ocean biogeochemical dynamics an introduction Annalisa Bracco Georgia Institute of Technology
Outline • Overview of biogeochemical cycles • Distribution of chemicals in the ocean • Tracer conservation equation and ocean transport: fundamentals of large scale circulation
‘Fundamentals’ of biogeochemical cycles • All matter cycles...it is neither created nor destroyed... • As the Earth is essentially a closed system with respect to matter, we can say that all matter on Earth cycles . • Biogeochemical cycles: the movement (or cycling) of matter through a system
by matter we mean: elements (carbon, nitrogen, oxygen) or molecules (water) so the movement of matter (for example carbon) between these parts of the system is, practically speaking, a biogeochemical cycle The Cycling Elements: macronutrients : required in relatively large amounts "big six": carbon , hydrogen , oxygen , nitrogen , phosphorous sulfur
other macronutrients: potassium , calcium , iron , magnesium micronutrients : required in very small amounts, (but still necessary) boron (green plants) copper (some enzymes) molybdenum (nitrogen-fixing bacteria)
Generalized Biogeochemical Cycle: Four of the most important are: • The nitrogen cycle • The oxygen cycle • The phosphorus cycle • The carbon cycle The circulation of chemicals in these biogeochemical cycles and interactions between cycles are critical for the maintenance of terrestrial, freshwater and marine ecosystems. Global climate change, temperature, precipitation and ecosystem stability are all dependent upon biogeochemical cycles
Nitrogen is essential to all living systems: Eighty percent of Earth's atmosphere is made up of nitrogen in its gas phase. • Atmospheric nitrogen becomes part of living organisms in two ways: • through bacteria in the soil that form nitrates out of nitrogen in the air. • through lightning. During electrical storms, large amounts of nitrogen are oxidized and united with water to produce an acid that falls to Earth in rainfall and deposits nitrates in the soil. • Plants take up the nitrates and convert them to proteins that travel up the food chain through herbivores and carnivores.
When organisms excrete waste, the nitrogen is released back into the environment. When they die and decompose, the nitrogen is broken down and converted to ammonia. • Nitrates may also be converted to gaseous nitrogen through a process called denitrification and returned to the atmosphere, continuing the cycle.
Human impacts: • by artificial nitrogen fertilization (through the Haber Process, using energy from fossil fuels to convert N2 to ammonia gas (NH3) and planting of nitrogen fixing crops (Vitousek et al., 1997). • transfer of nitrogen trace gases (N2O) to the atmosphere via agricultural fertilization, biomass burning, cattle and feedlots, and other industrial sources (Chapin et al. 2002). N2O in the stratosphere breaks down and acts as a catalyst in the destruction of atmospheric ozone. • NH3 in the atmosphere has tripled as the result of human activities. It acts as an aerosol, decreasing air quality and clinging on to water droplets (acid rain).
Fossil fuel combustion has contributed to a 6 or 7 fold increase in NOx flux to the atmosphere. NO alters atmospheric chemistry, and is a precursor of tropospheric (lower atmosphere) ozone production, which contributes to smog, acid rain, and increases nitrogen inputs to ecosystems (Smil, 2000). • Ecosystem processes can increase with nitrogen fertilization, but anthropogenic input can also result in nitrogen saturation, which weakens productivity and can kill plants (Vitousek et al., 1997) → algae blooms. • Decreases in biodiversity both over land and in the ocean can result if higher nitrogen availability increases nitrogen-demanding species (Aerts and Berendse 1988).
