Atlantic – Laboratoire LEMAR UMR 6539

The unaccounted dissolved iron (II) sink: Insights from dFe(II) concentrations in the deep Atlantic Ocean

Abstract

Hydrothermal vent sites found along mid-ocean ridges are sources of numerous reduced chemical species and trace elements. To establish dissolved iron (II) (dFe(II)) variability along the Mid Atlantic Ridge (between 39.5°N and 26°N), dFe(II) concentrations were measured above six hydrothermal vent sites, as well as at stations with no active hydrothermal activity. The dFe(II) concentrations ranged from 0.00 to 0.12 nmol L−1 (detection limit = 0.02 ± 0.02 nmol L−1) in non-hydrothermally affected regions to values as high as 12.8 nmol L−1 within hydrothermal plumes. Iron (II) in seawater is oxidised over a period of minutes to hours, which is on average two times faster than the time required to collect the sample from the deep ocean and its analysis in the onboard laboratory. A multiparametric equation was used to estimate the original dFe(II) concentration in the deep ocean. The in-situ temperature, pH, salinity and delay between sample collection and its analysis were considered. The results showed that dFe(II) plays a more significant role in the iron pool than previously accounted for, constituting a fraction >20 % of the dissolved iron pool, in contrast to <10 % of the iron pool formerly reported. This discrepancy is caused by Fe(II) loss during sampling when between 35 and 90 % of the dFe(II) gets oxidised. In-situ dFe(II) concentrations are therefore significantly higher than values reported in sedimentary and hydrothermal settings where Fe is added to the ocean in its reduced form. Consequently, the high dynamism of dFe(II) in hydrothermal environments masks the magnitude of dFe(II) sourced within the deep ocean.

Highlights

Considering oxidation, open ocean iron (II) concentrations are below 0.2 nmol L⁻¹.
The highest measured iron (II) concentration was 69.6 nmol L⁻¹ at the Rainbow vent.
In the open ocean iron (II) account for 20 % of the dissolved iron pool.
Oxygen variations within OMZ account for 60 % of iron(II) oxidation variability.

Reference

Gonzalez-Santana, D.; Lough, A. J. M.; Planquette, H.; Sarthou, G.; Tagliabue, A.; Lohan, M. C. The Unaccounted Dissolved Iron (II) Sink: Insights from DFe(II) Concentrations in the Deep Atlantic Ocean. Sci. Total Environ. 2023, 862, 161179.

https://doi.org/10.1016/j.scitotenv.2022.161179.

10/03/2023/by sherve@univ-brest.fr

On 17/12/2018 at 3pm in lecture hall A of IUEM, thesis defence of Pierre Poitevin (LEMAR)

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Title: Sclerochronological approaches in Saint-Pierre & Miquelon: from sub-hourly to multidecadal environmental reconstructions

The panel will be composed of:

Anne Lorrain (Examiner) IRD / LEMAR
Claire Lazareth (Examiner) IRD / LOCEAN
Stéphanie Thièbault (Examiner) CNRS / Director of INEE
Philippe Archambault (Rapporteur) Univ. Laval
Matthieu Carré (Rapporteur) CNRS / LOCEAN

Thesis supervisors:

Laurent Chauvaud and Pascal Lazure

Summary :

Coastal ecosystems are exposed to global climate change leading to modifications of their structure and functioning. However, little is known about the variability of their physical properties before 1950, mainly because of the lack of long-term instrumental measurements. The hard parts of long-lived marine biota hold the potential to extend instrumentally derived observations, at different temporal and spatial resolutions, in order to enhance our understanding of past environmental processes.

This PhD dissertation takes place on Saint-Pierre & Miquelon (SPM), a small archipelago at the confluence of major oceanic currents marking the boundary between the North Atlantic Ocean subtropical and subpolar gyres. In addition to its global key position, the abrupt bathymetric change occurring in the North West of Miquelon Island generates the anti-cyclonic propagation of a tidal coastal trapped wave around this archipelago. This local phenomenon, leads during the stratified period to the largest (up to 11.5°C amplitude) daily (25.8 h) temperature oscillations ever observed-at any frequency-on a stratified mid latitude continental shelf.

This work is based on the analyses of local marine biota hard parts to gain insights about past environmental variability at these two scales. First, I have learned different sclerochronological methods through Spisula solidissima study. Global and multi-decadal time scales were reached using the longest lived bivalve known to date Arctica islandica, and Clathromorphum compactum a newly discovered long-lived coralline alga. The relationships observed at SPM between A. islandica and C. compactum sclerochronological records and different geographical scales environmental datasets yield details about past large-scale oceanographic variability and ecosystem dynamics. Local, high-frequency (25.8 h) temperature oscillations were tracked using sclerochronological information contained in Placopecten magellanicus a fast growing (ca. 500 µm / day) bivalve species.

This study points out the relevant position of this archipelago for studying multiple scale oceanographic variability, biological responses and ecosystem dynamics facing global changes.

10/11/2018/by titodemora