Caribbean salt water is linked to our climate

Model simulation of surface currents in the Atlantic with representation of the Gulf Stream and the position of the investigated Caribbean sediment core. © GEOMAR

On the trail of salt as a climate factor: Past cold periods in Europe were characterized by increased salinity in surface water in the Caribbean, researchers report. These anomalies were in turn linked to the weaker North Atlantic currents at the time and the lower heat transport to the northeast, the study shows. The scientists say it contributes to knowledge about the important regulatory role of salt transfer through ocean currents on global climate systems.

Sometimes it was warm and friendly and then there were difficult periods with wet and cold weather: Over the last 2000 years, Europe's climate was characterized by climatic fluctuations that were due to natural effects. The most well-known climate anomaly was the so-called Little Ice Age from the 15th to the middle of the 19th century, which led to poor harvests and cold, wet summers. Which mechanisms have contributed to this and other developments in the past is an important question in climate research. “A look at the recent natural climate anomalies helps to understand the processes and mechanisms that human-caused global warming can trigger,” says first author Anastasia Zhuravleva from the GEOMAR Helmholtz Center for Ocean Research Kiel.

Climate anomalies due to flow changes

But the complex events still pose a number of mysteries for climate researchers. The important role of the North Atlantic currents, which include the so-called Gulf Stream, is generally known. They bring warm water from the southwest to the coasts of Europe, ensuring mild conditions there. The salinity of the water masses is known to be a factor in the pumping system. “Researchers often consider an increase in sea ice extent and sweetening in the subpolar North Atlantic as possible triggers for past cold periods, but processes in the tropical Atlantic may be just as important,” says Zhuravleva. “So far, there is little information about past climate events in the subtropical-tropical Atlantic Ocean and their effects on regions in the northern hemisphere,” adds co-author Henning Bauch from GEOMAR. “This is where our research comes into play.”

To understand the processes, the team examined sediment samples from the southern Caribbean. These were datable layer systems made of material that was deposited over the last 1,700 years. The researchers focused on the tiny shells of small marine organisms – so-called foraminifera. Based on certain isotope patterns in the shells, it was possible to draw conclusions about the temperature conditions under which they were formed. In addition, their elemental composition reflected the salinity of the water during the tiny creatures' lifetime.

Cooling and salinization detected

As the team reports, the results documented a cooling in the Caribbean sampling region of about one degree Celsius during the Little Ice Age. “This is a significant temperature change for this region,” emphasizes co-author Mahyar Mohtadi from MARUM – Center for Marine Environmental Sciences at the University of Bremen. “A further pronounced cooling for the period of the 8th to 9th centuries is also particularly notable. Lower temperatures in the otherwise warm tropical ocean resulted in lower regional rainfall, coinciding with severe droughts in the Yucatan Peninsula and the demise of the Classic Maya culture,” Mohtadi said.

But the most remarkable result was the evidence on salinity: the cold climate anomalies were therefore characterized by increased salinity in the Caribbean water. It stands to reason that this was due to weaker ocean circulation and associated feedback effects. “Advection, or movement, of tropical salt to high northern latitudes is essential for maintaining high surface densities in the subpolar North Atlantic. This is a prerequisite for the overall stability of large-scale ocean circulation, including the transfer of warm Gulf Stream water, which is responsible for our mild temperatures in Europe,” says Bauch.

Salty feedback mechanism

Specifically, there were probably the following developments in the cold periods: Volcanic eruptions, weak solar activity and the feedback between sea ice and the ocean in the north initially caused an initial cooling. The new study results now suggest that the associated effects in turn caused a decrease in salt movement to high northern latitudes. This then reduced the pumping effect of the flow system, thereby increasing and prolonging the climate events. Models have already suggested such a feedback of salinity. The study now provides clear evidence of this, the scientists explain.

The results can therefore be incorporated into assessments of what developments can be expected in the context of man-made climate change. In this context, there are already signs that the Gulf Stream is weakening. The study now shows that salt transport in a south-north direction could represent an important co-factor, the conclusion is.

Source: GEOMAR Helmholtz Center for Ocean Research Kiel, specialist article: Science Advances, doi: 10.1126/sciadv.adg2639

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