Seawater Chemistry: North Atlantic vs. North Pacific Ocean

This is Aleck Wang. I’m leading the chemistry group (Photo 1) during the cruise. We are in charge of measurements of seawater carbonate chemistry during this cruise. The seawater carbonate system can be characterized by four primary parameters: pH, partial pressure of carbon dioxide (pCO₂), total carbon dioxide (TCO₂), and total alkalinity (TA). We measured all of them. As a comparison, we also made similar measurements during a sister cruise last year in the North Atlantic Ocean at similar latitudes.

Photo 1 (by Taylor Crockford): The chemistry group: (from left) Kelly Knorr, Elliott Roberts, Aleck Wang, Nick Tuttle, Katherine Hoering, and Sophie Chu.

Let’s talk about the difference in seawater chemistry between the two ocean basins: North Atlantic vs. North Pacific. This is important because it sets the background and logic for this ocean acidification – pteropods project.

Naturally, seawater in the North Pacific Ocean is more acidic (lower in pH) than in the North Atlantic Ocean. This is related to the circulation, biology and chemistry in the ocean, which involve complicated processes that oceanographers have been studying for decades. The results of these processes are that seawater in the North Pacific in general has lower pH, but higher TCO₂ concentrations than that of the North Atlantic at similar latitude (Figure 1; data collected in August 2011 and August 2012 by Wang’s group through this project).

Figure 1. pH and TCO₂ profiles at two stations in the North Pacific vs. North Atlantic. Data were collected by Wang’s group through this project.

Such a difference causes profound differences in seawater chemistry between the two ocean basins. For example, aragonite compensation depth in the North Pacific is dramatically shallower than the North Atlantic. Aragonite, one type of calcium carbonate minerals, is required by many marine animals (e.g. pteropods, shrimps, and many species of bi-valves) to form their shells. Aragonite can dissolve or precipitate in seawater, depending on its solubility measured by saturation state: if aragonite saturation state is greater than 1, the condition favors aragonite precipitation and growth of shell-building animals; otherwise, aragonite would dissolve, which can have detrimental effects on shell building animals. The water depth where aragonite saturation state equals 1 is called aragonite compensation depth. Above this depth, aragonite saturation is greater than 1 and less than 1 otherwise.

Figure 2. Profiles of aragonite saturation state in the North Pacific vs. North Atlantic

Lower aragonite saturation state in the North Pacific is primarily due to lower pH condition as compared to the North Atlantic (Figure 2; data collected by Wang’s group through this project). As a result, the aragonite compensation depth is ~135 m at a North Pacific station (blue line in Figure 2) as compared to 2500 m at a North Atlantic station (green line in Figure 2). As the ocean continues acidifying as more CO₂ dissolves into the ocean due to the rise of atmospheric CO₂ concentration, seawater pH will continue dropping and aragonite compensation depth will become shallower in all ocean basins in the coming decades. In the North Pacific, this becomes an imminent problem for many shell-building animals, as the aragonite compensation depth inches up to the surface each year and the water layer supports their shell formation become narrower.

This ocean acidification project takes advantage of the very difference in carbonate chemistry between the North Atlantic and North Pacific to examine how such a difference affects pteropod’s life style and distribution. More about pteropod biology will follow. The results from this project will inform us what would happen to pteropods as ocean acidification continues. Essentially, the North Atlantic servers as a control case in this study to compared with, and the North Pacific is the acidified case.

The other goal of this project is to evaluate and compare the ocean acidification rates in the two ocean basins by comparing our measurements of carbonate parameters with historical data. Because of the difference in seawater chemistry between the two ocean basins, their acidification rates likely differ. This will help us to predict future changes in seawater chemistry.

The chemistry group (Photo 2) has done a marvelous job on making high quality measurements of carbonate chemistry during both North Atlantic and North Pacific cruises, as shown in Figures 1 and 2. Special thanks go to each group member.

Photo 2 (by Taylor Crockford): The chemistry group around the CTD-Rosette package.