63 research outputs found
On which timescales do gas transfer velocities control North Atlantic CO2 flux variability?
This is the final version of the article. Available from AGU via the DOI in this record.The North Atlantic is an important basin for the global ocean's uptake of anthropogenic and natural carbon dioxide (CO 2 ), but the mechanisms controlling this carbon flux are not fully understood. The air-sea flux of CO 2 , F, is the product of a gas transfer velocity, k, the air-sea CO 2 concentration gradient, ÎpCO 2 , and the temperature- and salinity-dependent solubility coefficient, α. k is difficult to constrain, representing the dominant uncertainty in F on short (instantaneous to interannual) timescales. Previous work shows that in the North Atlantic, ÎpCO 2 and k both contribute significantly to interannual F variability but that k is unimportant for multidecadal variability. On some timescale between interannual and multidecadal, gas transfer velocity variability and its associated uncertainty become negligible. Here we quantify this critical timescale for the first time. Using an ocean model, we determine the importance of k, ÎpCO 2 , and α on a range of timescales. On interannual and shorter timescales, both ÎpCO 2 and k are important controls on F. In contrast, pentadal to multidecadal North Atlantic flux variability is driven almost entirely by ÎpCO 2 ; k contributes less than 25%. Finally, we explore how accurately one can estimate North Atlantic F without a knowledge of nonseasonal k variability, finding it possible for interannual and longer timescales. These findings suggest that continued efforts to better constrain gas transfer velocities are necessary to quantify interannual variability in the North Atlantic carbon sink. However, uncertainty in k variability is unlikely to limit the accuracy of estimates of longer-term flux variability.This work was supported the RAGNARoCC NERC directed research program (NE/K002546/1, NE/K00249X/1, and NE/K002473/1)
Zooplankton Gut Passage Mobilizes Lithogenic Iron for Ocean Productivity
Iron is an essential nutrient for phytoplankton, but low concentrations limit primary production and associated atmospheric carbon drawdown in large parts of the worldâs oceans [1 and 2]. Lithogenic particles deriving from aeolian dust deposition, glacial runoff, or river discharges can form an important source if the attached iron becomes dissolved and therefore bioavailable [3, 4 and 5]. Acidic digestion by zooplankton is a potential mechanism for iron mobilization [6], but evidence is lacking. Here we show that Antarctic krill sampled near glacial outlets at the island of South Georgia (Southern Ocean) ingest large amounts of lithogenic particles and contain 3-fold higher iron concentrations in their muscle than specimens from offshore, which confirms mineral dissolution in their guts. About 90% of the lithogenic and biogenic iron ingested by krill is passed into their fecal pellets, which contain âŒ5-fold higher proportions of labile (reactive) iron than intact diatoms. The mobilized iron can be released in dissolved form directly from krill or via multiple pathways involving microbes, other zooplankton, and krill predators. This can deliver substantial amounts of bioavailable iron and contribute to the fertilization of coastal waters and the ocean beyond. In line with our findings, phytoplankton blooms downstream of South Georgia are more intensive and longer lasting during years with high krill abundance on-shelf. Thus, krill crop phytoplankton but boost new production via their nutrient supply. Understanding and quantifying iron mobilization by zooplankton is essential to predict ocean productivity in a warming climate where lithogenic iron inputs from deserts, glaciers, and rivers are increasing [7, 8, 9 and 10]
The distribution of lead concentrations and isotope compositions in the eastern Tropical Atlantic Ocean
