Carbonate chemistry in sediment porewaters of the Rhône River delta driven by early diagenesis (northwestern Mediterranean)
The Rhône River is the largest source of terrestrial organic and inorganic carbon for the Mediterranean Sea. A large fraction of this terrestrial carbon is either buried or mineralized in the sediments close to the river mouth. This mineralization follows aerobic and anaerobic pathways, with a range...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2016-09-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/13/5379/2016/bg-13-5379-2016.pdf |
| _version_ | 1818242345853779968 |
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| author | J. Rassmann B. Lansard L. Pozzato C. Rabouille |
| author_facet | J. Rassmann B. Lansard L. Pozzato C. Rabouille |
| author_sort | J. Rassmann |
| collection | DOAJ |
| description | The Rhône River is the largest source of terrestrial
organic and inorganic carbon for the Mediterranean Sea. A large fraction of
this terrestrial carbon is either buried or mineralized in the sediments
close to the river mouth. This mineralization follows aerobic and anaerobic
pathways, with a range of impacts on calcium carbonate precipitation and
dissolution in the sediment near the sediment–water interface. This study
focuses on the production of dissolved inorganic carbon (DIC) and total
alkalinity (TA) by early diagenesis, consequential pH variations and the
effect on calcium carbonate precipitation or dissolution. The sediment
porewater chemistry was investigated along a transect from the Rhône
River outlet to the continental shelf. TA and concentrations of DIC,
SO<sub>4</sub><sup>2−</sup> and Ca<sup>2+</sup> were analyzed on bottom waters and extracted
sediment porewaters, whereas pH and oxygen concentrations were measured in situ
using microelectrodes. The average oxygen penetration depth into the
sediment was 1.7 ± 0.4 mm close to the river mouth and 8.2 ± 2.6
mm in the continental shelf sediments, indicating intense respiration rates.
Diffusive oxygen fluxes through the sediment–water interface ranged between
3 and 13 mmol O<sub>2</sub> m<sup>−2</sup> d<sup>−1</sup>. In the first 30 cm of the sediment,
TA and DIC porewater concentrations increased with depth up to 48 mmol L<sup>−1</sup> near the river outlet and up to 7 mmol L<sup>−1</sup> on the shelf as a
result of aerobic and anaerobic mineralization processes. Due to aerobic
processes, at all stations pH decreased by 0.6 pH units in the oxic layer of
the sediment accompanied by a decrease of the saturation state regarding
calcium carbonate. In the anoxic layer of the sediments, sulfate reduction
was the dominant mineralization process and was associated with an increase of
porewater saturation state regarding calcium carbonate. Ultimately anoxic
mineralization of organic matter caused calcium carbonate precipitation
demonstrated by a large decrease in Ca<sup>2+</sup> concentration with depth in
the sediment. Carbonate precipitation decreased in the offshore direction,
together with the carbon turnover and sulfate consumption in the sediments.
The large production of porewater alkalinity characterizes these sediments
as an alkalinity source to the water column, which may increase the CO<sub>2</sub>
buffering capacity of these coastal waters. Estuarine sediments should
therefore receive more attention in future estimations of global carbon
fluxes. |
| first_indexed | 2024-12-12T13:43:46Z |
| format | Article |
| id | doaj.art-81a49237432345aabe7dd401eaea15fc |
| institution | Directory Open Access Journal |
| issn | 1726-4170 1726-4189 |
| language | English |
| last_indexed | 2024-12-12T13:43:46Z |
| publishDate | 2016-09-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Biogeosciences |
