Primary Porosity in Modern Coral Patch Reefs

Marjorie Crane and C. D. Burke. Department of Geology, Wichita State University, Wichita, KS  67260-0027  [email protected]

Mexico Rocks Patch Reef Complex (MRPRC) is located 0.5 km east of Ambergris Caye, Belize C.A., on the northern outer shelf in 3 m water depth. The Montastrea species-complex consists of over 100 patch reefs that trend northeast atop a local Pleistocene topographic ridge. In 2000, living and dead reef samples were collected from 19 out of 23 continuously monitored patch reefs. Samples were processed to remove organic constituents, dried, and slabbed for primary porosity measurements.

Both surface-area porosity (2-D) and total-volume porosity (3-D) were calculated. Using calipers and polar planimeter, porosity surface area was determined for living (2.0%) and dead samples (3.7%). Volume measurements were determined by fluid displacement. Living samples averaged 45.3% porosity by volume; whereas, dead samples averaged 33.7% total volume. Results indicate that living samples are more porous than dead samples. However, only the outer rind of the living coral samples consisted of healthy coral; beneath the rind exists non-living carbonate. When the non-living carbonate is added to the category of dead samples, total porosity for non-living samples is 74.5%; whereas, living samples decreased to an average of 4.5% porosity. Thus, porosity increases significantly (p<0.05) after coral death, or where coral growth ceases. At MRPRC, the increase of primary porosity in coral skeletons is predominately the result of bioerosion by boring sponges and annelids. In addition, massive bleaching events (1995 and 1998) have reduced the amount of living coral (-94%) on the patch reefs. Based on data from this study, coral death would ultimately lead to an increase in primary porosity.