A synthesis of upper ocean carbon and dissolved iron budgets for Southern Ocean natural iron fertilisation studies

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Abstract

For over a decade the relationship between iron addition and carbon export has been an active topic of Southern Ocean biogeochemical investigation. To study the iron-carbon interaction, a number of natural iron sources within the Antarctic Circumpolar Current have been targeted to quantify the resultant particulate organic carbon (POC) export. Three studies in particular near the Crozet Islands, Kerguelen Plateau, and Antarctic Peninsula have produced estimates of both dissolved iron (DFe) supply and POC export. For each area, we present a detailed synthesis of 234Thorium-derived POC export and of the DFe budgets that fuel the substantial phytoplankton blooms observed in these areas. Furthermore, we discuss the nuances of calculating seasonal POC export, which is required to estimate the seasonal response of POC export to DFe fertilisation. To conclude, we review the relationship between DFe supply and POC export (C:Fe ratio) for these areas, which provides an estimate of POC export efficiency. Daily rates of POC export from the mixed layer of naturally Fe-enriched Southern Ocean blooms range from 15 to 32 mmol C m−2 d−1, with associated control sites typically exporting ∼3-times less POC, (5 to 12 mmol C m−2 d−1). Within each project, the 3-fold trend is also observed in estimates of seasonal POC export; however, variation between study regions shows up to a 4-fold difference (range 1.4–5.0 mol m−2). DFe supply is dominated by horizontal processes and spans an order of magnitude depending on the location (190–2700 nmol m−2 d−1). Where the calculation of seasonal C:Fe ratios is possible, almost an order of magnitude variation (16,790–154,000) is observed between different Southern Ocean regions.

Introduction

Southern Ocean productivity is severely limited by a lack of iron, a hypothesis that has been repeatedly tested and verified by artificial iron enrichment experiments (Boyd et al., 2007, de Baar et al., 2005). To put the Southern Ocean's under-productivity into perspective, an initial estimate made during the advent of the ‘Iron Hypothesis’, suggested that with complete utilisation of nitrate and phosphate, plentiful iron could increase new production by approximately 30-fold (Martin, 1990a). However, phytoplankton in the Southern Ocean fail to tap the nutrient rich water, and consequently the region remains persistently low in chlorophyll. This condition is known as high-nutrient low-chlorophyll (HNLC), of which the Southern Ocean is the largest example in the world (Watson, 2001).

The idea that iron (Fe) is the micro-nutrient that limits biological production in the Southern Ocean was reported in the scientific literature as early as the 1930s (Gran, 1931, Harvey, 1933, Harvey, 1937, Ruud, 1930) and appears twice in the Discovery Reports from around the same time (Hart, 1934, Hart, 1942). However, the vanishingly low concentration of dissolved iron (DFe) in the surface water of the Southern Ocean was not fully recognised until the advent of trace-metal clean sampling techniques in the 1970s (review by Martin, 1991). This advance in understanding the distribution of DFe led to the formulation of the Iron Hypothesis, the idea that it may be possible to mitigate increasing atmospheric CO2 by stimulating phytoplankton blooms, and thus fixing and transporting carbon (C) to the deep ocean (Martin, 1990a). This hypothesis was first tested by Martin and Fitzwater (1988), who concluded that Fe was in fact the limiting micro-nutrient for phytoplanktonic growth. That same year, John Martin passed a casual comment at an informal seminar at the Woods Hole Oceanographic Institution (WHOI): “Give me half a tanker of iron, and I'll give you an ice age”, which although was met with laughter at the time (Chisholm and Morel, 1991, Martin, 1990b), later become a serious research topic as a potential means to sequester CO2 that is released by the burning of fossil fuels (Martin et al., 1990).

Since the early 1990s there have been 13 artificial iron-addition experiments. Although all of these have demonstrated that biological productivity can be enhanced through the artificial addition of Fe, usually in the form of iron sulphate, only four of them produced multiple lines of evidence of enhanced C export (Boyd et al., 2007, de Baar et al., 2005). However, there is even less evidence to support enhanced C sequestration (Buesseler et al., 2004), a depth at which C is isolated from atmospheric ventilation on a greater than seasonal time-scale. The finding of little or no significant C sequestration has led the scientific community to question the feasibility of using purposeful Fe fertilisation as a means to mitigate rising atmospheric CO2 levels (Buesseler and Boyd, 2003, Buesseler et al., 2008).

