The natural context of changes in the Great Barrier Reef - The Great Barrier Reef: An Environmental History - page 35

The modern coral reefs of the Great Barrier Reef evolved during the Holocene:
an interglacial epoch of dramatic changes in sea level and in the position of
the eastern Australian coastline. The north-eastern Australian continental shelf
experienced a rise in sea level of over 100 metres to around its present level, as
a result of glacial ice sheet melting and the isostatic response of the continental
shelf to water load, although the precise details of sea level history in the region are
complex and still debated (Hopley et al., 2007). As sea level rose during the early
Holocene, the north-eastern Australian coastline migrated laterally to its present
position as the continental shelf was inundated. The modern Great Barrier Reef
evolved on the newly-formed continental shelf, but in varying geomorphological
conditions that controlled the rate and location of coral reef development. In
particular, variations in the dominant geomorphological controls on the growth
of coral reefs - sea level and sedimentation - have created, in different places and
at different times, both favourable and hostile conditions for reef development.
As a result of these variations, over around 6,000 years, some reefs have been
brought close to thresholds of decline and may have experienced deterioration
for geomorphological reasons. Conversely, other reefs - especially some offshore
reefs - have flourished throughout the Holocene since they lie outside the region
of particular vulnerability to geomorphologically-controlled decline.
Hopley (1994) has argued that, as sea level rose along the eastern Australian
coast during the early Holocene, the corals of the Great Barrier Reef were able
to grow upwards at a similar rate, forming a barrier within which further reef
development was possible. However, during the early Holocene period of steady
sea level rise, the position of the shoreline migrated westwards, resulting in the
displacement of the Holocene sediment deposition zone across the shelf, with
the result that sediment did not accumulate to any great depth in any particular
area. However, once modern sea level was reached, around 6,000 years ago,
the pattern of sediment discharge from the mainland to the Great Barrier Reef
lagoon - which was dynamic during the period when the coastline was migrating
- subsequently stabilised. The result was that the zone of terrestrial influence
became static in its present position and sedimentation became concentrated in
the newly-formed nearshore zone. This led to the formation of an inshore mud-
silt wedge in the Great Barrier Reef lagoon, composed of sediments up to 15
metres in depth, as demonstrated by seismic surveys (Hopley, 1994, pp318-19;
Hopley et al., 2007).
The stability of sea level and sedimentation patterns after around 6,000
years ago initiated considerable changes in the reefs of the Great Barrier
Reef. Geomorphological descriptions of the Great Barrier Reef are based on
a classification of different reef types according to their morphology and stage
of development, and various classification schemes exist (Hopley et al., 1989).
Although some reef types, such as ribbon reefs and fringing reefs, do not fit
strictly within a developmental framework, in geomorphological terms most of
the reefs in the Great Barrier Reef can be classified as juvenile, mature or senile.
Hopley (1994, pp325-6) has argued that the progression of coral reefs from the