Seasonal variation in the stable carbon isotopic composition of algal lipids in a shallow anoxic fjord:: Evaluation of the effect of recycling of respired CO2 on the δ13C of organic matter

The sedimentary records of the Toarcian oceanic anoxic event display an up to 6 permil negative delta C-13 excursion in both carbonate and organic matter. One hypothesis to explain this excursion is increased recycling of respired CO2, formed by mineralization of organic matter and temporarily trapped below the chemocline of the shallow, stratified shelf seas of Europe (the Kuspert hypothesis). We attempted to evaluate the effect of recycling of CO2 on the C-13-contents of organic matter by studying a shallow marine fjord (Kyllaren, Norway) with persistent stratification and anoxic bottom waters. During a two-year field study the water column of the fjord was sampled for dissolved inorganic carbon (DIC) and particulate organic matter (POM). Earlier studies indicated that delta C-13(DIC) in bottom waters is indeed substantially depleted (delta C-13 up to -20 parts per thousand) and that this isotopically depleted DIC is advected to surface waters at times of mixing. However, delta C-13 of particulate organic carbon (POC) did not reveal isotopic depletion of POC at times of advection of respired CO2 as predicted in Kuspert's scenario, perhaps due to the isotopic heterogeneity of POM. Therefore, in this study we used the delta C-13 of biomarker lipids for specific photoautotrophic organisms in the surface water (above the chemocline) to further evaluate the effect of recycling of respired CO2 on the C-13-contents of organic matter. The concentrations of specific sterols and long-chain alkenones revealed a seasonal succession of dominant algal classes: diatoms bloomed in the spring, haptophytes proliferated in late summer and dinoflagellates thrived in fall. The change of sources of POM, and the taxon-specific differences in the carbon isotopic fractionation (epsilon(p)) may thus deteriorate the correlation between the delta C-13-CO2 and that of POC as we would expect if recycling of respired CO2 has an impact on the C-13-contents of organic matter. Similar to the epsilon(p) of POC, a large variation in the epsilon(p) of the sterols and alkenones was observed. Moreover, a strong co-variance existed between the epsilon(p) of POC and that of the individual biolipids, with high epsilon(p) in summer (up to 22 parts per thousand) and very low epsilon(p) (3 to 15 parts per thousand) in the fall. Most likely, a common environmental factor, such as an increased nutrient supply, caused the decrease in epsilon(p). Water column mixing by storms, the same mechanism that likely increased the C-13-depleted DIC in the oxygenated surface water of Kyllaren fjord in the fall, also explains the increase of nutrients. As a consequence, the short term effect of recycling of respired CO2 on the delta C-13 of phytoplankton lipids and total organic carbon is negligible in this anoxic marine system. This shows that, even by using compound-specific isotopes, it is difficult to reconstruct the impact of recycling of respired CO2 on the C-13-contents of organic matter.