Ocean Acidification is a Possible Contri-butor to Ancient Reef Crises

Ocean acidification<!–[if supportFields]> XE “acidification” <![endif]–><!–[if supportFields]><![endif]–> in recent years has been linked to decreases in calcification rates in reef-building corals. Little work has been done to analyze past coral<!–[if supportFields]> XE “coral” <![endif]–><!–[if supportFields]><![endif]–> reef crises and ocean acidification<!–[if supportFields]> XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–> events. Coral reefs have one of the most complete fossil records due to their calcium carbonate<!–[if supportFields]> XE “calcium carbonate (CaCO3)” <![endif]–><!–[if supportFields]><![endif]–> skeletons. Thus, the coral reef fossil record has been widely studied and documented. Ocean acidification in prehistoric times has been connected with several possible mechanisms resulting in large releases of CO2 into the marine environment. Like the fossil record, geologic evidence of these events has been collected in a database. Kiessling and Simpson (2011) investigated a possible relationship between past extinction<!–[if supportFields]> XE “extinction” <![endif]–><!–[if supportFields]><![endif]–> events, reef crises, and ocean acidification trends by examining data from the paleological databases. The authors found that four out of five past reef crises could be partially attributed to ocean acidification and global warming. Out of these five major events, only two had geological proof of a concurrent ocean acidification event. —Emily Putnam
Kiessling, W., Simpson, C., 2011. On the potential for ocean acidification<!–[if supportFields]>XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–> to be a general cause of ancient reef crises. Global Change Biology 17, 56–67.

Kiessling and Simpson from the Leibniz Institute for Research on Evolution and Biodiversity first determined major reef crises by calculating changes in reef volume per time interval and defining crises as significant outliers below the normal. Extinction intensities were found by calculating the rate of extinction<!–[if supportFields]>XE “extinction” <![endif]–><!–[if supportFields]><![endif]–> from the PaleoBiology Database. The selectivity of these extinctions was determined in two ways––by comparing calcified organisms with all others, and by comparing physiologically pH sensitive organisms with all others. The authors also took into consideration the possibility that ocean acidification<!–[if supportFields]>XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–> could have impacted the preservation of the fossil record by measuring the completeness of the record for groups at each time interval.
Kiessling and Simpson found that reef volume changes vary widely over time and only the most severe decreases in volume can be identified as crises. A total of five reef crises were found from the existing records––Late Devonian, Early Triassic, earliest Triassic, Early Jurassic and early Eocene. Four out of five of these crises followed elevated extinction<!–[if supportFields]> XE “extinction” <![endif]–><!–[if supportFields]><![endif]–> rates of calcifying organisms, with three of these showing a preferential extinction of these organisms. Five marine biodiversity<!–[if supportFields]>XE “biodiversity” <![endif]–><!–[if supportFields]><![endif]–> crises were found––the end-Ordivician, Late Devonian, end-Permian, end-Triassic, and end-Cretaceous. Degradation of the fossil record agreed with fossil preservation hypotheses for acidified conditions.
Analysis of the geologic evidence for possible ocean acidification<!–[if supportFields]> XE “ocean acidification” <![endif]–><!–[if supportFields]><![endif]–> events revealed that four out of five reef crises could have been at least partially due to ocean acidification<!–[if supportFields]> XE “acidification” <![endif]–><!–[if supportFields]><![endif]–>. Ocean acidification was not connected to mass extinctions. Thus, ocean acidification in today’s world may play a role in reef stress, but may not lead to mass extinctions. Despite the attempt to connect the past with the present, the authors could not offer more information relating the magnitude of the crises with CO2 levels to determine the potential impact of the current ocean acidification event on coral<!–[if supportFields]>XE “coral” <![endif]–><!–[if supportFields]><![endif]–> reefs. Global warming was considered to be an equally likely cause of reef degradation and crisis as ocean acidification. Kiessling and Simpson therefore concluded that the greatest danger to coral reefs comes from the double threat of global warming and ocean acidification.

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