Peter J. Hernes

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.

Glaciers as a source of ancient and labile organic matter to the marine environment

Riverine organic matter supports of the order of one-fifth of estuarine metabolism. Coastal ecosystems are therefore sensitive to alteration of both the quantity and lability of terrigenous dissolved organic matter (DOM) delivered by rivers. The lability of DOM is thought to vary with age, with younger, relatively unaltered organic matter being more easily metabolized by aquatic heterotrophs than older, heavily modified material. This view is developed exclusively from work in watersheds where terrestrial plant and soil sources dominate streamwater DOM. Here we characterize streamwater DOM from 11 coastal watersheds on the Gulf of Alaska that vary widely in glacier coverage (0–64 per cent). In contrast to non-glacial rivers, we find that the bioavailability of DOM to marine microorganisms is significantly correlated with increasing 14C age. Moreover, the most heavily glaciated watersheds are the source of the oldest (∼4 kyr 14C age) and most labile (66 per cent bioavailable) DOM. These glacial watersheds have extreme runoff rates, in part because they are subject to some of the highest rates of glacier volume loss on Earth. We estimate the cumulative flux of dissolved organic carbon derived from glaciers contributing runoff to the Gulf of Alaska at 0.13 ± 0.01 Tg yr-1 (1 Tg = 1012 g), of which ∼0.10 Tg is highly labile. This indicates that glacial runoff is a quantitatively important source of labile reduced carbon to marine ecosystems. Moreover, because glaciers and ice sheets represent the second largest reservoir of water in the global hydrologic system, our findings indicate that climatically driven changes in glacier volume could alter the age, quantity and reactivity of DOM entering coastal oceans.

Photochemical and microbial degradation of dissolved lignin phenols: Implications for the fate of terrigenous dissolved organic matter in marine environments

Received 29 March 2002; revised 27 February 2003; accepted 1 May 2003; published 12 September 2003. [1] Molecular level characterizations of dissolved lignin were conducted in Mississippi River plume waters to study the impact of various removal mechanisms (photooxidation, microbial degradation, and flocculation) on dissolved organic material (DOM) concentrations and compositions. Prior to analysis, dissolved ( 1 kDalton) and low molecular weight (LMW; 25 psu, photooxidation was a dominant factor influencing lignin compositions and concentrations. Diagnostic indicators of photooxidation include a sharp decrease in the percentage of lignin in the HMW size fraction, changes in ratios of syringyl to vanillyl phenols, and increases in LMW acid:aldehyde ratios for both vanillyl and syringyl phenols. A 10-day incubation experiment with plume water indicated rates of microbial degradation of dissolved lignin that were � 30% of photooxidation rates in surface waters. These results highlight the importance of microbial as well as photochemical processes in the cycling of terrigenous DOM in coastal waters. Neither flocculation nor microbial degradation significantly altered lignin composition, suggesting that composition is primarily determined by source and photochemical transformation. Overall, high removal rates indicate the potential importance of terrigenous DOM as a carbon and nutrient source in the coastal ocean. Strong correlations between absorption coefficients at 350 nm and dissolved lignin demonstrate the potential for using absorption to trace terrigenous DOM in coastal environments with significant riverine input. INDEX TERMS: 1055 Geochemistry: Organic geochemistry; 4219 Oceanography: General: Continental shelf processes; 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4847 Oceanography: Biological and Chemical: Optics; 4852 Oceanography: Biological and Chemical: Photochemistry; KEYWORDS: Mississippi River Plume, photooxidation, terrigenous organic matter, lignin, microbial degradation, CDOM

Photochemical degradation of dissolved organic matter and dissolved lignin phenols from the Congo River

[1] Photochemical degradation of Congo River dissolved organic matter (DOM) was investigated to examine the fate of terrigenous DOM derived from tropical ecosystems. Tropical riverine DOM receives greater exposure to solar radiation, particularly in large river plumes discharging directly into the open ocean. Initial Congo River DOM exhibited dissolved organic carbon (DOC) concentration and compositional characteristics typical of organic rich blackwater systems. During a 57 day irradiation experiment, Congo River DOM was shown to be highly photoreactive with a decrease in DOC, chromophoric DOM (CDOM), lignin phenol concentrations (S8) and carbon-normalized yields (L8), equivalent to losses of � 45, 85–95, >95 and >95% of initial values, respectively, and a +3.1 % enrichment of the d 13 C-DOC signature. The loss of L8 and enrichment of d 13 C-DOC during irradiation was strongly correlated (r = 0.99, p < 0.01) indicating tight coupling between these biomarkers. Furthermore, the loss of CDOM absorbance was correlated to the loss of L8 (e.g., a355 versus L8; r = 0.98, p < 0.01) and d 13 C-DOC (e.g., a355 versus d 13 C; r = 0.97, p < 0.01), highlighting the potential of CDOM absorbance measurements for delineating the photochemical degradation of lignin and thus terrigenous DOM. It is apparent that these commonly used measurements for examination of terrigenous DOM in the oceans have a higher rate of photochemical decay than the bulk DOC pool. Further process-based studies are required to determine the selective removal rates of these biomarkers for advancement of our understanding of the fate of this material in the ocean.

