African biomass burning is a substantial source of phosphorus deposition to the Amazon, Tropical Atlantic Ocean, and Southern Ocean

Abstract

The deposition of phosphorus (P) from African dust is believed to play an important role in bolstering primary productivity in the Amazon Basin and Tropical Atlantic Ocean (TAO), leading to sequestration of carbon dioxide. However, there are few measurements of African dust in South America that can robustly test this hypothesis and even fewer measurements of soluble P, which is readily available for stimulating primary production in the ocean. To test this hypothesis, we measured total and soluble P in long-range transported aerosols collected in Cayenne, French Guiana, a TAO coastal site located at the northeastern edge of the Amazon. Our measurements confirm that in boreal spring when African dust transport is greatest, dust supplies the majority of P, of which 5% is soluble. In boreal fall, when dust transport is at an annual minimum, we measured unexpectedly high concentrations of soluble P, which we show is associated with the transport of biomass burning (BB) from southern Africa. Integrating our results into a chemical transport model, we show that African BB supplies up to half of the P deposited annually to the Amazon from transported African aerosol. This observational study links P-rich BB aerosols from Africa to enhanced P deposition in the Amazon. Contrary to current thought, we also show that African BB is a more important source of soluble P than dust to the TAO and oceans in the Southern Hemisphere and may be more important for marine productivity, particularly in boreal summer and fall.

Keywords: Amazon Basin; Atlantic Ocean; biomass burning; dust; phosphorus.

Fig. 1.

Data are shown from spring only. A shows the dust concentration and percentage of SP in the Top and concentrations of TP and SP in the Bottom. Error bars for P measurements show one SD. B shows the correlation between dust and TP (Top), dust and SP (Middle), and percentage of soluble P and dust (Bottom). C shows HYSPLIT frequency plots of air mass back trajectories at 1,000 m initiated every 6 h from February 1 to March 31, 2016.

Fig. 2.

Data are shown for fall only. A shows the dust concentration and percentage of SP in the Top and concentrations of TP and SP in the Bottom. Error bars for P measurements show one SD. B shows the correlation between TP and dust (Top), dust and SP (Middle), and percentage of soluble P and dust (Bottom). C shows HYSPLIT frequency plots of air mass back trajectories at 1,000 m initiated every 6 h from September 1 to October 31, 2016.

Fig. 3.

A 315-h back trajectory initiated in Cayenne on September 28, 2016, at 1,000 m above ground level using HYSPLIT. B–D show the first 10 km of CALIPSO tracks that intersect the BT shown in A on (B) September 20, 2016, (C) September 22, 2016, and (D) September 28, 2016. The colored boxes in A correspond to the approximate location of where the CALIPSO overpass intersects the trajectory on September 20 (black box), September 22 (green box), and September 28 (purple box). Note that the altitude of the air mass is also boxed in corresponding colors. The black stars in A and D show the location of Cayenne.

Fig. 4.

Model predictions from CAM to estimate P deposition to the Amazon that were tuned to the observations to estimate the fraction of (A) TP deposition and (B) SP deposition from African dust (solid red line) and BB from both northern and southern Africa (dotted blue line). African dust and combustion sources of P are only included to investigate the relative importance of each transported source.

Fig. 5.

SP deposition (in milligrams per square meter per year) from all aerosol sources during boreal winter (DJF), spring (MAM), summer (JJA), and fall (SON). Model predictions in this figure were not tuned to our observations.

Fig. 6.

Percentage of SP deposited from global BB sources only in boreal winter (DJF), spring (MAM), summer (JJA), and fall (SON). Model predictions in this figure were not tuned to our observations.