Fredrick G. Prahl

Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions

Cultures of the marine coccolithophorid, Emiliania huxleyi, were grown in the laboratory at five temperatures (8°, 10°, 15°, 20°, 25°C) and monitored by capillary gas chromatography for their long-chain, unsaturated lipid compositions. The long-chain lipids of this plant comprise a series of C37, C38 and C39 di-, tri- and, in cells grown below 15°C, tetra-unsaturated methyl and ethyl ketones and a methyl and ethyl ester of a di-unsaturated C36 fatty acid. Systematic changes in the degree of unsaturation and in the overall carbon chain length distribution of the alkenones and in the proportion of fatty acid esters relative to alkenones are noted as a function of growth temperature. We present temperature calibrations for these changes in the lipid composition of laboratory cultures and compare these results with the compositions of this biomarker series measured in a variety of sediments accumulating beneath warm (⩾25°C) and cold (⩽12°C) surface waters in the tropical and temperate North Pacific Ocean, respectively. The comparisons demonstrate 1) this series of biomarkers is deposited in these oceanic sediments with minimal evidence of alteration to its original composition and 2) the strain of E. huxleyi used in this laboratory calibration is representative of the “average” marine phytoplankton supplying this novel series of biomarkers to contemporary sediments in these two environments and a wide variety of other oceanic environments. The long-chain alkenones constitute a major component (8.0 ± 2.9%) of the total organic carbon content of living cells of E. huxleyi. The high cellular abundance of these compounds appears to be relatively constant and independent of the growth temperature of the plant. These biomarkers provide a well-designed and useful geochemical tool for assessing variations not only in surface water temperatures but potentially also in the productivity of an important group of marine phytoplankton in oceans of the recent and distant past.

TERRESTRIAL ORGANIC CARBON CONTRIBUTIONS TO SEDIMENTS ON THE WASHINGTON MARGIN

Elemental and stable carbon isotopic compositions and biomarker concentrations were determined in sediments from the Columbia River basin and the Washington margin in order to evaluate geochemical approaches for quantifying terrestrial organic matter in marine sediments. The biomarkers include: an homologous series of long-chain n-alkanes derived from the surface waxes of higher plants; phenolic and hydroxyalkanoic compounds produced by CuO oxidation of two major vascular plant biopolymers, lignin and cutin. All marine sediments, including samples collected from the most remote sites in Cascadia Basin, showed organic geochemical evidence for the presence of terrestrial organic carbon. Using endmember values for the various biomarkers determined empirically by two independent means, we estimate that the terrestrial contribution to the Washington margin is ~ 60% for shelf sediments, ~ 30% for slope sediments, and decreases further to ≤15% in basin sediments. Results from the same geochemical measurements made with depth in gravity core 6705-7 from Cascadia Seachannel suggest that our approach to assess terrestrial organic carbon contributions to contemporary deposits on the Washington margin can be applied to the study of sediments depositing in this region since the last glacial period.

Terrestrial inputs of organic matter to coastal ecosystems: An intercomparison of chemical characteristics and bioavailability

Dissolved and particulate organic matter (DOM and POM) collected from rivers or groundwater feeding five estuaries along the east and west coasts of the USA were characterized with a variety of biogeochemical techniques and related to bioavailability to estuarine microbes. Surface water was sampled from the Columbia, Satilla, Susquehanna and Parker Rivers and groundwater was sampled from the Childs River. Several geochemical descriptors (percent organic matter of suspended particulate matter, C/N, lignin phenol content, ratio of vanillic acid to vanillin) suggested an ordering of the systems with respect to POM lability: Satilla < Parker < Columbia < Susquehanna.DOC concentrations in these systems ranged from <100 μM for the Columbia River to >2000 μM for the Satilla River. Elemental analysis of DOM concentrates (>1000 D) was used to predict organic matter composition and to calculate degree of substrate reduction using two different modeling approaches. Models predicted aliphatic carbon ranging between 43 and 60% and aromatic carbon between 26 and 36%, with aliphatic content lowest in the Satilla and highest in the Columbia River. The degree of substrate reduction of the organic matter concentrates followed a pattern similar to that for aliphatic C, being lowest in the Satilla (3.5) and highest in the Columbia (4.0). Extracellular enzyme activity varied broadly across the systems, but again ordered sites in the same way as did aliphatic content and degree of substrate reduction. Bacterial growth rates ranged from 1.3 ug mg-1 d-1 DOC in the Satilla to 1.7 ug mg-1 d-1 DOC in the Parker River. Bioassays confirmed patterns of dissolved organic matter lability predicted by the chemical models. Between 67% to 75% of the variation in bacterial growth could be explained by differences in organic matter composition.

