Saharan dust deposition in the eastern Mediterranean Sea: ballasting agent or fertilizer?

Dust deposition can increase the strength of the biological pump through fertilizing and ballasting effects of the deposited dust, in particular in (ultra-)oligotrophic oceans such as the eastern Mediterranean Sea (EMS). However, dust characteristics, such as nutrient content and bioavailability, organic-matter content, and grain-size distribution, and thus its fertilizing and ballasting potential, can vary between dust events.

Here, we present a long-term (1999-2011), high-resolution (14-21 days) sediment-trap record of dust fluxes, dust grain-size distributions, and fluxes of plant leaf waxes at 500, 1500, and 2500m water depth to assess seasonal and interannual variation in the amount and characteristics of dust deposited in the EMS.

We find that dust events mainly occur during late spring and summer, although their exact timing and magnitude varies between years. Differences in grain-size distribution and plant wax content between dust events indicate that the provenance, transportation, and/or deposition mode of the dust varied between events. The dust events archived in the sediment traps are preceded by atmospheric dust transport, indicated by increased Aerosol Optical Depth (AOD) values recorded by satellites in the weeks before dust fluxes increase. However, several major atmospheric dust outbreaks observed by satellites do not appear in the sediment trap record. This indicates that not all material that passes the EMS through the atmosphere is actually deposited on the sea surface and/or reaches the traps at larger water depths.

Most dust events in the sediment traps can be traced through the water column, indicating relatively rapid vertical export. The dust events coincide with increases in organic carbon flux, supporting the proposed role of dust in the biological pump through ballasting. However, while coarse-grained dust is consistently transferred to the deepest trap, regardless of the absolute flux, finer-grained dust is primarily detected in the upper trap. We will use our dataset to further investigate whether export of fine-grained dust is also linked to ballasting or is mediated by productivity in the surface ocean through the formation of organic aggregates and fecal pallets, either as a result of dust fertilization or natural processes.