Stuart G. Wakeham

A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals

Abstract In simulation studies of the oceans role in the global carbon cycle, predicting the depth-distribution for remineralization of particulate organic carbon (POC) is of particular importance. Following Sarmiento et al. (Global Biogeochemical Cycles 7 (1993) 417), most simulation models have the power-law curve of Martin et al. (Deep-Sea Research 34 (1987) 267) for this purpose. The Martin et al. curve is an empirical fit to data, most of which is from shallow floating sediment traps. Using such a fit implies that all the information necessary for prediction is contained in the carbon flux itself, so that the organic-carbon flux F OC ( z ) at any depth z can be predicted from the flux of organic carbon F OC ( z 0 ) at some near-surface depth z 0 . Here, we challenge this basic premise, arguing that fluxes of ballast minerals (silicate and carbonate biominerals, and dust) determine deep-water POC fluxes, so that a mechanism-based model of POC flux must simultaneously predict fluxes of both POC and ballast minerals. This assertion is based on the empirical observation that POC fluxes are tightly linked quantitatively to fluxes of ballast minerals in the deep ocean. Here, we develop a model structure that incorporates this observation, and fit this model to US JGOFS EqPac data. This model structure, plus the preliminary parameter estimates we have obtained, can be used to explore the implications of our model in studies of the ocean carbon cycle.

Poly cyclic aromatic hydrocarbons in Recent lake sediments—II. Compounds derived from biogenic precursors during early diagenesis

Abstract Five groups of polycyclic aromatic hydrocarbons (PAH) thought to be derived by early-diagenetic transformations of biogenic precursors are apparently present in Recent sediments of four lakes (Lake Lucerne. Lake Zurich, and Greifensee. Switzerland, and Lake Washington, northwest U.S.A.) These natural PAH include: 1. (1) perylene. 2. (2) an extended series of phenanthrene homologs. 3. (3) retene and pimanthrene derived from diterpenes. 4. (4) a series of tetra- and pentacyclic PAH derived from pentacyclic triterpenes of the amyrin-type. 5. (5) tetra- and pentacyclic PAH formed from pentacyclic triterpenes with five-membered E-rings. Since these PAH are abundant in very young sediment layers, the transformation reactions involved appear to be faster than previously thought and may be microbially mediated. There is no evidence that anthropogenic or petrogenic sources can account for the distributions of these groups of PAH in cores of Recent lake sediments.

Effect of Phytoplankton Cell Geometry on Carbon Isotopic Fractionation

The carbon isotopic compositions of the marine diatom Porosira glacialis and the marine cyanobacterium Synechococcus sp. were measured over a series of growth rates (μ) in a continuous culture system in which the concentration and carbon isotopic composition of CO2(aq) were determined. These data were compared with previously published isotopic results of growth rate experiments using the marine diatom Phaeodactylum tricornutum and the marine haptophyte Emiliania huxleyi. Systematic relationships were found to exist between μ/[CO2(aq)] and carbon isotopic fractionation (ϵP) for each species. Maximum isotopic fractionation (ϵf) for P. glacialis, E. huxleyi, and P. tricornutum was ∼25‰, suggesting that this value may be typical for maximum fractionation associated with Rubisco and β-carboxylases for marine eukaryotic algae. By contrast, ϵf determined for Synechococcus clone CCMP838 was ∼7‰ lower. The slopes of the lines describing the relationship between ϵP and μ/[CO2(aq)] for eukaryotic algal species were different by a factor of more than 20. This result can be accounted for by differences in the surface area and cellular carbon content of the cells. Comparison of chemostat experimental results with calculated results using a diffusion based model imply that the algae in the experiments were actively transporting inorganic carbon across the cell membrane. Our results suggest that accurate estimates of paleo-[CO2(aq)] from ϵP measured in sediments will require knowledge of growth rate as well as cell surface area and either cell carbon quota or cell volume. Given growth rate estimates, our empirical relationship permits reliable calculations of paleo-[CO2(aq)] using compound-specific isotopic analyses of C37 alkadienones (select haptophytes) or fossilized frustules (diatoms).

