Extreme Climate and Underwater Landslides
Published in the EarthSphere Blog
Eons ago, when the world was younger, something unusual was happening deep below the surface of our planet’s oceans. The first hominids had not yet climbed down from their trees to roam African savannas, and evolution was much closer to the age of dinosaurs than today’s planet of apes. No sunlight reached the deep ocean floors, so the changes taking place went unseen but not unfelt. Perpetual night in the deep oceans created a ‘dark forest’ scenario where survival depended on remaining undetected. But life on the seafloor faced another challenge in the form of massive underwater landslides caused by rapid environmental changes far above at the planet’s surface.
These underwater landslides, also known as gravity flows or turbidites, cascaded down continental slopes carrying sediments far into the abyssal depths of the world’s oceans. Swift-flowing water and powerful currents eroded away existing seafloors and swept their residents oceanward. When the sediment-laden waters finally lost their energy and came to rest, thick layers of sand and silt buried the benthic communities of the deep.
The time in question was the early Eocene, from 56 million to 48 million years ago. Earth was a hothouse, with global average temperatures spiking to 27°C, far above the 14.5°C we enjoy today. Rain forests extended into the polar regions of this ancient Earth, and the planet had no permanent ice caps keeping water locked away. All of Earth’s water was in its oceans, rivers, and lakes. Extra water and thermal expansion from warmer oceans raised sea levels by 150 meters, or almost 500 feet, compared to today. Much of the world as we know it was underwater.
The setting is important in understanding the unusual circumstances surrounding these underwater landslides and why the Eocene turbidites may be linked to extreme weather events from global warming.
Turbidites refer to sediments deposited in response to subsea landslides. Our common view of landslides naturally focuses on collapsing mountainsides and debris-covered roads because this is what we see in the news. But think about mudslides when we shift to a conversation about turbidites. The alternative names for turbidites, gravity flows or density flows, more aptly describe the mechanics at play in an underwater landslide.
Anyone who has tossed a rock into a clear puddle with a muddy bottom can appreciate how the disturbance causes the mud at the bottom to well up and mix with the water. After a while, the mud and silt particles will settle back to the bottom, leaving a clear puddle. The density of water increases when mud and fine sediments are entrained. A cup of muddy water weighs more than a cup of clear water.
Fill a wheelbarrow with clear water and slowly pour thick muddy water (at the same temperature) down the sloping front of the wheelbarrow, and you will find that while there is some mixing, most of the muddy water travels to the bottom. The muddy water is denser, and it sinks below the clear water. You can now picture the primary mechanics of a turbidite flow.
At a massive scale, disturbances high on the continental slopes cause water and sediment to mix. The denser water moves downslope under the influence of gravity, and as it picks up speed, it entrains more sediments, including sand, silt, and clay-sized particles. Soon, a massive volume of sediment is barreling down the slope at speeds of up to 30 kilometers per hour.
When this turbidity current reaches the flat abyssal plain, it loses energy, and the sediments slowly settle out. We are left with a turbidity deposit for geologists to examine millions of years in the future.
So, What’s Unusual?
Turbidite flows have occurred since oceans and mountains existed together on our planet. They are part of the natural geological cycle on a planet with seas and an atmosphere. The frequency of turbidite systems at any point in geological time is controlled by a number of environmental factors. Normally when sea levels are high, most sedimentation from erosion is captured in shallow continental oceans. When sea levels drop, these sediments are flushed onto the continental slopes and carried via turbidite flows to the deep ocean abyss.
The early Eocene conundrum arose because ocean levels were at an all-time high, yet today we still see evidence of very active Eocene turbidite systems around the globe. The data is a bit unusual. Anomalous data creates adrenaline rushes in wonky geologists, and they revel in extrapolating the data into interpretations and theories.
The critical question is what caused such active turbidite systems in the early Eocene. One possible and common explanation is increased seismic activity and mountain building. The higher the mountain, the quicker it erodes, meaning more sediment is shed into the oceans. But quite rightly, the authors of the report “Peak Cenozoic warmth enabled deep-sea sand deposition” point out that both passive and active margins were busy shedding sediments in the Eocene. Active margins are those where mountain building and seismic instability occur.
A second explanation was offered in the article, climate extremes. Erosion depends on weathering rates, and warm climates with lots of extreme storms will generate larger amounts of eroded sediments and more flooding to wash them into the ocean.
If some of this is beginning to sound familiar, it should. Epic flooding in Pakistan, massive spring rains flooding the central USA, fierce cyclones pounding the eastern and western coasts of North America, and all this in response to a 1°C rise in temperature. What would happen with a 12°C warmer world, as experienced in the Eocene?
If we want to know where global warming is headed, perhaps we should spend more time studying the early Eocene.