Coral reef and ocean acidification: effects on marine ecosystems
This course provides an overview of the two topics connecting the rise of atmospheric carbon dioxide with ocean acidity, coral reef destruction and past historical events that can be used as a proxy to extrapolate how nefarious anthropogenic carbon dioxide emissions are.
Additionally, the course explores the effects of ocean acidification and global warming on coral reef construction and maintenance, coral bleaching, dissolution and the destruction of essential marine ecosystems that harbour more than half of the ocean’s biodiversity.
Coral Reef and Ocean Acidification Course
Atmospheric carbon dioxide (CO2) levels have increased since preindustrial times.
The speed at which anthropogenic carbon dioxide emissions are happening is unprecedented.
Uptake of atmospheric carbon dioxide by the ocean occurs naturally; however, the concentrations present nowadays are causing a drastic reduction in pH, altering essential chemical balances -> ocean acidification.
The “other CO2 problem” has often been used as synonymous of ocean acidification. Human-made CO2 is responsible for not only global warming but also ocean acidification.
This course explores the effects of ocean acidification and global warming on coral reefs.
Coral Reef and Ocean Acidification
Scientists estimate that since preindustrial times, 30 to 40% of anthropogenic carbon dioxide has been absorbed by the oceans, rivers and lakes.
Uptake of atmospheric carbon dioxide reduces surface water pH and calcium carbonate (CaCO3) saturation unbalancing oceanic production of calcium structures such as shells and skeletons. The ocean’s capacity to absorb atmospheric CO2 depends on how fast calcium carbonate (CaCO3) dissolute in the water column or sediments.
The Aptian Oceanic Anoxic event (~120 Mya) is characterized by the widespread deposition of organic-rich sediments and represents a possible ocean acidification example. Scientists suggest that a marine calcification crisis occurred during this event. However, rather than being transient, the event was long-lasting, with a total duration of ~1Ma … The PETM (Paleocene-Eocene Thermal Maximum) (~55Mya), is probably, the closest analogue for the future that has been identified in the geologic record. The PETM was marked by both a global increase in surface temperatures by 5-9ºC and a substantial carbon release. These events resulted in ocean acidification and widespread dissolution of deep-sea carbonates.
PETM has several characteristics that are important for a meaningful comparison with our present situation.
1. It was a transient event with a rapid onset
2. It was associated with a large and fast carbon input
Nonetheless, it is important to keep in mind that the climatic and carbon cycle conditions before the PETM were different from those today.
Zeebe 2012 compared a PETM scenario with a Business-as-Usual scenario of fossil fuel emissions (See figure 4 for the PETM versus Anthropocene scenario). The results show that if the proposed PETM scenario roughly resembles the actual conditions during the onset of the event, then the effects on ocean chemistry, including surface-ocean saturation state, were less severe during the PETM than would be expected for the future.
The timescale of the anthropogenic carbon input is so short that the natural capacity of the surface reservoirs to absorb carbon is overwhelmed. As a result of a 5,000-Pg C (carbon), the surface-ocean calcite saturation state would drop approximately 5.4 to less than 2 within a few hundred years (figure 4). In contrast, the PETM scenario suggests a corresponding decline of saturation state from 5.5 to ~4 within a few thousand years.
The destruction of marine ecosystems
Ecologically, the perturbation of the surface-water saturation state across the PETM was not detrimental to the survival of the majority of calcareous species. In contrast, the Paleocene-Eocene boundary marks a major extinction event of benthic foraminifera, affecting 30 to 50% of species globally.
However, it is not clear whether the benthic extinction was caused by changes in oxygenation, bottom water temperatures, carbonate undersaturation or other factors. The reality is that the ocean acidification event that humans may cause over the next few centuries is unprecedented in the geological past. Therefore, simulation models using past events need to be evaluated with care. Nonetheless, the consequence of high carbon dioxide emission rates has a nefarious effect on ocean chemistry irrefutable