ALBERT

Description: New planktonic stable isotope and ice-rafted debris records from three core sites in the mid-latitude North Atlantic (IODP Site U1313, MD01-2446, MD03-2699) are combined with records of ODP Sites 1056/1058 and 980 to reconstruct hydrographic conditions during the middle Pleistocene spanning Marine Isotope Stages (MIS) 9–14 (300–540 ka). Together the study sites reflect western and eastern basin boundary currents as well as north to south transect sampling of subpolar and transitional water masses. Planktonic δ18O records indicate that during peak interglacial MIS 9 and 11 hydrographic conditions were similar among all the sites with relative stable conditions and confirm prolonged warmth during MIS 11c also for the mid-latitudes. Sea surface temperature (SST) reconstructions further reveal that in the mid-latitude North Atlantic MIS 11c is associated with two plateaus, the younger one of which is slightly warmer. Enhanced subsurface northward heat flux in the eastern boundary current system, especially during early MIS 11c, is denoted by the presence of tropical planktonic foraminifer species. MIS 13 was generally colder and more variable than the younger interglacials. The greatest differences between the sites existed during the glacial inceptions and glacials. Then a north-south trending hydrographic front separated the nearshore and offshore waters off Portugal. While offshore waters originated from the North Atlantic Drift as indicated by the similarities between the records of IODP Site U1313, ODP Site 980 and MD01-2446, nearshore waters as recorded in core MD03-2699 derived from the Azores Current and thus the subtropical gyre. A strong Azores Current influence is seen especially during MIS 12, when SST dropped significantly only during the Heinrich-type ice-rafting event at the onset of Termination V. Given the subtropical overprint on Portuguese nearshore sites such as MD03-2699 and MD01-2443 caution needs to be taken to interpret their records as basin-wide climate signals.

Description: A one-dimensional model of the biogeochemistry and speciation of iron is coupled with the General Ocean Turbulence Model (GOTM) and a NPZD-type ecosystem model. The model is able to simulate the temporal patterns and vertical profiles of dissolved iron (dFe) in the upper ocean at the Bermuda Atlantic Time-series Study site reasonably well. Subsurface model profiles strongly depend on the parameter values chosen for the loss processes for iron, colloidal aggregation and scavenging onto particles. Current estimates for these parameters result in depletion of dFe. A high stability constant of iron-binding organic ligands is required to reproduce the observed degree of organic complexation below the mixed layer. A solubility of atmospherically deposited iron higher than 2% lead to dFe concentrations incompatible with observations. Despite neglecting ultraviolet radiation, the model produces diurnal variations and mean vertical profiles of H2O2 and iron species that are in good agreement with observations.

Description: A one-dimensional model of Fe speciation and biogeochemistry, coupled with the General Ocean Turbulence Model (GOTM) and a NPZD-type ecosystem model, is applied for the Tropical Eastern North Atlantic Time-Series Observatory (TENATSO) site. Among diverse processes affecting Fe speciation, this study is focusing on investigating the role of dust particles in removing dissolved iron (DFe) by a more complex description of particle aggregation and sinking, and explaining the abundance of organic Fe-binding ligands by modelling their origin and fate. The vertical distribution of different particle classes in the model shows high sensitivity to changing aggregation rates. Using the aggregation rates from the sensitivity study in this work, modelled particle fluxes are close to observations, with dust particles dominating near the surface and aggregates deeper in the water column. POC export at 1000 m is a little higher than regional sediment trap measurements, suggesting further improvement of modelling particle aggregation, sinking or remineralisation. Modelled strong ligands have a high abundance near the surface and decline rapidly below the deep chlorophyll maximum, showing qualitative similarity to observations. Without production of strong ligands, phytoplankton concentration falls to 0 within the first 2 years in the model integration, caused by strong Fe-limitation. A nudging of total weak ligands towards a constant value is required for reproducing the observed nutrient-like profiles, assuming a decay time of 7 years for weak ligands. This indicates that weak ligands have a longer decay time and therefore cannot be modelled adequately in a one-dimensional model. The modelled DFe profile is strongly influenced by particle concentration and vertical distribution, because the most important removal of DFe in deeper waters is colloid formation and aggregation. Redissolution of particulate iron is required to reproduce an observed DFe profile at TENATSO site. Assuming colloidal iron is mainly composed of inorganic colloids, the modelled colloidal to soluble iron ratio is lower that observations, indicating the importance of organic colloids.

