Announcements
• First problem set due next Tuesday.
• Review for first exam next Thursday.
• Quiz on Booth (1994) after break today.
Intertidal, Lowes Cove, ME
Marine Sediments: Clues to the ocean’s past
There is more to mud than meets the eye
Learning goals:
1: Identify different types of sediments based on properties and origin
2: Determine factors that control distributions of sediment types
3: Interpret oceans history using sediment tracers
550’ Casco Bay, ME
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Classifying sediment (geologist’s perspective)
Grain Size: environment, transport
Grain Shape: sphericity: transport history
Sorting:
Well sorted: transport by selective agents (currents, wind)
Poorly sorted: non-selective agents (landslides, glaciers)
(or multiple sediment sources)
Surface texture: frosted: aeolian transport (wind)
striated (lined): transport by glaciers
Color: chemical properties – oxidation (dark=reduced, organic, small grains)
Grain Size
Size classification
Boulders
Cobbles
Gravel
Sand
Diameter
> 25 cm
6 cm – 25 cm
2 mm – 6 cm
0.063 mm – 2 mm
Silt
Clay
4 m – 63 m
0.2 m – 4 m
Colloidal
< 0.2 m
Electrostatic
forces become
important
“Particles”
no longer sink
Mineral composition: indicates source of sediment
Fossils: Indicate sediment age and past ecological conditions
Classifying sediment (biologist’s perspective)
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Biogenous sediments - primarily pelagic
Classifying mud (Global perspective)
Major sediment classes based on sediment source:
terrigenous, biogenous, hydrogenous, cosmogenous
Oozes (> 30% biogenic material)
Calcareous ooze
Composed of the remains of Formaminifera,
coccolithophores, pteropods, other calcareous organisms
Terrigenous sediments - primarily coastal
Siliceous ooze
Composed of the remains of diatoms, radiolarians
Origin:
Weathering of continental crust:
Physical weathering → physical breakup of minerals into grains
Chemical weathering → dissolution of minerals, produces clays
Transport: Rivers, wind (pelagic clays), glaciers
Diatoms
(centric and pennate)
Foraminifera
Non-biogenic sediment of open ocean (deep, low productivity)
- Abyssal clay -
Radiolaria
Coccolithophore
Sediments of slope and continental rise:
Mixture of neritic (terrigenous) and pelagic sediments
Plate-shaped, often negatively charged.
Nearshore, generated by chemical weathering or glaciers,
transported by rivers.
Offshore, transported by wind
NASA
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Manganese nodules: hydrogenous sediment
Fe/Mn crusts found near ridges
Mn nodules found in areas of with low rates of sedimentation
Growth rate: <1 cm / 106 years
Contains: Mostly Mn and Fe, but also Co, Ni, Cu, Zn, Cr
Continental rise and abyssal plain turbidite deposits: Alternating layers
of coarse and fine-grained terrigenous & biogenous sediment
Large-scale sediment distributions:
Supply > dilution + dissolution
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Large-scale sediment distributions:
Supply > dilution + dissolution
Distribution of calcareous oozes = f(water depth)
Factors controlling CCD (zcc):
Calcium carbonate compensation depth (CCD):
Geochemical definition: The depth at which the rate of supply of CaCO3 =
the rate of dissolution at the sediment surface
Geological definition:
The depth where CaCO3 drops to 10-20% by mass
Increasing pressure and increasing CO2 at depth shifts equilibrium,
increasing H+ (lowering pH) and reducing CO32-.
Little calcareous
ooze below
~ 4,500 m
Depth (saturation or CCD)
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
Calcite (CaCO3) solubility
CaCO3 ↔ Ca2+ + CO32- Dependent upon pressure (depth) and pH
(pressure, flux of CaCO3, deep [CO3-2])
Zsat = saturation depth
Zcc = CCD
Blue line is CCD
Current conditions
(avg CCD ~ 4500 m)
Figure from Boudreau et al. 2010
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Rates of sediment accumulation
River deltas: 10 - 100 mm/year (or greater in some cases)
Nearshore ~ 1-2 mm/year (same rate as sea level rise)
But, this varies depending upon local uplift or subsidence
Deep sea, 1-10 mm/103 years
(Rule of thumb: < 1cm / 1000 y)
Question: How to manganese nodules stay on the surface?
Mn-nodule growth rate: 1-10 mm/ 106 years
(Rule of thumb: < 1 cm / 1,000,000 y)
Size-selective Feeding by Cirriformia grandis
70
0
Sediment
Worm Gut
60
50
Direction of
Particle Movement
5
Burrow
Construction
Feeding Mode of
Cirriformia grandis
40
30
20
10
10
0
0
50
100
Bead Size (m)
150
200
6
0
0
Relative Bead Concentration
0.05
0.1
0.15
0.2
5
0
0
Relative Bead Concentration
0.05
0.1
0.15
0.2
5
Small particles
are transported
to depth
10
15
10
15
Size: 16 - 32  m
20
0
Size: 32 - 64  m
20
0
0.1
0.2
0.3
0.4
0
5
5
10
10
15
15
Size: 64 - 128  m
20
0
0.1
0.2
0.3
0.4
0.5
Big particles stay
near the surface
Size: 128 - 256
m
20
Sediments and paleoceanography
Requirements:
Means of dating sediment
(radionuclide dating, paleomagnetism)
Tracers for past environmental conditions
Paleothermometers:
Oxygen isotopes ratios (18O) in carbonate shells of foraminifera or in
glacial ice: 18O:16O (Temperature + Global Ice Volume)
Relative abundance of different species of foraminifera (Temp)
Mg/Ca ratios in foraminifera shells (Temp)
Alkenones found in marine organic material (Temp)
If you want to look deep into the earth’s past (~100 MY)
Where would you collect sediment cores?
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Paleotemperature comparisons
Effect of temperature on Mg/Ca ratio in
foraminfera CaCO3
Species of
Globigerinoides
Winter
Summer
Fauna
Temperature (°C)
Elderfield and Ganssen, 2000, Nature
Elderfield and Ganssen, 2000, Nature
Summary
Temperature change in southern
ocean over 0.5 million years
Sediments are combinations of terrigenous particles, organism parts, authigenic
(hydrogenous) and cosmogenic particles
Sediment distributions result from differences in supply, dilution, and dissolution of
sediment from different sources
Nearshore sediments: terrestrial origin (terrigenous)
Mg/Ca temperature
δ18O
temperature
Continental rise: may be composed of turbidites (alternating layers of
coarser terrigenous sediments and finer terrigenous & biogenous sediment)
Open ocean:
Biogenous sediments:
Calcareous oozes widespread in areas shallower than CCD.
Siliceous oozes found only in productive waters deeper than the CCD.
Clays:
Elderfield et al. 2009
Found deeper than the CCD in areas of relatively low productivity,
and downwind of large deserts.
Sediments record ocean history, read by measuring tracers to determine the age
of each stratum and indicators of past environmental conditions.
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