The role of nutrient availability in bioerosion: Consequences to carbonate buildups

Zooxanthellate organisms, which are among the major carbonate producers on coral reefs, are highly adapted to nutrient-deficient conditions and tend to be outcompeted by filamentous or fleshy algae if nutrients are abundant. Reef-dwelling bioeroding organisms, on the other hand, seem to increase in abundance with increasing availability of nutrient and food resources. Maximum rates of calcium carbonate production in a reef system are comparable in magnitude to maximum rates of bioerosion. The dynamic interplay between accretion destruction of coral reefs is therefore likely to be strongly influenced by nutrient availability.

Nutrient availability may also influence development of carbonate facies. In reef systems that develop in oligotrophic waters, skeletal sands and framework should dominate carbonate buildups. In systems that develop in mesotrophic waters, enhanced bioerosion should reduce much of the skeletal material to carbonate muds, increasing the prevalence of mud-rich facies. Eutrophic conditions should result in bioeroded hardgrounds and carbonate depositional hiatuses.

When carbonate accumulation rates fail to equal or exceed rates of local relative sea-level rise, reefs drown. Bored and lithified pavements with crusts of ferromanganese oxides, phosphates, or glauconite often separate shallow-reef from overlying deeper-water deposits. Nondeposition, bioerosion, and evidence for low redox potentials at the sediment-water interface all implicate excess nutrient availability as a factor in reef drowning.

Some carbonate depositional hiatuses and bioerosional surfaces are local phenomena, evidence of responses to local increases in nutrient availability resulting from submergence of soils, runoff, or topographic upwelling. Others appear to reflect regional or worldwide events. Because many carbonate-producing organisms have life history strategies specialized to nutrient-deficient conditions, they are unable to compete when nutrients become plentiful. Events that triggered ocean turnover or sharp increases in the rate of deep ocean circulation would have increased sea-surface nutrient availability worldwide. Such eutrophication could have caused widespread extinctions in reef communities. Because eutrophication promotes bioerosion, details of the fossil records of those events are probably lost in bioerosional hiatuses.