Gregory L. Cowie

Carbohydrate sources in a coastal marine environment

Abstract Individual neutral sugars in sediments, sediment trap materials and major biological sources of a coastal marine environment (Dabob Bay, Washington State) were analyzed by capillary gas chromatography of equilibrated isomeric mixtures. Plankton, bacteria, and vascular plant tissues of different types yielded reproducible and biochemically consistent compositional patterns. These patterns, when expressed in simple parameters, allowed distinctions between marine and terrestrial carbohydrate sources as well as among the major different types of vascular plant tissues. Plankton and bacteria, due to their compositional diversity, were not further distinguishable by carbohydrate compositions alone. Carbohydrate compositions of Dabob Bay sediments and sediment trap materials, interpreted using source-indicator parameters, indicate a predominantly marine origin with increased relative input of terrestrially-derived carbohydrates in winter periods of low phytoplankton productivity. Both plankton and grasses are indicated as major carbohydrate sources during spring. Glucose yield enhancement factors, determined by comparative acid pretreatments, confirm the general predominance of α-cellulose-poor marine polysaccharides and increased levels of α-cellulose-rich vascular plant remains in winter sediment trap samples.

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

SOURCES AND RELATIVE REACTIVITIES OF AMINO-ACIDS, NEUTRAL SUGARS, AND LIGNIN IN AN INTERMITTENTLY ANOXIC MARINE-ENVIRONMENT

A sediment-trap sample, representing an annual average particle flux at 50 m in Saanich Inlet, British Columbia, was analyzed for its elemental, amino acid, neutral sugar, and lignin composition. Amino acid analyses also were performed on underlying sediments which were analyzed previously for organic carbon, nitrogen, neutral sugars, and lignin. The results uniformly indicate primarily marine organic matter sources for all samples, although relatively higher terrigenous contributions are evident in the sediments. The δ13C values of trap materials also point to primarily autochthonous particle fluxes. Comparison of annual average water-column fluxes to sediment accumulation rates indicates under-sampling of sinking particles due to lateral sediment inputs at depth. The anoxic benthic interface appears to be an important site of diagenesis, and selective removal is observed both at compound-class and molecular levels. Cinnamyl and syringyl phenols are selectively removed relative to vanillyl phenols and loss patterns of the monosaccharides, and to a lesser degree the amino acids, strongly indicate preferential preservation of diatom cell-wall materials. Low flux ratios displayed by the nonprotein amino acids are consistent with their diagenetic origin. Preferential loss of marine organic material is indicated by the calculated δ13C value and biochemical composition of the substrate. Concentrations of all measured organic constituents decreased with depth in the uniformly varved 0–14 cm sediment interval, and suggest in situ degradation. Relative reactivities of the biochemical classes indicate a change in diagenetic substrate from that utilized above and at the benthic interface. With the exception of the amino acids, however, diagenesis is generally less selective in the sediments. The amino acid utilization pattern differs from that observed across the benthic interface, and down-core changes in protein and nonprotein amino acid compositions clearly indicate in situ degradation. The sedimentary degraded fraction also appears to be predominantly marine, but lignin yields and sugar compositions suggest a relative increase in the utilization of vascular plant remains. Protein, polysaccharide, and lignin contributions to total organic carbon decrease from 37% in the sediment-trap sample to 22% at the bottom of the 0–14 cm sediment interval. These biochemicals represent over 40 and 50–60% of the degraded carbon and nitrogen, respectively, and thus are important nutrients for the benthic and water-column communities.

Stable phytoplankton community structure in the Arabian Sea over the past 200,000 years

Glacial to interglacial climate changes have been related to organic carbon cycling in oceanic surface waters, and this possible link has led to the development of sedimentary tracers of past marine biological production. For example, sediment records of organic carbon, opal and biogenic barium have been used to reconstruct past variations in production in different oceanic regimes, but these tracers cannot be used to discriminate between the relative contributions of different phytoplankton groups. Such a discrimination would provide greater insight into the operation of the biological ‘pump’ transporting material down out of surface waters, and into the possible influence of the structure of oceanic food chains on carbon fluxes. Several organic biomarker compounds have now been established for tracing the contribution of different planktonic groups to organic carbon in sediments. Here we show that four such biomarkers—dinosterol, alkenones, brassicasterol and chlorins, which represent dinoflagellates, prymnesiophytes, diatoms and chlorophyll-producers, respectively—have concordant concentration maxima that coincide with organic carbon maxima over the past 200,000 years in a sediment core from the northeastern Arabian Sea. Not only do these organic tracers track changes in ocean production in this region, but the similar distributions of dinosterol and brassicasterol indicate that the relative contributions of the dominant members of the phytoplankton community (diatoms and dinoflagellates) to production were roughly uniform on timescales greater than 3,000–4,000 years over the past 200,000 years.

