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Science Highlights


Some recent GEOTRACES science findings are reported below.  
When getting older they are compiled in the Science Highlights Archive where the "Title Filter" search box will allow you to filter them by words in title (please note that only one-word search queries are allowed e.g. iron, Atlantic, etc.).

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A new application of aerosol iron isotopes: tracing anthropogenic iron; an example of the North Atlantic Ocean

Conway and co-authors (2019, see reference below) present the first evidence that anthropogenic iron (Fe) from combustion sources is visible at the basin scale, using iron isotopic composition (δ56Fe) analysis of the soluble aerosol phases collected during GEOTRACES cruise GA03 in the North Atlantic Ocean. Off Sahara, soluble aerosol samples have near-crustal δ56Fe whereas those from near North America and Europe display δ56Fe values as light as −1.6‰. Coupled to aerosol deposition modeling these results reveal that soluble anthropogenic aerosol Fe flux to the global surface oceans is highly likely to be underestimated.

 19 Conway

Figure. Tracing anthropogenic iron with iron isotopes (adapted from Conway et al., 2019). Panels a and b show that aerosols collected from near the Sahara have low solubility, a near-crustal iron isotope composition (beige circle) and a near-crustal Pb/Al composition (beige diamond). In contrast, those collected from near North America or Western Europe have very soluble iron, very light iron isotopes and are very enriched in Pb, indicating pollution from humans. When sampling points are overlain on output from dust modelling, it can be seen that the light iron isotopes correspond to where fossil fuel iron is expected to be important, and the crustal iron isotopes correspond to where natural dust iron is most important (panel c).

Reference:

Conway, T. M., Hamilton, D. S., Shelley, R. U., Aguilar-Islas, A. M., Landing, W. M., Mahowald, N. M., & John, S. G. (2019). Tracing and constraining anthropogenic aerosol iron fluxes to the North Atlantic Ocean using iron isotopes. Nature Communications, 10(1), 2628. DOI: https://doi.org/10.1038/s41467-019-10457-w
 

Upwelled hydrothermal iron stimulates massive phytoplankton blooms in the Southern Ocean

Joint Science Highlight with US-Ocean Carbon & Biogeochemistry (US-OCB).

In a recent study, Ardyna et al (2019, see reference below) combined observations of profiling floats with historical trace element data and satellite altimetry and ocean color data from the Southern Ocean to reveal that dissolved iron (Fe) of hydrothermal origin can be upwelled to the surface. Furthermore, the activity of deep hydrothermal sources can influence upper ocean biogeochemical cycles of the Southern Ocean, and in particular stimulate the biological carbon pump.

19 Ardyna

Figure: Southern Ocean phytoplankton blooms showing distribution, biomass (circle size) and type (color key). Adapted from Ardyna, et al., 2019. Click here to view the figure larger.

Reference:

Ardyna, M., Lacour, L., Sergi, S., d’Ovidio, F., Sallée, J.-B., Rembauville, M., Blain, S., Tagliabue, A., Schlitzer, R., Jeandel, C., Arrigo, K.R., Claustre, H. (2019). Hydrothermal vents trigger massive phytoplankton blooms in the Southern Ocean. Nature Communications, 10(1), 2451. DOI: https://doi.org/10.1038/s41467-019-09973-6

 

The causes of the 90-ppm glacial atmospheric CO2 drawdown still strongly debated

Joint Science Highlight with US-Ocean Carbon & Biogeochemistry (US-OCB).

Using an observationally constrained earth system model, S. Khatiwala and co-workers (2019, see reference below) compare different processes that could lead to the 90-ppm glacial atmospheric CO2 drawdown, with an important improvement on the deep carbon storage quantification (i.e. Biological Carbon Pump efficiency). They demonstrate that circulation and sea ice changes had only a modest net effect on glacial ocean carbon storage and atmospheric CO2, whereas temperature and iron input effects were more important than previously thought due to their effects on disequilibrium carbon storage. 

