Maureen H. Conte

Global temperature calibration of the alkenone unsaturation index (UK′37) in surface waters and comparison with surface sediments

In this paper, we compile the current surface seawater C37 alkenone unsaturation (UK′37) measurements (n = 629, −1 to 30°C temperature range) to derive a global, field-based calibration of UK′37 with alkenone production temperature. A single nonlinear “global” surface water calibration of UK′37 accurately predicts alkenone production temperatures over the diversity of modern-day oceanic environments and alkenone-synthesizing populations (T = −0.957 + 54.293(UK′37) − 52.894(UK′37)2 + 28.321(UK′37)3, r2 = 0.97, n = 567). The mean standard error of estimation is 1.2°C with insignificant bias in estimated production temperature among the different ocean regions sampled. An exception to these trends is regions characterized by strong lateral advection and extreme productivity and temperature gradients (e.g., the Brazil-Malvinas Confluence). In contrast to the surface water data, the calibration of UK′37 in surface sediments with overlying annual mean sea surface temperature (AnnO) is best fit by a linear model (AnnO = 29.876(UK′37) − 1.334, r2 = 0.97, n = 592). The standard error of estimation (1.1°C) is similar to that of the surface water production calibration, but a higher degree of bias is observed among the regional data sets. The sediment calibration differs significantly from the surface water calibration. UK′37 in surface sediments is consistently higher than that predicted from AnnO and the surface water production temperature calibration, and the magnitude of the offset increases as the surface water AnnO decreases. We apply the global production temperature calibration to the coretop UK′37 data to estimate the coretop alkenone integrated production temperature (coretop IPT) and compare this with the overlying annual mean sea surface temperature (AnnO). We use simple models to explore the possible causes of the deviation observed between the coretop temperature signal, as estimated by UK′37, and AnnO. Our results indicate that the deviation can best be explained if seasonality in production and/or thermocline production as well as differential degradation of 37:3 and 37:2 alkenones both affect the sedimentary alkenone signal.

Genetic and physiological influences on the alkenone/alkenoate versus growth temperature relationship in Emiliania huxleyi and Gephyrocapsa oceanica

Selected warm and cold water strains of the coccolithophorid Emiliania huxleyi and the closely related species Gephyrocapsa oceanica were cultured under controlled temperature conditions to assess genetic and physiological variability in the alkenone/alkenoate vs. temperature relationship. Differences in the strains’ growth rates over the 6–30°C experimental temperature range were small but consistent with their cold or warm water origins. E. huxleyi and G. oceanica had similar alkenone/alkenoate biochemistry, justifying the extension of alkenone stratigraphy to sediments predating the appearance of E. huxleyi. These species could not be distinguished by C38/C37 alkenone or alkenoate/alkenone ratios as previously suggested (Volkman et al. 1995; Sawada et al. 1996) but given samples from a range of temperatures may be distinguished by a plot of the C38 ethyl vs. C38 methyl unsaturation ratios (U38EtK and U38MeK, respectively). Biochemical responses to temperature and the C37 alkenone-based (U37K′) temperature calibrations differed significantly among the strains. The U37K′ temperature calibration was nonlinear for five of the six strains examined. A reduction in slope of the calibration at temperatures 21°C suggests the cell’s alkenone-based adaptation to temperature is limited at the extremes of its growth temperature range. The unsaturation ratios of the C38 methyl and ethyl alkenones (U38MeK and U38EtK) varied similarly with temperature and were strongly intercorrelated. The experiments also documented an influence of cell physiological state on both alkenone and alkenoate composition and on alkenone unsaturation. Cells in late logarithmic and stationary growth had significantly increased abundance of alkenoates and C38 ethyl alkenones relative to C38 methyl alkenone abundance. In some strains the unsaturation ratios of both C37 and C38 alkenones also significantly decreased when cells entered the late log phase. Comparison of culture results with field data indicates that the average physiological state of alkenone-synthesizers in the open ocean differs from cultured cells growing under exponential growth and appears to be more similar to cells in late log or stationary growth phases. Differences in alkenone/alkenoate ratios between cultured cells and sediments underlying waters of a similar temperature most probably reflect a difference in cell physiology between cultured cells and oceanic populations and not greater diagenetic losses of alkenones relative to alkenoates, as previously suggested (Prahl et al. 1995). Our experiments confirm that biogeographical variations observed in the alkenone vs. temperature relationship in natural waters reflect, at least in part, differences in genetic makeup and physiological status of the local alkenone-synthesizing populations. Hence, alkenone-based paleo sea surface temperature estimates are subject to errors, albeit small, which arise from genetic differences between modern-day and paleo-populations. The reduction in slope of the U37K′ temperature calibration for most strains at T >24°C indicate that linear U37K′ temperature calibrations (e.g., Prahl et al. 1988) which are currently used to estimate paleo SST, and which are poorly constrained at higher temperatures, probably underestimate the magnitude of SST change for tropical and subtropical regions.

