Nitrogen isotopes in chlorophyll and the origin of Eastern Mediterranean sapropels

Abstract

The goals of this thesis were: (1) to establish methods for the determination of nitrogen and carbon isotope ratios in marine particulate and sedimentary chlorophyll derivatives; (2) to establish chlorophyll ()15N and ol3C as proxies for the nitrogen and carbon isotopic composition of marine phytoplankton; and (3) to use chlorophyll nitrogen isotopic ratios to understand the origin of Late Quaternary Eastern Mediterranean sapropels.Techniques are presented for the determination of chlorin nitrogen and carbon isotopic ratios in marine particles and sediments with a precision greater than 0.15 per mil for both isotopes. The procedure can be performed in about 4 hours for particulate and 8 hours for sediment samples, and relies on multiple chromatographic purifications. About 20 g of a moderately organic-rich sediment are required.A technique is also presented for the determination of chlorin nitrogen and carbon isotopic ratios by isotope-ratio monitoring gas chromatography-mass spectrometry (irmGC-MS) by synthesizing bis-(tert.-butyldimethylsiloxy)Si(IV) chlorin derivatives. However, yields for the 4-step synthesis were only about 5-6% and there was a net isotopic depletion of 1.2 (± 0.3) per mil in the derivative, relative to the starting material.These techniques are then used to show that the nitrogen isotopic difference between chlorophyll and whole cells in six species of marine phytoplankton is 5.16 ± 2.40 per mil. For carbon, the isotopic difference between chlorophyll and whole cells in five species of marine phytoplankton is -0.02 ± 2.12 per mil. A model of the distribution of 15N in phytoplankton is constructed and it is demonstrated that the interspecies variability observed for the nitrogen isotopic difference between chlorophyll and whole cells can be attributed to differences in iii the partitioning of cellular nitrogen between non-protein biochemicals. In the field, where mixed assemblages of phytoplankton prevail, the isotopic difference beween chlorophyll and whole cells is expected to tend toward the average value of 5.16 per mil.Finally, the average nitrogen isotopic composition of chlorins from six Late Quaternary Eastern Mediterranean sapropels ( -5.01 + 0.38 per mil) was found to be very similar to the o15N of chlorophyll from the modem deep chlorophyll maximum ( -6.38 ± 1.80 per mil) in the Eastern Mediterranean. In addition, sapropel photoautotrophic material, calculated from the chlorin o15N, had the same isotopic composition (0.15 per mil) as both bulk sapropel sediments (-0.08 ± 0.53 per mil) and deep water nitrate (-0.05 per mil). These data suggest (a) that bottom waters were anoxic, (b) that organic matter burial efficiency was enhanced, and (c) that oligotrophic conditions similar to today persisted, in the Eastern Mediterranean during sapropel deposition. These results contradict earlier interpretations of Late Quaternary bulk sedimentary o15N in the Eastern Mediterranean. The latter concluded that the pattern of high o15N values in intercalated marl oozes and low values in ...

Distribution and sources of cyclic pheophorbides in the marine environment

“…Similar results were obtained by Falk et al (1975), who observed that CPP516 is extremely unstable in organic solvents in the presence of air. Sachs (1997) and Ocampo et al (1999) also observed that CPP516 degrades rapidly under a variety of conditions. Once care is taken to exclude oxygen from extracts, this degradation can be minimized.…”

Paleoenvironmental implications of taxonomic variation among δ15N values of chloropigments

“…1). These studies have focused mostly on eukaryotic taxa that are dominant in modern marine systems, as well as on terrestrial plants (Sachs, 1997;Kennicutt et al, 1992). measured an average value of e por = 5.1 ± 1.1& (i.e., chlorophyll was $5& depleted in 15 N relative to biomass) in seven species of eukaryotic phytoplankton and one cyanobacterium (Synechococcus sp., order Chroococcales).…”

Lack of isotopic fractionation of 15N of organic matter during long-term diagenesis in marine sediments, ODP Leg 202, Sites 1234 and 1235

“…Several field observations confirmed changes in δ 15 N during organic matter diagenesis. Sigman et al (1999), working with the sediments from the Southern Ocean, and Sachs (1997) and Sachs and Rapeta (1999), in the sapropels from Mediterranean Sea, found a 2‰-5‰ positive shift in δ 15 N of bulk sediments relative to unaltered organic matter. On the other hand, Altabet et al (1999a) and Pride et al (1999) found no evidence of diagenetic alterations in δ 15 N of preserved organic matter in California borderland basins and in the Gulf of California.…”