Palaeoenvironmental reconstruction of late Holocene climate dynamics in Southwest Africa using a multi-proxy characterization of Namaqualand mudbelt sediments

Despite the recognized importance of understanding late Holocene climatic changes in southern Africa, a dearth of available evidence has resulted in a frag- mented view of the recent past. South Africa has been identified as a key focus region for palaeoclimatic studies, as it reflects the dynamics of both tropical and temperate climate regimes. This study aims to create a catchment-integrated view of palaeoenvironmental conditions in the country's Winter Rainfall Zone (WRZ) and Southern Benguela region during the late Holocene, in order to establish the linkages between oceanic and terrestrial climate components. A multiproxy analysis was conducted on a west coast mudbelt sediment core, where continuous deposition over the last 2,250 years has aided the production of an uninterrupted age model. Alkenone and isotopic analyses were conducted at a multi-decadal resolution to assist in the reconstruction of sea-surface tem- perature (SST) in the St. Helena Bay region, and hydrological variation in the WRZ. Changes in moisture availability were inferred via the application of in- organic proxies, including grain size variation (promoted through the use of an end-member proxy algorithm) and the Fe/K ratio, a proxy interpreted to be rep- resentative of changes in chemical weathering. Furthermore, a newly-developed proxy, TEX 86 , was used as an alternative palaeothermometer. TEX 86 produced an additional record of SST, independent of alkenones, which elicited a com- parative study between SST TEX86 and SST UK'37 . The comparison facilitated a review of the effectiveness of TEX 86 within the Benguela Upwelling System, and assisted in quantifying reasons for the observed differences between the two methods. The results of the multiproxy analysis sheds new light on southwest African late Holocene climatic dynamics. This study documents a decrease in SST accompanied by increasing WRZ rainfall, which is hypothesized to be a result of large-scale changes in the position and/or intensity of the austral westerly wind belt. A northerly migration/increase in intensity of the winds acts to produce cooler SSTs and wetter west coast continental conditions, both of which were most acutely experienced during the so-called Little Ice Age (LIA) (1300 - 1850 CE). Zonal symmetry across the Southern Hemisphere is hypothesized to be a consequence of the large geographical extent of the westerly winds, as cooler and wetter conditions have been experienced in coastal, winter-rainfall areas of Chile and Australia

Organic-walled dinoflagellate cyst data of sediment core GeoB8323-2

The southern Benguela upwelling system near the St Helena Bay has been proposed to be affected by various factors, while few investigations about the late-Holocene oceanic conditions has been carried out in this area. To determine the oceanic variability and its forcing mechanisms in the southern Benguela region during the late-Holocene, we examined organic-walled dinoflagellate cyst (dinocyst) records from two marine sediment cores located in the southernmost and central Benguela upwelling system. We compare our results with other proxies including alkenone-derived SSTs, grain size, and coccolithophore assemblages from the same samples. The results indicate a distinctive behavior between the southernmost Benguela system and the central Benguela area. We infer that the oceanic conditions in these two regions are primarily governed by an interplay of wind-induced upwelling, fluvial discharge, and advection of cold sub-Antarctic waters, which is consistent with the current understanding of the paleoclimate conditions in this area. However, the findings also suggest that the southernmost Benguela system also receives additional effects of warm and saline waters via the Agulhas leakage, which has a clear influence on the oceanic conditions in this area

Late Holocene alkenone-derived sea-surface temperature data and grain size end-member model of sediment core GeoB8323-2 from the Benguela Upwelling System

