Strategies in times of crisis—insights into the benthic foraminiferal record of the Palaeocene–Eocene Thermal Maximum

Abstract

Climate change is predicted to alter temperature, carbonate chemistry and oxygen availability in the oceans, which will affect individuals, populations and ecosystems. We use the fossil record of benthic foraminifers to assess developmental impacts in response to environmental changes during the Palaeocene–Eocene Thermal Maximum (PETM). Using an unprecedented number of µ-computed tomography scans, we determine the size of the proloculus (first chamber), the number of chambers and the final size of two benthic foraminiferal species which survived the extinction at sites 690 (Atlantic sector, Southern Ocean, palaeodepth 1900 m), 1210 (central equatorial Pacific, palaeodepth 2100 m) and 1135 (Indian Ocean sector, Southern Ocean, palaeodepth 600–1000 m). The population at the shallowest site, 1135, does not show a clear response to the PETM, whereas those at the other sites record reductions in diameter or proloculus size. Temperature was similar at all sites, thus it is not likely to be the reason for differences between sites. At site 1210, small size coincided with higher chamber numbers during the peak event, and may have been caused by a combination of low carbonate ion concentrations and low food supply. Dwarfing at site 690 occurred at lower chamber numbers, and may have been caused by decreasing carbonate saturation at sufficient food levels to reproduce. Proloculus size varied strongly between sites and through time, suggesting a large influence of environment on both microspheric and megalospheric forms without clear bimodality. The effect of the environmental changes during the PETM was more pronounced at deeper sites, possibly implicating carbonate saturation.This article is part of a discussion meeting issue ‘Hyperthermals: rapid and extreme global warming in our geological past’.

Extensive morphological variability in asexually produced planktic foraminifera

“…In some benthic foraminifera, the haploid generation can reproduce either sexually or asexually (18,(22)(23)(24), and some species display a preference for asexual reproduction when population densities are too low for gamete fusion (23). By contrast, other species appear to increase the frequency of sexual reproduction in stressful or unstable environmental conditions (25,26). Planktic foraminifera are generally cultured in isolation once retrieved by scuba or net tow; thus, a preference for sexual reproduction at high population densities does not explain observations.…”

Placing our current ‘hyperthermal’ in the context of rapid climate change in our geological past

“…Benthic foraminifera are one of the few marine organisms to suffer significant extinction across the PETM [56]. Schmidt et al [49] show that the impact of the event in terms of benthic foram shell morphology is water-depthdependent, suggesting an important role for ocean acidification and ocean anoxia in driving the extinction of these species, as stress-related dwarfing and a potential change in reproductive strategy is evident at the deepest deep-ocean sites.…”

The origin and diversification of pteropods precede past perturbations in the Earth’s carbon cycle

“…Many pelagic groups disappeared during the well-known mass extinction at the K–Pg boundary ( 51 , 52 ), and, though it is difficult to pinpoint a specific cause of these changes ( 17 ), recent evidence from boron isotopes in foraminifers shows that rapid surface ocean acidification was associated with the impact ( 53 ). The PETM also had an impact on marine calcifiers, as shown by a rapid decrease in calcium carbonate content in marine sediments ( 54 ) and the recording of one of the largest extinctions among deep-sea benthic foraminifers ( 49 , 55 ). Planktonic calcifiers were also affected during the PETM, with shifts in species assemblages and changes in species abundance of calcareous nanoplankton, but no clear signal of increased extinction ( 56 – 59 ).…”