Controls on surface water carbonate chemistry along North American ocean margins.

Syntheses of carbonate chemistry spatial patterns are important for predicting ocean acidification impacts, but are lacking in coastal oceans. Here, we show that along the North American Atlantic and Gulf coasts the meridional distributions of dissolved inorganic carbon (DIC) and carbonate mineral saturation state (Ω) are controlled by partial equilibrium with the atmosphere resulting in relatively low DIC and high Ω in warm southern waters and the opposite in cold northern waters. However, pH and the partial pressure of CO2 (pCO2) do not exhibit a simple spatial pattern and are controlled by local physical and net biological processes which impede equilibrium with the atmosphere. Along the Pacific coast, upwelling brings subsurface waters with low Ω and pH to the surface where net biological production works to raise their values. Different temperature sensitivities of carbonate properties and different timescales of influencing processes lead to contrasting property distributions within and among margins.

The oceanic sink for anthropogenic CO2 from 1994 to 2007.

We quantify the oceanic sink for anthropogenic carbon dioxide (CO2) over the period 1994 to 2007 by using observations from the global repeat hydrography program and contrasting them to observations from the 1990s. Using a linear regression-based method, we find a global increase in the anthropogenic CO2 inventory of 34 ± 4 petagrams of carbon (Pg C) between 1994 and 2007. This is equivalent to an average uptake rate of 2.6 ± 0.3 Pg C year-1 and represents 31 ± 4% of the global anthropogenic CO2 emissions over this period. Although this global ocean sink estimate is consistent with the expectation of the ocean uptake having increased in proportion to the rise in atmospheric CO2, substantial regional differences in storage rate are found, likely owing to climate variability-driven changes in ocean circulation.

Climatic modulation of surface acidification rates through summertime wind forcing in the Southern Ocean.

While the effects of the Southern Annular Mode (SAM), a dominant climate variability mode in the Southern Ocean, on ocean acidification have been examined using models, no consensus has been reached. Using observational data from south of Tasmania, we show that during a period with positive SAM trends, surface water pH and aragonite saturation state at 60°-55° S (Antarctic Zone) decrease in austral summer at rates faster than those predicted from atmospheric CO2 increase alone, whereas an opposite pattern is observed at 50°-45° S (Subantarctic Zone). Together with other processes, the enhanced acidification at 60°-55° S may be attributed to increased westerly winds that bring in more "acidified" waters from the higher latitudes via enhanced meridional Ekman transport and from the subsurface via increased vertical mixing. Our observations support climatic modulation of ocean acidification superimposed on the effect of increasing atmospheric CO2.

Spectrophotometric Determination of Carbonate Ion Concentrations: Elimination of Instrument-Dependent Offsets and Calculation of In Situ Saturation States.

This work describes an improved algorithm for spectrophotometric determinations of seawater carbonate ion concentrations ([CO32-]spec) derived from observations of ultraviolet absorbance spectra in lead-enriched seawater. Quality-control assessments of [CO32-]spec data obtained on two NOAA research cruises (2012 and 2016) revealed a substantial intercruise difference in average Δ[CO32-] (the difference between a sample's [CO32-]spec value and the corresponding [CO32-] value calculated from paired measurements of pH and dissolved inorganic carbon). Follow-up investigation determined that this discordance was due to the use of two different spectrophotometers, even though both had been properly calibrated. Here we present an essential methodological refinement to correct [CO32-]spec absorbance data for small but significant instrumental differences. After applying the correction (which, notably, is not necessary for pH determinations from sulfonephthalein dye absorbances) to the shipboard absorbance data, we fit the combined-cruise data set to produce empirically updated parameters for use in processing future (and historical) [CO32-]spec absorbance measurements. With the new procedure, the average Δ[CO32-] offset between the two aforementioned cruises was reduced from 3.7 µmol kg-1 to 0.7 µmol kg-1, which is well within the standard deviation of the measurements (1.9 µmol kg-1). We also introduce an empirical model to calculate in situ carbonate ion concentrations from [CO32-]spec. We demonstrate that these in situ values can be used to determine calcium carbonate saturation states that are in good agreement with those determined by more laborious and expensive conventional methods.

