SECT. 3] SHALLOW-WATER CARBONATE SEDIMENTS 577 



aragonite in beach sands to biochemical precipitation by microorganisms. It is 

 certain that microorganisms are present within beach sands and within com- 

 posite grains or himps below sea-level. However, it seems likely that the 

 abmidance and rate of metabolism of these microorganisms are much less in 

 these sand-sized sediments than in finer-grained sediments rich in organic 

 matter. It therefore seems most likely that the fibrous aragonite is a physico- 

 chemical precipitate. 



We do not know precisely the amount of fibrous aragonite cement in the 

 submarine sediments of the Bahama Banks. However, it seems probable that it 

 occurs wherever there are composite grains (see Illing, op. cit., pp. 78-79; 

 Newell, Purdy and Imbrie, 1960) or numerous chambered forms. 



C. Aragonite Needles 



The acicular crystals of aragonite in the sediments west of Andros Island in 

 the Bahamas have long been considered a non-skeletal precipitate. This con- 

 clusion was based on the apparent similarity of these euhedral crystals and those 

 produced by artificial precipitation and on the unusual hydrographic conditions 

 associated with the Bahamian occurrence. Smith (1940) and Cloud (1955) have 

 shown that both the water conditions and the changes in the concentration of 

 the dissolved calcium carbonate suggest that precipitation does occur in the 

 area west of Andros Island. This conclusion implies that restriction of water 

 movement with attendant increase of temperature and salinity favors pre- 

 cipitation. 



Early workers suggested a microbiological origin for these aragonite needles 

 (Drew, 1914; Bavendam, 1932). However, it appears that the conditions 

 necessary for bacterial precipitation of calcium carbonate are only fulfilled in 

 areas of unusually high organic matter (Black, 1933, p. 465). High concentra- 

 tions of organic matter occur in the mangrove tidal swamps of Andros Island, 

 and it has been supposed that aragonite precipitated there is swept westward 

 into the sea by storms {op. cit., p. 466). 



Recently, the non-skeletal origin of aragonite needles has been seriously 

 questioned by Lowenstam (1955) and by Lowenstam and Epstein (1957). 

 Lowenstam {op. cit., pp. 270-271) described acicular crystals of aragonite from 

 several species of green algae that are similar in size and shape to those found 

 in the sediments. He and one of the present authors have observed similar 

 aragonite needles in sediments of the Florida reef tract, an area where there is 

 no major hydrographic restriction that would suggest precij)itation. More- 

 over, these marine occurrences are far removed from tidal swamps, where 

 bacterial precipitation is believed to be most active. 



The observed similarity between sediment and algal needles (Lowenstam, 

 op. cit.; Fig. IC) and the presence of these algae in the area indicate that some 

 of the needles must be derived from the algae. Whether or not the algae can 

 produce all the sediment needles depends on the rate of production and the 

 area of supply as compared with the amount and age of the sediment needles. 



