--> Halimeda Revisited: Where Did All the Needles Go?, by R. P. Reid and I. G. MacIntyre; #90986 (1994).

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Abstract: Halimeda Revisited: Where Did All the Needles Go?

R. Pamela Reid, Ian G. MacIntyre

The green alga Halimeda is an important geologic and biologic component of shallow water tropical marine ecosystems and is one of the most extensively studied calcareous algae. The crystal structure and mineralogy of carbonate deposits in Halimeda have been described in numerous papers. The dominant crystal form is thought to be aragonite needles that are up to 10 microns long. These needles have been considered an important source of lime mud.

Our recent studies show that aragonite needles are, in fact, relatively rare in living plants of H. incrassata, one of the most common species of Halimeda in the western Atlantic. Needles in these plants alter to anhedral equant aragonite crystals about 0.2 microns in size soon after they are formed. Crystal alteration seems to occur as an advancing front that moves in from the growth surface and from the filament walls. As the front passes, the original open needle fabric of the crystals is lost and a dense micritic texture is formed. Initial examination of other species of Halimeda suggests that similar processes of needle alteration also occur in these algae.

The discovery that aragonite needles alter in living plants of Halimeda confounds our notion of diagenesis, as we generally assume that biogenic crystals are stable until the death of an organism. It also has significant implications for physiological studies of calcification, which attempt to account for the organization and development of carbonate deposits in algae.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994