The reinvigoration of the Southern Ocean carbon sink.

Several studies have suggested that the carbon sink in the Southern Ocean-the ocean's strongest region for the uptake of anthropogenic CO2 -has weakened in recent decades. We demonstrated, on the basis of multidecadal analyses of surface ocean CO2 observations, that this weakening trend stopped around 2002, and by 2012, the Southern Ocean had regained its expected strength based on the growth of atmospheric CO2. All three Southern Ocean sectors have contributed to this reinvigoration of the carbon sink, yet differences in the processes between sectors exist, related to a tendency toward a zonally more asymmetric atmospheric circulation. The large decadal variations in the Southern Ocean carbon sink suggest a rather dynamic ocean carbon cycle that varies more in time than previously recognized.

Decadal acidification in the water masses of the Atlantic Ocean.

Global ocean acidification is caused primarily by the ocean's uptake of CO2 as a consequence of increasing atmospheric CO2 levels. We present observations of the oceanic decrease in pH at the basin scale (50 °S-36 °N) for the Atlantic Ocean over two decades (1993-2013). Changes in pH associated with the uptake of anthropogenic CO2 (ΔpHCant) and with variations caused by biological activity and ocean circulation (ΔpHNat) are evaluated for different water masses. Output from an Institut Pierre Simon Laplace climate model is used to place the results into a longer-term perspective and to elucidate the mechanisms responsible for pH change. The largest decreases in pH (∆pH) were observed in central, mode, and intermediate waters, with a maximum ΔpH value in South Atlantic Central Waters of -0.042 ± 0.003. The ΔpH trended toward zero in deep and bottom waters. Observations and model results show that pH changes generally are dominated by the anthropogenic component, which accounts for rates between -0.0015 and -0.0020/y in the central waters. The anthropogenic and natural components are of the same order of magnitude and reinforce one another in mode and intermediate waters over the time period. Large negative ΔpHNat values observed in mode and intermediate waters are driven primarily by changes in CO2 content and are consistent with (i) a poleward shift of the formation region during the positive phase of the Southern Annular Mode in the South Atlantic and (ii) an increase in the rate of the water mass formation in the North Atlantic.

Tracking the variable North Atlantic sink for atmospheric CO2.

The oceans are a major sink for atmospheric carbon dioxide (CO2). Historically, observations have been too sparse to allow accurate tracking of changes in rates of CO2 uptake over ocean basins, so little is known about how these vary. Here, we show observations indicating substantial variability in the CO2 uptake by the North Atlantic on time scales of a few years. Further, we use measurements from a coordinated network of instrumented commercial ships to define the annual flux into the North Atlantic, for the year 2005, to a precision of about 10%. This approach offers the prospect of accurately monitoring the changing ocean CO2 sink for those ocean basins that are well covered by shipping routes.

Advances in quantifying air-sea gas exchange and environmental forcing.

The past decade has seen a substantial amount of research on air-sea gas exchange and its environmental controls. These studies have significantly advanced the understanding of processes that control gas transfer, led to higher quality field measurements, and improved estimates of the flux of climate-relevant gases between the ocean and atmosphere. This review discusses the fundamental principles of air-sea gas transfer and recent developments in gas transfer theory, parameterizations, and measurement techniques in the context of the exchange of carbon dioxide. However, much of this discussion is applicable to any sparingly soluble, non-reactive gas. We show how the use of global variables of environmental forcing that have recently become available and gas exchange relationships that incorporate the main forcing factors will lead to improved estimates of global and regional air-sea gas fluxes based on better fundamental physical, chemical, and biological foundations.

Simultaneous spectrophotometric flow-through measurements of pH, carbon dioxide fugacity, and total inorganic carbon in seawater.

