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Ocean biogeochemical models have become critical tools for interpreting trace element and isotope (TEI) distributions observed during the GEOTRACES program and understanding their driving processes. Models stimulate new research questions that cannot be addressed with observations alone, for instance, concerning processes that occur over vast spatial scales and linkages between TEIs and other elemental cycles. A spectrum of modeling approaches has been applied to date, including (1) fully prognostic models that couple TEIs to broader biogeochemical frameworks, (2) simpler element-specific mechanistic models that allow for assimilation of observations, and (3) machine learning models that have no mechanistic underpinning but allow for skillful extrapolation of sparse data. Here, we evaluate the strengths and weaknesses of these approaches and review three sets of novel insights they have facilitated. First, models have advanced our understanding of global-scale micronutrient distributions, and their deviations from macronutrients, in terms of a "ventilation-regeneration-scavenging" balance. Second, models have yielded global-scale estimates of TEI inputs to and losses from the ocean, revealing, for instance, a rapid iron (Fe) cycle with an oceanic residence time on the order of decades. Third, models have identified novel links among various TEI cycling processes and the global ocean carbon cycle, such as tracing the supply of hydrothermally sourced Fe to iron-starved microbial communities in the Southern Ocean. We foresee additional important roles for modeling work in the next stages of trace element research, including synthesizing understanding from the GEOTRACES program in the form of TEI state estimates, and projecting the responses of TEI cycles to global climate change. [ABSTRACT FROM AUTHOR]

The ocean is responsible for taking up approximately 25% of anthropogenic CO2 emissions and stores >50 times more carbon than the atmosphere. Biological processes in the ocean play a key role, maintaining atmospheric CO2 levels approximately 200 ppm lower than they would otherwise be. The ocean's ability to take up and store CO2 is sensitive to climate change, however the key biological processes that contribute to ocean carbon storage are uncertain, as are how those processes will respond to, and feedback on, climate change. As a result, biogeochemical models vary widely in their representation of relevant processes, driving large uncertainties in the projections of future ocean carbon storage. This review identifies key biological processes that affect how ocean carbon storage may change in the future in three thematic areas: biological contributions to alkalinity, net primary production, and interior respiration. We undertook a review of the existing literature to identify processes with high importance in influencing the future biologically‐mediated storage of carbon in the ocean, and prioritized processes on the basis of both an expert assessment and a community survey. Highly ranked processes in both the expert assessment and survey were: for alkalinity—high level understanding of calcium carbonate production; for primary production—resource limitation of growth, zooplankton processes and phytoplankton loss processes; for respiration—microbial solubilization, particle characteristics and particle type. The analysis presented here is designed to support future field or laboratory experiments targeting new process understanding, and modeling efforts aimed at undertaking biogeochemical model development. Plain Language Summary: The storage of carbon in the ocean forms an essential component of the Earth's carbon cycle. The contribution of ocean biology to carbon storage is not well constrained by observations and as a result has large uncertainties in the future model projections. There are a multitude of processes involved in the uptake, remineralization and storage of carbon, many of which have high uncertainty. Here we assess significant processes in determining net primary production, the biological contribution to alkalinity, and interior respiration. Using an extensive literature review, expert assessment and community survey, we rank processes as having high, moderate or low importance to the future biologically‐mediated storage of carbon in the ocean. This analysis is intended to support future observational studies and biogeochemical model development. Key Points: Key processes needed to improve projections of the response of ocean carbon storage to climate change identifiedThree themes are addressed: net primary production, interior respiration, and biological contributions to alkalinityAn expert assessment and community survey used to rank processes according to importance and uncertainty levels [ABSTRACT FROM AUTHOR]

Climate change scenarios suggest that large‐scale carbon dioxide removal (CDR) will be required to maintain global warming below 2°C, leading to renewed attention on ocean iron fertilization (OIF). Previous OIF modelling has found that while carbon export increases, nutrient transport to lower latitude ecosystems declines, resulting in a modest impact on atmospheric CO2. However, the interaction of these CDR responses with ongoing climate change is unknown. Here, we combine global ocean biogeochemistry and ecosystem models to show that, while stimulating carbon sequestration, OIF may amplify climate‐induced declines in tropical ocean productivity and ecosystem biomass under a high‐emission scenario, with very limited potential atmospheric CO2 drawdown. The 'biogeochemical fingerprint' of climate change, that leads to depletion of upper ocean major nutrients due to upper ocean stratification, is reinforced by OIF due to greater major nutrient consumption. Our simulations show that reductions in upper trophic level animal biomass in tropical regions due to climate change would be exacerbated by OIF within ~20 years, especially in coastal exclusive economic zones (EEZs), with potential implications for fisheries that underpin the livelihoods and economies of coastal communities. Any fertilization‐based CDR should therefore consider its interaction with ongoing climate‐driven changes and the ensuing ecosystem impacts in national EEZs. [ABSTRACT FROM AUTHOR]

