Modelling the effects of CO2 on C3 and C4 grass competition during the mid-Pleistocene transition in South Africa

Abstract

Palaeoenvironmental reconstructions of the interior of South Africa show a wetter environment than today and a non-analogous vegetation structure in the Early Pleistocene. This includes the presence of grasses following both C3 and C4 photosynthetic pathways, whereas C3 grasses decline after the mid-Pleistocene transition (MPT, c. 1.2–0.8 Ma). However, the local terrestrial proxy record cannot distinguish between the potential drivers of these vegetation changes. In this study we show that low glacial CO2 levels, similar to those at the MPT, lead to the local decline of C3 grasses under conditions of decreased water availability, using a vegetation model (LPX) driven by Atmosphere–Ocean coupled General Climate Model climate reconstructions. We modelled vegetation for glacial climates under different levels of CO2 and fire regimes and find evidence that a combination of low CO2 and changed seasonality is driving the changes in grass cover, whereas fire has little influence on the ratio of C3:C4 grasses. Our results suggest the prevalence of a less vegetated landscape with limited, seasonal water availability, which could potentially explain the much sparser mid-Pleistocene archaeological record in the southern Kalahari.

Southern Hemisphere Subtropical Front impacts on Southern African Hydroclimate and early Hominins Across the Mid-Pleistocene Transition

“…Our factor analysis makes clear that the shifts in ODP 1084 δ 13 C wax over MIS 21 are related to the shift in rainfall seasonality inferred by δD p . While it is likely that large variability between glacial and interglacial pCO 2 also impacted C 3 and C 4 proportions on the landscape [44,45], we emphasize that both the changes to deep ocean carbon storage and rainfall seasonality are ultimately facilitated by a meridional migrations of the AFS [9,10]. Improved sampling resolution during the Late Pleistocene is needed to further examine the impacts of glacial/interglacial variability on δD p and δ 13 C wax after the MPT.…”

First evidence for human occupation of a lava tube in Arabia: The archaeology of Umm Jirsan Cave and its surroundings, northern Saudi Arabia

“…Local annual rainfall and temperature is correlated with the prevalence of plant photosynthetic pathways in the grass cover of arid regions [64][65][66], with typically a greater presence of C 4 plants in hotter and drier settings. Using δ 13 C values from the skeletal remains of grazing herbivores, which randomly sample from available grasses and are not subject to foddering by humans, it is possible to reconstruct past grass covers [67].…”

Eolian chronology reveals causal links between tectonics, climate, and erg generation

“…Additional sand availability was promoted by the sharp decrease in CO2 levels that began around 3 Ma ( Van de Wal et al, 2011), stimulating the dispersion and increasing the density of C4 grass cover during the Pleistocene, when concentrations fell below the 250 ppm threshold (Ecker et al, 2020). This in turn would have facilitated the mobilization of the freshly produced sand, by reducing the tree to grass ratio.…”