Lei Chou

Interactions of C, N, P and S Biogeochemical Cycles and Global Change

This is an up-to-date synthesis of the global biogeochemical cycles of C, N, P and S. Processes and models involving the flow of these elements in and between the ocean, atmosphere, biosphere and land are emphasized. Human-induced perturbations to the global cycles are discussed, and the role of these space scales of global change is considered, from the geological past to the present and the future. Feedback mechanisms that enhance or ameliorate global change are an important feature. The book is strongly interdisciplinary in scope.

The Portugal coastal counter current off NW Spain: new insights on its biogeochemical variability

Abstract Time series of wind-stress data, AVHRR and SeaWiFS satellite images, and in situ data from seven cruises are used to assemble a coherent picture of the hydrographic variability of the seas off the Northwest Iberian Peninsula from the onset (September–October) to the cessation (February–May) of the Portugal coastal counter current (PCCC). During this period the chemistry and the biology of the shelf, slope and ocean waters between 40° and 43°N have previously been undersampled. Novel information extracted from these observations relate to: 1. The most frequent modes of variability of the alongshore coastal winds, covering event, seasonal and long-term scales; 2. The conspicuous cycling between stratification and homogenisation observed in PCCC waters, which has key implications for the chemistry and biology of these waters; 3. The seasonal evolution of nitrite profiles in PCCC waters in relation to the stratification cycle; 4. The Redfield stoichiometry of the remineralisation of organic matter in Eastern North Atlantic Central Water (ENACW)—the water mass being transported by the PCCC; 5. The separation of coastal (mesotrophic) from PCCC (oligotrophic) planktonic populations by a downwelling front along the shelf, which oscillates to and fro across the shelf as a function of coastal wind intensity and continental runoff; and 6. The photosynthetic responses of the PCCC and coastal plankton populations to the changing stratification and light conditions from the onset to the cessation of the PCCC.

Mechanism of Kaolinite Dissolution at Room Temperature and Pressure: Part 1. Surface Speciation

Surface speciation of Georgia kaolinite was investigated by detailed potentiometric titrations (pH 2-12) at various ionic strengths in KCI04 solutions (0.1, 0.01, 0.001 M) under N2 atmosphere, corrected for Si and Al released due to dissolution. Proton adsorption or desorption was computed according to surface complexation models (nonelectrostatic model and constan! capacitance model), assuming the presence of multi-sites at the surfa AIOH 2 +,>AJO , >SiO- ). The pH of zero proton charge was found to be ~5.5. Below pH 5.5, the positive charge is due to the proton adsorption on aluminium sites of the octahedral sheet. The more acidic group corresponds to the externa! hydroxyls of the octahedral sheet, whereas the second protonation m ay take place either at the inner hydroxyl groups or at the edge aluminol groups. The corresponding intrinsic surface dissociation constants for these species (pKa 1 = 1.9 and pKa 1 =4.1, respective! y) are substantially lower than that for aluminium oxides (pKa 1 =4.5-8). Above pH 5.5, the kaolinite surface undergoes two successive deprotonations, the first ~tarts at pH around 5.5 and the second at pH approximately equal to 9. In comparison with the pK values for surface dissociation of silica and alumina, it is possible to deduce that the first deprotonation takes place at Si sites, while the second occurs at Al site~. Copyright© 1998 Elsevier Science Ltd l. INTRODUCTION The surface charge of kaolinite results from the existence of permanent (pH-independent) and nonpermanent (pH-depenxad dent) charges (Schoefield and Samson, 1954; Cashen, 1959; Bolland et aL, 1976, 1980). The isomorphic substitution of Si(IV) for Al(III) in tetrahedral positions develops a pennanent negative charge within the siloxane !ayer, which accounts for the cation exchange capacity (CEC) of kaolinite. lts value, very small when compared with other clay minerals, is in the range of 1-8 mEqllOO g (Newman and Brown, 1987). (t has been suggested that this Jow CEC value could also be due to the presence of a small amount of alumino-silicate gel coating (Ferris and Jepson, 1975) or of smectite contamination (Lim et aL, 1980). The origin of the nonpermanent charge is the consequence of the reactions occurring between the ionisable surface groups located at the edges or at the gibbsite basal plane and the ions or ligands present in the aqueous solution. The acid-base propxad erties of these groups are responsible for the positive/negative nature of the surface charge and its pH-dependence. The elecxad troneutrality of the crystal edges gives rise to the transformaxad tion of the dangling oxygens into silanol (>SiOH) or aluminol (>AlOH) groups and to the adsorption of water molecules (S pasito, 1984; Bleam et aL 1993 ). The silanol groups conxad tribute only to the negative charge. through the formation of >SiO- surface complexes by deprotonation (Abendroth, 1970; ller, 1979). The aluminol groups can undergo both protonation at low pH and deprotonation at high pH, resulting in the