The dispersal of dissolved iron (DFe) from hydrothermal vents is poorly constrained. Combining field observations and a modeling hierarchy, we find the dispersal of DFe from the Trans‐Atlantic‐Geotraverse vent site occurs predominantly in the colloidal phase and is controlled by multiple physical processes. Enhanced mixing near the seafloor and transport through fracture zones at fine‐scales interacts with the wider ocean circulation to drive predominant westward DFe dispersal away from the Mid‐Atlantic ridge at the 100 km scale. In contrast, diapycnal mixing predominantly drives northward DFe transport within the ridge axial valley. The observed DFe dispersal is not reproduced by the coarse resolution ocean models typically used to assess ocean iron cycling due to their omission of local topography and mixing. Unless biogeochemical models account for fine‐scale physics and colloidal Fe, they will inaccurately represent DFe dispersal from axial valley ridge systems, which make up half of the global ocean ridge crest. Plain Language Summary: Hydrothermal venting along mid ocean ridges supplies large quantities of the trace metal iron to the ocean. Once it mixed with oxygenated seawater, precipitation leads to iron being lost from the dissolved phase to generate seafloor metal deposits. However, a small fraction of iron supplied escapes precipitation and remains in the dissolved phase. The processes that control the retention and ocean transport of hydrothermal dissolved iron is important as it has a disproportionate influence on the global carbon cycle. In this work, we examined the processes driving the dispersal of dissolved iron from a major site of hydrothermal venting on the northern mid Atlantic ridge. We found that the complex topography of the mid Atlantic ridge was crucial in steering the escape of dissolved iron in the colloidal size range out of the immediate mid ocean ridge system. This raises challenges for the large scale ocean models used to represent the global ocean iron cycle as they are typically not parameterized at sufficient spatial resolution. The use of multiple grids, with higher resolution nests, may offer a solution to the challenge of representing the interactions of tracer dispersal with complex topography. Key Points: Iron is dispersed from trans Atlantic geotraverse predominantly northward within the axial valley and westward off axis, dominated by the colloidal size fractionA combination of fine‐scale processes are necessary to explain the dispersal both within and outside the axial valleyCoarse resolution models are impaired in their ability to constrain the broader influence of iron supplied from axial valley ridge systems [ABSTRACT FROM AUTHOR]

Although iron and light are understood to regulate the Southern Ocean biological carbon pump, observations have also indicated a possible role for manganese. Low concentrations in Southern Ocean surface waters suggest manganese limitation is possible, but its spatial extent remains poorly constrained and direct manganese limitation of the marine carbon cycle has been neglected by ocean models. Here, using available observations, we develop a new global biogeochemical model and find that phytoplankton in over half of the Southern Ocean cannot attain maximal growth rates because of manganese deficiency. Manganese limitation is most extensive in austral spring and depends on phytoplankton traits related to the size of photosynthetic antennae and the inhibition of manganese uptake by high zinc concentrations in Antarctic waters. Importantly, manganese limitation expands under the increased iron supply of past glacial periods, reducing the response of the biological carbon pump. Overall, these model experiments describe a mosaic of controls on Southern Ocean productivity that emerge from the interplay of light, iron, manganese and zinc, shaping the evolution of Antarctic phytoplankton since the opening of the Drake Passage. Plain Language Summary: Because of the Southern Ocean's unique role in ocean circulation, Antarctic phytoplankton profoundly influence the global carbon cycle. For instance, an increase in the supply of iron—the main nutrient limiting Antarctic phytoplankton—is thought to have lowered CO2 during past ice ages by increasing phytoplankton photosynthesis. However, the potential for other essential elements to limit Southern Ocean productivity is not well known. By accounting for requirements of several nutrients in a global model, we have identified that manganese, an essential cofactor in photosynthesis, can limit phytoplankton growth across the Southern Ocean. The enduring role of manganese deficiency will likely influence the response of Southern Ocean ecosystems to ongoing climate change. Key Points: Mn scarcity in the Southern Ocean limits phytoplankton growth in a global biogeochemical model, especially during austral springThe spatial extent of Mn limitation is sensitive to phytoplankton traits governing photophysiology and metal homeostasisGreater dust deposition to the Southern Ocean expands the role of Mn limitation and restricts carbon export from Fe fertilization [ABSTRACT FROM AUTHOR]

