Effects of eustatic sea-level change, ocean dynamics, and nutrient utilization on atmospheric <i>p</i>CO<sub>2</sub> and seawater composition over the last 130 000 years: a model study

Abstract

Abstract. We have developed and employed an Earth system model to explore the forcings of atmospheric pCO 2 change and the chemical and isotopic evolution of seawater over the last glacial cycle. Concentrations of dissolved phosphorus (DP), reactive nitrogen, molecular oxygen, dissolved inorganic carbon (DIC), total alkalinity (TA), 13 C-DIC, and 14 C-DIC were calculated for 24 ocean boxes. The bi-directional water fluxes between these model boxes were derived from a 3-D circulation field of the modern ocean (Opa 8.2, NEMO) and tuned such that tracer distributions calculated by the box model were consistent with observational data from the modern ocean. To model the last 130 kyr, we employed records of past changes in sea-level, ocean circulation, and dust deposition. According to the model, about half of the glacial pCO 2 drawdown may be attributed to marine regressions.

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

“…We test a hypothesis that the global PO 3− 4 inventory was larger during the LGM mainly due to reduced organic phosphorus burial on the exposed continental shelves from lower sea level (Broecker, 1982;Wallmann, 2010;Wallmann et al, 2016 Wallmann et al (2016) …”

“…Falkowski (1997) suggests that increased atmospheric Fe deposition to the glacial ocean stimulated additional N 2 fixation that could have increased the global NO − 3 inventory. Broecker (1982), and more recently (Wallmann et al, 2016) propose that a higher PO 3− 4 inventory, due to reduced loss on continental shelves, could have enhanced N 2 fixation during the LGM since PO 3− 4 is diazotrophs' other main limiting nutrient. On the other hand, Ren et al (2009Ren et al ( , 2012 interpret δ 15 N records from the western tropical North Atlantic and Pacific as reduced N 2 fixation, in response to lower N-loss in the glacial ocean.…”

“…Here we produce more realistic LGM simulations by considering spatially varying atmospheric Fe deposition and sea level effects on sedimentary N-loss, which were not accounted for in SS16. We also provide the first 3D quantitative estimates of effects of hypothesized increases in the ocean's PO 3− 4 inventory on the nitrogen cycle (Wallmann et al, 2016).…”

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

“…Among them are changes in the ocean circulation (Watson et al, 2015) and an increase in Southern Ocean stratification (e.g. Kobayashi et al, 2015), increase in sea ice area in the Southern Ocean (Stephens and Keeling, 2000) and a shift in the westerlies (Toggweiler et al, 2006), increase in nutrient inventory or change in the marine biota stoichiometry (Sigman and Boyle, 2000;Wallmann et al, 2016), changes in coral reefs accumulation and dissolution (Opdyke and Walker, 1992), accumulation of carbon in the permafrost regions (Ciais et al, 2012;Brovkin et al, 2016), variable volcanic outgassing (Huybers and Langmuir, 2009), and several other mechanisms. Most of these processes are not directly related to ice sheet area or volume, and thus should be considered as amplifiers or modifiers of the direct response of CO 2 to ice sheets operating through the climate-carbon cycle feedbacks.…”

“…Only a few have attempted to explain CO 2 dynamics during a part of (usually the glacial termination) or the entire last glacial cycle, with models of varying complexity -from simple box-type models (e.g. Köhler et al, 2010;Wallmann et al, 2016), to models of intermediate complexity (Brovkin et al, 2012;Menviel et al, 2012), or a stand-alone complex ocean carbon cycle model (Heinze et al, 2016). In all these studies, radiative forcing of CO 2 (or total GHGs) was prescribed based on paleoclimate reconstructions.…”