Holocene and Pleistocene calcareous red algae from various sites in the Caribbean show a wide spectrum of diagenetic modification. Processes of diagenesis begin while the organism is still living and proceed in numerous post-mortem pathways depending on diagenetic environment (i.e., hydrogeochemistry), residence time, and skeletal architecture. Of significance is porosity development and magnesium addition within the hypothallus region, with one result being selective dolomitization.

Holocene/Recent samples were collected as living specimens, seafloor sediments, and unconsolidated sediments from different diagenetic environments. Samples derive from the Caicos platform, northeastern Caribbean, and the San Blas Islands of Panama. Pleistocene red algae samples come from outcrops on the Caicos platform and from core samples from the subsurface of Barbados, West Indies.

Living specimens of calcareous red algae show intrazooecial cementation by high-Mg calcite (HMC), with most cement concentrated in the dense perithallus region. Electron probe microanalysis (EPMA) of these cements show comparable percentages of MgCO3 to that in the skeletal substrate (15-25 mol% MgCO3). Post-mortem marine sediment samples typically show greater addition of HMC cement in both the hypothallus and perithallus regions, with slightly lower Mg compared to the living specimens. The perithallus region is also commonly intensely bored by algae and fungi.

Freshwater lens diagenesis of Holocene red algal specimens includes incongruent dissolution of the skeletons and HMC cements, precipitation of low-Mg calcite (LMC), and/or dissolution of the hypothallus. In low-flow freshwater lenses and meteoric vadose zones, some hypothallus skeletal calcite has as high as 40 mol% MgCO3, indicating Mg addition likely resulting from Mg cannibalization. While there is no evidence for dolomite in these samples, this very high HMC is a probable dolomite precursor. Pleistocene red algae from Barbados mixing zones shows preferentially partially dolomitized hypothallus regions with increased porosity. Thus, dissolution-reprecipitation of the less dense hypothallus HMC in red algae is an important step in early dolomitization.