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. 

   

Image Source

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. 

Coral Reefs under the Acid Test Challenge

This photograph illustrates that coral reefs are the greatest victims of ocean acidification indeed!The picture on the left shows a healthy coral reef with living Acropora palmata (an important reef-building coral) under good water quality condition. In contrast, the other picture shows degraded coral reefs with dead  Acropora palmata, suffered from poor water quality caused by ocean acidification. This 'coral bleaching' is becoming more prominant in Puerto Rico, other Caribbean and western tropical Atlantic Ocean regions (Moyer, 2009)

The Acid Test

So, what is this Ocean’s Acid Test all about? Obviously, it refers to the ocean acidification phenomenon. In order to explore this global phenomenon further, we need to first understand the mechanism and chemistry of the ocean.

Source: http://na.oceana.org/en/our-work/climate-energy/ocean-acidification/learn-act/what-is-ocean-acidification

The above diagram gives a nice summary of a chain of chemical reactions happening in the ocean with the dissolved CO2 coming from the atmosphere. Like carbonated fizzy drinks, when more CO2 dissolves in the water, it becomes more acidic. When more atmospheric carbon dioxide is in contact with surface water of the ocean, more is dissolved to form carbonic acid. This carbonic acid dissociates to form bicarbonate ion and hydrogen ion (proton) which therefore decreases the seawater pH. These protons are likely to aggregate with carbonate ion present in seawater to form bicarbonate ion, which decreases the amount of carbonate ion available in seawater.  As most marine organisms, especially coccolithophorids, pteropods and foraminiferans require carbonate ions to build their calcium carbonate shells and skeletons, the decrease in carbonate ion makes them harder to calcify or even unable to survive. Therefore, less carbon can be deposited at the bottom of the ocean in the form of calcium carbonate via this carbonate pump. 

In fact, our ocean has been able to buffer upon this constant increase in acidity for the last 420,000 years at least (Hoegh-Guldberg et al., 2007). This natural mechanism is mainly driven by the biological and carbonate pump to transport carbon from the surface waters to the bottom of the ocean. Hence, the ocean’s deep water is rich in carbonate ion which is able to neutralise its natural acidity through the mixing of surface and deep waters via turbulences and ocean currents (Rahmstorf and Richardson, 2008).

However, this neutralisation process takes over 100,000 years! Since Industrial Revolution (1750), atmospheric CO2 concentration has increased from the pre-industrial level of 285ppm to the present level of 389ppm today in less than 300 years. Ocean pH has also decreased by 0.1 unit from a level of about 8.2 (Fenchell, 2011). According to the past geological records, our world’s ocean has never absorbed such vast amount of CO2 in the atmosphere and experienced this sharp decrease in pH in such a short timescale, which I am going to discuss in the following weeks.

Many scientists have projected a further decrease of 0.1 pH unit of our ocean by the end of this century based on the current anthropogenic CO2 emissions. Undoubtedly, this acid test is indeed our oceans’ biggest global challenge ever in history. Calcifying marine organisms and the coral communities are the first biggest victims. Next week, I will be looking at the effects of acidified oceans on these marine organisms and how corals can be used as an important proxy to understand the past ocean chemistry. Stay tuned!

Reference:

Rahmstorf, S. and K. Richardson (2008) Our Threatened Ocean, Haus Publishing: London 

Introduction

Welcome to my blog for GEOG3057: Global Environmental Change. In here, I am going to investigate one of the world’s most threatening global environmental problems – ocean acidification, another problem caused by anthropogenic carbon dioxide emissions. It seems that people tend to concern themselves with the Earth’s atmosphere only as the victim of burning fossil fuels, but often neglecting the ocean. In actual fact, the ocean is one of the major carbon sinks of our Earth, absorbing a quarter of the Earth’s CO2 emissions. However, scientists have discovered that with the ever-increasing level of CO2 in the atmosphere, the ocean has become more and more acidic which severely threatens marine life.  Some researchers even predict that if we continue to emit CO2 in the same way as we do now, ocean acidity is going to double by the end of this century. By then, nearly all marine life will be destroyed!

In the following 10 weeks, I will present to you my investigation on the problem of ocean acidification by analysing the ocean chemistry in the past, so as to understand and to predict our ocean’s destiny in the future. To start with, I would like to share with you a documentary produced by the Natural Resources Defense Council (NRDC) to present to you this most alarming global environmental problem from a group of ocean scientists.