Calibration of temperature-dependent ocean microbial processes in the cGENIE.muffin (v0.9.13) Earth system model

Abstract

Abstract. Temperature is a master parameter in the marine carbon cycle, exerting a critical control on the rate of biological transformation of a variety of solid and dissolved reactants and substrates. Although in the construction of numerical models of marine carbon cycling, temperature has been long recognised as a key parameter in the production and export of organic matter at the ocean surface, its role in the ocean interior is much less frequently accounted for. There, bacteria (primarily) transform sinking particulate organic matter (POM) into its dissolved constituents and consume dissolved oxygen (and/or other electron acceptors such as sulfate). The nutrients and carbon thereby released then become available for transport back to the surface, influencing biological productivity and atmospheric pCO2, respectively. Given the substantial changes in ocean temperature occurring in the past, as well as in light of current anthropogenic warming, appropriately accounting for the role of temperature in marine carbon cycling may be critical to correctly projecting changes in ocean deoxygenation and the strength of feedbacks on atmospheric pCO2. Here we extend and calibrate a temperature-dependent representation of marine carbon cycling in the cGENIE.muffin Earth system model, intended for both past and future climate applications. In this, we combine a temperature-dependent remineralisation scheme for sinking organic matter with a biological export production scheme that also includes a dependence on ambient seawater temperature. Via a parameter ensemble, we jointly calibrate the two parameterisations by statistically contrasting model-projected fields of nutrients, oxygen, and the stable carbon isotopic signature (δ13C) of dissolved inorganic carbon in the ocean with modern observations. We additionally explore the role of temperature in the creation and recycling of dissolved organic matter (DOM) and hence its impact on global carbon cycle dynamics. We find that for the present day, the temperature-dependent version shows a fit to the data that is as good as or better than the existing tuned non-temperature-dependent version of the cGENIE.muffin. The main impact of accounting for temperature-dependent remineralisation of POM is in driving higher rates of remineralisation in warmer waters, in turn driving a more rapid return of nutrients to the surface and thereby stimulating organic matter production. As a result, more POM is exported below 80 m but on average reaches shallower depths in middle- and low-latitude warmer waters compared to the standard model. Conversely, at higher latitudes, colder water temperature reduces the rate of nutrient resupply to the surface and POM reaches greater depth on average as a result of slower subsurface rates of remineralisation. Further adding temperature-dependent DOM processes changes this overall picture only a little, with a slight weakening of export production at higher latitudes. As an illustrative application of the new model configuration and calibration, we take the example of historical warming and briefly assess the implications for global carbon cycling of accounting for a more complete set of temperature-dependent processes in the ocean. We find that between the pre-industrial era (ca. 1700) and the present (year 2010), in response to a simulated air temperature increase of 0.9 ∘C and an associated projected mean ocean warming of 0.12 ∘C (0.6 ∘C in surface waters and 0.02 ∘C in deep waters), a reduction in particulate organic carbon (POC) export at 80 m of just 0.3 % occurs (or 0.7 % including a temperature-dependent DOM response). However, due to this increased recycling nearer the surface, the efficiency of the transfer of carbon away from the surface (at 80 m) to the deep ocean (at 1040 m) is reduced by 5 %. In contrast, with no assumed temperature-dependent processes impacting production or remineralisation of either POM or DOM, global POC export at 80 m falls by 2.9 % between the pre-industrial era and the present day as a consequence of ocean stratification and reduced nutrient resupply to the surface. Our analysis suggests that increased temperature-dependent nutrient recycling in the upper ocean has offset much of the stratification-induced restriction in its physical transport.

Resolving ecological feedbacks on the ocean carbon sink in Earth system models

“…In its default configuration (BIO+FPR), cGEnIE projects surface air temperature warming of 1.8, 2.6, 3.2, and 4.2 • C by 2100 relative to 1850-1900 in RCPs 3PD, 4.5, 6.0, and 8.5 respectively, which compares favourably with CMIP5 projections for these scenarios (1.6 ± 0.4, 2.4 ± 0.5, 2.8 ± 0.5, and 4.3±0.7 • C) (Collins et al, 2013). In the ocean, cGEnIE-BIO+FPR projects sea surface warming of 1.2, 1,8, 2.1, and 2.8 • C by 2100 in RCPs 3PD, 4.5, 6.0, and 8.5 respectively (see Fig.…”

“…By default, BIOGEM uses fixed remineralisation profiles similar to the Martin curve for the sinking labile fractions of both POC and PIC (Martin et al, 1987;Ridgwell et al, 2007) but includes an optional temperature-dependent remineralisation scheme which has previously been used to explore the biological pump in warm palaeo-oceans (John et al, 2014b). An updated calibration of this scheme which also couples TDR with temperature-dependent export production was also recently developed (Crichton et al, 2021) and is the version (cGEnIE.muffin v0.9.13) used in this paper.…”

“…We assess the differing impacts of replacing cGEnIE's fixed profile remineralisation (FPR) parameterisation with its temperature-dependent remineralisation (TDR) scheme (John et al, 2014b;Crichton et al, 2021) and replacing cGEnIE's original parameterised biogeochemistry BIOGEM module (BIO) with ecoGEnIE's trait-based ECOGEM module (ECO) (Ward et al, 2018). We test each new element both separately and in combination, analysing four cGEnIE and ecoGEnIE configurations:…”

“…Acidification could also lead to reduced particle ballasting (the hypothesised process by which denser falling PIC protects associated POC and increases POC export) by reducing the supply of PIC, resulting in reduced efficiency of POC export (Armstrong et al, 2001;Klaas and Archer, 2002). However, the overall effect of ocean acidification feedbacks remains uncertain (Doney et al, 2020), and many of these processes are not resolved by ESMs. Furthermore, the human-driven loss of organisms higher up the food chain as a result of overharvesting and habitat degradation has a considerable yet poorly quantified effect on the biological pump (Pershing et al, 2010).…”

Iron and sulfur cycling in the cGENIE.muffin Earth system model (v0.9.21)

“…However, it allows us to quickly diagnose spatial patterns in reducing Fe-S cycling at steady state, without a priori introducing a bias towards ferruginous or euxinic redox states. We initialize the model with an average ligand concentration of 1 nM (comparable to the modern ocean) and a semi-arbitrary marine phosphate inventory that is 50 % of today in order to reduce marine primary productivity (as productivity in the Precambrian ocean was likely lower than today; Crockford et al, 2018;Ozaki et al, 2019), and we run the ocean circulation to steady state for 20 000 years for each individual experiment and present model output from the 20 000th year of integration. It is important to emphasize that the idealized configuration we implement here is not meant to represent any specific period or event in Earth's history, but is rather meant to serve as a broadly plausible and computationally efficient set of boundary conditions for testing the extended model code.…”