Photosynthetic fractionation of 13C and concentrations of dissolved CO2 in the central equatorial Pacific during the last 255,000 years

Carbon isotopically based estimates of CO2 levels have been generated from a record of the photosynthetic fractionation of 13C [is equivalent to epsilon(p)] in a central equatorial Pacific sediment core that spans the last approximately 255 ka. Contents of 13C in phytoplanktonic biomass were determined by analysis of C37 alkadienones. These compounds are exclusive products of Prymnesiophyte algae which at present grow most abundantly at depths of 70-90 m in the central equatorial Pacific. A record of the isotopic composition of dissolved CO2 was constructed from isotopic analyses of the planktonic foraminifera Neogloboquadrina dutertrei, which calcifies at 70-90 m in the same region. Values of epsilon(p), derived by comparison of the organic and inorganic delta values, were transformed to yield concentrations of dissolved CO2 [is equivalent to c(e)] based on a new, site-specific calibration of the relationship between epsilon(p) and c(e). The calibration was based on reassessment of existing epsilon(p) versus c(e) data, which support a physiologically based model in which epsilon(p) is inversely related to c(e). Values of PCO2, the partial pressure of CO2 that would be in equilibrium with the estimated concentrations of dissolved CO2, were calculated using Henrys law and the temperature determined from the alkenone-unsaturation index U(K/37). Uncertainties in these values arise mainly from uncertainties about the appropriateness (particularly over time) of the site-specific relationship between epsilon(p) and 1/c(e). These are discussed in detail and it is concluded that the observed record of epsilon(p) most probably reflects significant variations in delta pCO2, the ocean-atmosphere disequilibrium, which appears to have ranged from approximately 110 microatmospheres during glacial intervals (ocean > atmosphere) to approximately 60 microatmospheres during interglacials. Fluxes of CO2 to the atmosphere would thus have been significantly larger during glacial intervals. If this were characteristic of large areas of the equatorial Pacific, then greater glacial sinks for the equatorially evaded CO2 must have existed elsewhere. Statistical analysis of air-sea pCO2 differences and other parameters revealed significant (p<0.01) inverse correlations of delta pCO2 with sea surface temperature and with the mass accumulation rate of opal. The former suggests response to the strength of upwelling, the latter may indicate either drawdown of CO2 by siliceous phytoplankton or variation of [CO2]/[Si(OH)4] ratios in upwelling waters.

A biomarker perspective on prymnesiophyte productivity in the northeast pacific ocean

Abstract Long-chain alkenones derived from prymnesiophyte algae were analysed in 1-year sediment trap time series (September 1987–1988) from three sites along a 630 km offshore transect at ∼42°N in the northeast Pacific Ocean. Biomarker flux monitored at 1000 m water depth was evident throughout the year at all sites and showed a consistent seasonal maximum in late spring which increased in amplitude with distance offshore. The integrated annual biomarker flux was constant along the transect, despite differences in seasonality between sites. Alkenone unsaturation patterns were remarkably uniform throughout the time series, reflecting an algal growth temperature of 10.6 ± 1.1°C. This value corresponds to regional water temterature at the sea-surface in winter. It recurs in seasonal upwelling near the coast and at the depth of the subsurface chlorophyll maximum offshore during seasons of stratification. These biomaker observations, interpreted in view of trap data for total organic (TOC) and inorganic carbon and ancillary hydrographic information, help to clarify seasonal productivity patterns for alkenone-producing prymnesiophytes in the northeast Pacific Ocean. Sediments accumulating with distance offshore along the sampling transect change from suboxic and Mn-reducing at the water-sediment interface to aerobic throughout the depths penetrated by box coring. Comparison of alkenone and TOC accumulation rates in surface (0–1 cm) sediments with corresponding annual fluxes integrated by the trap time series, shows that the fraction of both properties accounted at the seafloor is highest and similar under sub-oxic conditions (∼25%), and declines steeply and disproportionately as aerobic conditions are encountered farther offshore. Only 0.25 and 3.1% of the annual inventory for alkenones and TOC in traps are accountable in surface sediments from the slowest accumulating, most oxidizing site farthest offshore. Despite major loss of biomarker to early diagenesis, surface sediments and trap particles display consistent alkenone unsaturation patterns. Results from this study provide a necessary background for palaeoceanographic reconstruction of the northeast Pacific Ocean from stratigraphic analysis of alkenone abundances, unsaturation patterns and isotopic compositions in sediment cores.

