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.
Highlights
The cycle of carbon through the ocean is dominated by the production, destruction, and transformation of both dissolved and particulate organic matter (DOM and POM, respectively) (Legendre et al, 2015; Heinze et al, 2015)
We explore the role of temperature in the creation and recycling of dissolved organic matter (DOM) and its impact on global carbon cycle dynamics
We identify three primary parameters requiring joint retuning: (1) the maximum nutrient uptake rate Vmax (Eq 3) that scales export production; (2) the activation energy, Ea(1) (Eq 5), for which the labile POC1 dominates the export from the surface; and (3) the fraction of recalcitrant POC2 formed at the surface that reaches the very deep ocean
Summary
The cycle of carbon through the ocean is dominated by the production, destruction, and transformation of both dissolved and particulate organic matter (DOM and POM, respectively) (Legendre et al, 2015; Heinze et al, 2015). Nutrient uptake is instantaneously converted into organic matter export, both particulate organic matter (POM) and a fraction as dissolved organic matter (DOM), in a ratio of 1 : 2 following Najjar et al (2007), and this represents community production (see Fig. 2) This production encompasses the entire surface food web, including the action of primary producers (phytoplankton) and the effect of consumers (e.g. grazers). Where γ T is the temperature growth limitation term (see below), Vmax is the maximum net depletion rate multiplier (yr−1), PO4 is local PO4 concentration (mol kg−1), is the nutrient limitation term, KPO4 is the Michaelis–Menten half-saturation value (mol kg−1), 1 − Aice is the ice-free fraction of the cell, and Temperature growth limitation is represented by the Arrhenius equation, where T is local temperature (Eq 4) This is the Eppley curve, commonly applied to model metabolic response to temperature change (Table 1).
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