Alkenone Flux Research Articles

Influence of environmental variability and Emiliania huxleyi ecotypes on alkenone-derived temperature reconstructions in the subantarctic Southern Ocean

Long-chain unsaturated alkenones produced by haptophyte algae are widely used as paleotemperature indicators. The unsaturation relationship to temperature is linear at mid-latitudes, however, non-linear responses detected in subpolar regions of both hemispheres have suggested complicating factors in these environments. To assess the influence of biotic and abiotic factors in alkenone production and preservation in the Subantarctic Zone, alkenone fluxes were quantified in three vertically-moored sediment traps deployed at the SOTS observatory (140°E, 47°S) during a year. Alkenone fluxes were compared with coccolithophore assemblages, satellite measurements and surface-water properties obtained by sensors at SOTS. Alkenone-based temperature reconstructions generally mirrored the seasonal variations of SSTs, except for late winter when significant deviations were observed (3–10 °C). Annual flux-weighted averages in the 3800 m trap returned alkenone-derived temperatures ~1.5 °C warmer than those derived from the 1000 m trap, a distortion attributed to surface production and signal preservation during its transit through the water column. Notably, changes in the relative abundance of E. huxleyi var. huxleyi were positively correlated with temperature deviations between the alkenone-derived temperatures and in situ SSTs (r = 0.6 and 0.7 at 1000 and 2000 m, respectively), while E. huxleyi var. aurorae, displayed an opposite trend. Our results suggest that E. huxleyi var. aurorae produces a higher proportion of C37:3 relative to C37:2 compared to its counterparts. Therefore, the dominance of var. aurorae south of the Subtropical Front could be at least partially responsible for the less accurate alkenone-based SST reconstructions in the Southern Ocean using global calibrations. However, the observed correlations were largely influenced by the samples collected during winter, a period characterized by low particle fluxes and slow sinking rates. Thus, it is likely that other factors such as selective degradation of the most unsaturated alkenones could also account for the deviations of the alkenone paleothermometer.

Evaluation of reconstructed sea surface temperatures based on [formula omitted] from sediment surface samples of the North Pacific

The alkenone unsaturation index (U37k′) as proxy for sea surface temperature (SST) is an important tool in paleoclimatology for reconstructing past ocean temperature variability. Typically, U37k′ recorded in marine surface sediments shows a linear correlation with modern mean annual SST. However, in high-latitude oceanic regions, such as the subpolar Pacific, U37k′-based SSTs do overestimate the mean annual temperature by up to 6 °C, potentially leading to obscured paleoclimatic information drawn from stratigraphic U37k′-records. The reason for this “warm bias” is still not well understood. Here, we present a compilation of 97 sediment surface samples from Multicores collected in the Bering Sea, the Okhotsk Sea and the North Pacific to evaluate the alkenone-temperature proxy against observational data from the North Pacific. Sediment surface samples were analysed for alkenones and the derived U37k′-indices converted to water temperatures using different calibration equations established in the literature. U37k′-based SSTs were then compared to instrumental SST data, as well as modern alkenone flux data from sediment traps in the North Pacific. Our results confirm that most U37k′-based SSTs from the subpolar Pacific are 2–6 °C too warm compared to instrumental mean annual SSTs for calibrations applied. However, with an uncertainty at the level of ±1.5 °C or less reconstructed SSTs fit quite well to modern autumn temperatures north of the Subarctic Front (SAF), when maximum export flux of alkenones to the seafloor is indicated by sediment trap data. South of the SAF, reconstructed SSTs largely mimic the modern mean annual SST signal with an uncertainty of ±1.5 °C or less, which is likely due to the attenuation of seasonality and longer growth season of coccolithophorids according to sediment trap data. Our study further demonstrates that U37k′, when seasonality in alkenone production and export are known and considered, is able to provide reasonable estimates of SSTs in modern high-latitude ocean settings. We conduct a case study using available alkenone time-series derived from a sediment core collected from the south-western Okhotsk Sea to better understand the potential effect of seasonality in alkenone production on stratigraphic U37k′-record in the subpolar Pacific. The case study from the Okhotsk Sea indicates that even a small shift in seasonality may lead to strongly biased SSTs with broader regional implications for paleoclimate reconstructions in high-latitude ocean settings.

