Understanding the Distribution and Behavior of Si Isotopes in the Ocean Christina L. De La Rocha Alfred Wegener Institute

Diatoms diatom frustule made from opal aka amorphous, hydrated silica SiO 2.nH 2 O Fragilariopsis kerguelensis 10  m photo from G. Cortese

Diatoms Matter They carry out: >75% of the new production in coastal, high nutrient environments ~ half of all marine primary production ~ 20% of all primary production occurring on Earth each year and much of the production in the Southern Ocean

The Silica Cycle in Tmol y -1 6 weathering silica production240 dissolution river input 120 dissolution 91 upwelling 115 sedimentation 29 dissolution23 net burial6-7 weathering 0.4 hydrothermal 0.5 numbers from Tréguer et al., 1995 DeMaster 2002 Elderfield and Schultz, 1996 eolian deposition 0.5

Questions Can dissolved Si (DSi) utilization by diatoms be reconstructed from Si isotopes? –What is the distribution and behavior of Si isotopes in the modern ocean and how has it been in the past? –Do sediments faithfully record an annually integrated nutrient utilization signal from surface waters?

Notation Silicon has 3 stable isotopes: 28 Si 29 Si 30 Si 92.23% 4.67 % 3.10% Silicon isotope ratio variations are expressed in permil: where R SAM and R STD are the 30 Si/ 28 Si ratio in a sample and standard, respectively.

Fractionation of Si Isotopes During Opal Biomineralization  30 Si opal -  30 Si Si(OH)4 Marine Diatoms Skeletonema costatum–1.0 ± 0.4 ‰ Thalassiosira weissflogii–1.3 ± 0.4 ‰ –1.5 ± 0.2 ‰ Thalassiosira sp.–0.9 ± 0.3 ‰ data from De La Rocha et al., 1998; Milligan et al., 2004

Rayleigh Distillation of Isotopes

 30 Si vs Depth data from De La Rocha et al., 2000; Varela et al., 2004; Cardinal et al., 2005 blues = Pacific central and coastal N Pac Antarctic (Pac Sector) Pacific subantarctic pink/purple = Atlantic BATS and coastal N Atl Atlantic subantarctic

Fractionation of Si Isotopes Field Samples- Monterey Bay from De La Rocha et al., 2000

Fractionation of Si Isotopes: Southern Ocean from Varela et al., 2004

 30 Si in Southern Ocean Sediments data from De La Rocha et al., 1998 Brzezinski et al., 2002

Regional Variability but why? data from De La Rocha et al., 1998

Potential Influences on Signal (Other Than Silicic Acid Utilization) secular variation in whole ocean  30 Si regional/temporal variability in  30 Si upwelled to euphotic zone sediments that do not integrate equally over entire growing season (e.g. bias towards resting spores, more robustly silicified diatoms, etc)

Si Isotope Budget InputsT mol y -1  30 Si (‰) river to +3.4 hydrothermal low-T basalt weathering eolian total7.0? Outputs biogenic opal6.5 to 7.4–3.7 to +2.0

The Silica Cycle in Tmol y -1 6 weathering silica production240 dissolution river input 120 dissolution 91 upwelling 115 sedimentation 29 dissolution23 net burial6-7 weathering 0.4 hydrothermal 0.5 numbers from Tréguer et al., 1995 DeMaster 2002 Elderfield and Schultz, 1996 eolian deposition 0.5

Si Isotope Budget InputsT mol y -1  30 Si (‰) river to +3.4 hydrothermal low-T basalt weathering eolian total7.0? Outputs biogenic opal6.5 to 7.4–3.7 to +2.0 InputsT mol y -1  30 Si (‰) river hydrothermal low-T basalt weathering eolian total Outputs biogenic opal6.5 to

Examination With Simple 2-box Model from De La Rocha and Bickle, 2005

Impact of Changing Riverine Si Flux 2.5x modern 0.5x modern

Regional Variability but why? data from De La Rocha et al., 1998

 30 Si vs Depth data from De La Rocha et al., 2000; Varela et al., 2004; Cardinal et al., 2005 blues = Pacific central and coastal N Pac Antarctic (Pac Sector) Pacific subantarctic pink/purple = Atlantic BATS and coastal N Atl Atlantic subantarctic

Modeled Mixed Layer  30 Si Wischmeyer et al., 2003

Rayleigh Distillation of Isotopes

Modeled Mixed Layer  30 Si Wischmeyer et al., 2003

 30 Si vs DSi blues = Pacific central and coastal N Pac Antarctic (Pac Sector) Pacific subantarctic pink/purple = Atlantic BATS and coastal N Atl Atlantic subantarctic data from De La Rocha et al., 2000; Varela et al., 2004; Cardinal et al., 2005

SO Surface Waters- Model vs Reality model- Wischmeyer et al., 2003 data- Varela et al., 2004; Cardinal et al., 2005

Outlook Suggested areas of attack (samples + modeling) mapping of  30 Si at high spatial resolution especially upper water column relative to trace elements (Fe), N isotopes comparing data with models of Si isotopic composition of ocean, sediments do we understand all the processes controlling  30 Si? investigating links between species composition and isotopic composition of sediments improving methods for isolating diatoms from sediments