Major Element Cycles

Why This Matters

In geochemistry, understanding major element cycles means grasping how Earth functions as an integrated system. You're being tested on more than just memorizing which bacteria fix nitrogen or where phosphorus comes from—you need to understand reservoir dynamics, flux rates, and biogeochemical feedbacks. These cycles connect the lithosphere, hydrosphere, atmosphere, and biosphere, and exam questions frequently ask you to trace elements through multiple reservoirs or explain how perturbations in one cycle cascade through others.

The key concepts you'll encounter include residence time, limiting nutrients, redox transformations, weathering feedbacks, and anthropogenic perturbations. Each cycle illustrates different principles: some have significant gaseous phases while others are rock-bound; some operate on human timescales while others span millions of years. Don't just memorize the steps—know what makes each cycle unique and how they interconnect. That comparative understanding is what separates strong exam performance from mediocre recall.

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Cycles with Major Atmospheric Reservoirs

These cycles feature significant gaseous phases, allowing rapid global distribution and making them particularly sensitive to atmospheric chemistry changes. The presence of a gas phase means these elements can move quickly between hemispheres and respond rapidly to perturbations.

Carbon Cycle

• Photosynthesis and respiration drive short-term fluxes—plants fix $CO_2$ into organic matter, while respiration and decomposition return it to the atmosphere on annual to decadal timescales
• Long-term sequestration occurs through burial and weathering—organic carbon burial in sediments and silicate weathering (consuming $CO_2$) regulate atmospheric concentrations over millions of years
• Anthropogenic emissions have increased atmospheric $CO_2$ by ~50% since preindustrial times—fossil fuel combustion releases carbon stored over geological timescales, overwhelming natural sink capacity

Nitrogen Cycle

• Biological nitrogen fixation breaks the triple bond in $N_2$—specialized bacteria and archaea convert atmospheric nitrogen to bioavailable ammonia ($NH_3$), the rate-limiting step for ecosystem productivity
• Redox transformations control nitrogen speciation—nitrification oxidizes $NH_4^+$ to $NO_3^-$ under aerobic conditions, while denitrification reduces nitrate back to $N_2$ in anoxic environments
• Haber-Bosch process has doubled global nitrogen fixation rates—synthetic fertilizer production now rivals natural fixation, causing widespread eutrophication and nitrous oxide emissions

Oxygen Cycle

• Photosynthesis is the sole significant source of free $O_2$—oxygenic photosynthesis splits water molecules, releasing oxygen as a byproduct while fixing carbon
• Tightly coupled to the carbon cycle through stoichiometry—the ratio of $O_2$ produced to $CO_2$ consumed follows the Redfield ratio in marine systems
• Atmospheric $O_2$ has a residence time of ~4,500 years—this long residence time buffers against short-term perturbations but means changes reflect major shifts in Earth system processes

Sulfur Cycle

• Redox chemistry spans eight oxidation states—sulfur cycles between sulfide ($S^{2-}$), elemental sulfur ($S^0$), and sulfate ($SO_4^{2-}$), with microbial metabolism driving most transformations
• Volcanic emissions and DMS provide natural atmospheric sulfur—dimethyl sulfide from marine phytoplankton influences cloud condensation nuclei and climate feedbacks
• Anthropogenic $SO_2$ emissions cause acid deposition—fossil fuel combustion releases sulfur that oxidizes to sulfuric acid, though emissions have declined significantly since the 1970s due to regulations

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Rock-Bound Cycles (No Significant Gas Phase)

These cycles lack major atmospheric reservoirs, meaning elements move primarily through weathering, biological uptake, and sedimentation. Without a gas phase, these cycles operate more slowly and are strongly controlled by geological processes.

