Present-day production of CaCO3 in tne world ocean is calculated to be about 5 billion tons (bt) per year, of which about 3 bt accumulate in sediments; the other 40% is dissolved. Nearly half of the carbonate sediment accumulates on reefs, banks, and tropical shelves, and consists largely of metastable aragonite and magnesian calcite. Deep-sea carbonates, predominantly calcitic coccoliths and planktonic foraminifera, have orders of magnitude lower productivity and accumulation rates than shallow-water carbonates, but they cover orders of magnitude larger basin area. Twice as much calcium is removed from the oceans by present-day carbonate accumulation as is estimated to be brought in by rivers and hydrothermal activity (1.6 bt), suggesting that outputs have been overestimated or inputs underestimated, that one or more other inputs have not been identified, and/or that the oceans are not presently in steady state. One “missing” calcium source might be groundwater, although its present-day input is probably much smaller than that of rivers. If, as seems likely, CaCO3 accumulation presently exceeds terrestial and hydrothermal input, this imbalance presumably is offset by decreased accumulation and increased input during lowered sea level: shallow-water accumulation decreases by an order of magnitude with a 100 m drop in sea level, while groundwater influx increases because of heightened piezometric head and the diagenesis of metastable aragonite and magnesian calcite from subaerially exposed shallow-water carbonates.

Production and accumulation of calcium carbonate in the ocean: Budget of a nonsteady state

Stable carbon and oxygen isotopes (δ18O and δ13C values) and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years. As valuable as these insights have been, many problems regarding the interpretation of geochemical signals within mature rocks remain. For example, while the δ18O values of carbonate rocks are dependent both upon the temperature and the δ18O value of the fluid, and additional information including trace element composition aids in interpreting such signals, direct evidence of either the temperature or the composition of the fluids is required. Such information can be obtained by analysing the δ18O value of any fluid inclusions or by measuring the temperature using a method such as the ‘clumped’ isotope technique. Such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well-known tendency for δ18O values of carbonate rocks to decrease with increasing age. Unlike the δ18O, δ13C values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time. However, many studies have not sufficiently emphasized the effects of diagenesis and other post-depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored. Finally, many additional proxies are poised to contribute to the interpretation of carbonate diagenesis. Although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods, care should be taken to understand both old and new proxies before applying them to the ancient record.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/sed.12205

The geochemistry of carbonate diagenesis: The past, present and future

Photosynthetic uptake of inorganic carbon can raise the pH adjacent to cyanobacterial cells, promoting CaCO 3 precipitation. This effect is enhanced by CO 2 concentrating mechanisms that actively transport into cells for carbon fixation. CO 2 concentrating mechanisms presumably developed in response to atmospheric decrease in CO 2 and increase in O 2 over geological timescales. In present-day cyanobacteria, CO 2 concentrating mechanisms are induced when the atmospheric partial pressure of CO 2 (p CO2 ) falls below ∼ 0.4%. Reduction in p CO2 during the Proterozoic may have had two successive effects on cyanobacterial calcification. First, fall in p CO2 below ∼ 1% (33 times present atmospheric level, PAL) resulted in lower dissolved inorganic carbon (DIC) concentrations that reduced pH buffering sufficiently for isolated CaCO 3 crystals to begin to nucleate adjacent to cyanobacterial cells. As a result, blooms of planktic cyanobacteria induced precipitated ‘whitings’ of carbonate mud in the water column whose sedimentary accumulation began to dominate carbonate platforms ∼ 1400‐ 1300 Ma. Second, fall in p CO2 below ∼ 0.4% (10 PAL) induced CO 2 -concentrating mechanisms that further increased pH rise adjacent to cells and promoted in vivo cyanobacterial sheath calcification. Crossing of this second threshold is indicated in the fossil record by the appearance of Girvanella 750‐700 Ma. Coeval acquisition of CO 2 concentrating mechanisms by planktic cyanobacteria further stimulated whiting production. These inferences, that p CO2 fell below ∼ 1% ∼ 1400‐1300 Ma and below ~0.4% 750‐700 Ma, are consistent with empirical and modelled palaeo-atmosphere estimates. Development of CO 2 concentrating mechanisms was probably temporarily slowed by global cooling ∼ 700‐570 Ma that favoured diffusive entry of CO 2 into cells. Lower levels of temperature and DIC at this time would have reduced seawater carbonate saturation state, also hindering cyanobacterial calcification. It is suggested that as Earth emerged from ‘Snowball’ glaciations in the late Neoproterozoic, global warming and O 2 rise reactivated the development of CO 2 concentrating mechanisms. At the same time, rising levels of temperature, calcium ions and DIC increased seawater carbonate saturation state, stimulating widespread cyanobacterial in vivo sheath calcification in the Early Cambrian. This biocalcification event promoted rapid widespread development of calcified cyanobacterial reefs and transformed benthic microbial carbonate fabrics.

