TY - JOUR
T1 - Reverse weathering in marine sediments and the geochemical cycle of potassium in seawater
T2 - Insights from the K isotopic composition (41K/39K) of deep-sea pore-fluids
AU - Santiago Ramos, Danielle P.
AU - Morgan, Leah E.
AU - Lloyd, Nicholas S.
AU - Higgins, John Andrew
N1 - Funding Information:
The authors would like to thank Ian C. Bourg for his invaluable insights on the kinetics of cation exchange and K complexation in clay minerals, Satish C.B. Myneni for discussions on clay mineral structure, and Elizabeth Lundstrom for the experimental data on Ca fractionation discussed in Section 2.5.3. Reviews by Dr. L. Coogan and Dr. L. Lammers are also greatly appreciated and significantly improved this manuscript. All pore-fluid samples used in this study were sourced by the Ocean Drilling Program (ODP) and the Integrated Ocean Drilling Program (IODP). This work was funded through NSF OCE CAREER grant #1654571 to J.A. Higgins. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Funding Information:
The authors would like to thank Ian C. Bourg for his invaluable insights on the kinetics of cation exchange and K complexation in clay minerals, Satish C.B. Myneni for discussions on clay mineral structure, and Elizabeth Lundstrom for the experimental data on Ca fractionation discussed in Section 2.5.3 . Reviews by Dr. L. Coogan and Dr. L. Lammers are also greatly appreciated and significantly improved this manuscript. All pore-fluid samples used in this study were sourced by the Ocean Drilling Program (ODP) and the Integrated Ocean Drilling Program (IODP). This work was funded through NSF OCE CAREER grant # 1654571 to J.A. Higgins. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Publisher Copyright:
© 2018 Elsevier Ltd
PY - 2018/9/1
Y1 - 2018/9/1
N2 - In situ Al-silicate formation, also known as “reverse weathering,” is an important sink of many of the major and minor cations in seawater (e.g. Mg, K, and Li). However, the importance of this sink in global geochemical cycles and isotopic budgets of these elements remains poorly constrained. Here, we report on the potassium isotopic composition (41K/39K) of deep-sea sediment pore-fluids from four (Integrated) Ocean Drilling Program sites (1052, U1378, U1395 and U1403) to characterize potassium isotopic fractionation associated with the formation of authigenic Al-silicate minerals in marine sediments and its role in elevating the 41K/39K of seawater relative to bulk silicate Earth. Isotopic ratios are obtained by high-resolution multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in cold plasma conditions with a long-term external reproducibility of ca. 0.17‰. We find that, although all sites are characterized by pore-fluid K concentrations that decline with increasing depth, their K isotopic profiles vary systematically from site-to-site; at sites characterized by rapid sedimentation rates, pore-fluid profiles of 41K/39K are relatively invariant whereas at sites characterized by slow sedimentation rates, 41K/39K declines with depth by up to 1.8‰. Results from 1-D diffusion-advection-reaction models suggest that these differences may result from a complex interplay between sedimentation rate and fractionation of K isotopes during diffusion, Al-silicate authigenesis, and ion exchange. Model simulations suggest fractionation factors between 0.9980 and 1.0000 for reverse weathering reactions in deep-sea sediments. Although deep-sea sites do not constitute major sinks of K in seawater, some of the processes responsible for K isotopic fractionation at these sites (diffusion and Al-silicate authigenesis) likely play a role in determining the 41K/39K of seawater.
AB - In situ Al-silicate formation, also known as “reverse weathering,” is an important sink of many of the major and minor cations in seawater (e.g. Mg, K, and Li). However, the importance of this sink in global geochemical cycles and isotopic budgets of these elements remains poorly constrained. Here, we report on the potassium isotopic composition (41K/39K) of deep-sea sediment pore-fluids from four (Integrated) Ocean Drilling Program sites (1052, U1378, U1395 and U1403) to characterize potassium isotopic fractionation associated with the formation of authigenic Al-silicate minerals in marine sediments and its role in elevating the 41K/39K of seawater relative to bulk silicate Earth. Isotopic ratios are obtained by high-resolution multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in cold plasma conditions with a long-term external reproducibility of ca. 0.17‰. We find that, although all sites are characterized by pore-fluid K concentrations that decline with increasing depth, their K isotopic profiles vary systematically from site-to-site; at sites characterized by rapid sedimentation rates, pore-fluid profiles of 41K/39K are relatively invariant whereas at sites characterized by slow sedimentation rates, 41K/39K declines with depth by up to 1.8‰. Results from 1-D diffusion-advection-reaction models suggest that these differences may result from a complex interplay between sedimentation rate and fractionation of K isotopes during diffusion, Al-silicate authigenesis, and ion exchange. Model simulations suggest fractionation factors between 0.9980 and 1.0000 for reverse weathering reactions in deep-sea sediments. Although deep-sea sites do not constitute major sinks of K in seawater, some of the processes responsible for K isotopic fractionation at these sites (diffusion and Al-silicate authigenesis) likely play a role in determining the 41K/39K of seawater.
KW - Al-silicate authigenesis
KW - Ion exchange
KW - Marine diagenesis
KW - Pore-fluid chemistry
KW - Potassium cycle
KW - Potassium isotopes
KW - Reverse silicate weathering
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U2 - 10.1016/j.gca.2018.02.035
DO - 10.1016/j.gca.2018.02.035
M3 - Article
AN - SCOPUS:85043997740
SN - 0016-7037
VL - 236
SP - 99
EP - 120
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
ER -