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Biogeochemical cycles - The Hydrosphere The hydrosphere contains all the water on earth. Ocean represent 97% of it so we will focus mainly on oceans, but rivers will also be discussed

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Physical structure of the ocean (very brief ! ) Thermocline Surface Mixed layer Deep ocean Wind driven horizontal circulation Horizontal and vertical movements Thermohaline circulation Euphotic layer vertical Vertical structure Horizontal structure

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Ocean chemistry Motivation : global carbon cycle, ecosystem functioning Key questions : What controls the mean composition (and pH) of seawater ?

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Observed Mean Ocean Concentrations large range Can we explain todays mean ocean composition ? Major elements Trace elements.. Including nutrients (C,N,P,Si,Fe)

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River Water Sea Water Mainly Ca 2+ /HCO 3 - Mainly Na + /Cl - Both the composition and key ratios are different ppm = mg kg -1 Linking Ocean and river water composition ?

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Weathering (chemical) of limestone is considered a congruent reaction (all solid dissolves) CaCO 3 (s) + CO 2 (g) + H 2 O Ca 2+ + 2 HCO 3 - 1 2 Weathering of alumino-silicate minerals to clay minerals are examples of incongruent reactions (solid partially dissolves) silicate minerals + CO 2 (g) + H 2 O clay minerals + HCO 3 - + 2 H 4 SiO 4 + cations 1 1 2 KAlSi 3 O 8 (s) + CO 2 (g) + 11/2. H 2 O 1/2. Al 2 Si 2 O 5 (OH) 4 (s) + K + + HCO 3 - + 2.H 4 SiO 4 Weathering of rocks (orthoclase feldspar) (kaolinite) What Controls the chemical composition of Rivers? Minerals weather at different rates e.g.

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Variability in water vapor composition Europe, North America and Asia are more calcareous continents. Most of variability due to Ca 2+ and HCO 3 - which come from weathering of carbonate rock SO 4 2- and Cl - come from aerosols and weathering of evaporite rocks (e.g. Salt or NaCl). Na +, K +, Mg 2+, SiO 2 come from weathering of silicate rocks

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Can the ocean composition results from the evaporation of river water ? Examples: Mono Lake, CA Soap Lake, WA When river water evaporates, it makes a Na, HCO 3 -, CO 3-- brine and the resulting pH is very basic. pH Calculated from the charge balance H + + Na + + K + + 2Ca 2+ + 2Mg 2+ = Cl - + 2 SO 4 2- + HCO 3 - + 2CO 3 2-

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Can the ocean composition results from a chemical thermodynamical equilibrium ? Sillen hypothesis (1959, 1961) : Sources of dissolved ions = Weathering reactions and rivers Sinks = Reverse weathering reactions : formation of mineral (solid) in the ocean from dissolved compounds (reaching saturation) and sedimentation. Organizational framework: Gibbs Phase Rule f = c + 2 pf = degrees of freedom which means the number of intensive properties, which are independent of other intensive variables, and required to define the system. (e.g. T, P, concentrations Na +, Cl -, Ca 2+, SO 4 2- ) c = components (ingredients composing the system, e.g., HCl, NaOH, MgO ) p = phases at thermo equilibrium (domains of uniform composition, e.g. gas, liquid, pure solids) http://serc.carleton.edu/research_education/equilibria/phaserule.html

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Sillen ocean model : Nine component model (C = 9) Acids: HCl, H 2 O, CO 2 Bases: KOH, CaO, SiO 2, NaOH, MgO, Al(OH) 3 The ocean chemistry is viewed as a giant acid-base titration. Acids from the volcanoes and bases from the rocks weathering. If these phases are at equilibrium, then the system is determined by 2 independent intensive variables, taken as T and [Cl-]. The sea water composition is then fixed and it could only change if temperature or [Cl - ] changed. Kaolinite, illite, chlorite, montmorillonite and phillipsite are types of clay minerals

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Specific reverse weathering type reactions proposed to remove excess ions. In this approach newly formed clays would have to equal 7% of sedimentary mass to account for the river input and explain the observed sea water composition. The measured distribution of clay mineral in sediment does not support these reactions. chlorite in deep-sea sediments illite in deep-sea sediments Instead most clays are detrital, reflecting continental sources (so not locally found) rather than a inverse weathering source (which would produce clays in the ocean itself) detrital = particles of rock derived from pre-existing rock by weathering and erosion