New production of the NW Iberian shelf during the upwelling season over the period 1982–1999

New production (NP) is calculated for NW Iberian shelf waters from 421 to 431N (3500 km 2 ), at the fortnight, upwelling-season (March–October) and inter-annual time-scales. The time series used are (1) upwelling rates (daily values of offshore Ekman transport from 1982 to 1999), (2) bottom shelf temperatures (twice a week values from 1987 to 1999), and (3) the nutrient–temperature relationships ofupwelled Eastern North Atlantic Central Water (ENACW) obtained during 14 hydrographic cruises to the study area (between 1977 and 1998). Marked inter-annual variability is observed, both at the fortnight and the seasonal time-scales. Average NP over the upwelling-season ranged from 330 to 815 mg C m � 2 d � 1 (mean, 4907145 mg C m � 2 d � 1 ) in the 1982–1999 period. Large inter-annual changes ofupwelling rates are the reason behind the NP fluctuations: 83% ofthe variability ofNP can be explained by the offshore Ekman Transport ð� QX Þ: NP is compared with satellite-derived net microbial community production (NCP) during the 1998– 1999 upwelling seasons, when SeaWiFS images are available. An average upwelling-season NP/NCP ratio of0.33 was obtained, indicating that 67% of NCP is respired in situ and 33% is exported off-shelf to the surrounding oligotrophic ocean.

Southern Ocean CO2 sink: The contribution of the sea ice

We report first direct measurements of the partial pressure of CO2 (pCO2) within Antarctic pack sea ice brines and related CO2 fluxes across the air-ice interface. From late winter to summer, brines encased in the ice change from a CO2 large oversaturation, relative to the atmosphere, to a marked undersaturation while the underlying oceanic waters remains slightly oversaturated. The decrease from winter to summer of pCO2 in the brines is driven by dilution with melting ice, dissolution of carbonate crystals, and net primary production. As the ice warms, its permeability increases, allowing CO2 transfer at the air-sea ice interface. The sea ice changes from a transient source to a sink for atmospheric CO2. We upscale these observations to the whole Antarctic sea ice cover using the NEMO-LIM3 large-scale sea ice-ocean and provide first estimates of spring and summer CO2 uptake from the atmosphere by Antarctic sea ice. Over the spring-summer period, the Antarctic sea ice cover is a net sink of atmospheric CO2 of 0.029 Pg C, about 58% of the estimated annual uptake from the Southern Ocean. Sea ice then contributes significantly to the sink of CO2 of the Southern Ocean.

Biogeochemistry and microbial community composition in sea ice and underlying seawater off East Antarctica during early spring

Pack ice, brines and seawaters were sampled in October 2003 in the East Antarctic sector to investigate the structure of the microbial communities (algae, bacteria and protozoa) in relation to the associated physico-chemical conditions (ice structure, temperature, salinity, inorganic nutrients, chlorophyll a and organic matter). Ice cover ranged between 0.3 and 0.8xa0m, composed of granular and columnar ice. The brine volume fractions sharply increased above −4°C in the bottom ice, coinciding with an important increase of algal biomass (up to 3.9xa0mg C l−1), suggesting a control of the algae growth by the space availability at that period of time. Large accumulation of NH4+ and PO43− was observed in the bottom ice. The high pool of organic matter, especially of transparent exopolymeric particles, likely led to nutrients retention and limitation of the protozoa grazing pressure, inducing therefore an algal accumulation. In contrast, the heterotrophs dominated in the underlying seawaters.

