Nitrogen isotopes in chlorophyll and the origin of Eastern Mediterranean sapropels

Abstract

Abstract : HPLC and size-exclusion chromatography are used to purify chlorins from particles and sediments for delta 15N and delta 13C analysis with precisions greater than 0.15(1/1000). Chlorin delta 15N and delta 13C is also accomplished by irmGC-MS. Chlorophyll-whole phytoplankton DELTA delta 15N is 5.16 +/- 2.40(1/1000), while DELTA delta 13C is -0.02 +/- 2.12(1/1000). A model of 15N distribution in phytoplankton demonstrates that interspecies DELTA delta 15N variability is attributable to nitrogen partitioning between non-protein biochemicals. Chlorin delta 15N from six Late Quaternary Eastern Mediterranean sapropels averaged -5.01 +/- 0.38(1/1000), similar to chlorophyll delta 15N from surface waters of -6.38 +/- 1.80(1/1000). Chlorin-derived algal delta 15N in sapropels averaged 0.15(1/1000), similar to the whole-sediment average of -0.08 +/- 0.53(1/1000) and the delta 15N of contemporary deep-water nitrate, -0.05(1/1000). Non-sapropel sediment delta 15N averaged 5.3 +/- 1.0(1/1000), similar to core top delta 15N of 4.3(1/1000). We can thus conclude that surface waters were oligotrophic during sapropel events, as today, but that bottom waters were anoxic. Anoxia prevented the diagenetic enrichment of 15N observed in non-sapropel sediments and contributed to organic matter preservation. Finally, the very low delta 15N of chlorins and nitrate in the contemporary Eastern Mediterranean requires that nitrogen fixation is a significant new nitrogen source.