Chapter 6: The Biogeochemical Cycles

Overview Earth is a Peculiar Planet Life and Global Chemical Cycles General Aspects of Biogeochemical Cycles The Geologic Cycle Some Major Global Biogeochemical Cycles

Earth is a Peculiar Planet Four unique characteristics: Liquid water Water at its triple point (all three phases) Plate tectonics Life! Life has a large effect on biogeochemical cycling Why does earth have life, and not the nearby planets?

Planets near Earth Inner planets formed by gathering together of particle by gravitational force Would expect Earth, Mars, Mercury and Venus to have a similar atmosphere - they do not

Planets Near Earth Earth’s unique atmosphere indicates that it contains life Earth has “environmental fitness” Life evolved in an environment conducive for that to occur Life altered the environment at a global level

Rise of Oxygen Before 2.3 billion years ago, the atmosphere was low in oxygen Evidence - grains of pyrite in sedimentary rock (banded iron formations in figure on right) Oceans were filled with dissolved (unoxidized) iron

Rise of Oxygen Evolution of Photosynthesis brought about gaseous oxygen Photosynthesis - carbon dioxide and water are combined in presence of light to create sugars and free oxygen Took 2 billion years before oxygen started to accumulate in atmosphere First it oxidized all the unoxidized iron

Rise of Oxygen Early photosynthesizers included stromatolites (3.4 billion years old)

Early organisms on earth Prokaryotes Simple cell structure Lacked organelles and a nucleus (too much energy to maintain) Get energy from fermentation Low energy yield to organism Waste products of carbon dioxide and alcohol Live singly or on end-to-end chains Cannot form 3-D structures

Early Organisms of Earth Eukaryotes Use oxygen for respiration (energy efficient) Larger cells with nucleus and organelles Form 3-D colony of cells Permits multi-cellular body structure of plants, animals and fungi

Evolution of Biosphere After presence of eukaryotes and oxygenated atmosphere Biosphere started to change drastically Plants, Animals and Fungi Evolved 700–500 million years ago They, in turn, continued to alter the biogeochemical cycles on earth

Life and Global Chemical Cycles Micronutrients Elements required in small amounts by all life or moderate amounts by some forms of life Macronutrients 24 elements required by all organisms Include the “Big Six”, which are the building blocks of life Carbon, oxygen, hydrogen, nitrogen, phosphorus, sulfur Each plays a special role in organisms

Life and Global Chemical Cycles For life to persist elements must be available at the right time, in the right amount, and in right concentrations relative to one another Too much of some elements – can be toxic Too little of some element – can limit growth and development Neutral – some elements are neutral for life

Biogeochemical Cycles A biogeochemical cycle is the complete path a chemical takes through the four major components of Earth’s system Atmosphere Hydrosphere Lithosphere Biosphere

General Aspects of Biogeochemical Cycles Some chemicals cycle quickly and are readily regenerated for biological activity They typically have a gas phase, are soluble and carried by the hydrologic cycle Oxygen and nitrogen Other chemical elements are relatively immobile and returned by geological processes Typically lack a gas phase and insoluble Phosphorus

General Aspects of Biogeochemical Cycles Most required nutrients are light Heaviest is iodine with atomic weight of 53 Since life evolved it has altered biogeochemical cycles The continuation of processes that control biogeochemical cycles essential for maintenance of life

General Aspects of Biogeochemical Cycles Through modern technology transfer rate of elements into air, water, and soil altered May benefit society, but may also pose environmental hazard Must recognize the + and – consequences of altering cycles

General Aspects of Biogeochemical Cycles Simplest way to view a cycle is a box and arrow diagram Boxes represent places where a chemical is stored Donating compartment is a source Receiving compartment is a sink Amount of time an atom spends in any compartment is called its residence time Arrows represent pathways of transfer Flow is the amount moving from one box to another Flux is the rate of transfer

The Geologic Cycle Rocks and Soil Geologic cycle - group of cycles Continually created, maintained, changed and destroyed over the last 4.6 billion years Altered due to physical, chemical, and biological processes Geologic cycle - group of cycles Tectonic, Hydrologic Rock Biogeochemical

