Correlative isotope excursions driven by transport, not global environmental change

Carbonate sediments are widely used to reconstruct Earth's climatic and biogeochemical history. While pristine records come from deep-water settings with continuous vertical accumulation, much of the preserved carbonate record originates from shallow platforms, where horizontal transport, erosion, and diagenesis complicate this archive. We develop a numerical sediment transport model tracking the advection of conservative tracers (e.g., δ 13 C values of carbonate sediments) to explore the implications of redistribution on sedimentary archives. Results suggest that correlations between δ 13 C excursions and their vertical and lateral proximity to sequence boundaries can be explained by erosion and redeposition of older, isotopically distinct carbonate sediments, forming coarse-grained deposits above erosive surfaces. Such isotopic variability may reflect spatial gradients in the dissolved inorganic carbon (DIC) reservoir, with excursion magnitude constrained by the strength of those gradients. Alternatively, these excursions may record redeposition of older sediments, capturing past secular changes in δ 13 C D I C . We demonstrate that isotope excursions can be duplicated through redeposition, whereby initial shifts in DIC are re-expressed within younger stratigraphy with dampened magnitude. Applying these findings to the Hirnantian carbon isotope excursion (HICE) in Anticosti Island strata, we demonstrate that spatial isotopic gradients within basins will be recorded as excursions within progradational stratigraphy. These excursions will appear synchronous, despite being decoupled from global DIC change. Similar-magnitude δ 13 C excursions in coeval strata are often interpreted as evidence for global carbon cycle changes. We show that transport alone can produce such patterns, without invoking global mechanisms. Further, mapping intra-basin δ 13 C variability may help distinguish transport from global signals. • Isotope excursions created from sediment erosion and redeposition unrelated to DIC. • Excursions occur in predictable stratigraphic locations along sequence boundaries. • Redepositional excursions occur in coarse-grained transgressive deposits. • Redeposition along sequence boundaries can duplicate one change as two excursions.