3 resultados para EXCESS LOSS

em Publishing Network for Geoscientific


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In order to investigate production pathways of methyl iodide and controls on emissions from the surface ocean, a set of repeated in-vitro incubation experiments were performed over an annual cycle in the context of a time-series of in-situ measurements in Kiel Fjord (54.3 N, 10.1E). The incubation experiments revealed a diurnal variation of methyl iodide in samples exposed to natural light, with maxima during day time and losses during night hours. The amplitude of the daily accumulation varied seasonally and was not affected by filtration (0.2µm), consistent with a photochemical pathway for CH3I production. The methyl iodide loss rate during night time correlated with the concentration accumulated during daytime. Daily (24 hour) net production (Pnet) was similar in magnitude between in vitro and in situ mass balances. However, the estimated gross production (Pgross) of methyl iodide ranged from -0.07 to 2.24 pmol/day and were 5 times higher in summer than Pnet calculated from the in-situ study [Shi et al., 2014]. The large excess of Pgross over Pnet revealed by the in-vitro (incubation) experiments in summer is a consequence of large losses of CH3I by as-yet uncharacterized processes (e.g. biological degradation or chemical pathways other than Cl- substitution).

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In the years 2000 and 2001 we measured methane concentrations exceeding up to two orders of magnitude the equilibrium with the atmosphere in the water column on the SW-Spitsbergen continental shelf. This methane anomaly extended from its centre on the shelf westwards over the upper slope and eastwards well into the inner basins of the two southernmost Spitsbergen fjords, the Hornsundfjord and the van Mijenfjord. Methane concentrations and stable carbon isotopic ratios varied between 2 and 240 nM, and between -53 per mill and -20 per mill VPDB, respectively. Methane in high concentrations was depleted in 13C whereas in low concentrations d13CCH4 values were highly variable. On the continental shelf we found that methane discharged from seeps on top of sandy and gravelly banks is isotopically heavier than methane escaping from troughs filled with silty and clayey sediments. These distinct isotopic signatures suggest that methane is gently released from several inter-granular seepages or micro-seepages widely spread over the shelf. A potential migration path for thermogenic or hydrate methane may be the Hornsund Fracture Zone, a south-north running reactivated fault system created by stretching of the continental crust. After discharge into the water column, local water currents fed by Atlantic water, coastal water, and freshwater outflows from the fjords further determine pathways and fate of the methane. We used d18Owater and 222Rn data to trace origin and advection of the local water masses and water mixing processes. Methane spreads predominantly along pycnoclines and by vertical mixing. During transport methane is influenced simultaneously by oxidation and dilution, as well as loss into the atmosphere. Together these processes cause the spatial variability of the anomaly and heterogeneity in d13CCH4 in this polar shelf environment.

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Excess Thorium-230 (230Thxs) as a constant flux tracer is an essential tool for paleoceanographic studies, but its limitations for flux normalization are still a matter of debate. In regions of rapid sediment accumulation, it has been an open question if 230Thxs-normalized fluxes are biased by particle sorting effects during sediment redistribution. In order to study the sorting effect of sediment transport on 230Thxs, we analyzed the specific activity of 230Thxs in different particle size classes of carbonate-rich sediments from the South East Atlantic, and of opal-rich sediments from the Atlantic sector of the Southern Ocean. At both sites, we compare the 230Thxs distribution in neighboring high vs. low accumulation settings. Two grain-size fractionation methods are explored. We find that the 230Thxs distribution is strongly grain size dependent, and 50-90% of the total 230Thxs inventory is concentrated in fine material smaller than 10 µm, which is preferentially deposited at the high accumulation sites. This leads to an overestimation of the focusing factor Psi, and consequently to an underestimation of the vertical flux rate at such sites. The distribution of authigenic uranium indicates that fine organic-rich material has also been re-deposited from lateral sources. If the particle sorting effect is considered in the flux calculations, it reduces the estimated extent of sediment focusing. In order to assess the maximum effect of particle sorting on Psi, we present an extreme scenario, in which we assume a lateral sediment supply of only fine material (< 10 µm). In this case, the focusing factor of the opal-rich core would be reduced from Psi = 5.9 to Psi = 3.2. In a more likely scenario, allowing silt-sized material to be transported, Psi is reduced from 5.9 to 5.0 if particle sorting is taken into consideration. The bias introduced by particle sorting is most important for strongly focused sediments. Comparing 230Thxs-normalized mass fluxes biased by sorting effects with uncorrected mass fluxes, we suggest that 230Thxs-normalization is still a valid tool to correct for lateral sediment redistribution. However, differences in focusing factors between core locations have to be evaluated carefully, taking the grain size distributions into consideration.