Signal or noise? Isolating grain size effects on Nd and Sr isotope variability in Indus delta sediment provenance

Radiogenic elements, such as neodymium (Nd) and strontium (Sr), are commonly used to decipher the provenance and weathering histories of bulk siliciclastic sediments worldwide, although increasing evidence for diagenetic and sediment transport-driven bias of isotope signatures calls into question the utility of bulk isotope compositions as effective tools. This study evaluates grain size-dependence and variability of Nd and Sr compositions in the Indus delta of Pakistan, documenting both a significant isotopic evolution and coarsening upward sequence during the last deglaciation (similar to 15 ka). Grain size analysis, trace element geochemistry, and Nd and Sr geochemistry are determined for bulk and separate grain size fractions (< 63 mu m, 63-125 mu m, 125-250 mu m, >250 mu m) from four core sampling sites with similar provenance. Isotopic compositions are controlled by grain size distribution, in turn variably controlled by size fraction mineralogy. Fine-grained bulk sediment Nd isotope compositions are primarily dictated by the finest (< 125 mu m) sediments enriched in Nd-bearing monazite and allanite, with coarser bulk sediments influenced by mica. The most abundant grain size fraction drives bulk sediment Sr isotope compositions, with the finest grain size fractions indicating strong control by potassium feldspar, mica, epidote, and/or volcanogenic clay minerals. Over the last 15 k.y., resolvable provenance-driven trends are confidently identified in Nd isotope compositions. Long-term trends remain consistent with previous findings although the isotopic shift in delta compositions must be less than previously envisaged (similar to 4 vs. 0.69-1.91 epsilon(Nd) units). Although considerable variability is observed in bulk Sr isotope compositions,< 63 mu m and 63-125 mu m fraction Sr-87/Sr-86 compositions evolve in parallel through time and show fractionation by similar to 0.005-0.01 as a result of grain size. Within the isotopically diverse Indus drainage system, bulk isotopic compositions are estimated to deviate on average no than +/- 1.04 epsilon(Nd) units and +/- 0.0099 for Sr-87/Sr-86 values for any sediment as a result of mineralogy, grain size distribution, and analytical error. Isotopic excursions beyond these uncertainties can be unambiguously assigned to either a change in upstream provenance and/or chemical weathering.