Ocean chemistry and deep-sea sediments

Ocean chemistryanddeep-sea sediments

1. Introduction – type of deep-sea sediments2. Chemical cycles in the Ocean3. Accumulation of pelagic biogenic sediments4. Supply of terrigenous eolian and fluvial sediments5. Diagenesis of deep-sea sediments

1. Introduction – Type of deep-sea sediments

Global fluxes in the ocean constitute dissolved and particulate matter.We start with particulate matters. There are mainly two types ofsediment, biogenic and terrigenous (incl. volcanic) sediments. There is aminor amount of extraterrestrial sediments.• Biogenic sediment production of planktonic marine organism dependsupon both nutrients and sunlight and can be devided up in siliceousand calcareous oozes, made up by zooplankton and phytoplankton.Remember ocean circulation causes upwelling where highbiogenic sedimentation rates occur.

Biogenic sedimentsBeneath high-production upwelling cells the following sediment types concentrate:1. Siliceous ooze which consists of (phytoplankton) diatomsand/or (zooplankton) radiolaria.Diatom oozes are typical of high latitudes and some continental margin areas.Radiolarian oozes are characteristic for warmer equatorial regions.Siliceous oozes match closely very high productivity areas with highconcentrations of phosphate.

2. Carbonate oozes cover about 50 % or 140 x 10 6 km 2 of the ocean.Carbonate oozes consist of (phytoplankton) nanofossil ooze(coccolithophores), and (zooplankton) foraminifera, or pteropods.The concentration and distribution of biogenic sediments in the ocean iscontrolled by production, preservation and dissolution of the material.

Sediments in the open ocean are called pelagic and those influenced bycontinental margin and terrigenous sedimentation are calledhemipelagic sediments.

Where is the sediment core coming from, the ocean ridge or thecontinental margin? What about hemipelagic and pelagic sedimentation and seafloorspreading?

Carbonate oozePhytoplankton - Coccolithophores

Zooplankton - Foraminifera

Gastropod molluscs- pteropodsAragonite

Tropical coral reefs

Cold-water coral reefsExamples of different types of coral skeleton(morphotypes) from two sites in northern Norway.Cold-water coral reef consisting mainly of Lopheliapertusa are found in numerous locations andgeologic settings on the Norwegian shelf; from>500 to

Siliceous oozePhytoplankton - Diatoms

Zooplankton - Radiolaria

Ocean desertsThe central subtropical gyres show low ocean productivity

Calcareous and SiliceousmicrofossilsIdentify and name thezooplankton and thephytoplankton!

High productivity zones

Nutrients and δ 13 C13δ C =1313( C ) ( C12 − 12 )C sample C s13( C12 )s tan dardCtan dard18δ O =1818( O ) ( O16 − 16 )O sample O s18( O16 )s tan dardOtan dard

Nutrient rich intermediate waters

Modern and glacial Atlantic intermediate waters

Terrigenous sedimentsOcean circulations receive the terrigenous sediments – eolian, fluvial,ice rafted debris (IRD) - from the continents. The particles sink andcirculate within the different water masses and ultimately end up atthe seabed.Which regions most likely generate most of the wind blown dust?Which regions contain most of the IRD?What is the major contrast between eolian and fluvial sediments?Why are volcanic ash layers important in marine geology?Where would you expect to have sediment erosion and wheredeposition?

Clay mineralsKaolinite is formed by extremechemical weathering in low latitude.Chlorite dominates in areas ofphysical weathering in highlatitudes but is destroyed in areasof chemical weathering.Illite is the most common claymineral and occurs in both high andlow latitudes.Montmorillonite is an alteredvolcanic material.Red clays are a mixture of clayminerals.

