Scientists studying ancient fish teeth have now proposed that the passageway where the Pacific and Atlantic oceans join began opening around 41 million years ago, much earlier than originally estimated.
In spite of being the size of a grain of sand, fossil fish teeth have a wealth of a rare Earth element known as neodymium (Nd). The isotopic ratio of this element varies from ocean to ocean. The Pacific and Atlantic oceans have a significant difference in Nd ratios. “The presence of neodymium with a Pacific signature in the deep Atlantic suggests that Pacific surface waters flowed into the South Atlantic, where they cooled and sank,” explained Ellen E. Martin, study team member and associate professor of geology at University of Florida.
Gondwana, the southern supercontinent, combined the land of most of the continents in the Southern Hemisphere. About 160 million years ago, the same mechanisms that drive plate tectonics dented into the land. The dent grew and eventually became the Drake Passage, the body of water where the Atlantic and Pacific oceans meet between South America and Antarctica.
“The merging of the two oceans created a ring of cold water forming the largest ocean current in the world known as Antarctic Circumpolar Current (ACC),” said Howie Scher of University of Rochester and a study team member. The current, circling Antarctica, led to Antarctic’s frigid conditions after separating it from warmer climates. Ice sheets quickly appeared in the continent about 34 million years ago.
However, the debate continues on when the ACC formed. Prior to its formation, Antarctic’s temperatures most likely included the subtropical surface currents. “This transport of heat would have been interrupted as the ACC began to circulate following the opening of passageways around Antarctica,” Scher told meryl.net.
Searching for Clues in Fish Teeth and Rare Cruises
While on a different project, Martin found and analyzed fish teeth, which are made from the same material as human teeth. Scher and Martin verified the reliability of fossilized fish teeth for a better estimate of when the opening of the Drake Passage occurred. “The idea was to use a sediment core from the Atlantic Ocean to determine the earliest appearance of the isotopic signature of Pacific seawater, which would mark the opening of Drake Passage,” Scher said.
The team found increased Nd around 37 and 34 million years ago. They concluded that the ACC may have played a key role in Antarctic becoming covered with ice sheets, potentially explaining how the sheets formed and remained.
Martin said the samples for the research are based on those collected by the Ocean Drilling Program (ODP), an international consortium that supported drilling on a research vessel, in 1985. Research tours to the Southern Ocean are limited as it isn’t a hospitable place, Martin told meryl.net. In order to do such research, scientists spend years planning an expedition to a specific area of the ocean. A group of scientists spends two months on the ship and collects cores. After one year passes, any scientist can request samples.
Concerns of ocean and atmospheric warming
The researchers observed the Drake Passage opening coincided with growing biological production in the Southern Atlantic. “This is significant because biological productivity in the oceans pulls carbon dioxide out of the atmosphere, which can also lead to global cooling. The global significance of this linkage is an important question,” Scher explained.
Scher plans to study other sediment cores to see how different parts of the ocean responded to the changes. The study will provide information on how the climate system is affected by ocean circulation. Martin has another project to look at ocean circulation patterns in the Cretaceous period. This was the time when the Atlantic basin was opening up, global temperatures were much higher, and ocean circulation was different from what it is today.
Scientists are concerned about the collapsing of the ice sheets as they could cause the sea level to rise around the world. Evidence shows the West Antarctic Ice Sheet and the Greenland Ice Sheet are becoming less stable as they confront the ocean and atmospheric warming. “Our data highlights the role that ocean circulation plays in controlling climate. The West Antarctic ice sheet is very unstable today, and these data indicate that the circumpolar circulation is helping to slow the effects of global warming in this region. It also illustrates that we need to respect the power of the ocean to control climate,” Scher said.