IMPROVED AMINO-ACID QUANTIFICATION IN ENVIRONMENTAL-SAMPLES - CHARGE-MATCHED RECOVERY STANDARDS AND REDUCED ANALYSIS TIME

We describe an improved technique for hydrolytic extraction of amino acids from environmental samples and reverse-phase liquid chromatographic separation of their o-phthaldialdehyde derivatives. Modifications to existing procedures are employed to reduce sample hydrolysis and preparation times. Quantification is improved relative to existing methods by addition of a variety of non-protein amino acids before hydrolysis. They are used as charge-matched recovery standards which help to compensate for differential losses of acidic, neutral and basic amino acids during extraction and analysis. Two experiments demonstrate that selective amino acid losses can be important in the analysis of environmental samples and may have considerable effect on final hydrolysate composition. The results also indicate that the selected recovery standards accurately mimic the behaviour of their corresponding charge classes of protein amino acids. Use of multiple recovery standards is likely also to be of value in dissolved amino acid analyses, particularly in cases such as pore waters where the solution contacts mineral surfaces and associated organic materials.

A 36 kyr geochemical record from the Sea of Japan of organic matter flux variations and changes in intermediate water oxygen concentrations

Intervals of organic C- and carbonate-rich laminated sediments occur in the Sea of Japan with roughly the same frequency as temperature changes observed in Greenland ice cores, providing clear evidence of rapid oceanographic change during the past 36 kyr. Planktonic foraminiferal δ18O data suggest that only the laminated sediments deposited during the Last Glacial Maximum (LGM), and perhaps one other interval formed during a period of increased water column stratification. Sedimentary Re and Mo data are consistent with bottom waters that were sulfidic during the LGM and suboxic during other laminated intervals. Results of a numerical model of Corg and Re burial are consistent with a mechanism whereby an increased Corg flux to the seafloor drove oxygen concentrations toward depletion during times of deposition of the suboxic laminated intervals. Such a process could have resulted from increased upwelling driven either by increased deep water formation due to colder and/or more saline surface waters or by stronger northeasterly monsoonal winds.

Organic matter preservation through the oxygen-deficient zone of the NE Arabian Sea as discerned by organic carbon:mineral surface area ratios

Abstract It is debated whether there is a causal or even correlative relationship between the presence of a minimum in bottom water oxygen (BWO) concentration and the preservation of organic matter in underlying marine sediments. One relationship that has not been examined in this regard is the relationship between sedimentary organic carbon content and mineral surface area. Normalization of organic matter content to mineral surface area eliminates the mass-based problems associated with traditional weight percent carbon measurements, and thus may allow better evaluation of carbon preservation. We measured organic carbon to mineral surface area ratios (OC:SA) on 26 sediment samples from along the NE edge of the Arabian Sea, and on 21 hydrodynamically sorted fractions (using split-flow lateral transport thin [SPLITT] fractionation) from three of these stations. Samples spanned from above to well below the minimum in local BWO concentration. Sediments deposited under the oxygen minimum had OC:SA ratios in excess of 1.1 mg OC m −2 , indicative of enhanced preservation. A bioturbated sediment at the edge of the oxygen minimum (BWO ∼35 μM) also had a high OC:SA ratio. With the exception of the anomalous sample along the slope, all samples from shallower and deeper than the oxygen-depleted water mass (BWO>35 μM) have OC:SA ratios that fall within the typically observed range (0.5–1.1 mg OC m −2 ). While further work will be necessary to discern what causes this relationship, our data indicate that organic matter preservation (as estimated by OC:SA) is enhanced within the general locale of the BWO minimum in NE Arabian Sea sediments. If this relationship proves to be robust, repeatable and stable over time in other modern BWO minimum zones, surface area-normalized organic carbon loadings may be a useful paleoceanographic proxy for past BWO levels.