Fluvial input

Turbidites

Eolian input

LGM ice sheet extentIRD input

Hydrothermal vent sedimentinput

Sedimentation rates

Summary:Deep-sea sediments are a mixture of biogenic and terrigenous sediments, withminor volcanogenic and extraterrestrial contributions.Submarine topography and ocean currents determine their distribution.Pelagic sediments are mainly composed of the remains of calcareous and siliceoussediments with a mixture of eolian sediments.Hemipelagic sediments such as glaciomarine sediments are a mixture ofterrigenous sediment input from nearby continents and marine sediments.Wind blown dust can travel long distances while river derived sediment settledown on continental margins.Clay mineral compositions can be useful in determining climate zones ortransport pathways.IRD sediments help to identify major iceberg transport pathways and icesheetcollapse.

2. Chemical cycles in the Oceans

Dissolved constituentsAfter being introduced to particulatematter occurring in oceans we nowlearn about dissolved constituents inoceans.NitratenonconservativeSodiumConservativeBariumnonconservativeMajor constituents are those having aconcentration > 1x 10 -6 by weight.Salinity for example has 35 parts perthousand weight. The majority has aconservative behaviour.Minor and trace constituents have only 1x10 -9 by weight, and nearly all of themhave a non-conservative behaviour.

Steady state oceanWhat do the nutrients indicate?Do they exhibit conservative or non-conservative behaviour?Are they minor or major trace constituents?

Steady state oceanDefinition: The rate of supply of dissolved constituents equals the rate of removal.This concept allows to to define a mean oceanic residence time:Total mass of a dissolved element in the ocean/rate of supply (or removal) of this elementMost of the ocean residence times for elements are long ~10 5 years.The ocean mixing time, in contrast is much shorter between 250 – 500 or 1000 years.Why can a dissolved constituent with an ocean residence time of 100 years not be mixedthroughout the ocean?Note that the life span of most marine organism is short. Thus the dissolved constituate cango through biological cycles several times.

Dissolved constituents arecoming from:• Rivers (4 x10 9 tons/year)• Volcanic eruptions• Hydrothermal vents• Dissolution of sediments

• In the photic zone thephytoplankton is grazedby the zooplankton.• The dead particles aremainly decomposed by,for example bacteria.• The organic matter isrecycled to the photiczone.• The organic matterconsists of oxygen,hydrogen, carbon,nitrogen, phosphorus.• Some particles sinkthrough the water columnand reach the seabed.

A large part of the decomposed materialis recycled to the area above thethermocline.1% only fo the sinking particles reach theseabed and from this only a fractionsurvives.

Marine snowThe marine snow is composedof aggregates of smallparticles.Due to the larger size it is amechanisms to get particlessinking faster to the seabed.Marine snow is mostly foundwhere there is a highbiological productivity.

Phosphate distribution in oceansat 2000 m water depthWhy is the Pacific richer in nutrients than the Atlantic?

The answers are:• The nutrient-poor NADW flowssouthward in the western Atlanticand receives a steady rain ofnutrients.• They are sinking from the surfaceto the deep ocean where theyconcentrate.• In the south the AAIW andAABW contributes more nutrientsto the deep water before it flowsinto the Pacific.• Here, deep-water is furtherenriched in nutrients. This water isbecoming also one of the oldestwater masses.

Upper right: Nutrients areenriched in deeper watersfrom decomposition of organicmatter.Lower left: Nutrients aredepleted in surface waters as aresult of photosynthesisactivity by phytoplankton.

Summary:• Organic matter produced by primary productionin surface waters is a major regulator of seawatercomposition of minor and major constituents.• Major constituents - the nutrients are phophate,nitrate and silica.• They are depleted in surface waters andenriched in intermediate and deep waters.• Maximum oxygen depletion occurs inintermediate waters due to decomposition oforganic material at about 1 km water depth.• Oxygen is brought to the deep ocean bydownwelling (convergence) in polar regions.