The Tectonic Cycle Involves creation and destruction of the lithosphere (outer layer of Earth) ~100 km thick and broken in to several plates The slow movement of plates is called plate tectonics 2–15cm/yr Plate tectonics has large scales effects Location and size of continents Alterations in climate Ecological islands Areas of volcanic activity and earthquakes

Types of Plate Boundaries Divergent plate boundary Occurs at a spreading ocean ridge, where plates moving away from one another New lithosphere produced Known as sea floor spreading, produces ocean basins Convergent plate boundary Occurs when plates collide When heavier ocean plates meet lighter continental plates a subduction zone is present When two lighter continental plates collide a continental mountain range may form

The Tectonic Cycle Transform fault boundary Occurs where one plate slides past another San Andreas Fault in California Boundary of NA and Pacific plates LA moving towards SF

The Hydrologic Cycle The transfer of water from oceans to the atmosphere to the land and back to the oceans Driven by solar energy Evaporation of water from oceans Precipitation of water on land Transpiration of water by plants Evaporation of water from land Runoff from streams, rivers and subsurface groundwater

The Hydrologic Cycle Total water on earth = 1.3 billion km3 97% in oceans 2% in glaciers and ice caps 0.001% in atmosphere The rest in fresh water on land

The Hydrologic Cycle At the regional and local level, the fundamental unit of the landscape is the drainage basin The area that contributes surface runoff to a particular stream or river Vary greatly in size Usually named for main stream or river

The Rock Cycle Consists of numerous processes that produce rocks and soils Depends of the tectonic cycle for energy and the hydrologic cycle for water Rocks classified as Igneous Sedimentary Metamorphic

The Rock Cycle Physical weathering (freeze, thaw) produces sediment such as gravel, sand and silt. Chemical weathering occurs when weak acids in water dissolve chemicals from rocks.

The Carbon Cycle Carbon is the element that anchors all organic substances Carbon has a gaseous phrase Enters atmosphere (CO2 and CH4) through respiration, fires and diffusion Removed from the atmosphere by photosynthesis

The Carbon Cycle Carbon occurs in the ocean in several forms Dissolved CO2, carbonate and bicarbonate Marine organisms and their products, CaCO3 Enters the ocean by Simple diffusion then dissolves Transfer from land in rivers as dissolved carbon Wind

The Carbon Cycle Carbon enters the biota through photosynthesis and then returned by respiration or fire When organism dies decomposition releases carbon If buried under certain conditions carbon is not be released Transformed into fossil fuels

The Carbon-Silicate Cycle The cycling of carbon intimately involved with the cycling of silicon Weak carbonic acid falls as rain and weathers silicate rich rocks Releases Ca2+ and HCO3- Transferred to oceans and used by marine animals to construct shells Shells deposited on sea floor become part of sed rock layer and return to surface in subduction zones

The Carbon-Silicate Cycle Affects the levels of CO2 and O2 in the atmosphere

The Nitrogen Cycle N essential to life because it is necessary for the production of proteins and DNA Free N2 makes up 78% of atmosphere But most organisms can’t use it directly Relatively unreactive element must be converted to NO3- or NH4+ Performed by bacteria

The Nitrogen Cycle Nitrogen fixation - process of converting atmospheric N to NO3- or NH4+ Denitrification - process of releasing fixed N back to molecular N Almost all organisms depend on N converting bacteria Some have formed symbiotic relationships in the roots of plants or stomach on animals

The Nitrogen Cycle Industrial process can now convert molecular N into compounds usable by plants Main component of N fertilizers N in agricultural runoff potential source of water pollution N combines with O at high temperatures Oxides of N a source of air pollution

The Phosphorus Cycle P one of the “big six” required for life Often a limiting factor for plant and algal growth Does not have a gaseous phase Rate of transfer slow

The Phosphorus Cycle Enters biota through uptake as phosphate by plants, algae and some bacteria Returns to soil when plants die or is lost to oceans via runoff Returned to land via ocean feeding birds (guano) Guano deposits major source of P for fertilizers

Sulfur Cycle