Widespread occurrence of distinct alkenones from Group I haptophytes in freshwater lakes: Implications for paleotemperature and paleoenvironmental reconstructions

Alkenones are C35–C42 polyunsaturated ketone lipids that are commonly employed to reconstruct changes in sea surface temperature. However, their use in coastal seas and saline lakes can be hindered by species-mixing effects. We recently hypothesized that freshwater lakes are immune to species-mixing effects because they appear to exclusively host Group I haptophyte algae, which produce a distinct distribution of alkenones with a relatively consistent response of alkenone unsaturation to temperature. To evaluate this hypothesis and explore the geographic extent of Group I haptophytes, we analyzed alkenones in sediment and suspended particulate matter samples from lakes distributed throughout the mid- and high latitudes of the Northern Hemisphere (n=30). Our results indicate that Group I-type alkenone distributions are widespread in freshwater lakes from a range of different climates (mean annual air temperature range: −17.3–10.9 °C; mean annual precipitation range: 125–1657 mm yr−1; latitude range: 40–81°N), and are commonly found in neutral to basic lakes (pH > 7.0), including volcanic lakes and lakes with mafic bedrock. We show that these freshwater lakes do not feature alkenone distributions characteristic of Group II lacustrine haptophytes, providing support for the hypothesis that freshwater lakes are immune to species-mixing effects. In lakes that underwent temporal shifts in salinity, we observed mixed Group I/II alkenone distributions and the alkenone contributions from each group could be quantified with the RIK37 index. Additionally, we observed significant correlations of alkenone unsaturation (U37K) with seasonal and mean annual air temperature with this expanded freshwater lakes dataset, with the strongest correlation occurring during the spring transitional season (U37K=0.029⁎T−0.49; r2=0.60; p<0.0001). We present new sediment trap data from two lakes in northern Alaska (Toolik Lake, 68.632°N, 149.602°W; Lake E5, 68.643°N, 149.458°W) that demonstrate the highest sedimentary fluxes of alkenones in the spring transitional season, concurrent with the period of lake ice melt and isothermal mixing. Together, these data provide a framework for evaluating lacustrine alkenone distributions and utilizing alkenone unsaturation as a lake temperature proxy.

GDGT and alkenone flux in the northern Gulf of Mexico: Implications for the TEX86 and UK'37 paleothermometers

AbstractThe TEX86 and molecular biomarker proxies have been broadly applied in downcore marine sediments to reconstruct past sea surface temperature (SST). Although both TEX86 and have been interpreted as proxies for mean annual SST throughout the global ocean, regional studies of glycerol dibiphytanyl glycerol tetraethers (GDGTs) and alkenones in sinking particles are required to understand the influence of seasonality, depth distribution, and diagenesis on downcore variability. We measure GDGT and alkenone flux, as well as the TEX86 and indices in a 4 year sediment trap time series (2010–2014) in the northern Gulf of Mexico (nGoM), and compare these data with core‐top sediments at the same location. GDGT and alkenone fluxes do not show a consistent seasonal cycle; however, the largest flux peaks for both occurs in winter. covaries with SST over the 4 year sampling interval, but the ‐SST relationship in this data set implies a smaller slope or nonlinearity at high temperatures when compared with existing calibrations. Furthermore, the flux‐weighted value from sinking particles is significantly lower than that of underlying core‐top sediments, suggesting preferential diagenetic loss of the tri‐unsaturated alkenone in sediments. TEX86 does not covary with SST, suggesting production in the subsurface upper water column. The flux‐weighted mean TEX86 matches that of core‐top sediments, confirming that TEX86 in the nGoM reflects local planktonic production rather than allochthonous or in situ sedimentary production. We explore potential sources of uncertainty in both proxies in the nGoM but demonstrate that they show nearly identical trends in twentieth century SST, despite these factors.

