Light Limitation of Poleward Coral Reef Expansion During Past Warm Climates

The latitudinal range of modern shallow-water tropical corals is controlled by temperature, and presently limited to waters warmer than 16-18°C year-round. However, even during Cenozoic climates with such temperatures in polar regions, coral reefs are not found beyond >50° latitude. Here, we test the hypothesis that daily available solar radiation limited poleward expansion of coral reefs during warm climates, using a new box model of shallow marine coral calcification. Our results show that calcification rates start to decline beyond 40° latitude and drop severely beyond 50° latitude, due to decreasing winter light intensity and day length, irrespective of aragonite saturation. This suggests that light ultimately prohibits further poleward expansion in warm climates. In addition, fossil coral reef distribution is not a robust proxy for water temperatures and poleward expansion of reefs beyond 50° latitude is not an expected carbon cycle feedback of climate warming.