Preservation of chlorophyll-derived pigments in sedimentary organic matter

Abstract

The occurrence in sediments of chlorophyll-derived tetrapyrroles provides evidence for primary photosynthetic communities in palaeo water columns. In ancient sedimentary rocks the components occur mainly as nickel or vanadyl complexes of alkyl porphyrins, and to a lesser extent carboxylic acids. Although extensive loss of functional groups has occurred, the structures of a number of the components reveal that chlorophyll carbon skeletons can survive intact or virtually intact, although in other cases the skeletons have been modified by rearrangement. The structures of a few components indicate an origin from the chlorophyllsc, and thus an algal input, whereas an origin from photosynthetic bacterial chlorophylls is apparent from the carbon skeletons of other components. Such studies, taken with other distributional features (e.g. ratio of nickel to vanadyl components, extent of tetrapyrrole preservation) can provide information about the productivity and preservation of organic matter and presence of water column anoxia at the time of deposition. Studies of the tetrapyrrole components in one recent and two highly immature older sediments have provided further evidence for the pathway of defunctionalization, through the identification of the minimum number of components necessary to link a chlorophyll, such as chlorophylla, to the major alkyl porphyrin in sedimentary organic matter. Additionally, evidence for an early and unexpected transformation pathway comes from the recent identification in sediments of chlorin acids esterified with sterols.