Its importance can be seen clearly in the geological record in the form of vast deposits of limestones and chalk rocks. The mineral calcium carbonate (CaCO 3) is a fundamental building block for numerous marine organisms, from microscopic algae to reef-building corals. However, we will focus here on the better studied - and potentially more serious - implications of ocean acidification for calcifying marine organisms. This is because seawater borate ions (B(OH) 4 -), which absorb sound at these frequencies, become depleted with increasing absorption of CO 2 by the ocean (Brewer & Hester 2009), in much the same way that carbonate ions (CO 3 2-) are affected (as discussed earlier). Decreasing ocean pH also has rather more unexpected consequences - frequencies of sound, important for sonar and marine mammal (whale) communication, propagate more efficiently in waters with lower pH.
In fact, this greatly outweighs the negative feedback described above, meaning that as the ocean surface warms, even more of the emitted fossil fuel CO 2 will remain in the atmosphere.ĭecreasing ocean pH has the potential to affect life in the ocean because all organisms must expend metabolic energy in maintaining a particular pH inside of their cells to ensure biochemical processes operate efficiently (Raven et al. A well-known positive feedback in the carbon cycle arises due to the decrease in solubility of CO 2 gas in seawater at higher temperatures. For example, melting polar ice caps through global warming will reduce the amount of solar radiation that is reflected back out to space (the Earth's surface becomes less reflective), so producing more warming, which in turn will melt more ice, and so on - a positive feedback. Here, a feedback describes a mechanism that dimishes or amplifies an initial change and asribed the sign ‘negative' or ‘positive', respectively. The consequence of this is that, as the ocean warms, less DIC will be partitioned into the form of CO 2 (and more as CO 3 2-), hence enhancing the buffering and providing a ‘negative feedback' on rising atmospheric CO 2. The proportion of DIC present as CO 2 is also affected by temperature, as illustrated in Figure 2. Because of this, the proportion of CO 2 added to seawater that remains as CO 2(aq) increases as more CO 2 is added, an effect first recognised by Roger Revelle and Hans Suess (Revelle & Suess 1957) and quantified as the 'Revelle Factor'. It is then intuitive that the buffering capacity of seawater will decrease as more CO 2 is added and CO 3 2- is progressively consumed. All else being equal, as more CO 2 is added to seawater the pH will slowly decrease and the balance between the three carbonate species will change, with and increasing and decreasing - this is the fingerprint of anthropogenic (human caused) ocean acidification.įrom the relations above it can be seen that the ability of seawater to buffer changes in its pH as CO 2 is added depends on the amount (or concentration) of CO 3 2- present. Note that there is so little carbon in the form of carbonic acid (H 2CO 3) at any one moment in time, that the concentrations of CO 2(aq) and H 2CO 3 are usually combined as (also written: ). CO 3 2- is the next most abundant species (~10% of DIC), while CO 2(aq) represents less than 1% of DIC. Typically, the surface waters of today's ocean have a pH of around 8.1, meaning that HCO 3 - is the dominant carbonate species, representing about 90% of DIC. The distribution of DIC between these species varies with seawater pH (Figure 2). The total dissolved inorganic carbon (DIC) in seawater is defined as the sum of + +. 3 goes to the left to consume some of the excess H +, and in doing so, also consumes CO 3 2.
3) has now been unbalanced by excess acidity (H +), Eq. One can also think of the sequence of events resulting from dissolving CO 2 in seawater as firstly the production of HCO 3 - and H +, but because the equilibrium between HCO 3 - and CO 3 2- (Eq. 3), releasing protons and so decreasing the pH-which is where the ‘ocean acidification' actually comes from-but this drop is much smaller than for an un-buffered system. Where CO 2 is effectively neutralized by reaction with CO 3 2- to produce HCO 3. This is due to the natural capacity of seawater to buffer against changes in pH, which can be represented simply by: However, when CO 2 dissoves in seawater it does not fully dissociate into carbonate ions and the number of hydrogen ions produced (and the drop in pH) is therefore smaller than one might expect.
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