Using micro-computed tomography (µCT) to measure annually resolved sediment fluxes in varved sediments

Annually laminated sediments (varves) are excellent paleoclimate archives because of their high temporal resolution and because they contain their own chronology that can be converted to calendar years. However, their studies can be tedious because they can be disturbed, irregular or very thin, sometimes less than a mm. Here we explore the potential of micro-computed tomography (µCT) to unlock paleoclimate signals from varved records.First, we show the added value of performing varve counts and thickness measurements from a 3D volume instead of a randomly sampled single 2D plane that is commonly used.Second, we investigate the possibility to obtain density measurements for each varve, allowing calculating annually resolved sediment fluxes in gr cm-2 a-1. To achieve this, we µCT-scanned several varved sequences containing sediments covering a wide range of density and extracted their linear attenuation. Then, discrete volumetric samples were subsampled, weighted, gradually dried every 30 minutes and µCT-scanned each time at two different incident energies. This allowed establishing a calibration of water content based on linear attenuation coefficients. Once the density and water content were obtained, the next step was to calculate the density and the mass accumulation rate for each varve.This approach will pave the way for “forward modelling,” namely the modelling of climate indicators (proxy) contained in natural archives. This method is recognized as a way of improving paleoclimate reconstructions, as it is less sensitive to the non-linearity of the physical processes involved in the formation of proxies than the statistical models that are commonly used.