Elemental and major biochemical changes across an oxidation front in a relict turbidite: An oxygen effect

Abstract The elemental and major biochemical compositions of the relict f-turbidite sampled in two cores from the Madeira Abyssal Plain were determined. This fine-grained, distal sequence occurs at ca. 9 m core depth and includes a surficial oxidized horizon defined by a distinct color change. Oxygen diffused downward through sediments above this interface and in ca. 10 kyr destroyed 80% of the organic substances that below the front had survived degradation in the presence of porewater sulfate for ca. 140 kyr. These deposits provide an opportunity to establish the extent and selectivity of oxic sedimentary degradation under natural conditions without the usual complications of bioturbation and varying sources or sedimentation rates. In both cores, a sample from the upper oxidized layer was compared to two samples from the underlying unoxidized layers. The unoxidized sequences of both turbidities contained 0.93–1.02 wt% organic carbon (OC) and 0.10–0.11 wt% total nitrogen (TN). Approximately 20% of the initial OC and 40% of the initial TN remained in the oxidized horizons, with a consequent decrease in atomic C/N ratio from ca. 11 to ca. 5. All samples gave very low yields of lignin phenols and comparable OC-normalized yields of total aldoses and amino acids, and indicated predominantly marine organic matter (OM) and nonselective oxic degradation of these biochemical classes. Compositions of individual aldoses and amino acids generally were also similar in surface and deep sediments, except that the oxidized horizons yielded markedly elevated (3–5X) percentages of nonprotein amino acids. This study clearly demonstrates that prolonged exposure to OZ can lead to organic matter alteration which is far more extensive than that obtained with sulfate alone. In comparison to early diagenesis, however, alteration of the measured biochemicals was largely nonselective. Such oxidation reactions could control the distribution and composition of organic matter in slowly accumulating continental rise and deep-ocean environments.

Early Diagenesis: Consequences for Applications of Molecular Biomarkers

Detailed analyses of organic materials in soils, recent sediments, and natural waters invariably indicate the presence of complex mixtures of organic molecules (Eglinton and Murphy, 1969; Thurman, 1985). Typically, a major fraction of the total organic matter is highly degraded and occurs in structurally complex polymers such as humic substances (Christman and Frimmel, 1988). In spite of these complexities, organic materials in modern natural environments are of interest as sources of energy and nutrition, as recorders of past environmental conditions, and as precursors for the formation of fossil resources. The amounts and distributions of organic materials from different biological or geographic sources often are a fundamental consideration in studies of processes, such as production, transport, and degradation, that affect organic remains in natural settings. Thus, the development and use of dependable source indicators has been a major thrust in organic geochemistry.

