Some recent GEOTRACES science findings are reported below.
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As part of the GEOTRACES process study HEOBI (GIpr05) van der Merwe and co-workers (2019, see reference below) conducted a thorough characterization of the labile and refractory iron phases of the particles discharged to the local seawater by the glacial erosion and rivers of Heard and Mc Donald islands (Southern Indian Ocean). They demonstrate that, together with their specific lithology, the fraction of labile iron is significantly larger for particles that experienced glacial weathering processes than for particles of submarine hydrothermal origin. Moreover, they estimate that this labile particulate iron supplied from Heard and to a lesser extent, McDonald Island, is likely the missing iron required meeting biological demand over the plateau, downstream of the islands where traditionally an intense seasonal bloom develops.
Figure: Mixed layer, mean labile fraction of pFe (ratio of labile to refractory pFe) at each of the contrasting regions within the study. Heard Island displayed a significantly higher mean labile fraction compared to all other sites, including the reference station (one-way ANOVA, Games-Howell post hoc, p < 0.01). Furthermore, in addition to the ratio of labile to refractory Fe being higher at Heard Island, the absolute concentration of labile particulate Fe (pFelab) was also significantly higher at Heard Island (115 ± 34 nM, n = 9) compared to McDonald Island (79 ± 20 nM, n = 12) (one-way ANOVA, Games-Howell post hoc, p < 0.01). Standard boxplot median (centre black line), 25th and 75th percentiles (upper and lower box limits) and 95% confidence intervals (inner fences) are indicated. Outliers are shown as open circles and defined as less than 1.5 times the interquartile range. The number of independent data points for the reference, plateau, McDonald and Heard regions were 3, 2, 8 and 9 respectively. Click here to view the figure larger.
van der Merwe, P., Wuttig, K., Holmes, T., Trull, T. W., Chase, Z., Townsend, A. T., Goemann, K., Bowie, A. R. (2019). High Lability Fe Particles Sourced From Glacial Erosion Can Meet Previously Unaccounted Biological Demand: Heard Island, Southern Ocean. Frontiers in Marine Science, 6, 332. DOI: https://doi.org/10.3389/fmars.2019.00332
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.
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).
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
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.
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.
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
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.
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.
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
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.
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.
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