A molecular perspective on the ageing of marine dissolved organic matter

Abstract

Dissolved organic matter (DOM) was extracted with solid phase extraction (SPE) from 137 water samples from different climate zones and different depths along an Eastern Atlantic Ocean transect. The extracts were analyzed with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with electrospray ionization (ESI). Δ<sup>14</sup>C analyses were performed on subsamples of the SPE-DOM. In addition, the amount of dissolved organic carbon was determined for all water and SPE-DOM samples as well as the yield of amino sugars for selected samples. Linear correlations were observed between the magnitudes of 43 % of the FT-ICR mass peaks and the extract Δ<sup>14</sup>C values. Decreasing SPE-DOM Δ<sup>14</sup>C values went along with a shift in the molecular composition to higher average masses (<i>m/z</i>) and lower hydrogen/carbon (H/C) ratios. The correlation was used to model the SPE-DOM Δ<sup>14</sup>C distribution for all 137 samples. Based on single mass peaks a degradation index was developed to compare the degradation state of marine SPE-DOM samples analyzed with FT-ICR MS. A correlation between Δ<sup>14</sup>C, degradation index, DOC values and amino sugar yield supports that SPE-DOM analyzed with FT-ICR MS reflects trends of bulk DOM. A relative mass peak magnitude ratio was used to compare aged SPE-DOM and fresh SPE-DOM regarding single mass peaks. The magnitude ratios show a continuum of different reactivities for the single compounds. Only few of the compounds present in the FT-ICR mass spectra are expected to be highly degraded in the oldest water masses of the Pacific Ocean. All other compounds should persist partly thermohaline circulation. Prokaryotic (bacterial) production, transformation and accumulation of this very stable DOM occurs probably primarily in the upper ocean. This DOM is an important contribution to very old DOM, showing that production and degradation are dynamic processes

Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean

“…The relative peak magnitude distribution within a sample represents the intrinsic degradation state of the respective sample. The results from Flerus et al (2012) are strong evidence for a continuous (although not necessarily uniformly continuous) distribution of radiocarbon ages in DOM.…”

“…For the concept of a continuous degradation of DOM, Flerus et al (2012) used the intrinsic molecular signatures of 137 SPE-DOM samples from the East Atlantic Ocean analyzed with FT-ICR MS. Each peak from their set of samples for which an elemental composition could be determined (2850 distinct molecular formulae) was analyzed for a statistically significant correlation with the age of the bulk SPE-DOM sample. The relative peak magnitude distribution within a sample represents the intrinsic degradation state of the respective sample.…”

“…The mass spectra were evaluated in the range 200-600 m/z according to Flerus et al (2012). Briefly, molecular formulae were calculated from the m/z values with a home-built algorithm allowing for the elemental combinations 12 C 0-50 S 0-1 with a mass accuracy |Dm| 60.5 ppm.…”

“…A representative subset of FT-ICR MS peaks from 24 samples from the East Atlantic Ocean (Flerus et al, 2012) was used to model the age of bulk SPE-DOM. Peak magnitudes which showed a significantly positive (POS) or negative (NEG) correlation with measured D 14 C values (D 14 C meas ) were subsumed and implemented in a calibration function.…”

“…Recently, Flerus et al (2012) published the largest FT-ICR MS dataset of marine solid-phase extracted DOM (SPE-DOM) comprising 137 samples from the East Atlantic Ocean. Here we present a comprehensive ultra-high resolution dataset for the Atlantic sector of the Southern Ocean (Weddell Sea).…”