Oceanic dimethylsulfide:Production during zooplankton grazing on phytoplankton

About half the biogenic sulfur flux to the earths atmosphere each year arises from the oceans. Dimethylsulfide (DMS), which constitutes about 90% of this marine sulfur flux, is presumed to originate from the decomposition of dimethylsulfoniopropionate produced by marine organisms, particularly phytoplankton. The rate of DMS release by phytoplankton is greatly increased when the phytoplankton are subjected to grazing by zooplankton. DMS production associated with such grazing may be the major mechanism of DMS production in many marine settings.

Molecular indicators of diagenetic status in marine organic matter

The fluxes of individual carbohydrates, amino acids, lipids and pigments have been determined in net-plankton, particulate matter and sediments from three sites (9°N, 5°N, and 0°N) in the central equatorial Pacific to evaluate sources and reactivities of organic compounds. Although primary production rates vary markedly across this 9° swath, vertical trends in biochemical compositions remained remarkably parallel. Together these one hundred plus biochemicals account for 80% of the total organic carbon (Corg) in net-plankton and particles sinking from the euphotic zone, but represent only 24 and 20% of the organic carbon in deep-water particles and surface sediments, respectively. Scaled profiles of relative abundances, clearly illustrate (a) exponential losses of plankton remains and increases in heterotroph biomarkers throughout the water column, (b) elevated proportions of bacterial markers near the sediment surface, and (c) preservation of selected remains of bacteria, phytoplankton and vascular land plants deeper in the sediments. In spite of one of the most comprehensive analyses of major biochemicals yet applied to marine particulate samples, percentages of molecularly uncharacterized organic carbon increase progressively down the water column to values near 80% in the underlying sediments. The composition, formation pathway and information potential of this uncharatterized fraction are among the most fascinating questions in marine organic geochemistry.

Polycyclic aromatic hydrocarbons in Recent lake sediments—I. Compounds having anthropogenic origins

Abstract Polycyclic aromatic hydrocarbons (PAH) in sediment cores from Lake Lucerne, Lake Zurich, and Greifensee, Switzerland, and Lake Washington, northwest U.S.A., have been isolated, identified and quantified by glass capillary gas chromatography and gas chromatography/mass spectrometry. Surface sediment layers are greatly enriched in PAH—up to 40 times—compared to deeper layers. In addition, concentration increases in upper sediments generally correspond to increasing industrialization and urbanization in the catchment basins of the lakes. Few PAH could be detected in pre-industrial revolution sediments, indicating that background levels for most PAH in aquatic sediments are extremely low. These results are consistent with an anthropogenic source for most of the aromatic hydrocarbons present in the modern sediments. A comparison of PAH distributions in the sediments and in possible source materials shows that urban runoff of street dust may be the most important PAH input to these lacustrine sediments. There is evidence that a significant contribution to the PAH content of street dust comes from material associated with asphalt.

Consistent fractionation of 13C in nature and in the laboratory: Growth‐rate effects in some haptophyte algae

The carbon isotopic fractionation accompanying formation of biomass by alkenone-producing algae in natural marine environments varies systematically with the concentration of dissolved phosphate. Specifically, if the fractionation is expressed by epsilon p approximately delta e - delta p, where delta e and delta p are the delta 13C values for dissolved CO2 and for algal biomass (determined by isotopic analysis of C37 alkadienones), respectively, and if Ce is the concentration of dissolved CO2, micromole kg-1, then b = 38 + 160*[PO4], where [PO4] is the concentration of dissolved phosphate, microM, and b = (25 - epsilon p)Ce. The correlation found between b and [PO4] is due to effects linking nutrient levels to growth rates and cellular carbon budgets for alkenone-containing algae, most likely by trace-metal limitations on algal growth. The relationship reported here is characteristic of 39 samples (r2 = 0.95) from the Santa Monica Basin (six different times during the annual cycle), the equatorial Pacific (boreal spring and fall cruises as well as during an iron-enrichment experiment), and the Peru upwelling zone. Points representative of samples from the Sargasso Sea ([PO4] < or = 0.1 microM) fall above the b = f[PO4] line. Analysis of correlations expected between mu (growth rate), epsilon p, and Ce shows that, for our entire data set, most variations in epsilon p result from variations in mu rather than Ce. Accordingly, before concentrations of dissolved CO2 can be estimated from isotopic fractionations, some means of accounting for variations in growth rate must be found, perhaps by drawing on relationships between [PO4] and Cd/Ca ratios in shells of planktonic foraminifera.

