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A manipulative experiment in which a reef is alkalinized in situ shows that calcification rates are likely to be lower already than they were in pre-industrial times because of acidification. Ocean acidification is one of several factors projected to threaten coral reef ecosystems, but disentangling its effects from other factors such as temperature is difficult. These authors used a manipulative experiment in which sodium hydroxide was added to seawater flowing over a natural coral reef community in situ. When ocean chemistry was restored closer to pre-industrial conditions, net community calcification increased. This suggests calcification rates are already lower than they were in pre-industrial times because of acidification. Deliberate alkalinization has been proposed as a geoengineering technique to offset ocean acidification, and this work suggests that the method could be effective, but only on a small scale — in protected bays or lagoons, for example. Approximately one-quarter of the anthropogenic carbon dioxide released into the atmosphere each year is absorbed by the global oceans, causing measurable declines in surface ocean pH, carbonate ion concentration ([CO32−]), and saturation state of carbonate minerals (Ω)1. This process, referred to as ocean acidification, represents a major threat to marine ecosystems, in particular marine calcifiers such as oysters, crabs, and corals. Laboratory and field studies2,3 have shown that calcification rates of many organisms decrease with declining pH, [CO32−], and Ω. Coral reefs are widely regarded as one of the most vulnerable marine ecosystems to ocean acidification, in part because the very architecture of the ecosystem is reliant on carbonate-secreting organisms4. Acidification-induced reductions in calcification are projected to shift coral reefs from a state of net accretion to one of net dissolution this century5. While retrospective studies show large-scale declines in coral, and community, calcification over recent decades6,7,8,9,10,11,12, determining the contribution of ocean acidification to these changes is difficult, if not impossible, owing to the confounding effects of other environmental factors such as temperature. Here we quantify the net calcification response of a coral reef flat to alkalinity enrichment, and show that, when ocean chemistry is restored closer to pre-industrial conditions, net community calcification increases. In providing results from the first seawater chemistry manipulation experiment of a natural coral reef community, we provide evidence that net community calcification is depressed compared with values expected for pre-industrial conditions, indicating that ocean acidification may already be impairing coral reef growth.

Continental margins are extremely sensitive to climate change, so it is important to understand how they might respond to extreme warm conditions. Changes in productivity, ocean chemistry and carbon cycling in the equatorial Atlantic during the ‘greenhouse’ climate interval in the mid-Cretaceous period point to river runoff as an important factor in the coastal ocean ecosystem. High discharge rates of continental freshwater led to dramatic changes in ocean productivity and to a lack of oxygen in the coastal ocean. The tropics have been suggested as the drivers of global ocean and atmosphere circulation and biogeochemical cycling during the extreme warmth of the Cretaceous period1,2; but the links between orbital forcing, freshwater runoff and the biogeochemistry of continental margins in extreme greenhouse conditions are not fully understood. Here we present Cretaceous records of geochemical tracers for freshwater runoff obtained from a sediment core off the Ivory Coast that indicate that alternating periods of arid and humid African climate were driven by orbital precession. Our simulations of the precession-driven patterns of river discharge with a global climate model suggest that ocean anoxia and black shale sedimentation were directly caused by high river discharge, and occurred specifically when the northern equinox coincided with perihelion (the minimum distance between the Sun and the Earth). We conclude that, in a warm climate, the oceans off tropical continental margins respond rapidly and sensitively to even modest changes in river discharge.

An in situ experiment finds that reducing the acidity of the seawater surrounding a natural coral reef significantly increases reef calcification, suggesting that ocean acidification may already be slowing coral growth. See Letter p.362 Ocean acidification is one of several factors projected to threaten coral reef ecosystems, but disentangling its effects from other factors such as temperature is difficult. These authors used a manipulative experiment in which sodium hydroxide was added to seawater flowing over a natural coral reef community in situ. When ocean chemistry was restored closer to pre-industrial conditions, net community calcification increased. This suggests calcification rates are already lower than they were in pre-industrial times because of acidification. Deliberate alkalinization has been proposed as a geoengineering technique to offset ocean acidification, and this work suggests that the method could be effective, but only on a small scale — in protected bays or lagoons, for example.

The nitrogen cycle rarely features in the grim litany of things at risk from global warming. Nick Lane reports on research that might change this ? with grave consequences for ocean chemistry.

