Vertical flux of particulate matter in low productivity environments: A tale from the North

Abstract

The vertical export flux of particulate matter drives the transfer of carbon from ocean surface to depths and, thus, plays a key role in marine carbon sequestration. However, the mechanisms controlling the occurrence and intensity of this flux in oligotrophic and low-productivity environments are still actively debated. We present here original investigations on sediment cores from the low productivity Hudson Bay, northern Canada. Interestingly, despite a globally increased atmospheric import of anthropogenic lead, lead concentration in sediment cores from the Hudson Bay are relatively constant for the past centuries, as if the Anthropocene did not leave any track in this environment. This is not consistent with significant increased lead concentrations observed in nearby lakes, during the same period. Because atmospheric deposition is the source of lead for both the lakes and the Hudson Bay, the very low productivity condition that characterize the Hudson Bay is used to explain the reduced vertical export of lead. A reduced vertical export is also consistent with the trend observed in the lead isotopic composition. The 206Pb/207Pb isotopic value from bottom to top of cores indicates an apparent gradual overprint of anthropogenic lead, typical of mixed Canada-USA aerosol origins, during the 1900’s. In other words, the anthropogenic nature of lead is clearly registered in our record but not the total concentration, which suggests that something is limiting its export to the sediment. This is coherent with data from PCB accumulation in the sediment, which did not reflect actual inputs in the surface water of the Hudson Bay, again pointing to a limitation of the vertical export of contaminant. This supports the hypothesis that primary productivity is the most important factor to drive the vertical export of particulate matter, and thus contaminants, in low productivity environments.