The Sahel is highly sensitive to flooding, droughts, and wildfires, risking food and other resources on which nearly 100 million people depend. Understanding how natural variations of precipitation and vegetation fluctuate during high-amplitude glacial- interglacial cycles can help constrain the regional sensitivity to a wide range of external forcings. Further, the interactions between climate and ecosystem changes remain uncertain for sub-Saharan Africa due to the lack of long, highly-resolved, quantitative, terrestrial records. Here we present precipitation and vegetation records from ~215 ka to present, derived from long leaf wax hydrogen (δDwax) and carbon (δ13Cwax) isotopes, respectively. These geochemical records are derived from ODP Site 959 in the Gulf of Guinea, where westerly winds and major river systems transport Western Sahel-sourced terrestrial leaf waxes. We find that, unlike many African records that are precessionally- driven, obliquity plays an important role in West African late Pleistocene hydroclimate, suggesting that a cross-equatorial insolation gradient may be more important in this area and certainly that drivers of orbital-scale precipitation change are regionally-specific. Further, vegetation changes appear to have a complex relationship with hydroclimate over this mid-late Pleistocene interval. A potential shift in this climate-environment coupling at MIS6 ~130 ka, which is a time when there is also a shift in forcing mechanisms in East Africa, suggests that the global boundary condition changes associated with large glacial- interglacial cycling may affect equatorial climate.
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Orbital controls on eastern African hydroclimate in the Pleistocene
Abstract Understanding eastern African paleoclimate is critical for contextualizing early human evolution, adaptation, and dispersal, yet Pleistocene climate of this region and its governing mechanisms remain poorly understood due to the lack of long, orbitally-resolved, terrestrial paleoclimate records. Here we present leaf wax hydrogen isotope records of rainfall from paleolake sediment cores from key time windows that resolve long-term trends, variations, and high-latitude effects on tropical African precipitation. Eastern African rainfall was dominantly controlled by variations in low-latitude summer insolation during most of the early and middle Pleistocene, with little evidence that glacial–interglacial cycles impacted rainfall until the late Pleistocene. We observe the influence of high-latitude-driven climate processes emerging from the last interglacial (Marine Isotope Stage 5) to the present, an interval when glacial–interglacial cycles were strong and insolation forcing was weak. Our results demonstrate a variable response of eastern African rainfall to low-latitude insolation forcing and high-latitude-driven climate change, likely related to the relative strengths of these forcings through time and a threshold in monsoon sensitivity. We observe little difference in mean rainfall between the early, middle, and late Pleistocene, which suggests that orbitally-driven climate variations likely played a more significant role than gradual change in the relationship between early humans and their environment.
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- Award ID(s):
- 1826938
- NSF-PAR ID:
- 10350040
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 12
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Terrestrial‐marine dust fluxes, pedogenic carbonate δ13C values, and various paleovegetation proxies suggest that Africa experienced gradual cooling and drying across the Pliocene‐Pleistocene (Plio‐Pleistocene) boundary (2.58 million years ago [Ma]). However, the timing, magnitude, resolution, and relative influences of orbitally‐driven changes in high latitude glaciations and low latitude insolation differ by region and proxy. To disentangle these forcings and investigate equatorial eastern African climate across the Plio‐Pleistocene boundary, we generated a high‐resolution (∼3,000‐year) data set of compound‐specific
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Abstract In the early Pleistocene, global temperature cycles predominantly varied with ~41‐kyr (obliquity‐scale) periodicity. Atmospheric greenhouse gas concentrations likely played a role in these climate cycles; marine sediments provide an indirect geochemical means to estimate early Pleistocene CO2. Here we present a boron isotope‐based record of continuous high‐resolution surface ocean pH and inferred atmospheric CO2changes. Our results show that, within a window of time in the early Pleistocene (1.38–1.54 Ma), pCO2varied with obliquity, confirming that, analogous to late Pleistocene conditions, the carbon cycle and climate covaried at ~1.5 Ma. Pairing the reconstructed early Pleistocene pCO2amplitude (92 ± 13 μatm) with a comparably smaller global surface temperature glacial/interglacial amplitude (3.0 ± 0.5 K) yields a surface temperature change to CO2radiative forcing ratio of
S [CO2]~0.75 (±0.5) °C−1·W−1·m−2, as compared to the late PleistoceneS [CO2]value of ~1.75 (±0.6) °C−1·W−1·m−2. This direct comparison of pCO2and temperature implicitly incorporates the large ice sheet forcing as an internal feedback and is not directly applicable to future warming. We evaluate this result with a simple climate model and show that the presumably thinner, though extensive, northern hemisphere ice sheets would increase surface temperature sensitivity to radiative forcing. Thus, the mechanism to dampen actual temperature variability in the early Pleistocene more likely lies with Southern Ocean circulation dynamics or antiphase hemispheric forcing. We also compile this new carbon dioxide record with published Plio‐Pleistocene δ11B records using consistent boundary conditions and explore potential reasons for the discrepancy between Pliocene pCO2based on different planktic foraminifera.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41598-022-06826-z