Concentrations and abundance ratios of long-chain alkenones and glycerol dialkyl glycerol tetraethers in sinking particles south of Java

In this study, we obtained concentrations and abundance ratios of long-chain alkenones and glycerol dialkyl glycerol tetraethers (GDGTs) in a one-year time-series of sinking particles collected with a sediment trap moored from December 2001 to November 2002 at 2200m water depth south of Java in the eastern Indian Ocean. We investigate the seasonality of alkenone and GDGT fluxes as well as the potential habitat depth of the Thaumarchaeota producing the GDGTs entrained in sinking particles. The alkenone flux shows a pronounced seasonality and ranges from 1µgm−2d−1 to 35µgm−2d−1. The highest alkenone flux is observed in late September during the Southeast monsoon, coincident with high total organic carbon fluxes as well as high net primary productivity. Flux-weighted mean temperature for the high flux period using the alkenone-based sea-surface temperature (SST) index U37K′ is 26.7°C, which is similar to satellite-derived Southeast (SE) monsoon SST (26.4°C). The GDGT flux displays a weaker seasonality than that of the alkenones. It is elevated during the SE monsoon period compared to the Northwest (NW) monsoon and intermonsoon periods (approximately 2.5 times), which is probably related to seasonal variation of the abundance of Thaumarchaeota, or to enhanced export of GDGTs by aggregation with sinking phytoplankton detritus. Flux-weighted mean temperature inferred from the GDGT-based TEX86H index is 26.2°C, which is 1.8°C lower than mean annual (ma) SST but similar to SE monsoon SST. As the time series of TEX86H temperature estimates, however, does not record a strong seasonal amplitude, we infer that TEX86H reflects ma upper thermocline temperature at approximately 50m water depth.

Constraints on the applicability of the organic temperature proxies U&amp;lt;sup&amp;gt;K'&amp;lt;/sup&amp;gt;&amp;lt;sub&amp;gt;37&amp;lt;/sub&amp;gt;, TEX&amp;lt;sub&amp;gt;86&amp;lt;/sub&amp;gt; and LDI in the subpolar region around Iceland

Abstract. Subpolar regions are key areas for studying natural climate variability due to their high sensitivity to rapid environmental changes, particularly through sea surface temperature (SST) variations. Here, we have tested three independent organic temperature proxies (UK'37; TEX86; and the long-chain diol index, LDI) regarding their potential applicability for SST reconstruction in the subpolar region around Iceland. UK'37, TEX86 and TEXL86 temperature estimates from suspended particulate matter showed a substantial discrepancy with instrumental data, while long-chain alkyl diols were below the detection limit at most of the stations. In the northern Iceland Basin, sedimenting particles revealed a seasonality in lipid fluxes, i.e., high fluxes of alkenones and glycerol dialkyl glycerol tetraethers (GDGTs) were measured during late spring and during summer and high fluxes of long-chain alkyl diols during late summer. The flux-weighted average temperature estimates had a significant negative (ca. 2.3 °C for UK'37) and positive (up to 5 °C for TEX86) offset with satellite-derived SSTs and temperature estimates derived from the underlying surface sediment. UK'37 temperature estimates from surface sediments around Iceland correlate well with summer mean sea surface temperatures, while TEX86-derived temperatures correspond with both annual and winter mean 0–200 m temperatures, suggesting a subsurface temperature signal. Anomalous LDI-SST values in surface sediments and low mass flux of 1,13- and 1,15-diols compared to 1,14-diols suggest that Proboscia diatoms are the major sources of long-chain alkyl diols in this area rather than eustigmatophyte algae, and therefore the LDI cannot be applied in this region.

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.

