Biogeochemical controls on productivity and particle flux in the coastal Antarctic Sea Ice Environment

• Main Author: Carson, Damien Stuart
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Edinburgh 2009
• Subjects: Antarctic; Antarctic Peninsula; East Antarctica; Ryder; Ryder Bay; Southern Ocean; The Antarctic; Antarc*; Antarctica; Sea ice
• Online Access: https://archimer.ifremer.fr/doc/00494/60589/64073.pdf; https://archimer.ifremer.fr/doc/00494/60589/

In order to trace modern processes in the ocean and reconstruct past oceanic conditions, reliable tracers of oceanic processes such as productivity, nutrient utilisation and export production need to be developed. This study therefore assessed a suite of geochemical proxies in the coastal Antarctic sea ice environment over two growing seasons (2004-2006). Time series measurements of productivity and export production were carried out over two contrasting growing seasons in Ryder Bay, a coastal embayment on the Western Antarctic Peninsula with a heavy sea ice influence. Ryder bay exhibits intense primary production during the short ice-free spring and summer months and particle export occurs as an intense period of sedimentation following surface water productivity, similar to many other coastal Antarctic environments. The two growing seasons were preceded by different sea ice regimes, with lighter sea ice cover during winter 04 and extended sea ice cover during winter 05. This resulted in a dynamic water column during the 04/05 growing season with frequent nutrient inputs to the surface, and a more stable, stratified water column during the 05/06 season with a few minor nutrient inputs to the surface. In response to these changes in the water column, higher mass fluxes occurred during the 04/05 season due to higher nutrient availability, with much lower fluxes occurring during the 05/06 season after the extended sea ice cover during the preceding winter. Variability of 8 13Cpoc at this site is not caused by changes in [CO2(aq)] or 8 13C_ CO2, but appears to be affected by changes in diatom assemblages with different surface area to volume ratios and biochemical pathways for C fixation. Although the 8 13Cpoc signal in surface waters is transferred to depth, these findings preclude the use of 813 as a tracer of paleo-pCO2 concentration. 815 N, the proxy for nitrate utilisation in the Southern Ocean shows a predicted response to nutrient utilisation and the physical properties of the water column in each season, with each season displaying a different evolution of 815 over the course of spring and summer. However, this proxy for utilisation of nitrate does not reflect the absolute changes in C flux. It is therefore necessary to consider this scenario when making connections between nitrate utilisation and changes in atmospheric CO2 in sediment cores. This study also better constrained the biogeochemical cycling of Ba in the coastal Antarctic sea ice environment. Baxs formation in surface waters appears related to productivity, but appears to peak during periods of early decay of phytoplankton when barite is formed. Standing stocks of Baxs in sea ice are consistently an order of magnitude higher than surface waters and display higher than average Ba:POC ratios than surface waters. The formation of barite in the water column appears to be linked to the decay of organic matter and precipitation of barite in supersaturated microenvironments. Finally, the flux of Baxs relative to carbon in the coastal Antarctic sea ice environment is consistent with algorithms published for the open ocean. This study has successfully identified mechanisms that affect the attenuation of proxy signatures in coastal Antarctic sediments. However, extreme interannual variability also highlights the need for further long-term monitoring of elemental and isotopic tracers in the coastal Antarctic sea ice environment, in order to establish longer term trends and truly establish the effect of differences in sea ice extent. In addition to the work in Ryder Bay, this thesis also includes a case study on the Antarctic sediment core MD03-2601 from Adélie Land, East Antarctica. This 40 m Holocene core has an exceptionally high sedimentation rate and is heavily influenced by sediment focusing. Using careful corrections to remove the influence of lateral input of material, and redox sensitive tracers to identify any anoxic sections of the core, paleoproductivity reconstructions were carried out using carbon, opal and Ba tracers of export production. The results are consistent with the climatic phases of the Holocene and present a valuable case study suggesting that these proxies are useful, either quantitatively or qualitatively, for reconstructing paleoproductivity on a core heavily affected by focusing and containing periods of sedimentary oxygen deficiency. lv