Compositions and transport of lipid biomarkers through the water column and surficial sediments of the equatorial Pacific Ocean

A systematic investigation of fluxes and compositions of lipids through the water column and into sediments was conducted along the U.S. JGOFS EgPac transect from l2°N to l5°S at 140°W. Fluxes of lipids out of the euphotic zone varied spatially and temporally, ranging from ≈0.20 – 0.6 mmol lipid-C m−2 day−1. Lipid fluxes were greatly attenuated with increasing water column depth, dropping to 0.002-0.06 mmol lipid-C m−2 day−1 in deep-water sediment traps. Sediment accumulation rates for lipids were ≈ 0.0002 – 0.00003 mmol lipid-C m−2 day−1. Lipids comprised ≈ 11–23% of Corg in net-plankton, 10–30% in particles exiting the euphotic zone, 2–4% particles in the deep EgPac, and 0.1-1 % in sediments. Lipids were, in general, selectively lost due to their greater reactivity relative to bulk organic matter toward biogeochemical degradation in the water column and sediment. Qualitative changes in lipid compositions through the water column and into sediments are consistent with the reactive nature of lipids. Fatty acids were the most labile compounds, with polyunsaturated fatty acids (PUFAs) being quickly lost from particles. Branchedchain C15 and C17 fatty acids increased in relative abundance as particulate matter sank and was incorporated into the sediment, indicating inputs of organic matter from bacteria. Long-chain C39 alkenones of marine origin and long-chain C20-C30 fatty acids, alcohols and hydrocarbons derived from land plants were selectively preserved in sediments. Compositional changes over time and space demonstrate the dynamic range of reactivities among individual biomarker compounds, and hence of organic matter as a whole. A thorough understanding of biogeochemical reprocessing of organic matter in the oceanic water column and sediments is, thus, essential for using the sediment record for reconstructing past oceanic environments.

Tannin diagenesis in mangrove leaves from a tropical estuary: A novel molecular approach

Molecular-level condensed tannin analyses were conducted on a series of mangrove ( Rhizophora mangle) leaves at various stages of decomposition in a tropical estuary. Total molecular tannin yields ranged from 0.5% ash-free dry weight (AFDW) in the most highly degraded black leaves (6 -7 weeks in the water) up to .7% AFDW in fresh leaves (,1 week in the water). Total tannin exhibits an intermediate lability in these leaves relative to other measured biochemicals. Leaching is an important mechanism in tannin removal from leaves as indicated by the 30% loss of measurable tannin during a leaching experiment. Condensed tannin was .80% procyanidin (PC) with the remainder being prodelphinidin (PD). PD tannin, with its higher degree of hydroxylation, proved to be more labile than PC tannin. Average chain length of condensed tannin (degree of polymerization) exhibited an initial increase in response to leaching, but later decreased in the subsequent shift toward abiotic or microbially mediated chemical reactions. Several trends point toward a possible condensation reaction in which tannin plays a role in nitrogen immobilization. These include an apparent inverse correlation between molecular tannin and nitrogen, a positive correlation between molecular tannin and percent basic amino acids, 13 C-NMR data indicating transformation of tannin as opposed to remineralization, and 13 C-NMR data showing loss of condensed tannin B-ring phenolic carbons coupled with preservation of A-ring phenolic carbon. In addition to condensed tannin, the molecular method used also yielded several triterpenoids. Triterpenoids accounted for up to 3.5% AFDW of the leaf material and exhibited a threefold increase between yellow senescent leaves entering the estuary and black leaves. This trend is likely due to the weakening of protective cuticular membranes during leaf decomposition, which leads to increased yields in the acidic conditions used for tannin analyses. Copyright

Transport and diagenesis of dissolved and particulate terrigenous organic matter in the North Pacific Ocean

Lignin measurements were made on suspended particulate organic matter (POM), total dissolved organic matter (DOM), high-molecular-weight (HMW) DOM, and low-molecular-weight (LMW) DOM in the North Pacific at Station Aloha. Carbon-normalized yields of lignin and d 13 C measurements indicate that while terrigenous organic matter accounts only for B1% of DOM in seawater, submicron POM has a substantial terrigenous component. The apparent size distribution and terrigenous nature of the particles is consistent with an aerosol source, but these particles could also be delivered to the ocean via rivers. Ratios of syringyl to vanillyl phenols as well as the fraction of dissolved lignin in HMW DOM provide molecular evidence of photochemical oxidation. Several differences in lignin composition and concentrations were evident between water masses. In particular, concentrations in North Pacific Intermediate Water were a factor of two greater than at all other depths. This is likely a reflection of higher riverine input via the Sea of Okhotsk. These trends suggest that lignin could serve as a general circulation tracer in addition to tracing riverine input and diagenetic processing of terrigenous organic matter in the ocean. r 2002 Elsevier Science Ltd. All rights reserved.

