Episode 128 – the Deep Sea

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Far below the ocean surface, where light, heat, and food become scarce, there is a vast environment that supports strange organisms highly adapted for survival in this harsh underwater landscape. This episode, we discuss the mysteries, past and present, of the Deep Sea.

In the news
New early ankylosaur had a unique blade-like tail
Ostrich-like early dinosaur might not have eaten like an ostrich
The oldest record of shrimp hiding inside a clam
New hypercarnivorous Cretaceous ichthyosaur

Down to the Depths

Some say the “Deep Sea” begins at 200 meters below the surface; others say 1,000 meters. Whatever your definition, the deep sea accounts for the vast majority of Earth’s living space, including enormous stretches of open ocean and the seafloor beneath it. And yet, the conditions of deep water make it particularly difficult for us humans to study.

Scientists divide the ocean into a series of zones defined by their depth.
With increasing depth comes a gradual decrease in light, temperature, and food availability, along with an increase in pressure.
Image: Chris Huh, Public Domain

Most organisms in the deep sea live their lives either floating and swimming in the cold, dark water column or sitting and crawling upon the cold, dark seafloor. Underlying most of the ocean is the abyssal plain, the vast expanse of seafloor at depths of 3,000-6,000 meters. But the ocean floor is not featureless, it is adorned with various mountains, canyons, seamounts, and trenches which can reach nearly 11,000 meters in depth, so far from the surface that we humans have hardly ever visited them. Organisms in the deep often exhibit low-energy lifestyles, various forms of bioluminescence, and various adaptations for surviving at low temperatures and high pressures.

Food is rare in the deep. Across most of the deep sea, the main source of nutrition is marine snow, the constant rain of tiny organic particles from the surface. But there are occasional oases in the deep, dark desert: nutrients and organisms often gather around seamounts and canyons; cold-water corals can form reefs and support small communities on the abyssal plain; and there are the occasional and temporary boons provided by hydrothermal vents, cold seeps, and “food falls” such as whale falls and wood falls.

Hydrothermal vents are underwater geysers, where water from below the seafloor becomes superheated by nearby magma and erupts to the surface. This water is an excellent source of rare nutrients, and so these vents support diverse communities of sea life.
Depending on their mineral content, hydrothermal vents can be white smokers (top left) or black smokers (top right), and they can support everything from microbial life to colonies of large tubeworms (bottom).
All images from USGS and NOAA, Public Domain.
Any large-ish organic remains can become a food fall on the ocean floor, but the most famous and impactful are whale falls. These can provide a rich source of nutrients for decades.
The meat of a whale’s body is food for many mobile scavengers like fish and octopi, as seen in the left-side photo. Image by National Marine Sanctuaries, CC BY 2.0
The bones of whale contain nutritious chemicals that can be used by bacteria, bone worms, and other invertebrates, as seen in the right-side photo. Image from NOAA, Public Domain

The Deep Sea in Deep Time

Fossils from the deep sea are rare, but not unheard of. Ocean sediments can be uplifted onto continents during continental collisions and mountain-building, and the depth of an ancient ocean habitat can be estimated by comparing the fossilized sea life, especially micro-organisms like foraminifera, with modern deep-sea species. Paleontologists have even identified fossil food falls, including whale falls and, earlier than that, reptile falls such as turtles and ichthyosaurs.

Ancient deep sea research often focuses on the question of how the deep sea changes over time, how deep sea communities are impacted during times of global change or mass extinction, and how much of the diversity of deep sea life comes from colonization from more shallow water ancestors versus diversification of deep sea ancestors. Since deep sea life – fossil and modern – is relatively difficult to access for research, many of these questions are still largely unresolved.

These are some samples of fossils from a Jurassic deep sea site in Austria, estimated at a paleo-depth of over 1,000 meters. The fossils represent crinoids, sea stars, brittle stars, urchins, brachiopods, sea snails, and more. Image from Thuy et al 2014

There are also known examples of ancient hydrothermal vent communities, some containing evidence of life over 1 billion years ago, and potentially much older. Hydrothermal vents often come up in discussions about the origins of life, as the nutrient-rich waters of these habitats are considered a possible environment for the development of the very first true cells. Needless to say, this makes hydrothermal vents a topic of intense interest among early Earth geologists.

Dive Deeper

The Deep Sea, a great overview from the Smithsonian
Life in the Deep Sea
Map of known hydrothermal vent locations

Some deep sea paleontology papers:
Thuy et al 2021. Jurassic deep sea brittle stars (technical)
Thuy et al 2014. Jurassic deep sea community (technical)
Kiel & Goedert 2006. Early Cenozoic whale falls (technical)
Li & Kusky 2007. 1.4-billion-year-old black smoker ecosystem (technical, paywall)
Dodd et al 2017. Possible signs of life at early Earth hydrothermal vent (technical, paywall)

If you enjoyed this topic and want more like it, check out these related episodes:

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