Severity of ocean acidification following the end-Cretaceous asteroid impact Toby Tyrrella,1, Agostino Mericob,c, and David Ian Armstrong McKaya aOcean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton SO14 3ZH, United Kingdom; bSystems Ecology, Leibniz Center for Tropical Marine Ecology, 28359 Bremen, Germany; and cJacobs University, 28759 Bremen, Germany Edited by Henry J. Melosh, Purdue University, West Lafayette, IN, and approved April 16, 2015 (received for review October 1, 2014) Most paleo-episodes of ocean acidification (OA) were either too In contrast, at the end of the Cretaceous the asteroid impact slow or too small to be instructive in predicting near-future induced very sudden changes. Here we investigate the possibility impacts. The end-Cretaceous event (66 Mya) is intriguing in this that there was a sharp and sudden acidification event con- regard, both because of its rapid onset and also because many centrated in surface waters [deep waters experience delayed and pelagic calcifying species (including 100% of ammonites and more less severe acidification in response to an atmospheric source of than 90% of calcareous nannoplankton and foraminifera) went acidity (5)]. Because there are no paleo records with which to extinct at this time. Here we evaluate whether extinction-level OA constrain seawater chemistry changes during the critical few years could feasibly have been produced by the asteroid impact. Carbon following the impact (the slow speed at which most ocean sed- cycle box models were used to estimate OA consequences of iments accumulate limits the resolution of sediment records to (i) vaporization of up to 60 × 1015 mol of sulfur from gypsum rocks 15 thousands of years), we use models to calculate how dramatic at the point of impact; (ii) generation of up to 5 × 10 mol of NOx by the impact pressure wave and other sources; (iii) release of up the surface OA may have been at the end of the Cretaceous. to 6,500 Pg C as CO from vaporization of carbonate rocks, wild- 2 Extinctions of Calcifiers at the End of the Cretaceous. Another rea- fires, and soil carbon decay; and (iv) ocean overturn bringing high- son for being particularly interested in the Cretaceous/Paleogene CO2 water to the surface. We find that the acidification produced by most processes is too weak to explain calcifier extinctions. Sul- (K/Pg) boundary in the context of OA is that many surface- furic acid additions could have made the surface ocean extremely dwelling calcifiers went extinct at this time (6). Ammonites had EARTH, ATMOSPHERIC, existed on Earth for some 300 million years and had survived AND PLANETARY SCIENCES undersaturated (Ωcalcite <0.5), but only if they reached the ocean very rapidly (over a few days) and if the quantity added was at the previous extinction events, including the one at the end of the top end of literature estimates. We therefore conclude that severe Permian when more than 95% of all marine species were lost, ocean acidification might have been, but most likely was not, re- but they succumbed at the K/Pg (7). Within other groups of ma- sponsible for the great extinctions of planktonic calcifiers and am- rine organisms there seems also to have been a strong extinction monites at the end of the Cretaceous. bias toward calcifiers. Among autotrophs, for example, more than 90% of all calcareous nannoplankton (coccolithophore) species EVOLUTION ocean acidification | asteroid impact | K/Pg boundary | mass extinction went extinct at this time (8, 9). By contrast, there were much lower extinction rates for comparable noncalcareous groups, such as rom preindustrial times up to 2008, ca. 530 Pg of carbon were siliceous diatoms, of which at most 50% of species went extinct Fadded to the atmosphere through burning of fossil fuels and (10), organic-walled dinoflagellates, which experienced no signif- deforestation (1). This has led to an increase in atmospheric CO2 icant extinction (11), and noncalcifying haptophyte phytoplankton, of 40% (from 280 in 1750 to 400 ppm today). Simultaneously, of which many clades survived the K/Pg (12). Similarly, among about 160 Pg C has been taken up by the ocean, causing ocean heterotrophs, more than 95% of carbonate-shelled planktic fora- acidification (OA) (2). minifera were lost (10), whereas only a few planktic silica-shelled OA is of particular concern for calcifying organisms, because it radiolaria went extinct (10). The particular severity of extinctions 2− leads to lower CO3 concentrations and hence lower seawater saturation states with respect to CaCO3 (Ω). In theory, lower Ω Significance should make it energetically more costly for organisms to synthe- size CaCO3 shells and skeletons and, subsequently, if Ω falls below Ammonites went extinct at the time of the end-Cretaceous 1.0, to maintain them against dissolution. A large variety of