Abstract
Abstract. In high-nutrient–low-chlorophyll regions, phytoplankton growth is limited by the availability of water-soluble iron. The eruption of Kasatochi volcano in August 2008 led to ash deposition into the iron-limited NE Pacific Ocean. Volcanic ash released iron upon contact with seawater and generated a massive phytoplankton bloom. Here we investigate this event with a one-dimensional ocean biogeochemical column model to illuminate the ocean response to iron fertilisation by volcanic ash. The results indicate that the added iron triggered a phytoplankton bloom in the summer of 2008. Associated with this bloom, macronutrient concentrations such as nitrate and silicate decline and zooplankton biomass is enhanced in the ocean mixed layer. The simulated development of the drawdown of carbon dioxide and increase of pH in surface seawater is in good agreement with available observations. Sensitivity studies with different supply dates of iron to the ocean emphasise the favourable oceanic conditions in the NE Pacific to generate massive phytoplankton blooms in particular during July and August in comparison to other months. By varying the amount of volcanic ash and associated bio-available iron supplied to the ocean, model results demonstrate that the NE Pacific Ocean has higher, but limited capabilities to consume CO2 after iron fertilisation than those observed after the volcanic eruption of Kasatochi.
Highlights
Sensitivity studies with different supply dates gust 2008, volcanic ash was deposited into the iron-limited of iron to the ocean emphasise the favourable oceanic con- oceanic NE Pacific where atmospheric and oceanic condiditions in the NE Pacific to generate massive phytoplankton tions were favourable to genSeroatelida mEaassrivtehphytoplankton blooms in particular during July and August in comparison bloom as observed by satellite instruments (Fig. 1)
The first evidence of large-scale volcanic iron fertilisation of the surface ocean came from MODIS satellite data of chlorophyll a after the eruption of Kasatochi in August 2008 (Langmann et al, 2010a)
In situ measurements have become available to confirm the presence of the unusual large-scale phytoplankton bloom in the NE Pacific starting in August 2008 and persisting until about October 2008 (Hamme et al, 2010; Lockwood et al, 2012)
Summary
The Kasatochi volcano (52.17◦ N, 171.51◦ W) erupted on 7 August 2008. Three major explosive events occurred with eruption plumes rising to altitudes of about 15 km (Waythomas et al, 2010). The last explosive event on 8 August, which lasted for about 17 h, contained the highest amount of volcanic ash (Waythomas et al, 2010). Using an atmosphere-aerosol model, the release, dispersion and deposition of the Kasatochi ash was determined by Langmann et al (2010b). Based on the modelled tephra fallout, they estimated the amount of iron supplied to the NE Pacific Ocean attached to the volcanic ash from Kasatochi. Assuming an ocean mixed layer depth of 30 m (Whitney and Freeland, 1999) and a release of 200 nmol Fe per gram volcanic ash (a typical value for subduction zone volcanoes; see Olgun et al, 2011), the resulting surface ocean iron concentration at Papa (Fig. 3a) is 0.3–0.7 nmol L−1. Results from mesoscale iron enrichment experiments in the NE Pacific (Boyd et al, 2004) show that an increase of surface ocean iron concentrations by 1–2 nmol L−1 is sufficient for large diatoms to grow rapidly in this iron-limited region
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