ALBERT

Description: Stable isotope compositions can potentially be used to trace atmospheric Cd inputs to the surface ocean and anthropogenic Cd emissions to the atmosphere. Both of these applications may provide valuable insights into the effects of anthropogenic activities on the cycling of Cd in the environment. However, a lack of constraints for the Cd isotope compositions of atmospheric aerosols is currently hindering such studies. Here, we present stable Cd isotope data for aerosols collected over the Tropical Atlantic Ocean. The samples feature variable proportions of mineral dust-derived and anthropogenic Cd, yet exhibit similar isotope compositions, thus negating the distinction of these Cd sources using isotopic signatures in this region. Isotopic variability between these two atmospheric Cd sources may be identified in other areas, and thus warrants further investigation. Regardless, these data provide important initial constraints on the isotope composition of atmospheric Cd inputs to the ocean.

Description: Current climate models disagree on how much carbon dioxide land ecosystems take up for photosynthesis. Tracking the stronger carbonyl sulfide signal could help.

Description: Back-arc spreading centers (BASCs) form a distinct class of ocean spreading ridges distinguished by steep along-axis gradients in spreading rate and by additional magma supplied through subduction. These characteristics can affect the population and distribution of hydrothermal activity on BASCs compared to mid-ocean ridges (MORs). To investigate this hypothesis, we comprehensively explored 600 km of the southern half of the Mariana BASC. We used water column mapping and seafloor imaging to identify 19 active vent sites, an increase of 13 over the current listing in the InterRidge Database (IRDB), on the bathymetric highs of 7 of the 11 segments. We identified both high and low (i.e., characterized by a weak or negligible particle plume) temperature discharge occurring on segment types spanning dominantly magmatic to dominantly tectonic. Active sites are concentrated on the two southernmost segments, where distance to the adjacent arc is shortest (〈40 km), spreading rate is highest (〉48 mm/yr), and tectonic extension is pervasive. Re-examination of hydrothermal data from other BASCs supports the generalization that hydrothermal site density increases on segments 〈90 km from an adjacent arc. Although exploration quality varies greatly among BASCs, present data suggest that, for a given spreading rate, the mean spatial density of hydrothermal activity varies little between MORs and BASCs. The present global database, however, may be misleading. On both BASCs and MORs, the spatial density of hydrothermal sites mapped by high-quality water-column surveys is 2–7 times greater than predicted by the existing IRDB trend of site density versus spreading rate.

Description: The supply and bioavailability of iron (Fe) controls primary productivity and N2-fixation in large parts of the global ocean. An important, yet poorly quantified, source to the ocean is particulate Fe (pFe). Here we present the first combined dataset of particulate, labile-particulate (L-pFe) and dissolved Fe (dFe) from the (sub)-tropical North Atlantic. We show a strong relationship between L-pFe and dFe, indicating a dynamic equilibrium between these two phases whereby particles ‘buffer’ dFe and maintain the elevated concentrations observed. Moreover, L-pFe can increase the overall ‘available’ (L-pFe + dFe) Fe pool by up to 55%. The lateral shelf flux of this available Fe was similar in magnitude to observed soluble aerosol-Fe deposition, a comparison that has not been previously considered. These findings demonstrate that L-pFe is integral to Fe cycling and hence plays a role in regulating carbon cycling, warranting its’ inclusion in Fe budgets and biogeochemical models.

Description: We report a new synthesis of best estimates of the inputs of fixed nitrogen to the world ocean via atmospheric deposition and compare this to fluvial inputs and dinitrogen fixation. We evaluate the scale of human perturbation of these fluxes. Fluvial inputs dominate inputs to the continental shelf, and we estimate that about 75% of this fluvial nitrogen escapes from the shelf to the open ocean. Biological dinitrogen fixation is the main external source of nitrogen to the open ocean, i.e., beyond the continental shelf. Atmospheric deposition is the primary mechanism by which land-based nitrogen inputs, and hence human perturbations of the nitrogen cycle, reach the open ocean. We estimate that anthropogenic inputs are currently leading to an increase in overall ocean carbon sequestration of ~0.4% (equivalent to an uptake of 0.15 Pg C yr−1 and less than the Duce et al. (2008) estimate). The resulting reduction in climate change forcing from this ocean CO2 uptake is offset to a small extent by an increase in ocean N2O emissions. We identify four important feedbacks in the ocean atmosphere nitrogen system that need to be better quantified to improve our understanding of the perturbation of ocean biogeochemistry by atmospheric nitrogen inputs. These feedbacks are recycling of (1) ammonia and (2) organic nitrogen from the ocean to the atmosphere and back, (3) the suppression of nitrogen fixation by increased nitrogen concentrations in surface waters from atmospheric deposition, and (4) increased loss of nitrogen from the ocean by denitrification due to increased productivity stimulated by atmospheric inputs.

