DOI: 10.25921/kjmt-e068
title: NOAA/WDS Paleoclimatology - Global Ocean Model Data of Carbon and Nitrogen Cycles for Pre-industrial and Last Glacial Maximum Conditions

description: A three-dimensional, process-based model of the ocean’s carbon and nitrogen cycles, including 13C and 15N isotopes, is used to explore effects of idealized changes in the soft tissue biological pump. Results are presented from one pre-industrial control run (piCtrl) and six simulations of the Last Glacial Maximum (LGM) with increasing values of the spatially constant maximum phytoplankton growth rate μmax, which accelerates biological nutrient utilization mimicking iron fertilization. The default LGM simulation, without increasing μmax and with a shallower and weaker Atlantic Meridional Overturning Circulation and increased sea ice cover, leads to 280 Pg more respired organic carbon (Corg) storage in the deep ocean with respect to piCtrl. Dissolved oxygen concentrations in the colder glacial thermocline increase, which reduces water-column denitrification and, with delay, nitrogen fixation, thus increasing the ocean’s fixed nitrogen inventory and decreasing δ15NNO3 almost everywhere. This simulation already fits sediment reconstructions of carbon and nitrogen isotopes relatively well, but it overestimates deep ocean δ13CDIC and underestimates δ15NNO3 at high latitudes. Increasing μmax further enhances Corg and lowers deep ocean δ13CDIC, improving the agreement with sediment data. In the model’s Antarctic and North Pacific oceans modest increases in μmax result in higher δ15NNO3 due to enhanced local nutrient utilization, improving the agreement with reconstructions there. Models with moderately increased μmax fit both isotope data best, whereas large increases in nutrient utilization are inconsistent with nitrogen isotopes although they still fit the carbon isotopes reasonably well. The best fitting models reproduce major features of the glacial δ13CDIC, δ15NNO3, and oxygen reconstructions, while simulating increased Corg by 510-670 Pg compared with the pre-industrial ocean. These results are consistent with the idea that the soft-tissue pump was more efficient during the LGM. Both circulation and biological nutrient utilization are viable mechanisms. However, these conclusions are preliminary given our idealized experiments, which do not consider changes in benthic denitrification and spatially inhomogenous changes in aeolian iron fluxes. The analysis illustrates interactions between the carbon and nitrogen cycles as well as the complementary constraints provided by their isotopes. Whereas carbon isotopes are sensitive to circulation changes and indicate well the three-dimensional Corg distribution, nitrogen isotopes are more sensitive to biological nutrient utilization.
description: Model code, output, scripts used for analysis and plotting and an updated sediment dataset of nitrogen isotope data
     This dataset includes three directories:
       model/ includes data for the six LGM runs and the Pre-industrial Control run as well as the source code and well as input and output data for the model
      analysis/ includes ferret scripts used for analysis and to produce the figures
      sediment_data/ includes the updated nitrogen isotope dataset