Plant 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 plant waxes from source to sink is critical for improving paleoclimate interpretations from sedimentary plant waxes. However, there is limited knowledge about the controls on plant wax integration and transport in large catchments with multiple biomes and climates. To address this lack of understanding, we investigated the vegetation and climatic controls on plant 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 the geographic distribution of n-alkane carbon (delta C-13(alk)) and hydrogen (delta H-2(alk)) isotopic compositions (i.e. isoscapes) in the MRB using plant isotope fractionation calibrations from North America and similar climate regions for the pre-industrial. We utilized two different vegetation maps (i.e. biome map and %C-4 map) and two plant isotope fractionation estimation approaches, and discussed advantages and disadvantages of each method. Then, we developed mixing models weighting the isotope values by biological and climatic parameters (i.e. vegetation area, n-alkane concentration by chain lengths, net primary productivity (NPP), and runoff) and their combinations to test the sensitivity of basin-integrated plant wax isotopic compositions to these variables. Our approach does not factor in soil stocks of plant wax nor degradation in transit, as future work will need to constrain these factors for the MRB for the period of interest. Vegetation area weighting alone predicted relatively high C-4 plant contributions to the pool of waxes exported from the basin. When n-alkane concentration in leaves, NPP, or runoff was considered, the contribution of forest-derived plant waxes increased. Sensitivity of plant wax isotopic compositions to productivity and transport efficiency varied among models and chain lengths. For example, n-C-29 alkanes were sensitive to plant wax contribution increases from forests whereas n-C-33 alkanes were more sensitive to increases in C-4 grassland productivity. The plant wax concentration and NPP were combined as an estimate for total wax productivity, and a combination of this total wax productivity and runoff were used to simulate both production and transport. These multi-parameter models estimated the highest plant wax contribution from C-3 forests because these areas were calculated to have the highest productivity and greatest runoff. Despite large differences in delta C-13(alk) and delta H-2(alk) values across the MRB, variation in the predicted delta C-13 and delta H-2 of exported n-alkanes is small across all models. This small range is consistent with a calculated predominance of plant wax export from wet and tree-dominated biomes and suggests that drier and C-4 grass-dominated biomes may be under-represented in the sedimentary plant wax record. (C) 2019 Elsevier Ltd. All rights reserved.