Eocene's Hothouse Climate: Insights from Ancient Turbidites
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Chapter 1: An Early Eocene Hothouse
Long ago, during the early Eocene epoch, when our planet was still in its formative years, dramatic changes were occurring beneath the ocean's surface. At this time, the first hominids had yet to emerge from the trees, and evolution was still closely tied to the age of dinosaurs. The depths of the oceans remained untouched by sunlight, resulting in a hidden yet significant transformation. This perpetual darkness created a scenario akin to a 'dark forest' where survival hinged on stealth. However, life on the ocean floor faced another peril: enormous underwater landslides triggered by rapid shifts in the environment above.
These underwater landslides, commonly referred to as gravity flows or turbidites, surged down continental slopes, transporting sediments deep into the ocean's abyss. The powerful currents and fast-moving waters eroded the seafloor, sweeping its inhabitants away. Once the sediment-laden waters lost their momentum, they settled, burying the benthic communities under thick layers of sand and silt.
The period in question spans from 56 to 48 million years ago when Earth was a hothouse. Global average temperatures soared to approximately 27°C, significantly higher than today’s average of 14.5°C. Lush rainforests expanded into polar regions, and the planet lacked permanent ice caps to store water. All water was present in oceans, lakes, and rivers, and the additional water along with thermal expansion from warmer seas elevated sea levels by around 150 meters (nearly 500 feet), submerging much of the planet.
This context is crucial for understanding the extraordinary conditions that led to these underwater landslides and their potential links to extreme weather phenomena resulting from global warming.
Section 1.1: Understanding Turbidites
Turbidites are sediments deposited as a result of underwater landslides. While we often associate landslides with crumbling mountainsides and debris-covered roads, a shift in focus to mudslides offers a clearer analogy for turbidites. The alternative terms, gravity flows or density flows, aptly illustrate the processes involved in these underwater phenomena.
Imagine tossing a rock into a clear puddle with a muddy bottom—the disturbance causes the mud to rise and mix with the water. Eventually, the silt settles, restoring clarity. The density of the water increases when mud and fine sediments are mixed in. If you pour thick muddy water into a wheelbarrow filled with clear water, the denser muddy water will predominantly sink to the bottom, illustrating how turbidite flows operate.
On a grand scale, disturbances on continental slopes lead to the mixing of water and sediments. The denser, sediment-laden water moves downhill due to gravity, gaining speed and entraining additional sediments, including sand, silt, and clay-sized particles. This results in a sediment rush that can reach speeds of up to 30 kilometers per hour. When this turbidity current reaches the flat abyssal plain, it loses energy, and the sediments gradually settle, creating deposits for future geologists to study.
Section 1.2: The Unusual Eocene Conundrum
Turbidite flows have occurred throughout geological history wherever oceans and mountains coexist. The frequency of these flows is influenced by various environmental factors. Typically, when sea levels are elevated, most sedimentation from erosion is captured in shallow continental seas. Conversely, when sea levels recede, sediments are swept onto continental slopes and transported to the deep ocean via turbidite flows.
The early Eocene presents a paradox: ocean levels were at a historic high, yet evidence suggests highly active Eocene turbidite systems globally. This anomaly excites geologists, prompting them to delve into interpretations and theories.
A pivotal question arises: what drove such active turbidite systems during the early Eocene? One common hypothesis attributes it to increased seismic activity and mountain formation. Higher mountains erode more quickly, releasing greater amounts of sediment into the oceans. However, a recent study, “Peak Cenozoic warmth enabled deep-sea sand deposition,” highlights that both active and passive margins contributed to sediment shedding during the Eocene.
Another proposed explanation centers on climate extremes. Erosion rates are influenced by weathering, and warmer climates with more severe storms lead to greater sediment erosion and flooding, washing sediments into the ocean.
If this sounds familiar, it should. Recent extreme flooding in Pakistan, significant spring rains in the central USA, and intense cyclones battering North America's coasts are all linked to a mere 1°C rise in temperature. What implications might a 12°C increase, reminiscent of Eocene conditions, have?
To gain insights into the trajectory of global warming, we might benefit from examining the early Eocene more closely.
Chapter 2: Insights from the Eocene
The first video delves into the Paleocene-Eocene Thermal Maximum, exploring its significance for understanding today's climate dynamics.
The second video discusses a severe greenhouse effect experienced by Earth during the Paleocene-Eocene Thermal Maximum, providing context for current climate challenges.