DISENTANGLING TERRESTRIAL ORGANIC MOLECULAR RECORDS OF CLIMATE AND TOPOGRAPHY: HOW RIVERS, FLOODPLAINS AND CARBON STORAGE BIAS INTERPRETATIONS OF GLOBAL CLIMATE, TOPOGRAPHY AND ECOSYSTEM RESPONSE

Terrestrial records of climatic and topographic change are commonly derived from aggrading fluvial systems. These depositional environments record the accumulation of sediment over timescales of years to tens of millions of years. Records of paleoclimate or paleotopography can be preserved in the mixture of fossils, authigenic minerals and organic molecules that are frequently preserved in floodplain sediments and the paleosols that may develop in stable floodplains. For example, stable isotopes of materials such as volcanic glass, authigenic soil carbonates, or clay minerals provide a record of past water isotopes that can be related to regional or global temperature and the degree of isotopic fractionation due to orographic lifting. Similarly, paleosol geochemistry is used as a record of temperature and precipitation amount that can be linked to elevation. However, fluvial systems are dynamic; the rates at which they aggrade, migrate, and rework sediment vary as a result of climate and sediment flux. Changes in global or regional climate or topography are observed to produce fundamental changes in the dynamics of the very same river systems that are used to measure the climatic or tectonic event. This imposes a potentially significant bias in the application of fluvial sediments as a record of terrestrial environmental change. These biases are most strongly associated with variations in the degree of catchment integration of sediment that is stored in river floodplains, timescales of sediment stability, and the age of organic carbon or other materials preserved in floodplains and associated paleosols. Here we examine the temporal record of terrestrial climate change across the Eocene-Oligocene transition in western North America using the δ²H and δ¹³C of organic biomarkers preserved in floodplain sediments. We examine long term records of organic biomarker isotope data in the context of detailed analysis of isotopic heterogeneity across river floodplain systems at several key intervals of late Eocene to early Oligocene to disentangle the record of changing climate and environment in central North America from topographic signatures on hydrogen and carbon isotopes.