Closed-system dissolution experiment
Identifying the geochemical reactions occurring between the lithogenic matter and the seawater
Quantifying the processes with an emphasis on their kinetic
Checking if the particles reflect any modification as a result of these processes
Scientific and societal issues
Land to ocean transfer of solid material largely controls the chemical composition of seawater and the global cycles of the elements. Global riverine particulate flux to the oceans exceeds that of the dissolved flux by a factor of 30 to 50. Quantifying the fraction of element of lithogenic origin that is released at the particle/seawater contact and what controls this release is a major issue. Indeed, field observations and modelling demonstrate that the oceanic distribution of numerous tracers including Co, 143Nd/ 144Nd, 232Th/230Th, 56Fe/54Fe, 30Si/28Si, 87Sr/86Sr can only be interpreted by taking account of a substantial input to seawater from lithogenic material from the continental margins. This input could double the flux of macro- and micro-nutrients to the ocean. Solving this issue will impact i) our understanding of the element cycles, ii) the design of the climatic models and iii) the present knowledge of the contaminant dispersion. This is a major objective of the international program GEOTRACES.
Here we present results of closed system experiments, designed to constrain the rate and fate of the dissolution processes.
- 30 l containers filled by seawater (SW), which initial chemical and isotopic composition is known.
- Lithogenic material of different origin (river, estuarine or marine sediments) poured in this seawater, 4 month duration.
- Aliquots are collected regularly (every weeks or 3 weeks, depending on the element analyzed)
- Seawater chemistry is controlled along the experiment
- Thermodynamic modelling (with PHREEQC) allowing the mineral phases that could be saturated, potentially leading to secondary phase precipitations.
(Jones et al, EPSL 2012, GCA 2012 & Pearce et al, EPSL, 2013)
- Nd and Sr seawater isotopic compositions (IC) are modified within few weeks, eventually reaching the value of the solid material they are in contact with
- (here Icelandic riverine and estuarine sediments and top core from the shallow Kerguelen plateau). This reflects a transfer of elements from the solid to the liquid. In the figure Nd IC is expressed with the parameter Nd.
- Contrastingly the concentration increases are not consistent with what is required by the IC changes. This means that there is a scavenging from the water towards the solid. Coupling concentration and IC allows identifying exchange processes.
- Comparing the measured concentration increase (in blue) to that required by the change of IC (in red, here illustrated for Sr) allows evaluating the exchange rate and net resulting input of element from the particles to the seawater.
- We estimated that between 3 and 15% of Sr and ~ 0.5% of Nd content of the solid material are released in seawater. We although performed Scanning Electron Microscopy (SEM) and DRX analysis, which revealed that olivines and plagioclases are the minerals the most likely to dissolve
- Thermodynamic models suggest that secondary phases as rhabdophane (REE (PO4. nH20) could partly explain the Sr and Nd scavenging, although the net particle to dissolve flux remains positive.
- Last but not least seawater Si(OH)4 concentration increases by a factor of 10 (at 5°C°) or 20 (at 25°C) when the initial water is very Si depleted (0.3 ppm, Jones et al, 2012). Moreover, the figure here demonstrate that Si(OH)4 significantly increases although the initial Si seawater content is high (Pearce et al, 2013).
- The dissolution of lithogenic Si at the land-ocean interface due to silicate submarine weathering, never taken into account in the oceanic element budgets so far, could double the input fluxes, impacting the biological pump and therefore the climate.
Coupling concentration and isotopic composition is particularly powerful for process quantification. Our plans are to realize new designed system experiments –the only one allowing us to determine the processes at play and their kinetics- that time as open flow and measuring more isotopes. To say the least, the missing heroes of the preliminary experiences described above are Fe and Si isotopes. We also plan to select the minerals that are dissolving the most and to study the speciation of the particle surfaces (Emergence project submitted in 2014, Toulouse Idex; RTRA STAE Project 2014)
Participants: Catherine Jeandel, Eric Oelkers (GET), Morgan Jones (GET) & Chris Pearce (GET)
Collaborations and funding
LEGOS, GET, IRAP (for the future)
Financements: CNRS-INSU/LEFE, ANR, UE