We propose to develop an improved intermediate complexity box model for the study of ocean biogeochemical cycles in Deep Time. The rapidly improving data sets and the generation of novel hypotheses in Deep Time paleoceanography have created the need for more general and flexible models that can test a wide variety of hypotheses. The model is an advection-diffusion-reaction type box-model for the simulation of marine biogeochemical cycles, with emphasis on developing a mechanistic representation of the nitrogen cycle. Ocean circulation is currently represented in a 1.5-D model with three zones; a stratified "gyre" zone, an "upwelling" zone, and a "high latitude" zone. The gyre and upwelling zones are constructed with 34 vertical levels, ranging from 50 m resolution near the surface to 400 m resolution in the abyssal ocean, to provide realistic vertical profiles of biogeochemically relevant components. The model is designed to simulate the major biogeochemical processes in the CNPOS system. We propose to improve the functionality of the model, particularly the representation of dissolved organic carbon and of benthic exchange processes. We will calibrate it against modern ocean chemical data sets for both biogeochemical function and appropriate water exchange coefficients. We will extend its capabilities to include the calculation of the stable isotope ratios of C, N and S, to enable predictions that are testable against the growing database of isotopic data from sedimentary archives.
We plan to test three types of hypotheses: 1) reduced O2 supply to the deep ocean results in enhanced N losses via denitrification and anammox, which are compensated to an unknown extent by increased N fixation. We will compute the expected d15N values for N-replete and N-limited cases and expect that they will yield resolvably different isotopic signatures that can be tested against the sedimentary record. 2) We will investigate the dynamics of a DOC-rich ocean proposed for the Neoproterozoic. We will simulate vertical gradients of P, H2S and d13C in such an ocean, and the time scales and fluxes necessary to eliminate this large proposed DOC reservoir in a non-steady state fashion. 3) We will perform a model-model comparison with the GENIE EMIC model applied to the Permo-Triassic. We will use the results of the GENIE simulation to initialize the fully coupled CNPOS system in our model, and test the response of the N system to proposed photic zone sulfide fluxes.
Scientific merit: The proposed model development will result in a powerful, yet easy to use tool for testing hypotheses about the function of ancient marine biogeochemical cycles. While model cannot prove a particular hypothesis, they can be very useful in understanding the dynamics of biogeochemical systems. In particular, workers in Deep Time have begun to pose sophisticated hypotheses about the function of ancient coupled biogeochemical cycles, and adequate models are necessary to explore the consequences of these ideas. We will be able to test a series of hypotheses posed by Deep Time workers and examine their predicted impact on the coupled CNPOS system. We will be able to make testable predictions, particularly focused on the response of the N system which has been so far relatively little investigated in Deep Time studies.
Broader impacts: We plan to follow an "open source" approach, and make the model and appropriate documentation available to the scientific community. The model structure is deliberately modular, to enable easy modification of biogeochemical functions without having to modify the core structure. We believe that the model can readily used by a variety of researchers, for whom the barriers to developing a similar model would be high. We also expect that model development and hypothesis testing can proceed most rapidly when a community, rather than an individual research group, can readily carry out both development and simulations.