/pdf/cyanobacterial-calcification-carbon-dioxide-concentrating-ih4yz9kdhd.pdf

Cyanobacterial calcification, carbon dioxide concentrating mechanisms, and Proterozoic–Cambrian changes in atmospheric composition

Many guyots in the north Pacific are built of drowned Cretaceous shallow-water carbonates that rest on edifice basalt. Dating of these limestones, using strontium- and carbon-isotope stratigraphy, illustrates a number of events in the evolution of these carbonate platforms: local deposition of marine black shales during the early Aptian oceanic anoxic event; synchronous development of oolitic deposits during the Aptian; and drowning at different times during the Cretaceous (and Tertiary). Dating the youngest levels of these platform carbonate shows that the shallow-water systems drowned sequentially in the order in which plate-tectonic movement transported them into low latitudes south of the Equator (paleolati- tude 0°-10° south). The chemistry of peri-equatorial waters, rich in upwelled nutrients and carbon dioxide, may have been a contributory factor to the suppression of carbonate precipitation on these platforms. However, oceanic anoxic events, thought to reflect high nutrient availability and increased produc- tivity of planktonic organic-walled and siliceous microfossils, did not occasion platform drowning. Neither is there any evidence that relative sealevel changes were the primary cause of platform drowning, which is consistent with the established resilience of shallow-water carbonate systems when influenced by such phenomena. Comparisons with paleotemperature data show that platform drowning took place closer to the Equator during cooler intervals, such as the early Albian and Maastrichtian, and farther south of the Equator during warmer periods such as Albian-Cenomanian boundary time and the mid-Eocene. Initiation of one carbon- ate platform relatively close to the Equator, at paleolatitudes more northerly than those where others drowned, took place during the cool early mid-Aptian. These correlations are in accord with an interpretation that excess warmth in shallow peri-equatorial waters proved inimical for many carbonate-secreting organisms living on the platforms, allowing subsidence or eustatic sealevel rise to outpace sedimentation and guyots to form. A parallel may be drawn with the recent phenomenon of coral and foraminiferal bleaching, whereby photosynthetic sym- bionts succumb to prolonged high temperatures (G30°C) and the host organism dies. The fact that most Cretaceous guyots reside in the north Pacific may not be solely related to the age-distribution pattern of ocean floor but to their having run the gauntlet of a difficult and dangerous passage across the Equator. North Pacific guyots are relics from the Cretaceous (and Eocene) ''greenhouse'' Earth.

/pdf/stratigraphy-paleoceanography-and-evolution-of-cretaceous-2yajp5wznf.pdf

Stratigraphy, paleoceanography, and evolution of Cretaceous Pacific guyots; relics from a greenhouse Earth

U and Th concentrations and isotope ratios in modern carbonates and waters from the Bahamas

Sr concentrations in aragonitic muds on the banktop and leeward slope of Great Bahama Bank (GBB) are greater than in the needles produced by codiacean green algae but the same as in precipitated sand-size ooids and other nonskeletal grains. Results of two-part and three-part mixing models suggest that chemically precipitated aragonite contributes an average of 55-78% of the clay-size aragonite; individual samples contain as much as 95% chemically precipitated aragonite. Nonskeletal aragonitic needles apparently are precipitated ubiquitously over GBB but accumulate on the banktop only leeward of Andros Island. Much of the mud produced on the bank is exported to deeper waters: the thickness of mud-dominated Holocene sediments on the leeward slope can reach 90 m, and the aragonitic lysocline around the Bahamas is depressed by nearly 1500 m.

Great Bahama Bank Aragonitic Muds: Mostly Inorganically Precipitated, Mostly Exported

https://earth.ox.ac.uk/~gideonh/pdffiles/holocene.pdf

Glacial-to-Holocene sedimentation on the western slope of Great Bahama Bank

Carbonate-periplatforrn sedimentation by density flows: A mechanism for rapid off-bank and vertical transport of shallow-water fines: Comment and Reply

R. Jude Wilber

Papers

Great Bahama Bank Aragonitic Muds: Mostly Inorganically Precipitated, Mostly Exported

Glacial-to-Holocene sedimentation on the western slope of Great Bahama Bank

Carbonate-periplatforrn sedimentation by density flows: A mechanism for rapid off-bank and vertical transport of shallow-water fines: Comment and Reply