Anthropogenic emissions have dominated marine Pb sources during the past century. Here we present Pb concentrations and isotope compositions for ocean depth profiles collected in the eastern Tropical Atlantic Ocean (GEOTRACES section GA06), to trace the transfer of anthropogenic Pb into the ocean interior. Variations in Pb concentration and isotope composition were associated with changes in hydrography. Water masses ventilated in the southern hemisphere generally featured lower 206Pb/207Pb and 208Pb/207Pb ratios than those ventilated in the northern hemisphere, in accordance with Pb isotope data of historic anthropogenic Pb emissions. The distributions of Pb concentrations and isotope compositions in northern sourced waters were consistent with differences in their ventilation timescales. For example, a Pb concentration maximum at intermediate depth (600â900âŻm, 35âŻpmolâŻkgâ1) in waters sourced from the Irminger/Labrador Seas, is associated with Pb isotope compositions (206Pb/207PbâŻ=âŻ1.1818â1.1824, 208Pb/207PbâŻ=âŻ2.4472â2.4483) indicative of northern hemispheric emissions during the 1950s and 1960s close to peak leaded petrol usage, and a transit time of âŒ50â60âŻyears. In contrast, North Atlantic Deep Water (2000â4000âŻm water depth) featured lower Pb concentrations and isotope compositions (206Pb/207PbâŻ=âŻ1.1762â1.184, 208Pb/207PbâŻ=âŻ2.4482â2.4545) indicative of northern hemispheric emissions during the 1910s and 1930s and a transit time of âŒ80â100âŻyears. This supports the notion that transient anthropogenic Pb inputs are predominantly transferred into the ocean interior by water mass transport. However, the interpretation of Pb concentration and isotope composition distributions in terms of ventilation timescales and pathways is complicated by (1) the chemical reactivity of Pb in the ocean, and (2) mixing of waters ventilated during different time periods. The complex effects of water mass mixing on Pb distributions is particularly apparent in seawater in the Tropical Atlantic Ocean which is ventilated from the southern hemisphere. In particular, South Atlantic Central Water and Antarctic Intermediate Water were dominated by anthropogenic Pb emitted during the last 50â100âŻyears, despite estimates of much older average ventilation ages in this region
Sources of dissolved iron to oxygen minimum zone waters on the Senegalese continental margin in the tropical North Atlantic Ocean: Insights from iron isotopes
Oxygen minimum zones (OMZs) cover extensive areas of eastern boundary ocean regions and play an important role in the cycling of the essential micronutrient iron (Fe). The isotopic composition of dissolved Fe (dFe) in shelf and slope waters on the Senegalese margin was determined to investigate the processes leading to enhanced dFe concentrations (up to 2âŻnM) in this tropical North Atlantic OMZ. On the shelf, the ÎŽ56Fe value of dFe (relative to the reference material IRMM-014) was as low as â0.33â°, which can be attributed to input of dFe from both reductive and nonreductive dissolution of sediments. Benthic inputs of dFe are subsequently upwelled to surface waters and recycled in the water column by biological uptake and remineralisation processes. Remineralised dFe is characterised by relatively high ÎŽ56Fe values (up to +0.41â°), and the contribution of remineralised Fe to the total dFe pool increases with distance from the shelf. Remineralisation plays an important role in the redistribution of dFe that is mainly supplied by benthic and atmospheric inputs, although dust inputs, estimated from dissolved aluminium concentrations, were low at the time of our study (2â9âŻnmol dFe mâ2 dâ1). As OMZs are expected to expand as climate warms, our data provide important insights into Fe sources and Fe cycling in the tropical North Atlantic Ocean
Radium-228-derived ocean mixing and trace element inputs in the South Atlantic
Trace elements (TEs) play important roles as micronutrients in modulating marine productivity in the global ocean. The South Atlantic around 40âŠS is a prominent region of high productivity and a transition zone between the nitrate-depleted subtropical gyre and the iron-limited Southern Ocean. However, the sources and ïŹuxes of trace elements to this region remain unclear. In this study, the distribution of the naturally occurring radioisotope 228Ra in the water column of the South Atlantic (Cape Basin and Argentine Basin) has been investigated along a 40âŠS zonal transect to estimate ocean mixing and trace element supply to the surface ocean. Ra-228 proïŹles have been used to determine the horizontal and vertical mixing rates in the near-surface open ocean. In the Argentine Basin, horizontal mixingfromthecontinentalshelftotheopenoceanshowsan eddy diffusion of Kx =1.8±1.4 (106 cm2 sâ1) and an integrated advection velocity w=0.6±0.3cmsâ1. In the Cape Basin, horizontal mixing is Kx =2.7±0.8 (107 cm2 sâ1) andverticalmixing Kz=1.0â1.7cm2 sâ1 intheupper600m layer. Three different approaches (228Ra diffusion, 228Ra advection, and 228Ra/TE ratio) have been applied to estimate the dissolved trace element ïŹuxes from the shelf to the open ocean. These approaches bracket the possible range of off-shelf ïŹuxes from the Argentine Basin margin to be 4â21 (Ă103)nmolComâ2 dâ1, 8â19 (Ă104)nmolFemâ2 dâ1 and 2.7â6.3 (Ă104)nmolZnmâ2 dâ1. Off-shelf ïŹuxes from the Cape Basin margin are 4.3â6.2 (Ă103)nmolComâ2 dâ1, 1.2â3.1 (Ă104)nmolFemâ2 dâ1,
