Ocean geochemistry explores the chemical composition and processes in seawater. It covers major ions, trace elements, and dissolved gases that shape marine ecosystems. Understanding these components helps scientists study ocean processes, climate change impacts, and marine life.

This field also examines ocean circulation patterns, biogeochemical cycles, and phenomena like ocean acidification and hydrothermal vents. These topics reveal how oceans influence global chemical cycles, climate, and marine biodiversity.

Composition of seawater

• Seawater composition plays a crucial role in ocean geochemistry, influencing marine life, climate, and global chemical cycles
• Understanding seawater composition helps geochemists study ocean processes, climate change impacts, and marine ecosystem health

Major ions in seawater

• Sodium (Na+) and chloride (Cl-) dominate seawater composition, accounting for ~85% of dissolved ions
• Other major ions include sulfate (SO4^2-), magnesium (Mg^2+), calcium (Ca^2+), and potassium (K+)
• Concentrations of major ions remain relatively constant globally due to long residence times
• Total dissolved solids in seawater average ~35 g/kg, expressed as salinity in practical salinity units (PSU)

Trace elements in oceans

• Present in concentrations <1 ppm but crucial for biological processes and marine ecosystems
• Include iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn)
• Distributions affected by biological uptake, scavenging, and input from rivers and atmospheric deposition
• Some trace elements serve as proxies for past ocean conditions (cadmium for paleoproductivity)

Dissolved gases in seawater

• Oxygen (O2) essential for marine life, varies with depth and biological activity
• Carbon dioxide (CO2) plays a critical role in ocean acidification and carbon cycle
• Nitrogen (N2) relatively inert but important for nitrogen-fixing organisms
• Noble gases (helium, neon, argon) used as tracers for ocean circulation and mixing processes

Ocean circulation patterns

• Ocean circulation drives the distribution of heat, nutrients, and dissolved gases in the world's oceans
• Understanding circulation patterns helps geochemists interpret chemical distributions and fluxes in marine systems

Thermohaline circulation

• Global-scale circulation driven by differences in temperature and salinity
• Forms the "global conveyor belt" transporting water masses between ocean basins
• North Atlantic Deep Water (NADW) formation crucial for global circulation
• Circulation time scale of ~1000 years affects long-term climate and carbon storage

Surface currents vs deep currents

• Surface currents driven primarily by wind patterns (Ekman transport)
• Major surface currents include Gulf Stream, Kuroshio Current, and Antarctic Circumpolar Current
• Deep currents controlled by density differences and bathymetry
• Abyssal circulation important for nutrient transport and oxygen distribution in deep ocean

Upwelling and downwelling processes

• Upwelling brings nutrient-rich deep waters to the surface, supporting high productivity
• Coastal upwelling occurs along eastern boundaries of ocean basins (Peru Current, Benguela Current)
• Equatorial upwelling driven by trade wind divergence
• Downwelling occurs in subtropical gyres, creating oceanic "deserts" with low productivity

Biogeochemical cycles in oceans

• Oceans play a central role in global biogeochemical cycles, regulating atmospheric composition and climate
• Marine biogeochemical cycles involve complex interactions between biological, chemical, and physical processes

Carbon cycle in oceans

• Oceans contain ~50 times more carbon than the atmosphere
• Dissolved inorganic carbon (DIC) exists as CO2, bicarbonate (HCO3^-), and carbonate (CO3^2-)
• Biological pump transfers carbon from surface to deep ocean through sinking organic matter
• Carbonate pump involves formation and dissolution of calcium carbonate shells
• Air-sea gas exchange regulated by partial pressure differences and wind speed

Nitrogen cycle in marine environments

• Nitrogen essential for primary production, often limiting nutrient in oceans
• Major forms include dissolved N2, nitrate (NO3^-), ammonium (NH4+), and organic nitrogen
• Nitrogen fixation by cyanobacteria introduces new nitrogen to the system
• Nitrification and denitrification processes affect nitrogen availability and distribution
• Anammox (anaerobic ammonium oxidation) important in oxygen minimum zones

Phosphorus cycle in seawater

• Phosphorus critical for DNA, RNA, and ATP in marine organisms
• Primarily exists as dissolved inorganic phosphate (PO4^3-)
• No significant atmospheric component unlike carbon and nitrogen cycles
• Inputs from continental weathering and river runoff
• Removal through burial in marine sediments and formation of phosphorite deposits

