DFG-Schwerpunktprogramm 1158 "Antarktisforschung"

δ¹³C of particulate organic material in the Southern Ocean

Values of Southern Ocean δ¹³C org are frequently lighter than anywhere else, (δ¹³C_org -28 to -35 ‰ vs. values between -16 and -28 ‰). No such light material has been found in corresponding northern latitudes and thus the occurrence of very light material in the Southern Ocean cannot be explained by low temperatures or high CO₂ concentrations alone. This north-south asymmetry (Goericke and Fry, 1994) in δ¹³C_org has been an enigma for more than a decade, and the large variation of δ¹³C_org in the Southern Ocean present a major problem for the application of δ¹³C_org as a paleo-proxy.

Fig. 1: δ¹³C_org of particulate organic matter (POC) in the surface ocean (mixed layer, Goericke and Fry, 1994).

The purpose of this project is to identify the exact cause(s) of the anomalously low and variable Southern Ocean δ¹³C_org and in light of these, assess the reliability of δ¹³C as a paleo-proxy. It is hypothesized that specific phytoplankton species are primarily responsible for the light δ¹³C_org values. We aim to identify the responsible species and the growth conditions leading to the production of such isotopically light material.v Analysis of the size fractionated (5 size classes) samples for PO¹³C, PO¹⁵N, and phytoplankton as well as samples for total alkalinity, DIC and δ¹³CDIC collected from 21 stations during the IPY/Geotraces expedition “Zero and Drake” in the Southern Ocean (ANT-XXIV/3 with RV “Polarstern”) in spring 2008 is almost complete.

Isotopically extremely light values (δ¹³C_org less than -28 ‰) were consistently observed at stations south of the Polar Front, with values δ¹³C_org less than -30 ‰ in at least one size fraction at 4 stations. The lightest values were usually observed in the size class more than 100 µm, whereas the heaviest (highest) δ¹³C signal was found in the 5 µm to 20 µm fraction, with lower values in the size fractions less than 5 µm. The fractionations ε_p indicate that the produced organic carbon was appreciably lighter (+ 14 to 20 ‰) than the source DIC implying that biology is a key factor responsible for the isotope ratios. For example, at station PS71/150-1, the size fraction more than 100 µm, where the isotopic signal was extremely light (δ¹³C_org = -30.6 ‰), was dominated by Chaetoceros spp. and Phaeocystis colonies (40 % and 28 %, respectively), whereas the 5 µm to 20 µm size fraction, which had a “more normal” signal of δ¹³C_org = -25.9 ‰, consisted of 65 % Fragilariopsis spp. and of 14 % tintinids.

Moreover, culture experiments with the key Southern Ocean diatoms and Phaeocystis antarctica were conducted to confirm experimentally that species specific differences in fractionation of ¹³C /¹²C during growth and photosynthesis may explain the anomalous signals observed in the Southern Ocean. Interpretation of these results is premature, because the needed DIC signals are currently being analysed.

Phyto- and Protozooplankton composition in the (A) more than 100 µm and (B) the 5 µm to 20 µm size fraction in the chlorophyll maximum at station PS71/150-1, the southern most station of the Drake transect. The isotopic signal was extremely light (-30.6 ‰) in the size fraction more than 100 µm and the size fraction was dominated by Chaetoceros spp. and Phaeocystis colonies comprising 40 % and 28 % of the community respectively. In contrast, the 5 µm to 20 µm size fraction at the same station, which had a “more normal” signal of -25.9 ‰, consisted of 65 % Fragilariopsis spp.

Scientists

Uta Passow (Alfred Wegener Institute, Bremerhaven)

Dieter Wolf-Gladrow (Alfred Wegener Institute, Bremerhaven)

Elizabeth Sweet (Alfred Wegener Institute, Bremerhaven)

Research areas

Cruise participation: Polarstern 2008 ANT 24/3 “ZERO and DRAKE” - Geotraces expedition in the International Polar Year.

Publications

De La Rocha CL, Passow U, 2004. Recovery of Thalassiosira weissflogii from nitrogen and silicon limitation. Limnology and Oceanography 49, 245-255.

Garvey M, Moriceau B, Passow U, 2007. Applicability of the FDA assay to determine the viability of marine phytoplankton under different environmental conditions. Marine Ecology Progress Series 352, 17-26.

Passow U, 1991. Species specific sedimentation and sinking velocities of diatoms. Marine Biology 108, 449-455.

Rau GH, Riebesell U, Wolf-Gladrow D, 1996. A model of photosynthetic ¹³C fractionation by marine phytoplankton based on diffusive molecular CO₂ uptake, Mar. Ecol. Prog. Ser., 133, 275-285.

Zeebe RE, Wolf-Gladrow D, 2001. CO₂ in Seawater: Equilibrium, Kinetics, Isotopes, Elsevier Oceanography Book Series, 65, 346 pp, Amsterdam.

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