Description: During phytoplankton growth a fraction of dissolved inorganic carbon (DIC) assimilated by phytoplankton is exuded in the form of dissolved organic carbon (DOC), which can be transformed into extracellular particulate organic carbon (POC). A major fraction of extracellular POC is associated with carbon of transparent exopolymer particles (TEP; carbon content = TEPC) that form from dissolved polysaccharides (PCHO). The exudation of PCHO is linked to an excessive uptake of DIC that is not directly quantifiable from utilisation of dissolved inorganic nitrogen (DIN), called carbon overconsumption. Given these conditions, the concept of assuming a constant stoichiometric carbon-to-nitrogen (C:N) ratio for estimating new production of POC from DIN uptake becomes inappropriate. Here, a model of carbon overconsumption is analysed, combining phytoplankton growth with TEPC formation. The model describes two modes of carbon overconsumption. The first mode is associated with DOC exudation during phytoplankton biomass accumulation. The second mode is decoupled from algal growth, but leads to a continuous rise in POC while particulate organic nitrogen (PON) remains constant. While including PCHO coagulation, the model goes beyond a purely physiological explanation of building up carbon rich particulate organic matter (POM). The model is validated against observations from a mesocosm study. Maximum likelihood estimates of model parameters, such as nitrogen- and carbon loss rates of phytoplankton, are determined. The optimisation yields results with higher rates for carbon exudation than for the loss of organic nitrogen. It also suggests that the PCHO fraction of exuded DOC was 63±20% during the mesocosm experiment. Optimal estimates are obtained for coagulation kernels for PCHO transformation into TEPC. Model state estimates are consistent with observations, where 30% of the POC increase was attributed to TEPC formation. The proposed model is of low complexity and is applicable for large-scale biogeochemical simulations.

Description: During phytoplankton growth a fraction of dissolved inorganic carbon (DIC) assimilated by phytoplankton is exuded in the form of dissolved organic carbon (DOC), which can be transformed into extracellular particulate organic carbon (POC). A major fraction of extracellular POC is associated with carbon of transparent exopolymer particles (TEP; carbon content = TEPC) that form from dissolved polysaccharides (PCHO). The exudation of PCHO is linked to an excessive uptake of DIC that is not directly quantifiable from utilisation of dissolved inorganic nitrogen (DIN), called carbon overconsumption. Given these conditions, the concept of assuming a constant stoichiometric carbon-to-nitrogen (C:N) ratio for estimating new production of POC from DIN uptake becomes inappropriate. Here, a model of carbon overconsumption is analysed, combining phytoplankton growth with TEPC formation. The model describes two modes of carbon overconsumption. The first mode is associated with DOC exudation during phytoplankton biomass accumulation. The second mode is decoupled from algal growth, but leads to a continuous rise in POC while particulate organic nitrogen (PON) remains constant. While including PCHO coagulation, the model goes beyond a purely physiological explanation of building up carbon rich particulate organic matter (POM). The model is validated against observations from a mesocosm study. Maximum likelihood estimates of model parameters, such as nitrogen- and carbon loss rates of phytoplankton, are determined. The optimisation yields results with higher rates for carbon exudation than for the loss of organic nitrogen. It also suggests that the PCHO fraction of exuded DOC was 63±20% during the mesocosm experiment. Optimal estimates are obtained for coagulation kernels for PCHO transformation into TEPC. Model state estimates are consistent with observations, where 30% of the POC increase was attributed to TEPC formation. The proposed model is of low complexity and is applicable for large-scale biogeochemical simulations.

