PHYSICOCHEMICAL CONTROLS ON FORAMINIFERAL SIZE


PETM suggest a 3–4°C warming of deep waters. Thus, our results provide quantitative support for the hypothesis of Alegret et al. (2010) that size reduction in benthic foramini- fera during the PETM was driven, at least in part, by increased metabolic demands caused by warming of marine bottom waters. Climate warming may also have played a


role in foraminiferal size reduction across the end-Permian mass extinction. Rego et al. (2012) demonstrated that size reduction of benthic foraminifera across the Permian–Triassic mass extinction boundary was driven both by size-selective extinction and by size reduction in survivor species, with the size reduction within lineages accounting for the majority of overall size decrease. Small-sized foraminifera dominate postextinction fossil assemblages even in the shallowest marine settings (Groves et al. 2007; Payne et al. 2011; Song et al. 2011), where geochemical evidence for dysoxia is at best mixed (Loope et al. 2013), suggesting additional selective pressures on size beyond oxygen deficiency. Oxygen isotope data from conodont microfossils suggest warming of approximately 10°C across this extinction interval and persisting through the Early Triassic (Joachimski et al. 2012; Sun et al. 2012; Romano et al. 2013). Thus, elevated metabolic demand at higher environmental temperatures may help to explain the size reduction within survivors of the end-Permian extinction. The effects of temperature on long-term


evolution of foraminiferal morphology may also have been underappreciated. For example, a compilation of maximum test-size measure- ments of calcareous trochospiral benthic foraminifera over the past 120 Myr documents temporal variations in size related to global environmental fluctuations, which Kaiho (1999b) interpreted to result from variation in bottom-water oxygen concentrations. However, intervals marked by maximum test size also coincide with global climate cooling (Kaiho 1999b) and might reflect the direct influence of temperature on metabolic


demand. Similarly, Boltovskoy (1988) identi- fied five Rotallid foraminiferal species that show a trend of increasing mean and maximum size from the Oligocene through


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the Pleistocene and invoked Cope’s rule, the tendency for size increase over evolutionary time, to explain this trend. However, the late Cenozoic represents a period of gradual global climate cooling (Zachos et al. 2001). Thus, increasing foraminiferal size in these select species over the last ~34 Myr could alternatively result from temperature effects on metabolic demand, similar to the spatial and temporal trends toward larger size at cooler temperatures in Cenozoic deep-sea ostracodes (Hunt and Roy 2006).


Global Climate Change and Benthic Foraminifera


Concerns surrounding anthropogenic


climate change due to increasing atmospheric CO2 concentrations over the next century to millennium often focus on the effects of ocean warming, acidification, and deoxygenation on marine life (e.g., Gruber 2011; Bijma et al. 2013; Bopp et al. 2013). Of these expected changes, our results indicate that warming seawater temperatures will inflict the greatest metabolic stress on benthic foraminifera. Temperature acts both directly on metabolic rate and indirectly on the bioavailability of dissolved oxygen concentrations (Verberk et al. 2011; Verberk and Atkinson 2013). Warming oceans hold less dissolved oxygen (Gruber 2011; Bijma et al. 2013), thereby making it difficult for foraminifera, and other marine life, to meet increasing metabolic demands and subsequent oxygen requirements in warm waters. Foraminifera typically dominate low-oxygen environments and are thus ecologically important in the uptake and processing of organic matter deposited in these sediments (Moodley et al. 2000; Woulds et al. 2007; Gooday et al. 2009, 2010). However, as hypoxic conditions become more prevalent in the near future, foraminifera in waters with >3 ml/liter [O2] will have to devote a greater amount of energy to perform basic biological reactions or reduce their test volume–to–surface area ratios. Foraminifera that occur in modern oxygen minimum zones are typified by small, thin-shelled unornamented tests with low volume–to–surface area ratios; this morpho- logy decreases the oxygen demand of the cell


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