Seasonal and spatial dynamics of 234 Th/ 238 U disequilibria in southern Lake Michigan

Seasonal and spatial dynamics of 234 Th/ 238 U disequilibria in southern Lake Michigan

Measurements of 234 Th/ 238 U disequilibria were made on an approximately bimonthly basis over the course of a year in nearshore (10–40 m deep) and offshore (160 m deep) surface waters of the southern basin of Lake Michigan (18,100 km 2 ). The mean activity of 238 U in Lake Michigan measured 230 ± 2...

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Veröffentlicht in:Journal of Geophysical Research: Oceans 2004-10, Vol.109 (C10)
Hauptverfasser: Waples, James T., Orlandini, Kent A., Edgington, David N., Klump, J. Val
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container_title Journal of Geophysical Research: Oceans
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creator Waples, James T.
Orlandini, Kent A.
Edgington, David N.
Klump, J. Val
description Measurements of 234 Th/ 238 U disequilibria were made on an approximately bimonthly basis over the course of a year in nearshore (10–40 m deep) and offshore (160 m deep) surface waters of the southern basin of Lake Michigan (18,100 km 2 ). The mean activity of 238 U in Lake Michigan measured 230 ± 20 dpm m −3 , approximately 1 order of magnitude lower than what is typically found in marine systems. Measured median activities of dissolved (
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Val</creator><creatorcontrib>Waples, James T. ; Orlandini, Kent A. ; Edgington, David N. ; Klump, J. Val</creatorcontrib><description>Measurements of 234 Th/ 238 U disequilibria were made on an approximately bimonthly basis over the course of a year in nearshore (10–40 m deep) and offshore (160 m deep) surface waters of the southern basin of Lake Michigan (18,100 km 2 ). The mean activity of 238 U in Lake Michigan measured 230 ± 20 dpm m −3 , approximately 1 order of magnitude lower than what is typically found in marine systems. Measured median activities of dissolved (&lt;0.45 μm) and excess particle‐bound 234 Th were 8.0 and 60.6 dpm m −3 , respectively. Using a simple one‐dimensional model, median residence times for dissolved and particle‐bound 234 Th were ∼1 and ∼14 days, respectively. 234 Th‐based particle settling velocities had a median value of 0.4 m d −1 (range: ∼0.0–1.4 m d −1 ), and instantaneous 234 Th‐based estimates of the net vertical mass flux had a median value of 0.4 g m −2 d −1 (range: ∼0.0–5.2 g m −2 d −1 ). Average particle settling velocities were generally constant over time and increased only slightly in the shallowest (10 m) sampling stations. Calculated mass fluxes showed a strong correlation with temporal and spatial changes in the concentration of total suspended matter, which, in the nearshore area, closely followed seasonal variations in wind‐induced wave height. Using a simple two‐box model, the 234 Th‐based cross‐margin mass export rate for the entire southern basin of Lake Michigan was equal to 1.35 × 10 9 kg yr −1 , which is in excellent agreement with a 210 Pb − 137 Cs based, basin‐wide mass sedimentation rate of 1.28 × 10 9 kg yr −1 .</description><identifier>ISSN: 0148-0227</identifier><identifier>DOI: 10.1029/2003JC002204</identifier><language>eng</language><ispartof>Journal of Geophysical Research: Oceans, 2004-10, Vol.109 (C10)</ispartof><lds50>peer_reviewed</lds50><woscitedreferencessubscribed>false</woscitedreferencessubscribed><citedby>FETCH-LOGICAL-c804-e12e6d071ce25336e9a1ae66eb3b5ac4a1cb316a45bfd9c6ac80b14a2a0594063</citedby><cites>FETCH-LOGICAL-c804-e12e6d071ce25336e9a1ae66eb3b5ac4a1cb316a45bfd9c6ac80b14a2a0594063</cites></display><links><openurl>$$Topenurl_article</openurl><openurlfulltext>$$Topenurlfull_article</openurlfulltext><thumbnail>$$Tsyndetics_thumb_exl</thumbnail><link.rule.ids>314,776,780,27901,27902</link.rule.ids></links><search><creatorcontrib>Waples, James T.</creatorcontrib><creatorcontrib>Orlandini, Kent A.</creatorcontrib><creatorcontrib>Edgington, David N.