Christina Spence Morgan
Tens of million years in the past, sand tiger sharks hunted within the waters off the Antarctic Peninsula, gliding over a thriving marine ecosystem on the seafloor beneath.
All that is still of them at the moment is their sharp pointed tooth, however these tooth inform a narrative.
They’re serving to clear up the thriller of why the Earth, some 50 million years in the past, started shifting from a “greenhouse” local weather that was hotter than at the moment towards cooler “icehouse” circumstances.
Many theories about this local weather shift give attention to Antarctica. There’s geologic proof that each the Drake Passage, which is the water between South America and the Antarctic Peninsula, and the Tasman Gateway, between Australia and East Antarctica, widened and deepened throughout this time as Earth’s tectonic plates moved. The broader, deeper passages would have been obligatory for the waters of the main oceans to return collectively and the Antarctic Circumpolar Present to kind. That present, which flows round Antarctica at the moment, traps chilly waters within the Southern Ocean, retaining Antarctica chilly and frozen.
Copernicus Marine Companies/European Union
The now-extinct sand tiger shark species Striatolamia macrota was as soon as a relentless within the waters across the Antarctic Peninsula, and it left exquisitely preserved fossil tooth on what’s now Seymour Island close to the tip of the peninsula.
By finding out the chemistry preserved in these shark tooth, my colleagues and I discovered proof of when the Drake Passage opened, which allowed the waters of the Pacific and Atlantic oceans to combine, and what the water felt like on the time. The temperatures recorded in shark tooth are a number of the warmest for Antarctic waters and confirm local weather simulations with excessive atmospheric carbon dioxide concentrations.
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Oxygen captured in very sharp tooth
Sand tiger sharks have sharp tooth that protrude from their jaw to understand prey. A single shark has lots of of tooth in a number of rows. Over a lifetime, it sheds hundreds of tooth as new ones develop.
Essential environmental data is encoded throughout the chemistry of every tooth and preserved there over tens of millions of years.
For instance, the outer layer of a shark’s tooth consists of an enameloid hydroxyapatite, just like enamel in human tooth. It incorporates oxygen atoms from the water the shark lived in. By analyzing the oxygen, we are able to decide the temperature and salinity of the encircling water through the shark’s life.
The tooth from Seymour Island present that the Antarctic waters – a minimum of the place the sharks lived – stayed hotter longer than scientists had estimated.
Christina Spence Morgan
One other clue comes from the factor neodymium, which adsorbs and replaces different parts within the outer enameloid of the tooth throughout early fossilization. Every ocean basin has a definite ratio of two totally different neodymium isotopes based mostly on the age of its rocks. Wanting on the ratio within the shark tooth permits us to detect the sources of the water the place the shark died.
If circumstances are secure, the neodymium composition wouldn’t change. Nonetheless, if neodymium composition does change in fossil tooth over time, that signifies modifications in oceanography.
Large sharks, heat water
We studied 400 tooth from Seymour Island, from all ages of shark, juvenile to grownup, from people dwelling between 45 million to 37 million years in the past. The mix of tooth dimension and chemistry yielded some stunning clues to the previous.
A few of the tooth have been extraordinarily massive, suggesting these historic Antarctic sand tigers have been bigger than at the moment’s sand tiger shark, Carcharias taurus, which may develop to about 10 ft lengthy.
As well as, water temperatures the sharks lived in have been hotter than earlier research involving Antarctic clam shells advised. It’s doable the distinction was between waters nearer to the floor and deeper on the ocean ground, or the sharks whose tooth we discovered might have spent a part of their lives in South America. At present’s sand tiger sharks monitor heat waters. They spend summer time and early fall between coastal Massachusetts and Delaware, however when waters cool off, they migrate to coastal North Carolina and Florida. As a result of their tooth repeatedly kind and transfer ahead virtually like a conveyor belt, there are some tooth throughout the jaw that symbolize a unique habitat than the place a shark resides. It’s doable that the traditional sand tiger sharks additionally migrated, and when Antarctic waters cooled off, they headed north to hotter waters at decrease latitudes.
Jlencion/Wikimedia, CC BY-SA
The tooth advised that the sharks’ water temperature then was just like the water temperatures the place trendy sand tiger sharks will be discovered at the moment. Carbon dioxide concentrations have been additionally three to 6 instances increased than at the moment, so scientists would count on amplified temperatures within the areas.
Lastly, the neodymium within the fossil sand tiger shark tooth gives the earliest chemical proof of water flowing via the Drake Passage that aligns with tectonic proof. The early timing of the Drake Passage opening, however the delayed cooling impact, signifies there are advanced interactions between Earth’s methods that have an effect on local weather change.
What about their northern cousins?
Sand tiger sharks have been discovered world wide through the Eocene, suggesting they survived in a variety of environments. Within the Arctic Ocean, for instance, they lived in brackish waters which are much less salty than the open ocean 53 million to 38 million years in the past and have been a lot smaller than their southern cousins off Antarctica.
Variations within the saltiness of the tiger sharks’ habitat and dimension of the sharks additionally present up within the Gulf of Mexico throughout this time. That vary of environmental tolerance bodes nicely for the fashionable sand tiger sharks’ survival because the planet warms as soon as once more. Sadly, the tempo of warming at the moment is quicker and could also be past the sand tiger shark’s potential to adapt.
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Sora Kim receives funding from the Nationwide Science Basis (NSF-OPP #1842059).