Seasonality of [formula omitted] temperature estimates as inferred from sediment trap data

The seasonality of sea surface temperatures (SST) estimated from the alkenone- UK′37 index has been a debated issue since the development of the proxy. Using a compilation of sediment trap time series data from 34 sampling locations, we show that the seasonality of maximum alkenone flux in sediment traps varies markedly across the oceans, depending not only on latitude and light availability but also on local oceanographic conditions. The seasonality of the alkenone flux to sediments may also be shaped by the complexity of sedimentation processes and a consistent, globally applicable, seasonal pattern is not apparent. Nevertheless, UK′37 values display a world ocean scale correlation with mean annual SSTs (0 m) that closely resembles the standard calibration equation now established for modern surface sediment records. Thus, with a few notable exceptions at oceanographic locations proximate to major hydrographic fronts, it can be concluded that the integrated sedimentation patterns for UK′37 measured in sediment trap time series provide a measure of annual mean SST.

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

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

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&lt;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.

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.

Time-series sediment trap record of alkenones from the western Sea of Okhotsk

C 37–C 39 alkenones were measured in time-series sediment trap samples collected from August 1998 to June 2000 at two depths in the seasonal sea ice region of the western Sea of Okhotsk, off Sakhalin, in order to investigate alkenone production and water-column processes in the region. Measurable export fluxes of alkenones are ranged from < 0.1 to 5.8 μg/m 2/day and clearly showed that the alkenone production was restricted to autumn. In 1998, maximum export flux of alkenones occurred in September when surface water column was well stratified with low nutrients in the surface mixing layer. In the next year, the maximum flux is observed in October. Comparison between alkenone temperature and satellite based sea surface temperature (SST) shows that the estimated alkenone temperatures in August 1998 were found to be ∼ 10 °C lower than the temporal satellite SST, suggesting that alkenones are produced in surface to subsurface thermocline layers during the period. Annual mean flux of alkenones is lower in the lower traps than that of the upper traps, suggesting rapid degradation of alkenones in water column, but the U K 37′ value is not significantly altered. This study indicates that U K 37′ values preserved in the surface sediments off Sakhalin reflect the seasonal temperature signal of near surface water, rather than annual mean surface temperature.

Interannual variability of deep water particle flux in relation to production and lateral sources in the northeast Atlantic

Results of particle flux studies using sediment traps in the northeast Atlantic Ocean at 33oN, 22oW, a time-series station (L1/K276) operated within the German contribution to the international Joint Global Ocean Flux Study (JGOFS) between summer 1993 and summer 2001, are presented. The particle flux at 2000m depth is highly seasonal with only one main peak in February and March and is characterized by strong interannual variability. The comparison of the particle flux time series with the measurements of chlorophyll and phaeopigments in the sediment trap material, and chlorophyll-a concentrations at surface from SeaWiFS together with the similarity of the pattern observed in the alkenone flux, particulate organic carbon (POC) and calcium carbonate (CaCO3) leads to the conclusion that the particle flux at this position is fast and directly coupled to the phytoplankton development and decay in the overlying euphotic zone. SeaWiFS chlorophyll-a data (1997–2001) and characteristics of the upper water column (mixed layer depth, depth of nitracline, primary production) as predicted by an eddy-permitting model of the North Atlantic coupled with a simple nitrate–phytoplankton–zooplankton–detritus (NPZD) pelagic-ecosystem model are used to understand the observed interannual differences in particle flux at 2000m. Our results suggest that the interannual differences in the observed particle flux are a direct consequence of the variability of phytoplankton biomass and primary production in the euphotic zone in this region.

