Glacial/interglacial changes in southern Africa: Compound‐specific δ¹³C land plant biomarker and pollen records from southeast Atlantic continental margin sediments

Abstract

[1] This is part 2 of a study examining southwest African continental margin sediments from nine sites on a north-south transect from the Congo Fan (4°S) to the Cape Basin (30°S) representing two glacial (MIS 2 and 6a) and two interglacial stages (MIS 1 and 5e). Contents, distribution patterns, and molecular stable carbon isotope signatures of long-chain n-alkanes (C 27 -C 33 ) and n-alkanols (C 22 -C 32 ) as indicators of land plant vegetation of different biosynthetic types were correlated with concentrations and distributions of pollen taxa in sediments of the same time horizons. Selected single pollen type data reveal details of vegetation changes, but the overall picture is best illustrated by summing pollen known to predominantly derive from C 4 plants or C 4 plus CAM plants. The C 4 plant signals in the biomarkers are recorded in the d 13 C data and in the abundances of C 31 and C 33 n-alkanes, and the C 32 n-alkanol. Calculated clusters of wind trajectories for austral summer and winter situations for the Holocene and the Last Glacial Maximum afford information on the source areas for the lipids and pollen and their transport pathways to the ocean. This multidisciplinary approach provides clear evidence of latitudinal differences in leaf wax lipid and pollen composition, with the Holocene sedimentary data paralleling the current major phytogeographic zonations. The northern sites (Congo Fan area and northern Angola Basin) get most of their terrestrial material from the Congo Basin and the Angolan highlands dominated by C 3 plants. Airborne particulates derived from the western and central South African hinterland dominated by deserts, semideserts, and savannah regions are rich in organic matter from C 4 plants. As can be expected from the present and glacial positions of the phytogeographic zones, the carbon isotopic signatures of n-alkanes and n-alkanols both become isotopically more enriched in 13 C from north to south. In the northern part of the transect the relative importance of C 4 plant indicators is higher during the glacials than in the interglacials, indicating a northward extension of arid zones favoring grass vegetation. In the south, where grass-rich vegetation merges into semidesert and desert, the difference in C 4 plant indicators is small.

Climatic variability in Mfabeni peatlands (South Africa) since the late Pleistocene

“…The sharp excursion in core SL6 δ 13 C wax signal to more negative values suggests a large increase in C 3 plant input into the sediments during this same period, inferring a change to more temperate C 3 grasses during the LGM and late glaciation. Consistent with this, Dupont et al (2011), Rommerskirchen et al (2006, and Scott (2002) concluded that there is very little evidence for increased C 4 grasses during the LGM in sub-tropical Southern Africa, and that C 3 grasses more than likely dominated the region during this period. This is in direct contrast to records studied in the tropical regions of Africa where there is strong evidence for C 4 plant expansion during the LGM (Castañeda et al, 2009;Sinninghe Damsté et al, 2011).…”

“…3) recorded both 6 ± 2 °C cooler LGM temperatures and c. 50% less precipitation than the Holocene (Truc et al, 2013) during the LGM. These cooler and drier glacial conditions could arguably have been the cause for a shift to C 3 grass dominance in the Mfabeni, which tend to have an advantage over C 4 grasses at lower temperatures (Kotze and O' Conner, 2000;Sage et al, 1999), and appear to have dominated the sub-tropical and WRZ of Southern Africa during the LGM (Dupont et al, 2011;Rommerskirchen et al, 2006;Scott, 2002). These conditions persist with %TOC fluctuating at below average core levels, the δ 13 C bulk signal becoming increasingly more negative and dry grassland palynology signature continuing until c. 15 kcal yr BP.…”

“…On the other hand, Stock et al (2004) showed that temperature variations did not fully account for the C 3 and C 4 Cyperaceae (sedges) family distributions in South Africa, but seem to be more closely related to annual rainfall. They suggest that the C 4 sedges evolved under wetland conditions, but are more abundantly represented in warm and moist (sub) tropical areas (Rommerskirchen et al, 2006), although not at such high proportions as the C 4 Poaceae family (up to 95% in SRZ; Vogel et al, 1978).…”

Independent variations of CH4 emissions and isotopic composition over the past 160,000 years

“…For example, δ 13 C of plant biomarkers in sediment cores off the East Atlantic coast close to the river mouths of the Congo and Angola basins indicate 3-4 higher values during glacial times 41 , similar to evidence from the Guinea Plateau margin recording Sahara/Sahel vegetation 42 . A comparable marine geological study from the Cariaco Basin in the tropical West Atlantic reported a 4-5 δ 13 C decrease in leaf waxes from the Last Glacial Maximum (LGM) to the preboreal Holocene 43 .…”

Temporal Variability of Source-Specific Solvent-Extractable Organic Compounds in Coastal Aerosols over Xiamen, China

“…Long-chain n-alkanes (≥C27), n-FAs (≥C22) and n-alkanols (≥C22) are generally considered to predominantly derive from vascular plant waxes, which are also regarded as tracers of terrestrial vascular plant OM [26][27][28][29]. Figures 3b, 4b and 5b illustrate that more plant wax was detected in the cold season (December, February-March).…”