Basin‐wide particulate carbon flux in the Atlantic Ocean: Regional export patterns and potential for atmospheric CO2 sequestration

Particle flux data from 27 sites in the Atlantic Ocean have been compiled in order to determine regional variations in the strength and efficiency of the biological pump and to quantify carbon fluxes over the ocean basin, thus estimating the potential oceanic sequestration of atmospheric CO2. An algorithm is derived relating annual particulate organic carbon (POC) flux to primary production and depth that yields variations in the export ratio (ER = POC flux/primary production) at 125 m of between 0.08 and 0.38 over the range of production from 50 to 400 g C m−2 yr−1. Significant regional differences in changes of the export ratio with depth are related to the temporal stability of flux. Sites with more pulsed export have higher export ratios at 125 m but show more rapid decreases of POC flux with depth, resulting in little geographic variation in fluxes below ∼3000 m. The opposing effects of organic carbon production and calcification on ΔpCO2 of surface seawater are considered to calculate an “effective carbon flux” at the depth of the euphotic zone and at the base of the winter mixed layer. POC flux at the base of the euphotic zone integrated over the Atlantic Ocean between 65°N and 65°S amounts to 3.14 Gt C yr−1. Of this, 5.7% is remineralized above the winter mixed layer and thus does not contribute to CO2 sequestration on climatically relevant timescales. The effective carbon flux, termed Jeff, amounts to 2.47 Gt C yr−1 and is a measure of the potential sequestration of atmospheric CO2 for the area considered. A shift in the composition of sedimenting particles (seen in a decrease of the opal:carbonate ratio) is seen across the entire North Atlantic, indicating a basin-wide phenomenon that may be related to large-scale changes in climatic forcing.

Long-chain alkenones and alkyl alkenoates as palaeotemperature indicators: their production, flux and early sedimentary diagenesis in the Eastern North Atlantic

The production and fate of long-chain alkenones and alkenoates was examined with regard to their use as palaeotemperature indicators. Alkenoate abundances were higher than previously reported for other regions and more strongly correlated with temperature than was alkenone unsaturation (U37k), suggesting biogeographical variability in alkenone and alkenoate production in surface waters. We define the AA36 ratio to parameterize alkenoate abundance and present an empirical temperature calibration derived from our euphotic zone samples for use in the North Atlantic [T = 17.35 − 25.12 (U37k) − 26.73 (AA36 + 93.90 (U37k) (AA36)]. Significant diagenetic losses of alkenones and alkenoates were found within the water column and near surface sediments. In spite of diagenetic losses, the temperature signal recorded by these compounds in the sediments remains largely unaltered, and corresponds to the euphotic zone water temperature during the springtime period of maximum coccolithophorid production and flux rather than the average annual sea surface temperature.

Seasonal and interannual variability in deep ocean particle fluxes at the Oceanic Flux Program (OFP)/Bermuda Atlantic Time Series (BATS) site in the western Sargasso Sea near Bermuda

Abstract Since 1978, the Oceanic Flux Program (OFP) time-series sediment traps have measured particle fluxes in the deep Sargasso Sea near Bermuda. There is currently a 20+yr flux record at 3200-m depth, a 12+yr flux at 1500-m depth, and a 9+yr record at 500-m depth. Strong seasonality is observed in mass flux at all depths, with a flux maximum in February–March and a smaller maximum in December–January. There is also significant interannual variability in the flux, especially with respect to the presence/absence of the December–January flux maximum and in the duration of the high flux period in the spring. The flux records at the three depths are surprisingly coherent, with no statistically significant temporal lag between 500 and 3200-m fluxes at our biweekly sample resolution. Bulk compositional data indicate an extremely rapid decrease in the flux of organic constituents with depth between 500 and 1500-m, and a smaller decrease with depth between 1500 and 3200-m depth. In contrast, carbonate flux is uniform or increases slightly between 500 and 1500-m, possibly reflecting deep secondary calcification by foraminifera. The lithogenic flux increases by over 50% between 500 and 3200-m depth, indicating strong deep water scavenging/repackaging of suspended lithogenic material. Concurrent with the rapid changes in flux composition, there is a marked reduction in the heterogeneity of the sinking particle pool with depth, especially within the mesopelagic zone. By 3200-m depth, the bulk composition of the sinking particle pool is strikingly uniform, both seasonally and over variations in mass flux of more than an order of magnitude. These OFP results provide strong indirect evidence for the intensity of reprocessing of the particle pool by resident zooplankton within mesopelagic and bathypelagic waters. The rapid loss of organic components, the marked reduction in the heterogeneity of the bulk composition of the flux, and the increase in terrigenous fluxes with depth are most consistent with a model of rapid particle turnover and material scavenging from the suspended pool during new particle formation. We suggest that much of the deep mass flux is generated in situ by deep-dwelling zooplankton, and that mass flux, as well as scavenging of suspended materials from the deep water column, varies in proportion to changes in grazer activity. Labile, very rapidly sinking aggregates (e.g., salp fecal material) arriving in the bathypelagic zone within days of their upper ocean production may act to stimulate zooplankton grazing rates and increase large particle production and deep mass flux days to weeks in advance of the arrival of bulk of surface-produced material. This process could reconcile mean particle sinking rate estimates with the phase coherence observed between upper and deep ocean mass fluxes.