Generation, transport, and preservation of the alkenone‐based U37K′ sea surface temperature index in the water column and sediments of the Cariaco Basin (Venezuela)

Alkenone fluxes in the water column of the Cariaco Basin ranged from 12 to 20 μg m−2 d−1 and were inversely related to upwelling strength. The U37K′ ratios of sinking particles varied from 0.78 to 0.96 and exhibited seasonal changes that were coherent with a 7°C variation in sea surface temperature (SST). The correlation between SST and U37K′ ratios closely overlapped with the calibration of Prahl et al. [1988]. Alkenone burial fluxes in Cariaco Basin sediments varied markedly over the past ∼6000 years, ranging from 0.2 to 5 μg m−2 d−1. The U37K′ ratios of surface sediments indicate SST was higher (26.3°C) during the last 50 years of deposition than in the previous 300 years (∼25°C), signaling an upwelling decrease in the latter part of the twentieth century. The lowest U37K′‐derived temperatures (∼25°C) were measured in sediments deposited during the little ice age (LIA). These compositions, coupled with relatively low alkenone fluxes (≤2 μg m−2 d−1), are consistent with conditions of enhanced upwelling, decreased SST and reduced haptophyte production. The highest U37K′‐derived SST estimates (over 26.5°C) were measured during the Medieval Warm Period (MWP) and suggest reduced upwelling at this time. Prior to the MWP, the alkenone record indicates temperatures of ∼26°C and burial fluxes of ∼2 μg m−2 d−1. These compositions indicate stronger upwelling conditions during the Holocene relative to the last 50 years and the MWP but annual SSTs above those estimated for the LIA.

A 4-year sediment trap record of alkenones from the filamentous upwelling region off Cape Blanc, NW Africa and a comparison with distributions in underlying sediments

We analysed long-chain alkenones in sinking particles and surface sediments from the filamentous upwelling region off Cape Blanc, NW Africa, to evaluate the transfer of surface water signals into the geological record. Our study is based on time-series sediment trap records from 730 m (1990–1991) to 2195–3562 m depth (1988–1991). Alkenone fluxes showed considerable interannual variations and no consistent seasonality. The average flux of C 37 and C 38 alkenones to the deep traps was 1.9 μg m −2 d −1 from March 1988 to October 1990 and sevenfold higher in the subsequent year. Alkenone fluxes to the shallower traps were on average twice as high and showed similar temporal variations. The alkenone unsaturation indices U K′ 37, U K 38Me and U K 38Et closely mirrored the seasonal variations in sea-surface temperature (weekly Reynolds SST). Time lags of 10–48 days between the SST and unsaturation maxima suggest particle sinking rates of about 80 and 280 m d −1 for the periods of low and high alkenone fluxes, respectively. The average flux-weighted U K′ 37 temperature for the 4-year time series of the deeper traps was 22.1°C, in perfect agreement with the mean weekly SST for the same period. This and the comparison with seasonal temperature variations in the upper 100 m of the water column suggests that U K′ 37 records principally the yearly average of the mixed-layer temperature in this region. A comparison between the average annual alkenone fluxes to the lower traps (2400 μg m −2 yr −1) and into the underlying sediments (4 μg m −2 yr −1) suggests that only about 0.2% of the alkenones reaching the deep ocean became preserved in the sediments. The flux-weighted alkenone concentrations also decreased considerably, from 2466 μg gC −1 in the water column to 62 μg gC −1 in the surface sediments. Such a low degree of alkenone preservation is typical for slowly accumulating oxygenated sediments. Despite these dramatic diagenetic alkenone losses, the U K′ 37 ratio was not affected. The average U K′ 37 value of the sediments (0.796±0.010 or 22.3±0.3°C) was identical within error limits to the 4-year average of the lower traps. The unsaturation indices for C 38 alkenones and the ratio between C 37 and C 38 alkenones also revealed a high degree of stability. Our results do not support the hypothesis that U K′ 37 is biased towards higher values during oxic diagenesis.

