Living on the edge: How did shallow-marine ecosystems thrive during hothouse climates?

Earths climate history serves as a natural laboratory for testing the effect of (very) warm climates on the biosphere. The Cretaceous period featured prolonged intervals of greenhouse climates characterized by high atmospheric CO2 concentrations and mostly ice-free poles (OBrien et al., 2017; Ladant and Donnadieu, 2016). In such a climate, shallow seas in low latitudes likely became very hot, especially during the summer (Jones et al., 2022; de Winter et al., 2021). At the same time, life seems to have thrived there in reef-like ecosystems built by rudists, an extinct group of bivalve mollusks (Gili and Gtz, 2018). The summer temperatures documented for these ecosystems exceed the maximum tolerable temperatures of any marine mollusk alive today, which raises the question how these animals adapted to more extreme temperatures.Studying the growth of these ancient organisms and the extreme climate conditions they faced can yield valuable insight into the resilience of marine ecosystems to climate change. To tackle this climate resilience conundrum, we present a detailed sclerochronological (incrementally sampled) dataset of daily to seasonal scale variability in shell chemistry from fossil mollusks from a Campanian (75-million-year-old) low-latitude shallow marine ecosystem. With it, we investigate the extreme climate conditions these animals lived through and how this affected their growth or even the paleo-weather patterns they experienced. The goal of this study is to demonstrate how high-resolution geochemical records through fossil mollusk shells can shed light on the variability in past warm ecosystems and open the discussion about the limits of life in the shallow marine realm during a greenhouse climate.Gili, E. and Gtz, S.: Treatise Online no. 103: Part N, Volume 2, Chapter 26B: Paleoecology of rudists, 1, https://doi.org/10.17161/to.v0i0.7183, 2018.Jones, M. M., Petersen, S. V., and Curley, A. N.: A tropically hot mid-Cretaceous North American Western Interior Seaway, Geology, 50, 954958, https://doi.org/10.1130/G49998.1, 2022.Ladant, J.-B. and Donnadieu, Y.: Palaeogeographic regulation of glacial events during the Cretaceous supergreenhouse, Nat Commun, 7, 12771, https://doi.org/10.1038/ncomms12771, 2016.OBrien, C. L., Robinson, S. A., Pancost, R. D., Sinninghe Damst, J. S., Schouten, S., Lunt, D. J., Alsenz, H., Bornemann, A., Bottini, C., Brassell, S. C., Farnsworth, A., Forster, A., Huber, B. T., Inglis, G. N., Jenkyns, H. C., Linnert, C., Littler, K., Markwick, P., McAnena, A., Mutterlose, J., Naafs, B. D. A., Pttmann, W., Sluijs, A., van Helmond, N. A. G. M., Vellekoop, J., Wagner, T., and Wrobel, N. E.: Cretaceous sea-surface temperature evolution: Constraints from TEX 86 and planktonic foraminiferal oxygen isotopes, Earth-Science Reviews, 172, 224247, https://doi.org/10.1016/j.earscirev.2017.07.012, 2017.de Winter, N. J., Mller, I. A., Kocken, I. J., Thibault, N., Ullmann, C. V., Farnsworth, A., Lunt, D. J., Claeys, P., and Ziegler, M.: Absolute seasonal temperature estimates from clumped isotopes in bivalve shells suggest warm and variable greenhouse climate, Commun Earth Environ, 2, 18, https://doi.org/10.1038/s43247-021-00193-9, 2021.