Geochemical evidence for active tropical serpentinization in the Santa Elena Ophiolite, Costa Rica: An analog of a humid early Earth?. (14th May 2014)

• Record Type: Journal Article
• Authors: Sánchez‐Murillo, Ricardo; Gazel, Esteban; Schwarzenbach, Esther M.; Crespo‐Medina, Melitza; Schrenk, Matthew O.; Boll, Jan; Gill, Ben C.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 15:Number 5(2014:May)
• Page Start: 1783
• Page End: 1800
• Publication Date: 2014-05-14
• Subjects: Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1002/2013GC005213
• Languages: English
• ISSNs: 1525-2027

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>Serpentinization is a planetary process that has important consequences on geochemical cycles, supporting microbial activity through the formation of H<sub>2</sub> and CH<sub>4</sub> and having the potential to sequester atmospheric CO<sub>2</sub>. We present geochemical evidence of active serpentinization in the Santa Elena Ophiolite, Costa Rica which is sustained by peridotites with a degree of serpentinization less than 50% with no evidence of an internal heat source. Average spring water temperatures are 29.1°C. Two hyperalkaline spring systems were discovered, with a spring fluid pH up to 11.18. The fluids are characterized by low Mg (1.0–5.9 mg/L) and K (1.0–5.5 mg/L) and relative high Ca (29–167 mg/L), Na (16–27 mg/L), Cl (26–29 mg/L), hydroxide (41–63 mg/L), and carbonate (31–49 mg/L). Active CH<sub>4</sub> (24.3% v/v) vents coupled with carbonate deposits ( <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pghmgm1th2" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:15252027:media:ggge20449:ggge20449-math-0001" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi mathvariant="normal">δ</mml:mi><mml:mrow><mml:mn>13</mml:mn></mml:mrow></mml:msup><mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:mtext>CO2</mml:mtext></mml:msub><mml:mo> </mml:mo><mml:mo>=</mml:mo><mml:mo>−</mml:mo><mml:mn>27</mml:mn><mml:mo> </mml:mo><mml:mtext>to</mml:mtext><mml:mo> </mml:mo><mml:mo>−</mml:mo><mml:mn>14</mml:mn><mml:mo>‰</mml:mo></mml:mrow></mml:math></alternatives></inline-formula>; <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pghmgm1tf1" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:15252027:media:ggge20449:ggge20449-math-0003" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi mathvariant="normal">δ</mml:mi><mml:mrow><mml:mn>18</mml:mn></mml:mrow></mml:msup><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mtext>CO2</mml:mtext></mml:msub><mml:mo> </mml:mo><mml:mo>=</mml:mo><mml:mo>−</mml:mo><mml:mn>17</mml:mn><mml:mo> </mml:mo><mml:mtext>to</mml:mtext><mml:mo> </mml:mo><mml:mo>−</mml:mo><mml:mo> </mml:mo><mml:mn>6</mml:mn><mml:mo>‰</mml:mo></mml:mrow></mml:math></alternatives></inline-formula>) also provide evidence for active serpentinization and carbonation. Isotope ratios of the alkaline fluids (δ<sup>18</sup>O = −7.9‰, δ<sup>2</sup>H = −51.4‰) and groundwater (δ<sup>18</sup>O = −7.6‰; δ<sup>2</sup>H = −48.0‰) suggests that, during base flow recession, springs are fed by groundwater circulation. Methanogenic Archaea, which comprises a relatively high percentage of the 16S rRNA gene tag sequences, suggests that biological methanogenesis may play a significant role in the system. Santa Elena's extreme varying weather results in a scenario that could be of significant importance for (a) improving the knowledge of conditions on a humid early Earth or Mars that had periodic changes in water supply, (b) revealing new insights on serpentinizing solute transport, and (c) modeling hydrogeochemical responses as a function of recharge.</p> </abstract>