Controls on and paleoceanographic utility of the valve size frequency distribution of the Southern Ocean Diatom Fragilariopsis kerguelensis

Unraveling the role of the Southern Ocean's biological pump in regulating climate would be enhanced by the development of paleoceanographic proxies specific to carbon or silica cycling. Observations that the average size of valves of the diatom, Fragilariopsis kerguelensis, varies seasonally, with latitude, and over glacial-interglacial cycles in the Southern Ocean suggest that the valve size frequency distribution in sediments could be used to reconstruct aspects of paleoproductivity and silica cycling.



Fig.A: Diatoms underly a size reduction through mitotic division

We aim to develop this proxy by using culture, field and sediment samples to determine the exact controls on valve size frequency distributions, the most important of which is likely to be the frequency of auxospore formation within the population. The controls on auxospore formation are as yet unknown and both mating experiments in culture as well as in situ iron fertilization experiments are likely to unravel the mechanisms determining auxospore formation under both controlled and field conditions. The valve size frequency distribution can then be used to indicate the intensity of the environmental and/or biological conditions triggering auxospore formation.



Fig.B: Average valve area of F. kerguelensis present in surface sediments in the Southern Ocean. The lines show the position of the winter sea ice limit (blue), the polar front (pink), the subantarctic front (yellow), and the subtropical front (red).



Fig.C: Visual depiction of average-sized F. kerguelensis specimens versus latitude along a north-south transect at 30^oE. Examples of the morphological parameters measured are also shown; L is valve length, W is width, and 5C is the length of five costae. Oceanic front positions (name and color coding as in Fig.B) are also shown. Figure has been redrawn from Cortese and Gersonde, 2007.



Fig.D: Average F. kerguelensis valve area during the Holocene interglacial and the peak of the last glacial.

Scientists

Victor Smetacek
Philipp Assmy
Nike Fuchs
Rainer Gersonde
Alfred Wegener Institute for Polar and Marine Research
Bremerhaven

Christina De La Rocha
Université de Brest

Research areas

Circum Antarctica

Publications

Assmy P, Henjes J, Smetacek V, Montresor M, 2006. Auxospore formation by the silica-sinking, oceanic diatom Fragilariopsis kerguelensis(Bacillariophyceae). J. Phycol. 42, 1002-1006.

Cortese G, Gersonde R, 2007. Morphometric variability in the diatom Fragilariopsis kerguelensis: implications for Southern Ocean paleoceanography. Earth Planet. Sci. Lett. 257, 526-544.

De La Rocha CL, Brzezinski MA, DeNiro MJ, Shemesh A, 1998. Silicon-isotope composition of diatoms as an indicator of past oceanic change. Nature 395, 680-683.

Smetacek V, Assmy P, Henjes J, 2004. The role of grazing in structuring Southern Ocean pelagic ecosystems and biogeochemical cycles. Antarctic Sci. 16, 541–558.

Homepage

---

Research funding organisation

German Research Foundation

Project number: DE 1455/2
Funding period: 2008 - 2011