L4 - Chemical evolution of ocean water Flashcards
What percentage of the Earth’s surface is covered by oceans, and what is their average depth?
Oceans cover 71% of the Earth’s surface to an average depth of 3.8 km
What are clay minerals, and how are they formed?
Clay minerals are aluminosilicates formed by incongruent dissolution. They are very small particles (<0.002 mm) and dominate soils and sediments along with quartz.
What are the main properties of clays?
Clays are hydrous aluminum silicates with the following properties:
- Plasticity when wet, shrinkage when dry.
- Fine particle size, often forming colloidal suspensions in water but flocculating in saline solutions.
- Large surface area for ion exchange.
- Can be compacted into mudstone.
How do clay minerals interact with solutions?
Clays can exchange ions with solutions by absorbing ions onto mineral surfaces or exchanging structural ions. This modifies the composition of waters in contact with them.
What are the main structural components of clay minerals?
Clays have two main sheet structures:
- Silica sheets: Made of silicon tetrahedrons (SiO4).
- Alumina sheets: Made of aluminum octahedrons surrounded by six OH⁻ groups.
How are nutrient elements removed from the ocean?
Nutrient elements are taken up by plankton in surface layers and include organic carbon, H4SiO4, Ca²⁺, NO₃⁻, and PO₄³⁻.
How are scavenged elements removed from the ocean?
Scavenged elements are adsorbed onto solid particle surfaces and removed in sediments.
What are conservative elements?
Conservative elements are ions with small charges or anions unaffected by strong electrostatic forces. They are not influenced by biological processes and are only affected by evaporation, freezing, or dilution
What is residence time in the context of ocean chemistry?
Residence time measures the average time an element spends in the ocean before being removed. It is influenced by the element’s removal rate, not its crustal abundance.
How do sea-to-air fluxes occur, and what elements are involved?
Fluxes occur due to bubble bursting and breaking waves, removing Na⁺ and halogens from the ocean and contributing to river water salts.
What drives hydrothermal processes at mid-ocean ridges (MOR)?
Hydrothermal processes are driven by magma heat sources that create warm, buoyant fluids (hot springs, black smokers) and cold, dense descending seawater.
What are the effects of hydrothermal activity on ocean composition?
More MOR interaction lowers Ca²⁺ and K⁺ while raising Mg²⁺, sulfate, and bicarbonate. Less interaction reverses these effects.
What factors favor aragonite precipitation in the oceans?
Aragonite precipitation is favored by:
- High concentrations of CO₃²⁻ and SO₄²⁻.
- High temperatures.
- High Mg/Ca ratios (Mg inhibits calcite precipitation).
- Low PO₄³⁻ concentrations
How do Mg/Ca ratios influence carbonate precipitation?
- Low Mg/Ca (<1): Low-Mg calcite.
- Mg/Ca (1-2): High-Mg calcite.
- Mg/Ca (2-5): High-Mg calcite and aragonite.
- Mg/Ca (>5): Aragonite only.
What does the Hardie model explain about seawater chemistry?
The Hardie model explains the switch between calcite and aragonite seas over the past 550 Ma due to changes in MOR length and mixing with river water. It also accounts for changes in K-rich and Mg-rich evaporite deposits.
