Leah E. Morgan
Nicholas S. Lloyd
John A. Higgins
Danielle P. Santiago Ramos
2018
<p><span>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 (</span><sup><span class="math"><span id="MathJax-Element-1-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">41</mn></mrow></msup></mrow></math>"><span class="MJX_Assistive_MathML">41</span></span></span></sup><span>K/</span><sup><span class="math"><span id="MathJax-Element-2-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">39</mn></mrow></msup></mrow></math>"><span class="MJX_Assistive_MathML">39</span></span></span></sup><span>K) of<span> deep-sea sediment</span><span><span><span> </span>pore-fluids from four (Integrated)<span> Ocean Drilling Program</span><span> </span>sites (1052, U1378, U1395 and U1403) to characterize potassium isotopic fractionation</span><span><span> </span>associated with the formation of authigenic Al-silicate minerals in<span> marine sediments</span><span> </span>and its role in elevating the<span> </span></span></span></span><sup><span class="math"><span id="MathJax-Element-3-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">41</mn></mrow></msup></mrow></math>"><span class="MJX_Assistive_MathML">41</span></span></span></sup><span>K/</span><sup><span class="math"><span id="MathJax-Element-4-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">39</mn></mrow></msup></mrow></math>"><span class="MJX_Assistive_MathML">39</span></span></span></sup><span><span><span>K of seawater relative to bulk<span> silicate Earth. Isotopic ratios</span></span><span> </span>are obtained by high-resolution multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in<span> cold plasma</span></span><span><span> </span>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<span> sedimentation rates</span>, pore-fluid profiles of<span> </span></span></span><sup><span class="math"><span id="MathJax-Element-5-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">41</mn></mrow></msup></mrow></math>"><span class="MJX_Assistive_MathML">41</span></span></span></sup><span>K/</span><sup><span class="math"><span id="MathJax-Element-6-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">39</mn></mrow></msup></mrow></math>"><span class="MJX_Assistive_MathML">39</span></span></span></sup><span>K are relatively invariant whereas at sites characterized by slow sedimentation rates,<span> </span></span><sup><span class="math"><span id="MathJax-Element-7-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">41</mn></mrow></msup></mrow></math>"><span class="MJX_Assistive_MathML">41</span></span></span></sup><span>K/</span><sup><span class="math"><span id="MathJax-Element-8-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">39</mn></mrow></msup></mrow></math>"><span class="MJX_Assistive_MathML">39</span></span></span></sup><span><span>K 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<span> authigenesis</span><span>, and ion exchange. Model simulations suggest fractionation factors between 0.9980 and 1.0000 for reverse weathering reactions in<span> deep-sea</span><span> </span>sediments. Although deep-sea sites do not constitute major sinks of K in seawater, some of the processes responsible for K<span> isotopic fractionation </span></span></span>at these sites (diffusion and Al-silicate authigenesis) likely play a role in determining the<span> </span></span><sup><span class="math"><span id="MathJax-Element-9-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">41</mn></mrow></msup></mrow></math>"><span class="MJX_Assistive_MathML">41</span></span></span></sup><span>K/</span><sup><span class="math"><span id="MathJax-Element-10-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">39</mn></mrow></msup></mrow></math>"><span class="MJX_Assistive_MathML">39</span></span></span></sup><span>K of seawater.</span></p>
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10.1016/j.gca.2018.02.035
en
Elsevier
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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