Calibration of temperature-dependent ocean microbial processes in the cGENIE.muffin (v0.9.13) Earth system model
Authors: Crichton Katherine A. , Wilson Jamie D. , Ridgwell Andy , Pearson Paul N. 
Published: January 2021

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.