Resolving the ‘opal paradox’ in the Southern Ocean

Pondaven, Philippe; Ragueneau, Olivier; Tréguer, Paul; Hauvespre, Anne; Dezileau, Laurent; Reyss, Jean Louis

doi:10.1038/35012046

Cited by 226 publications

(203 citation statements)

References 21 publications

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“…We consider that accumulation rates in the Southern Ocean can be crucially influenced by syndepositional lateral sediment redistribution (winnowing and focussing) leading to misinterpretations of absolute sedimentary fluxes (e.g. Frank et al, 1996;Pondaven et al, 2000;Francois et al, 2004;Geibert et al, 2005). As these authors point out, normalization to 230 Th is the most appropriate tool for correcting accumulation rates to vertical rain rates, but such a correction is only possible for sediments younger than 300 ka (e.g.…”

Section: Methodsmentioning

confidence: 99%

“…the seasonally sea-ice covered zone (SIZ) and the permanent open-ocean zone (POOZ) (e.g. Pondaven et al, 2000), acted as a CO 2 source during preindustrial interglacial periods as a result of CO 2 -release to Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 240 -252 www.elsevier.com/locate/palaeo the atmosphere due to the upwelling of CO 2 -rich deep waters. This assumption contrasts with the present day situation (Louanchi et al, 1999;Takahashi et al, 2002), which may be a consequence of the modern high atmospheric CO 2 concentration (ca.…”

Section: Introductionmentioning

confidence: 99%

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Polar stratification: A critical view from the Southern Ocean

Hillenbrand

Cortese

2006

Palaeogeography, Palaeoclimatology, Palaeoecology

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Oceanic stratification represents an effective mechanism to reduce vertical mixing of the water column, thereby locking up carbon dioxide (CO 2 ) in the deep sea and preventing air-sea exchange of CO 2 . It has been proposed that enhanced stratification of the upper water column in polar oceans during late Cenozoic cooling episodes limited the upwelling of CO 2 -rich deep waters and thus CO 2 -release to the atmosphere, resulting in a net global drawdown of atmospheric CO 2 . Increased stratification in the Subarctic Northwest Pacific, during both the onset of Northern Hemisphere glaciation at 2.73 Ma and late Quaternary glacial periods, has been recently linked to enhanced ocean stratification south of the Antarctic Polar Front (APF) in the Pacific and Atlantic sectors of the Southern Ocean. Increased stratification of Southern Ocean surface waters was mainly deduced from a reduction of biological production during these cooling episodes, manifested by the decrease of geochemical proxies for productivity, such as biogenic opal and barium, in the marine sediment records. However, the records chosen from the Southern Ocean do not provide evidence for a more stratified upper water column, rather the observed decrease in biological productivity is likely to have resulted from an expansion of annual sea-ice coverage. The sediment records suggest that south of the APF in the Pacific and Atlantic sectors of the Southern Ocean, extensive sea ice may have contributed to the global CO 2 -drawdown during glacial periods, with hypothetical glacial increase of stratification in today's permanent open-ocean zone merely being an "add on" caused by this expansion.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Polar stratification: A critical view from the Southern Ocean

Hillenbrand

Cortese

2006

Palaeogeography, Palaeoclimatology, Palaeoecology

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“…Despite its reputation as a region of low annual productivity, the Southern Ocean clearly generates a substantial annual #ux of biogenic material which is exported to the deep sea. Results from other national JGOFS programs working in the Southern Ocean show that the SW Paci"c sector is not unique in this regard (Pondaven et al, 2000).…”

