Dissolved Organic Matter (DOM) in the Arctic Ocean: optical character-ization and biogeochemistry
PhD project (2013-) by Rafael Gonçalves-Araujo, CAPES (Brazil) stipend
The state of art regarding key processes controlling the carbon cycle and the transformation of carbon in ocean margins falls short a complete understanding, mostly because the dissolved organic matter (DOM) may originate from a range of sources. Some of this DOM is transported into the oceans through the rivers and its composition is influenced by the geology, land-use and hydrology. Some is produced in situ by microbial activity, which may be an independent source of organic matter or a recycling mechanism for that part which is transported to the system. In this context, rivers exert an important control on carbon dynamics carrying considerable amounts of terrigeneous dissolved organic matter (tDOM) to the coastal ocean. Although considerable progress has been made to understand the processes governing the dynamics of DOM in coastal and pelagic environments, the extremely heterogeneity of river-influenced ocean margins represents a major challenge for providing quantitative estimates of these processes. Furthermore, a more comprehensive understanding of the chemical composition of dissolved organic matter within the rivers and oceans would allow to better assess the export, transformation and fate of tDOM (the largest fraction of dissolved carbon in the Arctic) in the ocean.
Colored or chromophoric dissolved organic matter (CDOM) is the fraction of the DOM that absorbs light, which makes it important to the marine environment by absorbing light, mainly in the UV wavelengths, reducing the exposition of marine organisms to that harmful radiation. Since CDOM absorbs light in theUV and visible wavelengths, studying the optical properties of CDOM is also of great value for improving satellite algorithms and remote sensing products, which are often used as a proxy to model the carbon cycle in the oceans. This fact makes CDOM an important key on monitoring DOM in a spatial-temporal coverage necessary for a better understanding of the DOM dynamics in the aquatic ecosystems.
Fluorescent DOM (FDOM), which is the part of CDOM able to fluoresce, can be used to trace the supply, mixing and removal of different fractions of DOM. The traditional “peak-picking” method for EEM has identified humic-like (the so called A, C, M peaks) and protein-like fluorescence peaks (B, T peaks) using simple excitation-emission wavelength pairs of the fluorescence peak locations (Coble, 2007). However, with the recent adaptation of the Parallel Factor Analysis (PARAFAC) for analysis of DOM, a more holistic analysis of excitation-emission matrices (EEMs) allows for the differentiation of wider range of underlying DOM components [Stedmon and Bro, 2008]. By using the EEMs/PARAFAC technique, the distribution and dynamics of fluorescent DOM have been studied in a wide range of environments varying from lakes, estuaries, coastal and shelf to pelagic.
The Arctic Ocean receives considerable input of terrigenous carbon mobilized from high latitude carbon-rich soils and peatlands (Opsahl et al., 1999; Benner et al., 2004). This terrigenous material is supplied by Arctic rivers, which account for more than 10% of the total riverine and terrestrial organic carbon into the global ocean waters (Opshal et al., 1999; Benner et al., 2004). Most of the Arctic outflow and, as a consequence, the total carbon mobilized into the Arctic Ocean, leave the Arctic basin via the Fram Srait, following the East Greenland Current (EGC) and through the Davis Strait within the Baffin Current (BC) (e.g. Dodd et al., 2012). Recent studies have shown that the CDOM concentration (determined based on the absorption at 375nm) in the western part of the Fram Strait is strongly correlated to meteoric water (i.e. the fraction of fresh water signal, from continent) (Stedmon et al., 2011; Granskog et al., 2012; Stedmon et al., 2015), which presence and quantification are determined based on stable isotopic analysis (Dodd et al., 2012). Studies concerning the DOM characterization based on PARAFAC analysis have shown a strong humic-like signal, which was the dominant signal found in the six largest Arctic Rivers (Lena, Ob, Yenisei, Kolyma, Yukon and Mackenzie) (Walker et al., 2013) and also in shelf waters of the Laptev Sea under influence of the Lena River plume (Gonçalves-Araujo et al., in review). This dominant FDOM humic-like signal seems to be the dominant signal in the Arctic DOM-pool. Since CDOM and FDOM are strongly correlated in the Arctic Ocean and FDOM provides more qualitative information of DOM (e.g. Walker et al., 2013; Gonçalves-Araujo et al., in review), it is expected that the riverine humic-like signal detected by FDOM could be a reliable proxy to detect the riverine fingerprint (typical from the Arctic waters) and monitor its export to the Atlantic Ocean basin. However, no study addressing this hypothesis has been conducted/published so far.
The main goal of this project is to determine the composition/sources and fate of the dissolved organic matter (DOM), and use it to trace the riverine signal of DOM along the main Arctic outflow, the EGC, as a proxy of the Arctic water masses. In the framework of this purpose, the first task is to retrieve the composition of DOM from its optical properties (absorption and emission/fluorescence spectra) by applying the PARAFAC modeling. A second task is to relate the results obtained with the PARAFAC modeling to the physical/chemical properties of the water masses (e.g. temperature, salinity, potential density, dissolved inorganic nutrient concentrations and oxygen stable isotopes, for tracing water masses formation) and assess the biogeochemistry and fate of DOM in the Arctic Ocean. A practical application of those results would development of retrievals of the water masses fractions using DOM concentration and composition, based on autonomous fluorescence measurements in the ocean and, later on, based on hyperspectral remote sensing. Finally, a third and last task is it to study the optical structure of the water column and the impacts DOM exerts on it through optical profiles obtained with in situ radiometric measurements.
Benner, R., et al. (2004). Export of young terrigenous dissolved organic carbon from rivers to the Arctic Ocean. Geophys. Res. Lett. 31, L05305, doi: 10.1029/2003GL019251.
Coble, P.G. (2007). Marine optical biogeochemistry: the chemistry of ocean color. Chem. Rev. 107, 402–418.
Dodd, P.A. et al. (2012). The freshwater composition of the Fram Strait outflow derived from a decade of tracer measurements. J. Geophys. Res. 117, C11005, doi: 10.1029/2012JC008011.
Fichot, C.G. et al. Pan-Arctic distributions of continental runoff in the Arctic Ocean. Sci. Rep. 3, 1053; DOI:10.1038/srep01053 (2013).
Gonçalves-Araujo R., Stedmon, Heim, Dubinenkov, Kraberg, Moiseev, Bracher (revised). From fresh to marine waters: characterization and fate of dissolved organic matter in the Lena River Delta region, Siberia. Front. Mar. Sci.
Figure 1. August climatology (2002-2009) of CDOM slope (275-295nm; S275-295) using an algorithm applied to MODIS-Aqua. Increasing S275-295 values indicate decrease in tDOM relative contribution. The four largest Arctic rivers are labeled and ranked in order of decreasing discharge: Yenisei (1), Lena (2), Ob (3), and Mackenzie (4). River-influenced margins of the Arctic are labeled: Gulf of Ob (GO), Kara Sea (KS), Laptev Sea (LS), East Siberian Sea (ESS), Chukchi Sea (CS), Beaufort Sea (BS) and Amundsen Gulf (AG). The contour lines represent the 2000-m isobath. (modified from Fichot et al., 2013).