Reconstruct the ancient history of an ocean current

Ocean currents embody movement, meandering from the tropics to the poles and back again, moving large amounts of water from moment to moment. But they’re also incredibly old, following their basic course for millions of years.

Tracing a written history in water is the work of paleooceanographers such as Adriane Lam, postdoctoral researcher on presidential diversity in the Department of Geological Sciences and Environmental Studies at Binghamton University. Lam is the lead author of “Pliocene to early Pleistocene (5-2.5 Ma) Reconstruction of the Kuroshio Current Extension Reveals a Dynamic Current”, recently published in the journal Paleoceanography and Paleoclimatology. Co-authors include Assistant Professor of Geological Sciences and Environmental Studies Molly Patterson, as well as Kenneth MacLeod of the University of Missouri, Solveig Schilling of the University of Texas at Austin, R. Mark Leckie of the University of Massachusetts Amherst, Andrew Fraass from the University of Bristol in England and Nicholas Venti from the University of Delaware.

The main western border current in the northern Pacific Ocean, the Kuroshio Current and Extension, is analogous to the Gulf Stream, which flows along the east coast of North America. Wind-driven, border currents are the workhorses of the ocean, moving heat, salt and gas from equatorial seas to mid-latitudes, Lam said.

In other words, these currents help distribute heat from the tropics to higher latitudes. In fact, corals are found at their highest latitude of anywhere in the world in the Kuroshio Current because the waters are so warm, ”she said.

This heat comes from surface waters that collect in the Western Pacific Ocean along the equator, known as the Western Pacific Warm Water Basin. The Kuroshio Current carries these waters north, past the Japanese coast, then east to latitude 36 ° N, where it joins the open Pacific Ocean. At this point, it becomes the current extension of Kuroshio.

The current and extension carry away large amounts of heat and moisture evaporating from the hot water in the lower atmosphere of the northern hemisphere. For this reason, they help shape precipitation patterns over Japan and the west coast of North America, as well as the tracks of typhoons, which feed on warm waters. In addition to affecting the weather, the Kuroshio probably also affects the climate, although its impact over time scales of a thousand and million years is not yet clear.

The Kuroshio also plays a major role in ecosystems and the fishing industry. In the Pacific Northwest, it meets the Oyashio Current, which brings the cool waters of the polar region to the south. Where the two currents meet, a strong temperature gradient forms due to the mixing of hot and cold water. It also creates an upwelling region, where the nutrient-rich waters of the deep ocean are brought to the surface as the currents flow eastward.

It’s not just the waters that mix together: the hot and cold water organisms that live in the respective currents also flow together in a transition zone between ecosystems, known as an ecotone. Its inhabitants include several species of fish and plankton, which ultimately fuel Japan’s prolific fishing industry and form an important part of that country’s economy.

Because of their impact on biodiversity, weather and climate, understand how border currents such as the Kuroshio will respond to climate change and increased CO₂ levels in the atmosphere is critical. Today, those currents are warming two to three times faster than other areas of the ocean, Lam said.

Studies of ocean models and observational data also show that the extension of the Kuroshio Current is moving northward and increasing its carrying capacity, but researchers do not yet know how these changes will affect the organisms that live there. or local and regional weather and climate conditions.

The recently published research is the first of its kind to reconstruct the Kuroshio as it was 2.5 to 5 million years ago, a period that covered both periods of global warming and cooling, as well. as the closure of a major seaway in what is now Central. America. Looking at the distant past of the present can answer some of the questions about its future.

Oceans past and future

During the Pliocene, which stretches 2.5 to 5.3 million years ago, atmospheric CO₂ the levels were close to those we face today: around 350-450 parts per million. The current atmosphere contains about 415 parts per million CO₂.

“The funny part about this period is that the continents were laid out the same way they are today, which makes the Pliocene a great time to use as an analogue of how the earth system will respond to increased CO.₂ concentrations and warming, ”said Lam.

There were some differences when it came to land masses, she noted: until about 2.5 million years ago, a waterway existed between North America and North America. South which allowed the surface waters of the Pacific and Atlantic Oceans to mix. When the Central American Seaway closed, it may have brought the extension of the Kuroshio Current to its present configuration.

The Pliocene included a period of 3 to 3.3 million years known as the Mid Piacenzian Warm Period (mPWP), which saw increased carbon dioxide levels and global warming. Once this period was over, the cooling resumed, accompanied by the growth of glaciers and pack ice in the high latitudes of the northern hemisphere.

In the recently published study, researchers reconstructed the Kuroshio throughout the mPWP, using chemical signatures from fossilized marine plankton shells that once lived in the surface waters of the Kuroshio region.

“Our data indicate that during the first phase of the Pliocene mPWP warming, the current warmed up and potentially shifted its latitudinal position north. It then cooled and possibly shifted its position south. during a brief period of global cooling, “she added. noted.

Rebuild the current

Scientists use different techniques to reconstruct the history of an ocean current, depending on the time scale considered. For shorter timescales, they rely on observational data to see how the path of a current changes seasonally, year-to-year, or decade-to-decade. But when it comes to climate change, that data set may be insufficient.

“This is why it is useful and necessary to reconstruct the behavior of western frontier currents through deep time, using sedimentary records from millions of years ago,” Lam explained. “Through the lens of sediment recordings, short-term changes in current are ‘smoothed out’ or averaged, so we are essentially only able to recover signals that indicate longer and larger changes in currents.”

In the study, the researchers used chemical signals obtained from fossil plankton that lived on the ocean’s surface, as well as three deep-water sediment cores from Shatsky Rise, a location on the seabed of the Pacific Northwest. Plankctic foraminifera have lived in the oceans for 170 million years; their durable shells, called “tests”, are made of calcium carbonate and collect on the ocean floor when they die.

In a previous study, Lam calculated the diversity of fossil plankton at each site used in the subsequent chemical study. She found that diversity was highest at the northernmost site of Shatsky Rise, 12 million years ago until today. This finding indicates that the ecotone created by the current has been around for a very long time, and probably the Kuroshio as well.

Researchers don’t know how hot the current got during mPWP, or how much the chemical signal is affected by salinity as well as temperature changes. To get a better picture, Lam and his colleagues at other SUNY schools are currently working on a grant that would use different chemical methods to answer these questions.

“The ocean is greatly affected by climate change, and we need to think about ways to protect it and marine organisms. This is especially true for the extension of the Kuroshio Current, as this region is home to one of the greatest biodiversity in our world. ocean, ”Lam said.