Description: The coupled ocean circulation‐ecosystem model MITgcm‐REcoM2 is used to simulate biogeochemical variables in a global configuration. The ecosystem model REcoM2 simulates two phytoplankton groups, diatoms and small phytoplankton, using a quota formulation with variable carbon, nitrogen, and chlorophyll contents of the cells. To improve the simulation of the phytoplankton variables, chlorophyll‐a data from the European Space Agency Ocean‐Color Climate Change Initiative (OC‐CCI) for 2008 and 2009 are assimilated with an ensemble Kalman filter. Utilizing the multivariate cross covariances estimated by the model ensemble, the assimilation constrains all model variables describing the two phytoplankton groups. Evaluating the assimilation results against the satellite data product SynSenPFT shows an improvement of total chlorophyll and more importantly of individual phytoplankton groups. The assimilation improves both phytoplankton groups in the tropical and midlatitude regions, whereas the assimilation has a mixed response in the high‐latitude regions. Diatoms are most improved in the major ocean basins, whereas small phytoplankton show small deteriorations in the Southern Ocean. The improvement of diatoms is larger when the multivariate assimilation is computed using the ensemble‐estimated cross covariances between total chlorophyll and the phytoplankton groups than when the groups are updated so that their ratio to total chlorophyll is preserved. The comparison with in situ observations shows that the correlation of the simulated chlorophyll of both phytoplankton groups with these data is increased whereas the bias and error are decreased. Overall, the multivariate assimilation of total chlorophyll modifies the two phytoplankton groups separately, even though the sum of their individual chlorophyll concentrations represents the total chlorophyll.

Description: Antarctic Intermediate Water (AAIW) formation constitutes an important mechanism for the export of macronutrients out of the Southern Ocean that fuels primary production in low latitudes. We used quality-controlled gridded data from five hydrographic cruises between 1990 and 2014 to examine decadal variability in nutrients and dissolved inorganic carbon (DIC) in the AAIW (neutral density range 27 〈 γ n 〈  27.4) along the Prime Meridian. Significant positive trends were found in DIC (0.70 ± 0.4 μmol kg− 1 year− 1) and nitrate (0.08 ± 0.06 μ mol kg− 1 year− 1) along with decreasing trends in temperature (− 0.015 ± 0.01∘C year− 1) and salinity (− 0.003 ± 0.002 year− 1) in the AAIW. Accompanying this is an increase in apparent oxygen utilization (AOU, 0.16 ± 0.07 μ mol kg− 1 year− 1). We estimated that 75% of the DIC change has an anthropogenic origin. The remainder of the trends support a scenario of a strengthening of the upper-ocean overturning circulation in the Atlantic sector of the Southern Ocean in response to the positive trend in the Southern Annular Mode. A decrease in net primary productivity (more nutrients unutilized) in the source waters of the AAIW could have contributed as well but cannot fully explain all observed changes.

Description: Harde (2017) proposes an alternative accounting scheme for the modern carbon cycle and concludes that only 4.3% of today's atmospheric CO2 is a result of anthropogenic emissions. As we will show, this alternative scheme is too simple, is based on invalid assumptions, and does not address many of the key processes involved in the global carbon cycle that are important on the timescale of interest. Harde (2017) therefore reaches an incorrect conclusion about the role of anthropogenic CO2 emissions. Harde (2017) tries to explain changes in atmospheric CO2 concentration with a single equation, while the most simple model of the carbon cycle must at minimum contain equations of at least two reservoirs (the atmosphere and the surface ocean), which are solved simultaneously. A single equation is fundamentally at odds with basic theory and observations. In the following we will (i) clarify the difference between CO2 atmospheric residence time and adjustment time, (ii) present recently published information about anthropogenic carbon, (iii) present details about the processes that are missing in Harde (2017), (iv) briefly discuss shortcoming in Harde's generalization to paleo timescales, (v) and comment on deficiencies in some of the literature cited in Harde (2017).

Description: We investigate glacial–interglacial variations in the marine stable carbon-isotope record applying the marine ecosystem and biogeochemistry model RECOM, which is forced with model output from fully coupled climate simulations. Different to most other marine biogeochemistry models, RECOM does not rely on fixed stoichiomet- ric ratios of phytoplankton organic matter. Instead, the composition of phytoplankton organic matter is calculated as a response to light, temperature and nutrient supply, which allows for assessing potential stoichiometric shifts between the past and present. We consider carbon-isotopic fractionation of marine phytoplankton during photosynthesis, studying different biogenic fractionation parametrisations and their influence on model–data comparisons for the Last Glacial Maximum and the Holocene.

Description: Stable carbon isotopes have now been implemented in REcoM (Hauck et al., 2016), the marine ecosystem and biogeochemistry model applied at AWI. In an ocean-only setup imple- mented in the MITgcm 3D-OGCM we here show how changing boundary conditions influence the simulated δ13C fields. Different to most other marine biogeochemistry models, RECOM does not rely on fixed stoichiometric ratios of phytoplankton organic matter. Instead, the composition of phytoplankton organic matter is calculated as a response to light, tempera- ture and nutrient supply, which allows for assessing potential stoichiometric shifts between the past and present. We study different parametrisations of biogenic carbon-isotopic fractiona- tion of marine phytoplankton during photosynthesis (Laws versus Rau) and their influence on model–data comparisons for the Last Glacial Maximum and the Holocene. Furthermore, we perform simulations, in which the climatic boundary and initial condi- tions (SST, wind, precipitation, runoff, salinity) and / or the dust fluxes are prescribed for preindustrial or LGM conditions based on previous studies (Zhang et al., 2014; Völker & Köh- ler, 2013; Albani et al., 2016). This gives us four simulations, from which we will analyse how especially dust via iron fertilization of the marine biology versus mainly physical (ocean overturning) changes will influence simulated δ13C fields. In doing so we will quantify how relevant the silicic acid leakage hypothesis and ocean overturning changes are for glacial δ13C.

Description: The Southern Ocean (SO) has long been recognised as a key player in regulating atmospheric CO2 variations and hence global climate. Here, the biological utilisation of nutrients regulates the preformed nutrient inventory for most of the deep ocean and, therefore, the global average efficiency of the biological pump. Marine sediment records from the Subantarctic Atlantic and Pacific document that higher mineral dust flux, increased bioproductivity, and lower atmospheric CO2 co-varied on glacial-interglacial time scales, which has been associated with iron fertilisation. It has been suggested that up to 40-50 ppmv of past atmospheric CO2 changes are related to iron fertilisation in the Subantarctic Ocean. The main objective of DustIron is an improved characterisation of the modern and past dust cycle and its link to SO iron fertilisation and atmospheric CO2 through a closely coupled novel datamodel approach. Within this project we want to extend the available geographic coverage of modern dust deposition, provenance and marine productivity records as well as the spatial and temporal variability during past glacial-interglacial cycles across all SO sectors. For the modern ocean we will explore dust fluxes, grain-size, and geochemical source area fingerprints (iron mineralogy, isotopy). Iron fertilisation and productivity will be assessed with a variety of both traditional (e.g., fluxes of biogenic barium or opal) and novel proxies for nutrient utilisation (δ15N in foraminifera). Our paleostudies will focus on the last glacial-interglacial climate transitions from the western Atlantic proximal to the Patagonian sources and from the central Indian SO (Kerguelen) to the eastern Indian Ocean sector, in order to obtain a circum-Antarctic view of dust-productivity changes. This will be complemented by a modelling study, simulating glacial-interglacial changes of atmospheric dust concentrations, deposition fluxes and linked SO bioproductivity.