The biochemical and elemental compositions of marine plankton: A NMR perspective

The traditional Redfield–Ketchum–Richards (1963) equation for the production (or respiration) of ‘‘average marine plankton’’ 106 CO2 þ 16 HNO3 þ H3PO4 þ 122 H2O ¼ð CH2OÞ106ðNH3Þ16ðH3PO4 Þþ 138 O2 has long been a useful guideline for establishing the ratios and reaction extents of the bioactive elements in ocean systems. The empirical formula on the right of the above equation for marine plankton biomass adequately represents the C/N/P of mixed marine plankton collected in towed nets, but includes an impossibly elevated hydrogen content and a questionably high level of organic oxygen. An elevated estimate of oxygen content is particularly critical because it would lead to an underestimate of the amount of O2 required for complete respiration of plankton biomass. Although direct biochemical measurements have been used previously to constrain the compositions, and hence the reaction stoichiometries, of marine plankton and their remains, such analyses can be prone to error and analytical bias. To cast a new light on the chemical composition of marine plankton, we determined the major functional group distribution of organic carbon in mixed plankton tows from five contrasting ocean sites using cross-polarization, magic-angle spinning carbon-13 nuclear magnetic resonance (CP/MAS 13 C NMR). Using a mixing model that relates NMR spectral data to biochemical composition, we estimate an average major biochemical composition (weight basis) for these plankton samples of 65% protein, 19% lipid and 16% carbohydrate. This biochemical composition corresponds to an average elemental formula for plankton biomass of C106H177O37N17S0.4, whose complete oxidation requires 154 moles of O2. Although preliminary, this 13 C NMR-based estimate indicates elemental compositions and respiratory oxygen demands that are widely different from those indicated by the RKR composition (C106H260O106N16 and 138 O2, respectively) and those determined in many previous field studies. D 2002 Published by Elsevier Science B.V.

Reburial of fossil organic carbon in marine sediments

Marine sediments act as the ultimate sink for organic carbon, sequestering otherwise rapidly cycling carbon for geologic timescales. Sedimentary organic carbon burial appears to be controlled by oxygen exposure time in situ, and much research has focused on understanding the mechanisms of preservation of organic carbon. In this context, combustion-derived black carbon has received attention as a form of refractory organic carbon that may be preferentially preserved in soils and sediments. However, little is understood about the environmental roles, transport and distribution of black carbon. Here we apply isotopic analyses to graphitic black carbon samples isolated from pre-industrial marine and terrestrial sediments. We find that this material is terrestrially derived and almost entirely depleted of radiocarbon, suggesting that it is graphite weathered from rocks, rather than a combustion product. The widespread presence of fossil graphitic black carbon in sediments has therefore probably led to significant overestimates of burial of combustion-derived black carbon in marine sediments. It could be responsible for biasing radiocarbon dating of sedimentary organic carbon, and also reveals a closed loop in the carbon cycle. Depending on its susceptibility to oxidation, this recycled carbon may be locked away from the biologically mediated carbon cycle for many geologic cycles.

Biogeochemistry of particulate organic matter in the oceans: results from sediment trap experiments

Abstract Particulate organic matter collected in sediment traps from various oceanic regimes— Sargasso Sea, equatorial North Atlantic, central North Pacific, California Current, and Peru coastal upwelling—have been analyzed for their lipid and amino acid composition and flux. Despite rapid settling of the large particles through the water column and a relatively small depth gradient for total organic carbon flux, there are major changes in the composition and flux of lipids and amino acids associated with the particles. The rapid disappearance of the more labile compounds, such as amino acids and polyunsaturated fatty acids, with increasing depth indicates that the major sources of such compounds are in the upper part of the water column and that they are readily degraded as the particles sink. On the other hand, the intermittent appearance of large amounts of wax ester, along with the changing fatty acid composition of the particles, points to deep-water sources for some of these compounds.