19 Kathiwala

Figure: Illustration of the two main mechanisms identified by this study to explain lower atmospheric CO2 during glacial periods. Left: present-day conditions; right: conditions around 19,000 years ago during the Last Glacial Maximum. The obvious explanation for lower CO2 during glacial periods – cooler ocean temperatures (darker blue shade) making CO2 more soluble, much as a glass of sparkling wine will remain fizzier for longer when it is colder – has long been dismissed as not being a significant factor. However, previous calculations assumed that the ocean cooled uniformly and was saturated in dissolved CO2. The model, consistent with reconstructions of sea surface temperature, predicts more cooling at mid latitudes compared with polar regions and also accounts for undersaturation. This nearly doubles the effect of temperature change and accounts for almost half the 90 ppm glacial-interglacial atmospheric CO2 difference. Another quarter is explained in this model by increased growth of marine algae (green blobs and inset) in the waters off Antarctica. Algae absorb CO2 from the atmosphere during photosynthesis and “pump” it into the deep ocean when they die and sink. But their growth in the present-day ocean, especially the waters off Antarctica, is limited by the availability of iron, an essential micronutrient primarily supplied by wind-borne dust. In our model an increased supply of iron to the Southern Ocean, likely originating from Patagonia, Australia and New Zealand, enhances their growth and sucks CO2 out of the atmosphere. This “fertilization” effect was greatly underestimated by previous studies. The study also finds that, contrary to the current consensus, a large expansion of sea ice off Antarctica and reconfiguration of ocean circulation may have played only a minor role in glacial-interglacial CO2 changes. Credit: Illustration by Andrew Orkney, University of Oxford.

Reference:

Khatiwala, S., Schmittner, A., & Muglia, J. (2019). Air-sea disequilibrium enhances ocean carbon storage during glacial periods. Science Advances, 5(6), eaaw4981. DOI: https://doi.org/10.1126/sciadv.aaw4981

South Pacific particulate organic carbon fate challenges Martin’s Law

Joint Science Highlight with US-Ocean Carbon & Biogeochemistry (OCB).

Carbon storage in the ocean is sensitive to the depths at which particulate organic carbon (POC) is respired back to CO2 within the twilight zone (100-1000m). For decades, it has been an oceanographic priority to determine the depth scale of this regeneration process. To investigate this, GEOTRACES scientists are deploying new isotopic tools that provide a high-resolution snapshot of POC flux and regeneration across steep biogeochemical gradients in the South Pacific Ocean.

A recent paper in PNAS reported on particulate organic carbon (POC) fluxes throughout the water column (focusing on the upper 1000 m) along the GP16 GEOTRACES section between Peru and Tahiti (Figure 1A). POC fluxes (Figure 1B) were derived by normalizing concentrations of POC to 230Th following analysis of samples collected by in situ filtration. This work builds on a research theme initiated at the GEOTRACES-OCB synthesis workshop held at Lamont-Doherty Earth Observatory in 2016.

The study results show that POC regeneration depth is shallower than anticipated, especially in warm stratified waters of the subtropical gyre. Regeneration depth—expressed in terms of the Martin-curve power-law exponent “b” (Figure 1C)—is shown to be greater than previous estimates (horizontal dashed lines), but similar to values obtained using neutrally buoyant sediment traps at the Hawaii Ocean Time-series Station Aloha. In contrast to the rapid regeneration of POC in warm stratified waters, POC regeneration within the oxygen deficient zone (ODZ) is below our detection limits. Models have shown that shallower regeneration of POC leads to less efficient carbon storage in the ocean, making the authors speculate that global warming, yielding expanded and more stratified gyres, may induce a reduction of the ocean's efficacy for carbon storage via the biological pump.

Pavia, et al. 2019

Figure: Site map and POC flux characteristics from GEOTRACES GP16 section. Plot A) shows the GP16 station locations as white circles, with nearby sediment trap deployments as black stars, with 2013 MODIS satellite-derived net primary productivity in the background. Plot B) shows POC fluxes from particulate 230Th-normalization from selected stations spanning the zonal extent of the GP16 section. Plot C) shows power law exponent b values for each GP16 station (blue), compared to estimates from bottom-moored sediment traps in the South Pacific (black and red dashed lines), a compilation of sediment traps in the North Pacific (green dashed line), and neutrally buoyant sediment traps in the subtropical North Pacific (yellow shaded band). GP16 regeneration length scales from 230Th-normalization agree most closely with the estimates from neutrally buoyant sediment traps.

Reference: 

Pavia, F. J., Anderson, R. F., Lam, P. J., Cael, B. B., Vivancos, S. M., Fleisher, M. Q., Lu, Y., Zhang, P., Cheng, H., Edwards, R. L. (2019). Shallow particulate organic carbon regeneration in the South Pacific Ocean. Proceedings of the National Academy of Sciences of the United States of America, 116(20), 9753–9758. https://doi.org/10.1073/pnas.1901863116

 

About the decoupled fates of aluminium, manganese, cobalt and lead in the North Pacific Ocean

Did you know that each of these tracers could follow its own marine story, quite decoupled from the others?

