The flux and preservation of organic carbon in Baffin Island fjords

Abstract

Abstract Except for shell hash lags found along minor portions of fjord walls and on current swept sills, organic carbon dominates over inorganic carbon, accounting for between 95 and 1000f the total carbon composition within the fjords. The deposition of terrestrial organic carbon is of minor significance except proximal to river mouths; i.e. the atomic carbon/nitrogen ratio, both within the water column and within the fjord sediments, fluctuates around 6.4. Organic carbon is an important constituent of the suspended particulate matter throughout most of the year when the suspended sediment concentration is < 2 g m −3 . During the summer meltwater season, the organic fraction of the suspended sediment is overwhelmed by inorganic mineral matter. In the autumn season, the organic carbon content of the suspended particulate matter can exceed 30%, especially in the seasonal layer where productivity is highest. Bacterial biomass accounts for more than 500f the total biomass found at the sediment-water interface. Microbial respiration of organic carbon is highly effective, consuming > 900f the annual deposition of carbon to the sea floor. The remaining and mostly refractory carbon preserved within the sediment column is < 20y weight of the sediment and, in high sedimentation environments, such as proximal to tidewater glaciers, organic carbon is < 0.1%. The organic carbon content of sea floor samples is found inversely proportional to the modern suspended load delivered to these fjords. Anoxic degradation of this buried carbon appears to be a slow and ineffective process over the time period of 10 000 years. The abundance of organic carbon buried within Arctic fjord sediments is therefore considered as an inverse indicator of deglacial and post-glacial sedimentation rates. A consistent organic carbon stratigraphy relating to the deglacial and Hypsithermal period, the Neoglacial period culminating in the Little Ice Age, and the present-day warm conditions has been identified in all ten Baffin Island fjords investigated. A simple technique is presented to convert the content of organic carbon observed down core to relative or absolute sedimentation rates, depending on the availability of 14 C dates. Instantaneous deposition of sediment from slides or gravity flows remains problematic.