| spelling | doaj.art-81a49237432345aabe7dd401eaea15fc2022-12-22T00:22:44ZengCopernicus PublicationsBiogeosciences1726-41701726-41892016-09-0113185379539410.5194/bg-13-5379-2016Carbonate chemistry in sediment porewaters of the Rhône River delta driven by early diagenesis (northwestern Mediterranean)J. Rassmann0B. Lansard1L. Pozzato2C. Rabouille3Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ-Université Paris Saclay, 91198 Gif-sur-Yvette, FranceLaboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ-Université Paris Saclay, 91198 Gif-sur-Yvette, FranceLaboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ-Université Paris Saclay, 91198 Gif-sur-Yvette, FranceLaboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ-Université Paris Saclay, 91198 Gif-sur-Yvette, FranceThe Rhône River is the largest source of terrestrial organic and inorganic carbon for the Mediterranean Sea. A large fraction of this terrestrial carbon is either buried or mineralized in the sediments close to the river mouth. This mineralization follows aerobic and anaerobic pathways, with a range of impacts on calcium carbonate precipitation and dissolution in the sediment near the sediment–water interface. This study focuses on the production of dissolved inorganic carbon (DIC) and total alkalinity (TA) by early diagenesis, consequential pH variations and the effect on calcium carbonate precipitation or dissolution. The sediment porewater chemistry was investigated along a transect from the Rhône River outlet to the continental shelf. TA and concentrations of DIC, SO<sub>4</sub><sup>2−</sup> and Ca<sup>2+</sup> were analyzed on bottom waters and extracted sediment porewaters, whereas pH and oxygen concentrations were measured in situ using microelectrodes. The average oxygen penetration depth into the sediment was 1.7 ± 0.4 mm close to the river mouth and 8.2 ± 2.6 mm in the continental shelf sediments, indicating intense respiration rates. Diffusive oxygen fluxes through the sediment–water interface ranged between 3 and 13 mmol O<sub>2</sub> m<sup>−2</sup> d<sup>−1</sup>. In the first 30 cm of the sediment, TA and DIC porewater concentrations increased with depth up to 48 mmol L<sup>−1</sup> near the river outlet and up to 7 mmol L<sup>−1</sup> on the shelf as a result of aerobic and anaerobic mineralization processes. Due to aerobic processes, at all stations pH decreased by 0.6 pH units in the oxic layer of the sediment accompanied by a decrease of the saturation state regarding calcium carbonate. In the anoxic layer of the sediments, sulfate reduction was the dominant mineralization process and was associated with an increase of porewater saturation state regarding calcium carbonate. Ultimately anoxic mineralization of organic matter caused calcium carbonate precipitation demonstrated by a large decrease in Ca<sup>2+</sup> concentration with depth in the sediment. Carbonate precipitation decreased in the offshore direction, together with the carbon turnover and sulfate consumption in the sediments. The large production of porewater alkalinity characterizes these sediments as an alkalinity source to the water column, which may increase the CO<sub>2</sub> buffering capacity of these coastal waters. Estuarine sediments should therefore receive more attention in future estimations of global carbon fluxes.http://www.biogeosciences.net/13/5379/2016/bg-13-5379-2016.pdf |
| spellingShingle | J. Rassmann B. Lansard L. Pozzato C. Rabouille Carbonate chemistry in sediment porewaters of the Rhône River delta driven by early diagenesis (northwestern Mediterranean) Biogeosciences |
| title | Carbonate chemistry in sediment porewaters of the Rhône River delta
driven by early diagenesis (northwestern Mediterranean) |
| title_full | Carbonate chemistry in sediment porewaters of the Rhône River delta
driven by early diagenesis (northwestern Mediterranean) |
| title_fullStr | Carbonate chemistry in sediment porewaters of the Rhône River delta
driven by early diagenesis (northwestern Mediterranean) |
| title_full_unstemmed | Carbonate chemistry in sediment porewaters of the Rhône River delta
driven by early diagenesis (northwestern Mediterranean) |
| title_short | Carbonate chemistry in sediment porewaters of the Rhône River delta
driven by early diagenesis (northwestern Mediterranean) |
| title_sort | carbonate chemistry in sediment porewaters of the rhone river delta driven by early diagenesis northwestern mediterranean |
| url | http://www.biogeosciences.net/13/5379/2016/bg-13-5379-2016.pdf |
| work_keys_str_mv | AT jrassmann carbonatechemistryinsedimentporewatersoftherhoneriverdeltadrivenbyearlydiagenesisnorthwesternmediterranean AT blansard carbonatechemistryinsedimentporewatersoftherhoneriverdeltadrivenbyearlydiagenesisnorthwesternmediterranean AT lpozzato carbonatechemistryinsedimentporewatersoftherhoneriverdeltadrivenbyearlydiagenesisnorthwesternmediterranean AT crabouille carbonatechemistryinsedimentporewatersoftherhoneriverdeltadrivenbyearlydiagenesisnorthwesternmediterranean |