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Assessing oceanic paleo-productivity using silicon isotopes variations in seawater and the biogenic opal sedimentary record

English title Assessing oceanic paleo-productivity using silicon isotopes variations in seawater and the biogenic opal sedimentary record
Applicant Reynolds Ben Christopher
Number 116473
Funding scheme Project funding (Div. I-III)
Research institution Institut für Geochemie und Petrologie ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Geochemistry
Start/End 01.09.2007 - 31.08.2010
Approved amount 186'936.00
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Keywords (12)

marine geochemistry; stable isotope; mass spectrometry; water masses; nutrients; paleoproductivity; oceanography; silicon isotopes; seawater; carbon cycle; biological pump; paleoceanography

Lay Summary (English)

Lead
Lay summary
It has been known for more than twenty years that the concentration of carbon dioxide (CO2) in the Earth’s atmosphere was lower (by about one-third) during ice ages than during interglacial times. It is believed that these variations in atmospheric CO2 have played an important role in magnifying the magnitude of climate change between glacial and interglacial times, since CO2 is a greenhouse gas that causes atmospheric warming.
There is, however, no general agreement as to why atmospheric CO2 has varied, but many scientists believe that the so-called “biological pump” could be the answer. The biological pump is driven by the growth of innumerable microscopic photosynthesising algae in the surface of the oceans, which use the CO2 dissolved in seawater for their growth. When these algae die, a small proportion of them sink through the water column into the deep ocean, where they either decompose to release the carbon incorporated in their tissues to the deep ocean, or are deposited on the seafloor. This process thus ‘pumps’ CO2 from the surface ocean (and ultimately from the atmosphere, which exchanges CO2 with the surface ocean) into the deep ocean and sediment. An increase in the amount of algae growing in the surface ocean (the ‘productivity’ of the ocean), and in the amount of dead algae being exported into the deep ocean, thus removes more CO2 from the surface ocean and the atmosphere, reducing the atmospheric warming effect of CO2.
It is believed that such an increase in oceanic productivity could have taken place in the Southern Ocean around Antarctica during glacial times. Here, oceanic productivity is dominated by diatoms, unicellular algae that form a shell (or frustule) of opal (hydrous silicon dioxide). Diatoms thus require considerable amounts of silicon (Si) in order to grow. The mechanism of formation of the frustules discriminates between the different isotopes of Si, such that the lighter isotopes are preferentially incorporated into the frustules, while the heavier isotopes preferentially remain in the water in which the diatoms grow. Thus, as the diatoms remove more and more Si from seawater, the isotopic composition of the Si remaining in seawater becomes increasingly heavy, which in turn leads to a heavier isotopic composition of newly-growing diatom frustules. Measurement of the Si isotopic composition of diatom frustules thus gives us information about how strongly the diatoms depleted Si in seawater - or in other words, how high the productivity of the surface ocean was.
Creating silicon isotope records of diatoms recovered from sediment cores in the Southern Ocean will thus help us to reconstruct changes in Southern Ocean productivity over glacial-interglacial transitions, an important step in assessing the role of the biological pump in controlling glacial-interglacial variations in atmospheric CO2. This data will also help to further our understanding of the interrelationships between the oceanic and atmospheric systems and their implications for the global carbon cycle, a vital prerequisite for the assessment of future CO2 sequestration strategies.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
130361 Quantifying diatom paleo-productivity using Si isotopes 01.09.2010 Project funding (Div. I-III)
130361 Quantifying diatom paleo-productivity using Si isotopes 01.09.2010 Project funding (Div. I-III)

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