Wei-Li Hong

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The taste of an ocean in the past

(Left) Metal-free clean room for sample preparation. (Right) The MC-ICPMS (multi-collector inductively-coupled-plasma mass spectrometers) from the Isotope Geochemistry laboratory in University of St Andrews. Photo credit: STAiG laboratory.
pH, the measure of "sourness", is a scientific term that is used very often in our daily lives. For example, when you accidently taste lemon juice, you will notice the sourness of it. You can say the pH of lemon juice is low, lower than our drinking water which is obviously not sour.

By Wei-Li Hong & Haoyi Yao

pH has to be just right otherwise it is not good for the environment and people. Rain water containing too much acid ("acid rain") poses great threat to the buildings and our health as it can be corrosive (i.e., eats up the building or make you bald). When the pH in the ocean is getting lower, due to the dissolution of increasing atmosphere CO2, animals with carbonate outer shields, such as oysters and other shellfish, will be threatened as their shells will become thinner under lower pH.

In the NORCRUST project, we are interested in the pH of the past porewater (water in the sediments) from methane seeps. Why are we interested in this? Even though there is not much visible life in the sediments, the tiny microbes in the sediments are able to change the pH of porewater. Especially when there is a large quantity of methane in the sediments, the microbes can oxidize methane and increase the pH of porewater. With a higher pH, carbonate minerals start to precipitate in the sediments (the opposite of a thinner shell of an oyster in a sour ocean). Knowing the pH of the past porewater can help us understand when and how the carbonate minerals form in the sediments of a methane seep.

How do we know the taste of an past ocean?

We can study past pH from carbonate minerals with a secret tool: boron isotopes. Boron is an element used to make glasses and ceramics. For example, boron fiber is a high-strength and light-weight material used in aerospace structures and sports equipments such as golf clubs and fishing rods. We specifically care about two siblings in the boron family: 10B and its slightly heavier sibling 11B. In the water, boron is forced to join with "balls" that are made with OH (hydroxide). The amount of balls boron carries depends on the pH of the water: boron tends to carry four OH balls (borate; B(OH)4-) when the pH is higher and only three OH balls under lower pH (boric acid; B(OH)3) (Figure 1). The twin siblings of boron, 10B and 11B, also tend to carry different amount of OH balls. The heavier sibling, 11B, is the lazier one who prefers carrying less OH balls whereas the skinner sibling, 10B, tends to carry more balls with him (or her).

The taste of an ancient ocean from carbonate rocks

There is a fundamental difference between B(OH)4- and B(OH)3: the former is negative charged while the later is not. This is important because the negative charged one is attractive to the carbonate minerals. Therefore, what happens when a condition changes from low pH to high pH is, there will be more carbonate mineral formation and more B(OH)4- in the water that can be easily be incorporated by the carbonate minerals (Figure 1). As we introduced in the previous paragraph, 10B and 11B have different tendency for B(OH)4-, hence we can then measure the ratio of 10B/11B in the B(OH)4- of carbonate minerals to calculate the pH when such carbonate minerals formed.

Boron has two stable isotopes, 11B (80.1%) and 10B (19.9%), and the two soluble boron species, boric acid (B(OH)3) and borate (B(OH)4-).The equilibration between these two species is pH dependent and B(OH)4- is more abundant when the pH is higher. Due to the negative charge of B(OH)4-, it is incorporated carbonate mineral more easily. By measuring the d10B from the carbonate, we can determine the ancient pH when this carbonate mineral formed.

Not an easy task

Even though the principle may sound straight forward, there are a few problems that stop us from getting the answer we are looking for. One big problem is, no matter how careful we are when treating the carbonate minerals, we cannot avoid the sediment particles in the carbonate which are known to have boron with very different boron isotopic ratios. Measuring the sediment-contaminated samples can result in misleading outcome. We are working with Dr. James Rae from the University of St. Andrews. With the state-of-art facilities in his laboratory (Figure 2), we are looking for ways to solve this problem so that we can know the taste of an ocean in the past.