As marine scientists we should know: why the oceans are salty

1. Origin and Steady State Composition of Seawater – Geochemical Mass Balance of the Oceans Why is the sea salty, and how does it stay that way? • As marine scientists we should know: • why the oceans are salty • why they contain the salts that they do • why the composition of the oceans has been stable for very long periods of time • the processes that govern ocean stability

2. The earth is about 4.8 x 109 y old. Micro fossil records (stromatolites) going back 3.8 x 109 y, indicate that Earth quickly evolved conditions not too different than those of today. Modern Stromatolites – Shark Bay Australia Ancient stromatolites http://www2.nature.nps.gov/geology/parks/glac/car0484.jpg from http://www2.nature.nps.gov/geology/parks/glac/

3. Ocean Origin - Quick soak theory Rapid outgassing of volatiles (H2O, CO2, H2S, HCl, HBr, HF, B, Ar) from the primitive earth. As Earth cooled water condensed and volatiles dissolved, leading to acid ocean. Highly acidic conditions caused rapid leaching of Juvenile or primordial rock material -first circuit through crustal-ocean-atmosphere factory - yielding many of the cations (Na+, K+, Mg2+, Ca2+ etc) and alkalinity in the primitive ocean. These early weathering reactions consumed acid

4. After quick soak, the ocean composition was maintained in a pseudo-steady state, implying that: salt inputs = salt exports. Thermodynamic equilibrium model of Sillen (ca 1960) Major ion concentrations are controlled by reactions that reach equilibrium with the various phases, rock, sediment and ocean waters. Flux mass balance model of MacKenzie and Garrels (1966) Inputs of materials to ocean are balanced by outputs through various mechanisms. Their theory invoked a process of reverse weathering to remove certain elements and alkalinity

5. What are the present day sources of matter to the oceans?: - Rivers - Wind (aeolian transport) - Bedrock and sediment leaching - Hydrothermal reactions at vents Rivers are by far the largest source for most materials What are the sinks? • Evaporite salt precipitation • Sedimentation (e.g. CaCO3) • Pore water burial with sediments • Hydrothermal reactions at vents

6. Sources and sinks for major salt elements in seawater These same mechanisms affect many minor elements as well

7. Review - the major salt ions in seawater Six ions constitute > 99.8% of all dissolved solids in seawater Na+ sodium Cl- chloride K+ potasium SO42- sulfate Mg2+ magnesium Anions Ca2+ calcium Cations

8. Major ion elements Residence time vs. concentration of different elements in seawater. For the most part - the elements with the longest residence times have the highest concentrations - Na+ and Cl- are VERY unreactive chemicals in aqueous solution! C Concentration (M) Mg Ca S K C P Fe Cu 102 106 108 Residence Time (y)

9. Rivers are the major source of material to the ocean. Is the ocean merely evaporated river water? Not quite. Rivers contain the same salts as seawater but the proportions are not the same in river vs. ocean water. Per gram of salt, river water contains more carbonate and bicarbonate than seawater and thus evaporated river water would have a much higher alkalinity than the ocean. Most lakes aren’t salty because they are geologically young. Ancient lakes like Mono Lake, CA are indeed salty and highly alkaline (soda lakes; pH’s ~9.5). Mono Lake

10. The ionic composition of seawater is distinct from that of river water Per gram of dissolved salt, river water has a much higher amount of Ca2+ and HCO3- than seawater. Rivers also contain very little Na+ or Cl-, whereas seawater has little H2SiO4-. Source: Open University These are averages - River water varies considerably depending on drainage basin

11. Pore water burial is a significant sink for Cl- and Na+ but a relatively insignificant sink for other major ions and alkalinity

12. Evaporite Basins: Where net evaporation of water takes place, salts become more concentrated. If concentrations of some ion species become high enough to exceed the solubility products of the least soluble mineral forms, precipitation will occur and selectively remove those species. The sequence of precipitation due to evaporation of a fixed volume of sea water: 1) Calcite (CaCO3) (least soluble) 2) Aragonite (CaCO3) 3) Gypsum (CaSO4.2H2O) 4) Anhydrite (CaSO4) 5) Halite (NaCl) and some Mg salts. 6) Sylvite (KCl) (most soluble) Density (g/cm3) Lines perpendicular to curve represent concentration of salts precipitated Proportion of original volume remaining

13. Evaporite salt formation – reverse estuary system Continuous input of salts from ocean allow continuous precipitation of salts in basin

14. Do marginal seas evaporate to dryness and deposit salt? Continuous input model. Mediterranean example. Evaporation Fluctuating sea level Interglacial sea level sw Glacial sea level Precipitation of evaporite salts during low sea levels Mediterranean Sill Atlantic Ocean Mud layers deposited during non-salt precipitating periods Salt deposits No, not really. Mediterranean Sea

15. Evaporites- Contain a large fraction of the crustal Na, Ca, Cl, SO42-, K and Mg. • The amount of NaCl in evaporites (as halite) might be as much as 2-4 times the amount in seawater presently. • Much of the river runoff of ions is from sedimentary deposits and evaporites (a form of sedimentary deposit). • Evaporites are the main mechanism of removing chloride (also some pore water losses which are significant for Na and Cl). • Sulfate is also removed in large amounts - therefore evaporites are the major anion sink.

