Oceanographic controls on the carbon isotopic compositions of sinking particles from the Cariaco Basin

This study examined the relationship between carbon isotopic composition of sinking organic matter (OM) and the biological, physical and chemical properties of the surface ocean in the Cariaco Basin. The 13C/ 12C ratio of OM ( δ 13C org) in sinking particles was determined on sediment trap samples from four depths collected from 1996 to 1999 as part of the CArbon Retention In A Colored Ocean time series. Water column properties, including temperature, productivity, chlorophyll and concentration of dissolved CO 2, were concurrently measured on monthly cruises. The δ 13C org varied from a high of –17.7‰ to a low of –22.6‰ during the study period. The variation of the δ 13C org throughout seasonal cycles was directly proportional to the strength of upwelling and was negatively correlated with temperature ( r 2=0.64). During the 1996–1997 upwelling event, the strongest during the study period, the δ 13C org increased by ∼4.4‰ whereas during the 1998–1999 upwelling event, the weakest during the study period, the δ 13C org only increased by ∼3.3‰. Contrary to most previous studies, we observed a negative relationship ( r 2=0.53) between [CO 2 aq ] and the estimated isotopic fractionation factor ( ε p). However, there was no correlation between ε p and the calculated growth rates indicating that there was non-diffusive uptake of carbon into phytoplankton cells. It thus appears that [CO 2 aq ] does not control the δ 13C org in the water column of the study site. The best explanation for the isotopic enrichment observed is a carbon concentrating mechanism (CCM) in phytoplankton. The existence of a CCM in phytoplankton has major implications for the interpretation of the δ 13C org in the Cariaco Basin.