</creatorcontrib><creatorcontrib>Klump, J. Val</creatorcontrib><title>Seasonal and spatial dynamics of 234 Th/ 238 U disequilibria in southern Lake Michigan</title><title>Journal of Geophysical Research: Oceans</title><description>Measurements of 234 Th/ 238 U disequilibria were made on an approximately bimonthly basis over the course of a year in nearshore (10–40 m deep) and offshore (160 m deep) surface waters of the southern basin of Lake Michigan (18,100 km 2 ). The mean activity of 238 U in Lake Michigan measured 230 ± 20 dpm m −3 , approximately 1 order of magnitude lower than what is typically found in marine systems. Measured median activities of dissolved (&lt;0.45 μm) and excess particle‐bound 234 Th were 8.0 and 60.6 dpm m −3 , respectively. Using a simple one‐dimensional model, median residence times for dissolved and particle‐bound 234 Th were ∼1 and ∼14 days, respectively. 234 Th‐based particle settling velocities had a median value of 0.4 m d −1 (range: ∼0.0–1.4 m d −1 ), and instantaneous 234 Th‐based estimates of the net vertical mass flux had a median value of 0.4 g m −2 d −1 (range: ∼0.0–5.2 g m −2 d −1 ). Average particle settling velocities were generally constant over time and increased only slightly in the shallowest (10 m) sampling stations. Calculated mass fluxes showed a strong correlation with temporal and spatial changes in the concentration of total suspended matter, which, in the nearshore area, closely followed seasonal variations in wind‐induced wave height. 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Val</creatorcontrib><collection>CrossRef</collection><jtitle>Journal of Geophysical Research: Oceans</jtitle></facets><delivery><delcategory>Remote Search Resource</delcategory><fulltext>fulltext</fulltext></delivery><addata><au>Waples, James T.</au><au>Orlandini, Kent A.</au><au>Edgington, David N.</au><au>Klump, J. Val</au><format>journal</format><genre>article</genre><ristype>JOUR</ristype><atitle>Seasonal and spatial dynamics of 234 Th/ 238 U disequilibria in southern Lake Michigan</atitle><jtitle>Journal of Geophysical Research: Oceans</jtitle><date>2004-10</date><risdate>2004</risdate><volume>109</volume><issue>C10</issue><issn>0148-0227</issn><abstract>Measurements of 234 Th/ 238 U disequilibria were made on an approximately bimonthly basis over the course of a year in nearshore (10–40 m deep) and offshore (160 m deep) surface waters of the southern basin of Lake Michigan (18,100 km 2 ). The mean activity of 238 U in Lake Michigan measured 230 ± 20 dpm m −3 , approximately 1 order of magnitude lower than what is typically found in marine systems. Measured median activities of dissolved (&lt;0.45 μm) and excess particle‐bound 234 Th were 8.0 and 60.6 dpm m −3 , respectively. Using a simple one‐dimensional model, median residence times for dissolved and particle‐bound 234 Th were ∼1 and ∼14 days, respectively. 234 Th‐based particle settling velocities had a median value of 0.4 m d −1 (range: ∼0.0–1.4 m d −1 ), and instantaneous 234 Th‐based estimates of the net vertical mass flux had a median value of 0.4 g m −2 d −1 (range: ∼0.0–5.2 g m −2 d −1 ). Average particle settling velocities were generally constant over time and increased only slightly in the shallowest (10 m) sampling stations. Calculated mass fluxes showed a strong correlation with temporal and spatial changes in the concentration of total suspended matter, which, in the nearshore area, closely followed seasonal variations in wind‐induced wave height. Using a simple two‐box model, the 234 Th‐based cross‐margin mass export rate for the entire southern basin of Lake Michigan was equal to 1.35 × 10 9 kg yr −1 , which is in excellent agreement with a 210 Pb − 137 Cs based, basin‐wide mass sedimentation rate of 1.28 × 10 9 kg yr −1 .</abstract><doi>10.1029/2003JC002204</doi></addata></record>
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title Seasonal and spatial dynamics of 234 Th/ 238 U disequilibria in southern Lake Michigan
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