted for by the water δD increase. Consequently, no net change in alkenone-water D/H fractionation occurred as a result of the salinity increase from 10 to 29. This observation is at odds with results from culture studies with alkenone-producing marine coccolithophorids, and from two field studies, one with a dinoflagellate sterol in the CB, and one with a wide variety of lipids in saline ponds on Christmas Island, that indicate a decline in D/H fractionation with increasing salinity. Why D/H fractionation in alkenones in the CB showed no dependence on salinity, while D/H fractionation in CB dinsoterol decreased by 1‰ per unit increase in salinity remains to be determined. Two hypotheses we consider to be valid are that (i) the assemblage of alkenone-producing haptophytes changes along the Bay and each species has a different sensitivity to salinity, such that no apparent trend in α_{alkenone-water} occurs along the salinity gradient, and (ii) greater osmoregulation capacity in coastal haptophytes may result in a diminished sensitivity of alkenone-water D/H fractionation to salinity changes.     

A High-Resolution Sea-Surface Temperature Record from the Tropical South China Sea (16,500–3000 yr B.P.)

The timing and magnitude of sea-surface temperature (SST) changes in the tropical southern South China Sea (SCS) during the last 16,500 years have been reconstructed on a high-resolution, ¹⁴C-dated sediment core using three different foraminiferal transfer functions (SIMMAX28, RAM, FP-12E) and geochemical (U^k′₃₇) SST estimates. In agreement with CLIMAP reconstructions, both the FP-12E and the U^k′₃₇ SST estimates show an average late glacial–interglacial SST difference of 2.0°C, whereas the RAM and SIMMAX28 foraminiferal transfer functions show only a minor (0.6°C) or no consistent late glacial–interglacial SST change, respectively. Both the U^k′₃₇ and the FP-12E SST estimates, as well as the planktonic foraminiferal δ¹⁸O values, indicate an abrupt warming (ca. 1°C in <200 yr) at the end of the last glaciation, synchronous (within dating uncertainties) with the Bølling transition as recorded in the Greenland Ice Sheet Project 2 (GISP2) ice core, whereas the RAM-derived deglacial SST increase appears to lag during this event by ca. 500 yr. The similarity in abruptness and timing of the warming associated with the Bølling transition in Greenland and the southern SCS suggest a true synchrony of the Northern Hemisphere warming at the end of the last glaciation. In contrast to the foraminiferal transfer function estimates that do not indicate any consistent cooling associated with the Younger Dryas (YD) climate event in the tropical SCS, the U^k′₃₇ SST estimates show a cooling of ca. 0.2–0.6°C compared to the Bølling–Allerød period. These U^k′₃₇ SST estimates from the southern SCS argue in favor of a Northern Hemisphere-wide, synchronous cooling during the YD period.     

Alkenones in Cretaceous black shales, Blake-Bahama Basin, western North Atlantic

We here report the discovery of unusual distributions of long-chain alkenones (C₃₇-C₄₂) in two Cretaceous black shales from the Blake-Bahama Basin, western North Atlantic. These sediments are Cenomanian (c. 95 Ma) and mid-Albian (c. 105 Ma) in age, thus significantly extending the geological range of these compounds. The precise source of these lipids is, as yet, unknown, although they may derive from an ancient ancestor of Emiliania huxleyi.     

Unusual C35 and C36 alkenones in a paleoceanographic benchmark strain of Emiliania huxleyi

Our specimen of the cultured Emiliania huxleyi strain (CCMP1742, also known as NEPCC55a) that provides the benchmark for -based paleothermometry has started producing, for reasons yet unclear, major amounts of three new alkenones identified as ω15,22-C₃₅ methyl ketone, ω15,22-C₃₆ ethyl ketone and ω16,23-C₃₆ methyl ketone. Comparison of these structures with those established now by the same OsO₄ derivatization method applied to the di-unsaturated C₃₇, C₃₈ and C₃₉ alkenones typically found in this organism provides insight into the possible pathway for their biosynthesis. Isothermal batch culture experiments also show the content and composition of these new compounds change systematically and quite significantly in cells when subjected to environmental conditions such as nutrient depletion, variation in light availability and prolonged darkness. Alkenones of similarly unusual short-chain length are evident in suspended particulate materials from present day surface waters in the Ligurian Sea (Mediterranean) and in two different Holocene time horizons (Unit I and Unit II deposits) in Black Sea sediments. However, the positions of the double bonds are different from those that we now report in our culture, implying a different biosynthetic sequence. These alkenones are most likely derived from another, as yet unknown, haptophyte species. If this other organism accounts for all documented occurrences of these compounds in natural samples, then either it has a capacity for growth over a remarkably wide salinity range or surface water salinity in the early Holocene Black Sea may not have been as low as is currently believed.     

