Orbital precession modulates interannual rainfall variability, as recorded in an Early Pleistocene speleothem

Abstract

Interannual variability of African rainfall impacts local and global communities, but its past behavior and response in future climate projections are poorly understood. This is primarily due to short instrumental records and a lack of long high-resolution palaeoclimate proxy records. Here we present an annually resolved 91,000 year Early Pleistocene record of hydroclimate from the early hominin-bearing Makapansgat Valley, South Africa. Changes in speleothem annual band thickness are dominated by precession over four consecutive orbital cycles with strong millennial-scale periodicity. The frequency of interannual variability (2.0–6.5 yr oscillations) does not change systematically, yet its amplitude is modulated by the orbital forcing. These long-term characteristics of interannual variability are reproduced with transient climate model simulations of water balance for South Africa from the Late Pleistocene to Recent. Based on these results, we suggest that the frequency of interannual variations in southern African rainfall is likely to be stable under anthropogenic warming, but that the size of year-to-year variations may increase. We see an orbitally forced increase in the amplitude of interannual climate variability between 1.8 Ma and 1.7 Ma coincident with the first evidence for the Acheulean stone tool technology.

Speleothem evidence for C3 dominated vegetation during the Late Miocene (Messinian) of South Africa

“…= 3.5, n = 27) for the spring, well and cave waters of northern South Africa reported byKronfeld et al (1994). High initial 234 U/ 238 U values of 5.56 to 6.19 (mean = 5.9, n = 16) do occur in U-Th dated South African speleothems from the Makapansgat Valley and from Wolkberg Cave(Hopley et al, 2018; Holzkamper et al, 2009), but these sites are located approximately 300 km north east of the Cradle of Humankind where the bedrock and hydrological conditions are different. Using the Cradle of Humankind values (…”

A Window into Africa’s Past Hydroclimates: The SISAL_V1 Database Contribution

“…While the initial spread of C4 grasses was probably caused by dropping global levels of atmospheric pCO2, a subsequent expansion of C4 grasses in the South African summer rainfall region at ~1.7 Ma was caused by drying climates due to the reorganization of tropical circulation [12,13]. Changes in the net water budget, indicated by δ 18 O and lamina thickness, follow a precessional periodicity at this time [88,13].…”

A Window into Africa’s Past Hydroclimates: The SISAL_v1 Database Contribution

“…While the initial spread of C 4 grasses was probably caused by falling levels of atmospheric pCO 2 , a subsequent expansion of C 4 grasses in the summer rainfall region at~1.7 Ma was caused by drying climates due to the reorganization of tropical circulation [10,11]. Changes in the net water budget, indicated by δ 18 O and lamina thickness, follow a precessional periodicity at this time [13,111].…”