Locally Produced Sedimentary Biomarkers in High-Altitude Catchments Outweigh Upstream River Transport in Sedimentary Archives.

Sedimentary records of lipid biomarkers such as leaf wax n -alkanes are not only influenced by ecosystem turnover and physiological changes in plants, they are also influenced by earth surface processes integrating these signals. The integration of biomarkers into the sedimentary record and the effects of integration processes on recorded environmental signals are complex and not fully understood. To determine the depositional constraints on biomarker records in a high-altitude small catchment system, we collected both soil and stream sediments along a 1000 m altitude transect (1500 – 2500 masl) in the Areguni Mountains, a subrange of the Lesser Caucasus Mountains in Armenia. We utilize the existence of a treeline at ~ 2000 masl, which separates alpine meadow above from deciduous forest below, to assess the relative contribution of upstream biomarker transport to local vegetation input in the stream. We find that average chain length (ACL), hydrogen isotope (δD) and carbon isotope (δ¹³C) values of n -alkanes are significantly different in soils collected above and below the treeline. However, samples collected from the stream sediments do not integrate these signals quantitively. As the stream drops below the treeline, the ACL, δD and δ¹³C values of n -alkanes preserved in streambed sediments reflect a bias toward n -alkanes sourced from trees. This suggests that there is either 1) minimal transportation of organic matter from the more open vegetation in higher elevations, or 2) greater production of target biomarkers by trees and shurbs found at lower elevations results in overprinting of stream signals by local vegetation. Though this latter observation may preclude using n -alkanes to measure past treeline movement in these mountains, δD values of biomarkers in fluvial deposits in these settings are more likely to record local hydrological changes rather than changes in upstream fractionation differences associated with vegetation turnover. [ABSTRACT FROM AUTHOR]