Role of suspended matter in controlling beryllium-7 (7Be) in the Black Sea surface layer

Abstract

Spatial variability of 7Be activity in both particulate and dissolved phases in the Black Sea was studied using field observations from the 95th cruise of R/V Professor Vodyanitsky in the summer of 2017. The total (particulate + dissolved) 7Be activity varied spatially from 2.8 to 8.3 Bq m−3 and averaged to 5.3 ± 1.9 Bq m−3. The particulate fraction of 7Be activity varied spatially between 5% and 13%, with an average value of 8% ± 2%. This paper provides an analysis of the influence of atmospheric precipitation, mixed layer depth, total suspended matter concentration (TSM), seawater temperature and salinity on the spatial variability of 7Ве activity. It is shown that the variability of the TSM might determine the spatial heterogeneity of total 7Be activity in the study area. Quantitative estimates of the distribution coefficient (Kd) of 7Be for the dissolved and particulate phases were obtained, and the functional dependence of Kd on the TSM was established. The average Kd value was determined to be (1.6 ± 0.3) × 102 m3 kg−1. Kd decreased with an increase in TSM. The obtained estimates suggested that the average 7Be sinking rate on the particulate matter was (1.3 ± 0.7) × 10−5 m s−1. Sinking particles removed approximately 7% of total 7Be activity from the surface layer offshore and reached 29% on the Black Sea shelf. The influence of the TSM, its sinking rate, vertical exchange intensity, and bathymetry on the 7Be sinking flux in the surface mixed layer (SML) were also investigated. A statistically significant correlation was established for both the particle sinking rate and TSM: an increase in the sinking rate and TSM led to an increase in the contribution of the sinking process to the total 7Be flux from SML.