Iron controls phytoplankton growth in large tracts of the global ocean, and thereby influences carbon dioxide drawdown. Recent advances reveal the importance of iron-binding ligands and organic matter remineralization in regulating ocean iron levels.

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The biogeochemical cycle of iron in the ocean

The deep sea, the largest ecosystem on Earth and one of the least studied, harbours high biodiversity and provides a wealth of resources. Although humans have used the oceans for millennia, technological developments now allow exploitation of fisheries resources, hydrocarbons and minerals below 2000 m depth. The remoteness of the deep seafloor has promoted the disposal of residues and litter. Ocean acidification and climate change now bring a new dimension of global effects. Thus the challenges facing the deep sea are large and accelerating, providing a new imperative for the science community, industry and national and international organizations to work together to develop successful exploitation management and conservation of the deep-sea ecosystem. This paper provides scientific expert judgement and a semi-quantitative analysis of past, present and future impacts of human-related activities on global deep-sea habitats within three categories: disposal, exploitation and climate change. The analysis is the result of a Census of Marine Life – SYNDEEP workshop (September 2008). A detailed review of known impacts and their effects is provided. The analysis shows how, in recent decades, the most significant anthropogenic activities that affect the deep sea have evolved from mainly disposal (past) to exploitation (present). We predict that from now and into the future, increases in atmospheric CO2 and facets and consequences of climate change will have the most impact on deep-sea habitats and their fauna. Synergies between different anthropogenic pressures and associated effects are discussed, indicating that most synergies are related to increased atmospheric CO2 and climate change effects. We identify deep-sea ecosystems we believe are at higher risk from human impacts in the near future: benthic communities on sedimentary upper slopes, cold-water corals, canyon benthic communities and seamount pelagic and benthic communities. We finalise this review with a short discussion on protection and management methods.

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Man and the last great wilderness: human impact on the deep sea.

Zooplankton fecal pellets, marine snow, phytodetritus and the ocean’s biological pump

Presented here is a combined historical account, current synthesis and a perspective of how the modern Fe cycle functions, and how this cycle has evolved through geologic time. We begin by highlighting how new developments in nanogeoscience demonstrate the importance of nanoparticulate Fe (oxyhydr) oxide aggregates in the modern iron cycle. We further document how these aggregates are supplied from shelf sediments, aeolian dust and icebergs to the global ocean. Based on these observations, we present a kinetic model evaluating the supply of bioavailable Fe to surface seawater by ferrihydrite dissolution, photoreduction and siderophore-aided dissolution. The model indicates that the rate of delivery of bioavailable Fe from icebergs to the Southern Ocean is at least as large as that by wind-blown dust. However estimates of all the main aqueous, nanoparticulate and colloidal (and potentially bioavailable) Fe inputs to the ocean are poorly-constrained.

We provide a historical perspective on the evolution of ideas as to how sedimentary pyrite formation is controlled and how these ideas led to the development of the Fe-based palaeoenvironmental proxies widely used today. This provides a springboard into our discussion of the ancient Fe cycle, which begins with a survey of how Fe interacts with a variety of other elements of biogeochemical interest including sulphur, oxygen and nitrogen. We highlight how interactions between these elements have evolved through geologic time, and how these interactions define the evolution of ocean and atmospheric chemistry. It is clear that the Fe cycle has gained a prominent role in regulating the biogeochemical function of the oceans through time. We offer, in the end, suggestions and a geochemical perspective as to how recent momentum in our understanding of the Fe cycle may be harnessed into catalysing future progress in the field.

The Iron Biogeochemical Cycle Past and Present

. The natural environment is a major source of atmospheric aerosols, including dust, secondary organic material from terrestrial biogenic emissions, carbonaceous particles from wildfires, and sulphate from marine phytoplankton dimethyl sulphide emissions. These aerosols also have a significant effect on many components of the Earth system such as the atmospheric radiative balance and photosynthetically available radiation entering the biosphere, the supply of nutrients to the ocean, and the albedo of snow and ice. The physical and biological systems that produce these aerosols can be highly susceptible to modification due to climate change so there is the potential for important climate feedbacks. We review the impact of these natural systems on atmospheric aerosol based on observations and models, including the potential for long term changes in emissions and the feedbacks on climate. The number of drivers of change is very large and the various systems are strongly coupled. There have therefore been very few studies that integrate the various effects to estimate climate feedback factors. Nevertheless, available observations and model studies suggest that the regional radiative perturbations are potentially several Watts per square metre due to changes in these natural aerosol emissions in a future climate. Taking into account only the direct radiative effect of changes in the atmospheric burden of natural aerosols, and neglecting potentially large effects on other parts of the Earth system, a global mean radiative perturbation approaching 1 W m−2 is possible by the end of the century. The level of scientific understanding of the climate drivers, interactions and impacts is very low.

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A review of natural aerosol interactions and feedbacks within the Earth system

The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial-interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization--an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below--as invoked in some palaeoclimatic and future climate change scenarios--may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

Effect of natural iron fertilization on carbon sequestration in the Southern Ocean

. The fate of the organic matter (OM) produced by marine life controls the
major biogeochemical cycles of the Earth's system. The OM produced through
photosynthesis is either preserved, exported towards sediments or degraded
through remineralisation in the water column. The productive eastern
boundary upwelling systems (EBUSs) associated with oxygen minimum zones
(OMZs) would be expected to foster OM preservation due to low O2 
conditions. But their intense and diverse microbial activity should enhance
OM degradation. To investigate this contradiction, sediment traps were
deployed near the oxycline and in the OMZ core on an instrumented moored
line off Peru. Data provided high-temporal-resolution O2 series
characterising two seasonal steady states at the upper trap: suboxic
( [O2] µ mol kg −1 ) and hypoxic–oxic ( 15 [ O 2 ] 160 µ mol kg −1 ) in austral summer and
winter–spring, respectively. The OMZ vertical transfer efficiency of
particulate organic carbon (POC) between traps ( Teff ) can be classified
into three main ranges (high, intermediate, low). These different Teff 
ranges suggest that both predominant preservation (high Teff>50  %) and remineralisation (intermediate Teff 
20   50 % or low Teff  %) configurations can occur. An efficient OMZ vertical transfer
( Teff>50  %) has been reported in summer and winter associated with extreme
limitation in O2 concentrations or OM quantity for OM degradation.
However, higher levels of O2 or OM, or less refractory OM, at the
oxycline, even in a co-limitation context, can decrease the OMZ transfer
efficiency to below 50 %. This is especially true in summer during
intraseasonal wind-driven oxygenation events. In late winter and early
spring, high oxygenation conditions together with high fluxes of sinking
particles trigger a shutdown of the OMZ transfer ( Teff  %). Transfer efficiency of chemical elements composing the majority of
the flux (nitrogen, phosphorus, silica, calcium carbonate) follows the same
trend as for carbon, with the lowest transfer level being in late winter and
early spring. Regarding particulate isotopes, vertical transfer of
 δ15N suggests a complex pattern of 15N 
impoverishment or enrichment according to Teff modulation. This
sensitivity of OM to O2 fluctuations and particle concentration calls
for further investigation into OM and O2 -driven remineralisation
processes. This should include consideration of the intermittent behaviour of
OMZ towards OM demonstrated in past studies and climate projections.

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L. Scouarnec

Papers

Effect of natural iron fertilization on carbon sequestration in the Southern Ocean

Modulation of the vertical particle transfer efficiency in the oxygen minimum zone off Peru