Sources of organic matter in two contrasting tropical coastal environments: The Caribbean Sea and the eastern Pacific

Abstract

In this study, we evaluate sources of organic matter (OM) in two contrasting tropical coastal environments in Colombia, using a combination of glycerol dialkyl glycerol tetraethers (GDGTs) and n-alkane lipid biomarkers. A clear difference between the less productive Caribbean Sea and the more productive eastern Pacific is observed, with higher terrestrial contributions into the eastern Pacific strongly associated with riverine inputs. Although higher land plant-derived n-alkanes can be transported as eolian dust, our results suggest mainly riverine sources for our samples. When looking at the branched GDGT compositions, in situ production could be identified in most Caribbean stations and some of the more coastal sites in the Pacific with #rings(tetra) index values of above 0.7. The remaining stations in the eastern Pacific display soil-eroded GDGTs inputs associated with river discharges and deposited in the proximity of the coast. Marine production as measured by short chain n-alkanes and crenarchaeol are also higher in the eastern Pacific. It appears that the terrestrial inputs would bring nutrients as well as OM fueling both auto- and heterotrophs making the eastern Pacific overall more productive. We also observe OM of petrogenic origin, albeit in a low amount, especially in coastal Caribbean sites located near main harbors. This study helps improve the understanding of the complex mixture of OM inputs, origins and transport mechanisms into Colombian coastal areas. The present work emphasizes the need to integrate climate, geological setting and hydrology to fully understand the carbon cycle at a regional scale.