short- asteroid impact, as did more than 90% of species of calcium term experiments have been carried out to test for such conse- carbonate-shelled plankton (coccolithophores and foraminifera). quences (2). It is widely recognized, however, that one aspect that Comparable groups not possessing calcium carbonate shells these experiments generally do not address is the degree to which were less severely affected, raising the possibility that ocean organisms can evolve in response to the changing carbonate chem- acidification, as a side effect of the collision, might have been istry and thereby become more tolerant of the new conditions. As responsible for the apparent selectivity of the extinctions. We a result, there is a need for approaches that reveal the long-term investigated whether ocean acidification could have caused the response to OA with evolutionary adaptation factored in. disappearance of the calcifying organisms. In a first detailed modelling study we simulated several possible mechanisms from Motivation impact to seawater acidification. Our results suggest that acidi- fication was most probably not the cause of the extinctions. Using Earth History to Understand Ocean Acidification Impacts. Events in the past could potentially shed more light on the evolutionary Author contributions: T.T. designed research; T.T., A.M., and D.I.A.M. performed research; response to OA. However, a recent review (3) highlighted a major T.T., A.M., and D.I.A.M. analyzed data; and T.T., A.M., and D.I.A.M. wrote the paper. difficulty: During most suspected OA events, CO2 levels rose so The authors declare no conflict of interest. slowly that the carbonate compensation process, that is, the auto- This article is a PNAS Direct Submission. Ω matic stabilizing mechanism opposing changes in (4), must have 1To whom correspondence should be addressed. Email: [email protected]. interposed to alter the nature of the impacts, making them less This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. useful for understanding the future. 1073/pnas.1418604112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1418604112 PNAS Early Edition | 1of6 Downloaded by guest on October 1, 2021 for calcifiers has led to suggestions (e.g., refs. 13 and 14) that they pH or the possible loss of SO3 through recombining with CaO were caused by OA. within the plume to reform solid CaSO4 (22, 23). We used biogeochemical box models of the global carbon cycle After being injected into the atmosphere, SO3 would have been (simulating particle fluxes, mixing, and air–sea CO2 exchange, transformed to sulfuric acid (H2SO4), which would have rained explained in greater detail in Methods and Supporting Information) out to the ocean. There is debate about the rapidity of this trans- to assess whether severe OA might have occurred. Because of the port. Sulfur dynamics following the eruption of Mount Pinatubo absence of accompanying paleodata at this timescale, our aim is in 1991 suggest (24, 25) a timescale of a few years, with injection not to pin down the exact pattern of carbon chemistry changes of S into the dry stratosphere delaying its return to Earth’s sur- that took place at the K/Pg. Instead, we focus our attention on face. In line with this, earlier K/Pg modeling studies (23, 26) used delineating the upper bound of OA severity. Our aim is to calculate atmospheric residence times of S of several months to a few years. the maximum degree of OA that might plausibly have occurred, However, an alternative and very different scenario has recently not the most likely. been proposed for the K/Pg. It is suggested (14) that, immediately after the impact, most of the sulfuric acid aerosols could have Previous Work on Carbon Chemistry at the K/Pg been scavenged by large silicate particles falling rapidly back to A few other studies (14–18) have previously addressed carbon Earth, delivering the H2SO4 to the ocean within only one or a cycle and OA changes at the K/Pg. Beerling et al. (16) used few days. data (the stomatal index of land plant leaves) and a box model; D’Hondt et al. (18) estimated total sulfuric acid production to – × 15 – they suggested that atmospheric pCO2 increased from 350 to have been in the range 10 130 10 mol H2SO4 (320 4,160 Pg 500 to 2,300 ppmv across the K/Pg boundary [although such a S), based on several earlier studies (27–29). Later studies suggested ∼ – × 15 rise is not seen in other proxy data (17)], from which they 10-fold smaller total production of only 0.9 9 10 mol H2SO4 – – × 15 – inferred an instantaneous transfer of ca.4,600PgCfromrocks (30 300PgS,ref.23)and2.4 11 10 mol H2SO4 (78 364 to the atmosphere. Pg S, ref. 30). D’Hondt et al. (18) used calculations rather than models to We implemented this hypothesis in the model through a family × 15 investigate the severity of surface OA at the K/Pg boundary.