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<front>
<journal-meta>
<journal-id journal-id-type="publisher">EGUsphere</journal-id>
<journal-title-group>
<journal-title>EGUsphere</journal-title>
<abbrev-journal-title abbrev-type="publisher">EGUsphere</abbrev-journal-title>
<abbrev-journal-title abbrev-type="nlm-ta">EGUsphere</abbrev-journal-title>
</journal-title-group>
<issn pub-type="epub"></issn>
<publisher><publisher-name>Copernicus Publications</publisher-name>
<publisher-loc>Göttingen, Germany</publisher-loc>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="doi">10.5194/egusphere-2025-2493</article-id>
<title-group>
<article-title>Balancing water column and sedimentary &lt;sup&gt;234&lt;/sup&gt;Th fluxes to quantify coastal marine carbon export</article-title>
</title-group>
<contrib-group><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Healey</surname>
<given-names>Madeline G.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Black</surname>
<given-names>Erin E.</given-names>
<ext-link>https://orcid.org/0000-0003-2781-7531</ext-link>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Algar</surname>
<given-names>Christopher K.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Armstrong</surname>
<given-names>Maria</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Kienast</surname>
<given-names>Stephanie S.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
</contrib-group><aff id="aff1">
<label>1</label>
<addr-line>Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada</addr-line>
</aff>
<aff id="aff2">
<label>2</label>
<addr-line>Department of Earth and Environment, University of Rochester, Rochester, New York, USA</addr-line>
</aff>
<pub-date pub-type="epub">
<day>17</day>
<month>06</month>
<year>2025</year>
</pub-date>
<volume>2025</volume>
<fpage>1</fpage>
<lpage>27</lpage>
<permissions>
<copyright-statement>Copyright: &#x000a9; 2025 Madeline G. Healey et al.</copyright-statement>
<copyright-year>2025</copyright-year>
<license license-type="open-access">
<license-p>This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit <ext-link ext-link-type="uri"  xlink:href="https://creativecommons.org/licenses/by/4.0/">https://creativecommons.org/licenses/by/4.0/</ext-link></license-p>
</license>
</permissions>
<self-uri xlink:href="https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2493/">This article is available from https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2493/</self-uri>
<self-uri xlink:href="https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2493/egusphere-2025-2493.pdf">The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2493/egusphere-2025-2493.pdf</self-uri>
<abstract>
<p>Quantitative estimates of particulate organic carbon (POC) flux and burial in coastal systems are critical for constraining coastal carbon budgets and understanding their role in regional and global carbon cycling. In this study, POC export fluxes were quantified in the Bedford Basin, a coastal inlet in the Northwest Atlantic, based on measurements of &lt;sup&gt;234&lt;/sup&gt;Th/&lt;sup&gt;238&lt;/sup&gt;U disequilibria and the POC:&lt;sup&gt;234&lt;/sup&gt;Th ratio on small (1&amp;ndash;51 &amp;micro;m) and large (&amp;gt; 51 &amp;micro;m) particles. These water column export fluxes were compared to sediment accumulation fluxes of &lt;sup&gt;234&lt;/sup&gt;Th and POC to refine the carbon budget in the Bedford Basin. The coupled water column-surficial sediment sampling approach, which is relatively new, was applied quasi-seasonally throughout 2021&amp;ndash;2024, including for the first time in boreal winter. Total &lt;sup&gt;234&lt;/sup&gt;Th activities reveal persistent deficits with respect to &lt;sup&gt;238&lt;/sup&gt;U throughout the water column, likely due to extensive particle scavenging, which is also indicated by high &lt;sup&gt;234&lt;/sup&gt;Th activities on particles. Here, we find that the removal of &lt;sup&gt;234&lt;/sup&gt;Th in the water column is typically balanced, within uncertainties, by the inventory of excess &lt;sup&gt;234&lt;/sup&gt;Th in underlaying marine sediments. This finding reveals that on the timescale of ~100 days, the &lt;sup&gt;234&lt;/sup&gt;Th budget in the Bedford Basin is in balance, and no major particle loss is occurring. Using the POC:&lt;sup&gt;234&lt;/sup&gt;Th ratio on sinking particles and integrated &lt;sup&gt;234&lt;/sup&gt;Th water column fluxes, we report a mean (&amp;plusmn; s.d.) depositional flux of 20 &amp;plusmn; 14 mmol C m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt; (range 3.6 to 44.5 mmol C m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;) to the seafloor. This mean flux translates to an annual molar flux of 7.2 &amp;plusmn; 0.7 mol C m&lt;sup&gt;-2&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;, within a factor of 2 to model estimates and previous sediment trap results at this site. Our findings contribute to ongoing research efforts in the Bedford Basin, and aid in the evaluation of coastal regions in local carbon budgets.</p>
</abstract>
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