Fertilisation of an Ocean Desert by Volcanic Eruption
I-I Lin,1* Yuan-Hui Li,2 Chuanmin Hu,3 Chi-Wei Hwang,1,5 D. Allen Chu,4
George T. F. Wong,5 Dong-Shan Ko,6 Jingfeng Wu,7Jen-Ping Chen1
1
Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan.
2
Department of Oceanography, School of Ocean and Earth Science and Technology,
University of Hawaii, USA. 3College of Marine Sciences, University of South Florida, St.
Petersburg, Florida, 33701, USA. 4Goddard Earth Sciences and Technology Center, NASA
Goddard Space Flight Center, Greenbelt, Maryland, USA.
5
Research Centre for
Environmental Change, Academia Sinica, Taipei, Taiwan. 6Naval Research Laboratory,
Stennis Space Center, Mississippi, USA. 7International Arctic Research Center, University
of Alaska, Fairbanks, USA.
*To whom correspondence should be addressed. Email: iilin@as.ntu.edu.tw
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On May 10 2003, the Anatahan volcano in the Northern Mariana Islands erupted for
its first time in the recorded history. Using five types of satellite data and numerical
modeling, here we present evidence on the enhancement of ocean biological activities
from fallout of the Anatahan eruption in the western North Pacific subtropical gyre, a
well-known Low Nutrient Low Productivity (LNLP) ocean desert. Results show more
than doubling of phytoplankton fluorescence through the supply of iron and
phosphorus from the eruption. Spectral analyses further suggest the species involved
include the primary N2 fixer, Trichodesmium. This work provides evidence on the
fertilisation potential to promote nitrogen fixation in the LNLP waters by volcanic
eruption.
The fertilisation potential of volcanic aerosols to promote nitrogen fixation (1-4) in the low
nutrient low productivity (LNLP) waters of the world and the subsequent impact on climate
is a subject at its infancy (5-8). N2 fixation in the vast LNLP waters on Earth (including the
Earth’s largest biomes, the subtropical North Pacific Ocean and the subtropical/tropical
Atlantic) plays a key role in the earth’s nitrogen, carbon, and biogeochemical cycling, thus
2
impact the climate (1-4). In these oligotrophic waters, N2 fixation is limited by the
availability of essential nutrients such as iron and phosphorus (9,10). Research on nutrient
supply has been focused on desert dust for few decades (11-16). Nutrient supply from
volcanic eruption has been speculated since long, but real world evidence is lacking (5-8).
In the evening of 10 May 2003, the Anatahan volcano (146E, 16N) in the Northern
Mariana Islands (Figure 1a) erupted for its first time in the recorded history (17-18). The area
is in the midst of the western North Pacific Subtropical Gyre (WNPSG), a well-known
oligotrophic ocean desert (19). Historical data suggest that nutrients are predominantly
undetectable in the top 100m together with surface chlorophyll-a (chl-a) concentration as low
as ~0.05 mgm-3 (19). Within a week after the eruption, clear enhancement in biological
activities was detected by NASA’s Moderate Resolution Imaging Spectroradiometer
(MODIS) ocean colour sensor (20). Using multi-satellite data together with numerical
modelling, this work documents this unprecedented volcanic event and provides evidence
that N2 fixation can indeed by promoted in the LNLP ocean desert through nutrient inputs
from volcanic ash. The 5 types of satellite data used include MODIS aerosol and ocean
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colour data, TOPEX/Poseidon and JASON-1 altimetry sea surface height anomaly (SSHA)
data, Tropical Rainfall Measuring Mission/Microwave Imager (TMI) sea surface
temperature (SST) data, and QuikSCAT sea surface wind vectors while the numerical model
used is the US Naval Research Lab’s EASNFS (East Asian Seas Nowcast/Forecast System)
41 sigma-z level full-physics ocean model (20).
As illustrated in Fig. 1b, before eruption the WNPSG was characterised by very clean, typical
marine atmospheric condition of low aerosol loading (aerosol optical depth or AOD < 0.1).
Immediately after the eruption on 11 May 2003, MODIS-derived AOD abruptly shot up to
1.0-2.0, nearly a 20-fold increase of aerosol loading at the downwind of the island (Fig. 1c).
