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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 ¹³C of Organic Matter

Yvonne Van Breugel^*, Stefan Schouten^*,, Matthias Paetzel^** and Jaap S. Sinninghe Damsté^*

^* Royal Netherlands Institute for Sea Research (NIOZ), Department of Marine Biogeochemistry and Toxicology, PO Box 59, 1790 AB, Den Burg, The Netherlands
^** The Faculty of Science, HSF University College, PO Box 133, N-6851 Sogndal, Norway

Corresponding author: schouten{at}nioz.nl

The sedimentary records of the Toarcian oceanic anoxic event display an up to 6 permil negative ¹³C 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 Küspert hypothesis"). We attempted to evaluate the effect of recycling of CO₂ on the ¹³C-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 ¹³C_DIC in bottom waters is indeed substantially depleted (¹³C up to –20) and that this isotopically depleted DIC is advected to surface waters at times of mixing. However, ¹³C of particulate organic carbon (POC) did not reveal isotopic depletion of POC at times of advection of respired CO₂ as predicted in Küspert’s scenario, perhaps due to the isotopic heterogeneity of POM. Therefore, in this study we used the ¹³C 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 ¹³C-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 (_p) may thus deteriorate the correlation between the ¹³C-CO₂ and that of POC as we would expect if recycling of respired CO₂ has an impact on the ¹³C-contents of organic matter. Similar to the _p of POC, a large variation in the _p of the sterols and alkenones was observed. Moreover, a strong co-variance existed between the _p of POC and that of the individual biolipids, with high _p in summer (up to 22 ) and very low _p (3 to 15 ) in the fall. Most likely, a common environmental factor, such as an increased nutrient supply, caused the decrease in _p. Water column mixing by storms, the same mechanism that likely increased the ¹³C-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 ¹³C 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 ¹³C-contents of organic matter.