Control of acid polysaccharide production and ²³⁴Th and POC export fluxes by marine organisms

Abstract

Ratios of particulate organic carbon (POC) to particulate 234Th activities (POC/234Th) in the ocean are used to determine the POC export flux, and thus, the power of the biological pump. In order to understand the main reasons why this ratio frequently varies as a function of depth, size, and kind (suspended vs. sinking particles), we measured vertical profiles in a cold core ring and warm core ring in the Gulf of Mexico in May 2001. Here we show that particulate 234Th/POC ratios in the Gulf of Mexico are positively correlated to the content of different Th(IV)‐binding polysaccharide fractions (uronic acids, total acid polysaccharides, total polysaccharides) in both suspended and sinking particles as well as to prymnesiophyte abundance, but negatively correlated to bacterial production. Variations in acid polysaccharide compounds, produced by both algae and bacteria, but degraded only by bacteria, can account for observed variations in POC/234Th ratios seen in the ocean.

Organic carbon to 234Th ratios of marine organic matter

“…This implies that in the upper 15 m 234 Th binding was a function of the OC fraction, which consists of TEP or TEP-precursors, independent of the size of the material, ranging from the colloidal to large particles. A linear relationship between the 234 Th : OC ratio and the fraction of acidic polysaccharides within the pool of OC was also found for sinking and suspended particles in the Gulf of Mexico (Santschi et al, 2003), confirming that thorium binds preferentially to the acidic polysaccharide fraction of OC (Santschi et al, 2006-this issue). A direct comparison of results with regards to the relationships between the OC :…”

“…234 Th ratio and marine organisms has also been suggested to be based on the production of acidic polysaccharides by phytoplankton (Santschi et al, 2003). Contradictory findings regarding the relationship of the OC : 234 Th ratio versus particle size for particles N 1 Am may thus easily reflect differences in the dominating particle type, even if no differences in the affinity of 234 Th to different substances are assumed.…”

Thorium speciation in seawater

“…2) and colloidal polysaccharide, with values reaching up to 10 8 ml/g-polysaccharides when extrapolated to 100% polysaccharides. Polysaccharide content is used as a proxy for the presence of strong Th-binding ligands, which are present at considerably lower abundance than total polysaccharides (Hung et al, 2003a,b;Santschi et al, 2003). Interestingly, purified Th(IV)-binding colloid material with 100% polysaccharide content had modern D 14 C values (Santschi et al, 1998), suggesting that the 234 Th(IV) binding polysaccharide entity is of recent origin.…”

An assessment of particulate organic carbon to thorium-234 ratios in the ocean and their impact on the application of 234Th as a POC flux proxy

“…If the number of surface sites or affinity for 234 Th changes with particle size or type this would impact C / 234 Th ratios as well . For example, it has been proposed that changes in solution chemistry associated with complexation of 234 Th by natural acid polysaccharide (APS)-rich TEP ligands (e.g., Guo et al, 2002a;Hung et al, 2004;Santschi et al, 2003) alters the exchange of 234 Th (relative to POC) with the surrounding particle regime. While distribution coefficients of 234 Th for different inorganic particle types are on the order of 10 3 -10 6 , L/kg, affinity for APS ligands, which are part of the polysaccharide fraction, is more than two orders of magnitude higher for colloidal size classes (1 kDa to 0.2 Am; Guo et al, 2002b;Quigley et al, 2002) when expressed as L/kg polysaccharides, with similar abundance of polysaccharides and organic sorbents.…”