3. Accumulation of pelagic biogenic sediments

Which factors control the distribution of biogenic sediments:• Biological productivity of surface waters• Water depth and dissolution of particles• Dilution by terrigenous sediments• Marine snow or fecal pellets causing a speedy transport to the sea bed

Preservation of pelagic carbonatesedimentsBiological productivity of calcareous oozedominate in lower productivity zones whileforaminiferal oozes increase faster in highproductivity zones.CO 2 plays a major role in carbonatepreservation. First note that CO 2 is moresoluble in cold than in warm water. Therefore,higher CO 2 concentrations in colder watercause more carbonate dissolution (polarwaters have no or only minor amounts ofcarbonate)!Second, CO 2 combined with water produces aweak acid. It produces hydrogen andbicarbonate ions:Typical profile of sum (marked left) of total dissolvedinorganic carbon Σ CO 2 . CO 2 is removed from solution insurface waters by photosynthesis. It is returned tosolution in deep water as organic matter is decomposed.CO 2 gas + H 2 O = H 2 CO 3 = H + + HCO 3-HCO 3- = H + + CO 32-

Preservation of pelagic carbonatesedimentsΣCO 2 is total dissolved inorganic matter, as this sum increases, so does the ratio of bicarbonate tocarbonate ions. CO 2 gas + H 2 O = H 2 CO 3 = H + + HCO 3- ; HCO 3- = H + + CO 32-[H + ] = K [HCO 3- ]/[CO 32-]The terms are in molar concentrations and K is the equilibrium konstant.• If H + increases, then there are more hydrogen ions in the water, and the water becomes more acid(pH decreases)!• If H + decreases, then the ΣCO 2 decreases, and the water becomes less acid (pH increases)!• Dissolution of calcium carbonate skeletons occurs where ΣCO2 is high that means the water ismore acid. We can use two equations for the carboanet dissolution.1. CaCO 3 + H + Ca 2+ + HCO 32-2. CaCO 3 Ca 2+ + CO 32-

1 2Ca 2+ concentrations inseawater are virtuallyconstant.The concentration of CO 32-ions determines whethercarbonate will dissolve ornot.unstablestableThe saturation curve (2)shows that calcite willdissolve to the left but it isstable to the right of thesolid line.Would you expect carbonate dissolution to be higherin Atlantic than in Pacific waters?In order to predict thedepth at which calciteskeletal begin to dissolvewe need to know thecarbonate ionconcentrations.

Seawater has a pH of ~ 8.2

Preservation of pelagic carbonatesedimentsCO 2 combined with water produces a weak acid.CO 2 gas + H 2 O = H 2 CO 3 = H + + HCO 3-HCO 3- = H + + CO 32-Inorganic carbon occurs in solution as: CO 2 gas, H 2 CO 3 , HCO 3 -, and CO 32-. The last two arequantitatively the most important.Note: CO 2 is reduced in surface water due to photosynthesis and increases in deep water dueto decomposition of organic matter. In addition, carbon dioxide comes from cold deep waterthat sinks in high latitudes to the deep sea. Nearly all carbon dioxide is in the form ofcarbonate ions and bicarbonate.

Carbonate ion concentrationsBy direct measurements of ΣCO 2 and alkalinity ofwater the CO 32-can be calculated. To the left you seethe experimentally derived saturation curves forAragonite and Calcite and the measured carbonate ionconcentration at one station in the Atlantic. Note thatAragonite is less stable than calcite!• Aragonite starts to dissolve a little deeper than3000 m water depth.• Calcite starts to dissolve at nearly 4500 m waterdepth.

The Lysoclineand the Carbonate Compensation Depth (CCD)Lysocline: Water depth where thedissolution of skeletal materialbegins. Below the Lysoclinedissolution of particles occurs atincreasing rates.CCD: Water depth where lessthan 20% of the carbonatematerial is preserved.Theoretically the depth of theLysocline is defined by the depthwhere the carbonate ionconcentration curves intersectswith the saturation curves (seefigure on previous page)

We now know that:• The rate of carbonate dissolution increases with depth.• The Pacific shows more carbonate dissolution than the Atlantic.• The CCD is depressed beneath the equator because of highbiological productivity and increased flux of calcarous materialto the seabed.• If calcite concentrations in sediments are more than 20% thanthe area is above the CCD; if they contain less, it is below it.• The CCD rises towards the continental margins, where biologicalproductivity is in general greater than in open oceans.This seems to contradict point three! However, rich supply oforganic matter causes a consumption of organic matter at theseabed. As a result, CO 2 is released, and thus CO 2 increases inthe bottom water, thus it becomes more acid.• The CCD changes in space and time since the oceanproductivity, depth, temperature, and water mass ventilationchanges.