Ocean acidification

• Ocean acidification represents a major threat to marine ecosystems and global geochemical cycles
• Understanding this process is crucial for predicting future impacts on ocean chemistry and biology

Causes of ocean acidification

• Primarily driven by increased atmospheric CO2 from human activities
• CO2 dissolution in seawater forms carbonic acid (H2CO3)
• Carbonic acid dissociates, releasing hydrogen ions (H+) and lowering pH
• Oceans have absorbed ~30% of anthropogenic CO2 emissions since the industrial revolution
• Rate of acidification ~100 times faster than any time in the last 300 million years

Effects on marine ecosystems

• Reduced calcification rates in coral reefs, mollusks, and some plankton species
• Altered behavior and physiology of fish and invertebrates
• Potential disruption of food webs and ecosystem functions
• Increased dissolution of calcium carbonate sediments
• Changes in nutrient availability and speciation of trace metals

Future projections and impacts

• Models predict further pH decrease of 0.3-0.4 units by 2100 under high emission scenarios
• Potential for major shifts in marine biodiversity and ecosystem services
• Feedbacks on global carbon cycle and climate system
• Socioeconomic impacts on fisheries, aquaculture, and coastal communities
• Need for mitigation strategies and adaptation measures to address ocean acidification

Hydrothermal vents

• Hydrothermal vents represent unique geochemical environments in the deep ocean
• These systems play important roles in element cycling and support diverse ecosystems

Formation and characteristics

• Form along mid-ocean ridges and back-arc basins due to tectonic activity
• Seawater percolates through oceanic crust, heated by magma chambers
• High-temperature reactions between seawater and rock alter fluid composition
• Vent types include black smokers (>300°C) and white smokers (<300°C)
• Chimney structures form from precipitating minerals as hot fluids mix with cold seawater

Chemical composition of vent fluids

• Enriched in dissolved metals (iron, manganese, copper, zinc)
• High concentrations of reduced compounds (H2S, CH4, H2)
• Depleted in magnesium and sulfate relative to seawater
• pH ranges from highly acidic (2-3) to alkaline (9-11) depending on vent type
• Fluid composition reflects subsurface rock types and reaction conditions

Microbial communities in vents

• Chemosynthetic bacteria form the base of unique food webs
• Sulfur-oxidizing bacteria dominate in many vent ecosystems
• Methanogens and methanotrophs important in methane cycling
• Thermophilic and hyperthermophilic archaea adapted to extreme temperatures
• Microbial communities influence mineral precipitation and element cycling

Marine sediments

• Marine sediments serve as important archives of past ocean conditions and play crucial roles in global geochemical cycles
• Understanding sediment processes is essential for interpreting paleoceanographic records and element cycling in the oceans

Types of marine sediments

• Terrigenous sediments derived from continental weathering and erosion
• Biogenic sediments from skeletal remains of marine organisms (foraminifera, coccolithophores)
• Authigenic sediments formed in situ through chemical precipitation (manganese nodules)
• Volcanogenic sediments from submarine volcanic activity and ash falls
• Cosmogenic sediments from extraterrestrial sources (micrometeorites)

Sediment composition and sources

• Lithogenic components include quartz, feldspars, and clay minerals
• Biogenic components consist of calcium carbonate, opal, and organic matter
• Authigenic minerals form through diagenetic processes (pyrite, glauconite)
• Sediment distribution controlled by distance from shore, water depth, and ocean productivity
• Continental margins dominated by terrigenous input, deep ocean by biogenic sediments

Diagenesis in marine sediments

• Early diagenesis involves microbial degradation of organic matter
• Redox zonation develops with depth (oxic, suboxic, anoxic zones)
• Carbonate dissolution occurs below the lysocline and carbonate compensation depth
• Silica diagenesis involves dissolution and reprecipitation of opal
• Authigenic mineral formation (pyrite, phosphorites) alters sediment composition over time

Isotope geochemistry in oceans

• Isotope geochemistry provides powerful tools for studying ocean processes and past environmental conditions
• Understanding isotope systematics is crucial for interpreting paleoceanographic records and tracing element cycles

Stable isotopes in oceanography

• Oxygen isotopes (δ18O) used as proxies for temperature and global ice volume
• Carbon isotopes (δ13C) trace carbon sources and biological productivity
• Nitrogen isotopes (δ15N) indicate nutrient utilization and nitrogen fixation
• Silicon isotopes (δ30Si) reflect silicic acid utilization by diatoms
• Boron isotopes (δ11B) serve as proxies for paleo-pH and ocean acidification