As also clearly depicted in the corresponding MODIS true colour image, a large plume of
volcanic ash was found to spread westward across approximately 500 km over the WNPSG
from the Mt. Anatahan (Fig. 1a). In the ocean, before eruption the MODIS ocean colour
image depicts the typical WNPSG oligotrophic ocean condition with chl-a concentration of
about ~0.05 mg m-3 (Fig. 2a). On 15 May (five days after the eruption), the first-available
cloud-free MODIS chl-a image illustrates a well-defined ‘bloom-like’ patch in the area of
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144-146.5∘ E, and 16.5-17.5∘ N (Fig. 2b) co-located at the ash plume location (Fig. 1a).
However, chl-a image alone is not sufficient to show that the ‘bloom-like’ patch is indeed a
phytoplankton bloom because such type of standard chl-a image uses the blue/green band
ratio to estimate the chl-a concentration (20). Non-living particles like suspended volcanic
ash in the water can also cause a decrease in the blue/green ratio and therefore a false increase
in the estimated chl-a (8,21). As such, additional information from the MODIS fluorescence
line height (FLH) at 678nm is used. This is because among all water constituents including
phytoplankton, coloured dissolved organic matter, detritus, and other non-living particles,
only phytoplankton has the local peak at 678nm unless the concentrations of non-living
particles are very high (e.g., > 5 mg L-1). It has been found that this MODIS FLH signal is
effective in delineating phytoplankton blooms in complex waters, especially in CDOM-rich
waters (22-23).
As depicted in Fig. 2d, elevated FLH signal is also detected in the ‘bloom-like’ patch (boxed
region in Fig. 2b) but the magnitude of increase is smaller than in the standard chl-a image
(Fig. 2b). As seen in Fig. 2c, the FLH value is ~ 0.002-0.005 mWcm-2μm-1sr-1 in the ambient
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oligotrophic water and ~0.005-0.014 mWcm-2μm-1sr-1 in the bloom patch, indicating around
doubling in the biological activities. However, the standard chl-a image shows a 2-15 times
increase in biological activities, from the ambient chl-a of ~ 0.05-0.07 mg m-3 to chl-a of ~
0.1-1 mg m-3 in the patch, suggesting the false amplification in chl-a increase due to the
presence of the suspended volcanic ash. On 17 May 2003, the next-available MODIS cloud
free chl-a image reveals that the bloom patch elongated towards northeast at the location
between 145.-147.∘ E, and 16.5-18.5∘ N (Fig. 2c). The corresponding flow field from the
EASNFS also shows NE dispersion, supporting the elongated pattern of the bloom (Fig. 3e).
Similar to the situation on 15 May, the FLH image depicts a 2-3 times increase in the
biological activities (Fig. 2e) while the standard chl-a image reveals the ~ 10 times false
increase situation (Fig. 2c). These results suggest that enhanced biological activities, based
on the 2-3 times increase in the FLH signals, did occur one week after the eruption. By early
June 2003, the bloom patch had mostly disappeared.
To rule out the possibility of the observed bloom patch being fuelled by deep-water nutrients
through wind mixing or upwelling (24-25), data collected by other satellite sensors (ambient
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wind, SST, and altimetry SSHA) are examined. As depicted in Fig. 3a, the bloom area is
dominated by positive SSHA of ~ 10 cm, indicating a clear down-welling condition (24).
Consistent observations are also found in the corresponding SST and sea surface wind that
during the bloom period, the SST is dominant by uniformly warm SST of ~ 29∘ C (Fig. 3c),
and weak easterly winds of around 5-7 m/s (Fig. 3d). Fig. 3b shows that the bloom patch is
located in the midst of the gyre and little (~ 0 μM) nutrient is detectable in the top 100 m of
the water (28). Therefore, it is nearly impossible that the observed bloom is fuelled by
deep-water nutrients. Furthermore, to ensure that the observed MODIS ocean colour signal
(Figs. 2a-e) originated only from the ocean and not atmosphere, co-incident and co-located
MODIS AOD images are examined (Fig. 1d). It is clear that during the bloom period aerosol
loading in the atmosphere had subsided, with AOD fell back to the pre-eruption level of
around 0.1, suggesting the observed bloom in the SeaWiFS images was indeed an ocean
feature and not an artifact from the suspended ashes in the atmosphere.