SUMMARY:• The solubility of calcium carbonate increases with depth.• The concentration of ΣCO 2 increases with depth and aging water masses (respiration of organicmatter and depletion of oxygen and increases in dissolved CO 2 ).• Calcite is more stable than Aragonite, therefore the Lysocline and the CCD is shallower forAragonite than for Calcite.• The average level of the CCD is an indicator for the removal of atmospheric CO 2 .• The ocean will become more acid and the level of the CCD will rise!• Ocen acidity may have changed through time and space. A relative carbonate-poor ocean mighthave been caused by a shallow CCD and vice versa.• A very high productivity in a warm house world, for example, produces high organic materialwhich is consuming much of the oxygen in the ocean, and coverting it to CO 2 causing anincrease in the acidity of water and a shallow CCD.

Preservation of pelagic siliceousparticlesThere are fundamentaldifferences betweencarbonate and silica!• Seawater is everywhereundersaturated with respect toSiO 2 .• The solubility of silicadecreases with a decrease intemperature. Thus silica tendsto concentrate in polar watermasses.• The more silica productionthe better the chances toaccumulate on the seabed.• Silicous particles are relatedto areas of high productivity.

Transport and deposition of dissolved ions

Only a very few particles reach the seabed

Water mass stratification hinderparticles to sink

Currents cause erosion, transport ordeposition of material

Sedimentation ratesdetermine time resolution

After the lectures a student askedWhat causes the NAO andthe Gulfstream?Intrumental records Historical records Paleo records

What causes the Gulfstream?Circulation• The sun and its solar radiation causes the Gulf Stream and all other global wind andocean circulation systems on Earth. The sun causes the Gulf stream by heating theEarth’s equator more than its poles. The Gulf stream redistributes the heat.- Heat passes from warmer to cooler areas causing convection cells• The earth rotates from west to east causing the Coriolis force.- This makes the poleward current appear to move to the right onthe northern and to the left on the southern hemisphere.• In the northern hemisphere, great warm winds sweep up from theequator toward the pole, curve to the right and move clockwise (H-pressure; anticyclone).- The wind drags the surface water into a similar gyre.- The Gulf stream is part of this clockwise anticylonic system, a subtropical gyre.Warm water• Warm water from the North Equatorial current forks into two branches (Carribean, westIndies). They rejoin and pass through the Straits of Florida at an rate of ~30 Sv.High solar radiation and evaporation increases its heat and salt transport as a westernboundary current. It flows along the east coast of United States until is flows across theAtlantic.

4. Supply of terrigenous sediments to the deep sea

• Transport of sediments to the ocean:Fluvial and/or Eolian• Transport of sediments within the ocean:Alongslope transport by currents orDownslope transport by gravitational processes• Down slope transport of sedimentsMass wasting:slidesdebris flowsturbiditesChannels:Canyons:• Along slope transport of sedimentsSediment drifts - Contourites

Alongslope transport by currents

Sediment drifts• A major step in understanding the magnitude of bottom water currents wasthe discovery of large, elongated sediment ridges – sediment drifts.• Sediment drifts are elongated parallel to bottom currents and are often mantledwith sediment waves.• Sediment drifts can be hundred of kilometres long and tens of kilometres wide• Sedimentation rates can be high approx. 12 cm/ka.• Sediment waves can have wavelength of several km andamplitudes of a few tens of metres

In- outflow regime at the Faeroe-Shetland channeland Rockall Trough

Sediment waves or mud waves

Sediment drift

Bottom currents and sediment driftoff Norway

Seismic Image

Downslope transport by gravitation

Passive Continental Margin

IntroductionTwo factors are crucial for continental margins which maylead to submarine mass movements downslope:(1) predisposition factors such as”weak layers” , highsedimentation rates, zones of seismic activity, gas hydratesetc.(2) triggering factors such as earth quakes and/or gashydrate dissociation, excess pore pressure etc.

