Deep-Sea Biogenic Sediments
Calcareous Ooze
Biogenic calcareous ooze composed of precipitated CaCO3 (usually calcite,
but occasionally aragonite) shells of microscopic plankton
Dominant calcifying plankton evolved in Mesozoic
(coccolithophores in Triassic, forams in Jurassic) so
deep-sea carbonate likely rare prior to that time
Planktonic foraminifera
Coccolithophore
Chalk – carbonate rock made nearly entirely from
coccolithophores (calcareous nannofossils)
What controls accumulation of calcareous ooze in the ocean?
What factors promote accumulation of carbonate sediment?
1. Concentration of Ca and CO3
Ca+2 + CO3-2 ↔ CaCO3
Ca+2 CO3 −2
Ω=
K sp
W is the saturation state of calcite (or aragonite)
When W < 1, carbonate is undersaturated and will dissolve
When W > 1, carbonate is supersaturated and will precipitate
2. Temperature
W higher in warm water (shallow, tropical water)
3. Pressure
W higher at low pressure (shallow water)
[Ca] is relatively constant because of calcium’s long residence time
Carbonate concentration [CO3] is most variable and is the main control on W
Carbonate speciation is pH-sensitive
CO2 + H2O ↔ H2CO3
H2CO3 ↔ H+ + HCO3HCO3- ↔ H+ + CO3-2
Adding CO2 reduces pH and [CO3]
W decreases with depth because of decreasing pH, decreasing T, increasing P
Lysocline: depth at which calcite becomes undersaturated (W<1)
W>1
W<1
Calcite lysocline
But carbonate sediments can still accumulate below lysocline as long as
sediment supply exceeds the rate of dissolution
W>1
W<1
Calcite compensation depth (CCD)
Depth where dissolution rate exceeds sedimentation rate and no calcite is
preserved in sediment (like a snow line)
Typically 3-5 km depth today
There is also an “aragonite compensation depth” which is slightly
shallower (because aragonite is more soluble than calcite)
Siliceous Ooze
Skeletons constructed from opal-A,
amorphous hydrous silica: SiO2 · nH2O
Diatoms
Radiolarians
Siliceous ooze – radiolarian chert
A biogenic sedimentary rock formed from silica (SiO2) skeletons of microscopic
radiolarians (marine protists; Cambrian-Recent)
Radiolarians
Siliceous ooze – diatomite
A biogenic sedimentary rock formed from silica (SiO2) skeletons of microscopic
diatoms (marine/freshwater; Cretaceous-Recent)
Diatoms
Miocene, Gulf of California
Highly undersaturated in surface ocean
(concentration <10 mM)
Still undersaturated in deep ocean but
with substantial inter-ocean variation
SiO2(OH)2-2, SiO(OH)3-, Si(OH)4
1. Preserved where silica flux (sedimentation rate) is high so that silica supply
exceeds silica dissolution
2. Siliceous microfossils covered by organic coating, protecting their frustules
from dissolution even after death
Biogenic silica production (g/m2/yr)
Abyssal Red Clay
Primary source: windblown dust
Deep-sea clay deposited everywhere but dominates wherever carbonate or
siliceous sedimentation rates are low (below CCD, mid-latitudes)