Biogeochemical Cycles

• Biogeochemical cycles refer to the complete pathways chemicals take through the Earth's major components, or reservoirs, including the atmosphere, lithosphere, hydrosphere, and biosphere.

Reservoirs of Earth's System:

• Atmosphere is the layer of gases surrounding the Earth and is divided into four layers: troposphere, stratosphere, mesosphere, and thermosphere.
• Lithosphere is the Earth's outer layer, approximately 100km thick, containing the ocean basin and continents.
• Hydrosphere is the water portion of the planet, consisting of surface freshwater, oceans, glaciers, groundwater, and atmospheric water like clouds.
• Biosphere is the part of Earth where life exists and includes the planetary system that sustains life.
• The biogeochemical cycle include the atmosphere, hydrosphere, rocks, and soil,

Key Concepts:

• Some chemical elements such as O and N cycle quickly and regenerate easily for biological activity, typically existing in a gas phase, occurring in the atmosphere and easily dissolving and carried in the hydrologic cycle.
• Other chemicals are tied up in relatively immobile forms and return slowly through geologic processes, typically lacking a gas phase appearing in significant concentrations in the atmosphere, besides being insoluble in water, like P.
• All living things consist of chemicals, with 24 out of over 103 known chemicals required for life.
• Macronutrients are elements required in large amounts by living things, including the "big 6" (C, H, O, N, P, S).
• Micronutrients are chemical elements required in trace amounts by at least some life forms, such as B, Cu, and Mo.
• A limiting factor is the single requirement for growth that is available in the least supply relative to an organism's needs.
• Certain elements can be toxic at low concentrations such as Hg, while others essential in small concentrations, such as Cu can be toxic at higher concentrations.
• Required nutrient elements typically have a light atomic weight, the heaviest required micronutrient is iodine.
• Since the evolution of life, biogeochemical cycles have been greatly altered, changing the planet.
• The processes that control biogeochemical cycles are essential for maintaining life on Earth long term.
• A source refers to a compartment that donates materials to another compartment, while a sink is a compartment that receives materials.

Geological Cycle:

• Geologic Cycle involves the processes responsible for the formation and changes of Earth materials, encompassing the tectonic, rock, and hydrologic cycles.

Tectonic Cycle:

• Tectonic cycle involves the creation/destruction of the lithosphere, which is broken into large plates via plate tectonics, moving relative to each other at a rate of 2 to 15 cm per year.
• The tectonic process is driven by heat from the Earth's interior, with cooler, denser slabs sinking into the crust and setting plates in motion.
• The tectonic cycle impacts the position, size, and shape of continents.
• Tectonic cycle changes in the patterns related to ocean and atmospheric circulation, which has consequences to environmental changes.
• The tectonic cycle create ecological islands that lead to the development of new species.
• Tectonic boundaries are active geologically where earthquakes and volcanoes occur.
• Types of plate boundaries include convergent, divergent, and transform.
• Convergent boundaries involve plates moving into each other, leading to the destruction of the lithosphere.
• Convergent boundaries include types such as continental crust colliding with oceanic crust.
• Convergent boundaries colliding a continental crust with an oceanic crust see the oceanic crust subducting, whereas the continental floats, which can lead to volcanism such as in Andes mountains.
• Convergent boundaries also involve continental crust colliding with continental crust, with each crust being of equal density which leads to buckling, fracturing, and deformation, such as in the Himalayas.
• Collisions between oceanic crusts during convergent boundaries leads to one being thrust underneath another, also leading to extensive volcanism, which can lead to volcanic island arcs, as with the Mariana Islands.
• Divergent boundaries involve plates moving apart, leads to the production of new lithosphere, and occur at mid-ocean ridges where seafloor spreading occurs, e.g. Atlantic ocean.
• Transform boundaries happen where plates grind past each other, where lithosphere is neither made or destroyed, as with the San Andreas Fault.

