Leaf wax integration and transport from the Mississippi River Basin to the Gulf of Mexico inferred from GIS enabled isoscapes and mixing models

Abstract

Understanding the fate of terrestrial leaf waxes from source to sink is critical for improving paleoclimate interpretations from sedimentary leaf waxes. However, there is a limited knowledge about the controls on leaf wax integration and transport in large catchments with multiple biomes and climates. This study constrained vegetation and climatic controls on leaf wax integration and transport from the Mississippi River Basin (MRB), the largest river in the U.S., to the Gulf of Mexico (GOM). We first estimated geographic representation of n-alkane carbon (δ13Calk) and hydrogen (δDalk) isotopic compositions (i.e. isoscapes) in the MRB using plant isotope fractionation calibrations from North America and similar climate regions for the modern and mid-Holocene. Then, we weighted the biological and climatic parameters (i.e. area of the vegetation, n-alkane production by chain lengths, net primary productivity, and runoff) to constrain and quantify their influence on integration and transport of the leaf waxes to the GOM sediments. The parameters-weighted mixing model results were then compared to measured δ13Calk and δDalk values in core sediment samples from the northeastern GOM (ODP 625B). The area of the vegetation was not a significant influence on the resulting leaf wax isotopic composition in the sediments. The leaf wax production, NPP, and runoff influenced the leaf wax integration, and were generally higher iansport in large catchments with multiple biomes and climates. This study constrained vegetation and climatic controls on leaf wax integration and transport from the Mississippi River Basin (MRB), the largest river in the U.S., to the Gulf of Mexico (GOM). We first estimated geographic representation of n-alkane carbon (δ13Calk) and hydrogen (δDalk) isotopic compositions (i.e. isoscapes) in the MRB using plant isotope fractionation calibrations from North America and similar climate regions for the modern and mid-Holocene. Then, we weighted the biological and climatic parameters (i.e. area of the vegetation, n-alkane production by chain lengths, net primary productivity, and runoff) to constrain and quantify their influence on integration and transport of the leaf waxes to the GOM sediments. The parameters-weighted mixing model results were then compared to measured δ13Calk and δDalk values in core sediment samples from the northeastern GOM (ODP 625B). The area of the vegetation was not a significant influence on the resulting leaf wax isotopic composition in the sediments. The leaf wax production, NPP, and runoff influenced the leaf wax integration, and were generally higher in forests, leading to higher C3 plant carbon isotope signals especially in n-C29 alkanes. The C4 grassland-derived leaf wax contribution in the sediment increased in higher chains (n-C33) due to their high production in grasses. However, the single parameter-weighted models could not capture the strong C3 carbon isotope signal recorded in the sediments. We combined leaf wax production and NPP as a total wax biomass, and also combined the total wax biomass with runoff to consider both productivity and transport. The combination models better matched with the measured values, confirming that both productivity and runoff affect wax integration and transport. Our results suggest that leaf wax export is not spatially uniform in large basins and will be biased to areas with high productivity and greatest runoff. Therefore, when interpreting sedimentary leaf wax records, it is important to take into account the source and transport bias. Otherwise, we may underestimate low productivity and dry biomes, especially when only looking at n-C29 alkanes and not the other chain lengths.