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So, a thermodynamical equilibrium approach doesn t work : The phases suggested do not appear to be at equilibrium The composition of seawater has changed in the past (evidence from paleo-climatic studies) But there is some evidence that inverse weathering reactions do occur especially in near shore sediments Reverse weathering not totally eliminated but cannot be a control for an equilibrium global ocean. Current models for seawater composition emphasize the balance between inputs and removals. The balance sheet has become more important that solubility relationships for explaining ocean chemistry. The difference has many important ramifications. We would expect a thermodynamic ocean to have a constant composition of the ocean and its sediments over geological time. According to the kinetic view we would expect changes in paleo ocean chemistry as inputs and removals varied in the geologic past. Kinetic Model of Seawater

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What is the origin of seawater s composition? Sources Rivers?? Mid-Ocean Ridges?? Other?? Aerosols Sinks Sediments?? Mid-Ocean Ridges?? Other?? Aerosols Kinetic Model of Seawater - A Mass Balance Approach Q = input rate (e.g. moles y -1 ) S = output rate (e.g. moles y -1 ) [M] = total dissolved mass in the box (moles) = mass / input or removal flux = M / Q = M / S Sites of Hydrothermal Vents on Mid-Ocean Ridges

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Kinetic model of seawater mass balance model Main input and removal fluxes for major ions in seawater (from McDuff and Morel, 1980)

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Group Ia Cl short term cycle = aerosols and rivers main sink over geological time = evaporites = controlled by tectonics, geometry of marginal seas residence time is so long (~100 My) that changes in concentration are hard to see. Group Ib Mg, SO 4, probably K input from rivers main sink through ocean crust / hydrothermal activity The dominant control of mass balance is hydrothermal activity, thus tectonics. Classification of elements according to cycling processes Group II (e.g. Ca, Na, the remaining cations with long residence times) Consider the charge balance for seawater: 2[Ca 2+ ] + [Na + ] + 2[Mg 2+ ] + [K + ] = [HCO 3 - ] + [Cl - ] + 2[SO 4 2- ] or rearranged: 2[Ca 2+ ] + [Na + ] - [HCO 3 - ] = [Cl - ] + 2[SO 4 2- ] - 2[Mg 2+ ] - [K + ] This side is controlled by tectonics Therefore this sum is also controlled by tectonics The controls on the relative proportions of elements on the left hand side are complicated but include: a) Ca/Na ion exchange in estuaries b) Ca/HCO 3 regulation by calcium carbonate equilibrium

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Group III (e.g. nutrients (Si, P, C, N) and trace metals) The main balance is input from rivers and removal as biological debris to sediments

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Dissolved O 2 is important ! New production Recycling

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Overview of nutriment cycling and time scales

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Regional variability of ocean chemical composition: : Salinity Why does surface salinity vary? S = 30 to 37 In oceanography, it has been traditional to express salinity not as percent, but as per mille (parts per thousand) (), which is approximately grams of salt per kilogram of solution

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Evaporation and Precipitation Effects on Surface Salinity

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Regional variability : Ocean currents control on marine biology Basin, planetary scale ~ 10,000 km Large-scale atmospheric wind patterns Upwelling Downwelling Upwelling

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Regional variability : Nutrient limitation on marine biology. The example of iron Fe is necessary nutrient for marine phytoplankton Major source is atmospheric dust deposition Southern Ocean, far from continental dust sources, is depleted in Fe Illustration : SOIREE (Boyd et al. 2001) Purposeful iron addition experiment Increased marine productivity after iron addition It is not yet clear its long-term impact on the oceanic carbon uptake Some organisms can utilize N from dissolved N 2 gas (N 2 fixers) Increased Fe input to the ocean can stimulate N 2 fixation (Falkowski 1997)

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Antarctic ice core data Increased dust deposition during cold and arid ice age climates (Vostok ice core data) Iron hypothesis (Martin, 1990) .. The Iron example illustrates complex geochemical couplings between different elements, geospheres and climate evolution

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Summary Salinity of seawater is determined by the major elements. Early ideas were that the major composition was controlled by equilibrium chemistry. Modern view is of a kinetic ocean controlled by sources and sinks. River water is main source composition from weathering reactions. Evaporation of river water does not make seawater. Reverse weathering was proposed but the evidence is weak. Sediments are a major sink. Hydrothermal reactions are a major sink. Biological control on nutriment concentration and sedimentation.