Rock Cycle:

• Rock cycle is a group of processes produce igneous, sedimentary and metamorphic rocks formation.
• The rock cycle can serve as a sink for carbon through the formation of coal or anthracite.
• Additional elements are also incorporated during the rock cycle, for example, incorporation into limestone (a sink for Ca).
• Additionally, gases may be released into atmosphere on the formation of igneous rocks, like extrusive volcanic variety.
• Rocks, through weathering, act as a source of chemical elements being released into the environment.

Hydrologic Cycle:

• Hydrologic cycle transfers from the oceans, to the atmosphere, to land and back to the oceans.
• Hydrologic cycle components include evaporation from the ocean, precipitation on land, evaporation from land, transpiration by vegetation, surface runoff, subsurface flow, storage on land, and evaporation from lakes and dams.
• This cycle is driven by solar energy, of which the oceans hold 97%, glaciers and ice caps hold 2%, shallow ground water constitutes 0.8%, lakes and rivers 0.01%, and atmosphere at 0.001%.
• The rate of water transferred from land to oceans is slow.
• Land and ocean are somewhat isolated from the hydrologic cycle.
• Rainfall in the ocean comes from evaporated ocean water, and rainfall over land comes from evaporated land water.
• Approximately 60% of the water that precipitates on land evaporates to atmosphere, with 40% flowing back to the ocean.
• Water's distribution over land not even, leading to environmental issues.

Carbon Cycle:

• In the atmosphere, carbon exists in a gaseous state as CO2 and CH4.
• Photosynthesis and diffusion with oceans removes carbon from the atmosphere.
• Respiration, wildfires, burning of fossil fuels, decomposition of organic materials, and volcanic eruptions introduces carbon into the atmosphere.
• In the hydrosphere, carbon exists as dissolved CO2, CO3-2, and HCO3.
• Photosynthesis of algae and photosynthetic organisms, diffusion with the atmosphere and formation of CaCO3 in sea shells takes carbon compounds from an ocean environment.
• The respiration of marine organisms, rivers with dissolved carbon dioxide and organic particles, plus wind introduce carbon compounds to the ocean.
• In the biosphere, plants consumes carbon during photosynthesis.
• Higher trophic levels obtain carbon by consuming those at lower ones.
• The decomposition of the materials is broken down to organic compounds, as well as CO2.
• If low in oxygen or temperature, the rate of decomposition slows, causing organic material accumulation.
• In the lithosphere, carbon has a very small presence within the general crust (0.035% by weight).
• Accumulated organic material in land systems gets compressed over time via geologic forces.
• This compression squeezes out compounds such as O and H from organic material, which leads to a higher accumulation of C and formation of coal.
• In marine systems, compression of mostly phytoplankton leads to the formation of oil and natural gas.
• The formation of limestone (CaCO3) in marine environments is a carbon "sink".

Phosphorous Cycle:

• The atmosphere has no gaseous phase of P, with it only being found in small particles of dust.
• In the hydrosphere, P tends to form relatively insoluble compounds in water.
• P is transported by rivers to oceans in soluble forms or as suspended particles.
• P might be introduced to systems from the "runoff" of industrial or agricultural waste, and that can lead to eutrophication.
• In the biosphere, Phosphorous is transferred to biota through the uptake of phosphate by the plants, algae, and photosynthetic bacteria.
• Phosphorous can be transferred to land through guano birds that eat marine life, then nest offshore.
• Birds that nest on offshore islands will have their P rich excrement accumulate (guano) in concentrations up to 40m thick.
• In the lithosphere, P is usually in an oxidised state as phosphate and connects to form minerals.
• Phosphorous is bound to minerals, like in oxidized states such Ca, Mg, K and Fe.
• The rate of transfer is slower compared to rates for other elements like C, O, and N.
• Phosphorous often finds itself in marine environments, eventually taking part in marine sediments and the rock cycle.