Downslopesedimenttransport•TMF• Slides• Debris flows•Channelsfrom Vorren et al.

Tsunami deposits on landIcelandStoreggaSlideTrænadjupetSlideNyk SlideTsunami deposits relatedto the Storegga Slide,approx. 8200 years BPTsunami depositsyoungerthan the Storegga SlideSearched for “2000 event”tsunami deposits in coastallakes without any findingsFaeroe Is.ShetlandSearched forTrænadjupet Slidetsunami depositswithout any findingsReferences:Bondevik, S. 2001/2002: Reports to Norsk Hydro onmapping and dating of tsunami depositsDenmarkBondevik, S., Svendsen, J.I., Johnsen, G.,Mangerud, J., & Kaland, P.E. 1997: The Storeggatsunami along the Norwegian coast, its age andrunup. Boreas 26, 29-53.IrelandUKDawson, A.G., Long, D., Smith, D.E., Shi, S., &Foster, I.D.L. 1993: Tsunamis in the Norwegian Seaand North Sea caused by the Storegga submarinelandslides. In Tinti, S. (ed.): Tsunamis in the world,228. Kluwer Academic Publishers, Netherlands.Grauert, M., Björck, S., & Bondevik, S. 2001:Storegga tsunami deposits in a coastal lake onSuderøy, the Faroe Islands. Boreas 30, 263-271.

Slide headwall shapes and run out

Continental margin slope failuresDown slope transport of sediments: slides - debris flows -turbidites• Down slope transport of sedimentsMass wasting:headwall slidesdebris flowsturbiditesChannels:Canyons:• Along slope transport of sedimentsSediment drifts - Contourites

Slide headwalls StoreggaWhat do we observe?Glide planes of Storegga SlidesShelfWhat do we model?Slide mechanisms

Slide headwalls ArcticShelf

Down slope transport of sediments: slides - debris flows -turbidites• Down slope transport of sedimentsMass wasting:slidesdebris flowsturbiditesChannels:Canyons:• Along slope transport of sedimentsSediment drifts - Contourites

Rotational slumps/faults at Storneset1 2 31H=140 m23

Glide planes in the Storegga SlideShaded relief of the seabedExposed andpolished glide planINO3TNRTNS

Glide planes and sediment typesThe glide planes consist of marine clay– The marine clays are homogenous and have a large lateralcontinuation– Characteristics of the marine clays in the area are• the clays are weaker than the glacial deposits (lowerinternal friction angle)• more compressible and produces more excess waterduring compression

Continental margin slope failuresDown slope transport of sediments: slides - debris flows -turbidites• Down slope transport of sedimentsMass wasting:slidesdebris flowsturbiditesChannels:Canyons:• Along slope transport of sedimentsSediment drifts - Contourites

Debris flow

Submarine slides and sea level change

Downslopesedimenttransport•TMF• Slides• Debris flows•Channels

Bear Island Trough Mouth Fan

Sidescan Sonar and Seismic Images

I present the seafloor morphologyof the Storegga Slide area startingat the lower slope moving upthrough the slide scar to theheadwall

Continental margin slope failuresDown slope transport of sediments: slides - debris flows -turbidites• Down slope transport of sedimentsMass wasting:slidesdebris flowsturbiditesChannels:Canyons:• Along slope transport of sedimentsSediment drifts - Contourites

Turbidites

Solsikke – Seabed topographySlide depositsThe shape of thebackwall is controlledby deeper polygonalfaultsSolsikkeDebris flows

Ormen Lange - Seabed TopographyStoregga, 8200 yBP

Key questions for the understanding of slopefailure mechanisms• Does the initial slide start in the upper orlower part of the slope?• Which triggers are relevant for slide initiationin the upper or the lower slope?

