Grain size control on Sr-Nd isotope provenance studies and impact on paleoclimate reconstructions: An example from deep-sea sediments offshore NW Africa

Five sediment cores from offshore NW Africa were analyzed for strontium and neodymium isotope ratios to reconstruct temporal variations in continental weathering regimes. Sediments were taken from three time slices with well-known and distinctive environmental conditions: present-day (dry and warm), similar to 6 ka (wet and warm), and similar to 12 ka (dry and cold). Terrigenous sediment samples were split into two size fractions to distinguish between the two dominant transport mechanisms offshore NW Africa: fluvial (0-10 mu m) and aeolian (10-40 mu m). Sr isotope data record evidence of marked grain size control with higher isotopic ratios in the fine fraction. In contrast, epsilon(Nd) values are largely unaffected by grain size. Minor variability in Nd isotope data at each sampling site indicates near constant sources of terrigenous matter over the last similar to 12 ka. Variations in Sr isotope ratios are interpreted to reflect major changes in the evaporation-precipitation balance. We suggest that the Sr-Nd isotope data record a latitudinal shift of the northern limit of the African rainbelt and associated wind systems causing changes in the humidity and rate of chemical weathering over NW Africa. While hyperarid conditions prevailed similar to 12 ka, more humid conditions and intensified monsoonal rainfall at similar to 6 ka resulted in greater breakdown of easily weathered K-bearing phases and increased Sr-87/Sr-86 in the detritus. In late Holocene times the monsoonal circulation diminished resulting in a return to arid conditions. Our results clearly show that it is of vital importance in paleoenvironmental studies to carry out isotopic analyses on individual sediment fractions that were carried to the studied deposition site by distinct sediment transport mechanisms. If isotopic analyses are carried out on bulk sediments, the observed variability in isotopic values most likely represents changes in the particle size and mixing proportions of the sediment subpopulations.