How old are oceanic sediments?

• Can be millions of years old - until the sediment is lithified
• However, usually the time resolution is not so good, cannot study fine-scaled events in time
  • Resolvable detail in lakes is around 10-100 years
  • Shallow seas or coasts, 100-1000 years
  • Deep ocean, 1000-5000 years resolution - 1cm deposited per 1000 y
• Mixing (bioturbation, currents) occurs in the top layers of lake and marine sediments, but the rate is much higher in coasts and shallow seas due to currents and animals

Formation of ocean archives

• Typically the shallowest oceanic sediments are at the new seafloor formed near mid-oceanic ridges (divergent plate boundaries), then the deepest and oldest sediments are closer to the convergent (subduction) zones
• Different types of sediments occur in different marine depositional environments:
  !Lecture_Ocean archives, p.13

Composition and sources of marine archives

Composition

Lithogenous fraction

(From rocks)

• Fluvial (stream)
  • This is the biggest source of oceanic sediments, especially on the coast, and is responsible for most sedimen thickness in these areas
  • The Ganges, Yangtze and Yellow rriver systems contribute a ton
• Eolian (wind blown)
  • Dust and smoke, especially from deserts
  • Crazy image of a dust storm off of Australia:
• Ice-rafted sediment
  • This is affected by the climate and its rates of deposition are associated with climate warming
  • You can tell from the shape of the grains and also its chemical composition (can tell you the origin and allow reconstructing the path it took)
• Volcanic (kind of the same as Eolian)

Biogenous fraction

(from organisms)
Planktonic and Planktic are both accepted terms for biogenic sedment composed from remains of plankton

Calcareous (CaCO3)

• Mostly from plankton like foraminifera (a microzooplankton) and coccolithophores
• Locally from coral, bryozoans, molluscs
• Can be affected by local patterns of acidification
• CaCO3 is more soluble in lower temperature, higher pressure and CO2 rich waters, where it will not be deposited as much and can't really be studied.

!Lecture_Ocean archives, p.27

Siliceous (diatoms)

• Radiolaria and diatoms, common in high-productivity regions
• So these kinds of sedimens are most often found in regions wih very high productivity
• Somehow diatoms off the coast of Greenland aren't really preserved in the sediment, we don't really know why

Organic material

• Can come from both marine and terrestrial sources of fixed carbon
• Most oceanic produced fixed carbon (from phytoplankton) is consumed at the surface so not much of it reaches the sea floor, even though hald of global primary production occurs in the seas
• Relatively minor component (0.1 - 2%), but since there is so much marine sediment it still functions as a major carbon sink because of how big the ocean is
  • In lakes, 2-70% of the material is organic (measured by Loss-on-ignition)

Coastal blue carbon

• There is also some carbon deposited in coastal areas from seagrass meadows, mangrove forests, kelp forests, etc. This is referred to as "coastal blue carbon". 45% of total oceanic burial comes from vegetated habitats.

Organic walled microfossils

• Some tiny plankton spores like dinoflagellates can create organic shells that are very resistant to degradation
• Fungal spores and pollen are sometimes also transported to oceans and preserved
• In areas where calcium and silicon based organisms dissolve, these can be used instead
• Do these get fossilized (lithified) in old sediments?

Hydrogenous fraction

• Solidified chemically from seawater
• What exactly are thse?

Global distribution of sediment types

• Red clay - common around Africa from wind blown sand and dust from the Sahara

Distribution map

• Continental sediment of course most common on the continental shelves, near river deltas or ice areas (the Arctic)
• Radiolarian ooze is found close to the equator
• Diatom ooze is most common near Antarctica
• Calcareous ooze common elsewhere
• The deepest and least bioactive parts of the sea are mostly just abyssal clay
  This is a bit of a simplification, these are all found everywhere, but this is a description of where each one dominantes.

Processes influencing sediments after deposition

• Tidal and wave action can move sediments at the bottom of the sea
• Erosion due to slope morphology - these can cause currents on continental margins
• Bioturbation - benthic fauna mizing things around (nematodes)
  • This impacts carbon deposition
    On a longer timescale, sea level changes, tectonics, and burial also have some effects

Conclusion

• Marine sediments are powerful long term paleoarchives, but interpreting them requires consideration of lots of different factors like sedimentation rates and preservation processes.