CHANGES in ocean chemistry and circulation have been invoked to explain the lower atmospheric CO2 concentrations of glacial periods observed in ice-core records1. The processes that modulate these concentrations are not well understood, but an increase in the nutrient inventory of the ocean is one mechanism that could lower atmospheric CO2 levels by enhancing oceanic biological productivity and CO2 storage1-3. The oceanic concentrations of one such nutrient, nitrate, may be regulated by changes in the rate at which it is degraded by bacteria (denitrification) in oxygen-deficient subsurface waters. Denitrification constitutes a significant global sink for oceanic nitrate4, and the eastern tropical North Pacific Ocean is particularly important in this respect as it accounts for at least a third of global oceanic fixed-nitrogen removal by water-column denitrification4,5. Here we present 15N/14N records from marine sediment cores, which show that water-column denitrification in the eastern tropical North Pacific Ocean was greatly diminished during glacial periods. We suggest that, because nitrate limits biological productivity in much of the modern ocean, a consequent increase in the oceanic nitrate inventory during glacial periods could have contributed to the observed decrease in atmospheric CO2 concentration.

Lack of dissolved iron in the sea limits biological productivity and the uptake of carbon dioxide. The sources of dissolved iron in the North Atlantic Ocean have been identified from isotopic variations of this trace nutrient. See Letter p.212 Iron availability limits phytoplankton growth throughout the oceans, acting as a key influence on the global carbon cycle and the oceanic response to changing climate. But large uncertainties remain as to the relative importance of the various sources of iron, including windblown dust and hydrothermal vents. This paper presents a high-resolution transect of seawater dissolved stable iron isotope ratios and iron concentrations in the North Atlantic Ocean. Saharan dust aerosol emerges as the dominant source of dissolved iron along the section, with sediments and hydrothermal vents also significant. Changes in these sources through time may have wide-ranging implications for the global carbon cycle.

The observation of non-mass-dependent sulphur isotope ratios in sedimentary rocks more than ∼2.4 billion years old and the disappearance of this signal in younger sediments is taken as evidence for the transition from an anoxic to oxic atmosphere around 2.4 Gyr ago. But now, the preservation of a non mass-dependent signal that differs from that of preceding and following periods in the Archean is demonstrated. The findings support the original idea of an anoxic early atmosphere before 2.4 Gyr ago, and at the same time identifies variability within the isotope record that suggests changes in pre-2.4 Gya atmospheric pathways for non-mass-dependent chemistry and in the ultraviolet transparency of an evolving early atmosphere. The evolution of the Earth’s atmosphere is marked by a transition from an early atmosphere with very low oxygen content to one with an oxygen content within a few per cent of the present atmospheric level. Placing time constraints on this transition is of interest because it identifies the time when oxidative weathering became efficient, when ocean chemistry was transformed by delivery of oxygen and sulphate, and when a large part of Earth’s ecology changed from anaerobic to aerobic1. The observation of non-mass-dependent sulphur isotope ratios in sedimentary rocks more than ∼2.45 billion years (2.45 Gyr) old and the disappearance of this signal in younger sediments is taken as one of the strongest lines of evidence for the transition from an anoxic to an oxic atmosphere around 2.45 Gyr ago1,2,3,4,5. Detailed examination of the sulphur isotope record before 2.45 Gyr ago also reveals early and late periods of large amplitude non-mass-dependent signals bracketing an intervening period when the signal was attenuated5,6,7,8,9. Until recently, this record has been too sparse to allow interpretation, but collection of new data has prompted some workers8 to argue that the Mesoarchaean interval (3.2–2.8 Gyr ago) lacks a non-mass-dependent signal, and records the effects of earlier and possibly permanent oxygenation of the Earth’s atmosphere. Here we focus on the Mesoarchaean interval, and demonstrate preservation of a non-mass-dependent signal that differs from that of preceding and following periods in the Archaean. Our findings point to the persistence of an anoxic early atmosphere, and identify variability within the isotope record that suggests changes in pre-2.45-Gyr-ago atmospheric pathways for non-mass-dependent chemistry and in the ultraviolet transparency of an evolving early atmosphere.

RECENT observations of abundant, nonliving, submicrometre particles in the upper ocean1–3 and new measurements of 'dissolved9organic carbon4–7 have fuelled speculation concerning the role of colloidal matter in ocean chemistry and biology. Colloids may act as reactive intermediates in the marine geochemistry of trace metals8–12, and a biologically labile pool of colloidal matter would affect models of ocean carbon cycling13–16. Here we report the use of naturally occurring 234Th as an in situ tracer to estimate the residence time of colloidal matter in the surface waters near Bermuda. The 234Th activity of colloidal matter (size range 10,000 nominal molecular weight to 0.2 μm) is similar to that of small particles (0.2–53 μm). Modelling of our results indicates a mean residence time of colloidal 234Th with respect to aggregation into small particles of 10 days, which is roughly the same as for small-particle 234Th, yet a factor of ∼6 less than for the dissolved pool. These results suggest that, more generally, macromolecular colloidal matter has a short residence time and hence a rapid turnover rate in the upper open ocean.

