H.J.L. van der Lubbe

Neodymium isotope constraints on provenance, dispersal, and climate-driven supply of Zambezi sediments along the Mozambique Margin during the past ∼45,000 years

Marine sediments deposited off the Zambezi River that drains a considerable part of the southeast African continent provide continuous records of the continental climatic and environmental conditions. nHere we present time series of neodymium (Nd) isotope signatures of the detrital sediment fraction during the past ~45,000 years, to reconstruct climate-driven changes in the provenance of clays deposited along the Mozambique Margin. Coherent with the surface current regime, the Nd isotope distribution in surface sediments reveals mixing of the alongshore flowing Zambezi suspension load with sediments supplied by smaller rivers located further north. To reconstruct past changes in sediment provenances, Nd isotope signatures nof clays that are not significantly fractionated during weathering processes have been obtained from core 64PE304-80, which was recovered just north of the Zambezi mouth at 1329 m water depth. Distinctly unradiogenic clay signatures (ENd values <214.2) are found during the Last Glacial Maximum, Heinrich Stadial 1, and Younger Dryas. In contrast, the Nd isotope record shows higher, more radiogenic isotope signatures during Marine Isotope Stage 3 and between ~15 and ~5 ka BP, the latter coinciding with the timing of the northern hemisphere African Humid Period. The clay-sized sediment fraction with the least radiogenic Nd isotope signatures was deposited during the Holocene, when the adjacent Mozambique Shelf became completely flooded. In general, the contribution of the distinctly unradiogenic Zambezi suspension load has followed the intensity of precession-forced monsoonal precipitation and enhanced during periods of increased southern hemisphere insolation and high-latitude northern hemispheric climate nvariability.

87Sr/86Sr as a quantitative geochemical proxy for 14C reservoir age in dynamic, brackish waters: Assessing applicability and quantifying uncertainties

Accurate geochronologies are crucial for reconstructing the sensitivity of brackish and estuarine environments to dynamic external impacts of the past. Radiocarbon (C-14) dating is commonly used for palaeoclimate studies, but its application in brackish environments is severely limited by an inability to quantify spatiotemporal variations in C-14 reservoir age, or R(t), due to dynamic interplay between river runoff and marine water. Additionally, old carbon effects and species-specific behavioral processes also influence C-14 ages. Using the worlds largest brackish water body (the estuarine Baltic Sea) as a test bed, combined with a comprehensive approach that objectively excludes both old carbon (using GIS) and species-specific C-14 effects, we demonstrate the use of Sr-87/Sr-86 ratios for quantifying R(t) in ubiquitous mollusc shell material, leading to almost an order of magnitude increase in Baltic Sea C-14 geochronological precision over the current state of the art. We propose that similar proxy methods can be developed for other brackish water bodies worldwide.