The name of our expedition “Hawaiian Drowned Reefs” has raised some questions. How does an ecosystem that thrives underwater “drown”?
According to our science team, shallow water reefs are considered “drowned” when sea level rises, and the reef is submerged to a depth where it can no longer conduct photosynthesis.
“When sea level goes down, coral becomes exposed and dies,” Kenna Rubin says. “But if the sea level goes up faster than coral can grow then we call it drowning.”
The exact depth where reefs “drown” depends on the species that live there, but generally no shallow water corals can survive below 40 meters of water.
At our study sites around the Big Island, our science team believes there are two main factors that caused reefs to be submerged to these depths – fast sea level change following ice ages, and the slow, natural subsidence of the Big Island itself.
At the peak of the last ice age, sea level was approximately 120 – 130 meters below what it is now.
“To a non-geologist, it might not seem fast,” Rubin says. “But when it comes back up to modern sea level in only 10,000 years – that’s a very fast period of rise.”
The youngest island in the Hawaiian chain, the Big Island is approximately a million and half years old. But we still have a lot to learn about the mechanics of its subsidence.
“We don’t know the growth history of the volcanoes and the magma supply levels from below,” Rubin says. “If those are constant then it would be sinking at a constant rate, and if those factors vary then the island will sink at different rates over time.”
Looking at the bathymetry of the Big Island, in some areas we can see stair step formations of reef platforms. As you descend the “staircase”, each step is older and older.
“We know that from early dating work done in the 80s,” Rubin says. “It wasn’t very precise, but it was good enough to tell us these steps go down to a million years old.”
Approximately 24,000 years ago, at the peak of the last ice age, the Big Island was probably sinking a little bit faster than it is today – assuming Mauna Loa was more active back then.
“That variation is something we should be able to see in the coral records we recover,” Rubin says.
As we start to bring cores on board, different members of our science team are excited about various features of the cores. “Some of us will work on determining the ages, some of us will focus on identifying types of coral that grew at different times, and some of us will examine the coral record of past climate change,” Rubin says. “All of these components are essential for predicting future climate impacts on Hawaii’s coral reefs.”
Blog of IODP Expedition 389: Hawaiian Drowned Reefs 