Description: A one-dimensional model of iron speciation and biogeochemistry, coupled with the General Ocean Turbulence Model (GOTM) and a NPZD-type ecosystem model, is applied for the Tropical Eastern North Atlantic Time-series Observatory (TENATSO) site. Aimed at investigating the role of organic complexation and dust particles in Fe speciation and bioavailability, the model is extended in this study by a more complex description of the origin and fate of organic ligands and of particle aggregation and sinking. Model results show that the profile of dissolved iron is strongly influenced by the abundance of organic ligands. Modelled processes controlling the source and fate of ligands can well explain the abundance of strong ligands. However, a restoring of total weak ligands towards a constant value is required for reproducing the observed nutrient-like profile of weak ligands, indicating that decay time of weak ligands might be too long for a 1d-model. High dust deposition brings not only considerable input of iron into surface waters but also fine inorganic particles for particle aggregation and Fe scavenging. Simulated profiles of dissolved iron show high sensitivity to re-dissolution of colloidal and particulate iron. The colloidal to soluble iron ratio is underestimated assuming that colloidal iron is mainly composed of inorganic colloids. That strongly argues for introducing organic colloids into the model in future work.

Description: The effect of artificial acidic polysaccharides (PS) and exudates of Phaeodactylum tricornutum on the half-life of Fe(II) in seawater was investigated in laboratory experiments. Strong photochemical hydrogen peroxide (H2O2) production of 5.2 to 10.9 nM (mg C)−1 h−1 was found in the presence of PS and diatom exudates. Furthermore when illuminated with UV light the presence of algal exudates had a net stabilising effect on ferrous iron in seawater (initial value 100 nmol L−1) above that expected from oxidation kinetics. In the dark the PS gum xanthan showed no stabilising effect on Fe(II). The photochemical formation of superoxide (O2−) in presence of diatom exudates and its reducing effect on Fe(III) appears to result in greater than expected concentrations of Fe(II). A model of the photochemical redox cycle of iron incorporating these processes supported the observed data well. Diatom exudates seem to play an important role for the photochemistry of iron in coastal waters.

Description: A one-dimensional model of the biogeochemistry and speciation of iron is coupled with the General Ocean Turbulence Model (GOTM) and a NPZD-type ecosystem model. The model is able to simulate the temporal patterns and vertical profiles of dissolved iron (dFe) in the upper ocean at the Bermuda Atlantic Time-series Study site reasonably well. Subsurface model profiles strongly depend on the parameter values chosen for the loss processes for iron, colloidal aggregation and scavenging onto particles. Estimates for these parameters based on observations in particle-rich waters result in depletion of dFe. A high stability constant of iron-binding organic ligands is required to reproduce the observed degree of organic complexation below the mixed layer. The scavenging residence time for iron in the model is shortest in spring and summer, because of the larger abundance of particles, and increases with depth towards values on the order of a hundred years. A solubility of atmospherically deposited iron higher than 2% lead to dFe concentrations incompatible with observations. Despite neglecting ultraviolet radiation, the model produces diurnal variations and mean vertical profiles of H2O2 and iron species that are in good agreement with observations.

Description: Offshore south central Chile (35° S–42° S), the morphology of the lowermost continental slope and trench floor witnesses a voluminous submarine mass-wasting event. The blocky slide body deposited in the Chile Trench at 73°46´ W 35°35´ S was targeted for study during RRS JAMES COOK Cruise JC23 and termed Reloca Slide. Its size of about 24 km3, its steep and high headscarp, the spatial distribution of slide deposits and the cohesive nature of major slide blocks make it interesting to address the issue of tsunami generation. We have obtained seismic reflection data that partly reveal the internal structure of the slide body. Gravity core samples were retrieved that will allow the slide to be dated and linked to the history of sedimentation and slope stability along this particular segment of the Chilean convergent margin. At present we assume a Holocene age for the sliding event.