Section: Antarctic Circumpolar Currentmentioning

confidence: 95%

The US Southern Ocean Joint Global Ocean Flux Study: an introduction to AESOPS

Smith

Anderson

Moore

et al. 2000

Deep Sea Research Part II: Topical Studies in Oceanography

120

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The United States Southern Ocean Joint Global Ocean Flux Study (JGOFS), also known as AESOPS (Antarctic Environment and Southern Ocean Process Study), focused on two distinct regions. The "rst was the Ross-Sea continental shelf, where a series of six cruises collected a variety of data from October 1996 through February 1998. The second area was the southwest Paci"c sector of the Southern Ocean, spanning the Antarctic Circumpolar Current (ACC) at &1703W. Data were collected within this region during "ve cruises from September 1996 through March 1998, as well as during selected transits between New Zealand and the Ross Sea. The "rst results of these cruses are described in this issue. The Ross-Sea investigation extensively sampled the area along 76330 S to elucidate the temporal patterns and processes that contribute to making this one of the Antarctic's most productive seas. Hydrographic distributions con"rm that strati"cation is initiated early in October within the polynya, generating an environment that is favorable for phytoplankton growth. Signi"cant spatial variations in mixed-layer depths, the timing of the onset of strati"cation, and the strength of the strati"cation existed throughout the growing season. Nutrient concentrations re#ected phytoplankton uptake, and reached their seasonal minimal in early February. Chlorophyll concentrations were maximal in early January, whereas productivity was maximal in late November, which re#ects the temporal uncoupling between growth and biomass accumulation in the region. Independent estimates of biogenic export suggest that majority of the #ux occurred in late summer and was strongly uncoupled from phytoplankton growth. The ACC region exhibited seasonal changes that in some cases were greater than those observed in the Ross Sea.0967-0645/00/$ -see front matter 2000 Published by Elsevier Science Ltd. PII: S 0 9 6 7 -0 6 4 5 ( 0 0 ) 0 0 0 5 9 -X Sea ice covered much of the region south of the Polar Front in winter, and retreated rapidly in late spring and early summer. Mixed layers throughout the region shoaled in summer due to surface heating, while the addition of freshwater from melting sea ice enhanced strati"cation in the Seasonal Ice Zone, creating conditions favorable for phytoplankton growth. For example, silicic acid concentrations decreased from initial values as high as 65 to less than 2 M within approximately 100 km (from 65.7 to 64.83S). Fluorescence values, however, showed less than a two-fold variation over the same distance. The vertical #ux of carbon in the Polar Front area is substantial, and marked variations in the composition of exported material exited over the region. The results provide a means whereby the controls of phytoplankton growth and organic matter #ux and remineralization can be analyzed in great detail. Additional results of the AESOPS project are discussed.

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“…In addition to iron supply, silicic acid availability also plays an important role in diatom productivity (Mengelt et al, 2001;Poulton et al, 2007). Lower diatom fluxes of 2.5 Â 10 7 valves m À 2 d À 1 were observed (Salter et al, 2012) in the iron-fertilized productive waters downstream of Crozet (Polar Frontal Zone, PFZ) and may be explained by the lower silicic acid concentration and higher temperature north of the Polar Front (Pondaven et al, 2000;Whitehouse et al, 2012).…”

Section: Seasonality Of Diatom Fluxesmentioning

confidence: 99%

Strong contribution of diatom resting spores to deep-sea carbon transfer in naturally iron-fertilized waters downstream of South Georgia

Rembauville

Manno

Tarling

et al. 2016

Deep Sea Research Part I: Oceanographic Research Papers

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a b s t r a c tBiogeochemical and diatom export fluxes are presented from two bathypelagic sediment trap deployments in the Antarctic Zone of the Southern Ocean. One of the sediment traps was deployed in very productive, naturally iron-fertilized waters downstream of South Georgia (P3, 2000 m) and compared to a deployment in moderately productive waters upstream of the island system (P2, 1500 m). At both sites significant diatom export events occurred in spring (November) and contained mostly empty cells that were associated with low particulate organic carbon (POC) fluxes. A summer export pulse occurred one month later at P2 (end February/March) compared to P3 (end January). Diatom fluxes at P3 were one order of magnitude higher than at P2, a difference mainly attributed to the short and intense export of resting spores from Chaetoceros Hyalochaete and Thalassiosira antarctica species. Aside from these resting spores, diatom export assemblages at both sites were dominated by empty Fragilariopsis kerguelensis frustules. The fraction of diatoms exported as empty frustules was considerably lower at P3 (52%) than P2 (91%). This difference was related to the flux of intact diatom resting spores at P3 and may partially explain the lower Si:C export stoichiometry observed at P3 (1.1) compared to P2 (1.5). Through the enumeration of full diatom frustules and subsequent biomass calculations we estimate that diatom resting spores account for 42% of annual POC flux in the productive waters downstream of South Georgia. At both sites the contribution of diatom vegetative stages to POC fluxes was considerably lower (o 5%). From these analyses we conclude that resting spore export contributes towards the slightly higher bathypelagic (POC) flux at P3 (40.6 mmol m À 2 y À 1 ) compared to P2 (26.4 mmol m À 2 y À 1 ). We compared our sediment trap records with previously published diatom assemblage data from the mixed layer and surface sediments (3760 m) around South Georgia. The relative proportion of diatom resting spores within diatom assemblages increases as a function of depth and is explained by selective preservation of their robust frustules. Our study highlights the significance of diatom resting spore export as a carbon vector out of the mixed layer. Furthermore, the contribution or resting spores to POC flux in the bathypelagic ocean and sediments suggests they play a particularly important role in sequestering biologically fixed CO 2 over climatically relevant timescales.

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Resolving the ‘opal paradox’ in the Southern Ocean

Cited by 226 publications

References 21 publications

Polar stratification: A critical view from the Southern Ocean

Polar stratification: A critical view from the Southern Ocean

The US Southern Ocean Joint Global Ocean Flux Study: an introduction to AESOPS

Strong contribution of diatom resting spores to deep-sea carbon transfer in naturally iron-fertilized waters downstream of South Georgia

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