Description: The relationships between tectonic processes, magmatism, and hydrothermal venting along ∼600 km of the slow-spreading Mariana back-arc between 12.7°N and 18.3°N reveal a number of similarities and differences compared to slow-spreading mid-ocean ridges. Analysis of the volcanic geomorphology and structure highlights the complexity of the back-arc spreading center. Here, ridge segmentation is controlled by large-scale basement structures that appear to predate back-arc rifting. These structures also control the orientation of the chains of cross-arc volcanoes that characterize this region. Segment-scale faulting is oriented perpendicular to the spreading direction, allowing precise spreading directions to be determined. Four morphologically distinct segment types are identified: dominantly magmatic segments (Type I); magmatic segments currently undergoing tectonic extension (Type II); dominantly tectonic segments (Type III); and tectonic segments currently undergoing magmatic extension (Type IV). Variations in axial morphology (including eruption styles, neovolcanic eruption volumes, and faulting) reflect magma supply, which is locally enhanced by cross-arc volcanism associated with N-S compression along the 16.5°N and 17.0°N segments. In contrast, cross-arc seismicity is associated with N-S extension and increased faulting along the 14.5°N segment, with structures that are interpreted to be oceanic core complexes—the first with high-resolution bathymetry described in an active back-arc basin. Hydrothermal venting associated with recent magmatism has been discovered along all segment types.

Description: The process of plate accretion at mid-ocean ridges, once thought to occur in a relatively simple, magmatic system, has been shown in recent years to possess unexpected layers of complexity [e.g., Cannat, 1996; Escartín and Lin, 1998; Jokat et al., 2003; Michael et al., 2003]. Particularly at lower spreading rates, the magma supply to some or all of the ridge decreases, with the plate spreading motion being taken up instead on faults. The balance between these magmatic and tectonic processes governs such features as the topography, seismic activity location of hydrothermal vents, and degree of chemical exchange between crust and ocean at spreading axes. It therefore has important implications for the hydrothermal marine biosphere and global chemical budgets.

Description: H2O2 was measured in the upper water column (0–200 m) along a west-east transect through the Equatorial Atlantic as part of the German SOLAS (Surface Ocean Lower Atmosphere) cruise Meteor 55 (M55). Vertical profiles of H2O2 showed characteristic exponential decay consistent with light profiles and rainwater inputs. Integrated (0–100 m) water column H2O2 inventories ranged from 1.1–8.9 mmol m−2 with the highest values in the Amazon Plume. H2O2 inventories were also higher at the Equatorial Upwelling and after heavy rain showers in the region of the Inter Tropical Convergence Zone (ITCZ). Analysis of rain water samples collected during the cruise gave a volume weighted mean of 10.8 μmol L−1 (range 1.5–22.3 μmol L−1). This work highlights the importance of rainwater as a major source for H2O2 in the surface waters under the ITCZ.

Description: The global tropospheric budget of gaseous and particulate non-methane organic matter (OM) is re-examined to provide a holistic view of the role that OM plays in transporting the essential nutrients nitrogen and phosphorus to the ocean. A global 3-dimensional chemistry-transport model was used to construct the first global picture of atmospheric transport and deposition of the organic nitrogen (ON) and organic phosphorus (OP) that are associated with OM, focusing on the soluble fractions of these nutrients. Model simulations agree with observations within an order of magnitude. Depending on location, the observed water soluble ON fraction ranges from similar to 3% to 90% (median of similar to 35%) of total soluble N in rainwater; soluble OP ranges from similar to 20-83% (median of similar to 35%) of total soluble phosphorus. The simulations suggest that the global ON cycle has a strong anthropogenic component with similar to 45% of the overall atmospheric source (primary and secondary) associated with anthropogenic activities. In contrast, only 10% of atmospheric OP is emitted from human activities. The model-derived present-day soluble ON and OP deposition to the global ocean is estimated to be similar to 16 Tg-N/yr and similar to 0.35 Tg-P/yr respectively with an order of magnitude uncertainty. Of these amounts similar to 40% and similar to 6%, respectively, are associated with anthropogenic activities, and 33% and 90% are recycled oceanic materials. Therefore, anthropogenic emissions are having a greater impact on the ON cycle than the OP cycle; consequently increasing emissions may increase P-limitation in the oligotrophic regions of the world's ocean that rely on atmospheric deposition as an important nutrient source.