Stephen G. Bray

Moored sediment trap measurements of carbon export in the Subantarctic and Polar Frontal Zones of the Southern Ocean, south of Australia

Sediment trap moorings were deployed from September 21, 1997 through February 21, 1998 at three locations south of Australia along 140°E: at ∼47°S in the central Subantarctic Zone (SAZ) with traps at 1060, 2050, and 3850 m depth, at ∼51°S in the Subantarctic Front with one trap at 3080 m, and at ∼54°S in the Polar Frontal Zone (PFZ) with traps at 830 and 1580 m. Particle fluxes were high at all the sites (18–32 g m−2 yr−1 total mass and 0.5–1.4 g organic carbon m−2 yr−1 at −1000 m, assuming minimal flux outside the sampled summer period). These values are similar to other Southern Ocean results and to the median estimated for the global ocean by Lampitt and Antia [1997], and emphasize that the Southern Ocean exports considerable carbon to the deep sea despite its “high-nutrient, low chlorophyll” characteristics. The SAZ site was dominated by carbonate (>50% of total mass) and the PFZ site by biogenic silica (>50% of total mass). Both sites exhibited high export in spring and late summer, with an intervening low flux period in December. For the 153 day collection period, particulate organic carbon export was somewhat higher in all the traps in the SAZ (range 0.57–0.84 gC m−2) than in the PFZ (range 0.31–0.53), with an intermediate value observed at the SAF (0.60). The fraction of surface organic carbon export (estimated from seasonal nutrient depletion, Lourey and Trull [2001]) reaching 1000 m was indistinguishable in the SAZ and PFZ, despite different algal communities.

Latitudinal and temporal distributions of diatom populations in the pelagic waters of the Subantarctic and Polar Frontal zones of the Southern Ocean and their role in the biological pump

Abstract. The Subantarctic and Polar Frontal zones (SAZ and PFZ) represent a large portion of the total area of the Southern Ocean and serve as a strong sink for atmospheric CO2. These regions are central to hypotheses linking particle fluxes and climate change, yet multi-year records of modern flux and the organisms that control it are, for obvious reasons, rare. In this study, we examine two sediment trap records of the flux of diatoms and bulk components collected by two bottom-tethered sediment traps deployed at mesopelagic depths (~ 1 km) in the SAZ (2-year record; July 1999–October 2001) and in the PFZ (6-year record; September 1997–February 1998, July 1999–August 2000, November 2002–October 2004 and December 2005–October 2007) along the 140° E meridian. These traps provide a direct measure of transfer below winter mixed layer depths, i.e. at depths where effective sequestration from the atmosphere occurs, in contrast to study of processes in the surface ocean. Total mass fluxes were about twofold higher in the PFZ (24 ± 13 g m−2 yr−1) than in the SAZ (14 ± 2 g m−2 yr−1). Bulk chemical composition of the particle fluxes mirrored the composition of the distinct plankton communities of the surface layer, being dominated by carbonate in the SAZ and by biogenic silica in the PFZ. Particulate organic carbon (POC) export was similar for the annual average at both sites (1.0 ± 0.1 and 0.8 ± 0.4 g m−2 yr−1 for the PFZ and SAZ, respectively), indicating that the particles in the SAZ were relatively POC rich. Seasonality in the particle export was more pronounced in the PFZ. Peak fluxes occurred during summer in the PFZ and during spring in the SAZ. The strong summer pulses in the PFZ are responsible for a large fraction of the variability in carbon sequestration from the atmosphere in this region. The latitudinal variation of the total diatom flux was found to be in line with the biogenic silica export with an annual flux of 31 ± 5.5 × 108 valves m−2 yr−1 at the PFZ compared to 0.5 ± 0.4 × 108 m−2 yr−1 at the SAZ. Fragilariopsis kerguelensis dominated the annual diatom export at both sites (43 % at the SAZ and 59 % in the PFZ). POC fluxes displayed a strong positive correlation with the relative contribution of a group of weakly silicified and bloom-forming species in the PFZ. Several lines of evidence suggests that the development of these species during the growth season facilitates the formation of aggregates and carbon export. Our results confirm previous work suggesting that F. kerguelensis plays a major role in the decoupling of the carbon and silicon cycles in the high-nutrient low-chlorophyll waters of the Southern Ocean.

The Australian Integrated Marine Observing System Southern Ocean Time Series facility

The CSIRO, Bureau of Meteorology, University of Tasmania, and Antarctic Climate and Ecosystems CRC operate the Integrated Marine Observing System (IMOS) Southern Ocean Time Series (SOTS) facility with funding from the National Collaborative Research Infrastructure Strategy (NCRIS)- a set of moorings designed to quantify physical, chemical, and biological processes important to the transfer of heat, moisture, momentum, oxygen and carbon dioxide between the atmosphere and ocean. There are 3 mooring platforms at the SOTS site near 140°E, 47°S in ∼4500m water depth, in the Subantarctic Zone (SAZ) ∼36 hours by ship southwest of Tasmania: i) the Southern Ocean Flux Station (SOFS) - a large surface tower buoy that focuses on meteorological measurements, ii) the Pulse surface mixed layer mooring focusing on biological nutrient and carbon transformations using sensors and an automated water sampler, and iii) the deep SAZ sediment trap mooring (below 1000m depth) that quantifies sinking carbon fluxes to the ocean interior and returns particle samples for a broad range of biogeochemical studies. Additional applications include evaluation of wave models, calibration of isotopic proxies for past ocean conditions, and quantification of impacts of ocean acidification on foraminiferal zooplankton.

