We interview the ICM researcher Eva Calvo to know what the water chemistry tells us about the oceans of the past and which has been her contribution to the TRANSMOW campaign, an expedition that circled last month the Iberian Peninsul.
1. When we talk about water chemistry, what do we mean?
Water chemistry refers to the complex mixture of elements and chemical compounds dissolved in seawater (nutrients, C compounds, trace elements, etc.) and the processes that affect their transformation. Nowadays, the chemistry of seawater is being greatly altered as a result of the absorption of a large part of anthropogenic CO₂ emissions, which is causing a progressive acidification of the water, i.e. a decrease in pH.
2. What information can parameters such as pH tell us about the oceans of the past?
The oceans play a very important role in the global carbon cycle and pH is one of the most appropriate parameters to study the oceanic CO₂ system. This gas, once dissolved in seawater, forms a weak acid that is transformed into bicarbonate, carbonate and proton ions. Thus, if we are able to reconstruct pH changes in the past, we will be able to detect changes caused by the anthropogenic increase in atmospheric CO₂ and go beyond what the instrumental data show us. In the Mediterranean, for example, we do not yet have such a record. Finally, the pH of the water is also modified by biology (respiration/production) and by the circulation or ventilation of the waters, so the reconstruction of pH in periods prior to the industrial revolution will allow us to obtain information on changes in the different processes that modify this parameter.
3. What about the oceans of the future, and what information do parameters such as pH provide us with?
It is essential to know the present and past variability of these parameters in order to be able to establish ranges, trends and possible processes that could be behind, not only the changes that the oceans are experiencing today, but also those that the oceans of the future will experience.
4. How are samples taken from hundreds and thousands of metres deep?
The marine water and sediment samples we work with are obtained by participating in campaigns on board oceanographic vessels with the instrumentation and technical staff to facilitate the various operations that need to be carried out. For example, last month, we participated in the TRANSMOW campaign on the BIO Hespérides, led by the UB's Marine Geosciences Group, where a rosette was used to collect water at different depths, boxcorers and multicorers to collect surface sediments, and the gravity corer to recover longer sediment cores.
5. What analyses are done afterwards in the laboratory?
On the ship we already perform some of the analyses, mainly on the water samples, where we analyse the pH and also the alkalinity, another parameter of the CO₂ system that, together with the pH, will allow us to derive the rest of the variables: dissolved inorganic carbon, pCO₂ or carbonate ions. For sediment samples, however, a more laborious treatment and analysis is necessary, so they are processed later in the laboratory.
6. What will be done with the samples collected during the TRANSMOW campaign?
One of the objectives of the TRANSMOW campaign is to characterise the geochemical signal of the Mediterranean waters that are exported to the Atlantic Ocean, through the Strait of Gibraltar, by measuring a series of chemical tracers such as pH and alkalinity, but also neodymium isotopes, among others. All these tracers are characteristic of each water mass, which will allow us to differentiate them. On the other hand, the analysis of the marine sediments recovered during the campaign will allow to reconstruct some of these variables in the past. This is the case of the boron isotopic composition in the shells of fossil foraminifera, which we will use as a proxy for past pH changes.
7. What innovative results do you expect to obtain from this campaign?
Once the water masses surrounding the peninsula are well characterised geochemically, we hope to be able to identify this signal also in the sediment and thus reconstruct changes in the export of these waters to the Atlantic in the past. In addition, the recovery of surface sediments, together with the extensive sampling of the water column, will allow us to calibrate and validate boron isotopes as a proxy for paleopH, a methodology that we have recently set up in the context of the HICCUP project of the Plan Estatal.
8. Will these results provide insight into the sensitivity of the oceans to climate oscillations?
We hope that the integration of several proxies and the enhanced precision of the analyses in the different measurements that we will carry out will improve the capacity to identify changes in these water masses and allow us to better understand their relationship with past climate changes.