Dust flux in the Arabian Sea

Eolian grain size and flux were measured on samples from 11 Arabian Sea sediment traps deployed 200-1250 km offshore. The timing of increased grain size is coincident with the onset of strong summer monsoon winds and dust storm activity over the Arabian Peninsula and Middle East. Data spanning a full annual cycle show that eolian grain size is highly correlated with barometric pressure (r=-0.91) and wind speed (r=0.84), enabling calibration of the downcore record in terms of these primary meteorological variables. Eolian flux is highly correlated with organic carbon flux (r=0.80); both increase 6-8 weeks after the grain size increase and summer monsoon onset. This lag, and the low correlation between eolian grain size and eolian flux (r=0.36), likely result from the differential sinking rates of large and small dust particles in the surface waters as well as biological scavenging associated with monsoon-induced productivity.

Particle flux studies of sediment traps from the northern and equatorial Indian Ocean

The Asian monsoon system governs seasonality and fundamental environmental characteristics in the study area from which two distinct peculiarities are most notable: upwelling and convective mixing in the Arabian Sea and low surface salinity and stratification in the Bay of Bengal due to high riverine input and monsoonal precipitation. The respective oceanography sets the framework for nutrient availability and productivity. Upwelling ensures high nitrate concentration with temporal/spatial Si limitation; freshwater-induced stratification leads to reduced nitrogen input from the subsurface but Si enrichment in surface waters. Ultimately, both environments support high abundance of diatoms, which play a central role in the export of organic matter. It is speculated that, additional to eddy pumping, nitrogen fixation is a source of N in stratified waters and contributes to the low-d15N signal in sinking particles formed under riverine impact. Organic carbon fluxes are best correlated to opal but not to carbonate, which is explained by low foraminiferal carbonate fluxes within the river-impacted systems. This observation points to the necessity of differentiating between carbonate sources for carbon flux modeling. As evident from a compilation of previously published and new data on labile organic matter composition (amino acids and carbohydrates), organic matter fluxes are mainly driven by direct input from marine production, except the site off Pakistan where sedimentary input of (marine) organic matter is dominant during the NE monsoon. The explanation of apparently different organic carbon export efficiency calls for further investigations of, for example, food web structure and water column processes.