Rivers to the Sea/Hydrological Cycle/Particles/Pollution

Readings (in Sverdrup, 2005):
1. Section 2.6
2. Sections 6.1 - 6.2
3. Chapter 13

Note: terms to understand are underlined

1. Hydrologic Cycle

-         Mean temperature on earth surface (16°C) is such that variations allow water to exist as liquid, solid, and gas (The Water Planet)

-         Atmosphere acts a shield to evaporation, preventing loss of water to space

-         Most of water on earth is in oceans (98% of total)

● Ocean surface area = 361 million km2

● Ocean volume = 1.37 billion km3

● Other reservoirs: ice (1.6%), ground water (0.36%), rivers+lakes (0.04%), atmosphere (0.001%)

NOTE: Water in rocks is left out of this inventory (adds another 20%)

-         Hydrologic cycle includes well-defined oceanic inputs and outputs (see figure in text)

-         Characteristic water residence times in each reservoir

● Large reservoirs usually have long residence times, smaller have  shorter

● Remember the bucket analogy for reservoir size and residence time

● Effect on sea level of changing the balance of the hydrologic cycle

● Remember that the concept of residence time implies steady state

Now let’s go back to salts in the sea for a minute, and explore what seawater contains besides the 6 major ions (remember the 6? Na⁺, Mg⁺, K⁺, Ca⁺, Cl^- and SO₄^2- account for 99% of the dissolved matter in seawater).

2. Seawater constituents other than the 6 big major ions

-         Minor ions: Conc. range ~ 0.05 – 50 µmol/kg

● Some conservative, many non-conservative: i.e. vary independent of salinity, undergo rapid chemical reactions, affected by biological organisms                 

ex.: non-conservative: nutrients -> phosphate (PO43-) and nitrate (NO3-), and also silicate (H₄SiO₄) which is needed by some phytoplankton (diatoms) to make their shells (frustules)

● Trace elements: Conc. range ~ 0.05 – 50 nmol/kg

most of periodic table, including "heavy metals"

ex.: Mn, Fe, Ni, Cu, Zn, Cd, Pb

-         Organic constituents: mostly large molecules containing C, H, O, N, S

● Major group of chemicals that are distinct from salts

● DOC (95%) and POC (5%)

● Total sum of all organics are 30,000 times less than inorganic salts in seawater

● Types of organic compounds: carbohydrates (sugars), proteins,

hydrocarbons (C and H only), lipids (fats), and humics (big, complex molecules, mostly C and O, yellow color)

source: mostly from organisms living in water

-         Other substances in seawater: dissolved gases, particles (bits of rock + living or dead bits of organisms), colloids (very small particles and large macromolecules)

NOW: Let's make a BUDGET for salts (ions) in seawater (i.e. identify inputs + outputs)

3. How do salts get into the ocean?

-         Cations from weathering of rocks, anions from outgassing of earth through volcanoes (especially early earth)

-         Rivers: carry dissolved rocks and suspended solids, a big input

● Weathering: mechanical and chemical

     Q: Are oceans simply dissolved rocks?

        A: No! (See handout figures)

     Q: Are oceans like concentrated river water?

        A: No! (See handout again)

-         Atmosphere: rain and falling dust

● Highest inputs near continents

● Also source of gasses

● ex. Iron (Fe) in surface ocean comes from dust

-         Hydrothermal: Hot Springs in the deep ocean

● More on this October 21^th

4. How do salts get out of the ocean?

-         Sea spray: from breaking waves and bubbles

● Important near coasts

-         Evaporites: precipitation of salts in small closed basins

● Major control of ocean salinity (only major sink for Na⁺ and Cl-)

-         Adsorption: chemical adhesion to a surface

● Ions go on particles which sink to sediments

● Very important for many trace metals (mostly cations)

● Can lead to ion exchange in clay minerals

-         Biological removal: uptake by organisms near surface; death -> sinking

● Organic tissue: carbon eventually becomes fossil fuel

● Shells: calcium carbonate (CaCO3) and silica (SiO₂•2H₂O)

● Purposeful uptake (nutrients) and unintentional (e.g. Hg)

-         Hydrothermal: reactions with basalt remove some elements

Note that removal of many dissolved chemicals from the ocean involves uptake by or adsorption onto biogenic, authigenic, or lithogenic particles

5. Particles in the Ocean

-         How many are there, and where are they?

● Many thousands per liter of small particles, much fewer big ones

● Usually, we speak of mass per liter for particles of all sizes

● In typical open ocean - surface water: ~100µg/L ; deep water: ~5µg/L

● Deep ocean water is some of the "cleanest" on earth (similar to South Pole snow)

-         What are ocean particles made of?

● Two kinds of material: inorganic (minerals) and organic (live and dead tissue of organisms)

● Inorganics are mostly of two types: biogenic (made by organisms) and lithogenic (made from rocks)

● Biogenic inorganics are mostly shells of small plants and animals made of calcium carbonate (CaCO3) and silica (called opal, amorphous SiO2°2H₂O)

● Calcium carbonate shells are made by plants (e.g. Coccolithophorids) and animals (e.g. Foraminifera).

● Silica shells are also made by plants (e.g. Diatoms) and animals (e.g. Radiolaria)

● Lithogenic inorganics are small pieces of rock dust (clays and oxides) which enter surface waters either from rivers or blown in from the atmosphere

● Organic material is 95% of particles in surface waters, and is made of the same compound classes as DOC (see last lecture), including dead tissue and living organisms

-         How big are ocean particles?

● Huge range: from very small particles like colloids and viruses all the way to whales (taking the definition of particles very broadly)

● Most of the mass of suspended material is in a size range of a few micrometers (µm)

● Very few particles are larger than 100µm, but they sink very fast (see below) and therefore contribute most of the vertical sinking flux

● Large organic aggregates ("marine snow") are one important class of large particles

-         How fast do particles sink?

● Also huge range: small ones as slow as 1 m/day, large aggregates at ~300m/day (Q: What is residence time of average particle in ocean)

● They sink faster than simple passive sinking (Stokesian sinking) due to aggregation of small particles (bigger=faster sinking)

● Generally, sinking of surface-produced material is fast enough that there is little time for horizontal transport, therefore particles in bottom sediments reflect what was produced immediately above

6. Pollution in the sea

-         General Principles (read chapter on pollution in text!)

● Pollutant may be entirely new human-made (e.g. CFCs) or simply an addition to natural concentrations (e.g. nutrients, metals, CO2)

● Pathways: many pollutants follow pathways through the environment that are known from study of the unpolluted system.

● Biological effects: disruption of food web balance, bioaccumulation (e.g. PCBs, Hg), reproductive failure

● Proving toxic effects and setting concentration limits: surprisingly difficult, many variables at play, LD₅₀ is often all we have to go by.

● Most effects in coastal ocean via rivers– globally dispersed pollution usually requires atmospheric transport (e.g. Pb, Hg, PCB, CFCs)

● Major concerns now: nutrients (and low oxygen), metals, toxic organics, oil, exotic species