Southwest Africa is an important region for paleo-climatic studies, being influenced by both tropical and temperate climate systems and thus reflecting the interplay of variable controls. The aim of this study was to unravel the interaction of sea-surface temperature (SST) changes in the southernmost Benguela upwelling system with precipitation changes in South Africa's winter rainfall zone (WRZ) during the late Holocene. Therefore, a marine sediment core from the southernmost Benguela upwelling system was investigated to reconstruct climate changes in this region for the past ~2000 years. Grain size and geochemical analyses were conducted to reconstruct changes in fluvial sediment discharge and weathering intensity, while SST changes were estimated using alkenone paleo-thermometry. Results show that the southernmost Benguela behaves distinctly in comparison with the rest of the Benguela system reflecting amplified SST changes. Decreasing SSTs accompanied increasing river discharge during times of increased precipitation in the WRZ, reflecting northerly shifted westerly winds during austral winter. We infer a control of past SST changes by processes not analogous to modern processes driving seasonal SST changes by changes through upwelling intensity. The findings suggest that late-Holocene SST changes in the southernmost Benguela upwelling system and the precipitation in the WRZ were both driven by latitudinal shifts of the austral westerly wind belt and associated changes in advection of cold sub-Antarctic waters and/or changes in Agulhas leakage of warm Indian Ocean waters

n-Alkane content and compound-specific δ¹³C values of soil samples, river samples, and marine surface samples

Southwestern Africa's coastal marine mudbelt, a prominent Holocene sediment package, provides a valuable archive for reconstructing terrestrial palaeoclimates on the adjacent continent. While the origin of terrestrial inorganic material has been intensively studied, the sources of terrigenous organic material deposited in the mudbelt are yet unclear. In this study, plant wax derived n-alkanes and their compound-specific d13C in soils, flood deposits and suspension loads from regional fluvial systems and marine sediments are analysed to characterize the origin of terrestrial organic material in the southwest African mudbelt. Soils from different biomes in the catchments of the Orange River and small west coast rivers show on average distinct n-alkane distributions and compound-specific d13C values reflecting biome-specific vegetation types, most notably the winter rainfall associated Fynbos Biome of the southwestern Cape. In the fluvial sediment samples from the Orange River, changes in the n-alkane distributions and compound-specific d13C compositions reveal an overprint by local vegetation along the river's course. The smaller west coast rivers show distinct signals, reflecting their small catchment areas and particular vegetation communities. Marine surface sediments spanning a transect from the northern mudbelt (29°S) to St. Helena Bay (33°S) reveal subtle, but spatially coherent, changes in n-alkane distributions and compound-specific d13C, indicating the influence of Orange River sediments in the northern mudbelt, the increasing importance of terrigenous input from the adjacent western coastal biomes in the central mudbelt, and contributions from the Fynbos Biome to the southern mudbelt. These findings indicate the different sources of terrestrial organic material deposited in the mudbelt, and highlight the potential the mudbelt has to preserve evidence of environmental change from the adjacent continent

Sources, transport and deposition of terrestrial organic material: A case study from southwestern Africa

International audienceSouthwestern Africa's coastal marine mudbelt, a prominent Holocene sediment package, provides a valuable archive for reconstructing terrestrial palaeoclimates on the adjacent continent. While the origin of terrestrial inorganic material has been intensively studied, the sources of terrigenous organic material deposited in the mudbelt are yet unclear. In this study, plant wax derived n-alkanes and their compound-specific d 13 C in soils, flood deposits and suspension loads from regional fluvial systems and marine sediments are analysed to characterize the origin of terrestrial organic material in the southwest African mudbelt. Soils from different biomes in the catchments of the Orange River and small west coast rivers show on average distinct n-alkane distributions and compound-specific d 13 C values reflecting biome-specific vegetation types, most notably the winter rainfall associated Fynbos Biome of the southwestern Cape. In the fluvial sediment samples from the Orange River, changes in the n-alkane distributions and compound-specific d 13 C compositions reveal an overprint by local vegetation along the river's course. The smaller west coast rivers show distinct signals, reflecting their small catchment areas and particular vegetation communities. Marine surface sediments spanning a transect from the northern mudbelt (29 S) to St. Helena Bay (33 S) reveal subtle, but spatially coherent, changes in n-alkane distributions and compound-specific d 13 C, indicating the influence of Orange River sediments in the northern mudbelt, the increasing importance of terrigenous input from the adjacent western coastal biomes in the central mudbelt, and contributions from the Fynbos Biome to the southern mudbelt. These findings indicate the different sources of terrestrial organic material deposited in the mudbelt, and highlight the potential the mudbelt has to preserve evidence of environmental change from the adjacent continent