Holocene hydrologic and vegetation developments in the Orange River catchment (South Africa) and their controls

Abstract

The summer rainfall zone (SRZ) in the South African interior experienced pronounced hydrological and vegetation changes during the Holocene inferred to be driven mainly by shifts in atmospheric and oceanic circulations systems. The exact mechanisms controlling these changes are still debated. To gain better insights into the Holocene environmental changes in the South African SRZ and their driving factors, we analysed compound-specific carbon and hydrogen isotopes of plant wax n-alkanes (δ13Cwax and δDwax) from a marine sediment core covering the last 9900 years. The core has been recovered offshore the mouth of the Orange River, predominantly draining the South African summer rainfall region. Our data indicate a dry early Holocene and a gradual increase of wetter conditions with a higher abundance of C4 vegetation towards the middle Holocene. Wettest conditions occurred around 3900 cal. yr BP. The last 3900 years were characterised by a gradual aridification overlain by variable wetter conditions. During the ‘Little Ice Age’ (LIA: ca. 640–310 cal. yr BP), relatively dry conditions with elevated C4 plant contributions occurred. This opposite behaviour, that is, more C4 plant contribution during drier conditions compared to the remainder of the Holocene, points towards an influence of winter rainfall in the lower Orange River catchment during the late-Holocene and a decline in summer rainfall. We emphasise the importance of changes in the latitudinal insolation gradient (LIG) as a potentially important controlling mechanism for hydrologic and vegetation changes in the SRZ.

Ecological stability of Late Pleistocene-to-Holocene Lesotho, southern Africa, facilitated human upland habitation

“…3). The average chain length (ACL [25][26][27][28][29][30][31][32][33][34][35] ) ranges from 29.3 to 31.2 (30.7 ± 0.5, n = 16), consistent with previous reports of ACL from African terrestrial plants 71,72 . The carbon preference index (CPI) of the C 25 -C 35 n-alkanes ranges between 3.0 and 11.5 (6.7 ± 2.5, n = 16).…”

“…Average chain length (ACL) is another metric used as a vegetation proxy because it has been shown to be higher in C 4 grasses 83 , but also correlates with higher growing season temperature and aridity [85][86][87][88][89] . Although there was no significant difference in ACL [25][26][27][28][29][30][31][32][33][34][35] values between Pleistocene and Holocene samples in a Student's t test (two-tailed, p = 0.506) and Mann-Whitney U-test (p = 0.958), there are observable relationships between the ACL 25-35 and δ 13 C and δD throughout the sequence (Fig. 3, Table 1).…”

“…However, given the historic emphasis placed on sequences from rockshelters and caves at or near the coast 25,26 , it is crucial to complement this with observations from the sub-continent's topographically variable and biologically diverse interior which experienced significant climatic, environmental, and demographic changes throughout the Pleistocene [27][28][29][30][31] . Long-term proxy records from southern Africa show the potential impacts of changing plant landscape composition and hydroclimate on human populations over the Quaternary [32][33][34][35][36][37][38][39][40][41][42] . Because these environmental records are often located far from archaeological sites, however, it is important to compare these data to on-site and catchment scale (i.e., proximal) records to develop highly spatially and temporally resolved palaeoclimate and palaeoenvironmental information relevant to human evolution and behavioural change [43][44][45][46] .…”

The Provenance of Terrigenous Components in Marine Sediments Along the East Coast of Southern Africa

“…Within a particular catchment, the provenance of inorganic and organic material is likely to be distinct, especially if there is large variability in relief and vegetation cover. This has been shown in overview studies of riverine transport (Leithold et al, 2016) as well as in individual catchment studies (e.g., Burdanowitz et al, 2018;Bouchez et al, 2014;Häggi et al, 2016;Galy et al, 2011;Hahn et al, 2016;Herrmann et al, 2016;Zhao et al, 2015). Furthermore, the transport pathways and deposition of the various terrigenous fractions will be a function of their grain size, shape, density, and durability as shown by several on-shelf sediment distribution studies (e.g., Govin et al, 2014;Petschick et al, 1996;Rogers & Rau, 2006).…”

A Joint Soil‐Vegetation‐Atmospheric Modeling Procedure of Water Isotopologues: Implementation and Application to Different Climate Zones With WRF‐Hydro‐Iso

“…The effect of more dominant factors controlling the ratio between transpiration and direct evaporation, such as plant types (e.g., Hou et al., 2007), should be considered in a future sensitivity analysis. This is particularly relevant for the interpretation of paleo records associated with pronounced vegetation changes, such as in Southern Africa (e.g., Burdanowitz et al., 2018).…”