In parallel to the numerous artificial Fe fertilisation experiments in the early 2000s, several groups approached the topic of iron fertilisation from that of naturally iron fertilised systems. The two most prominent projects were the CROZet natural iron bloom and EXport experiment (CROZEX) (Pollard et al., 2009), and the KErguelen Ocean and Plateau compared Study (KEOPS) (Blain et al., 2007). In addition to these, there are four other noteworthy studies: the Blue Water Zone (BWZ) project (Zhou et al., 2010; 2013), the Discovery 2010 cruises (Tarling et al., 2012), the Dynamic Light on Fe limitation (DynaLiFe) program (Arrigo and Alderkamp, 2012), and most recently a return to the Kerguelen Plateau in late 2011 referred to as KEOPS 2 (Table 1). Productivity in these settings appears to be stimulated by Fe supplied from sources such as shelf and plateau sediments, glacial melting and, to a lesser extent, vertical mixing enhanced by ocean currents flowing over topographic features. These six projects represent the most thorough and integrative initiatives to investigate the impact that natural Fe supply has on the ecology, productivity and geochemical cycles in these HNLC Southern Ocean systems.

The aim of this synthesis is to focus on the current state of understanding of the C and iron budgets, and export efficiencies in naturally Fe fertilised Southern Ocean systems. Consequently, CROZEX, KEOPS and BWZ will be the focus of this synthesis as these three studies have the most comprehensive datasets of both DFe supply and particulate organic carbon (POC) export. Firstly, a brief description of each project will be presented, followed by a discussion of the rates of daily and seasonal POC export. Secondly, up-to-date DFe budgets will be presented so that the C:Fe ratio can be calculated; the molar ratio between DFe added and the response of C to this addition. Whilst presenting these geochemical budgets the nuances and limitations of the various approaches will be discussed, and the varying results will be compared. The motivation for this paper arose from a workshop at WHOI in July 2011 (Charette et al., 2011) called ‘Modeling and Synthesis of Southern Ocean Natural Iron Fertilzation’ (http://www.whoi.edu/sbl/liteSite.do?litesiteid=48732).

Section snippets

Description of natural Fe fertilisation studies

The underlying structure of a natural Fe fertilisation field campaign is to survey the Fe-enriched bloom region and compare it to a nearby Fe-limited, non-bloom, HNLC region. Then by comparing the Fe-enriched and Fe-depleted regions, it is possible to gauge the impact of Fe supply on the subject of interest, in this case POC export. To ensure successful field campaigns, the chosen study site must support a reproducible bloom with a similar timing and magnitude each year. The first three

POC export

The most common approach for measuring upper ocean POC export is the 234Th/238U technique, an approach that measures the disequilibrium between particle reactive 234Th and its conservative parent 238U (Rutgers van der Loeff et al., 2006). The measured 234Th flux (t½=24.1 d) is then converted to a POC flux using a POC:234Th ratio measured on the >∼50 μm particulate material (Buesseler et al., 2006). This larger size fraction typically represents the sinking component of the particulate pool (

Supply of dissolved iron

An underlying hypothesis of studies that investigate the effects of natural Fe fertilisation is that where Fe enrichment is observed there will be increased POC export. Consequently, considerable effort was made to measure DFe distributions and fluxes into the study systems. For CROZEX, Planquette et al. (2007) constructed a DFe budget in the bloom and HNLC regions so that the upper ocean, pre-bloom DFe inventory could be estimated. The DFe budget was comprised of vertical mixing, atmospheric

Summary and recommendations

The budgeting exercises presented here aim to highlight the strengths and weaknesses of the various approaches used, and attempt to shed light on the inherent nuances when comparing natural Fe fertilisation studies. Any budgeting exercise will benefit from complete quantification of the inventories involved, and where applicable, the fluxes in and out of these inventories. In the case of a C budget that uses the ΔDIC approach, it is essential to make sure that adequate quantification of the

Acknowledgments

The motivation for this manuscript arose from the workshop: Modeling and Synthesis of Southern Ocean Natural Iron Fertilization (SOFe), for this, and all the stimulating discussions during the workshop, we are greatly thankful to the SOFe participants. Funding for this synthesis paper and the SOFe workshop was provided by a grant from the National Science Foundation (NSF) to MAC (NSF-ANT 0948442). BWZ 234Th samples were collected and processed by Paul Henderson, Henrieta Dulaiova, Prae

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    Present address: School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK.

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