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Marine primary producers are largely dependent on and shape the Earth's climate, although their relationship with climate varies over space and time. The growth of phytoplankton and associated marine primary productivity in most of the modern global ocean is limited by the supply of nutrients, including the micronutrient iron. The addition of iron via episodic and frequent events drives the biological carbon pump and promotes the sequestration of atmospheric carbon dioxide (CO2) into the ocean. However, the dependence between iron and marine primary producers adaptively changes over different geological periods due to the variation in global climate and environment. In this review, we examined the role and importance of iron in modulating marine primary production during some specific geological periods, that is, the Great Oxidation Event (GOE) during the Huronian glaciation, the Snowball Earth Event during the Cryogenian, the glacial–interglacial cycles during the Pleistocene, and the period from the last glacial maximum to the late Holocene. Only the change trend of iron bioavailability and climate in the glacial–interglacial cycles is consistent with the Iron Hypothesis. During the GOE and the Snowball Earth periods, although the bioavailability of iron in the ocean and the climate changed dramatically, the changing trend of many factors contradicted the Iron Hypothesis. By detangling the relationship among marine primary productivity, iron availability and oceanic environments in different geological periods, this review can offer some new insights for evaluating the impact of ocean iron fertilization on removing CO2 from the atmosphere and regulating the climate. [ABSTRACT FROM AUTHOR]

Approximately one-quarter of the CO 2 emitted to the atmosphere annually from human activities is absorbed by the ocean, resulting in a reduction of seawater pH and shifts in seawater carbonate chemistry. This multi-decadal process, termed "anthropogenic ocean acidification" (OA), has been shown to have detrimental impacts on marine ecosystems. Recent years have seen a globally coordinated effort to measure the changes in seawater chemistry caused by OA, with best practices now available for these measurements. In contrast to these substantial advances in observing physicochemical changes due to OA, quantifying their biological consequences remains challenging, especially from in situ observations under real-world conditions. Results from 2 decades of controlled laboratory experiments on OA have given insight into the likely processes and mechanisms by which elevated CO 2 levels affect biological process, but the manifestation of these process across a plethora of natural situations has yet to be fully explored. This challenge requires us to identify a set of fundamental biological and ecological indicators that are (i) relevant across all marine ecosystems, (ii) have a strongly demonstrated link to OA, and (iii) have implications for ocean health and the provision of ecosystem services with impacts on local marine management strategies and economies. This paper draws on the understanding of biological impacts provided by the wealth of previous experiments, as well as the findings of recent meta-analyses, to propose five broad classes of biological indicators that, when coupled with environmental observations including carbonate chemistry, would allow the rate and severity of biological change in response to OA to be observed and compared. These broad indicators are applicable to different ecological systems, and the methods for data analysis suggested here would allow researchers to combine biological response data across regional and global scales by correlating rates of biological change with the rate of change in carbonate chemistry parameters. Moreover, a method using laboratory observation to design an optimal observing strategy (frequency and duration) and observe meaningful biological rates of change highlights the factors that need to be considered when applying our proposed observation strategy. This innovative observing methodology allows inclusion of a wide diversity of marine ecosystems in regional and global assessments and has the potential to increase the contribution of OA observations from countries with developing OA science capacity. [ABSTRACT FROM AUTHOR]

Approximately one quarter of the CO2 emitted to the atmosphere annually from human activities is absorbed by the ocean, resulting in a reduction of seawater pH and shifts in seawater carbonate chemistry. This multi-decadal process, termed "anthropogenic ocean acidification" (OA) has been shown to have detrimental impacts on marine ecosystems. Recent years have seen a globally coordinated effort to measure the changes in seawater chemistry caused by OA, with best practices now available for these measurements. In contrast to these substantial advances in observing physico-chemical changes due to OA, quantifying their biological consequences remains challenging, especially from in-situ observations under real-world conditions. Results from two decades of controlled laboratory experiments on OA have given insight into the likely processes and mechanisms by which elevated CO2 levels affect biological process, but the manifestation of these process across a plethora of natural situations has yet to be explored fully. This challenge requires us to identify a set of fundamental biological and ecological indicators that are i) relevant across all marine ecosystems, ii) have a strongly demonstrated link to OA, and iii) have implications for ocean health and the provision of ecosystem services with impacts on local marine management strategies and economies. This paper draws on the understanding of biological impacts provided by the wealth of previous experiments, as well as the findings of recent meta-analyses, to propose five broad classes of biological indicators that, when coupled with environmental observations, including carbonate chemistry, would allow the rate and severity of biological change in response to OA to be observed and compared. These broad indicators are applicable to different ecological systems, and the methods for data analysis suggested here would allow researchers to combine biological response data across regional and global scales by correlating rates of biological change with the rate of change in carbonate chemistry parameters. Moreover, a method using laboratory observation to design an optimal observing strategy (frequency and duration) and observe meaningful biological rates of change highlights the factors that need to be considered when applying our proposed observation strategy. This innovative observing methodology allows inclusion of a wide diversity of marine ecosystems in regional and global assessments and has the potential to increase the contribution of OA observations from countries with developing OA science capacity. [ABSTRACT FROM AUTHOR]

