The Weakening of the North Atlantic Current During the Late Pliocene and Early Pleistocene

The role of the North Atlantic Current (NAC) in regulating the Northern Hemisphere sea surface temperature (SST) gradient and the air temperature is huge. If the current were to weaken or stop, the effects could be drastic. Roughly 2.6 Ma that is exactly what happened, resulting in a major glacial event. Naafs et al. (2010) address how big an impact the NAC had on changing the climate, and show how the NAC may have weakened. Using coring samples from the middle of the Atlantic Ocean, Naafs et al. reproduce SST and productivity, to show how both change over time, and what those changes mean for the NAC. The shift of the Arctic Front southward, and the slow weakening of the NAC led to a major glacial event, changing the face of the planet. The findings of the authors highlight the long-term nature of change in Earth systems, while also showing how drastically they can be changed. –Mathew Harreld

Naafs, B.D.A., Stein, R., Hefter, J., Khélifi, N., Schepper, S.D., Haug, G.H. 2010. Late Pliocene changes in the North Atlantic Current. Earth and Planetary Science Letters 298, 434-442.

The North Atlantic Current (NAC) is hugely important in regulating the northern hemisphere climate. Warmer surface waters are pulled northward toward the Arctic Ocean, and some is downwelled into the deeper ocean, while some is sent back southward. This explains why England is more temperate than the Eastern Seaboard of the United States even though it is at a higher latitude. It is the warmer tropical waters from the south that keep the northern Atlantic Ocean at temperate temperatures. A topic of debate in current research is how might the slowing and stopping of this current affect local temperatures, and how might the NAC slow down. Naafs et al. raise this question in a previous epoch of Earth’s history. During the early Pliocene epoch, roughly 5 million years ago (Ma), there were warmer sea surface temperatures (SST) in the Northern Atlantic Ocean than there are today by nearly 10°C. This higher temperature meant that the NAC was more powerful than it is presently. However we also know that around 3.6 Ma an intense Northern Hemisphere glacial and interglacial cycle period began. Around 2.6 Ma, at the beginning of the Pleistocene epoch, the Earth fell into its first major glacial period that exhibited large Norther Hemisphere ice sheets.

During this shift into a deep glaciation the NAC is thought to have shifted dramatically. The extent of of the Arctic Front extends into lower latitudes, as ice sheets expand, and therefore there must be some change in NAC, which is carrying warmer waters that would otherwise melt the forming ice. In order for the ice to be expanding, and temperatures to be falling the NAC must have shifted. Naafs et al. hypothesize that that the NAC flattens out at a lower latitude, possibly even shrinking, while the Arctic Front expands southward, weakening the NAC. To test their hypothesis Naafs et al. use Deep Sea Drilling Project core samples from a site in the middle of the Atlantic Ocean on a latitude of 41°00’N. The core samples are used to remodel ocean productivity, which acts as a proxy for global average to temperature, and SST. They purposely chose a site that is currently right in the middle of the NAC because they believed it would show a greater effect of change in glacial conditions.

Having acquired the core samples, the authors used oxygen-18 data and organic compound data to remap SST and productivity. Oxygen-18 allowed the authors to remap SST because in colder temperatures there is more oxygen-18 present in oceans because oxygen-16 (a lighter isotope than oxygen-18) is frozen in the ice. The organic compounds allowed the authors to see levels of productivity because a higher level of organic compounds meant there were more organisms living in that area at that time. It should also be noted that there are higher levels of productivity when there are lower SSTs because that means there is less influence from the NAC, which would allow more nutrient-rich waters to stay in the area. Using both of these variables the authors were able to put together changes in SST and productivity, giving a better understanding of how the NAC might have changed over the million-year period being studied.

During mid/late Pliocene there were generally low levels of oxygen-18 in the oceans, reflecting high SST. This combined with the generally low amounts of organic compounds meant that there was a highly intense NAC affecting the coring area about 3.6 to 3.4 Ma. During this time, however, there are also periods of lowering SST and increasing productivity, but not enough to suggest any major shift.

In the period between 3.4 and 3.29 Ma there began a shift toward lower SSTs, and lower productivity. This suggests that the NAC was weakening of this time, but not enough to allow for nutrients to flow back into the area. This period also saw the beginning of the move of the Arctic Front southwards, and the initial build up of glaciers.

The next 0.5 million years saw the ocean return to a warmer climate. SSTs rose and productivity decreased, suggesting an increased influence of the NAC and a northern Arctic Front. This period did see a gradual shift downward of SST, but nothing like what was seen between 3.4 and 3.29 Ma.

Between 2.9 and 2.45 Ma SSTs plummeted and an increase in productivity was seen. The authors mention that there may be many factors that may have influenced an increase in productivity, but that such a large increase—more than 10 times—in productivity could only be attributed to the weakening of the NAC. This period at the end of Pliocene saw a significantly reduced NAC and a much lower latitude Arctic Front. The changes observed in the core sample are remarkable, and are supported by other findings in other core samples in the Northern Atlantic region.

The change that is expected in the NAC during a glaciation period is exactly what the authors predicted. The combination of increased ice albedo (reflectivity) and eastward movement of the NAC could have brought on the glaciation period. However there are still some variables that are not well understood about how the NAC could change so drastically. The authors suggest that perhaps changes in wind forcings may have played an important role, as might have orbital forcings. Either way it seems certain that the slowing and weakening of the NAC, pushing it southward, played a pivotal role in producing ice sheet growth southward, and drastic temperature drops in the higher and mid latitudes. The article also highlights the long-term nature of the changes that affect our planet greatly. And while it may be possible to predict such changes in their beginning stages, the full effects will not be observed for thousands or millions of years.

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