Reverse weathering in marine sediments and the geochemical cycle of potassium in seawater; insights from the K isotopic composition (41K/39K) of deep-sea pore-fluids

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doi: 10.1016/j.gca.2018.02.035
Author(s): Santiago Ramos, Danielle P.; Morgan, Leah E.; Lloyd, Nicholas S.; Higgins, John A.
Author Affiliation(s): Primary:
Princeton University, Department of Geosciences, Princeton, NJ, United States
U.S. Geological Survey, United States
Thermo Fisher Scientific, Germany
Volume Title: Chemistry of oceans past and present; a special issue in tribute to Harry Elderfield
Volume Author(s): Marchitto, Thomas M., editor; de Baar, Hein; Erez, Jonathan; Hodell, David A. V.; Mills, Rachel; Rickaby, Rosalind; Rohling, Eelco
Source: Chemistry of oceans past and present; a special issue in tribute to Harry Elderfield, edited by Thomas M. Marchitto, Hein de Baar, Jonathan Erez, David A. V. Hodell, Rachel Mills, Rosalind Rickaby and Eelco Rohling. Geochimica et Cosmochimica Acta, Vol.236, p.99-120. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. 65 refs.; illus., incl. 4 tables
Summary: 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 ppm. 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 ppm. 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.
Year of Publication: 2018
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Alkali metals; Alkaline earth metals; Aluminum; Atlantic Ocean; Authigenic minerals; Blake Nose; Blake Plateau; Caribbean Sea; Costa Rica Seismogenesis Project; East Pacific; Expedition 334; Expedition 340; Expedition 342; Geochemical cycle; Geochemistry; ICP mass spectra; IODP Site U1378; IODP Site U1395; IODP Site U1403; Integrated Ocean Drilling Program; Isotope ratios; K-41/K-39; Leg 171B; Lithium; Magnesium; Marine sediments; Mass spectra; Metals; North Atlantic; North Pacific; Northeast Pacific; ODP Site 1052; Ocean Drilling Program; One-dimensional models; Pacific Ocean; Potassium; Sea water; Sediments; Spectra; Weathering
Record ID: 2019040421
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands

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