Deep-Sea Biogenic Sediments

Calcareous Ooze

Biogenic calcareous ooze composed of precipitated CaCO3 (usually calcite, but occasionally aragonite) shells of microscopic plankton

Planktonic foraminifera

Coccolithophore

Dominant calcifying plankton evolved in Mesozoic (coccolithophores in Triassic,

forams in Jurassic) so deep-sea carbonate likely rare prior to that time

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?

Ca+2 + CO3-2 ↔ CaCO3

1. Concentration of Ca and CO3

Ω=[Ca+2 ] [CO3−2 ]

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 precipitate2. Temperature

W higher in warm water (shallow, tropical water)W higher at low pressure (shallow water)

3. Pressure

[Ca] is relatively constant because of calcium’s long residence time

Carbonate concentration [CO3] is most variable and is the main control on WCarbonate speciation is pH-sensitive

CO2 + H2O ↔ H2CO3

H2CO3 ↔ H+ + HCO3-

HCO3- ↔ 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>1Calcite lysocline

W<1

W>1

But carbonate sediments can still accumulate below lysocline as long as sediment supply exceeds the rate of dissolution

Calcite compensation depth (CCD)

Depth where dissolution rate exceeds sedimentation rate and no calcite is preserved in sediment (like a snow line)

There is also an “aragonite compensation depth” which is slightly shallower (because aragonite is more soluble than calcite)

Typically 3-5 km depth today

Diatoms

Radiolarians

Skeletons constructed from opal-A, amorphous hydrous silica: SiO2 · nH2O

Siliceous Ooze

Siliceous ooze – radiolarian chert

A biogenic sedimentary rock formed from silica (SiO2) skeletons of microscopic radiolarians (marine protists; Cambrian-Recent)

Radiolarians

Diatoms

Miocene, Gulf of California

Siliceous ooze – diatomite

A biogenic sedimentary rock formed from silica (SiO2) skeletons of microscopic diatoms (marine/freshwater; Cretaceous-Recent)

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)