Abstract. The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world's ocean. In contrast to those of the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We investigate the water mass distributions based on temperature, salinity and oxygen data, and the concentrations of water column nutrients and the stable isotope composition of nitrate, using water samples collected between ∼30∘ S and the Equator during two expeditions: MSM 59/2 in 2016 and SO 259 in 2017. Our results are the first from this oceanic region and provide new information on nitrogen sources and transformation processes. We identify the thick layer of nutrient-depleted surface waters of the oligotrophic IOSG with nitrate (NO3-) and phosphate (PO43-) concentrations of < 3 and < 0.3 µmol kg−1, respectively (< 300 m; σ < 26.4 kg−1 m−3). Increased nutrient concentrations towards the Equator represent the northern limb of the gyre, which is characterized by typical strong horizontal gradients of the outcropping nutriclines. The influx of the Subantarctic Mode Water (SAMW) from the Southern Ocean injects oxygen-saturated waters with preformed nutrients, indicated by the increased N and O isotope composition of nitrate (δ15N > 7 ‰; δ18O > 4 ‰) at 400–500 m (26.6–26.7 kg−1 m−3), into the subtropical thermocline. These values reflect partial N assimilation in the Southern Ocean. Moreover, in the northern study area, a residue of nitrate affected by denitrification in the Arabian Sea is imported into intermediate and deep water masses (> 27.0 kg−1 m−3) of the gyre, indicated by an N deficit (N* ∼-1 to −4 µmol kg−1) and by elevated isotopic ratios of nitrate (δ15N > 7 ‰; δ18O > 3 ‰). Remineralization of partially assimilated organic matter, produced in the subantarctic, leads to a decoupling of N and O isotopes in nitrate and results in a relatively low Δ(15–18) value of < 3 ‰ within the SAMW. In contrast, remineralization of 15N-enriched organic matter from the Arabian Sea indicates higher Δ(15–18) values of > 4 ‰ within the Red Sea–Persian Gulf Intermediate Water (RSPGIW). Thus, the subtropical southern Indian Ocean is supplied by preformed nitrate from the lateral influx of water masses from regions exhibiting distinctly different N-cycle processes documented in the dual isotope composition of nitrate. Additionally, a significant contribution of N2 fixation between 20.36 and 23.91∘ S is inferred from reduced δ15N–NO3- values towards surface waters (upward decrease of δ15N ∼2.4 ‰), N* values of > 2 µmol kg−1 and a relatively low Δ(15–18) value of < 3 ‰. A mass and isotope budget implies that at least 32 %–34 % of the nitrate in the upper ocean between 20.36 and 23.91∘ S is provided from newly fixed nitrogen, whereas N2 fixation appears to be limited by iron or temperature south of 26∘ S.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; Vast subtropical gyres are important areas for the exchange of carbon between atmosphere and ocean in spite of low nutrient concentrations, and supposedly for the influx of reactive nitrogen to the ocean by dinitrogen fixation. To identify sources and transformation processes in the nitrogen cycle of the southern Indian Ocean subtropical gyre, we investigated concentrations of water column nutrients and stable isotope composition of nitrate of samples from two expeditions in 2016 (MSM 59) and 2017 (SO 259) in the subtropical gyre between ~&amp;#8201;30&amp;#8201;&amp;#176;S and the equator. Low nitrate and phosphate concentrations mark the thick mixed layer of the oligotrophic gyre with values of &lt;&amp;#8201;5.9&amp;#8201;&amp;#181;M&amp;#8201;NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; and &lt;&amp;#8201;0.5&amp;#8201;&amp;#181;M PO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;3&amp;#8722;&lt;/sup&gt; (&lt;&amp;#8201;310&amp;#8201;m; &amp;#963;&amp;#8201;&lt;&amp;#8201;26.4&amp;#8201;kg/m&amp;#179;). Increased nutrient concentrations towards the equator represent the northern end of the gyre, characterized by typical strong horizontal gradients of the outcropping nutriclines. Measurements of stable isotopes of nitrate (&amp;#948;&lt;sup&gt;15&lt;/sup&gt;N and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O) indicate isotopic maxima of &amp;#948;&lt;sup&gt;15&lt;/sup&gt;N (&gt;&amp;#8201;7&amp;#8201;&amp;#8240;) and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O (&gt;&amp;#8201;4&amp;#8201;&amp;#8240;) centred at 400&amp;#8211;500&amp;#8201;m, representing the preformed nitrate exported from the Southern Ocean with mode water and induced by partial N-assimilation there. Additionally, a residue of nitrate affected by denitrification in the Arabian Sea is imported into the sub-thermocline of the gyre, indicated by a strong N deficit (N*&amp;#8201;&lt;&amp;#8201;&amp;#8722;1&amp;#8201;&amp;#181;M) within the northern study area, accompanied by elevated isotopic ratios of nitrate (&amp;#948;&lt;sup&gt;15&lt;/sup&gt;N&amp;#8201;&gt;&amp;#8201;7&amp;#8201;&amp;#8240;; &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&amp;#8201;&gt;&amp;#8201;3&amp;#8201;&amp;#8240;). The subtropical South Indian Ocean is thus supplied by nitrate from lateral influx of water masses that have similar isotopic character, but antagonistic origin (preformed versus regenerated). A significant contribution of N&lt;sub&gt;2&lt;/sub&gt;-fixation within the Indian Ocean subtropical gyre (17&amp;#176;&amp;#8201;S&amp;#8211;25&amp;#176;&amp;#8201;S) is promoted by low nitrate to phosphate ratios in the surface layer, where approximately one-third of the nitrate in the upper ocean is derived by newly fixed N.&lt;/p&gt;

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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.

Abstract. Amino acids (AA) 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 as well as 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 bulk δ15N ratios. This new indicator furthermore is 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.