The roles of climate and human land-use in the late Holocene rainforest crisis of Central Africa

Abstract

There is increasing evidence that abrupt vegetation shifts and large-scale erosive phases occurred in Central Africa during the third millennium before present. Debate exists as to whether these events were caused by climate change and/or intensifying human activities related to the Bantu expansion. In this study, we report on a multi-proxy investigation of a sediment core (KZR-23) recovered from the Congo submarine canyon. Our aim was to reconstruct climate, erosion and vegetation patterns in the Congo Basin for the last 10,000 yrs, with a particular emphasis on the late Holocene period. Samples of modern riverine suspended particulates were also analyzed to characterize sediment source geochemical signatures from across the Congo watershed. We find that a sudden increase of bulk sediment aluminium-to-potassium (Al/K) ratios and initial radiocarbon ages of bulk organic matter occurred after 2,200 yrs ago, coincident with a pollen-inferred vegetation change suggesting forest retreat and development of savannas. Although hydrogen isotope compositions of plant waxes (δD wax) do not reveal a substantial hydroclimate shift during this period, neodymium isotopes and rare earth elements in detrital fractions indicate provenance changes for the sediment exported from the Congo Basin at that time, hence suggesting a reorganization of spatial rainfall patterns across Central Africa during this event. Taken together, these findings provide evidence for changing landscapes in Central Africa from about 2,200 yrs ago, associated with synchronous events of vegetation changes and enhanced erosion of preaged and highly weathered soils. These events coincided remarkably well with the arrival of Iron Age communities into the rainforest, as inferred from comparison to regional archaeological syntheses. While the human impact on the environment remains difficult to quantify at the scale of the vast Congo Basin, we tentatively propose that strengthening of El Niño-Southern Oscillation (ENSO) variability at that time played a key role in triggering the observed environmental changes, and possibly acted as a driver for the eastward migration of Bantu-speaking peoples across Central Africa.

Accelerated Contribution of the Paleo–Congo River to Global Seawater 87Sr/86Sr Change Following Eocene‐Oligocene Collapse of the African Surface

“…This range of εNd values is most characteristic of Precambrian terrains from the Oubanguides flanking the northern Congo Basin, and which is coherent with the dated detrital zircons that shows predominant 1100‐ to 500‐Ma source regions for the sediments (Figures b and c; De Wit & Linol, ; Thiéblemont et al, ). Our results are also in good agreement with those from suspended sediments in the modern river systems (−22.7 < εNd < −9.8), and Holocene deposits in the Congo submarine canyon (−15.6 in average; Bayon et al, ).…”

Determining the Pace and Magnitude of Lake Level Changes in Southern Ethiopia Over the Last 20,000 Years Using Lake Balance Modeling and SEBAL

“…This conclusion is supported by two arguments: (1) We expect a different climatic forcing mechanism than during the AHP, where a precession minimum caused enhanced humidity and the shift of rain belts over the study region (e.g., Costa et al, 2014;Junginger et al, 2014;Bloszies et al, 2015;Beck et al, 2019). For the past ∼5 ka, for example, short-term changes in sea surface temperatures and associated circulation systems could account for these short-term increases in precipitation (e.g., Nicholson, 2017;Bayon et al, 2019). (2) The second argument against maximum water levels in Chew Bahir is that K concentrations do not reach AHP minima.…”

The Congo deep-sea fan: Mineralogical, REE, and Nd-isotope variability in quartzose passive-margin sand

“…The studied samples, independently of grain size, display a negative Ce anomaly (Ce/Ce* 0.98, 0.78 for 5863) and notably lower Ce/La ratios ( 1.95, 1.53 for 5863) than observed in any component of the geochemical flux in the Congo basin (Gaillardet et al 1995;Dupré et al 1996). Bayon et al (2019) reported negative Ce anomalies from suspended load in several Congo tributaries and Ce/Ce* values as low as 0.73-0.74 in the Ubangui and Kasai rivers. In Congo sediments, Ce depletion can be ascribed to REE remobilization in soils followed by preferential incorporation of Ce into Fe-oxides, as suggested by the marked positive Ce anomalies shown by river-borne oxyhydroxides (Bayon et al 2004).…”

“…In Congo Fan sand, the strong grain-size control on e Nd values is ascribed to mixing of monazite and other Nd carriers mainly derived from Proterozoic orogenic belts with quartz grains carrying a more negative signature because more durable and thus ultimately derived in larger proportions from Archean cratons. respectively) has not drastically changed throughout the Holocene (Bayon et al 2019).…”