Interdecadal changes in intensity of the oxygen minimum zone off Concepción, Chile (∼ 36° S), over the last century

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Abstract

We reconstructed oxygenation changes in the upwelling ecosystem off Concepción (36° S), Chile, using inorganic and organic proxies in a sediment core covering the last ca. 110 years of sedimentation in this area. Authigenic enrichments of Mo, U and Cd were observed between ca. 1935 and 1971 CE, implying a prolonged period with predominantly more reduced conditions in bottom waters and surface sediments. Significant positive correlations between redox-sensitive metals, algal sterols, biomarkers of micro-aerophilic and anaerobic microorganisms, and archaeal glycerol dialkyl glycerol tetraethers point to a tight coupling among bottom water O2 depletion and increased primary and export production. The time interval with low O2 of ca. 35 years seems to follow low-frequency interdecadal variation of the Pacific Decadal Oscillation, and it may have resulted in O2 depletion over the entire continental shelf off Concepción. Taking this together with the concurrent increase in sedimentary molecular indicators of micro-aerophilic and anaerobic microbes, we can suggest that changes in oxygenation of the water column are reflected by changes in microbial community. This study can inform our understanding of ecological consequences to projected trends in ocean deoxygenation.

Figures

  • Table 1. Lipid biomarkers used in this study and their paleobiological interpretation.
  • Figure 1. Location of sampling site Station 18 in the upwelling ecosystem off Concepción, central-southern Chile. Bathymetry in shades of blue, scale on right-hand side.
  • Figure 2. Geochronology estimated from 210Pbxs inventories (black line) and 14C measurements ± standard deviation. Ages are years before present (1950). Dotted line shows fitted values from curve (r2= 0.99).
  • Figure 3. Downcore excess content (ppm) of redox-sensitive metals (a) Mo, (b) U, and (c) Cd. Shaded area and black bar correspond to a period of ca. 35 years of enhanced authigenic precipitation of redox-sensitive metals compared to periods of higher oxygenation (white bars) and low authigenic precipitation. CE: Common Era. Samples for interval 1957–1969 were lost.
  • Table 2. Spearman rank order correlations. Significant values (p < 0.05) are highlighted in bold.
  • Figure 4. Downcore contents of (a) sterols; (b) archaeal GDGTs; (c) 17α-22, 29, 30-trinorhopene (C27 TNH); (d) 17β,21β-homohopanol (C31 hopanol); (e) 17β, 21β-bishomohopanol (C32 hopanol); (f) MAGEs; and (g) Pacific Decadal Oscillation (PDO) index (http://jisao. washington.edu/pdo/PDO.latest). Units are micrograms per gram dry weight. Shaded area and black bar as in Fig. 3. Gaps in the record indicate that biomarker content was under detection limit.
  • Table 3. Molecules identified in m/z 191 mass chromatogram of aliphatic hydrocarbon and alcohol fractions from shelf sediments off Concepción (36◦ S).

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APA

Srain, B., Pantoja, S., Sepúlveda, J., Lange, C. B., Muñoz, P., Summons, R. E., … Salamanca, M. (2015). Interdecadal changes in intensity of the oxygen minimum zone off Concepción, Chile (∼ 36° S), over the last century. Biogeosciences, 12(20), 6045–6058. https://doi.org/10.5194/bg-12-6045-2015

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