Phosphorus Cycle

• No stable gaseous phase makes this cycle uniquely slow—phosphorus moves through weathering, biological uptake, and sedimentation without atmospheric shortcuts, with residence times of 10,000+ years in ocean reservoirs
• Rock weathering is the ultimate source—apatite and other phosphate minerals release $PO_4^{3-}$ through chemical weathering, the primary input to the biologically active pool
• Frequently the limiting nutrient in freshwater systems—low solubility and lack of atmospheric input make phosphorus availability a key control on primary productivity

Silicon Cycle

• Silicate weathering consumes $CO_2$ over geological timescales—the reaction $CaSiO_3 + CO_2 \rightarrow Citeite CaCO_3 + SiO_2$ represents Earth's primary long-term climate thermostat
• Diatoms and radiolarians control marine silicon cycling—these organisms build siliceous frustules and tests, exporting biogenic silica to sediments
• Reverse weathering in marine sediments returns silicon to minerals—authigenic clay formation in seafloor sediments represents a significant silicon sink

Calcium Cycle

• Carbonate precipitation links calcium to carbon sequestration—formation of $Cite cite CaCO_3$ in shells and coral removes both calcium and carbon from seawater
• Weathering of silicates and carbonite rocks provides dissolved $Ca^{2+}$—rivers deliver ~0.5 Gt of calcium to oceans annually
• Saturation state controls carbonate mineral stability—ocean acidification reduces carbonate ion concentrations, threatening calcifying organisms and shifting the cycle

Potassium Cycle

• Clay minerals are the primary reservoir and buffer—potassium adsorbs strongly to clay surfaces, creating a slowly exchangeable pool that sustains plant nutrition
• Weathering of feldspars and micas releases $K^+$—these potassium-bearing silicates break down through hydrolysis and cation exchange
• No atmospheric phase limits global redistribution—unlike nitrogen, potassium deficiencies cannot be corrected through atmospheric deposition, making soil mineralogy critical

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Cycles Controlled by Redox and Micronutrient Dynamics

These cycles involve elements that, while less abundant, exert disproportionate control over ecosystem productivity and other biogeochemical processes. Redox conditions and trace availability often determine the pace of major element cycling.

Iron Cycle

• Redox state determines solubility and bioavailability—$Fe^{2+}$ (ferrous) is soluble under reducing conditions, while $Fe^{3+}$ (ferric) precipitates as oxyhydroxides in oxic environments
• Iron limitation controls productivity in HNLC ocean regions—High-Nutrient, Low-Chlorophyll zones have abundant nitrogen and phosphorus but lack sufficient iron for phytoplankton growth
• Couples to sulfur cycle through pyrite formation—$FeS_2$ precipitation in anoxic sediments sequesters both iron and sulfur, influencing redox buffering

Water Cycle (Hydrologic Cycle)

• Drives weathering and nutrient transport across all other cycles—precipitation initiates chemical weathering, and runoff delivers dissolved and particulate materials to oceans
• Residence times vary enormously by reservoir—atmospheric water (~9 days), rivers (~2 weeks), deep ocean (~1,000 years), and groundwater (100–10,000 years)
• Evapotranspiration links to vegetation and carbon cycling—plant water use connects hydrologic fluxes to ecosystem productivity and land-atmosphere feedbacks

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Quick Reference Table

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Self-Check Questions

1. Which two cycles lack significant atmospheric reservoirs, and how does this affect their response time to perturbations?

2. Compare the role of microorganisms in the nitrogen cycle versus the sulfur cycle—what types of transformations do they mediate, and why are redox conditions critical for both?

3. Why is phosphorus typically limiting in freshwater systems while nitrogen limits marine productivity? How do the characteristics of each cycle explain this pattern?

4. If an FRQ asked you to explain how silicate weathering acts as Earth's long-term climate thermostat, which cycles would you need to integrate, and what is the key reaction?

5. Iron is a micronutrient, yet iron limitation can control carbon fixation across vast ocean regions. Explain this paradox and identify what distinguishes iron cycling from macronutrient cycles like calcium or potassium.