Seasonal variations, origin, and fate of settling diatoms in the Southern Ocean tracked by silicon isotope records in deep sediment traps

The Southern Ocean plays a pivotal role in the control of atmospheric CO 2 levels, via both physical and biological sequestration processes. The biological carbon transfer to the ocean interior is tightly coupled to the availability of other elements, especially iron as a trace-limiting nutrient and dissolved silicon as the mineral substrate that allows diatoms to dominate primary production. Importantly, variations in the silicon cycling are large but not well understood. Here we use δ 30 Si measurements to track seasonal flows of silica to the deep sea, as captured by sediment trap time series, for the three major zones (Antarctic, AZ; Polar Frontal, PFZ; and Sub-Antarctic, SAZ) of the open Southern Ocean. Variations in the exported flux of biogenic silica (BSi) and its δ 30 Si composition reveal a range of insights, including that (i) the sinking rate of BSi exceeds 200 m d −1 in summer in the AZ yet decreases to very low values in winter that allow particles to remain in the water column through to the following spring, (ii) occasional vertical mixing events affect the δ 30 Si composition of exported BSi in both the SAZ and AZ, and (iii) the δ 30 Si signature of diatoms is well conserved through the water column despite strong BSi and particulate organic carbon (POC) attenuation at depth and is closely linked to the Si consumption in surface waters. With the strong coupling observed between BSi and POC fluxes in PFZ and AZ, these data provide new constraints for application to biogeochemical models of seasonal controls on production and export.

Indices based on silicoflagellate assemblages offer potential for paleo-reconstructions of the main oceanographic zones of the Southern Ocean

This study reports detailed silicoflagellate assemblage composition and annual seasonal flux from sediment traps at four locations along a transect across the Southern Ocean frontal systems. The four traps sampled the central Subantarctic Zone (SAZ, 47°S site), the Subantarctic Front (SAF, 51°S site), the Polar Frontal Zone (54°S site) and the Antarctic Zone (61°S site) across the 140°E longitude. Annual silicoflagellate fluxes to the deep ocean exhibited a similar latitudinal trend to those of diatom fluxes reported in previous work, with maxima in the Antarctic Zone and minima in the Subantarctic Zone. The data suggest that, along with diatoms, silicoflagellates are important contributors to biogenic silica export at all sites, particularly in the Subantarctic Zone. Two main silicoflagellate genera were observed, with Stephanocha sp. (previously known as Distephanus) dominating polar waters and Dictyocha sp. important in sub-polar waters. This is consistent with previous use of the Dictyocha / Stephanocha ratio to infer paleotemperatures and monitor shifts in the position of the Polar Frontal Zone in the sedimentary record. It appears possible to further refine the application of this approach by using the ratio between two Dictyocha species, because Dictyocha aculeata dominated at the Subantarctic Front, while Dictyocha stapedia dominated in the central Subantarctic Front. Given the well-defined environmental affinities of both species, a new SAF silicoflagellate index (SAF-SI) based on this ratio is proposed as a useful diagnostic for SAF and SAZ water mass signatures in the Plio-Pleistocene and Holocene sedimentary record.

Ocean acidification impacts on southern ocean calcifiers

Adding carbon dioxide (CO2) to the ocean alters the carbonate chemistry and lowers pH, making surface waters more acidic and decreasing the carbonate ions available to calcifiers for calcite and aragonite production. Both forms of calcium carbonate dissolve more easily under conditions of higher CO2, lower temperatures and higher pressures due to depth. As the uptake rate of anthropogenic CO2 is at a maximum in the Southern Ocean we have a unique opportunity to observe marine calcifiers’ responses to changing carbonate chemistry. The Southern Ocean provides an excellent setting for the analysis of current and past ocean carbon-cycle changes because it crosses major surface-ocean gradients in carbonate chemistry and calcium carbonate production and spans the latitudes of maximum oceanic uptake of anthropogenic CO2. Through in situ sustained monitoring sediment traps deployed in the Southern Ocean we infer a reduction in calcification of one morphotype of shelled pteropod of ~ 35% over the past decade, consistent with the continuing lowering of aragonite saturation. Through a comparison of surface-sediment foraminifera, representing pre-industrial conditions, and modern foraminifera collected in sediment traps, we estimate a ~ 38% reduction in calcification since the industrial revolution. Planktonic foraminifera preserved in sediments are the same species living in the modern ocean, and provide a pre-industrial baseline to estimate the effects of acidification on shell formation in the modern high-CO2 ocean. As the magnitude of the anthropogenic CO2 increase is similar to deglacial increases in CO2, the geological record provides a means of scaling for the ecological response to ocean carbonate chemistry changes. The recent reduction in calcification is similar to deglacial calcification changes during the Late Pleistocene. The responses of these Southern Ocean calcifiers represent one of the earliest sets of field evidence of the impacts of CO2 on pelagic ecosystems. Furthermore, as the Southern Ocean contains a disproportionate amount of the oceanic inventory of anthropogenic CO2 it is a biogeochemical harbinger for the impacts of acidification, which will spread throughout the global ocean. Our results point to the importance of field observations on marine ecosystems as the ocean continues to absorb CO2 as a means of detecting impacts as early as possible.