Timescale-dependent dust accumulation measurements in high alpine lakes can masquerade as human-induced increases in dust emissions

Abstract:

High-elevation lakes are quiescent environments subject to little direct human influence. Sediment archival analysis suggests human land use changes lead to rapid increases in sediment accumulation rates in distant arid regions. While local sediment input can remain constant over long timescales, exogenous dust input can be intermittent. This intermittency results in smaller average rates with increasing timescale, as periods of zero input are incorporated into the sedimentary archive. That is, accumulation rates may be significant when dust arrives at a lake, but incorporation of intermittent dust accumulation will result in lower long-term rates.

We evaluate sediment archives from ten lakes that have apparent sediment accumulation increases. Sediment ages and deposition rates in lakes cluster into two distinct populations based on dating method; large rate differences tend to occur depending on whether the dating methods is ²¹⁰Pb or ¹⁴C. ²¹⁰Pb samples are young with small inter-sample time intervals, while ¹⁴C samples are older and more widely spaced. Individual rates between dated intervals of sediment are correct for the scale at which they are measured, but linear rate measurements of different timescales are not comparable. We demonstrate the process by which even constant deposition rates can be misinterpreted as change. A simulated core with a known, constant deposition rate produces the same behavior as the alpine lake cores, where simulated ²¹⁰Pb dates expressed higher sedimentation rates than simulated ¹⁴C dates with larger inter-sample time intervals. Age-depth modelling software, such as rbacon, do not account for or correct these timescale biases. Our results demonstrate that dust accumulation rates in alpine lakes must be placed in the context of measurement timescale to determine whether it is possible to make conclusions based on comparison of the historical record of a core to contemporary dust accumulation.