Geochemistry on core 202-1240A

The modern Eastern Equatorial Pacific (EEP) Ocean is a large oceanic source of carbon to the atmosphere1. Primary productivity over large areas of the EEP is limited by silicic acid and iron availability, and because of this constraint the organic carbon export to the deep ocean is unable to compensate for the outgassing of carbon dioxide that occurs through upwelling of deep waters. It has been suggested that the delivery of dust-borne iron to the glacial ocean could have increased primary productivity and enhanced deep-sea carbon export in this region, lowering atmospheric carbon dioxide concentrations during glacial periods. Such a role for the EEP is supported by higher organic carbon burial rates documented in underlying glacial sediments but lower opal accumulation rates cast doubts on the importance of the EEP as an oceanic region for significant glacial carbon dioxide drawdown. Here we present a new silicon isotope record that suggests the paradoxical decline in opal accumulation rate in the glacial EEP results from a decrease in the silicon to carbon uptake ratio of diatoms under conditions of increased iron availability from enhanced dust input. Consequently, our study supports the idea of an invigorated biological pump in this region during the last glacial period that could have contributed to glacial carbon dioxide drawdown. Additionally, using evidence from silicon and nitrogen isotope changes, we infer that, in contrast to the modern situation, the biological productivity in this region is not constrained by the availability of iron, silicon and nitrogen during the glacial period. We hypothesize that an invigorated biological carbon dioxide pump constrained perhaps only by phosphorus limitation was a more common occurrence in low-latitude areas of the glacial ocean.

Physical oceanography and ADCP measurements during METEOR cruise M71/3

Work on M71-3 aimed to answer the question whether phosphate loss and incomplete utilisation of nitrate on the one hand, or dinitrogen fixation on the other hand are responsible for unusual nutrient ratios in the water column of the eastern Mediterranean Sea. We investigated the reason for unusually depleted 15N/14N ratios in dissolved nitrate, suspended matter and surface sediments of that oligotrohic ocean basin. During a total of 18 days of ship time on R/V METEOR, Leg 3 OF M71 performed water column and surface sediment work in the eastern Mediterranean Sea (Heraklion-Istanbul). Stations will be aligned on one E-W and two N-S transects and sampled all major basins and water masses. On stations, CTD casts were obtained and water samples for nutrient concentrations were taken. These samples and those of suspended matter and surface sediment will be analysed for the stable nitrogen isotope composition of dissolved nitrate, suspended matter, and surface sediments. In addition, phytoplankton samples were be taken and experiments were be carried out to determine N2 fixation rates, genetic expressions of nitrogen fixation, and the composition of microbial and algal assemblages. One mooring of sediment traps (Ierapetra Deep) was be deployed and will be recovered in September 2007 to monitor changes in particle flux and its isotope signature over a seasonal cycle.

Investigation of time series stations along a productivity gradient north of the Canary Islands

Three stations along a productivity gradient north of the Canary Islands were investigated for surface-water properties, particle flux, and composition (biogenic and lithogenic components, and stable nitrogen isotope composition, delta15N) and export production. Investigation sites along the east-west transect off the NW African upwelling margin included the European Station for Time-Series in the Ocean, Canary Islands (ESTOC), one location contiguous to the NW African upwelling zone in the Eastern Boundary Current (EBC) and one station north of the island La Palma (LP). The seasonality of surface-water properties along the transect was mainly influenced by the winter cooling and simultaneous phytoplankton maximum and, in addition at EBC, by nearby upwelling. Accordingly, particle flux and composition along the transect were closely linked to the winter bloom sedimentation and upwelling related enhanced plankton biomass stemming from the primary upwelling and the Cape Yubi filament at EBC. During all seasons, particle flux was highest at EBC and had the highest contribution of biogenic opal and lithogenic components, and the lowest delta15N compared to the offshore stations. But contrary to what would be expected from the productivity gradient, particle flux did not decrease from ESTOC to LP. Below the upper several hundred meters, particle flux was enhanced by additional particle input along the entire transect, manifested by an increase of flux with depth and lower delta15N values. We offer a scenario in which intermediate nepheloid layers originating from the primary upwelling as well as particle dispersion from upwelling filaments, mainly the Cape Ghir filament, impact on the trap stations as far as 700 km into the open ocean. This study contributes to our understanding of the poorly resolved biogeochemical transition between the productive shelf and subtropical gyre provinces.