10.5 Ocean geochemistry
9 min read•august 21, 2024
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 and . 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
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- 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
- 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
- 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
- (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, (NO3^-), (NH4+), and organic nitrogen
- 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 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
- (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
Sources of trace metals
- Atmospheric deposition (dust, )
- 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 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
Key Terms to Review (27)
Ammonium: Ammonium is a positively charged polyatomic ion with the formula NH₄⁺, formed when ammonia (NH₃) gains a proton. This ion plays a crucial role in various biological and environmental processes, especially in the nitrogen cycle and marine chemistry. Ammonium is vital for plant nutrition as it serves as a key nitrogen source, and its behavior in ocean waters affects both nutrient dynamics and biogeochemical cycles.
Anthropogenic emissions: Anthropogenic emissions refer to the release of pollutants and greenhouse gases into the atmosphere as a result of human activities. These emissions significantly impact climate change, air quality, and ocean chemistry, as they alter natural cycles and contribute to the accumulation of harmful substances in the environment.
Biogeochemical processes: Biogeochemical processes refer to the natural cycles and interactions that involve biological, geological, and chemical components of the Earth system. These processes play a critical role in the cycling of nutrients and elements like carbon, nitrogen, and phosphorus, which are essential for sustaining life. In the context of ocean geochemistry, these processes help regulate the chemistry of seawater and influence marine ecosystems, nutrient availability, and global climate.
Carbon cycle: The carbon cycle is the series of processes by which carbon atoms circulate through the Earth's atmosphere, oceans, soil, and living organisms. This cycle plays a crucial role in regulating Earth's climate and supporting life by facilitating the transfer of carbon in various forms such as carbon dioxide, organic matter, and carbonate minerals.
Carbonates: Carbonates are chemical compounds that contain the carbonate ion, CO₃²⁻, which consists of one carbon atom covalently bonded to three oxygen atoms. They are significant in ocean geochemistry as they play a crucial role in regulating the carbon cycle and influencing ocean acidity. Carbonates can precipitate to form sediments and are key components in marine organisms' shells, impacting both the ecosystem and global climate.
Chemosynthesis: Chemosynthesis is the process by which certain organisms, known as chemosynthetic organisms, convert inorganic compounds into organic matter using chemical energy instead of sunlight. This biological process is critical for life in extreme environments where sunlight is absent, such as deep-sea hydrothermal vents and other extreme habitats. Chemosynthesis supports unique ecosystems that thrive in these conditions by providing a primary energy source for various extremophiles and contributes significantly to oceanic nutrient cycling.
David Archer: David Archer is a prominent climate scientist and geochemist known for his work on the carbon cycle and its implications for climate change. He has contributed significantly to our understanding of how carbon dioxide interacts with ocean chemistry and how these processes influence global warming and climate patterns. His research emphasizes the importance of the oceans in regulating atmospheric CO2 levels and the potential impacts of human activity on this delicate balance.
Dissolved Inorganic Carbon: Dissolved inorganic carbon (DIC) refers to the total concentration of carbon species in water that include carbon dioxide (CO₂), bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻). DIC plays a crucial role in regulating the acidity of water bodies and is an essential component in the biogeochemical cycling of carbon, influencing both terrestrial and aquatic ecosystems. Its interaction with various biological and chemical processes drives the dynamics of carbon cycling, particularly in relation to climate change and ocean health.
Dissolved Oxygen: Dissolved oxygen (DO) refers to the amount of oxygen that is present in water, which is essential for the survival of aquatic organisms. It plays a critical role in maintaining water quality and supports various biological processes, including respiration in fish and other marine life. Factors like temperature, pressure, and the presence of organic matter can affect DO levels, making it an important parameter in assessing both freshwater and marine ecosystems.
Eutrophication: Eutrophication is the process by which water bodies, like lakes and rivers, become overly enriched with nutrients, leading to excessive growth of algae and depletion of oxygen. This process is often triggered by runoff from agricultural land, urban areas, and industrial discharges, which introduces high levels of nitrogen and phosphorus into aquatic ecosystems.
Hydrothermal vents: Hydrothermal vents are fissures on the ocean floor that emit heated water enriched with minerals, primarily due to volcanic activity beneath the Earth's crust. These vents play a crucial role in ocean geochemistry by providing a unique environment for various chemical reactions, influencing nutrient cycling and supporting diverse ecosystems that thrive in extreme conditions.
ICP-MS: Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is an analytical technique used to detect and quantify trace elements in various samples, including environmental, geological, and biological materials. It combines the ionization capabilities of inductively coupled plasma with the mass analysis of a mass spectrometer, allowing for highly sensitive detection of elements at parts per trillion levels. This technique is essential in understanding trace element geochemistry and ocean geochemistry by providing accurate data on elemental composition and distribution.
