Complementary constraints from carbon (¹³C) and nitrogen (¹⁵N) isotopes on the glacial ocean's soft‐tissue biological pump

Abstract

A three-dimensional, process-based model of the ocean's carbon and nitrogen cycles, including 13 C and 15 N isotopes, is used to explore effects of idealized changes in the soft-tissue biological pump. Results are presented from one preindustrial control run (piCtrl) and six simulations of the Last Glacial Maximum (LGM) with increasing values of the spatially constant maximum phytoplankton growth rate μ max , which accelerates biological nutrient utilization mimicking iron fertilization. The default LGM simulation, without increasing μ max and with a shallower and weaker Atlantic Meridional Overturning Circulation and increased sea ice cover, leads to 280 Pg more respired organic carbon (C org ) 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 δ 15 N NO3 almost everywhere. This simulation already fits sediment reconstructions of carbon and nitrogen isotopes relatively well, but it overestimates deep ocean δ 13 C DIC and underestimates δ 15 N NO3 at high latitudes. Increasing μ max enhances C org and lowers deep ocean δ 13 C DIC , improving the agreement with sediment data. In the model's Antarctic and North Pacific Oceans modest increases in μ max result in higher δ 15 N NO3 due to enhanced local nutrient utilization, improving the agreement with reconstructions there. Models with moderately increased μ max 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 δ 13 C DIC , δ 15 N, and oxygen reconstructions while simulating increased C org by 510-670 Pg compared with the preindustrial 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 could contribute. 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 C org distribution, nitrogen isotopes are more sensitive to biological nutrient utilization.

Simulation of climate, ice sheets and CO<sub>2</sub> evolution during the last four glacial cycles with an Earth system model of intermediate complexity

“…However, even if we consider only the LGM (as most previous works did), the composition of the "carbon stew" remains highly uncertain, even though there is a growing awareness that both physical and biological processes must have played a comparably important role in glacial CO 2 drawdown (e.g. Schmittner and Somes, 2016;Galbraith and Jaccard, 2015). Obviously, the choice of the "carbon stew" is crucially important for suc- cessful simulations of glacial cycles.…”

A Three-Dimensional Model of the Marine Nitrogen Cycle during the Last Glacial Maximum Constrained by Sedimentary Isotopes

“…δ 15 N records are typically interpreted qualitatively as changes to nutrient utilization in modern High Nitrate Low Chlorophyll (HNLC) regions, water column N-loss, or N 2 fixation (e.g., Altabet and Francois, 1994;Altabet et al, 1995;Ren et al, 2009). However, nitrogen isotope ratios are influenced by a variety of biogeochemical processes, sometimes at the same location, including transport by the circulation and mixing (Schmittner and Somes, 2016), which complicates their ability to quantitatively constrain nitrogen cycling processes based on individual sediment cores alone.…”

“…We build upon the isotope modeling work of Schmittner and Somes (2016), herein denoted as SS16, which used a global database of sedimentary nitrogen isotopes from the LGM (Tesdal et al, 2012) to constrain idealized 3D LGM simulations. In our global 3D model-data analysis, observations are directly compared to model results at the same locations they represent, rather than defining large, basin-scale averages that previous box modeling studies had to impose.…”

Coupled climate-carbon cycle simulation of the Last Glacial Maximum atmospheric CO<sub>2</sub> decrease using a large ensemble of modern plausible parameter sets

“…A dominant assumption is also that the terrestrial biosphere carbon inventory was reduced (Crowley et al, 1995;Adams and Faure, 1998;Ciais et al, 2012;Peterson et al, 2014), in line with independent estimates of an ocean carbon inventory that was 30 3 enhanced by several hundred petagrams (Goodwin and Lauderdale, 2013;Sarnthein et al, 2013;Allen et al, 2015;Schmittner and Somes, 2016). The decrease in terrestrial carbon is generally attributed to unfavourable climatic conditions for photosynthesis, and the destruction of organic material by moving ice sheets (e.g.…”

“…Brovkin et al, 2002;Bopp et al, 5 2003;Brovkin et al, 2007;Chikamoto et al, 2012;Palastanga et al, 2013;Schmittner and Somes, 2016;Buchanan et al, 2016) (Fig. 12).…”