Palaeoproductivity at the Antarctic continental margin: opal and barium records for the last 400 ka

Abstract

Records of biogenic opal and barium were measured in sediment cores at the Antarctic continental margin in the area of the Weddell, Lazarev and Cosmonaut seas. These records provide a qualitative and quantitative tool to estimate changes in paleoproductivity over the last 400 ka. The stratigraphy of the investigated cores is calibrated to a lithostratigraphy, adjusted to a stable isotope record from the eastern Weddell Sea, which is supported by a Th-dating method. We present evidence that interglacial productivity along the Antarctic continental margin is twice as high compared to subantarctic sites near South Orkney. A glacial/interglacial pattern with high productivity during peak warm stages can be observed back to 400 ka.High interglacial productivity is linked to a reduced sea ice coverage, which is regulated by the heat flux introduced by North Atlantic Deep Water (NADW) to the Antarctic Ocean. Generally, good correlations between the barium and opal records of the sediment cores indicate that dissolution of opal in the water column and the sediment does not obscure the surface productivity signal. Therefore, in this area biogenic opal in combination with other proxies, can be used for paleoproductivity estimates. Paleoproductivity is also assessed quantitatively from the barium record using the approaches of Dymond et al. (1992) and Francois et al. (1995).Paleoproductivity rates obtained by both methods show a good correspondence. In peak warm stages higher values are computed with the approach of Dymond et al. (1992). However, both methods provide values, which are representative of a highproductivity area. They are drastically reduced during glacial times. The extent and duration of sea ice coverage and the persistence of coastal polynyas is considered to be of primary importance in controlling the flux of biogenic material to sediments of the Antarctic continental margin.

Polar stratification: A critical view from the Southern Ocean

“…Consequently, any glacial shut-down of the CO 2 exchange between ocean and atmosphere in the region corresponding to the modern SIZ cannot have influenced the global CO 2 budget noticeably, as even under modern conditions CO 2 outgassing in this area is heavily impaired by either the presence of sea ice or by a strongly stratified water column. Similarly to the late Pliocene decline in biogenic opal deposition, the reduction of biological productivity in the SIZ during late Quaternary glacial periods is more likely to have originated from a prolonged seasonal (or even permanent) sea-ice coverage as previously suggested (Nürnberg et al, 1997;Bonn et al, 1998;Hillenbrand and Fütterer, 2001;Hillenbrand et al, 2002;cf. Gersonde et al, 2005).…”

“…1), lack direct evidence for increased stratification of the water column both during the late Pliocene and late Quaternary. Rather, the drop in biological productivity recorded at these sites is likely to have resulted from an expansion of the Antarctic sea-ice coverage as previously suggested (Frank et al, 1995;Nürnberg et al, 1997;Bonn et al, 1998;Frank et al, 2000;Hillenbrand and Fütterer, 2001;Hillenbrand et al, 2002).…”

“…With the exception of site PS1772, which is located south of the present maximum winter sea-ice boundary, and site PS1768, the core sites presented by Jaccard et al (2005a) from the Atlantic and Pacific sectors of the Southern Ocean are located within the modern SIZ (Fig. 1), where the average sea-ice season lasts for five to eight months per year (Bonn et al, 1998;Hillenbrand and Fütterer, 2001;Hillenbrand et al, 2002). Accordingly, air-sea exchange of CO 2 is reduced during these periods of time.…”

“…These Ba bio minima have been interpreted as reflecting minima in export production (e.g. Nürnberg et al, 1997;Bonn et al, 1998;Hillenbrand and Fütterer, 2001;Hillenbrand et al, 2002). With the exception of site PS1772, which is located south of the present maximum winter sea-ice boundary, and site PS1768, the core sites presented by Jaccard et al (2005a) from the Atlantic and Pacific sectors of the Southern Ocean are located within the modern SIZ (Fig.…”

“…Therefore, the Ba bio record at site PS1768 bears an element of uncertainty in terms of interpreting it as an unequivocal proxy for export production. (Hillenbrand et al, 2002), PS1565 (Hillenbrand and Fütterer, 2001), PS1648 and PS1821 (Bonn et al, 1998) in the present seasonal sea-ice zone (SIZ) and at sites PS1772 and PS1768 in the modern permanent open-ocean zone (POOZ) (Nürnberg et al, 1997). For sites PS1772 and PS1768 relative concentrations of the sea-ice indicator diatoms Fragilariopsis cylindrus and Fragilariopsis curta (light-grey shaded area) and of the deep-temperature adapted diatom Fragilariopsis obliquecostata (black area) are also shown.…”