Abstract
Abstract. Amino acids (AAs) mainly bound in proteins are major constituents of living biomass and non-living organic material in the oceanic particulate and dissolved organic matter pool. Uptake and cycling by heterotrophic organisms lead to characteristic changes in AA composition so that AA-based biogeochemical indicators are often used to elucidate processes of organic matter cycling and degradation. We analyzed particulate AA in a large sample set collected in various oceanic regions covering sinking and suspended particles in the water column, sediment samples, and dissolved AA from water column and pore water samples. The aim of this study was to test and improve the use of AA-derived biogeochemical indicators as proxies for organic matter sources and degradation and to better understand particle dynamics and interaction between the dissolved and particulate organic matter pools. A principal component analysis (PCA) of all data delineates diverging AA compositions of sinking and suspended particles with increasing water depth. A new sinking particle and sediment degradation indicator (SDI) allows a fine-tuned classification of sinking particles and sediments with respect to the intensity of degradation, which is associated with changes of stable isotopic ratios of nitrogen (δ15N). This new indicator is furthermore sensitive to sedimentary redox conditions and can be used to detect past anoxic early diagenesis. A second indicator emerges from the AA spectra of suspended particulate matter (SPM) in the epipelagic and that of the meso- and bathypelagic ocean and is a residence time indicator (RTI). The characteristic changes in AA patterns from shallow to deep SPM are recapitulated in the AA spectra of the dissolved organic matter (DOM) pool, so that deep SPM is more similar to DOM than to any of the other organic matter pools. This implies that there is equilibration between finely dispersed SPM and DOM in the deep sea, which may be driven by microbial activity combined with annealing and fragmentation of gels. As these processes strongly depend on physico-chemical conditions in the deep ocean, changes in quality and degradability of DOM may strongly affect the relatively large pool of suspended and dissolved AA in the ocean that amounts to 15 Pg amino acid carbon (AAC) and 89 ± 29 Pg AAC, respectively.
Highlights
Amino acids (AAs) are ubiquitous in living organisms and comprise a major share of characterized organic matter in the particulate and dissolved pool in the ocean (Lee, 1988; Wakeham et al, 1984; Zhang et al, 2016; Davis et al, 2009; Lee et al, 2004)
The degradation index (DI), originally derived from sediment samples of different degradation states (Dauwe et al, 1999; Dauwe and Middelburg, 1998), decreases from positive values in fresh plankton and most sinking particles to negative values in sediments as it integrates the products of Asp and Gly multiplied with negative factors and the products of glutamic acid (Glu), Met, Ile, Leu, Tyr, and Phe multiplied with positive factors (Dauwe et al, 1999)
A principal component analysis (PCA) of individual AAs (Mol %, Fig. 6a) of all samples compiled in this study results in two factors which explain 59 % of the total variance within the data set
Summary
Amino acids (AAs) are ubiquitous in living organisms and comprise a major share of characterized organic matter in the particulate and dissolved pool in the ocean (Lee, 1988; Wakeham et al, 1984; Zhang et al, 2016; Davis et al, 2009; Lee et al, 2004). In two studies of AA composition in the Benguela upwelling system and in the Arabian Sea, it was shown that the degradation pathways of SPM and sinking particles differ as their AA compositions diverge with depth (Gaye et al, 2013b; Nagel et al, 2009). These studies suggested that there is only little interaction between suspended and sinking particle pools below the euphotic zone. Whereas information on the composition of sediment trap samples has been compiled in comprehensive studies (Honjo et al, 2008; Wilson et al, 2012; Rixen et al, 2019a, b), similar compilations of the profuse literature on suspended matter are still missing
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