Production of biogenic silica during the EBENE cruise, Equatorial Pacific

During the EBENE cruise (November 1996), distributions of biogenic silica concentration and production rates were investigated in the surface waters of the equatorial Pacific (180°W, from 8°S to 8°N), with particular emphasis on the limitation of the biogenic silica production by ambient silicic acid concentrations. Integrated over the depth of the euphotic layer, concentrations of biogenic silica and production rates were maximum at the Equator (8.0 and 2.6 mmol/m**2/d) and decreased more or less symmetrically polewards. Contribution of diatoms to the new production was estimated indirectly, comparing biogenic silica production rates and available data of new and export production in the same area. This comparison shows that new production in the equatorial area could mostly be sustained by diatoms, accounting for the major part of the exported flux of organic carbon. Kinetics experiments of silicic acid enrichment were performed. Half saturation constants were 1.57 µM at 3°S and 2.42 µM at the Equator close to the ambient concentrations. The corresponding Vmax values for Si uptake were 0.028/h at 3°S and 0.052/h at the equator. Experiments also show that in situ rates were restricted to 13-78% of Vmax, depending on ambient silicic acid concentrations. This work provides the first direct evidence that the rate of Si uptake by diatom populations of the equatorial Pacific is limited by the ambient concentration of silicic acid. However, such Si limitation might not be sufficient in itself to explain the low diatom growth rates observed, and additional limitation is suggested. One hypothesis that is consistent with the results of Fe limitation studies is that Fe and Si limitations may interact, rather than just being a mutually exclusive explanation for the HNLC character of the system.

Measurements of organic complexation of iron during the CARUSO-EISENEX experiment

The speciation of strongly chelated iron during the 22-day course of an iron enrichment experiment in the Atlantic sector of the Southern Ocean deviates strongly from ambient natural waters. Three iron additions (ferrous sulfate solution) were conducted, resulting in elevated dissolved iron concentrations (Nishioka, J., Takeda, S., de Baar, H.J.W., Croot, P.L., Boye, M., Laan, P., Timmermans, K.R., 2005, Changes in the concentration of iron in different size fractions during an iron enrichment experiment in the open Southern Ocean. Marine Chemistry, doi:10.1016/j.marchem.2004.06.040) and significant Fe(II) levels (Croot, P.L., Laan, P., Nishioka, J., Strass, V., Cisewski, B., Boye, M., Timmermans, K.R., Bellerby, R.G., Goldson, L., Nightingale, P., de Baar, H.J.W., 2005, Spatial and Temporal distribution of Fe(II) and H2O2 during EisenEx, an open ocean mescoscale iron enrichment. Marine Chemistry, doi:10.1016/j.marchem.2004.06.041). Repeated vertical profiles for dissolved (filtrate < 0.2 µm) Fe(III)-binding ligands indicated a production of chelators in the upper water column induced by iron fertilizations. Abiotic processes (chemical reactions) and an inductive biologically mediated mechanism were the likely sources of the dissolved ligands which existed either as inorganic amorphous phases and/or as strong organic chelators. Discrete analysis on ultra-filtered samples (< 200 kDa) suggested that the produced ligands would be principally colloidal in size (> 200 kDa-< 0.2 µm), as opposed to the soluble fraction (< 200 kDa) which dominated prior to the iron infusions. Yet these colloidal ligands would exist in a more transient nature than soluble ligands which may have a longer residence time. The production of dissolved Fe-chelators was generally smaller than the overall increase in dissolved iron in the surface infused mixed layer, leaving a fraction (about 13-40%) of dissolved Fe not bound by these dissolved Fe-chelators. It is suggested that this fraction would be inorganic colloids. The unexpected persistence of such high inorganic colloids concentrations above inorganic Fe-solubility limits illustrates the peculiar features of the chemical iron cycling in these waters. Obviously, the artificial about hundred-fold increase of overall Fe levels by addition of dissolved inorganic Fe(II) ions yields a major disruption of the natural physical-chemical abundances and reactivity of Fe in seawater. Hence the ensuing responses of the plankton ecosystem, while in itself significant, are not necessarily representative for a natural enrichment, for example by dry or wet deposition of aeolian dust. Ultimately, the temporal changes of the Fe(III)-binding ligand and iron concentrations were dominated by the mixing events that occurred during EISENEX, with storms leading to more than an order of magnitude dilution of the dissolved ligands and iron concentrations. This had strongest impact on the colloidal size class (> 200 kDa-< 0.2 µm) where a dramatic decrease of both the colloidal ligand and the colloidal iron levels (Nishioka, J., Takeda, S., de Baar, H.J.W., Croot, P.L., Boye, M., Laan, P., Timmermans, K.R., 2005, Changes in the concentration of iron in different size fractions during an iron enrichment experiment in the open Southern Ocean. Marine Chemistry, doi:10.1016/j.marchem.2004.06.040) was observed.

Stable oxygen isotope record and age determination of benthic foraminiferal faunas of the Norwegian-Greenland Sea

Fluctuations in benthic foraminiferal faunas over the last 130,000 yr in four piston cores from the Norwegian Sea are correlated with the standard worldwide oxygen-isotope stratigraphy. One species, Cibicides wuellerstorfi, dominates in the Holocene section of each core, but alternates downcore with Oridorsalis tener, a species dominant today only in the deepest part of the basin. O. tener is the most abundant species throughout the entire basin during periods of particularly cold climate when the Norwegian Sea presumably was ice covered year round and surface productivity lowered. Portions of isotope Stages 6, 3, and 2 are barren of benthic foraminifera; this is probably due to lowered benthic productivity, perhaps combined with dilution by ice-rafted sediment; there is no evidence that the Norwegian Sea became azoic. The Holocene and Substage 5e (the last interglacial) are similar faunally. This similarity, combined with other evidence, supports the presumption that the Norwegian Sea was a source of dense overflows into the North Atlantic during Substage 5e as it is today. Oxygen-isotope analyses of benthic foraminifera indicate that Norwegian Sea bottom waters warmer than they are today from Substage 5d to Stage 2, with the possible exception of Substage 5a. These data show that the glacial Norwegian Sea was not a sink for dense surface water, as it is now, and thus it was not a source of deep-water overflows. The benthic foraminiferal populations of the deep Norwegian Sea seem at least as responsive to near-surface conditions, such as sea-ice cover, as they are to fluctuations in the hydrography of the deep water. Benthic foraminiferal evidence from the Norwegian Sea is insufficient in itself to establish whether or not the basin was a source of overflows into the North Atlantic at any time between the Substage 5e/5d boundary at 115,000 yr B.P. and the Holocene.