Physiological impacts on alkenone paleothermometry

We conducted isothermal (15°C) batch culture experiments with the coccolithophorid Emiliania huxleyi (strain NEPCC 55a) to evaluate the extent to which nutrient and light stress contribute to variability in the alkenone unsaturation index U37K′. Alkenone content and composition were constant throughout exponential growth in both experiments when nutrients (nitrate and orthophosphate) were replete. Stationary phase (nutrient-starved) cells continued to produce alkenones, amassing concentrations (ΣAlk) ≥ 3 times higher than those dividing exponentially (1.5–2 pg cell−1), and the U37K′ of “excess” alkenone dropped by 0.11 units. In contrast, 5 days of continuous darkness resulted in a 75% decrease in cellular ΣAlk and a significant U37K′ increase (+0.11 units). Given an established 0.034 unit/°C response for exponentially growing cells of this strain, the observed range of U37K′ variability at 15°C corresponds to an uncertainty of ±3.2°C in predicted growth temperature. This level of variability matches that of the global U37K′ annual mean sea surface temperature calibration for surface marine sediments, begging the question: What is the physiological condition of alkenone-producing cells exported to marine sediments? Comparison of our laboratory results for a strain of E. huxleyi isolated from the subarctic Pacific Ocean with depth profiles for alkenones in surface waters from two contrasting sites in the northeast Pacific Ocean suggests that the answer to this question depends on the ocean regime considered, a possibility with significant bearing on how stratigraphic U37K′ records in marine sediments are to be interpreted paleoceanographically.

A case of post-depositional aerobic degradation of terrestrial organic matter in turbidite deposits from the Madeira Abyssal Plain

Abstract A set of oxidized and unoxidized sediment intervals from the f-turbidite identified in two piston cores from the Madeira Abyssal Plain (MAP) was analyzed for calcium carbonate, total organic carbon (TOC), total nitrogen (TN), stable carbon composition of TOC (δ13C toc ), total hydrolyzable amino acids (THAA), total neutral sugars (TSUG), lignin phenols (LIG), two different lipid biomarkers of marine phytoplankton origin and vascular plantwax n-alkanes and n-acids. Comparison shows changes for all properties depicting the effect of post-depositional, aerobic oxidation. The oxidation process re-mineralized approximately 80% of the TOC and approximately 60% of the TN in the original deposit and left the residual organic matter depleted in 13C by 1.7–2.9‰ THAA and TSUG account for 10% or less of the TOC in all samples leaving approximately 90% of the TOC chemically unidentified. THAA, TSUG, LIG, marine phytoplankton biomarkers and plantwax n-acids were lost from the oxidized deposit to an extent as great as that for TOC. Two non-protein amino acids (β-alanine and γ-aminobutyric acid) and plant wax n-alkanes displayed much less degradation (9–37%) and concentrated 3.3–4.8 times relative to TOC by the oxidation process. These observations show that both marine and terrestrial components contribute to the re-mineralized TOC fraction. A binary mixing approach was used to model quantitatively the change in δ13C toc and plantwax n-alkane concentration. Results suggest that terrestrial organic carbon contributes approximately 15% to the TOC content of the original turbidite, its abundance increases approximately 2–4 times as a consequence of the oxidation process and approximately 40 and 90% of the terrestrial and marine component of TOC in the original turbidite, respectively, was destroyed by the oxidation process. Although selective preservation of terrestrial relative to marine organic carbon is a well-documented phenomenon in sedimentary processes, this study represents the first attempt to assess the sensitivity of terrestrial organic matter to oxidative degradation in a sedimentary environment.

Biomarker temperature estimates for modern and last glacial surface waters of the California Current System between 33° and 42°N

Sea surface temperature (SST) estimates were made using data for C37-39 alkenones analyzed in modern and glacial-age intervals of sediment cores collected along an east-west (∼42°N from 125° to 132°W) and a north-south (41°–33°N) transect of the California Current system (CCS). The prymnesiophyte biomarker results suggest that surface waters warmed significantly throughout this region since the last glacial maximum (LGM) but the magnitude of warming varied spatially. Stratigraphic profiles from four sediment cores along the north-south transect indicate the warming period was confined to the glacial/interglacial transition (15-10 ka), with SST reaching a maximum value at ∼10 ka and maintaining a uniformly high value throughout the Holocene. Comparison of estimates derived from analysis of modern and LGM sediment intervals indicates the magnitude of the SST change was minimal for locations south of about 36°N (≤1°–2°C) and increased significantly (3°–5°C) north of this latitude. Using a simple heat balance model, we calculate from the latitudinal gradient in SST that southward flow in the California Current system during the LGM was about 60% of that measured today at 36°N. Our findings support the conclusion of others based on pollen data that coastal upwelling in the region of the northeast Pacific off northern California was significantly reduced or even completely shutdown during the LGM.