Bulk geochemical, biomarker and leaf wax isotope records of Mfabeni peatland, KwaZulu Natal, South Africa since the late Pleistocene

[摘要] ENGLISH ABSTRACT: Southern Africa is a topographically diverse region that is influenced by temperate, sub-tropical and tropical climates encompassing varying rainfall zones. The core regional contemporary climatic drivers are the large sea surface temperature (SST) gradients between the Atlantic and Indian Oceans, and seasonal fluctuations in the Inter Tropical Convergence Zone (ITCZ). Our understanding of how these two mechanisms interacted in the past and how ecosystems responded to these climate drivers is ambiguous, mainly due to a lack of continuous archives as a consequence of the region's semi-arid climate. The Mfabeni peatland is a 11 m thick continuous peat sequence that has been dated to ca. 47 kcal yr BP. It is the only known coastal peatland record in the summer rainfall zone of Southern Africa to transcend the Last Glacial Maximum (LGM), and gives us the opportunity to reconstruct high-resolution palaeoenvironment records under both glacial and interglacial conditions on the south eastern coastline of the African continent.A diverse set of geochemical techniques and analysis (bulk C and N elemental and stable isotopes; different biomarkers and leaf wax δ13C isotope) was undertaken on a 810 cm long core to reconstruct primary productivity, organic matter (OM) sources, rates of OM remineralisation, peatland hydrology and relative contributions of C3 and C4 plant matter into the peat deposit. These geochemical climatic indicators were used to infer precipitation intensities and relative temperatures at time of sedimentation and, in conjunction with other regional archives, the dominant mechanisms (Indian Ocean SST changes versus changes in the position of ITCZ) driving climatic fluctuations since the late Pleistocene were explored.We established the Mfabeni peatland to be a well-preserved and unique palaeoecological archive that recorded both environmental and climatic signals throughout the depositional history of the peatland. Even though the dominant OM source of the peat was terrestrial and emergent plants, there were definitive periods of predominant submerged macrophyte input, suggesting elevated water levels. A general positive trend was observed between the temperature and moisture proxies, however the local plant physiology (n-alkane chain lengths; ACLalk) and plant types(terrestrial vs aquatic and their influence on OM lability; CPIalk) was dominated by moisture availability as opposed to temperature variations, arguably due to the relatively moderate cooling experience in the sub-tropics during the LGM. The leaf wax C isotope data set established variability in the proportional balance of C3 and C4 plants, with interchanges between plant clades and inter family C3 and C4 switches in response to changes in environmental conditions. However, plant assemblage shifts were absent during some of the more ephemeral climatic events which we concluded was due to local hydrological overprinting.The Mfabeni archive correlates strongly with Mozambique Channel SST records, suggesting the dominant climate forcing factor in south eastern Africa to be the evaporation potential and advection of moisture from the adjacent Indian Ocean since the late Pleistocene. It was also noted that the Mfabeni record exhibited overall opposite environmental responses to Northern Hemisphere climatic events, suggesting an anti-phase coupling between the two hemispheres.

[发布机构] Stellenbosch University