The transfer of bomb radiocarbon and anthropogenic lead to the deep North Atlantic Ocean observed from a deep sea coral

Abstract

Deep-ocean, Δ¹⁴C, Pb concentrations, and Pb isotopes were reconstructed from a deep-sea coral Enallopsammia rostrata from 1410 m depth off of Bermuda. Our high-resolution time series is created from closely spaced radial cross sections, with samples taken from the center of concentric coral growth bands that we show to be the oldest portion of the section. Prebomb radiocarbon ages from the coral demonstrate that the vertical growth rate of the coral is linear, and the age of the coral is estimated to be 560–630 yr old based on the growth rate. Using this age model to reconstruct Δ¹⁴C in deep seawater, we first detect bomb radiocarbon at the coral growth site around 1980, and show that Δ¹⁴C increased from −80 ± 1%₀(average 1930–1979) to a plateau at −39 ± 3%₀ (1999–2001). Pb/Ca of the coral ranges between 1.1–4.5 nmol/mol during the 16th and 17th centuries, and Pb isotope ratios (²⁰⁶Pb/²⁰⁷Pb =1.21, ²⁰⁸Pb/²⁰⁷Pb = 2.495) in this period agree with pre-anthropogenic values found in the pelagic sediments of the North Atlantic Ocean basin. Coral Pb/Ca is slightly elevated to 6.2 ±0.9nmol/molbetween the 1740s and the 1850s and then increases to 25.1 ±0.2nmol/molin the 1990s. The increase in coral Pb/Ca is accompanied by a decrease in coral ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb, indicating that the increase was caused by the infiltration of anthropogenic Pb to the coral growth site. Comparing our data to the surface coral Δ¹⁴C and Pb records from Bermuda reveals a time scale of tracer transport from the surface ocean to the coral growth site. Some characteristic features, e.g., the bomb-derived Δ¹⁴C increase, appear in the deep ocean approximately 25 yr later than the surface, but the overall increase of Δ¹⁴C and Pb in the deep ocean is smaller and slower than the surface, showing the importance of mixing during the transport of these tracers. Keywords: deep-sea coral; time-series; lead; lead isotopes; radiocarbon; anthropogenic