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

Baker, Andrea (2016-03)

Thesis (PhD)--Stellenbosch University, 2016.

Thesis

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.

AFRIKAANSE OPSOMMING: Suidelike Afrika is a topografies diverse streek wat beïnvloed word deur gematigde, sub-tropiese en tropiese climate omvattende wisselende groeiseisoen reënval sones. Die kern streeks kontemporêre klimaatsdrywers is die groot see-oppervlak temperatuur (SST) gradiënte tussen die Atlantiese en Indiese Oseane, asook seisoenale skommelinge in die Intertropiese Konvergensiesone (ITCZ). Ons begrip van hoe die interaksie tussen die twee meganismes in die verlede was en hoe ekostelsels reageer het teenoor hierdie klimaatsdrywers is dubbelsinnig grootliks as gevolg van ‗n tekort aan deurlopende argiewe as ‗n gevolg van die streek se semi-droë klimaat. Die Mfabeni veenland is ‗n 11 meter dik aaneenlopende veen reeks gedateer ongeveer 47 kcal jaar BP. Dit is die enigste bekende kus veenland rekord in die somerreënval sone van Suider-Afrika wat die Laaste Ystydperk Maksimum (LGM) oortref het en gee ons die geleentheid om hoë resolusie paleo-omgewings rekords te rekonstrueer onder beide ystydperk en tussen-ystydperk toestande op die suid-oostelike kuslynvan die Afrika continent. ‗n Diverse stel geochemiese tegnieke en analise was (grootmaat C en N elementele en stabiele isotope; biomerkers en blaarwas δ13C isotoop) onderneem op ‗n 810 cm lang kern om primêre produktiwiteit , organiese material (OM) bronne, tempo van OM hermineralisasie, veenland hidrologie en relatiewe bydraes van C3 en C4 plantmateriaal tot die veen afsetting te rekonstrueer. Ek het toe hierdie geochemiese klimaat aanwysers gebruik om neerslag intensiteite en relatiewe temperature gedurende sedimentasie af te lei en, in samewerking met ander streeksargiewe, het ek die dominante meganisme (Indiese Oseaan SST veranderinge vs. veranderinge in die posisie van ITCZ) wat die klimaatskommelinge vanaf die laat Pleistocene tydperk dryf afgelei. Ons het vasgestel dat die Mfabeni veenland ‗n goed gepreserveerde/bewaarde en unieke paleo-ekologiese argief is wat beide omgewings- en klimaatsseine deurlopende die afsettings geskiedenis van die veenland opneem. Alhoewel die dominante OM bron van die veen land- en ontluikende plante was, was daar definitiewe periodes van oorheersende onderwater makrofiet insette wat verhoogde watervlakke voorstel. ‗n Algemene positiewe tendens was opgemerk tussen die temperature en vog gevolgmatigdes. Die plaaslike plantfisiologie (n-alkaan ketting lengtes; ACLalk) en plantsoorte (land vs. akwaties en hulle invloed op OM labiliteit) was egter gedomineer deur vogbeskikbaarheid daarteenoor temperatuur variasies, waarskynlik as gevolg van die relatiewe matige afkoeling wat ervaar was in die subtropiese areas gedurende die LGM. Die blaarwas C isotoop datastel het vasgestel dat variasie in die proporsionele balans van C3 en C4 plante plaasvind, met verwisseling tussen plant klades en interfamilie C3 en C4 skakelaars in reaksie tot veranderinge in omgewings toestande. Die plantsamestelling-skuif was egter afwesig gedurende sommige van die meer efemere klimaatstoestand gebeurtenisse en ons het vasgestel dat dit ‗n nagevolg van plasslike hidrologiese oordrukking is. Die Mfabeni argief korreleer sterk met Mozambiek Kanaal SST rekords en stel voor dat die dominante klimaat dwingende faktor in suid-oostelike Afrika die verdampingspotensiaal en adveksie van vog is van die aangrensende Indiese Oseaan vanaf die laat Pleistocene tydperk. Dit was ook opgemerk dat die Mfabeni rekord algeheel teenoorgestelde omgewings reaksies toon teenoor die Noordelike Halfrond klimaatsgebeurtenisse, wat daarop dui dat daar ‗n anti-fase koppeling is tussen die twee Halfronde.

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