Oceanographic considerations for the application of the alkenone-based paleotemperature U 37K′ index in the Gulf of California

Alkenone concentrations were determined in sediment trap samples and surficial sediments collected from Guaymas Basin (Gulf of California) to investigate the factors that control the relationship between the U 37 K′ index and SST. The results from the sediment trap study reveal a positive correlation ( r 2 = 0.5) between alkenone fluxes and satellite AVHRR sea surface temperature (SST) and a strong correlation ( r 2 = 0.9) between U 37 K′ and SST over an annual temperature range of 20°C. Although SST and U 37 K′ are tightly coupled throughout two upwelling seasons, the relationship deviates from the expression developed by Prahl et al. (1988) at temperatures higher than 26°C. The possible reasons for this deviation are varied but include the observed changes in the coccolithophore species assemblage and the evolution of the thermocline during periods of high SST. The seasonal pattern of alkenone fluxes (higher fluxes during warm periods) observed throughout the 1996 to 1997 study period causes a “bias” in the annually averaged U 37 K′ record, resulting in temperature estimates that are ∼1°C higher than the annual AVHRR SST average. Furthermore, the U 37 K′ ratios obtained from core-top sediments indicate an additional ∼2°C “warm bias” in the sedimentary record that may be related to the lateral inputs of alkenones from other regions of the Gulf. These results highlight the importance of oceanographic processes in determining the alkenone temperature signal that is preserved in sediments. After accounting for such effects, U 37 K′ ratios from a box core were used to reconstruct a SST record extending back to the 18th century for the Guaymas Basin. These results indicate a steady 2°C warming of the surface waters of the Gulf from early 1700s to the 1950s, followed by a rapid apparent 1°C cooling between the 1950s and the 1980s.

Alkenones and particulate fluxes in sediment traps from the central equatorial Pacific

We investigated a year-long (September 1992 to August 1993) time series of total mass, calcium carbonate, organic carbon, opal, and alkenone fluxes in sinking particles collected with sediment traps moored at 1770 and 4220 m in the central equatorial Pacific. The total mass fluxes varied from 14.7 to 68.7 mg/m 2/day at 1770 m, with greater fluxes in October–November and February–April, and from 14.6 to 50.4 mg/m 2/day with peak fluxes during October–November at 4220 m. High flux in the spring season shown at 1770 m was not indicated at 4220 m; instead, a slight increase was shown during a broad period from March to June. The calcium carbonate fluxes varied from 10.8 to 49.1 mg/m 2/day with higher fluxes in October–November and March–April at 1770 m, and from 8.9 to 37.0 mg/m 2/day with a higher flux in October–November at 4220 m. The organic carbon fluxes varied from 0.36 to 5.91 mg/m 2/day, with higher fluxes in October–November and March–April at 1770 m, and from 0.72 to 2.58 mg/m 2/day at 4220 m. The annual mean organic carbon flux was 1.84 and 1.28 mg/m 2/day at 1770 and at 4220 m, respectively. These values were less than half of those reported for the EqPac sediment trap experiment. The opal fluxes varied from 0.55 to 4.4 mg/m 2/day at 1770 m and from 1.23 to 2.95 mg/m 2/day at 4220 m. Alkenone fluxes varied significantly from 0.05 to 0.84 μg/m 2/day, with high values in November, February–March, and June at 1770 m. For the 4220 m trap, these values ranged from 0.05 to 0.25 μg/m 2/day, with slightly higher fluxes in April–May and June–July, which followed periods of high alkenone fluxes observed in February–April and June–July, respectively, at 1770 m depth. These values were remarkably low compared with those reported by the previous studies at other sites. U 37 K′ values were constantly high >0.95 throughout the collection period. However, relatively low U 37 K′ values (0.92 and 0.93) were occasionally observed during February to March. Estimated alkenone temperatures from those U 37 K′ values were about 27–29°C and consistent with the observed temperature of the upper layer at ca.100 m depth. The seasonal change of the U 37 K′ values could be affected by not only water temperature but also the relative amount of ‘warm’ and ‘cold’ types of alkenone producer in the central equatorial Pacific.