16. Weathering: • The simplest weathering reaction can be viewed as the dissolution of calcium carbonate rock by acid solution (acid derived from CO2 in atmosphere). • CO2 + H2O <=> H2CO3 • H2CO3 + CaCO3 <=> Ca2+ + 2HCO3- • Note this reaction: • consumes CO2 (e.g. carbonic acid) • releases cations (e.g. Ca2+) • releases alkalinity in the form of bicarbonate

17. Weathering of continental igneous rocks often refers to alteration of aluminosilicates (clays): • 2CO2 + 3 H2O + CaAl2Si2O8 --> Al2Si2O5(OH)4 + Ca2+ + 2 HCO3- • Weathering: • consumes CO2 • releases cations • generates alkalinity The dissolution of carbonate rocks during weathering supplies >80% of the alkalinity of river water with the remainder coming from weathering of shales and igneous silicates. If igneous rock weathering goes to completion, all the cations will be removed leaving quartz (SiO2) - hence our sand beaches!

18. From Emerson & Hedges

19. Reverse weathering: Reverse weathering (RW) is a misnomer in that the original products are not reformed - rather the reactions are similar to the reverse of the weathering reaction. Cation-poor alumino silicate + silicic acid + bicarbonate + cations ---> Cation rich alumino silicate + CO2 RWForms illites, montmorilinites and chlorites (types of aluminosilicate clays • Reverse weathering: • generates acid (CO2) • consumes cations • consumes alkalinity

20. Weathering and reverse weathering - long term balance of elements, pH and alkalinity in the oceans (Feedbacks in the ocean-atmosphere-crust connection) Seawater remains electronically neutral. Weathering releases alkalinity (HCO3-) which can absorb protons, causing OH- and pH to go up. Increased [OH-] would shift the carbonate equilibrium toward precipitation of carbonates, thereby removing cations like Ca2+ (returning to steady state). It would also lower pCO2 in ocean. Decreased pCO2 in seawater (and hence atmosphere) would slow weathering because pH of rain would rise. With these feedbacks, the whole system is balanced within relatively narrow limits.

21. Clays: Crystalline solids made of aluminosilicate with diameters of < 1 m. Sheet like structure - think of something like mica or vermiculite. Two layered clays (alternating octahedron and tetrahedron. - Kaolinites. The octahedral layer contains Al or Mg while the tetrahedral layer contains Si. Three layered clays have an octahedral layer sandwiched between two tetrahedral layers. - Montmorilinites, illites and chlorites. Vermiculite is a 3 layer clay which can expand due to formation of a water layer between the sheets of clay. This is why vermiculite is good to have in the garden and why it is used for spills (holds water).

22. **Important Characteristics of Clay minerals** • High surface area • High cation exchange capacity • Net negative charge on surface • Small size – highly mobile (winds & currents; highly compressible in sediments) • Some types are rich in Fe • Cool names Clays are important “chemicals” in marine systems

23. Characteristics of some Clay minerals (aside of having some great names)

24. Clay Transport - River transport - most deposited on shelf or slope areas, but some is transported offshore. - Wind transport is significant due to the small particle size. Aeolian transport of clays is important from the standpoint of clay geochemistry but also because of the trace elements associated with it, particularly Fe. Some regions of the open ocean have sediments composed mainly of atmospheric-derived clay! - Ice transport. - Glaciers (ice rafted detrius deposited in central ocean basin – includes clays and larger particles that can only get offshore by transport in floating ice)

26. McKenzie & Garrels Mass Balance model for ocean Missing sinks Incorrect assumptions about location of sinks in original model – e.g. FeS2 ppt in sediments Removal of these ions occurs at Hydrothermal vents Total bar length is total input per 108 years

27. Hydrothermal Vent Systems History: Geochemists and geophysicists predicted some sort of hydrothermal input to the deep sea as far back as the mid to early sixties (Elder, 1965). Evidence included extensive metaliferous sediments, altered mid-ocean ridge basalts and temperature anomalies. 3He/4He anomalies pointed toward conductive and convective heat flow from ridges.

28. Discovery of hydrothermal vents in the 1970’s revolutionized understanding of ocean geochemistry Hydrothermal vent systems and sea-floor spreading centers. Here reactions of seawater constituents with basalts at high and low temperature accelerate reverse weatheringand other reactions. There is now good evidence that Mg2+ is completely removed from circulating water at these vents.

