210Pb and 239,240Pu in the Northeast Water Polynya, Greenland: particle dynamics and sediment mixing rates

Abstract

Abstract Distributions of the radionuclides 210Pb and 239,240Pu in sediment cores from the Northeast Water Polynya, Greenland, showed that these nuclides reached depths of 5–15 cm by particle mixing and sediment accumulation. End-member average values of the particle mixing coefficient and sediment accumulation rate were 0.13 cm2 y−1 and 0.06 cm y−1, obtained from the 210Pb profiles by assuming that each process is dominant relative to the other. Both 210Pb and 239,240Pu were measured on four cores; using the Pu data to constrain mixing rates produced corrected sediment accumulation rates that were 20–80% of the values calculated by neglecting mixing. Organic carbon burial in the polynya sediments was ≤0.4 mmol m−2 d−1, based on measured POC values at depth in the sediments and sediment accumulation rates corrected for mixing. This value is about 1% of the independently measured POC flux leaving the euphotic zone and compares with benthic carbon remineralization rates of 7% calculated by others from O2 uptake in the sediments. The inventories of excess 210Pb in the sediments ranged from 6 to 28 dpm cm−2. Relative to the atmospheric input of 210Pb and in situ production from decay of 226Ra, approximately 5 dpm cm−2 of 210Pb was being removed from the water column. The difference between the removal from the water column and sediment inventories suggests a net import of 210Pb to the polynya. This may occur by input of dissolved 210Pb from offshore waters or by input of 210Pb carried by sea ice. Particulate matter in land-derived fast ice adjacent to the polynya contained 330 ± 14 dpm of excess 210Pb g−1. If particles transported in sea ice are comparable to those extracted from fast ice, then sea ice transport into the polynya followed by melting may be an important source of excess 210Pb to the area. Fast ice also may contribute 210Pb if portions break off and melt within the polynya, as occurred in 1993.