The Once and Future Ocean

Rashid, H., D. J. W. Piper, K. B. Lazar, K. McDonald, and F. Saint-Ange (2017), The Holocene Labrador Current: Changing linkages to atmospheric and oceanographic forcing factors, Paleoceanography, 32, 498–510, doi:10.1002/2016PA003051.

Trying to predict the future climate is a complex business, fraught with computational issues and, occasionally, accusations of witchery (Google ‘climate witch.’ The results are…interesting). To help inform and contextualize these predictions, scientists look to the past. People working in the fields of paleo-climate and oceanography have dedicated themselves to reconstructing what Earth looked like in past millennia, well before humanity began influencing the climate.

Figure 1 – Map of the northern part of the Atlantic meridional overturning circulation. The Labrador Current is on the left-hand side of the map in light blue. The dot labeled Hu-2006040-40 is where the core was drilled (Adapted from Rashid et al., 2017).

Ocean circulation is an important part of Earth’s climate system since it exerts control on atmospheric carbon by regulating the amount of gas that is sequestered, or stored, in the water moving along the sea floor. Painting a complete picture of ancient Earth requires filling in the details of these water movement patterns. But, as you might imagine, measuring what transient water movements looked like thousands of years ago is no easy task.

Dr. Harunur Rashid of the Memorial University of Newfoundland sought to fill in a piece of the puzzle by examining what the Labrador Current (LC) might have looked like centuries ago. The LC is a part of the Atlantic meridional overturning circulation (AMOC), the pattern of water movement in the Atlantic basin. In particular, the LC delivers cold, freshwater from Arctic melt (Fig 1).

Scientists have hypothesized that the LC might become stronger as the Earth warms and northern ice sheets melt more rapidly. The effect of a stronger, fresher LC on the AMOC is unknown. But, as Dr. Rashid points out, there have been several melt water events in the past that could give some clues of what a fresher North Atlantic will look like.

Figure 2 – An acoustic image of subsea sediment layers from the core site. The red line indicates where the core fits in. The left y-axis shows the depth and the right describes some of the layers (adapted from Rashid et al., 2017).

To tease apart the LC’s historic strength, Dr. Rashid and a group of international collaborators took a 1010 cm long sediment core from the Karlsefni Trough, right in the middle of the current. The core, labeled Hu-2006040-40, was collected from a carefully selected location at the confluence of several branches of the LC. The researchers choose this site in the hopes of detecting a substantial freshwater signal. Back in the lab, the scientists took samples every 10 cm in the core, measured the grain size of the particles, and analyzed them for oxygen and carbonate content (Fig. 2).

Dr. Rashid used these measurements to reconstruct how fast the LC moved 10 thousand years ago. To do so, he treated the average particle size in each core sample as a proxy for current speed. The approach is based on the physical ability of water to move particles of a given size: the faster the current, the larger the sand size it can sweep along, and the larger a grain must be to sink. He divided the record into three distinct periods based on how mean sediment size was changing.  From 9.2 to 5 ka the grain size increased, was steady between 5 to 3.1 ka, and declined from 3.1 ka to present (where ka is a unit of a thousand years). In terms of LC speed, these periods correspond to accelerating, steady, and slowing down respectively (Fig. 3).

Based on these data, Dr. Rashid argues that high melt water and increased LC vigor contributed to an acceleration and north-south elongation of the AMOC. More recently, the slowing of the LC is partially responsible for the more lethargic, east-west oriented AMOC of today. These subtle changes on the currents can have major effects on the marine life in these regions, the efficiency of the local carbon pump, and the broader climate system.

Figure 3 – Graph of the mean grain size in the core. The x-axis is the age and is read from right to left. The right most section is where the grain size, and LC speed, is increasing. The yellow bars denote areas of major melt events (adapted from Rashid et al., 2017).

Dr. Rashid’s results do not directly tell scientists anything about what might happen to the LC in the future. But they do suggest that as the climate warms and ice melts, the LC and, therefore, the AMOC will get faster. What this means for the global climate is tough to say; it is quite a complicated system after all. The little hints offered by Dr. Rashid’s study do, however, give us a little peek.

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