Organic Molecular Paleohypsometry: A New Approach to Quantifying Paleotopography and Paleorelief

Abstract

Stable isotope paleoaltimetry is one of the most commonly used approaches for quantifying the paleoelevation history of an orogen yet this methodology is often limited to arid to semi-arid climates, mountain systems with a clear orographic rainshadow and terrestrial basins. We present a new approach to reconstructing past topography and relief that uses the catchment-integrated signature of organic molecular biomarkers to quantify the hypsometry of fluvially-exported biomass. Because terrestrially-produced biomolecules are synthesized over the full range of global climate conditions and can be preserved in both terrestrial and marine sediments, the geochemistry of fluvially-transported sedimentary biomarkers can provide a means of interrogating the evolution of topography for a range of environments and depositional settings, including those not well suited for a traditional isotope paleoaltimetry approach. We show an example from Taiwan, a rapidly eroding tropical mountain system that is characterized by high rates of biomass production and short organic residence time and discuss key factors that can influence molecular isotope signal production, transport and integration. Data show that in high relief catchments of Taiwan, river sediments can record integration of biomass produced throughout the catchment. Sedimentary biomarker δ2HnC29 in low elevation river deposition sites is generally offset from the δ2HnC29 value observed in local soils and consistent with an isotope composition of organics produced at the catchment mean elevation. We test the effect of distinct molecular production and erosion functions on the expected δ2HnC29 in river sediments and show that elevation-dependent differences in the production and erosion of biomarkers/sediment may yield only modest differences in the catchment-integrated isotopic signal. Relating fluvial biomarker isotope records to quantitative estimates of organic source elevations in other global orogens will likely pose numerous challenges, with a number of variables that influence molecular production and integration in a river system. We provide a discussion of important parameters that influence molecular biomarker isotope signatures in a mountain system and a framework for employing a molecular paleohypsometry approach to quantifying the evolution of other orogenic systems.

Rapid topographic growth of the Taiwan orogen since ~1.3–1.5 Ma

“…Model predictions closely match modern observed soil and river sediments Δδ 2 H n C29 and water Δδ 2 H data in Taiwan reported by Hren and Ouimet ( 23 ). Note that modern soil Δδ 2 H n C29 are plotted against sample site elevation, while the modern fluvial sediments Δδ 2 H n C29 and river water Δδ 2 H are plotted against catchment mean elevation.…”

“…1) to constrain changes in organic source elevation through time. These data are related to catchment hypsometry ( 23 ) and provide the first constraints on the evolution of paleotopography in the Taiwan mountain range.…”

“…In this study, we focus on the isotopic record of n -alkanes with 29 carbon atoms ( n C 29 ), as this compound is the most abundant molecular species in the soil and sediments in Taiwan ( 23 ). Modern Taiwan soils show a δ 2 H n C29 (hydrogen isotope composition of leaf waxes) of approximately −150 to −155 per mil (‰) near sea level and less than −200‰ at elevations of >2500 m, yielding an ~20‰ per km decrease in δ 2 H n C29 that mirrors the Δδ 2 H of precipitation across the entire elevation range of the Taiwan orogen ( 23 ). These molecules can be accumulated in soil and be eroded, ablated by wind/rain, or mobilized by rain that moves organics through surface and groundwater as well as transported through river networks and deposited in fluvial and marine environments ( 24 ).…”

“…These molecules can be accumulated in soil and be eroded, ablated by wind/rain, or mobilized by rain that moves organics through surface and groundwater as well as transported through river networks and deposited in fluvial and marine environments ( 24 ). These leaf waxes molecules are capable of carrying and preserving isotopic signatures from their high-elevation sources ( 23 , 27 – 29 ). In Taiwan, the average residence time of organic carbon and, in particular, plant biomarkers is generally short (<1000 years) so the carbon that is exported is representative of the integrated carbon produced across the landscape ( 30 ).…”

“…In Taiwan, the average residence time of organic carbon and, in particular, plant biomarkers is generally short (<1000 years) so the carbon that is exported is representative of the integrated carbon produced across the landscape ( 30 ). Studies have shown that modern river sediments δ 2 H n C29 data in Taiwan generally reflects the hypsometry of leaf waxes produced throughout a catchment (i.e., catchment mean elevation) ( 23 ).…”