This is what is shown and discussed by Zheng and co-workers (2019, see reference below) after having analysed about 500 samples for aluminium (Al), manganese (Mn), lead (Pb) and cobalt (Co) along three sections in the North Pacific Ocean. They demonstrate that the distribution of each element is uniquely related to ocean circulation; that the subsurface Pb maximum has been sustained in the North Pacific Ocean through the growth of anthropogenic sources in Asia and Russia, contrasting with the decrease observed in the Atlantic Ocean (please also read the science highlight from Bridgestock et al., 2016); that the labile fraction of particulate Al is larger than that of particulate lead; and finally that while the Pb enrichment factor confirms its predominant atmospheric origin, those of Mn and Co clearly attest that sources other than the aerosol deposition are more significant contributors to the concentrations of these two tracers.

19 Zheng l

Figure: Sectional distributions of dissolved metals (dM) and potential density anomaly at depths of 0–1200 m along 160°W (section highlighted in red in the map). Dissolved aluminium (dAl) is high in Equatorial Under Current (EQ, 175 m depth) and North Equatorial Current (20°N, surface). Although dissolved manganese (dMn) and dissolved cobalt (dCo) have a concurrent source at the continental shelf of the Aleutian Islands, dCo is more widely distributed via North Pacific Intermediate Water (NPIW, ~600 m). Dissolved lead (dPb) is concentrated in Subtropical Mode Water and Central Mode Water above the NPIW. Adapted from Zheng et al., 2019. Click here to view the figure larger.

Reference:

Zheng, L., Minami, T., Konagaya, W., Chan, C.-Y., Tsujisaka, M., Takano, S., Norisuye, K., Sohrin, Y. (2019). Distinct basin-scale-distributions of aluminum, manganese, cobalt, and lead in the North Pacific Ocean. Geochimica et Cosmochimica Acta, 254, 102–121. DOI: http://doi.org/10.1016/J.GCA.2019.03.038

Bridgestock, L., van de Flierdt, T., Rehkämper, M., Paul, M., Middag, R., Milne, A., Lohan, M.C., Baker, A.R., Chance, R.,, Khondoker, R., Strekopytov, S., Humphreys-Williams, E., Achterberg, E.P., Rijkenberg, M.J.A., Gerringa, L. J.A., de Baar, H. J. W. (2016). Return of naturally sourced Pb to Atlantic surface waters. Nature Communications, 7, 12921. doi: http://doi.org/10.1038/ncomms12921

On the optimal use of the seaFAST system

Do you wish to improve your recoveries, blanks or any other parameter of your seawater preconcentration system (seaFAST)? Or are you simply curious about it? Wuttig and co-workers (2019, see reference below) propose a critical evaluation of this system’s capabilities. They perform an impressive list of tests including system conditioning, improving blank levels, finding the optimal pH of the buffer, improving preconcentration factors for different sample matrices, estimating memory effects, the initial sample salinity and UV oxidation effects on trace element concentrations. These tests considered an array of trace elements (cadmium, cobalt, copper, iron, gallium, manganese, nickel, lead, titanium and zinc) using SF-ICP-MS with data validation for some of these trace elements by flow injection analysis (iron, manganese) and/or GEOTRACES (n=42 for GSP and GSC) reference samples. They eventually make a long and useful list of recommendations for an optimal use of the system.

19 Wuttig

Figure: During this work, a commercially available seaFAST preconcentration system combined with sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS) was utilised to measure six reference seawaters (SAFe S, D1 and D2; GEOTRACES GD, GSC and GSP) 3-42 times each. In this figure, our measured values were compared to the consensus values for copper (Cu), iron (Fe), manganese (Mn) and Titanium (Ti). Titanium is a novel element for this system with limited consensus values available and was compared to values determined with voltammetry by Croot, 2011. Errors are presented as 1 standard deviation (σ) for both consensus and the measured values. Note different scales for Ti. 

Reference:

Wuttig, K., Townsend, A. T., van der Merwe, P., Gault-Ringold, M., Holmes, T., Schallenberg, C., Latour, P., Tonnard, M., Rijkenberg, M. J.A., Lannuzel, D., Bowie, A. R. (2019). Critical evaluation of a seaFAST system for the analysis of trace metals in marine samples. Talanta, 197, 653–668. DOI: http://doi.org/10.1016/J.TALANTA.2019.01.047

Croot, P.L., Rapid determination of picomolar titanium in seawater with catalytic cathodic stripping voltammetry, Anal Chem 83(16) (2011) 6395-400.

A treasure of geochemical data to trace ocean circulation, ventilation, mixing, biogeochemical and hydrothermal processes

This treasure is made of approximately 60,000 valid tritium measurements, 63,000 valid helium isotope determinations, 57,000 dissolved helium concentrations, and 34,000 dissolved neon concentrations, including their metadata (geographic location, date and sample depth). It was compiled by Bill Jenkins and co-workers (2019, see reference below) who describe the nature of the data, discuss their quality, list the contributors and pioneers, and of course are giving free access to this huge dataset (https://doi.org/10.25921/c1sn-9631). They also provide some figures illustrating how powerful this new tool is as for example the figure below. 

