Diatoms are microscopic, single-celled phytoplankton, which secrete walls of silica, called frustules, on an organic template. The frustules of growing diatoms and their organic template record the environmental conditions in which they grow. Once frustules sink and become sediment, paleoceanographers are able to use their chemical and isotopic composition to study the past carbon cycle and climate. Despite great progress, researchers investigating diatom-based proxies have faced numerous setbacks. One such setback has been the inability to radiocarbon date the organic matter in sedimentary diatoms. Without a reliable method for radiocarbon dating diatom-rich deposits, it is not possible to develop reliable sediment core age models in regions where foraminifera are absent, such as many parts of the Southern Ocean and the North Pacific. In addition, even in locations where foraminifera are present, without a diatom dating method, the age and provenance of diatoms remain unknown, resulting in uncertainty surrounding the origin of sedimentary diatoms. Here we present results of a recently developed method for compound-specific radiocarbon dating of organic compounds in diatom frustules. In this method, sediments from diatom-rich locations is first cleaned to remove excess organic matter that is not associated with frustules. The cleaned sediment is then digested in hydrofluoric acid to dissolve siliceous frustules. The organic compounds liberated by HF treatment are then purified using preparative LC-MS. Frustule biomarker compounds targeted for purification include long-chain polyamines, as well as several newly identified compounds. Purified compounds are radiocarbon dated using microscale AMS. We applied this method to several Southern Ocean sediment cores. Our results show that diatoms may be subjected to extensive redistribution before final burial and generally have an older radiocarbon age than co-occurring foraminifera. This new method shows promise for refining sedimentary age models and for improving our understanding of how carbon associated with diatoms is ultimately preserved in sediments.
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