Collaborative Proposal: A Field and Laboratory Examination of the Diatom N and S Isotope Proxies: Implications for Assessing the Southern Ocean Biological Pump

Award Period
Award Amount
Agency Name
National Science Foundation
Award Number
PI First Name
PI Last Name
MSI People
Area/s of Research

Project Summary

Overview: Diatom nitrogen and silicon isotopes, δ15NDB and δ30SiBSi, respectively, are important

paleoceanographic tools used to investigate the role of the Southern Ocean biological pump in

regulating atmospheric CO2 concentrations. Existing calibrations, including culture experiments,

surface sediment data and downcore reconstructions, all suggest that nutrient utilization is the

primary driver of δ15NDB and δ30SiBSi in the Southern Ocean. However, a strong species effect is

implied by recent culture results. Moreover, field and laboratory studies present contradictory

results on the relative importance of interspecific variation and diagenetic alteration of the

nutrient isotope signals to isotope records.

We propose a first-order test of the N and Si nutrient paleo-proxies using both field and

laboratory studies. Field work will evaluate species effects at 13 stations across the Antarctic

Circumpolar Current measuring the concentration and isotopic composition of nutrients, bulk

particulates, water-column diatom frustules and sedimentary diatoms relative to changes in

diatom species composition to investigate species-related variability in fractionation and its

relationship to surface nutrient fields. Field work will also address alteration during early

diagenesis by collecting multicores at each station. For N we will test the hypothesis that

observed isotopic differences between fresh material and sedimentary material reflects

syndepositional processing that fractionates diatom-bound organic matter. For Si isotopes we

will investigate alteration of the primary isotope signal by dissolution in the water column and in

sediments to determine the relative importance of congruent dissolution of all frustules versus

the complete loss of some species on sedimentary δ30SiBSi. Culture studies will evaluate the

importance of interspecific variation in N and Si isotope fractionation to sediment records by

targeting diatom species that dominate Southern Ocean sediment records.

Intellectual Merit: Nitrogen and silicon isotope reconstructions of nutrient utilization in the

Southern Ocean provide key constraints on the biological pump and large scale overturning

circulation changes. The proposed expedition will improve reconstructions of the nutrient status

of the Southern Ocean through quantification of in-situ isotopic relationships and culture studies

will evaluate species effects for sedimentologically important diatoms. The main outcomes will

be a catalog of species-specific culture-based estimates of the degree of Si and N fractionation

during frustule formation, assemblage-based estimates of the same fractionations for the

important zones of the Southern Ocean and a greater understanding of how the culture results

relate to in-situ observations and sedimentary reconstructions.

Broader Impacts: Reconstructing the magnitude of glacial-interglacial changes in the Southern

Ocean biological pump and its potential role in observed pCO2 changes is of great interest, to not

only the paleoclimate/paleoceanographic community, but to the larger earth sciences community

as a whole. Improving the nutrient isotope proxies may allow for a more quantitative

understanding of the role of polar biology in glacial-interglacial CO2 change. The proposed

international collaboration with diatom taxonomist Dr. Christina Riesselman of the University of

Otago (New Zealand), will foster the career of a young female scientist (Riesselman) and

develop a potentially fruitful collaboration between a diatom taxonomist and diatom isotope

specialists. This work will provide training and research opportunities for undergraduate and

graduate students at the University of Rhode Island and University of California Santa Barbara.

Public interest in STEM disciplines in general and in climate and polar regions specifically will

be fostered in a cooperative program with Rhode Island schools and through the development of

a K-12 teaching module for UCSB’s Oceans2Classroom program.