A fine‐scale record of 130 years of organic carbon deposition in an anoxic fjord, Saanich Inlet, British Columbia

Abstract

A varved sediment sequence from an anoxic temperate fjord is examined for a 130‐yr record of organic carbon content and stable carbon isotope composition. Saanich Inlet, British Columbia, receives most detrital input from a logged and reforested watershed. Winter and summer laminae differences are significant and are isotopically depleted with respect to water column organic carbon. Within the core, organic carbon content increased from 2.5% in the 1860s to 3.3% in the 1990s, whereas carbon isotopes become depleted by 1.2‰ toward the late 1990s. Homogenization of large sample volumes yields the most repeatable carbon results. Overlaid on the trends are cycles of roughly a 38‐yr period for both records. River outflow has not changed, but logging rates that peaked midcentury in the watershed match the organic carbon shifts. This influence, plus the interdecadal Pacific‐climate oscillation, likely explain the periodicity observed.