Linking organic carbon sedimentation, burial efficiency, and long-term accumulation in boreal lakes

Abstract

Carbon (C) storage in lakes is now recognized as a significant sink of C at a global scale, but the pathways that lead to this storage remain poorly understood. In this study, we attempt to reconstruct and connect the processes that lead to long-term C accumulation in boreal lakes. These include the rate of particulate organic C (POC) sedimentation in the water column and sediment metabolism operating at a temporal scale of weeks to months, organic C accumulation in the top sediment layers integrated over scales of tens of years, and long-term organic C burial in lake sediment integrated over hundreds to thousands of years. The sinking POC flux was tenfold higher than the short-term sediment C accumulation rates in all systems, and we found no direct relationship between this downward C flux and either the short-term or long-term C accumulation rates. However, the resulting C burial efficiency (which ranged from 5 to 62%) was strongly related to lake shape, which ultimately constrains the time freshly deposited material that is exposed to oxygen and thereby regulates the fraction of the carbon sinking flux that is mineralized back to the atmosphere or permanently buried in the sediments. Small and deep lakes act as more efficient C sinks than large and flat lakes. We also show that long-term burial rates are nearly identical to current centennial-scale accumulation rates and that therefore, little degradation occurs after a few decades. Sediment C storage tends to be small (<5%) relative to lake C emissions, but that this balance is also strongly related to lake morphometry. Key Points Sinking C flux is higher than recent and Holocene sediment C accumulation rates C burial efficiency is strongly related to lake shape Little degradation occurs after a few decades ©2014. American Geophysical Union. All Rights Reserved.