Carbon export and transfer to depth across the Southern Ocean Great Calcite Belt

S Z Rosengard,S Pike,W M Balch,B Bowler,P J Lam,D Drapeau,M E Auro

doi:10.5194/bg-12-3953-2015

S Z Rosengard, S Pike + Show 5 more

Open Access

https://doi.org/10.5194/bg-12-3953-2015

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Abstract

Abstract. Sequestration of carbon by the marine biological pump depends on the processes that alter, remineralize, and preserve particulate organic carbon (POC) during transit to the deep ocean. Here, we present data collected from the Great Calcite Belt, a calcite-rich band across the Southern Ocean surface, to compare the transformation of POC in the euphotic and mesopelagic zones of the water column. The 234Th-derived export fluxes and size-fractionated concentrations of POC, particulate inorganic carbon (PIC), and biogenic silica (BSi) were measured from the upper 1000 m of 27 stations across the Atlantic and Indian sectors of the Great Calcite Belt. POC export out of the euphotic zone was correlated with BSi export. PIC export was not, but did correlate positively with POC flux transfer efficiency. Moreover, regions of high BSi concentrations, which corresponded to regions with proportionally larger particles, exhibited higher attenuation of > 51 μm POC concentrations in the mesopelagic zone. The interplay among POC size partitioning, mineral composition, and POC attenuation suggests a more fundamental driver of POC transfer through both depth regimes in the Great Calcite Belt. In particular, we argue that diatom-rich communities produce large and labile POC aggregates, which not only generate high export fluxes but also drive more remineralization in the mesopelagic zone. We observe the opposite in communities with smaller calcifying phytoplankton, such as coccolithophores. We hypothesize that these differences are influenced by inherent differences in the lability of POC exported by different phytoplankton communities.

Highlights

The biological pump sequesters atmospheric carbon dioxide (CO2) in the ocean (Volk and Hoffert, 1985) by way of phytoplankton-driven CO2 fixation, followed by the sinking of this fixed particulate organic carbon (POC) as aggregates and fecal pellets down the water column (Riley et al, 2012)
The quantity per unit area and time of POC exiting the base of the euphotic zone defines the export flux, while export efficiency represents the fraction of bulk primary production comprising this flux (Buesseler, 1998)
Each activity profile is associated with two metrics that have been used in previous studies to define the export depth: the base of the euphotic zone, which we define at 0.3 % surface photosynthetically available radiation (PAR) (e.g., Buesseler and Boyd 2009), and zTh/U, where 234Th and 238U activities re-establish secular equilibrium (Table 1)

Summary

Introduction

The biological pump sequesters atmospheric carbon dioxide (CO2) in the ocean (Volk and Hoffert, 1985) by way of phytoplankton-driven CO2 fixation, followed by the sinking of this fixed particulate organic carbon (POC) as aggregates and fecal pellets down the water column (Riley et al, 2012). Export and sequestration flux vary widely by region, as do export efficiencies and attenuation of export flux (Buesseler and Boyd, 2009; Buesseler et al, 2007; Henson et al, 2011, 2012b; Martin et al, 1987; Thomalla et al, 2008) Such variations may drive observed differences in the weight percent of organic carbon deposited at the sediment surface (Hedges and Oades, 1997), suggesting that the overall strength of the biological pump as a carbon sink is not globally uniform. These geographical differences have spurred decades of research into how mechanisms

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