Chlorophyll diagenesis in the water column and sediments of the Black Sea

Abstract

This thesis examines the degradation pathways of chlorophyll in the Black Sea water column and sediments. Measurements are made of total chlorophyll in sediment traps from two locations and depths in the water column, and at two locations in surface sediments. Individual chlorophyll degradation products are also identified. This data is used to construct a mass balance of chlorophyll production and sedimentation showing the major pathways for chlorophyll loss and the ultimate sedimentary sinks. The distribution of chlorophyll degradation products is also analyzed down core and related to environmental changes in the Black Sea Several new sinks for chlorophyll degradation products are identified. Steryl esters of pyropheophorbide-a are identified in sediment trap and sediment samples. It is thought that these compounds are formed during grazing. In sediment traps it is found that the distribution of the sterols esterified to pyropheophorbide-a change with season and that the sterols esterified are related to the distribution of sterols synthesized by the phytoplankton living in the photic zone at the time of production. Analysis of pyropheophorbide-a steryl esters in sediments shows the distribution of sterols to be quantitatively and qualitatively more similar to the distribution of free sterols in sediment traps than in sediments. The esterification of the sterols to pyropheophorbide-a apparently prevents the preferential removal of 4-desmethylsterols relative to 4-methylsterols during degradation of the sterol esters. Chlorophyll degradation products which are incorporated into high molecular weight material and material which is only accessible with strong acid are also identified. The chlorophyll degradation products incorporated into these structures represent only a few percent of the total structure. In the high molecular weight material, only phorbins derived from chlorophyll-a are identifies, where as in the acid extractable material, porphyrins are also identified. In surface sediments, the acid extractable chlorophyll degradations products and the solvent extractable macromolecular chlorophyll degradation products each comprise approximately 30% of total sedimentary chlorophyll degradation products. The acid extractable chlorophyll degradation products are identified in sediment trap samples, and evidence is presented for the occurrence of the solvent extractable macromolecular chlorophyll degradation products in sediment trap samples. Using data from sediment traps, sediments, and the literature, a mass balance of chlorophyll flux, degradation, and accumulation in the Black Sea is presented. In the photic zone, chlorophyll degradation products are either destroyed by photo-oxidation and grazing, or they are transported into the anoxic water column in large, rapidly sinking particles. Once the chlorophyll degradation products have reached the anoxic water column, they survive to be deposited in the underlying sediments. As a comparison, 25 times more total organic carbon reaches the anoxic water column than does total phorbin, but 75% of total organic carbon which reaches the anoxic water column is degraded, either in the anoxic water column or in the very surface sediments. Though a larger percentage of total organic carbon passes out of the photic zone, the phorbin macrocycle appears to be more stable under anoxic conditions than is total organic carbon. The chlorophyll which can be detected below the chemocline of the Black Sea in the form of chlorophyll degradation products will survive to be deposited in surface sediments. Once in sediments, chlorophyll degradation products are found in four different reservoirs: phorbin steryl esters, free phorbins, solvent extractable macromolecular chlorophyll degradation products, and acid extractable chlorophyll degradation products. Evidence for the occurrence of porphyrins in surface Black Sea sediments is also presented. The distribution of chlorophyll degradation products in Unit I Black Sea sediments varies greatly with sediment depth. The concentration of total phorbin generally increases with increasing burial depth, but the concentrations of the individual chlorophyll degradation products vary in a manner which is both dissimilar to total phorbin and to each other. No parent/daughter relationships for the chlorophyll degradation products are indicated by the data. The distribution of sterols esterified to pyropheophorbide-a changes with sediment depth with the largest qualitative changes occurring in strata where the total phorbin concentration shows the largest quantitative changes. It is suggested that the variations seen in the esterified sterols are related to changes in the phytoplankton community over time. From the presented data, it is also suggested that total phorbin concentration, normalized to total organic carbon, in Black Sea Unit I sediments is related to paleoprimary production. Several conclusions are drawn from the work presented in this thesis. There is approximately 3 times more chlorophyll-derived phorbin in Black Sea sediments than can be accounted for when considering only individual pheopigments, and therefore the sedimentary degradation of chlorophyll is much more complex than previously thought. In the anoxic sediments of the Black Sea, the total phorbin distribution can be accounted for with organically extractable high molecular weight degradation products, pyropheophorbide steryl esters, pheopigments, and acid extractable chlorophyll degradation products. The sterol distribution in the pyropheophorbide steryl esters may preserve the sterol distribution in surface waters as synthesized by the phytoplankton, and pyropheophorbide steryl esters are preserved in sediments over the long term.