Radioisotopes in marine systems

• Carbon-14 used for dating marine carbonates and organic matter (up to ~50,000 years)
• Thorium-230 and Protactinium-231 applied in paleoproductivity and circulation studies
• Radium isotopes trace groundwater inputs and coastal mixing processes
• Tritium and Helium-3 used as tracers for ocean circulation and mixing rates
• Beryllium-10 indicates changes in cosmic ray flux and magnetic field strength

Applications in paleoceanography

• Reconstruction of past ocean temperatures using δ18O in foraminifera shells
• Tracing changes in ocean circulation using Nd isotopes in ferromanganese crusts
• Estimating past atmospheric CO2 levels using boron isotopes in coral skeletons
• Determining rates of ocean overturning using radiocarbon age differences
• Reconstructing past productivity patterns using barite accumulation rates

Nutrient dynamics in oceans

• Nutrient dynamics play a crucial role in controlling marine primary productivity and biogeochemical cycles
• Understanding nutrient distributions and cycling is essential for predicting ecosystem responses to environmental changes

Nutrient distribution patterns

• Vertical profiles show depletion in surface waters and enrichment at depth
• Horizontal gradients exist between coastal and open ocean environments
• High-latitude regions generally have higher nutrient concentrations
• Nutrient ratios (N:P:Si) vary spatially and temporally
• Redfield ratio (C:N:P = 106:16:1) describes average elemental composition of marine organic matter

Limiting nutrients in oceans

• Nitrogen often limits productivity in much of the global ocean
• Phosphorus can be limiting on geological timescales
• Iron limits productivity in high-nutrient, low-chlorophyll (HNLC) regions
• Silicon limitation affects diatom growth in some areas
• Co-limitation by multiple nutrients occurs in some marine ecosystems

Eutrophication in coastal waters

• Excess nutrient input from anthropogenic sources (agriculture, sewage)
• Leads to increased primary productivity and potential harmful algal blooms
• Can result in oxygen depletion (hypoxia) in bottom waters
• Alters ecosystem structure and function
• Management strategies include reducing nutrient inputs and improving wastewater treatment

Trace metal cycling

• Trace metals play essential roles in marine ecosystems despite their low concentrations
• Understanding trace metal cycling is crucial for interpreting marine productivity and biogeochemical processes

Sources of trace metals

• Atmospheric deposition (dust, anthropogenic emissions)
• Riverine input from continental weathering
• Hydrothermal vents release metals to deep ocean
• Sediment resuspension in coastal areas
• Anthropogenic sources (industrial effluents, mining activities)

Scavenging processes in oceans

• Adsorption onto sinking particles removes trace metals from solution
• Biological uptake and incorporation into organic matter
• Co-precipitation with iron and manganese oxides
• Complexation with organic ligands affects metal solubility and reactivity
• Residence times vary widely among different trace metals

Biological importance of trace metals

• Iron essential for photosynthesis and nitrogen fixation
• Zinc required for carbonic anhydrase and alkaline phosphatase enzymes
• Copper used in electron transport chains and oxidative enzymes
• Cobalt necessary for vitamin B12 synthesis
• Manganese involved in oxygen-evolving complex of photosystem II

Organic matter in oceans

• Marine organic matter plays a crucial role in ocean biogeochemistry and global carbon cycling
• Understanding organic matter dynamics is essential for interpreting marine productivity and carbon sequestration

Sources of marine organic matter

• Phytoplankton primary production in surface waters
• Terrestrial inputs from rivers and coastal runoff
• Atmospheric deposition of organic aerosols
• Chemosynthetic production at hydrothermal vents and cold seeps
• Viral lysis and zooplankton grazing release dissolved organic matter

Degradation of organic compounds

• Microbial remineralization in water column and sediments
• Photochemical degradation of chromophoric dissolved organic matter
• Enzymatic hydrolysis of particulate organic matter
• Preferential degradation of labile compounds (sugars, amino acids)
• Refractory dissolved organic matter persists for thousands of years

Role in carbon sequestration

• Biological pump transfers organic carbon from surface to deep ocean
• Burial of organic matter in marine sediments removes carbon from active cycle
• Dissolved organic carbon represents large oceanic carbon reservoir
• Microbial carbon pump produces refractory dissolved organic matter
• Organic matter-mineral interactions enhance preservation in sediments