Biogeochemical Impacts of a Black Carbon Wet Deposition Event in Halong Bay, Vietnam

Xavier Mari,Justine Brune,Benjamin Guinot,Pradeep Ram Angia Sriram,Patrick Raimbault,Chu Van Thuoc,Thorsten Dittmar,Jutta Niggemann,Jean-Pierre Lefebvre

doi:10.3389/fmars.2019.00185

Abstract

Black carbon (BC) is emitted to the atmosphere during biomass, biofuel and fossil fuel combustion, and leaves the atmosphere via dry or wet deposition on land and on the ocean. On a global scale, wet deposition accounts for about 80% of the total atmospheric BC inputs to the ocean. The input of BC particles to the ocean can enrich surface waters with carbon and associated elements, and owing to high porosity and surface-active properties, BC can alter biogeochemical cycles by sorbing dissolved compounds and promoting aggregation. The rain-mediated input of BC to the ocean and its consequences on nutrient concentrations and particle dynamics were studied in Halong Bay, Vietnam, during a 24 h cycle impacted by short and heavy rainfall events. This study suggests that once introduced in the surface ocean via wet deposition, BC sorbs dissolved organic matter (DOM) and stimulates aggregation processes. The observed wet deposition events were characterized by sudden and pulsed inputs of BC particles that created a thin layer of sinking surface-active aggregates, acting as a net-like scavenger for DOM, nutrients (especially phosphate), and small particles. In addition, the wet deposition events coincided with an enrichment of nutrients in the surface microlayer, with an excess input of nitrogen relative to phosphorus leading to an increase of the molar N:P ratio from 24:1 to 37:1. In the underlying water, the molar N:P ratio also increased (i.e., from 39:1 to 64:1), and this can be attributed to the preferential scavenging of dissolved P-compounds on sinking BC-aggregates.

Highlights

Black carbon (BC) is the product of incomplete combustion of fossil fuels, biofuels and biomass, and is a major component of soot
At the start of the 24-h cycle, at 08:00 on 19 October, atmospheric BC concentration was high until the heavy rainfall event of 4 L m−2 that occurred after the first seawater sampling (Figure 2)
This timing is reflected in the measured timing of atmospheric BC (Figure 2) and the variations of salinity, temperature and density of seawater (Figure 3)

Summary

Introduction

Black carbon (BC) is the product of incomplete combustion of fossil fuels, biofuels and biomass, and is a major component of soot. In addition to BC particles emitted to the atmosphere, aged BC that has been oxidized in soil can return into the atmosphere associated with soil dust in the form of water soluble BC (WSBC; Bao et al, 2017). Both forms of BC leave the atmosphere via dry or wet deposition, on land and oceans. The global atmospheric deposition of WSBC associated with dust is estimated to ∼1.8 Tg yr−1, with about 70% in the form of wet deposition (Bao et al, 2017). Wet deposition is the dominant input mechanism of atmospheric BC to the ocean

Methods

Results

Conclusion