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Diagenesis in tephra-rich sediments from the Lesser Antilles Volcanic Arc: pore fluid constraints

Diagenesis in tephra-rich sediments from the Lesser Antilles Volcanic Arc: pore fluid constraints
Diagenesis in tephra-rich sediments from the Lesser Antilles Volcanic Arc: pore fluid constraints
We present sediment pore fluid and sediment solid phase results obtained during IODP Expedition 340 from seven sites located within the Grenada Basin of the southern Lesser Antilles Volcanic Arc region. These sites are generally characterized as being low in organic carbon content and rich in calcium carbonate and volcanogenic material. In addition to the typical reactions related to organic matter diagenesis, pore fluid chemistry indicates that the diagenetic reactions fall within two broad categories; (1) reactions related to chemical exchange with volcanogenic material and (2) reactions related to carbonate dissolution, precipitation, or recrystallization. For locations dominated by reaction with volcanogenic material, these sites exhibit increases in dissolved Ca with coeval decreases in Mg. We interpret this behavior as being driven by sediment-water exchange reactions from the alteration of volcanic material that is dispersed throughout the sediment package, which likely result in formation of Mg-rich secondary authigenic clays. In contrast to this behavior, sediment sequences that exhibit decreases in Ca, Mg, Mn, and Sr with depth suggest that carbonate precipitation is an active diagenetic process affecting solute distributions. The distributions of pore fluid 87Sr /86Sr reflect these competitive diagenetic reactions between volcanic material and carbonate, which are inferred by the major cation distributions. From one site where we have solid phase 87Sr /86Sr (site U1396), the carbonate fraction is found to be generally consistent with the contemporaneous seawater isotope values. However, the 87Sr /86Sr of the non-carbonate fraction ranges from 0.7074 to 0.7052, and these values likely represent a mixture of local arc volcanic sources and trans-Atlantic eolian sources. Even at this site where there is clear evidence for diagenesis of volcanogenic material, carbonate diagenesis appears to buffer pore fluid 87Sr /86Sr from the larger changes that might be expected given the high abundance of tephra in these sediments. Part of this carbonate buffering, at this site as well as throughout the region, derives from the fact that the Sr concentration in the non-carbonate fraction is generally low (< 200 ppm), whereas the carbonate fraction has Sr concentrations approaching ∼1000 ppm.
0016-7037
119-135
Murray, Natalie A.
036f7a1a-a481-416f-84a1-b2ec2052ee75
McManus, James
2eefd888-eea8-46df-bf71-202bf70b49f8
Palmer, Martin R.
d2e60e81-5d6e-4ddb-a243-602537286080
Haley, Brian
edc3444f-978a-465e-ae7e-18276b3d601c
Manners, Hayley
3f515c77-e9d4-4374-ac4e-71df19507876
Murray, Natalie A.
036f7a1a-a481-416f-84a1-b2ec2052ee75
McManus, James
2eefd888-eea8-46df-bf71-202bf70b49f8
Palmer, Martin R.
d2e60e81-5d6e-4ddb-a243-602537286080
Haley, Brian
edc3444f-978a-465e-ae7e-18276b3d601c
Manners, Hayley
3f515c77-e9d4-4374-ac4e-71df19507876

Murray, Natalie A., McManus, James, Palmer, Martin R., Haley, Brian and Manners, Hayley (2018) Diagenesis in tephra-rich sediments from the Lesser Antilles Volcanic Arc: pore fluid constraints. Geochimica et Cosmochimica Acta, 228, 119-135. (doi:10.1016/j.gca.2018.02.039).

Record type: Article

Abstract

We present sediment pore fluid and sediment solid phase results obtained during IODP Expedition 340 from seven sites located within the Grenada Basin of the southern Lesser Antilles Volcanic Arc region. These sites are generally characterized as being low in organic carbon content and rich in calcium carbonate and volcanogenic material. In addition to the typical reactions related to organic matter diagenesis, pore fluid chemistry indicates that the diagenetic reactions fall within two broad categories; (1) reactions related to chemical exchange with volcanogenic material and (2) reactions related to carbonate dissolution, precipitation, or recrystallization. For locations dominated by reaction with volcanogenic material, these sites exhibit increases in dissolved Ca with coeval decreases in Mg. We interpret this behavior as being driven by sediment-water exchange reactions from the alteration of volcanic material that is dispersed throughout the sediment package, which likely result in formation of Mg-rich secondary authigenic clays. In contrast to this behavior, sediment sequences that exhibit decreases in Ca, Mg, Mn, and Sr with depth suggest that carbonate precipitation is an active diagenetic process affecting solute distributions. The distributions of pore fluid 87Sr /86Sr reflect these competitive diagenetic reactions between volcanic material and carbonate, which are inferred by the major cation distributions. From one site where we have solid phase 87Sr /86Sr (site U1396), the carbonate fraction is found to be generally consistent with the contemporaneous seawater isotope values. However, the 87Sr /86Sr of the non-carbonate fraction ranges from 0.7074 to 0.7052, and these values likely represent a mixture of local arc volcanic sources and trans-Atlantic eolian sources. Even at this site where there is clear evidence for diagenesis of volcanogenic material, carbonate diagenesis appears to buffer pore fluid 87Sr /86Sr from the larger changes that might be expected given the high abundance of tephra in these sediments. Part of this carbonate buffering, at this site as well as throughout the region, derives from the fact that the Sr concentration in the non-carbonate fraction is generally low (< 200 ppm), whereas the carbonate fraction has Sr concentrations approaching ∼1000 ppm.

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Murray_et_al_2018 - Accepted Manuscript
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Accepted/In Press date: 24 February 2018
e-pub ahead of print date: 5 March 2018
Published date: 1 May 2018

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Local EPrints ID: 418564
URI: http://eprints.soton.ac.uk/id/eprint/418564
ISSN: 0016-7037
PURE UUID: 0764b25f-ad14-4f49-832a-5a641d8b3248

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Date deposited: 12 Mar 2018 17:30
Last modified: 17 Dec 2019 05:33

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