Variations in sea surface temperatures based on alkenones in Korea Plateau sediments of the East Sea (Sea of Japan) over the last 300,000 years

Variations in long chain alkenone-based sea surface temperature (SST) from a piston core (M04-PC1A) collected from the Korea Plateau in the East Sea (Sea of Japan) were investigated to understand paleoceanographic variations over the last 300,000 years. By combining sedimentological and geochemical proxies (the lithological marker of crudely laminated mud, alkenone SST, foraminiferal oxygen isotope values, and ¹⁴C age determination) and by comparison with previous works, we examined paleoceanographic variations back to Marine Isotope Stage (MIS) 8, approximately 300,000 years B.P. In particular, analysis of alkenones suggests that SSTs were about 8 °C and 11 °C lower during MIS 8 and MIS 6 than that in the present-day SST, respectively. Furthermore, SST was estimated to be 5 °C lower during the Last Glacial Maximum. These significant SST differences among MIS 8, 6, and 2 may be attributable to not only the formation of distinctive water masses, but also to differential alkenone synthesis under different environmental conditions. These results suggest that SSTs in the East Sea during the last three glacial periods (MIS 8, 6, and 2) were different, but rather were closely linked with regional oceanographic conditions overlapped with sensitive responses to the intensity of the East Asian monsoon. Surface-water freshening was a local paleoceanographic consequence that was imprinted in the core during MIS 2 and MIS 6, and potentially during MIS 4. Furthermore, alkenone-based SST data suggested that surface water circulation and biological productivity were strongly associated with the inflow of the Tsushima Warm Current during interglacial periods.     

Pronounced occurrence of long-chain alkenones and dinosterol in a 25,000-year lipid molecular fossil record from Lake Titicaca, South America

Our analysis of lipid molecular fossils from a Lake Titicaca (16° S, 69° W) sediment core reveals distinct changes in the ecology of the lake over an ∼25,000-yr period spanning latest Pleistocene to late Holocene time. Previous investigations have shown that over this time period Lake Titicaca was subject to large changes in lake level in response to regional climatic variability. Our results indicate that lake algal populations were greatly affected by the changing physical and chemical conditions in Lake Titicaca. Hydrocarbons are characterized by a combination of odd-numbered, mid- to long-chain (C₂₁-C₃₁) normal alkanes and alkenes. During periods when lake level was higher (latest Pleistocene, early Holocene, and late Holocene), the C₂₁n-alkane, and the C₂₅ and C₂₇ alkenes dominate hydrocarbon distributions and indicate contribution from an algal source, potentially the freshwater alga Botryococcus braunii. The C₃₀ 4 α-methyl sterol (dinosterol) increases sharply during the mid-Holocene, suggesting a greatly increased dinoflagellate presence at that time. Long-chain alkenones (LCAs) become significant during the early Holocene and are highly abundant in mid-Holocene samples. There are relatively few published records of LCA detection in lake sediments but their occurrence is geographically widespread (Antarctica, Asia, Europe, North America). Lake Titicaca represents the first South American lake and the first low-latitude lake in which LCAs have been reported. LCA abundance and distribution may be related to the temperature-dependent response of an unidentified algal precursor. Although the LCA unsaturation indices cannot be used to determine absolute Lake Titicaca temperatures, we suspect that the published LCA U₃₇^K unsaturation calibrations can be applied to infer relative temperatures for early to mid-Holocene time when LCA concentrations are high. Using these criteria, the U₃₇^K unsaturation indices suggest relatively warmer temperatures in the mid-Holocene. In contrast to previous speculation, lipid analysis provides little evidence of a greatly increased presence of aquatic plants during the mid-Holocene. Instead, it appears that a few algal species were dominant in the lake. Based on the dramatic rise in abundances of LCAs and dinosterol during the early to mid-Holocene, we suspect that the algal producers of these compounds rose in response to a combination of physical and chemical changes in the lake. These include temperature, salinity, and alkalinity changes that occurred as lake level dropped sharply during a multi-millennial drought affecting the Central Andean Altiplano.