Many theories about this climate shift focus on Antarctica. There is geologic evidence that both 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 during this time as the earth's tectonic plates moves. The wider, deeper passages would have been necessary for the waters of the major oceans to come together and the Antarctic circumpolar current to form. That current, which flows around Antarctica today, traps cold waters in the Southern Ocean, keeping Antarctica cold and frozen.
By studying the chemistry preserved in these shark teeth, evidence was found that when the Drake Passage opened, which allowed the waters of the Pacific and Atlantic oceans to mix, and what the water felt like. The temperatures recorded in shark teeth are some of the warmest for Antarctic waters and verify climate simulations with high atmospheric carbon dioxide concentrations.
Oxygen captured in very sharp teeth:
Sand tiger sharks have sharp teeth that protrude from their jaw to grasp prey. A single shark has hundreds of teeth in multiple rows. Over a lifetime, it sheds thousands of teeth as new ones grow.
Important environmental information is encoded within the chemistry of each tooth and preserved there over millions of years.
For example, the outer layer of a shark’s tooth is composed of an enameled hydroxyapatite, similar to enamel in human teeth. It contains oxygen atoms from the water the shark lived in. By analyzing the oxygen, we can determine the temperature and salinity of the surrounding water during the shark’s life.
The teeth from Seymour Island show that the Antarctic waters – at least where the sharks lived – stayed warmer longer than scientists had estimated.
Another clue comes from the element neodymium, which adsorbs and replaces other elements in the tooth's outer enamel during early fossilization. Each ocean basin has a distinct ratio of two different neodymium isotopes based on the age of its rocks. Looking at the ratio in the shark teeth allows us to detect the water sources where the shark dies.
If conditions are stable, the neodymium composition will not change. However, if neodymium composition does change in fossil teeth over time, that indicates changes in oceanography.
Big sharks, warm water:
We studied 400 teeth from Seymour Island, from all ages of shark, juvenile to adult, from individuals living between 45 million to 37 million years ago. The combination of tooth size and chemistry yielded some surprising clues to the past.
Some of the teeth were extremely large, suggesting these ancient Antarctic sand tigers were larger than today’s sand tiger shark, Carcharias taurus, which can grow to about 10 feet long.
In addition, the water temperatures the sharks lived in were warmer than prev studies involving Antarctic clam shells suggested. It’s possible the difference was between waters closer to the surface and deeper on the seafloor or the sharks whose teeth we found may have spent part of their lives in South America. Today’s sand tiger sharks track warm waters. They spend summer and early fall between coastal Massachusetts and Delaware, but they migrate to coastal North Carolina and Florida when the waters cool off. Because their teeth continuously form and move forward almost like a conveyor belt, some teeth within the jaw represent a different habitat than where a shark is living. It is possible that the ancient sand tiger sharks also migrated, and when Antarctic waters cooled off, they headed north to warmer waters at lower latitudes.
The teeth suggested that the sharks’ water temperature was similar to the water temperatures where modern sand tiger sharks can be found today. Carbon dioxide concentrations were also three to six times higher than today, so that scientists would expect amplified temperatures in the regions.
Finally, the neodymium in the fossil sand tiger shark teeth provides the earliest chemical evidence of water flowing through the Drake Passage that aligns with tectonic evidence. The early timing of the Drake Passage opening, but the delayed cooling effect, indicates there are complex interactions between Earth’s systems that affect climate change.
What about their northern cousins?
Sand tiger sharks were found worldwide during the Eocene, suggesting they survived in a wide range of environments. For example, in the Arctic Ocean, they lived in brackish waters that are less salty than the open ocean 53 million to 38 million years ago and were much smaller than their southern cousins off Antarctica.
Differences in the saltiness of the tiger sharks’ habitat and the size of the sharks also show up in the Gulf of Mexico during this time. That range of environmental tolerance bodes well for the modern sand tiger sharks’ survival as the planet warms once again. Unfortunately, the pace of warming today is faster and maybe beyond the sand tiger shark’s ability to adapt.
You must be logged in to post a comment.