29. Location of confirmed (circles) and probable (triangles) hydrothermal vents

30. Seawater is drawn through fissures in the rock/sediment. It is heated thereby becoming less dense and buoyant, therefore it rises by convection, exiting through vents. Meanwhile the water chemistry is changed dramatically, with addition of some compounds and removal of others. Magma (Hot!)

31. Within the fissure of vent systems Heated seawater interacts with basaltic rocks at 350oC and 500 atm pressure. Some water is incorporated into the rock but a host of chemical reactions occur. • Acidic vent water (CO2 rich) leaches metals and other minerals • Reduction of metals and sulfate, forming highly soluble metal ions and hydrogen sulfide • The sulfide originates primarily from seawater sulfate that is reduced as it is drawn through the circulation cells in the rock/vent system. • Precipitation of gypsum and anhydrite - removes calcium • Reverse weathering type reactions consume alkalinity and Mg2+ • Heated seawater dissolves quartz leading to saturated H2SiO4 at 350 oC end-member temperature

33. http://oceanexplorer.noaa.gov/explorations/06fire/background/chemistry/media/arcvolcano.htmlhttp://oceanexplorer.noaa.gov/explorations/06fire/background/chemistry/media/arcvolcano.html

34. Hydrothermal vent processes and chemistry http://www.oceanexplorer.noaa.gov/explorations/02fire/background/vent_chem/ventchem.html

35. Black smoker vents Acidic hot water circulating through rocks is rich in H2S and Fe2+. When this water mixes with the cold, relatively alkaline (pH 8) seawater, metal sulfides precipitate. Fe2+ + S2- -> FeS (s) Reduced iron + sulfide -> pyrhotite Clouds of metal sulfide precipitates The vent chimneys are often made of metal sulfides and CaSO4 (gypsum and anhydrite)

36. Seawater end-member Pore water evidence that Mg2+ ions are consumed (removed from solution) in sediments near mid ocean ridges. The number near each line represent the upward convective flow of pore water. Removal of Mg2+ occurs below these depths at the interface of hot basalt and sediments

37. Chemosynthetic communities at vents • Food chain based on chemical energy - not light • Oxidation of reduced chemicals (H2S, CH4, Fe2+, Mn2+) by chemoautotrophic bacteria forms the basis of the food chain. • Chemosynthetic bacteria often live in symbiosis with higher organisms

38. Black smoker vents(photo by Emory Kristof - National Geographic Society

39. Vent community(photo by Emory Kristof - National Geographic Society

40. Vent Crab on chiminey(photo by Emory Kristof - National Geographic Society

41. Types and locations of “vents”: Hot vents - spreading centers (Galapogos, East Pacific Rise, Juan deFuca ridge) - seamount volcanos (Loihi) -Cool vents/diffuse flow Aegean Sea (accessible by snorkeling!) Seeps - cold - CH4-rich (Gulf of Mexico) - Mud volcanoes (CH4 seeps) - brine seeps (Florida escarpment) - oil seeps - Santa Barbara Loihi seamount vent – Fe rich

42. In Review Vents are: -Sources of gases and solutes -sites of heat input and dissipation -sources of polymetalic sulfides -sites of geochemical exchange

43. End

44. Origin and Steady State Composition of Seawater – Geochemical Mass Balance of the Oceans (Outline) • Origin and Evolution of the Oceans • Where did the water and major dissolved constituents come from? • Sources and Sinks for Material in the Present Day Oceans • Sources - Rivers, Atmosphere, Vents/seeps • Sinks - Sediments, Vents, Evaporites • Weathering/Reverse Weathering

45. How is pseudo-steady state composition of the oceans maintained?

46. The McKenzie and Garrels (1966) flux balance model was generally accepted despite questionable assumptions about significant removal of dolomite (CaMgCO3) and pyrite (FeS2) as sedimentary deposits - evidence does not support large current deposition of these minerals. Later discovery of hydrothermal vents makes these assumptions unnecessary. At vents, sufficient Mg and S are removed to eliminate the need to invoke dolomite and pyrite sink proposed by MacKenzie and Garrels.

47. Geological evidence for the long-term stability of ocean composition: • sedimentary rocks nearly the same composition • evaporites nearly the same composition over geologic time (though some significant variations) • organisms appear to have been similar over time

49. Seawater Salts – The major ions of seawater include 4 cations (Na+, K+,Ca2+, Mg2+), and two anions (Cl-, SO42-), and together make up > 99.8% of salinity. Chloride is by far the major anion, with a concentration of ~540 mM compared to sulfate 28 mM. Seawater is about 86% NaCl in terms of mass. Charge balance - Seawater, like all solutions, is electronically neutral. The negative charge of anions is balanced by the positive charges of cations. This must be satisfied everywhere.