Isotopic Signatures: Isotopic signatures refer to the unique ratios of stable or radioactive isotopes found in various substances, which can reveal information about their origin, age, and processes that have affected them. These signatures can be used to trace environmental changes, biological processes, and geological events, helping scientists understand complex systems in both contemporary and ancient contexts.
John Martin: John Martin was a prominent oceanographer and geochemist known for his influential work on the role of oceanic processes in global biogeochemical cycles. He is particularly recognized for the 'Martin Curve,' which describes the relationship between nutrient availability and primary productivity in marine environments, emphasizing the importance of nutrient supply in regulating ocean ecosystems.
Marine sedimentation: Marine sedimentation refers to the process by which sediments are deposited in ocean environments, forming layers on the seafloor over time. This process plays a crucial role in ocean geochemistry, as sediments can influence the chemical composition of seawater, the distribution of nutrients, and the cycling of elements within marine ecosystems.
Mass spectrometry: Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions, allowing for the identification and quantification of chemical compounds. This method is crucial for understanding the composition and behavior of elements and compounds in various contexts, including natural processes and industrial applications.
Nitrate: Nitrate is a chemical compound containing one nitrogen atom bonded to three oxygen atoms, represented as NO₃⁻. It plays a vital role in various biological and geological processes, particularly in the nitrogen cycle where it acts as a crucial nutrient for plants and a key component in the movement of nitrogen through ecosystems. Nitrate's presence in water bodies also influences aquatic chemistry and the overall health of marine environments.
Nitrogen Cycle: The nitrogen cycle is the continuous process through which nitrogen is converted between its various chemical forms, playing a crucial role in sustaining life on Earth. This cycle involves processes such as nitrogen fixation, nitrification, denitrification, and ammonification, impacting ecosystems, agriculture, and atmospheric chemistry.
Nitrogen fixation: Nitrogen fixation is the process through which atmospheric nitrogen (N₂) is converted into a more reactive form, such as ammonia (NH₃), by certain bacteria and archaea. This process is crucial for making nitrogen accessible to living organisms, as most cannot utilize atmospheric nitrogen directly. It plays a key role in biogeochemical cycles, particularly in the nitrogen cycle, influencing soil fertility and ecosystem productivity, and also affects ocean geochemistry by impacting nutrient availability in marine environments.
Nutrient cycling: Nutrient cycling refers to the process through which essential nutrients move through ecosystems, being recycled and reused by various biological and geological processes. This cycle is crucial for maintaining ecosystem productivity and health, as it involves the transformation of nutrients through different forms and reservoirs, linking organic and inorganic matter in a continuous loop that sustains life.
Ocean acidification: Ocean acidification is the process by which the pH levels of the ocean decrease due to the absorption of excess carbon dioxide (CO2) from the atmosphere. This phenomenon impacts marine ecosystems, particularly organisms that rely on calcium carbonate for their shells and skeletons, and is intricately linked to various biogeochemical cycles, the carbon cycle, and climate change dynamics.
PH levels: pH levels measure the acidity or alkalinity of a solution, expressed on a scale from 0 to 14, with 7 being neutral. In ocean geochemistry, pH levels are crucial as they influence chemical reactions, biological processes, and the overall health of marine ecosystems. Changes in pH levels can affect carbonate chemistry, which is vital for organisms that rely on calcium carbonate for their shells and skeletons.
Photosynthesis: Photosynthesis is the biochemical process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose, using carbon dioxide and water. This process plays a crucial role in the global cycling of carbon and oxygen, influencing various biogeochemical cycles and the overall health of ecosystems.
Pycnocline: A pycnocline is a layer in a body of water where the density changes rapidly with depth. This gradient in density is often influenced by temperature and salinity, leading to distinct stratification in oceanic and freshwater systems. The presence of a pycnocline affects the mixing of water layers and has significant implications for marine life and geochemical processes.
Redfield Ratio: The Redfield Ratio is a term in oceanography that describes the consistent ratio of nutrients found in oceanic phytoplankton, specifically carbon, nitrogen, and phosphorus, typically expressed as 106:16:1. This ratio is significant because it reflects the relative abundance of these nutrients needed for phytoplankton growth and is crucial for understanding ocean productivity and nutrient cycling.
Thermohaline circulation: Thermohaline circulation refers to the large-scale ocean circulation driven by differences in temperature (thermo) and salinity (haline), which affects the density of seawater. This process is crucial in regulating global climate, as it facilitates the transport of heat and nutrients throughout the world's oceans, linking oceanic and atmospheric systems.
Upwelling: Upwelling is a process in oceanography where deep, cold, and nutrient-rich water rises to the surface, often resulting in enhanced biological productivity. This phenomenon occurs along coastlines and in open ocean regions where winds or currents cause surface waters to be displaced, allowing deeper waters to ascend. The nutrients brought to the surface support phytoplankton growth, which forms the basis of the marine food web.