Trends in historical mercury deposition inferred from lake sediment cores across a climate gradient in the Canadian High Arctic

Recent climate change may be enhancing mercury fluxes to Arctic lake sediments, confounding the use of sediment cores to reconstruct histories of atmospheric deposition. Assessing the independent effects of climate warming on mercury sequestration is challenging due to temporal overlap between warming temperatures and increased long-range transport of atmospheric mercury following the Industrial Revolution. We address this challenge by examining mercury trends in short cores (the last several hundred years) from eight lakes centered on Cape Herschel (Canadian High Arctic) that span a gradient in microclimates, including two lakes that have not yet been significantly altered by climate warming due to continued ice cover. Previous research on subfossil diatoms and inferred primary production indicated the timing of limnological responses to climate warming, which, due to prevailing ice cover conditions, varied from ∼1850 to ∼1990 for lakes that have undergone changes. We show that climate warming may have enhanced mercury deposition to lake sediments in one lake (Moraine Pond), while another (West Lake) showed a strong signal of post-industrial mercury enrichment without any corresponding limnological changes associated with warming. Our results provide insights into the role of climate warming and organic carbon cycling as drivers of mercury deposition to Arctic lake sediments.

Linking sedimentary total organic carbon to 210Pbex chronology from Changshou Lake in the Three Gorges Reservoir Region, China

The influences of total organic carbon (TOC) and total nitrogen (TN) on Lead-210 (²¹⁰Pb) dating have recently been of increasing concern in lacustrine research. Sediment core from Changshou Lake in the Longxi catchment was investigated for influence of TOC on ²¹⁰Pb dating. Lead-210 excess (²¹⁰Pb_ex), Cesium-137 (¹³⁷Cs) activities, TOC, TN, and particle size were measured. We proposed a dating index based on ¹³⁷Cs chronology and particle size distribution of the lake sediment profile and rainfall erosivities calculated from Longxi catchment metrological records. Increasing trends in TOC and TN were specifically caused by commercial cage fish farming after 1989. The statistically significant correlation between ²¹⁰Pb_ex activity, TOC (0.61, p = 0.04) and TN (0.51, p = 0.04), respectively explained post-1989 ²¹⁰Pb scavenging. The ²¹⁰Pb_ex activity was closely related with coupled peaks of TOC and TN from mass depth 5-10 g cm^-2. Higher TOC/TN ratio (8.33) indicated submerged macrophytes and native aquatic algal growth as main source of carbon from enhanced primary productivity because of massive fertilizer use and coherent climate warming. The study supported key hypothesis on vital role of fertilizer usage and algal derived TOC in controlling sedimentary ²¹⁰Pb_ex activity at Changshou Lake sediment. ¹³⁷Cs profile and erosive events as time markers provided reliable and consistent sedimentation rate of (1.6 cm y^-1). ²¹⁰Pb_ex activity decayed exponentially after peak at mass depth 5.68 g cm^-2. Therefore, violation of ²¹⁰Pb dating primary assumptions made it inappropriate for sediment dating at Changshou Lake. TOC content must be considered while using ²¹⁰Pb as dating tool for lake sediment profiles.