We present a new approach for quantifying the bioavailability of dissolved iron (dFe) to oceanic phytoplankton. Bioavailability is defined using an uptake rate constant (kin‐app) computed by combining data on: (a) Fe content of individual in situ phytoplankton cells; (b) concurrently determined seawater dFe concentrations; and (c) growth rates estimated from the PISCES model. We examined 930 phytoplankton cells, collected between 2002 and 2016 from 45 surface stations during 11 research cruises. This approach is only valid for cells that have upregulated their high‐affinity Fe uptake system, so data were screened, yielding 560 single cell kin‐app values from 31 low‐Fe stations. We normalized kin‐app to cell surface area (S.A.) to account for cell‐size differences. The resulting bioavailability proxy (kin‐app/S.A.) varies among cells, but all values are within bioavailability limits predicted from defined Fe complexes. In situ dFe bioavailability is higher than model Fe‐siderophore complexes and often approaches that of highly available inorganic Fe′. Station averaged kin‐app/S.A. are also variable but show no systematic changes across location, temperature, dFe, and phytoplankton taxa. Given the relative consistency of kin‐app/S.A. among stations (ca. five‐fold variation), we computed a grand‐averaged dFe availability, which upon normalization to cell carbon (C) yields kin‐app/C of 42,200 ± 11,000 L mol C−1 d−1. We utilize kin‐app/C to calculate dFe uptake rates and residence times in low Fe oceanic regions. Finally, we demonstrate the applicability of kin‐app/C for constraining Fe uptake rates in earth system models, such as those predicting climate mediated changes in net primary production in the Fe‐limited Equatorial Pacific. Plain Language Summary: In many oceanic regions, iron exerts strong control on phytoplankton growth, ecosystem structure, and carbon cycling. Yet, iron bioavailability and uptake rates by phytoplankton in the ocean are poorly constrained. Recently, Shaked et al. (2020) established a new approach for quantifying the availability of dissolved Fe (dFe) in natural seawater based on its uptake kinetics by Fe‐limited cultured phytoplankton. Here, we extend this approach to in situ phytoplankton, establishing a standardized proxy for dFe bioavailability in low‐Fe oceanic regions. Bioavailability is estimated through single cell Fe uptake constants, calculated by combining measured Fe contents of individual phytoplankton cells collected from multiple regions with concurrently measured dFe concentrations, as well as modeled growth rates. We then utilize this proxy for: (a) comparing dFe bioavailability among organisms and regions; (b) calculating dFe uptake rates and residence times in low‐Fe oceanic regions; and (c) constraining Fe uptake parameters of earth system models to better predict ocean productivity in response to climate change. Key Points: A proxy for dissolved Fe bioavailability in low‐Fe regions is established from Fe quotas, dissolved Fe concentrations, and modeled growth ratesIn situ phytoplankton cells record high and relatively uniform dissolved Fe bioavailability across many low‐Fe oceanic regionsThe new proxy is applicable for calculating in situ Fe uptake rates and biological Fe residence times and for validating global model output [ABSTRACT FROM AUTHOR]

Phytoplankton iron contents (i.e., quotas) directly link biogeochemical cycles of iron and carbon and drive patterns of nutrient limitation, recycling, and export. Ocean biogeochemical models typically assume that iron quotas are either static or controlled by dissolved iron availability. We measured iron quotas in phytoplankton communities across nutrient gradients in the Pacific Ocean and found that quotas diverged significantly in taxon‐specific ways from laboratory‐derived predictions. Iron quotas varied 40‐fold across nutrient gradients, and nitrogen‐limitation allowed diatoms to accumulate fivefold more iron than co‐occurring flagellates even under low iron availability. Modeling indicates such "luxury" uptake is common in large regions of the low‐iron Pacific Ocean. Among diatoms, both pennate and centric genera accumulated luxury iron, but the cosmopolitan pennate genus Pseudo‐nitzschia maintained iron quotas 10‐fold higher than co‐occurring centric diatoms, likely due to enhanced iron storage. Biogeochemical models should account for taxonomic and macronutrient controls on phytoplankton iron quotas. [ABSTRACT FROM AUTHOR]

Numerical models of ocean biogeochemistry are relied upon to make projections about the impact of climate change on marine resources and test hypotheses regarding the drivers of past changes in climate and ecosystems. In large areas of the ocean, iron availability regulates the functioning of marine ecosystems and hence the ocean carbon cycle. Accordingly, our ability to quantify the drivers and impacts of fluctuations in ocean ecosystems and carbon cycling in space and time relies on first achieving an appropriate representation of the modern marine iron cycle in models. When the iron distributions from 13 global ocean biogeochemistry models are compared against the latest oceanic sections from the GEOTRACES program, we find that all models struggle to reproduce many aspects of the observed spatial patterns. Models that reflect the emerging evidence for multiple iron sources or subtleties of its internal cycling perform much better in capturing observed features than their simpler contemporaries, particularly in the ocean interior. We show that the substantial uncertainty in the input fluxes of iron results in a very wide range of residence times across models, which has implications for the response of ecosystems and global carbon cycling to perturbations. Given this large uncertainty, iron fertilization experiments based on any single current generation model should be interpreted with caution. Improvements to how such models represent iron scavenging and also biological cycling are needed to raise confidence in their projections of global biogeochemical change in the ocean. [ABSTRACT FROM AUTHOR]