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General Session 2: Marine & estuarine biogeochemistry |
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Pretreatments and data interpretation of dissolved organic matter (DOM) characterized by Excitation Emission Matrix Spectroscopy (EEMs) in subseafloor pore water
GS2-08-S
Shuchai Gan* , MARUM – Center for Marine Environmental Sciences, University of Bremen
Verena Heuer, MARUM – Center for Marine Environmental Sciences, University of Bremen
Frauke Schmidt, MARUM – Center for Marine Environmental Sciences, University of Bremen
Kai-Uwe Hinrichs, MARUM – Center for Marine Environmental Sciences, University of Bremen
Presenter Email: sgan@uni-bremen.de
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| Excitation Emission Matrix Spectroscopy (EEMs, also known as 3-D fluorescence Spectroscopy) has shown its powerful applications in source identification and diagenesis process of dissolved organic matter (DOM) in recent studies of water columns, soils and surface sediment. Compared to these oxic environments, pore water in deep sediment is unique with highly concentrated DOC and suboxic/anaerobic conditions. It is unclear how to pretreat the samples and to what extent the DOM characterization and implications of pore water are different from samples from oxic environments. This study aims to investigate firstly the impacts of pretreatments on the fluorescence spectra and secondly how to interpret the EEMs of sub-seafloor samples. We measured more than 100 samples from one site in the North Sea and three sites across different depths and distances to the coast in the Mediterranean Sea; for reconfirmation, we conducted incubations and used Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) as complementary method. By parallel factor analysis (PARAFAC), 5 peaks were identified, including humic-like peaks (A, C, M) and protein-like peaks (B, T). With regard to pretreatments, results showed impacts of the dilution ratio, ion strength, iron, long exposure to O2 on the fluorescence spectra and indexes, e.g., FI (contribution of terrigenous DOM), BIX (autochthonous DOM), HIX (humification). Micro-liquid sampling (50-300 µL, µg carbon) is enough before pretreatment, which is a distinct advantage for pore water in deep sediments with limited recovery of liquid phase samples. With regard to data implications, results showed humic-like DOM (H) was transformed differently in oxic and anoxic layer during diagenesis of organic matter; FI, BIX and HIX oscillated significantly within few centimeters at certain layers, incubations confirmed these indexes derived from water column and soil should be less applicable in anaerobic sediment due to fluctuations of redox conditions. We propose to use simply the P/H, AC/M (or C/M) and B/T for the subseafloor samples, which imply the proportion of labile DOM, autochthonous transformation of conjugation and size, state of labile DOM, separately. This study suggests the EEMs is an informative method in DOM characterization in anaerobic environment for biogeochemical processes with consistent pretreatments suggested in this study. The rapid variations of EEMs during diagenesis in the deep sediment are different from oxic environments, which could interfere the signal of source identification and humification indicated by FI, BIX and HIX. Thus, care should be taken for the selection and interpretation of appropriate indexes. |
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