Using the radium quartet (228Ra, 226Ra, 224Ra and 223Ra) to estimate water mixing and radium inputs in Loch Etive, Scotland

The radium (Ra) quartet (228Ra, 226Ra, 224Ra, and 223Ra) has been investigated in Loch Etive, a Scottish fjord, to provide new constraints on water mixing rates and on the inputs of Ra from sediments. Maximum water transport rates for the inflowing estuarine layer at 5 m depth, determined from the excess 223Ra (223Raex), indicate that this water travels at no more than 2.4 ± 0.2 cm s−1 net and that it takes 17 ± 2 d for waters to travel from the mouth to the head of the loch if no horizontal mixing is taken into account. Alternatively, neglecting advection, the short-lived Ra distribution could be explained by horizontal mixing rates of 6.1 × 106 cm2 s−1 (223Raex) or 9.1 × 106 cm2 s−1 (224Raex). Periodic overturning circulation plays an important role in resetting chemical cycles in the isolated deep basin of the inner loch. Sediment in this deep basin provides the major input of 228Ra to the isolated deep water, and the accumulation of 228Ra in deep waters allows an assessment of sedimentary fluxes of 228Ra, a poorly constrained aspect of the 228Ra input to the global ocean. The calculated sedimentary 228Ra flux of 2.1 ± 0.2 (× 109) atoms m−2 yr−1 in the inner deep basin is comparable with previous measurements of sedimentary 228Ra inputs from shelf sediments, supporting existing global 228Ra budgets, which are used to assess global rates of groundwater discharge to the ocean.