Hypoxia in the changing marine environment

The predicted future of the global marine environment, as a combined result of forcing due to climate change (e.g. warming and acidification) and other anthropogenic perturbation (e.g. eutrophication), presents a challenge to the sustainability of ecosystems from tropics to high latitudes. Among the various associated phenomena of ecosystem deterioration, hypoxia can cause serious problems in coastal areas as well as oxygen minimum zones in the open ocean (Diaz and Rosenberg 2008 Science 321 926–9, Stramma et al 2008 Science 320 655–8). The negative impacts of hypoxia include changes in populations of marine organisms, such as large-scale mortality and behavioral responses, as well as variations of species distributions, biodiversity, physiological stress, and other sub-lethal effects (e.g. growth and reproduction). Social and economic activities that are related to services provided by the marine ecosystems, such as tourism and fisheries, can be negatively affected by the aesthetic outcomes as well as perceived or real impacts on seafood quality (STAP 2011 (Washington, DC: Global Environment Facility) p 88). Moreover, low oxygen concentration in marine waters can have considerable feedbacks to other compartments of the Earth system, like the emission of greenhouse gases to the atmosphere, and can affect the global biogeochemical cycles of nutrients and trace elements. It is of critical importance to prediction and adaptation strategies that the key processes of hypoxia in marine environments be precisely determined and understood (cf Zhang et al 2010 Biogeosciences 7 1–24).

Carbon and Phosphorus Cycling in Arabian Sea Sediments across the Oxygen Minimum Zone

Several studies have focused on carbon, oxygen, and phosphorus dynamics across the modern oxygen minimum zone (OMZ) to constrain how signals of modern systems get “locked in” upon burial. In this study, a sequential phosphorus fractionation technique was applied to surficial and sub-surface sediments from stations at depths spanning the OMZ on the Pakistan margin of the Arabian Sea in order to test the oxygen-carbon-phosphorus connection in modern marine sediments. Some early diagenetic loss of phosphorus compared to organic carbon was observed, but a significant portion of the released phosphorus was retained by uptake on oxyhydroxides and by the formation of an authigenic phosphorus-bearing phase. This process is unaffected by station location relative to the OMZ, and results in an effective organic carbon-to-reactive-phosphorus sediment ratio that is close to the average observed for open-ocean sediments, regardless of bottom water oxygen content.

Rates and Regulation of Microbially Mediated Aerobic and Anaerobic Carbon Oxidation Reactions in Continental Margin Sediments from the Northeastern Arabian Sea (Pakistan Margin)

Rates of microbially mediated C oxidation were measured at sites above, within, and below the oxygen minimum zone (OMZ) on the Pakistan margin of the Arabian Sea, before and after the southwest monsoon, with the goal of assessing how low bottom water O 2 concentration affects microbial C oxidation processes. Rates of C oxidation coupled to aerobic and anaerobic processes were measured at five depths: 140 m (seasonally hypoxic), 300 m (OMZ core), 940 m (OMZ transition), 1200 m (OMZ transition), and 1850 m (non-OMZ). Rates and mechanisms of C oxidation did not vary significantly between seasons. However, an exception was found at the 140-m site, which became hypoxic during the southwest monsoon. Considering both seasons, C oxidation rates ranged from 0.73 to 4.86 mmol C m ―2 d ―1 . Generally, OMZ sites and those on the OMZ transition had lower C oxidation rates (0.73―2.90 mmol C m ―2 d ―1 ) than those located below the OMZ (3.13―4.86 mmol C m ―2 d ―1 ). The relative importance of C oxidation via different terminal electron acceptors varied between sites according to the position and intensity of the OMZ. At all sites, a large proportion of measured O 2 consumption (30―100%) was coupled to the oxidation of reduced species; consequently, aerobic processes were essentially absent at low-0 2 sites. In contrast, under higher bottom water O 2 concentrations, aerobic processes accounted for 4―64% of C oxidation. Denitrification largely dominated carbon oxidation at all sites (36―99%). Rates of C oxidation coupled to microbial Mn 4+ and Fe 3+ reduction were quantitatively unimportant. Measured sulphate reduction rates at all sites across the margin were surprisingly low (0―0.45 mmol m ―2 d ―1 ) compared to rates measured on other margin environments.