Organic matter preservation in turbidites on the Madeira Abyssal Plain

Oxidized intervals of five organic-rich Madeira Abyssal Plain (MAP) turbidites deposited during the Miocene, Pliocene, and Pleistocene all displayed comparable major loss of total organic carbon (TOC) (84 ± 3.1%) accompanied by a negative isotopic (d13C) shift ranging from -0.3 to -2.9 per mil. Major but significantly lower loss of total nitrogen (Ntot, 61 ± 7.1%) also occurred, leading to a decrease in TOC relative to Ntot (C/Ntot) and a +1.3 to 2.7 per mil Ntot isotopic (d15N) shift. Compound specific isotopic measurements on plant wax n-alkanes indicate the terrestrial organic component in the unoxidized deposits is 13C-enriched owing to significant C4 contribution. Selective preservation of terrestrial relative to marine organic carbon could account for the d13C behavior of TOC upon oxidation but only if a 13C-depleted component of the bulk terrestrial signal is selectively preserved in the process. Although the C/Ntot decrease and positive d15N shift seems inconsistent with selective terrestrial organic preservation, results from analysis of a Modern eolian dust sample collected in the vicinity indicate these observations are compatible. Regardless of the specific explanation for these isotopic observations, however, our findings provide evidence that paleoreconstruction of properties such as pCO2 using the d13C of TOC is a goal fraught with uncertainty whether or not the marine sedimentary record considered is 'contaminated' with significant terrestrial input. Nonetheless, despite major and selective loss of both marine and terrestrial components as a consequence of postdepositional oxidation, intensive organic geochemical proxies such as the alkenone unsaturation index, UK'37, appear resistant to change and thereby retain their paleoceanographic promise.

Chemical characterization of dissolved organic compounds from coastal sea surface microlayers (Baltic Sea, Germany)

The physicochemical properties of the sea surface microlayer (SML), i.e. the boundary layer between the air and the sea, and its impact on air-sea exchange processes have been investigated for decades. However, a detailed description about these processes remains incomplete. In order to obtain a better chemical characterization of the SML, in a case study three pairs of SML and corresponding bulk water samples were taken in the southern Baltic Sea. The samples were analyzed for dissolved organic carbon and dissolved total nitrogen, as well as for several organic nitrogen containing compounds and carbohydrates, namely aliphatic amines, dissolved free amino acids, dissolved free monosaccharides, sugar alcohols, and monosaccharide anhydrates. Therefore, reasonable analytical procedures with respect to desalting and enrichment were established. All aliphatic amines and the majority of the investigated amino acids (11 out of 18) were found in the samples with average concentrations between 53 ng/l and 1574 ng/l. The concentrations of carbohydrates were slightly higher, averaging 2900 ng/l. Calculation of the enrichment factor (EF) between the sea surface microlayer and the bulk water showed that dissolved total nitrogen was more enriched (EF: 1.1 and 1.2) in the SML than dissolved organic carbon (EF: 1.0 and 1.1). The nitrogen containing organic compounds were generally found to be enriched in the SML (EF: 1.9-9.2), whereas dissolved carbohydrates were not enriched or even depleted (EF: 0.7-1.2). Although the investigated compounds contributed on average only 0.3% to the dissolved organic carbon and 0.4% to the total dissolved nitrogen fraction, these results underline the importance of single compound analysis to determine SML structure, function, and its potential for a transfer of compounds into the atmosphere.

Simulated Methyl iodide emission and concentration using the ocean biogeochemistry model MPIOM/HAMOCC forced by OMIP data

Production pathways of the prominent volatile organic halogen compound methyl iodide (CH3I) are not fully understood. Based on observations, production of CH3I via photochemical degradation of organic material or via phytoplankton production has been proposed. Additional insights could not be gained from correlations between observed biological and environmental variables or from biogeochemical modeling to identify unambiguously the source of methyl iodide. In this study, we aim to address this question of source mechanisms with a three-dimensional global ocean general circulation model including biogeochemistry (MPIOM-HAMOCC (MPIOM - Max Planck Institute Ocean Model HAMOCC - HAMburg Ocean Carbon Cycle model)) by carrying out a series of sensitivity experiments. The simulated fields are compared with a newly available global data set. Simulated distribution patterns and emissions of CH3I differ largely for the two different production pathways. The evaluation of our model results with observations shows that, on the global scale, observed surface concentrations of CH3I can be best explained by the photochemical production pathway. Our results further emphasize that correlations between CH3I and abiotic or biotic factors do not necessarily provide meaningful insights concerning the source of origin. Overall, we find a net global annual CH3I air-sea flux that ranges between 70 and 260 Gg/yr. On the global scale, the ocean acts as a net source of methyl iodide for the atmosphere, though in some regions in boreal winter, fluxes are of the opposite direction (from the atmosphere to the ocean).

Organic compounds in surface sediments from the Pakistan continental margin

In order to assess whether the oxygen-minimum zone (OMZ) in the Arabian sea has an effect on the preservation and composition of organic matter in surface sediments we investigated samples from three different transects on the Pakistan continental margin across the OMZ. In addition to determining the total amount of organic carbon (TOC), we analyzed the extractable lipids by gas chromatography, combined gas chromatography/mass spectrometry, and compound-specific stable carbon isotope measurements. The extractable lipids are dominated by marine organic matter as indicated by the abundance of lipids typical of marine biota and by the bulk and molecular isotopic composition. Sediments from within the OMZ are enriched in organic carbon and in several extractable lipids (i.e. phytol, n-alcohols, total sterols, n-C35 alkane) relative to stations above and below this zone. Other lipid concentrations, such as those of total n-fatty acids and total n-alkanes fail to show any relation to the OMZ. Only a weak correlation of TOC with mineral surface area was found in sediments deposited within the OMZ. In contrast, sediments from outside the OMZ do not show any relationship between TOC and surface area. Among the extractable lipids, only the n-alkane concentration is highly correlated with surface area in sediments from the Hab and Makran transects. In sediments from outside the OMZ, the phytol and sterol concentrations are also weakly correlated with mineral surface area. The depositional environment of the Indus Fan offers the best conditions for an enhanced preservation of organic matter. The OMZ, together with the undisturbed sedimentation at moderate rates, seems to be mainly responsible for the high TOC values in this area. Overall, the type of organic matter and its lability toward oxic degradation, the mineral surface area, the mineral composition, and possibly the secondary productivity by (sedimentary) bacteria also appear to have an influence on organic matter accumulation and composition.

Geochemical data and radiocarbon ages of organic matter fractions and bacteriohopanepolyol data of sediment core GeoB6518-1 from the Congo River basin

The age of organic material discharged by rivers provides information about its sources and carbon cycling processes within watersheds. While elevated ages in fluvially-transported organic matter are usually explained by erosion of soils and sediments, it is commonly assumed that mainly young organic material is discharged from flat tropical watersheds due to their extensive plant cover and high carbon turnover. Here we present compound-specific radiocarbon data of terrigenous organic fractions from a sedimentary archive offshore the Congo River in conjunction with molecular markers for methane-producing land cover reflecting wetland extent in the watershed. We find that the Congo River has been discharging aged organic matter for several thousand years with increasing ages from the mid- to the Late Holocene. This suggests that aged organic matter in modern samples is concealed by radiocarbon from nuclear weapons testing. By comparison to indicators for past rainfall changes we detect a systematic control of organic matter sequestration and release by continental hydrology mediating temporary carbon storage in wetlands. As aridification also leads to exposure and rapid remineralization of large amounts of previously stored labile organic matter we infer that this process may cause a profound direct climate feedback currently underestimated in carbon cycle assessments.