Dissolved aluminium dynamics in response to dust storms, wet deposition, and sediment resuspension in the Gulf of Aqaba, northern Red Sea

Abstract

Dissolved aluminium (Al) is a primary tracer of atmospheric deposition to the open ocean. However, the impact of short-term environmental perturbations such as dust storms, sediment resuspension and rainfall events on the oceanic water column is poorly constrained due to the typically low temporal resolution sampling in open ocean settings. The Gulf of Aqaba (GoA), northern Red Sea, is a highly accessible deep oligotrophic water body featuring exceptionally high atmospheric deposition fluxes delivered by dust storms, which constitutes as the main terrigenous input to the GoA surface water.Here, we present a time series of dissolved Al and silicate (Si) concentration profiles sampled during 2017 and 2018, with a particular focus on daily time scale dust storms, episodes of sediment resuspension and rain events. We evaluate the results in conjunction with high temporal resolution measurements of airborne aerosols and sediment trap -based water column sinking particulate fluxes.Dissolved Al and Si concentrations ranged between 22 and 91 nmol kg−1 and 0.6 and 3.2 µmol kg−1, respectively. These two elements correlated at depth but decoupled in the upper water column. Counter intuitively, mixed layer Al (AlML) inventories decrease with increasing aerosol loads, with dust storms promoting intense Al scavenging, causing scavenging rates to surpass dissolution rates and abruptly driving down AlML by up to 14 %. Concurrently, post dust storm AlML change rates increase linearly with increasing theoretical dissolution rates and thus net dissolution is predicted for higher dust loads than observed in this study. However, low seawater particle loads during low magnitude dust storms and deep mixing depths will result in conditions that favor scavenging. Similarly, a sediment resuspension event triggered a decrease of 34 % in the Al water column inventory. By contrast, wet deposition may enhance the soluble Al flux from mineral dust by a factor of 11. Atmospheric deposition flux estimates (29.8 ± 4.4 g m−2 year−1) calculated using long-term average AlML and mixed layer depths agree with independent flux estimations. Conversely, fluxes calculated using discrete profiles yielded a wide range of values (8–93 g m−2 year−1).The combined results demonstrate that atmospheric deposition in the oceans acts as a long-term source for Al while concomitantly serving as a short-term sink through scavenging. The in-situ rates and insights presented here may be used to understand and quantify the true impact of abrupt environmental events on water column chemical compositions.