Colonial coral resilience by decreasing size: reaction to increased detrital influx during onset of the late Palaeozoic Ice Age

Modern coral reefs and associated biodiversity are severely threatened by
increasing terrestrial runoff. Similar scenarios could be suspected for
geological times, but reef coral resilience is still an enigma. In late
Visean-Serpukhovian (Mississippian foraminiferal zones/MFZ 14-16) times, a
major glaciation phase of the late Palaeozoic Ice Age (LPIA) associated
with enhanced terrestrial weathering and runoff coincides with a
biodiversity crisis and coral reef decline. In this study, the impact of
enhanced terrestrial runoff is tested on size variations of colonial
corals Aulina rotiformis and Lithostrotion decipiens along a gradient of
contemporaneous (Serpukhovian) open marine carbonate to nearshore
siliciclastic facies in South China. Along this gradient, their sizes
decrease from carbonate, through intermediate carbonate-siliciclastic, to
siliciclastic facies. This is consistent with increasing abundance of
terrestrial materials of high silicon, aluminium, and phosphorus values.
On a larger million-year-long interval (MFZ14-16) and for several
palaeocontinents, size data of Lithostrotion decipiens and Siphonodendron
pauciradiale show a distinct decline in late Visean, when enhanced
terrestrial weathering occurred commonly with palaeosols developed during
regression. This suggests that terrestrial sediment and nutrient input may
have mainly controlled phenotypic plasticity in Mississippian reef corals,
with a decrease in size as a component of resilience across the LPIA
onset.