Marine nitrogen fixers mediate a low latitude pathway for atmospheric CO2 drawdown: Biogeochemical and physical output v1.0

Full description

The data included in this repository includes both the physical and biogeochemical fields that were generated by 89 simulations over 10,000 years with an Ocean General Circulation Model (OGCM) and attached ocean biogeochemistry: CSIRO Mk3L-COAL v1.0. The 89 simulations alter one of three major processes to understand the relationship between N₂fixation and CO₂drawdown from the atmosphere into the ocean. The three major processes are (1) different circulation states, (2) different representations of the marine nitrogen (N) cycle, and (3) different rates of aeolian iron (Fe) deposition to the ocean surface.

1. Four physical states were generated. Three were of preindustrial (PI; 1850 CE) climate and one was of the Last Glacial Maximum (22,000 BCE). The preindustrial states were generated by forcing the OGCM with the boundary conditions produced by piControl runs of CSIRO Mk3L v1.2, GFDL-ESM2G, and HadGEM2-CC. The Last Glacial Maximum state was generated by forcing the OGCM with the boundary conditions produced by a glacial run of CSIRO Mk3L v1.2 (Buchanan et al., 2016). These boundary conditions were sea surface temperature, salinity and the meridional and zonal components of surface wind stresses, and the final 10 years of these runs were averaged and regridded onto the CSIRO Mk3L-COAL grid space.
2. Numerous alterations to the parameters governing the N cycle were undertaken. These include:
   • Deactivating the N cycle and holding mean NO₃content stable
   • Increasing/decreasing the Fe half saturation coefficient for N₂fixers
   • Increasing/decreasing the PO₄half saturation coefficient for N₂fixers
   • Altering the C:P ratio of N₂fixers organic matter
   • Increasing/decreasing the base rate of sedimentary denitrification to alter mean ocean NO₃
3. Iron deposition rates to the ocean surface vary between 25% and 2500% of the modern rate, which is taken to be the climatology produced by Mahowald et al. (2005). All deposition rates are made to be a factor of this modern field, expect for the two highest fields of 500% and 2500%, which represent the Last Glacial Maximum dust deposition field of Lambert et al.(2015) assuming 3.5% Fe content and 0.4% or 2% solubility.

These three major processes are altered in isolation and in combination to thoroughly elucidate the relationship between N₂fixation and the drawdown of CO₂into the ocean.

Furthermore, some simulations were undertaken with an additional model development step, which involved allowing atmospheric CO₂to freely evolve as the ocean absorbed or release carbon. These are noted by having file names with “openCO2”.

Climatologies of sea surface temperature, sea surface salinity, and x and y vectors of sea surface wind stresses were produced by both the PI and LGM coupled experiments and were used to force the ocean general circulation model. Additional climatologies of sea ice fractional cover, sea surface wind speeds, net incident short-wave radiation, and the aeolian deposition of iron and reactive nitrogen were important for forcing the biogeochemical model. These climatologies are made available here.

Also available are the three-dimensional global annual averages of oceanic properties for all simulations at their steady-state solutions. These include the physical properties of temperature, salinity, ideal water mass age, velocities, overturning streamfunctions and the various fluxes of heat, salt and momentum at the surface. Three-dimensional (3D) fields of oxygen, apparent oxygen utilisation, dissolved inorganic carbon, total alkalinity, phosphate, nitrate, nitrate-15, and iron are provided by the biogeochemical model output. The biogeochemical model also provides 3D fields of the sources and sinks of the N cycle (N₂fixation, Water column denitrification and sedimentary denitrification), as well as 2D fields of export production of organic and inorganic carbon, atmospheric Fe and NO₃deposition, the elemental stoichiometry of organic matter and the bexponent of the Martin Curve used to predict remineralisation rates.

A full description of the CSIRO Mk3L v1.2 climate system model can be found in both:

• Phipps, S. J., Rotstayn, L. D., Gordon, H. B., Roberts, J. L., Hirst, C., and Budd, W. F. (2012). The CSIRO Mk3L climate system model version 1.0 - Part 2: Response to external forcings. Geosci. Model Dev.5, 649–682. doi:10.5194/gmd-5-649-2012.
• Phipps, S. J., Rotstayn, L. D., Gordon, H. B., Roberts, J. L., Hirst, A. C., and Budd, W. F. (2011). The CSIRO Mk3L climate system model version 1.0 - Part 1: Description and evaluation. Model Dev.4, 483–509. doi:10.5194/gmd-4-483-2011.

Chronological descriptions of the developing biogeochemical ocean model within the CSIRO Mk3L-COAL v1.0 Earth System Model that was used can be found in:

• Buchanan, P. J., Matear, R. J., Chase, Z., Phipps, S. J., and Bindoff, N. L. (2018). Dynamic Biological Functioning Important for Simulating and Stabilizing Ocean Biogeochemistry. Global Biogeochem. Cycles. doi:10.1002/2017GB005753.
• Buchanan, P. J., Matear, R. J., Chase, Z., Phipps, S. J., and Bindoff, N. L. (2019). Ocean carbon and nitrogen isotopes in CSIRO Mk3L-COAL version 1.0: a tool for palaeoceanographic research. Model Dev.12, 1491–1523. doi:10.5194/gmd-12-1491-2019.
• Matear, R. J., and Lenton, A. (2014). Quantifying the impact of ocean acidification on our future climate. Biogeosciences11, 3965–3983. doi:10.5194/bg-11-3965-2014.