As reported in Wade et al. (2005) (17), soon after the eruption there was a well-organised
field campaign by the US National Science Foundation’s MARGINS team to the Anatahan to
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collect eruption samples for major and trace element analyses. Iron and phosphorus were
found together with silica and other elements (see sup_Table 1 in the supplementary material)
(17). As this is in an LNLP region, it would be interesting to see whether the observed bloom
is associated with the N2 fixation species like Trichodesmium (26). Therefore, MODIS ocean
colour spectra of the normalised water-leaving radiance were analysed and compared with
the known Trichodesmium spectra (27). Also, as suggested by Duggen et al. (8), spectra from
suspended volcanic ash in the water are often associated with significant raise in the 555nm
band. Thus to avoid taking spectra from regions dominant by the volcanic ash, each pixel is
screened and only spectra without such possible contamination are used. The screened
spectra are then compared with the reference oligotrophic ocean background spectra outside
the bloom and the well-known reference Trichodesmium spectra from the Atlantic (27). As in
Fig. 4, the spectra in the reference region outside the bloom (in black and locations depicted
as black box in Fig. 2e) clearly depict the oligotrophic blue water condition of peak in the
412-nm band. The reference Trichodesmium spectra are apparently different from the
oligotrophic reference spectra and are characterised by a clear single peak at 490nm (green
spectra in Fig. 4). Comparing with the observed spectra in various parts of the bloom patch
(red spectra in Fig. 4, location depicted by black arrows in Figs. 2d and 2e), one can found
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that they are in close similarity in their spectral shapes and peak locations (Fig. 4). This
provides further support that through the supply of the Anatahan volcanic ash, enhancement
of biological activities through nitrogen fixation can be promoted in one of the most
oligotrophic ocean deserts on Earth, the WNPSG.
Nitrogen fixation in the vast LNLP waters on earth is critical for maintaining ocean’s
biological productivity but many mechanistic links remain unknown (1-4,9-10).
For
example, the supply of critical nutrient, iron, has been focused on desert dust (11-16).
However, this can not explain why high N2 fixation rate can be found in the Pacific gyre
where iron supply through desert dust is low (4). In this work, we present several lines of
evidences to show that not only desert dust, volcanic eruption can indeed be an effective
nutrient source to promote nitrogen fixation. For long such potential has only been
speculated. Through multi-satellite observations and numerical modelling, this work
provides early evidence confirming such potential.
References and Notes
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29. Thanks to NASA and Remote Sensing Systems for data provision. This work is
primarily supported by the National Science Council, Taiwan through NSC
95-2119-M-002-019-AP1. Additional supports are given to Wong through NSC
96-2611-M-001-003-MY3, and to Wong and Lin through a thematic research grant titled
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‘Atmospheric Forcing on Ocean Biogeochemistry (AFOBi)’ by the Academia Sinica,
Taiwan.
30. Correspondence and requests for materials should be addressed to Dr. I-I Lin (email:
iilin@as.ntu.edu.tw).
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Fig. 1: (a): MODIS true colour image showing the volcanic plume erupted (in the red box;
140-150°E, 15-20°N) from Anatahan on 11 May 2003. The box location is depicted in the
geographical map below. (b)-(d): MODIS AOD images of the box; (b) 6-8 May 2003 (before
eruption), (c) 11 May 2003 (during eruption), and (d) 14-19 May 2003 (after eruption).
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Fig. 2: (a)-(c): MODIS chl-a images on (a) 7-9 May 2003 (i.e., pre-eruption), (b) 15 May
2003 (5 days after the eruption), and (c) 17 May 2003 (7 days after the eruption). (d)-(e):
MODIS FLH images of the boxed region in (b) and (c), respectively. Black arrows in (d) and
(e) depict the locations where the bloom spectra in Fig. 4 are plotted, while the black box in (e)
depicts the location where the reference background spectra in Fig. 4 are plotted.
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Fig. 3: (a) Sea surface height anomaly from the TOPEX/Poseidon and JASON-1 satellite
altimetry measurements for 1 cycle between 10 and 19 May 2003 (location depicted in the
boxed region from the map of Fig. 1a). (b) Climatological depth-nitrate profile in May. (c)
Corresponding SST map from the TRMM microwave imager on 15-17 May. (d)
Corresponding QuikSCAT ocean surface wind speed and direction on 15-17 May. (e)
Corresponding surface current field from NRL’s EASNFS ocean nowcast system on 16 May.
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Fig. 4: (a) MODIS ocean colour spectra for a: reference oligotrophic ocean background
spectra (in black) (location depicted by black square in Fig. 2e), b: reference Trichodesmium
spectra from the Atlantic (in green) (location see the green box of sup_Fig. 1 in the
supplementary material), c: spectra from various parts of the bloom patch (red spectra,
locations see arrows in Figs. 2d and 2e). [why not show the full spectral until 760 nm? This
way the fluorescence peak should show up]
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