Deep-Sea Biogenic Sediments

Calcareous Ooze

Biogenic calcareous ooze composed of precipitated CaCO 3 (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 + CO 3 -2 ↔ CaCO 3 1.Concentration of Ca and CO 3  is the saturation state of calcite (or aragonite) When  < 1, carbonate is undersaturated and will dissolve When  > 1, carbonate is supersaturated and will precipitate 2.Temperature  higher in warm water (shallow, tropical water)  higher at low pressure (shallow water) 3.Pressure

[Ca] is relatively constant because of calcium’s long residence time Carbonate concentration [CO 3 ] is most variable and is the main control on  Carbonate speciation is pH-sensitive CO 2 + H 2 O ↔ H 2 CO 3 H 2 CO 3 ↔ H + + HCO 3 - HCO 3 - ↔ H + + CO 3 -2 Adding CO 2 reduces pH and [CO 3 ]

 decreases with depth because of decreasing pH, decreasing T, increasing P Lysocline: depth at which calcite becomes undersaturated (  <1)  <1  >1 Calcite lysocline

 <1  >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: SiO 2 · nH 2 O Siliceous Ooze

Siliceous ooze – radiolarian chert A biogenic sedimentary rock formed from silica (SiO 2 ) skeletons of microscopic radiolarians (marine protists; Cambrian-Recent) Radiolarians

Diatoms Miocene, Gulf of California Siliceous ooze – diatomite A biogenic sedimentary rock formed from silica (SiO 2 ) skeletons of microscopic diatoms (marine/freshwater; Cretaceous-Recent)

Highly undersaturated in surface ocean (concentration <10  M) Still undersaturated in deep ocean but with substantial inter-ocean variation SiO 2 (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/m 2 /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)