Impact of nuclear fuel reprocessing on the temporal evolution of marine radiocarbon

Abstract

Radiocarbon (14C) is broadly used in oceanography to determine water ages, trace water circulation, and develop sediment- and sclerochronologies. These applications require an accurate knowledge of marine 14C levels, which have been largely perturbed by human activities. Globally during the last century the above-ground nuclear weapon testings have been the primary cause of the increased atmospheric and marine 14C. However, other anthropogenic sources may have caused important regional deviations from the bomb pulse. For the last 70 years European nuclear fuel reprocessing plants have been major contributors of 14C to air and oceans, yet, their regional impact on surrounding marine 14C has been largely overlooked. Here we use a collection of bivalve shells of known capture date and age collected from various locations, including the North Sea, the Irish Sea, Norway, and the Bay of Biscay to reconstruct the sea surface 14C over the last five decades. The measured 14C values for the period 1969–2019, reported in fraction modern, ranged from 1.1 to 1.6 in coastal waters of the Netherlands and from 1.2 to 3.2 along the coast of the UK, indicating significantly higher levels of 14C than those expected for the marine bomb pulse (0.950–1.150). The 14C peaks revealed by the shells coincide with the increase of liquid 14C releases reported from the reprocessing plants of La Hague into the English Channel, and from Sellafield into the Irish Sea. Conversely, the shells from Norway and Spain showed 14C values close to the range of the global marine bomb pulse. The observed large spatial and temporal differences in sea surface 14C show that 14C dating and tracing studies could become problematic in the English Channel, Irish Sea and North Sea for the time period covering the discharge of liquid 14C from the reprocessing plants.