Oceanic Sediments

Question 1

Advantages of Oceanic Evidence

Answer 1

• Long, continuous records of deposition.
• Contain multiple proxies
• Reflects global oceanic & temperature changes.

Question 2

Disadvantages of oceanic evidence

Answer 2

Some proxies lack a real representation.

- Low temporal resolution due to low sedimentation rates (1cm/1000yrs), and sediment mixing.

Question 3

How do we attain ocean sediment cores?

Answer 3

Ocean drilling cruises extract cores from the ocean bottom. 2m of core = 20,000 years.

Question 4

What are the types of biogenic ocean sediments?

Answer 4

Calcereous oozes (CaCO3) that are the remains of foraminifera.
Siliceous oozes (SiO2) that are remains of diatoms (algae) and radiolarians (protozoa). 
These live both at the ocean surface (planktonic) and in deep water (benthic). We expect planktonic organisms to be more responsive to climatic changes, e.g. temperature. Benthic records give a longer term global record.

Question 5

How to we infer ocean temperature from ocean sediments?

Answer 5

As some species have narrow temperature ranges, we can use a mutual climatic range approach to find the specific temperatures.
Neogloboquadrina Pachyderma live in polar water, Globorotalia Menardii loves warm water.

Question 6

What are Terrigenous Sediments?

Answer 6

These are terrestrial sediments bought by rivers into the world’s oceans, tending to be fine sediments. Coarser sediments can be introduced from ice sheets and ice bergs. Aeolian activity introduces dust to the oceans, and turbidities introduces coarse sediment to the deeper ocean.

Question 7

How do we determine age from ocean cores?

Answer 7

We use radiometric methods to determine the age of deposition. We can extract proxies from biogenic remains to compile the histories of proxy changes.

Question 8

How do biogenic sediments have oxygen isotopes within them?

Answer 8

Fractionation of forams mean they absorb oxygen-16 and -18, reflecting the ratio found In the ocean.
This ratio reflects the planet temperature at the time of the organisms life.

Question 9

What is fractionation?

Answer 9

This is the preferential evaporation of oxygen-16, leaving oxygen 19 in the ocean. For every 1 degree rise in water temp, there is a decrease in the foraminiferal o-18 ratio value relative to the ocean water value of 0.2%.

Question 10

How do ice sheets interrupt the fractionation cycle?

Answer 10

When ice sheets expand, evaporaton preferentially removes O-16 from the ocean, enriching the O-18 content. Water gets locked up in the ice sheets, the colder the climate the greater the O18/O16 ratio in the ocean is.

Question 11

Cesare Emiliani (1955)

Answer 11

Associated major changes in the amount of o-18 going down marine cores with the idea that there were past glaciations. Found a difference in O-18 between glacials and interglacials.

Question 12

Nick Shackleton (1967 & 77)

Answer 12

Realised forams in ocean sediments are benthic and not strongly affected by sea surface temp change.

Question 13

Reconstructing ocean temperatures with foram assemblages

Answer 13

We can use species assemblages to reconstruct SSTs by finding forams of the same age and looking at the diversity of species and their relative abundances.

Question 14

GLAMAP 2000 (Pflaumann et al., 2003)

Answer 14

This is a sea surface temp reconstruction for the Glacial North Atlantic Sea, using 947 cores for reconstruction with 26 species.

• Modern day reconstruction from tops of cores, assessing relationship with current SSTs, found good correlation between estimates and measurements.
• This enabled them to weork out SST for LGM using relative distribution and abundance of forams.
• Compared LGM with today.
• Found SW of UK has had the biggest temperature difference since LGM (due to gulf stream).

Question 15

Geochemical Proxies

Answer 15

There are a range of molecules found in plankton and diatoms that can be used as proxies of several quantities (SST from Alkenones, Sea ice).
Relative proportions of different molecules occur under regimes of different environmental conditions.

Question 16

Alkenones as a proxy for sea surface temperature

Answer 16

The ratio of Alkenones measure SST. These long chain hydrocarbons are produced by coccolithopores.

Question 17

Other oceanic sediment proxies:

Answer 17

• Volcanic Ash
• Ice-rafted debris
• Large scale signatures.

Question 18

Volcanic Ash as a proxy

Answer 18

Eruptions have a distinct geochemical signature. If we can date the eruption on land, the ocean ash layer with that signature gives a dating horizon, which can be vital in areas with little carbonate signatures (E.g. North Pacific).
Ramsey et al., (2015) produced a map recording eruptions from N. Atlantic ash records up to 12,500 ya.

Question 19

Ice rafted debris as a proxy

Answer 19

Icebergs and sea ice drop sediment as they melt. This sediment is usually more coarse than oceanic sediment.
Their mineralogy and geochemistry can be used to suggest the origin of the debris, and we can infer ocean currents at the time of deposition.

Question 20

Large Scale Oceanic Signatures

Answer 20

• Icebergs in contact with sea floor leave scour marks. Depth of these can indicate past ocean currents. (Found in Hudson Strait)
• Turbidites are features where sediment fines out with distance from source. This can provide a record of river deltas, submarine landslides or large terrestrial floods. (Found in bay of Biscay)