An autonomous multi-parameter flow-through CO2 system has been developed to simultaneously measure surface seawater pH, carbon dioxide fugacity (fCO2), and total dissolved inorganic carbon (DIC). All three measurements are based on spectrophotometric determinations of solution pH at multiple wavelengths using sulfonephthalein indicators. The pH optical cell is machined from a PEEK polymer rod bearing a bore-hole with an optical pathlength of approximately 15 cm. The fCO2 optical cell consists of Teflon AF 2400 (DuPont) capillary tubing sealed within the bore-hole of a PEEK rod. This Teflon AF tubing is filled with a standard indicator solution with a fixed total alkalinity, and forms a liquid core waveguide (LCW). The LCW functions as both a long pathlength (approximately 15 cm) optical cell and a membrane that equilibrates the internal standard solution with external seawater. fCO2 is then determined by measuring the pH of the internal solution. DIC is measured by determining the pH of standard internal solutions in equilibrium with seawater that has been acidified to convert all forms of DIC to CO2. The system runs repetitive measurement cycles with a sampling frequency of approximately 7 samples (21 measurements) per hour. The system was used for underway measurements of sea surface pH, fCO2, and DIC during the CLIVAR/CO2 A16S cruise in the South Atlantic Ocean in 2005. The field precisions were evaluated to be 0.0008 units for pH, 0.9 microatm for fCO2, and 2.4 micromol kg(-1) for DIC. These field precisions are close to those obtained in the laboratory. Direct comparison of our measurements and measurements obtained using established standard methods revealed that the system achieved field agreements of 0.0012+/-0.0042 units for pH, 1.0+/-2.5 microatm for fCO2, and 2.2+/-6.0 micromol kg(-1) for DIC. This system integrates spectrophotometric measurements of multiple CO2 parameters into a single package suitable for observations of both seawater and freshwater.

Farfield tracing of a point source discharge plume in the coastal ocean using sulfur hexafluoride.

Pathways and dilution of a point source ocean discharge in the farfield (approximately to 10-66 km) were measured using the deliberate tracer sulfur hexafluoride (SF6). The injection of SF6 was performed by bubbling the gas over a period of 6 days into an ocean outfall pipe discharging into the southeast Florida coastal ocean. The surface SF6 concentrations show that the discharged water flowed northward parallel to the coast with a broadening of the width of the plume to about 3 km at the farthest point sampled, 66 km from the outfall. The discharge was fully mixed throughout the water column within 13 km of the outfall terminus. In the first 20 km from the outfall, SF6 surface concentrations were highly variable, while beyond this the SF6 concentrations decreased monotonically going northward. The currents were measured during the study with a bottom-mounted acoustic Doppler current profiler (ADCP) located 5.5 km from the outfall. Velocities were variable in magnitude and direction but showed a net northward flow during the 6-day study. Maximum concentrations decreased by about 200-fold per kilometer from the outfall to the northern end of the study area. The study shows that SF6 is an effective method to trace point source releases far from their origin.

The oceanic sink for anthropogenic CO2.

Using inorganic carbon measurements from an international survey effort in the 1990s and a tracer-based separation technique, we estimate a global oceanic anthropogenic carbon dioxide (CO2) sink for the period from 1800 to 1994 of 118 +/- 19 petagrams of carbon. The oceanic sink accounts for approximately 48% of the total fossil-fuel and cement-manufacturing emissions, implying that the terrestrial biosphere was a net source of CO2 to the atmosphere of about 39 +/- 28 petagrams of carbon for this period. The current fraction of total anthropogenic CO2 emissions stored in the ocean appears to be about one-third of the long-term potential.

Southern Ocean iron enrichment experiment: carbon cycling in high- and low-Si waters.

The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the Southern Ocean is particularly important because of its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may be differentially expressed between waters with high silicic acid in the south and low silicic acid in the north, where diatom growth may be limited by both silicic acid and iron. Two mesoscale experiments, designed to investigate the effects of iron enrichment in regions with high and low concentrations of silicic acid, were performed in the Southern Ocean. These experiments demonstrate iron's pivotal role in controlling carbon uptake and regulating atmospheric partial pressure of carbon dioxide.

Long-term storage of natural water samples for dissolved oxygen determination.

A method for preserving natural water samples for dissolved oxygen analysis is recommended. The conventional method of using greased glass stoppers have been found to cause an increase in oxygen concentration by 12% over 1-month period as a result of evaporation of water sample through micro-gaps and concurrent intrusion of air into the water sample bottles. Sealing the sample bottles with water has been found to be the optimal storage method. It permits a 100.2 +/- 0.3% recovery of dissolved oxygen concentration from storage seawater samples over 4 months.