Neutral carbohydrate geochemistry of particulate material in the central equatorial Pacific

Abstract Neutral carbohydrate compositions were determined for particulate samples from plankton net tows, shallow floating sediment traps, mid-depth and deep moored sediment traps, and sediment cores collected along a north-south transect in the central equatorial Pacific Ocean during the U.S. JGOFS EqPac program. Total neutral carbohydrate depth profiles and patterns along the transect follow essentially the same trends as bulk and organic carbon (OC) fluxes—attenuating with depth, high near the equator and decreasing poleward. OC-normalized total aldose (TCH 2,O ) yields along the transect and with depth do not show any consitent patterns. Relative to a planktonic source, neutral carbohydrate compositions in sediment trap and sediment core samples reflect preferential loss of ribose and storage carbohydrates rich in glucose, and preferential preservation of structural carbohydrates rich in rhamnose, xylose, fucose, and mannose. There is also evidence for an intermediately labile component rich in galactose. It appears that compositional signatures of neutral carbohydrates in sediments are more dependent upon their planktonic source than on any particular diagenetic pathway. Relative to other types of organic matter, neutral carbohydrates are better preserved in calcareous oozes from 12°S to 5°N than in red clays at 9°N based on OC-normalized TCH 2 O yields, due to either differing sources or sorption characteristics. Weight per cent glucose generally decreases with increased degradation of organic material in the central equatorial Pacific region. Based on weight per cent glucose, comparisons of samples between Survey I (El Nion) and Survey II (non-El Nino) indicate that during Survey I, organic material in the epipelagic zone in the northern hemisphere may have undergone more degradation than organic material in the southern hemisphere.

Molecular Trickery in Soil Organic Matter: Hidden Lignin

Binding to minerals is one mechanism crucial toward the accumulation and stabilization of organic matter (OM) in soils. Of the various biochemicals produced by plants, lignin-derived phenols are among the most surface-reactive compounds. However, it is not known to what extent mineral-bound lignin-derived phenols can be analytically assessed by alkaline CuO oxidation. We tested the potential irreversible binding of lignin from three litters (blue oak, foothill pine, annual grasses) to five minerals (ferrihydrite, goethite, kaolinite, illite, montmorillonite) using the CuO-oxidation technique, along with bulk organic carbon (OC) sorption. Up to 56% of sorbed lignin could not be extracted from the minerals with the CuO-oxidation technique. The composition of the irreversibly bound lignin component differed markedly between minerals and from that of the parent litter leachates, indicating different bonding strengths related to individual monomers and conformations. The difference in extractability of individual phenols suggests that abiotic processes, such as sorption/desorption, should be taken into account when using CuO oxidation data for assessing lignin turnover in mineral matrixes. However, given the apparent relationship between aromaticity as indicated by carbon-specific UV absorbance (SUVA) and bulk OC sorption, it is likely that irreversible sorption is a concern for any technique that addresses the broad class of aromatic/phenolic compounds in soils and sediments.

Pan-arctic trends in terrestrial dissolved organic matter from optical measurements

Climate change is causing extensive warming across arctic regions resulting in permafrost degradation, alterations to regional hydrology, and shifting amounts and composition of dissolved organic matter (DOM) transported by streams and rivers. Here, we characterize the DOM composition and optical properties of the six largest arctic rivers draining into the Arctic Ocean to examine the ability of optical measurements to provide meaningful insights into terrigenous carbon export patterns and biogeochemical cycling. The chemical composition of aquatic DOM varied with season, spring months were typified by highest lignin phenol and dissolved organic carbon (DOC) concentrations with greater hydrophobic acid content, and lower proportions of hydrophilic compounds, relative to summer and winter months. Chromophoric DOM (CDOM) spectral slope (S275-295) tracked seasonal shifts in DOM composition across river basins. Fluorescence and parallel factor analysis identified seven components across the six Arctic rivers. The ratios of ‘terrestrial humic-like’ versus ‘marine humic-like’ fluorescent components co-varied with lignin monomer ratios over summer and winter months, suggesting fluorescence may provide information on the age and degradation state of riverine DOM. CDOM absorbance (a350) proved a sensitive proxy for lignin phenol concentrations across all six river basins and over the hydrograph, enabling for the first time the development of a single pan-arctic relationship between a350 and terrigenous DOC (R2 = 0.93). Combining this lignin proxy with high-resolution monitoring of a350, pan-arctic estimates of annual lignin flux were calculated to range from 156 to 185 Gg, resulting in shorter and more constrained estimates of terrigenous DOM residence times in the Arctic Ocean (spanning 7 months to 2½ years). Furthermore, multiple linear regression models incorporating both absorbance and fluorescence variables proved capable of explaining much of the variability in lignin composition across rivers and seasons. Our findings suggest that synoptic, high-resolution optical measurements can provide improved understanding of northern high-latitude organic matter cycling and flux, and prove an important technique for capturing future climate-driven changes.