1 2 3 4 5 6 7 Mechanisms and models of iridium anomaly shape across the Cretaceous - 8 Paleogene boundary 9 10 11 12

47 The interpretation of the Cretaceous-Paleogene (K-Pg) iridium anomaly – and other 48 impact ejecta – as the result of a single, large asteroid impact has been the subject of much 49 debate, in part due to the distribution of impact markers beyond the narrow confines of the K-Pg 50 boundary sedimentary layer. Here, we revisit the hypothesized processes leading to the shape of 51 K-Pg iridium profiles including geochemical remobilization and/or diffusion, prolonged 52 deposition, volcanism, multiple impacts, and sediment mixing. Using evidence from the 53 literature and modeling of one North Pacific site, we find that sediment mixing of a single impact 54 event provides the most parsimonious mechanism for iridium profile shape in open ocean oxic 55 sediments, while the increase in background iridium bracketing the boundary likely has a 56 volcanic origin. In some past studies, a sediment mixing mechanism for iridium profile shape 57 was ruled out based on an overly simplified set of expectations for the effect of sediment mixing 58 on markers of geologically instantaneous events. Thus, we introduce and use a Lagrangian 59 sediment-mixing model to illustrate the theoretical effects of mixing on records of rapid events. 60 The sediment mixing origin for iridium anomaly shape, the correspondence in mixing extent 61 between iridium and microfossils, and the fit of sediment mixing models to an empirical iridium 62 profile indicate that iridium may provide a better tracer of mixing than previously proposed K-Pg 63 mixing tracers such as Ni-spinels. 64