Age of major submarine landslides4500 yrs BP8200 yrs BP

Slope angles - Section B

Why is the continental slope angle so similaron passive continental margins of theOcean?The role of oceandynamics in shapingContinental margins:• internal waves• contour currents• ocean productivity

Summary•Geological, morphological and geotechnical understandings of thearea is the key for understanding the slide occurrences and slidedevelopments.•Slides can have developed from the distal part and failed along thesediment boundaries. Slides stepped up to shallow levels until the resistancein terms of thickness and degree of consolidation became too large for afailure.•Slides can have developed in the gas hydrate dissociation areafrom the proximal part and failed then afterwards with or withoutretrogressive sliding.•Changes in sea level and climatic driven processes from low to highlatitudes are important for a regional or global assessment of downslopeand alongslope transport of sediments.

Mienert, J. & Weaver, P.P.W., eds., European Margins Sediment dynamics, Springer Verlag,New York, 2003.

Mienert, J. & Weaver, P.P.W., eds., European Margins Sediment dynamics, Springer Verlag,New York, 2003.

Ice sheets and submarine canyonson high-latitude continental margins

Ice margin 18 000 BPGrand BanksSource: D. Piper

ScotianSlopeIce margin 18 000 BP

Scotian Slope multibeamDownloaded fromwww.cctechnol.comSource: D. Piper

Scotian Slope: Details West of the Logan CanyonSource: D. Piper

From Pickrill et al., 2001. OTC paper 12995Lower slopeLogan CanyonthalwegOld failure scar5 km

Head of Logan CanyonFrom Pickrill et al., 2001. OTC paper 12995

Upper and middle slope canyonseast of Logan CanyonFrom Pickrill et al., 2001. OTC paper 12995

3D seismic seabed rendercourtesy of EnCanaSource: D. Piper

Single channel airgun profile over failure (source: D. piper)

Slope canyon incisions and sediment failures

Slope canyon incisions and sediment failurein Andøya canyon

5. Diagenesis of deep-sea sediments

Diagenesisof sediments after burialBiogenic sediments:Carbonate oozeSiliceous oozeTerrigenous sediments:SandsClayAll the sediments areinfluenced byburial with age,compaction,temperature,pore water chemistry,.......How many years does it take to develop from a carbonate ooze to limestone?

Diagenesisof sediments after burialSilica diagenesisBiogenic Opal ADifferential solutionof skeletonsDissolved Si(OH)4carbonate diagenesisBiogenic carbonatesCompactionDissolution of fossilsPrecipitation of opal CTcementing poresOpal CTOpal-CT/quartz transformationwith increasing burial depthQuartzChalkPrecipitation of calcite,Development of interstial cementLimestone

Diagenesisof sediments after burialCompaction increases with depthBiogenic sediments:Carbonate oozeAll sediments areinfluenced byburial with age,compaction,temperature,pore water chemistry,.......

Diagenetic potential(Schlanger & Douglas, 1974):Depth and time of a diagenetic pathwaybefore the sediment reaches the finaldiagenetic aggregate condition.

Diagenesisof sediments after burialBiogenic sediments:Carbonate oozeSiliceous oozePore water chemistry changes with depthTerrigenous sediments:SandsClayAll sediments areinfluenced byburial with age,compaction,temperature,pore water chemistry,.......

How many years does it taketo develop from a carbonate ooze to limestone?Compaction of calcareous ooze:0 - 70 mbsf 0 - 5 myBreakup and dissolution of foraminifera: 70 – 200 mbsf 5 - 13 myChalk build up: 200 – 400 mbsf 13 - 23 myContinuing cementation: 400 – 800 mbsf 23 - 60 myLimestone: 800 – 1400 mbsf 60 – 135 myDissolution and compaction of siliceous ooze:Opal-A (diatomite)Opal-CT (Porcellanite)Quartz (Chert)