Silica burial enhanced by iron limitation in oceanic upwelling margins

In large swaths of the ocean, primary production by diatoms may be limited by the availability of silica, which in turn limits the biological uptake of carbon dioxide. The burial of biogenic silica in the form of opal is the main sink of marine silicon. Opal burial occurs in equal parts in iron-limited open-ocean provinces and upwelling margins, especially the eastern Pacific upwelling zone. However, it is unclear why opal burial is so efficient in this margin. Here we measure fluxes of biogenic material, concentrations of diatom-bound iron and silicon isotope ratios using sediment traps and a sediment core from the Gulf of California upwelling margin. In the sediment trap material, we find that periods of intense upwelling are associated with transient iron limitation that results in a high export of silica relative to organic carbon. A similar correlation between enhanced silica burial and iron limitation is evident in the sediment core, which spans the past 26,000 years. A global compilation also indicates that hotspots of silicon burial in the ocean are all characterized by high silica to organic carbon export ratios, a diagnostic trait for diatoms growing under iron stress. We therefore propose that prevailing conditions of silica limitation in the ocean are largely caused by iron deficiency imposing an indirect constraint on oceanic carbon uptake

The Congolobe project, a multidisciplinary study of Congo deep-sea fan lobe complex: Overview of methods, strategies, observations and sampling

The presently active region of the Congo deep-sea fan (around 330,000 km(2)), called the terminal lobes or lobe complex, covers an area of 2500 km(2) at 4700-5100 m water depth and 750-800 km offshore. It is a unique sedimentary area in the world ocean fed by a submarine canyon and a channel-levee system which presently deliver large amounts of organic carbon originating from the Congo River by turbidity currents. This particularity is due to the deep incision of the shelf by the Congo canyon, up to 30 km into the estuary, which funnels the Congo River sediments into the deep-sea. The connection between the river and the canyon is unique for major world rivers. In 2011, two cruises (WACS leg 2 and Congolobe) were conducted to simultaneously investigate the geology, organic and inorganic geochemistry, and micro- and macro-biology of the terminal lobes of the Congo deep-sea fan. Using this multidisciplinary approach, the morpho-sedimentary features of the lobes were characterized along with the origin and reactivity of organic matter, the recycling and burial of biogenic compounds, the diversity and function of bacterial and archaeal communities within the sediment, and the biodiversity and functioning of the faunal assemblages on the seafloor. Six different sites were selected for this study: Four distributed along the active channel from the lobe complex entrance to the outer rim of the sediment deposition zone, and two positioned cross-axis and at increasing distance from the active channel, thus providing a gradient in turbidite particle delivery and sediment age. This paper aims to provide the general context of this multidisciplinary study. It describes the general features of the site and the overall sampling strategy and provides the initial habitat observations to guide the other in-depth investigations presented in this special issue. Detailed bathymetry of each sampling site using 0.1-1 m resolution multibeam obtained with a remotely operated vehicle (ROV) shows progressive widening and smoothing of the channel-levees with increasing depth and reveals a complex morphology with channel bifurcations, erosional features and massive deposits. Dense ecosystems surveyed in the study area gather high density clusters of two large-sized species of symbiotic Vesicomyidae bivalves and microbial mats. These assemblages, which are rarely observed in sedimentary zones, resemble those based on chemosynthesis at cold-seep sites, such as the active pockmarks encountered along the Congo margin, and share with these sites the dominant vesicomyid species Christineconcha regab. Sedimentation rates estimated in the lobe complex range between 0.5 and 10 cm yr(-1), which is 2-3 orders of magnitude higher than values generally encountered at abyssal depths. The bathymetry, faunal assemblages and sedimentation rates make the Congo lobe complex a highly peculiar deep-sea habitat driven by high inputs of terrigenous material delivered by the Congo channel-levee system. (c) 2016 Elsevier Ltd. All rights reserved.ZAIANGOANR Congolobe (ANR Blanc SIMI5-6) [11 BS56 030]IFREMERCEA through LSCEU.S. National Science Foundation [OCE-0831156]info:eu-repo/semantics/acceptedVersio

Evidence for reduced biogenic silica dissolution rates in diatom aggregates

International audienceBecause aggregated diatoms sink rapidly through the water column, leaving little time for dissolution, aggregation influences the balance between recycling of biogenic silica (bSiO2) and its sedimentation and preservation at the seafloor. Additionally, aggregation may directly impact dissolution rates of opal. Laboratory experiments were conducted to investigate the influence of aggregation on bSiO2 dissolution rates using 3 different batch cultures of the diatoms Chaetoceros decipiens, Skeletonema costatum, and Thalassiosira weissflogii. Specific dissolution rates of bSiO2 of aggregated and freely suspended diatoms were compared. Additionally, the influences of the dissolved silicon (dSi) concentration in the pore water of aggregates, the viability of diatoms, and the concentrations of transparent exopolymer particles (TEP) and of bacteria on bSiO2 dissolution rates were determined. Initial specific dissolution rates of diatom frustules were significantly lower for aggregated diatoms (2.9% d–1) than for freely suspended diatoms (6.6% d–1). Lower specific dissolution rates in aggregates were attributed to elevated dSi concentrations in aggregate pore water (maximum 230 vs. 20 µmol l–1) and to the fact that aggregated diatoms remained viable for longer than freely suspended diatoms. Specific bSiO2 dissolution was significantly correlated to viability of cells independent of treatment. Bacterial concentrations in both treatments appeared high enough, so that after cell death the coating protecting the silica frustule was degraded without measurable delay. The TEP content of aggregates appeared to affect dissolution rates, possibly by retaining solutes within aggregates

Sensitivity of the marine biospheric Si cycle for biogeochemical parameter variations

A systematic quantitative assessment of the marine silicon cycle is presented, based on a prognostic coupled water column-sediment global biogeochemical ocean general circulation model (HAMOCC). The resulting tracer distributions are compared with a comprehensive marine Si database of measurements. The model parameters which govern the Si cycle within the model world are optimized through a linear response model. The functional relationships between the Si cycle parameters and the Si tracer distributions are derived from a series of sensitivity experiments addressing opal export production, particle flux through the water column, porewater chemistry, and external biogeochemical forcing. The most important parameters for a further quantitative improvement of the simulation are depth-dependent opal dissolution kinetics, a productivity-dependent opal settling velocity, a general change in maximum Si uptake velocity Vmax opal, and the clay as well as the Si input from continental weathering. The modeled Si budget shows a larger global export production, larger opal deposition rates onto the sediment surface and higher diffusive transports of porewater silicic acid into the open water column as estimated by Tre´guer et al. [1995]