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 continues to be of them as we speak is their sharp pointed enamel, however these enamel 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 as we speak towards cooler “icehouse” circumstances.
Many theories about this local weather shift deal with 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 essential for the waters of the key oceans to return collectively and the Antarctic Circumpolar Present to type. That present, which flows round Antarctica as we speak, traps chilly waters within the Southern Ocean, maintaining 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 enamel on what’s now Seymour Island close to the tip of the peninsula.
By learning the chemistry preserved in these shark enamel, 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 enamel are among 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 enamel
Sand tiger sharks have sharp enamel that protrude from their jaw to know prey. A single shark has lots of of enamel in a number of rows. Over a lifetime, it sheds hundreds of enamel as new ones develop.
Vital environmental data is encoded throughout the chemistry of every tooth and preserved there over thousands and thousands of years.
For instance, the outer layer of a shark’s tooth consists of an enameloid hydroxyapatite, much like enamel in human enamel. It incorporates oxygen atoms from the water the shark lived in. By analyzing the oxygen, we will decide the temperature and salinity of the encircling water through the shark’s life.
The enamel 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 component neodymium, which adsorbs and replaces different components within the outer enameloid of the tooth throughout early fossilization. Every ocean basin has a definite ratio of two completely different neodymium isotopes based mostly on the age of its rocks. Trying on the ratio within the shark enamel permits us to detect the sources of the water the place the shark died.
If circumstances are steady, the neodymium composition wouldn’t change. Nevertheless, if neodymium composition does change in fossil enamel over time, that signifies adjustments in oceanography.
Huge sharks, heat water
We studied 400 enamel from Seymour Island, from all ages of shark, juvenile to grownup, from people residing between 45 million to 37 million years in the past. The mix of tooth dimension and chemistry yielded some shocking clues to the previous.
A number of the enamel have been extraordinarily giant, suggesting these historic Antarctic sand tigers have been bigger than as we speak’s sand tiger shark, Carcharias taurus, which may develop to about 10 toes lengthy.
As well as, water temperatures the sharks lived in have been hotter than earlier research involving Antarctic clam shells urged. It’s potential the distinction was between waters nearer to the floor and deeper on the ocean ground, or the sharks whose enamel we discovered might have spent a part of their lives in South America. Right this moment’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 enamel constantly type and transfer ahead virtually like a conveyor belt, there are some enamel throughout the jaw that signify a distinct habitat than the place a shark resides. It’s potential 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 enamel urged that the sharks’ water temperature then was much like the water temperatures the place trendy sand tiger sharks may be discovered as we speak. Carbon dioxide concentrations have been additionally three to 6 occasions greater than as we speak, so scientists would count on amplified temperatures within the areas.
Lastly, the neodymium within the fossil sand tiger shark enamel offers the earliest chemical proof of water flowing by means of 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 programs 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 might be 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 trendy sand tiger sharks’ survival because the planet warms as soon as once more. Sadly, the tempo of warming as we speak is quicker and could also be past the sand tiger shark’s skill to adapt.
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Sora Kim receives funding from the Nationwide Science Basis (NSF-OPP #1842059).