Evaluation of long-chain alkenones as paleo-temperature indicators in the Mediterranean Sea

Long-chain alkenones were analyzed in samples collected from February to July 1989 by a time-series sediment trap located at 200 m depth in the northwestern Mediterranean Sea. The alkenone temperature signal was calibrated using water column particles collected at the sediment trap mooring site. The calibration (U37K′ = 0.041T−0.21, r2 = 0.97, 13°C < T < 19°C), differed from the culture calibration of Prahl et al. (1988, Geochimica et Cosmochimica Acta, 52, 2303–2310) and the calibration for the eastern North Atlantic waters between 16°C and 25°C (Cenre and Eglinton, 1993, Deep-Sea Research 1, 40, 1935–1961). The slope of the calibration was, however, similar to the slope of the calibration established using all Pacific Ocean samples (Sikes and Volkman, 1993, Geochimica et Cosmochimica Acta, 57, 1883–1889). The correlation of U38MeK and water temperature was also significant (U38MeK =0.042T−0.25, r2=0.87). Sediment trap results revealed that the production of alkenones during high flux periods predominantly occurred in subsurface waters. This was further supported by the depth distribution of alkenones observed during the monthly cruises: C37 alkenone concentrations in the upper 100 m of the water column indicated a subsurface maximum at approximately 50 m depth in May, June and July, whereas in March and November alkenones were more abundant in the upper 50 m. These results emphasize that accurate reconstruction of SST in core-tops must take into account the seasonality and depth variations of alkenone production. An intercalibration exercise, conducted on Mediterranean and Norwegian Sea water samples, demonstrated that high resolution chromatographic methods are essential to obtain reliable values of U37K′ and avoid the overestimation of production temperatures, particularly for colder waters where alkyl alkenoates are abundant.

LIPID BIOMARKER DIVERSITY IN THE COCCOLITHOPHORID EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) AND THE RELATED SPECIES GEPHYROCAPSA OCEANICA1

Twenty‐four strains of Emiliania huxleyi and two strains of Gephyrocapsa oceanica were grown at 15°C under identical culture conditions to assess genetic variability in key lipid biomarker profiles (C37‐C39 alkenones, C36 and C37 alkyl alkenoates, and C31‐C38 alkenes). Under our culture conditions, little divergence an biomarker composition was detected between E. huxleyi strains from different oceanic regions or between E. huxleyi and G. oceanica even though the strains originated from biogeographical regions as diverse as the subpolar North Atlantic and subtropical Western Pacific. The major differences observed were in tetraunsaturated alkenone abundance and alkene profiles, which tended to separate neritic from open ocean strains. Different strains from the same locality were as different as strains originating from widely separated ocean basins, indicating extreme genotypic diversity within a population. Replicate cultures of the same strain showed significant variability in their biomarker profiles even though the culture temperature varied by only ±0.3°C, indicating that their synthesis ratios are influenced by environmental and/or physiological variable(s), as yet unidentified, in addition to temperature. Strong covariance in C37 and C38 methyl alkenone unsaturation ratios (Uk37 and Uk38Mρ respectively) and, in coastal strains, C33, alkene and alkenone unsaturation ratios indicates that these compounds are biochemically linked.

Alkenone and alkenoate distributions within the euphotic zone of the eastern North Atlantic: correlation with production temperature