An alkenone (U37K′) quasi-annual sea surface temperature record (A.D. 1440 to 1940) using varved sediments from the Santa Barbara Basin

Analyses for C37 alkenones (U37K′) and total organic carbon (TOC) in a varved core from the depocenter of the Santa Barbara Basin (34°13.5′N, 120°01′W, 590 m water depth) have been made on an approximately 1–2 year basis from A.D. 1440 to 1940. The U37K′-based sea surface temperature (SST) record oscillates around 15.5°C with an amplitude of less than 3°C at approximately interannual, decadal, and centennial frequencies. Historical El Niño/Southern Oscillation (ENSO) events are matched only partially by warmer U37K′ SSTs. Three records: our SST, a published Pacific coral δ18O, and a published tree-ring temperature anomaly from the California valleys, show no evidence of colder temperatures during the Little Ice Age (ca. 1500 to 1840 A.D.). Significantly, there is a strong centennial-scale cyclicity in the U37K′ SST record which approaches 1°C in amplitude. TOC burial and alkenone fluxes are largely controlled by sedimentation rate as expressed in varve thickness.

Alkenone fluxes and anomalous U37K′ values during 1989–1990 in the Northeast Atlantic (48°N 21°W)

We report and discuss new data on alkenone fluxes from the North East Atlantic (48°N 21°W), in a site investigated by the Joint Global Ocean Flux Study and the North Atlantic Bloom Experiment from April 1989–March 1990. The investigated sediment trap station is situated within the North Atlantic Transition Zone in a region influenced by upwelling events induced by mesoscale eddy activity, for which branches of the North Atlantic Current act as major sources. The flux of alkenones in the NE Atlantic was consistent with the monthly evolution of particulate fluxes, rapidly increasing from April 1989 to June 1989 (peak bloom), and gradually declining until August 1989. The timing of the only bloom observed is not equivalent to the timing of alkenone flux maxima recorded elsewhere in the world's oceans. Hence, we further confirm that there is not a world-wide regular season of alkenone blooms. The annual alkenone flux profile in our trap resembled that of the coccospheres, but not so much that of coccoliths. This discrepancy may indicate that alkenones were mainly transported downwards to the sea-floor by coccospheres, as part of the intact structure of the dead coccolithophorid. The sedimentary concentration of alkenones, which is related to productivity and efficiency of preservation, may then be also partly dependent on the rate of disaggregation of coccospheres in the water column, and the factors that control them. Comparison of the ratios of amounts of alkenones per coccosphere and coccoliths, with those of amounts of alkenones per cell numbers from culture and field data, suggests that there could have been a contribution of alkenones to the traps by Coccolithus pelagicus, despite this species considered devoid of alkenones. We suggest, hence, that further laboratory tests of this species are carried out. U37K′ in the sediment trap material showed a poor correlation with sea surface temperature during spring 1989. We propose that the anomalously low U37K′ values reflects an input of alkenones from elsewhere to the trap, not from the overlying surface waters, associated to the presence of several eddies and phytoplanktonic material trapped within. The origin of the eddy-bound alkenones may be found in subpolar waters, as indicated by the presence of C. pelagicus tests in the trap, and also by the low U37K′ values which are similar to the values found in surface sediments near the polar front. Our data, thus, further illustrates the challenge of using short-term studies of annual water-column alkenone fluxes to understand average sedimentation patterns of alkenones to the deep ocean.

Alkenones in the northwestern Mediterranean Sea: Interannual variability and vertical transfer

C37 alkenone fluxes were measured with sediment traps at 200 m depth over the years 1989/1990 and 1993/1994 to assess the interannual variability of the alkenone flux from the surface waters of the Mediterranean Sea. Fall and spring were identified as the high flux periods. SST estimates derived from the U37K′ index indicated 50 m and 30 m as major production depths in spring and fall, respectively. Althought interannual variation of alkenone fluxes was notable, the seasonality and depth of production appeared to be recurrent features of the coccolithophorid cycle of production. Alkenone fluxes at 1000 m measured over the year 1993/1994 were about 5 times lower than at 200 m and show no evidence of preferential preservation relative to the organic carbon between these depths. SST predicted at 200 m and 1000 m indicated a remarkably good transfer of the surface temperature signal to deeper layers.