Understanding the Ocean’s Past: Finding the evidence

Knowing the ocean’s history is vital as it provides us an analogue, if not, deeper understanding of today’s ocean. With this, past environment reconstruction often comes into play using various paleo-records. In my last post, I mentioned about corals being the potential proxy for past ocean chemistry reconstruction.  In fact, after doing more research about the subject this week, corals are actually of a low potential to document chemical changes of the ocean in the past. There is still a lack of evidence proofing the decline in coral calcification is directly related to ocean acidification. My apologies here! Nevertheless, corals are important marine carbonate organisms in marine ecosystems and biodiversity, which I’ll come back to that later on. As for now, I’m going to introduce some common proxies used in palaeoceanography to reconstruct past seawater chemistry.  

Boron Isotope (δ¹¹B­) proxy: The paleo-seawater pH meter

The chemical element, Boron, exists in two molecular species in the ocean: boric acid B(OH)₃ and borate ion B(OH)₄^-. The proportion of the two species varies strongly with oceanic acidity. As calcifying organisms incorporate boron in their structures, the charged borate species, B(OH)₄^- is predominantly incorporated into marine carbonates, substituting HCO₃^- or CO₃^2-. In other words, boron isotopic composition of marine carbonates will be changed accordingly. Hence, by calculating the changes in boron isotope (δ¹¹B­) ratio of marine carbonates, oceanic pH can be inferred. The figure below shows the concentration and isotopic composition of the two boron species with seawater pH. 

   

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Foraminifera are commonly used in the analysis of boron isotopic ratios as a proxy. It is a calcareous plankton species with shells made of calcium carbonate, exists in both planktonic and benthic form. Foraminifera samples are often taken from deep-sea sediment cores for ancient oceanic pH reconstruction.

However, δ¹¹B is not always a perfect proxy for past pH reconstruction, especially in the context of deep time scales, i.e. beyond 10 to 20 million years. Over this timescale,  δ¹¹B  of seawater cannot be considered as constant due to the residence time of boron in seawater (Pelejero et al., 2010). Furthermore, the concentration of boron in foraminifera is often low (~10ppm), hence measuring its boron isotopic composition can be very difficult (Rae et al., 2011).

Boron/Calcium Ratio of benthic foraminifera : deep water carbonate saturation

Carbonate ion (CO₃^2-) concentration is another important component to understand the ocean carbonate chemistry. It is highly correlated with atmospheric CO₂ (pCO₂). When CO₂ dissolves in seawater, it reduces the available carbonate ion in surface water via the release of protons. This directly decreases the amount of carbonate precipitated at the ocean seafloor. Yu and Elderfield (2007) have successfully reconstructed past deep water carbonate using the measurements of B/Ca ratio of four benthic foraminifera species, in which a strong linear correlation between B/Ca and deep water CO₃^2- is shown.

In recent years, several scientists have attempted to reconstruct past ocean pH using B/Ca shell ratio of marine organisms. Yu et al. (2007) have proved that B/Ca measurements of planktonic foraminifera is a promising proxy for detecting variations in past ocean pH and pCO₂. However, it is not necessarily the case for other calcifying organisms. For example, B/Ca ratio of a California mussel species does not strongly correlate with its seawater pH but largely due to its specific biological control (McCoy et al., 2007). 

Ice core records for atmospheric CO2

And of course, a well documentation of atmospheric CO₂ in the past is essential as it is the main driver of the ocean’s chemical changes. Past CO₂ concentration in the atmosphere can be reconstructed from the composition of air bubbles trapped in ice cores, mostly taken from Greenland or Antarctica. For studying past oceanic chemistry changes,  Antarctic ice cores are common proxies used as it can be dated back to 800,000 years ago (glacial-interglacial timescale). Pelejero et al. (2010) have illustrated that atmospheric CO₂ and ocean surface pH almost synchronise with each other over the last 800,000 years, shown in the diagram below.

 

I have only listed a few common proxies for past ocean chemistry reconstruction in here, but there are a lot more out there worth to explore! As each of the proxies are subject to uncertainties and constraints, analysis using multiple proxies is often a common practice to encounter for spatial and temporal constraints for a better past reconstruction. Next week, I’ll further explore what these proxies actually tell us about the oceans, focusing on the abrupt ocean acidification event at the Paleocene-Eocene Thermal Maximum (PETM) 55Mya.