CORAL GROWTH VERSUS REEF ACCRETION: PROBLEMS OF SCALE, TAPHONOMY AND PERCEPTION

The recent decline in coral reefs begs for geologic context to separate “unprecedented” decline from natural but cyclic change. The issue is not whether anthropogenic stress bears heavily on the present problem, but rather how reefs have responded in the past to natural factors, and how we might use a better understanding of larger-scale processes to create better management and mitigation schemes under a regime of known human impact. Fundamental to this effort is an honest evaluation of how the things we observe on a small temporal or spatial scale (e.g., monitoring) compare to what we see in outcrop or cores. Coral growth decreases exponentially with depth and light. However, recently compiled data from coring investigations show that reef accretion does not change significantly with either water depth or dominant coral species within the upper 20-30 m of the water column. This apparent disconnect between coral growth and reef accretion argues for caution when comparing modern and ancient; reef accretion is not constrained solely by coral growth. While taphonomy can skew preservation away from branching or platy forms, time averaging can exaggerate the absolute importance of live corals in an assemblage preserved in core or outcrop. Equally important, spatial persistence need not infer temporal continuity. Because environments shift on any reef, changes in community structure within a few cores or across limited outcrop exposure need not reflect a significant change in community structure. At the same time, large time gaps may exist within continuous intervals of similar corals, especially when those species turn over rapidly. Methodological differences (e.g., quadrat vs. line transect vs chain transect) can produce significantly different results from the same modern reef. When combined with the realization that vertical outcrop transects (or cores) span time while modern measurements cross space, we are faced with some fundamental problems that must be overcome. While these might seem daunting reasons to abandon the whole exercise, such comparisons provide our only means of understanding reef development on time spans longer than a funding cycle or term limit. Realizing these pitfalls and quantifying their effects are of paramount importance if we are to bridge this critical gap between modern decline and past ecological change.