Plants use the energy of sunlight to convert carbon dioxide and water into carbohydrates and oxygen via photosynthesis. 6CO2 + 6H2O + energy → C6H12O6 + 6O2 Photosynthesizing organisms include the plant life of the land areas as well as the phytoplankton of the oceans. The tiny marine cyanobacteriaProchlorococcus was discovered in 1986 and accounts for more than half of the photosynthesis of the open ocean. Animals form the other half of the oxygen cycle breathing in oxygen used to break carbohydrates down into energy in a process called respiration. O2 + carbohydrates → CO2 + H2O + energy
The phosphorus cycle describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. The atmosphere does not play a significant role, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth. • Phosphorus normally occurs in nature as part of a phosphate ion, consisting of a phosphorus atom and some number of oxygen atoms, the most abundant form (called orthophosphate) having four oxygens: PO43-. Most phosphates are found as salts in ocean sediments or in rocks. Over time, geologic processes can bring ocean sediments to land, and weathering will carry terrestrial phosphates back to the ocean.
Plants absorb phosphates from the soil and phosphate enters the food chain. After death, the animal or plant decays, and the phosphates are returned to the soil. Runoff may carry them back to the ocean or they may be reincorporated into rock. • The primary biological importance of phosphates is as a component of nucleotides, which serve as energy storage within cells (ATP) or when linked together, form the nucleic acids DNA and RNA. Phosphorus is also found in bones, and in phospholipids (found in all biological membranes). • Phosphates move quickly through plants and animals; however, the processes that move them through the soil or ocean are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.
Human influence: Artificial fertilizers and other wastes not absorbed by plants mostly enter the groundwater and collect in streams, lakes and ponds. The extra phosphates are a major contributor to the process called eutrophication, which causes excessive growth of water plants and algae populations and subsequent depletion of dissolved oxygen potentially suffocating fish and other aquatic fauna.
Usually thought of as four major reservoirs of carbon (the atmosphere, the terrestrial biosphere - which includes freshwater systems and non-living organic material, such as soil carbon -, the oceans with dissolved inorganic carbon and living and non-living marine biota, and the sediments which includes fossil fuels) interconnected by pathways of exchange. • The exchanges between reservoirs, occur because of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth, but the deep ocean part of this pool does not rapidly exchange with the atmosphere.
The global carbon budget is the balance of the exchanges (incomes and losses) of carbon between the carbon reservoirs or between one specific loop (e.g., atmosphere - biosphere) of the carbon cycle. IN THE OCEAN: • The seas contain around 36000 gigatonnes of carbon, mostly in the form of bicarbonate ion. Inorganic carbon, that is carbon compounds with no carbon-carbon or carbon-hydrogen bonds, is important in its reactions within water. This carbon exchange becomes important in controlling pH in the ocean and can also vary as a source or sink for carbon.
Carbon is readily exchanged between the atmosphere and ocean. In regions of oceanic upwelling, carbon is released to the atmosphere. Conversely, regions of downwelling transfer carbon (CO2) from the atmosphere to the ocean. When CO2 enters the ocean, carbonic acid is formed: • CO2 + H2O ⇌ H2CO3 • This reaction has a forward and reverse rate, that is it achieves a chemical equilibrium. Another reaction important in controlling oceanic pH levels is the release of hydrogen ions and bicarbonate. This reaction controls large changes in pH: • H2CO3 ⇌ H+ + HCO3−
"Present day" (1990s) sea surface DIC concentration (from the GLODAP climatology)
focusing on the ocean: Distribution of chemicals vertical profiles of various element in the Pacific ocean following the periodic table
some elements (for example sodium, magnesium, potassium, calcium, sulfur, chlorine….) do not vary through the water column. They also have the longest residence time → the rate at which they are modified/used is slower than the rate at which the water of the ocean mixes completely (~ 1000ys). They are called biounlimitedelements. (Small variations at the surface are driven by evaporation and rainfall) • others have distributions far from uniform. In general concentrations lower at the surface than at depth (exceptions: cobalt and lead) → to explained reduced surface concentrations: biological processes and sinking particles. Divided in biolimiting elements (nitrogen in form of nitrate, phosphorous-phosphate, silicon-silicic-acid) very depleted at surface by biological processes, and biointermidiate (carbon, barium) only partially depleted. (Broecker and Peng, 1982)