Distribution and characterization of dissolved and particulate organic matter in Antarctic pack ice

Distribution and composition of organic matter were investigated in Antarctic pack ice in early spring and summer. Accumulation of organic compounds was observed with dissolved organic carbon (DOC) and particulate organic carbon (POC) reaching 717 and 470xa0μM C, respectively and transparent exopolymeric particles (TEP) up to 3,071xa0μg Xanthan gum equivalentxa0l−1. POC and TEP seemed to be influenced mainly by algae. Particulate saccharides accounted for 0.2–24.1% (mean, 7.8%) of POC. Dissolved total saccharides represented 0.4–29.6% (mean, 9.7%) of DOC, while dissolved free amino acids (DFAA) accounted for only 1% of DOC. Concentrations of TEP were positively correlated with those of saccharides. Monosaccharides (d-MCHO) dominated during winter–early spring, whereas dissolved polysaccharides did in spring–summer. DFAA were strongly correlated with d-MCHO, suggesting a similar pathway of production. The accumulation of monomers in winter is thought to result from limitation of bacterial activities rather than from the nature of the substrates.

Distribution and Fluxes of Calcium Carbonatealong the Continental Margin in the Gulf of Biscay

Compositions of major components in suspended matter,collected by centrifugation, in situ pumping andsediment traps, in the Gulf of Biscay during the OMEXproject were determined and compared. The resultsshow a strong and rapid decrease in the concentrationof biogenic fraction in the upper 200 m of the watercolumn which may be attributed to the preferentialremoval of this component due to the production offaecal pellets and to the formation of marine snow. Concurrent decrease with depth of the organic andinorganic carbon contents demonstrate the importanceof the respiration of organic matter and thedissolution of calcium carbonate in the oceanic carboncycling. Lithogenic and carbonate fluxes across thecontinental slope in the Goban Spur area wereevaluated based on sediment trap records. The rate ofproduction of calcium carbonate in the surface waters(100 g CaCO3 m-2y-1), deduced from theprimary production measurements, is much largercompared to the fluxes of this component observed inthe sediment traps (5–21 g CaCO3 m-2y-1)and to its rate of burial (9–31 gCaCO3 m-2y-1). It strongly suggeststhe occurrence of carbonate dissolution, even inwaters oversaturated with respect to the mineral phaseconsidered. This is likely to be associated with therespiration of organic matter within the faecalpellets, or at the surface of biogenic calcite oraragonite

Comparative Geochemistry of Marine Saline Lakes

Saline lakes may be classified into two groups on the basis of the primary water source for the lake. These groups are the athalassic (nonmarine) and the marine saline lakes. Athalassic saline lakes receive freshwater input and become saline owing to the high evaporation rates of the arid or semiarid climatic regions in which these lacustrine systems are predominantly found. This group of lakes includes most of the small inland salt lakes and playas, as well as larger lakes such as the Dead Sea in Israel and the Greqt Salt Lake of the United States. The second group of saline lakes is the marine lakes, which are filled primarily with water having a seawater or chemically modified seawater composition. These coastal lakes are usually of small areal extent and are found in a wide variety of climatic regions ranging from temperate to arid.

Biocalcification by Emiliania huxleyi in batch culture experiments

Abstract Coccolithophores, among which Emiliania huxleyi is the most abundant and widespread species, are considered the most productive calcifying organism on earth. The export of organic carbon and calcification are the main drivers of the biological CO2 pump and are expected to change with oceanic acidification. Coccolithophores are further known to produce transparent exopolymer particles (TEP) that promote particle aggregation. As a result, the TEP and biogenic calcium carbonate (CaCO3) contribute to the export of carbon from the surface ocean to deep waters. In this context, we followed the development and the decline of E. huxleyi using batch experiments with monospecific cultures. We studied the link between different processes such as photosynthesis, calcification and the production of TEP. The onset of calcification was delayed in relation to photosynthesis. The timing and the general feature of the dynamics of calcification were closely related to the saturation state of seawater with respect to calcite, Ωcal. The production of TEP was enhanced after the decline of phytoplankton growth. After nutrient exhaustion, particulate organic carbon (POC) concentration increased linearly with increasing TEP concentration, suggesting that TEP contributes to the POC increase. The production of CaCO3 is also strongly correlated with that of TEP, suggesting that calcification may be considered as a source of TEP precursors.