Experimental evidence of formation of transparent exopolymer particles (TEP) and POC export provoked by dust addition under current and high pCO2 conditions

Abstract

The evolution of organic carbon export to the deep ocean, under anthropogenic forcing such as ocean warming and acidification, needs to be investigated in order to evaluate potential positive or negative feedbacks on atmospheric CO2 concentrations, and therefore on climate. As such, modifications of aggregation processes driven by transparent exopolymer particles (TEP) formation have the potential to affect carbon export. The objectives of this study were to experimentally assess the dynamics of organic matter, after the simulation of a Saharan dust deposition event, through the measurement over one week of TEP abundance and size, and to evaluate the effects of ocean acidification on TEP formation and carbon export following a dust deposition event. Three experiments were performed in the laboratory using 300 L tanks filled with filtered seawater collected in the Mediterranean Sea, during two ‘no bloom’ periods (spring at the start of the stratification period and autumn at the end of this stratification period) and during the winter bloom period. For each experiment, one of the two tanks was acidified to reach pH conditions slightly below values projected for 2100 (~ 7.6–7.8). In both tanks, a dust deposition event of 10 g m-2 was simulated at the surface. Our results suggest that Saharan dust deposition triggered the abiotic formation of TEP, leading to the formation of organic-mineral aggregates. The amount of particulate organic carbon (POC) exported was proportional to the flux of lithogenic particles to the sediment traps. Depending on the season, the POC flux following artificial dust deposition ranged between 38 and 90 mg m-2 over six experimental days. Such variability is likely linked to the seasonal differences in the quality and quantity of TEP-precursors initially present in seawater. Finally, these export fluxes were not significantly different at the completion of the three experiments between the two pH conditions.

Depth is relative: the importance of depth for transparent exopolymer particles in the near-surface environment

“…Such precursor material may have been deposited from the atmosphere; Cabo Verde is known for its Saharan dust deposition events and indeed dust events were observed during our campaign (supporting data will be shown in this special issue). This dust deposition has been shown to increase the abiotic formation of TEPs (Louis et al, 2017) and is potentially a large contributing factor to higher enrichment of TEPs in Cabo Verde. If true, this presents interesting implications for the residence time of dust which ends up floating in the SML with sufficient time for photochemical processing.…”

Depth is Relative: The Importance of Depth on TEP in the Near Surface Environment

“…Due to operational interference from research vessels and the use of large rosette water samples, most studies sample at 3-5 meters for the shallowest depth and assume this surface water to be 255 homogenous towards the surface, and therefore equally representative. However, the importance and influence of the SML has been thoroughly supported (Cunliffe et al, 2013;Engel et al, 2017;Hardy, 1982;Liss and Duce, 1997;Wurl et al, 2011b) and thus there is a need to better understand the biogeochemical cycling occurring in the near surface water and how they relate to organic matter transfer to deeper water masses. This study is the first to take a higher resolution look at the vertical distribution of TEP and other related parameters in the upper 2 meters of the 260 ocean.…”

Tuffaceous Mud is a Volumetrically Important Volcaniclastic Facies of Submarine Arc Volcanism and Record of Climate Change

“…However, we suggest a different transport mechanism: vitriclast aggregation by Transparent Exopolymer Particle (TEP) gels. Experiments have found that such gels form abiotically from the addition of clay or dust particles to sea water, and can settle 70–80% of the particles in days as organo‐mineral aggregates (Louis et al, ; Passow et al, ). We speculate that the porous, sinuous, and rough character of submarine arc vitriclasts would be especially effective in this regard, and that the turbulent character of submarine eruption plumes would add to the scavenging potential of the gels.…”