Characterization of diatom-derived lipids and chlorophyll within Holocene laminites, Saanich Inlet, British Columbia, using conventional and laser scanning fluorescence microscopy

Abstract

Conventional fluorescence microscopy and visible light region fluorescence microspectrometry have been combined with laser scanning fluorescence microscopy (LSFM) to examine and characterize soluble sedimentary organic matter (SOM) in Holocene diatomaceous laminites from ODP core 1034, Saanich Inlet, British Columbia, Canada. Microscopic SOM in the laminites is dominated by two components: red-fluorescing chlorophyllinite, and yellow-fluorescing, diatom-derived oils. Both are commonly preserved within the interior of siliceous diatom auxospores and appear as an abundant and intense stain on mineral matter throughout the laminites. The mineral stain is classified as matrix chlorophyllinite and matrix oil, respectively. Accessory insoluble SOM consists of dinoflagellate, prasinophyte and coccoidal alginites, and rare woody huminite, funginite, and sporinite. Visible light region fluorescence microspectrometry of chlorophyllinite reveals a predominance at λ max 670–675 nm, which is consistent with a chlorophyll a source based on comparison with reference samples of chlorophyll a and b. The consistency in λmax and spectral character of chlorophyllinite with increasing depth in Saanich Inlet correlates with a high degree of chlorophyll preservation. Its distribution in the laminites is readily imaged by LSFM using either UV (363 nm) or blue (488 nm) laser excitation and >665 nm emission. Yellow-fluorescing diatom oils have λmax ranging from 485 to 520 nm, suggesting that they may comprise up to 70% saturates. The distribution of entrapped yellow-fluorescing diatom oils as well as matrix oils is also readily imaged with LSFM using a combination of UV laser excitation (363 nm) and 510 ±40 nm emission. Laminae in the Saanich Inlet samples which are highly enriched in both diatom oils and ‘reproductive’ auxospore cells are interpreted as recording intense episodic spring diatom blooms. The observations reported here document microscopically, in situ, possibly pristine preservation of diatom oils (including possible fatty acids) at appreciable depths within Holocene organic-rich laminites. Their recognition in recent and older strata is important, for they serve as potential biomarkers for assessing long term geo-environmental changes such as water temperature. The periodic presence of anomalously high amounts of diatom-derived biological oils in laminae within the recent sediments may mark times of unusually high nutrient levels and paleoproductivity.