The seasonal variability of the lateral flux of total alkalinity (TAlk) and dissolved inorganic carbon (DIC) of the tropical Hooghly estuary is analyzed in this work. In situ observations of water temperature, salinity, dissolved oxygen, TAlk, and pH were measured in four different stations of the Hooghly estuary. It was measured once every month during 2015–2016, and subsequently, DIC was estimated. A carbon budget was constructed to quantify carbon flows through the freshwater-marine continuum of the Hooghly estuary, and plausible impacts on the adjacent coastal ocean, the northern Bay of Bengal, were examined. The biogeochemical mass balance box model was used to compute the seasonal flow of carbon flux, and subsequently, the annual budgeting of lateral fluxes of TAlk and DIC to the adjacent coastal ocean was carried out. The net annual TAlk and DIC flux from the Hooghly estuary to the adjacent coastal ocean were 4.45 ± 1.90 × 1011 mol and 4.59 ± 1.70 × 1011 mol, respectively. The net annual DIC flux of the Hooghly estuary is about 30 to 60 times higher than surface area integrated air–water CO2 flux, which is an indication of promoting acidification in the adjacent coastal ocean. The present study indicates that the lateral DIC flux has increased substantially in the Hooghly estuary during the last two decades. The increase in inorganic carbon load in the Hooghly estuary due to the enhanced discharge of inorganic and organic matter load in the upper reaches of the estuary led to this increase in lateral DIC flux. The results strongly establish the need of having such regional studies for better understanding the estuarine carbon dynamics, and its role in controlling the adjacent coastal ocean dynamics. [ABSTRACT FROM AUTHOR]

Key Points: Synoptic‐scale winter blooms and mixed‐layer pump events were observed by a BGC‐Argo float in the midlatitude Northwest Pacific OceanThe repetitive mixed‐layer pump episodes generated a very high particulate organic carbon export of ~110 mg C m−2 day−1 in wintertimeThe current BGC‐Argo protocols captured only 20%, or entirely missed, substantial pulsed carbon export driven by mixed‐layer pump events The winter‐ and spring‐time mixed‐layer pump (MLP) significantly augments the global carbon transport from the surface mixed layer to deeper waters when ephemeral surface phytoplankton blooms are repeatedly mixed to depth. Exploiting unusual 190 + daily BGC‐Argo profiling measurements within a recirculation gyre, we show repetitive MLP episodes generating a January–March averaged particulate organic carbon (POC) export of ~110 mg C m−2 day−1 in the midlatitude (31°N) Northwest Pacific. Subsampling this dataset on a 5‐ or 10‐day cycle yielded an order of magnitude less export, or even totally missed all the MLP events. The evidence here supports the need for new strategies if the BGC‐Argo program is to adequately quantify ocean carbon cycling and its effects on biological systems. We propose that a handful of floats be tasked with daily profiling, and machine learning strategies be used to link these data with satellite derived measurements to estimate the synoptic‐scale MLP export. Plain Language Summary: The Biogeochemical‐Argo program (BGC‐Argo) is an essential tool for quantifying carbon export from surface waters to the ocean interior, which is critical for understanding and forecasting changes in atmospheric CO2. Using novel, high‐frequency (1‐day cycle during 9‐ month period) BGC‐Argo data, we estimated the carbon export in the midlatitude Northwest Pacific Ocean stemming from unexpectedly continuous mixed‐layer pump (MLP) events. Such events occur when wintertime phytoplankton blooms developed during calm wind periods then are injected to depth by storms. This process generated remarkably high carbon export over winter, with values ranging from 18% to 30% of that from the massive North Atlantic spring bloom. These events, however, are largely (80%) or entirely missed using the longer standard float cycling intervals (5–10 days) accepted by the BGC‐Argo program. The findings here suggest that some high‐frequency BGC‐Argo observations are necessary for more accurately estimating the global carbon export. [ABSTRACT FROM AUTHOR]

In the context of global climate change, ocean iron fertilization (OIF) has been suggested as a potential geoengineering strategy to enhance the growth of marine phytoplankton, subsequently promote the ocean carbon sink, and ultimately regulate atmospheric carbon dioxide (CO 2) and mitigate climate change. However, in past artificial OIF experiments, the elevation of both net primary production and carbon export to the deep ocean is far less than anticipated. This discrepancy can be attributed to several key factors that strongly influence the efficiency of OIF and biological carbon pump (BCP) including iron bioavailability and retention time, the sensitivity of phytoplankton to iron limitation, the influence of viruses and other microorganisms comprising the microbial carbon pump (MCP), and non-biological carbon cycles. Although previous reviews have extensively summarized the outcomes of the past OIF experiments, the mechanisms and key factors that influence climate regulation mediated by OIF have not been elucidated. In this review, we analyze these mechanisms throughout the whole process from OIF, to phytoplankton physiological responses, to carbon dioxide removal (CDR) and sequestration in the ocean, and ultimately to CDR in the atmosphere and its impact on climate change. By examining both natural and artificial OIF cases, as well as recent advancements in phytoplankton physiological ecology and carbon pump theory, possible reasons leading to the inconsistencies between OIF and carbon export are explored. These comprehensive analyses are helpful to identify the key issues in regulating climate change by providing a reference and scientific approach to guide potential OIF attempts in the future. [ABSTRACT FROM AUTHOR]