Organisms in Carbonate Precipitation

Although carbonate precipitation is fundamentally a chemical process, it is aided in a variety of ways by organisms. Purely inorganic precipitation of calcium carbonate from normal-salinity seawater or freshwater can occur, but it may be less common today than precipitation aided in some way by organic processes. ^[1] Geologic evidence suggests that organisms have played a significant role in carbonate sedimentation throughout most of Phanerozoic time, and some organisms (such as bacteria) may also have mediated carbonate precipitation during Precambrian time. ^[1] The organic pathways fall into five broad categories, each with a distinct immediate effect and ultimate carbonate product.

Direct Skeletal Extraction

The most important organic role is the direct removal of dissolved carbonate constituents from water to build skeletal structures — shells, tests, and internal stiffeners. ^[1] Marine invertebrates range from freely drifting planktonic species such as foraminifers and pteropods to bottom-dwelling benthonic organisms such as calcareous algae, corals, molluscs, and echinoderms, and they can remove CaCO₃ not only from calcium-carbonate-saturated tropical surface waters but also from less saturated waters in temperate and colder regions. ^[1] Shell sands and gravels are important deposits of the modern seafloor in shallow, cool water at high latitudes. ^[1]

On death, skeletal organisms release carbonate particles of varying sizes. Large shells fragment into sand-size or smaller pieces through biogenic activity or physical breakage; foraminifers and pteropod tests are sand-size; coccolith nannofossils are fine silt-size. ^[1] Some red and green calcareous algae — species of Halimeda, Penicillis, and Udotea — have skeletal elements made of tiny needlelike aragonite crystals deposited within intercellular spaces as stiffeners for soft tissue; when these organisms die, decomposition releases the crystals, producing a fine lime mud of elongated aragonite crystals 3–10 mm long and very small (<1 mm) equant crystals. ^[1]

Photosynthesis

Aquatic photosynthesizing plants — blue-green algae (cyanobacteria), photosynthesizing bacteria, and small phytoplankton such as diatoms, dinoflagellates, and coccoliths — remove CO₂ from water, which increases pH and promotes carbonate precipitation. ^[1] The photosynthesis reaction — 6H₂O + 6CO₂ → C₆H₁₂O₆ + 6O₂ — consumes CO₂, and because photosynthetic activity peaks in sunlight and falls to a minimum in the dark, the CO₂ content of water can vary measurably between day and night in productive areas. ^[1]

Bacterial Mediation

Bacteria may promote carbonate precipitation both directly and indirectly. Some marine peloids appear to have originated as fine-grained precipitates of high-magnesian calcite within and around active bacterial clumps, and bacteria can promote CaCO₃ precipitation on dead cyanobacteria, lithifying microbial mats into stromatolites. ^[1] The mechanism involves calcification just outside cell walls in an alkaline microenvironment generated when Ca²⁺ is exported from the cell in exchange for uptake of 2H⁺, with calcification resulting from uptake of CO₂ (microalgae) or HCO₃^- (cyanobacteria). ^[1]

Decay of Dead Organisms

Decay of dead organisms has a complex and bidirectional effect on pH. Decay releases organic acids and CO₂, which increases acidity; but some decay products are alkaline, and alkalinity may be increased by organic matter degradation through sulfate reduction by bacteria. ^[1] An increase in alkalinity favors CaCO₃ precipitation. ^[1]

Feeding and Pellet Generation

Sea cucumbers, mollusks, and worms ingest calcium carbonate muds to extract nutrients and extrude the remains as pellets; this process does not generate new carbonate sediment but reshapes existing carbonate mud into peloids that then enter the sediment record in a new form. ^[1]

Whitings: Inorganic or Organic?

The origin of large volumes of nonfossiliferous carbonate mud (micrite) in the stratigraphic record is contested. Whitings — milky patches of surface and near-surface water caused by dense concentrations of suspended aragonite crystals, seen in warm-water areas such as the Bahamas, the Persian Gulf, and the Dead Sea — have been invoked as evidence for either spontaneous inorganic nucleation of aragonite or for resuspension of seafloor mud by waves, tidal flow, or bottom-feeding fish. ^[1] Isotopic evidence suggests that whitings are probably not produced by resuspension, and the weight of opinion is shifting toward microbially mediated precipitation by photosynthesizing microalgae or cyanobacteria as the likely origin. ^[1]

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