The ocean chemistry of 1.5 billion years ago, inferred from rocks of that age, supports the view that marine conditions then were very different from those that pertained at earlier and later times.

The aragonite compensation depth fluctuated considerably during the last 20,000 yr. This fluctuation culminated in a preservation spike at about 14,000 yr BP. The aragonite preservation stratigraphy parallels that in calcitic foraminifera, and constitutes part of a worldwide phenomenon signalling considerable changes in ocean chemistry and providing a useful tool for correlation.

The potential impact of high carbon dioxide partial pressure on ocean chemistry is examined in order to investigate what constraints are imposed by the known record of chemical sedimentation through time. The evidence consists of the persistence of calcium carbonate and sulfate precipitation throughout almost the entire sedimentary rock record. A uniformitarian point of view that assumes no very great change in the conditions for the deposition of these chemical sediments. The methods of Holland (1972) are used to set limits on the composition of the water from which precipitation occurred. No inconsistencies between the sedimentary rock record and presumed higher partial pressure of carbon dioxide early in earth history, provided that high partial pressure was accompanied by a generally lower pH for seawater, higher concentrations of calcium and biocarbonate ions, and lower concentrations of carbonate and sulfate ions.

The depth–time pattern of calcite accumulation recorded at Deep Sea Drilling Project (DSDP) sites beneath the oligotrophic subtropical gyre in the south-east Pacific Ocean allows us to define here both the calcite compensation depth (CCD) and the lysocline. The Neogene CCD history is one of shoaling before 15 or 20 Myr and subsequent deepening to the present level of 4,100 m. A broad zone of increased calcite dissolution, 600 m thick, formed during early to middle Miocene time and has remained. This zone, the lysocline, denotes an important change in ocean chemistry and may reflect either an increase in the volume of more corrosive deep water beginning about 18 Myr and reaching steady state by 14 Myr, the time of rapid ice build-up in Antarctica, or an increase in calcite rain rate caused by an increase in surface-water productivity.

Cyanophytes range from at least 2,200 Myr (ref. 1) to the Recent, but they only produced common marine shelly fossils during the Palaeozoic and Mesozoic (570–80 Myr) (Fig. 1). This contrasts with the pattern of metazoan evolution in which rapid diversification near the Precambrian–Cambrian boundary was closely accompanied by skeletonization2 which has been retained in marine environments to the Recent. Attempts to explain this unusual geological distribution of marine calcareous cyanophytes cannot be made solely by reference to biological processes because these algae are mainly dependent on environmental conditions for their calcification3. Thus, the presence or absence of calcified cyanophytes may be a general indication of long-term changes in seawater chemistry. This likelihood has been recognized previously4 but has not been explored in any detail. Here I outline some possible explanations and suggest that cyanophyte calcification was facilitated by enhancement of marine CaCO3 precipitation rates in the late Precam-brian because of decrease in the Mg2+/Ca2+ ratio, linked to falling levels and extensive dolomite formation. Scarcity of calcareous cyanophytes in Cenozoic marine environments again implicates the Mg2+/Ca2+ ratio, inferred from ooid mineralogy to have increased in the late Mesozoic5, as a factor influencing cyanophyte calcification in the sea.

PERHAPS the most perplexing problem of biological evolution involves explanation of why diversification of life began only after 85% of the lifetime of the Earth while simple life seems to have been present for virtually all of geological time. Recent observations1–2 on the presence of complex organic species in meteoritic debris and in the interstellar medium show that some of the basic biological building blocks may have been available from the earliest times. Chamberlain and Marland3 have commented on the biological consequences of the Hargraves4 model of ocean-crust evolution while others5–6 have reassessed evidence on atmospheric evolution and indicated that changes in oxygen partial pressure may not have been critical to the diversification of life. In this note I suggest that important changes in ocean chemistry must have occurred if our current models of crustal evolution and igneous events have any validity. The evolution of complex life forms requires a high degree of environmental stability and constancy of chemical environment. It is suggested that this condition is closely linked to the spacing of major volcanic centres, and that the modern long wavelength pattern of ocean ridges and subduction zones was necessary to achieve this stability. The distribution of ophiolites or ocean-floor structures of modern type7 indicates that the present pattern may have been initiated about one billion years ago. Furthermore, large scale continental emergence4 may have been necessary to provide oceans with stable chemistry.