This paper reports the concentrations and within-class distributions of long-chain alkenones and alkyl alkenoates in the surface waters (0–50 m) of the eastern North Atlantic, and correlates their abundance and distribution with those of source organisms and with water temperature and other environmental variables. We collected these samples of >0.8 μm particulate material from the euphotic zone along the JGOFS 20°W longitude transect, from 61°N to 24°N, during seven cruises of the UK-JGOFS Biogeochemical Ocean Flux Study (BOFS) in 1989–1991; the biogeographical range of our 53 samples extends from the cold ( 25°C) oligotrophic subtropical waters off Africa. Surface water concentrations of total alkenone and alkenoates ranged from <50 ng l−1 in oligotrophic waters below 40°N to 2000–4500 ng l−1 in high latitude E. huxleyi blooms, and were well correlated with E. huxleyi cell densities, supporting the assumption that E. huxleyi is the predominant source of these compounds in the present day North Atlantic. The within-class distribution of the C37 and C38 alkenones and C36 alkenoates varied strongly as a function of temperature, and was largely unaffected by nutrient concentration, bloom status and other surface water properties. The biosynthetic response of the source organisms to growth temperature differed between the cold ( 16°C) waters below 47°N, the relative proportions of alkenoates and alkenones synthesized remained constant with increasing temperature while the unsaturation ratios of the C37 and C38 methyl alkenones (U37k and U38Mek, respectively) increased linearly. The fitted regressions of U37k and U38Mek versus temperature for waters >16°C were both highly significant (r2 > 0.96) and had identical slopes (0.057) that were 50% higher than the slope (0.034) of the temperature calibration of U37k reported by Prahl and Wakeham (Nature, 330, 367–369, 1987) over the same temperature range. These observations suggest either a physiological adjustment in biochemical response to growth temperature above a 16–17°C threshold and/or variation between different E. huxleyi strains and/or related species inhabiting the cold and warm water regions of the eastern North Atlantic. Using our North Atlantic data set, we have produced multivariate temperature calibrations incorporating all major features of the alkenone and alkenoate data set. Predicted temperatures using multivariate calibrations are largely unbiased, with a standard error of approximately ±1°C over the entire data range. In contrast, simpler calibration models cannot adequately incorporate regional diversity and nonlinear trends with temperature. Our results indicate that calibrations based upon single variables, such as U37k, can be strongly biased by unknown systematic errors arising from natural variability in the biosynthetic response of the source organisms to growth temperature. Multivariate temperature calibration can be expected to give more precise estimates of Integrated Production Temperatures (IPT) in the sedimentary record over a wider range of paleoenvironmental conditions, when derived using a calibration data set incorporating a similar range of natural variability in biosynthetic response.

Early diagenesis of lipid biomarker compounds in North Atlantic sediments

Near-surface sediments (0–45 mm depth interval) from the Iceland Basin (59°N, 21°W, 3070-m water depth) and Biscay Abyssal Plain (48°N, 17°W, 4105-m water depth) were sectioned at millimeter-scale resolution to assess alterations in key lipid biomarkers during early diagenesis. Inventories (µg cm−2 in the topmost 45 mm) of biomarkers from presumed terrestrial sources (>C20 linear n-alkanes, n-alkanoic acids, n-alkanols) and from phytoplankton sources (phytosterols, long-chain alkenones and alkenoates) were 1.5–4 times more abundant in the Iceland Basin core than in the Biscay Abyssal Plain core, indicating greater sedimentary inputs and/or preservation at the Iceland Basin site. Biomarker concentrations varied significantly over millimeter-scale depth intervals. Pronounced subsurface concentration maxima were present at both sites. At the Biscay Abyssal Plain site, steep concentration gradients in the uppermost few mm of the sediment column indicate extensive diagenetic losses at or near the sediment/water interface. Strong covariance was seen in the concentrations of various compounds within each class (e.g., long-chain n-alkanes, n-alkanols, n-alkanoic acids and long-chain alkenones and alkyl alkenoates), indicating that individual compounds within each class are associated with the same organic matrix and have similar degradation and mixing rates. Thus biomarker indices for assessment of land-derived inputs (CPI, C29/C31, ACL) and sea surface temperature (IPT) remain largely unaltered by early diagenetic processes in oxic, abyssal sediments in spite of extensive bioturbation and degradative losses within the uppermost few centimeters of the sediment column.

Plant biomarkers in aerosols record isotopic discrimination of terrestrial photosynthesis.

Carbon uptake by the oceans and by the terrestrial biosphere can be partitioned using changes in the 12C/13C isotopic ratio (δ13C) of atmospheric carbon dioxide, because terrestrial photosynthesis strongly discriminates against 13CO2, whereas ocean uptake does not. This approach depends on accurate estimates of the carbon isotopic discrimination of terrestrial photosynthesis (Δ; ref. 5) at large regional scales, yet terrestrial ecosystem heterogeneity makes such estimates problematic. Here we show that ablated plant wax compounds in continental air masses can be used to estimate Δ over large spatial scales and at less than monthly temporal resolution. We measured plant waxes in continental air masses advected to Bermuda, which are mainly of North American origin, and used the wax isotopic composition to estimate Δ simply. Our estimates indicate a large (5–6‰) seasonal variation in Δ of the temperate North American biosphere, with maximum discrimination occurring in late spring, coincident with the onset of production. We suggest that the observed seasonality arises from several factors, including seasonal shifts in the proportions of production by C3 and C4 plants, and environmentally controlled adjustments in the photosynthetic discrimination of C3-plant-dominated ecosystems.