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Quantifying diatom paleo-productivity using Si isotopes

English title Quantifying diatom paleo-productivity using Si isotopes
Applicant Reynolds Ben Christopher
Number 130361
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 01.09.2010 - 31.08.2011
Approved amount 61'920.00
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All Disciplines (2)

Discipline
Geochemistry
Oceanography

Keywords (4)

paleoproductivty; stable isotope; mass-spectrometry; stable isotopes

Lay Summary (English)

Lead
Lay summary
Marine diatoms play a major role in carbon fixation, thereby sequestering atmospheric carbon dioxide (CO2) into the deep-sea. However, the effect of changes in diatom productivity on the modern carbon cycle and past variations in pCO2 is poorly known. Diatoms extract dissolved nutrients, especially at high-latitudes like the Southern Ocean, and also require silicon (Si) for the formation of their biogenic silica (opal) frustules. The formation of biogenic opal leads to a mass-dependent Si isotope fractionation, such that the Si isotope composition of seawater and biogenic opal reflects the degree of Si utilization in ocean surface waters by siliceous algae. The magnitude of productivity in the oceans is a product of this biological nutrient utilization and physical vertical mixing of waters that supply nutrients to the surface ocean. Hence, the Si isotope composition of diatoms can be used to trace changes in diatom productivity and help elucidate the links between marine productivity and climate change.Quantifying the modern Si cycle enables characterization of the dynamical interactions of circulation, mixing, seasonality, utilization and export productivity. Such interactions must be taken into account for quantitative estimates of paleo-proxies, and were investigated within the previous SNF funded project 200021-116473 ("Assessing oceanic paleo-productivity using silicon isotope variations in seawater and the biogenic opal sedimentary record", from 01.09.07 to 31.08.2010). This project will follow-up the previous work and will continue supporting the PhD student, Gregory de Souza, for his final (4th) year. We expect to quantify how the interplay of global circulation and surface productivity affects the oceanic Si and Si isotope distributions, and thus answer the question of "how does the interaction between physical circulation and biological uptake control the paleoceanographic proxy record", and "what constraints does this place of changes in diatom productivity place on the global carbon cycle during climatic oscillations".
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

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Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
V. M. Goldschmidt Conference 2011 15.08.2011 Prague, Czech Republic


Associated projects

Number Title Start Funding scheme
116473 Assessing oceanic paleo-productivity using silicon isotopes variations in seawater and the biogenic opal sedimentary record 01.09.2007 Project funding (Div. I-III)
116473 Assessing oceanic paleo-productivity using silicon isotopes variations in seawater and the biogenic opal sedimentary record 01.09.2007 Project funding (Div. I-III)

Abstract

Marine diatoms play a major role in carbon fixation, thereby sequestering CO2 into the deep-sea. However, the role of changes in diatom productivity on the modern carbon cycle and past variations in pCO2 is poorly known. Diatoms extract dissolved nutrients, especially at high-latitudes like the Southern Ocean, but require silicon (Si) for the formation of their biogenic silica (opal) frustules. The formation of biogenic opal leads to a mass-dependent Si isotope fractionation, such that the Si isotope composition of seawater and biogenic opal reflects the degree of Si utilization in ocean surface waters by siliceous algae. The magnitude of productivity in the oceans is a product of this biological nutrient utilization and physical vertical mixing of waters that supply nutrients as part of the oceanic circulation. Hence, the Si isotope composition of diatoms can be used to trace changes in diatom productivity and help elucidate the links between marine productivity and climate change. This projects aims at exploiting the potential of this proxy to understand global climate change in the geological record.Once the mechanisms controlling the modern Si isotope distribution and the fidelity of the opal composition of diatoms to reflect Si utilization have been characterized, it will be possible to quantify how past changes in Si utilization are linked to the efficiency of diatom production and CO2 draw-down. To date, the few published paleoceanographic Si isotope records infer the degree of Si utilization assuming a simplistic fractionation model that is unaffected by ocean circulation and mixing. The dynamical interaction of circulation, mixing, seasonality, utilization and export productivity on regional and global scales, should be taken into account for quantitative estimates of nutrient utilization and paleo-productivity from such records. This dynamical interaction can be assessed by modeling the modern marine Si cycle. We are currently evaluating how the modern oceanic nutrient dynamics (amount of nutrient recycling and export) controls the Si isotope distribution in the Atlantic Ocean and the composition within the underlying sediments, within an on-going project entitled “Assessing oceanic paleo-productivity using silicon isotope variations in seawater and the biogenic opal sedimentary record” (SNF funded project 200021-116473, from 01.09.07 to 31.08.2010). The project proposed here will follow-up this work and will continue supporting the PhD student, Gregory de Souza, for his final (4th) year, and then support a new post-doctoral researcher to further exploit the use of Si isotope records to understand climate change. The project will continue to take place within the Isotope Geochemistry Group at ETH Zurich led by Prof. B. Bourdon, and under the supervision of Dr. Ben Reynolds. Whilst Gregory de Souza is focusing on the modern oceanographic distribution of Si isotopes, the post-doctoral researcher will measure the Si isotope variations of diatoms from marine sediment cores in order to reconstruct past changes in silicon utilization, and estimate paleo-productivity using geochemical models based upon the currently funded research. This post-doctoral researcher will work alongside an ETH funded PhD student, Florian Wetzel, who is investigating paleo-records of Si utilization in complementary studies focusing on a different region.The key questions we will address are as follows:(i)How does the interaction between physical circulation and biological uptake control the Si isotope distribution in the modern oceans, and opal sediments under differing conditions?(ii)Does an increased supply of Si to low-latitudes during glacial periods lead to increased diatom production and draw-down of CO2, thereby proving the “Silicic Acid Leakage Hypothesis” (Matsumoto et al., 2002)? We expect to deliver the following outcomes: (i) quantify how the global thermohaline circulation and surface water productivity affects the Si and Si isotope distributions; (ii) generate new paleoceanographic records of Si utilization over glacial-interglacial timescales, outside the Antarctic Zone of the Southern Ocean; and (iii) establish Si isotopes as a routine paleoceanographic proxy. At each stage, we will present our results at conferences and publish them in international journals, in order to ensure the widest dissemination of our results.
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