Sources and accumulation of sediment and particulate organic carbon in a subarctic fjard estuary: Pb-210, Cs-137, and delta C-13 records from Lake Melville, Labrador

摘要

The sources and distribution of sediment and particulate organic carbon (OC) to Lake Melville, Labrador, were characterized to better understand impacts from climate and hydrological changes to the system. Mass accumulation rates (MARs) across the Lake Melville System (LMS) were established from 15 sediment cores collected in 2013 and 2014 by fitting excess Pb-210 (Pb-210(ex)) profiles to a two-layer advection-diffusion model. MARs, validated using Cs-137, varied between 0.04 and 0.41 g cm(-2) a(-1), and overall decreased with increasing distance from the Churchill River, which drains into Goose Bay, a western extension of Lake Melville. The Churchill River is the greatest source of sediment to the system, but surprisingly, MARs were greatest in western Lake Melville rather than Goose Bay, reflecting the contribution of fine material carried eastward in the Churchill River plume and inputs from nearby tributaries. A comparison of Cs-137 and Pb-210(ex) inventories to expected atmospheric fallout (1.5 and 23.6 disintegrations per minute (dpm) cm(-2), respectively) in sediment across the LMS suggests particles are largely sourced from the watershed. In eastern Lake Melville, elevated Pb-210(ex) inventories and marine OC point to particle scavenging of dissolved Pb-210 from inflowing marine water. A transient tracer mixing model was used to determine the depth in each core where > 90% of sediment was deposited before and after hydroelectric development at Churchill Falls (1970) and applied to down-core profiles of OC and organic carbon isotopes (delta C-13(org)). We observed a significant increase of terrestrial OC to Lake Melville post 1970, which we interpret as change from climate or hydrology of the Churchill River.