and 0.9â1.2 (Ă104)nmolZnmâ2 dâ1. On average, at 40âŠS in the Atlantic, vertical mixing supplies 0.1â 1.2nmolComâ2 dâ1, 6â9nmolFemâ2 dâ1, and 5â 7nmolZnmâ2 dâ1 to the euphotic zone. Compared with atmospheric dust and continental shelf inputs, vertical mixing is a more important source for supplying dissolved trace elements to the surface 40âŠS Atlantic transect. It is insufïŹcient, however, to provide the trace elements removed by biological uptake, particularly for Fe. Other inputs (e.g. particulate or from winter deep mixing) are required to balance the trace element budgets in this region
Seasonal cycling of zinc and cobalt in the south-eastern Atlantic along the GEOTRACES GA10 section
Abstract. We report the distributions and stoichiometry of dissolved zinc (dZn) and cobalt (dCo) in sub-tropical and sub-Antarctic waters of the south-eastern Atlantic Ocean during austral spring 2010 and summer 2011/2012. In sub-tropical surface waters, mixed-layer dZn and dCo concentrations during early spring were 1.60â±â2.58ânM and 30â±â11âpM, respectively, compared with summer values of 0.14â±â0.08ânM and 24â±â6âpM. The elevated spring dZn concentrations resulted from an apparent offshore transport of elevated dZn at depths between 20â55âm, derived from the Agulhas Bank. In contrast, open-ocean sub-Antarctic surface waters displayed largely consistent inter-seasonal mixed-layer dZn and dCo concentrations of 0.10â±â0.07ânM and 11â±â5âpM, respectively. Trace metal stoichiometry, calculated from concentration inventories, suggests a greater overall removal for dZn relative to dCo in the upper water column of the south-eastern Atlantic, with inter-seasonally decreasing dZnâ/âdCo inventory ratios of 19â5 and 13â7âmolâmolâ1 for sub-tropical surface water and sub-Antarctic surface water, respectively. In this paper, we investigate how the seasonal influences of external input and phytoplankton succession may relate to the distribution of dZn and dCo and variation in dZnâ/âdCo stoichiometry across these two distinct ecological regimes in the south-eastern Atlantic. </jats:p
Toward a Harmonization for Using in situ Nutrient Sensors in the Marine Environment
Improvedcomparabilityofnutrientconcentrationsinseawaterisrequiredtoenhancethe quality and utility of measurements reported to global databases. SigniïŹcant progress has been made over recent decades in improving the analysis and data quality for traditional laboratory measurements of nutrients. Similar efforts are required to establish high-quality data outputs from in situ nutrient sensors, which are rapidly becoming integral components of ocean observing systems. This paper suggests using the good practices routine established for laboratory reference methods to propose a harmonized setofdeploymentprotocolsandofqualitycontrolproceduresfornutrientmeasurements obtained from in situ sensors. These procedures are intended to establish a framework to standardize the technical and analytical controls carried out on the three main types of in situ nutrient sensors currently available (wet chemical analyzers, ultraviolet optical sensors, electrochemical sensors) for their deployments on all kinds of platform. The routine reference controls that can be applied to the sensors are listed for each step of sensor use: initial qualiïŹcation under controlled conditions in the laboratory, preparation of the sensor before deployment, ïŹeld deployment and ïŹnally the sensor recovery. The fundamental principles applied to the laboratory reference method are then reviewed in termsofthecalibrationprotocol,instrumentalinterferences,environmentalinterferences, external controls, and method performance assessment. Data corrections (linearity, sensitivity, drifts, interferences and outliers) are ïŹnally identiïŹed along with the concepts and calculations for qualiïŹcation for both real time and time delayed data. This paper emphasizes the necessity of future collaborations between research groups, referenceaccredited laboratories, and technology developers, to maintain comparability of the concentrationsreportedforthevariousnutrientparametersmeasuredbyinsitusensors
- âŠ