Authors invite anyone with knowledge of additional tritium, helium, or neon data that has not been included, to please contact wjenkins@whoi.edu with details for inclusion in future versions of the data set.

19 Jenkins
Figure:
(top) A map of helium values at approximately 2500 m depth. (bottom) A map of helium values at approximately 4000 m depth. The values plotted are simply an average of all measurements within a 1’ square between 3750 and 4250 dbar. Depths shallower than 4000 m are masked in gray, and sampling locations are indicated by light gray dots. Click here to view the figure larger.
3He is an extremely rare isotope that is a sensitive tracer of hydrothermal processes. Since it is both stable and chemically inert, it is detectable over great distances in the ocean. The two maps shown above are of the distribution of δ3He, a tracer of hydrothermal activity, at two levels in the deep ocean. The shallower one roughly corresponds to the depth of the mid-ocean ridge system, where the bulk of this hydrothermal injection takes place. One can see the dominant role of the fast-spreading ridges in the eastern Pacific, which drive two massive, westward reaching plumes north and south of the equator. The deeper horizon shows the spreading of δ3He-impoverished bottom waters from the northern and southern polar regions into the deep ocean basins.

Reference:

Jenkins, W. J., Doney, S. C., Fendrock, M., Fine, R., Gamo, T., Jean-Baptiste, P., Key, R., Klein, B., Lupton, J. E., Newton, R., Rhein, M., Roether, W., Sano, Y., Schlitzer, R., Schlosser, P. Swift, J. (2019). A comprehensive global oceanic dataset of helium isotope and tritium measurements. Earth System Science Data, 11(2), 441–454. DOI: http://doi.org/10.5194/essd-11-441-2019

Fate of different iron phases along the Lena river plume in the Arctic Ocean

Separation of the particulate (> 0:22 μm), colloidal (0.22 μm–1 kDa), and truly dissolved (< 1 kDa) fractions of iron (Fe) was carried out along the 600 km mixing between the Lena river and the Laptev seawater. While 99% of the particulate Fe and 90% of the colloidal one are disappearing across the shelf, the truly dissolved Fe stays relatively constant along the Lena River plume. This could indicate that this truly dissolved fraction is an important source of bioavailable Fe, along with colloidal Fe, for the local Arctic phytoplankton. Conrad and co-workers (2019, see reference below) also determined the Fe isotopes (δ56Fe) on these fractions. Negative colloidal δ56Fe values close to the river mouth are evolving to positive values at the outermost stations (see figure below). The shelf is thus interpreted as a sink for Fe, the negative values representing reduced ferrihydrites. Contrastingly, the positive values would correspond to oxidized Fe oxyhydroxydes, that are remaining in the water column and tag the Fe isotopic signature of the Arctic Ocean.

19 Conrad

Figure: A) Sampling stations in the Arctic Ocean: The black dots mark the sampling stations in the Arctic Ocean. Along the Lena River–Laptev Sea transect (next to the map) we filtered the water samples with different techniques to separate the particles from the colloids and the truly dissolved phase. Iron concentrations and isotope ratios were measured on the different sizes, as well as in the sediments along the East Siberian Sea Shelf. B) The colloidal (CFe) and particulate (PFe) iron concentrations plotted vs. salinity: Particulate Fe is dominating the Fe system close to the river mouth at low salinities, but the increasing salinity along the freshwater plume promotes the flocculation and settling of particles and colloids. Therefore, the concentrations of PFe and CFe are equal at higher salinities. C) Iron-isotope values along the Lena River freshwater plume and the uppermost sediment of the East Siberian Arctic Shelf (ESAS): Negative particulate and colloidal Fe-isotope values are lost during estuarine mixing and buried in the sediment, which shows a similar range of Fe isotope ratios. The positive colloidal Fe isotope values found in the outer plume are resistant to estuarine mixing and can be found in the Arctic Ocean. (Reprinted from Conrad, et al, 2019. Click here to view the image larger. 

Reference:

Conrad, S., Ingri, J., Gelting, J., Nordblad, F., Engström, E., Rodushkin, I., Andersson, P. S., Porcelli, D., Gustafsson, Ö., Semiletov, I., Öhlander, B. (2019). Distribution of Fe isotopes in particles and colloids in the salinity gradient along the Lena River plume, Laptev Sea. Biogeosciences, 16(6), 1305–1319. DOI: http://doi.org/10.5194/bg-16-1305-2019

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