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

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-dominated communities produce large and labile POC aggregates, which 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.

Nutrient Controls on Export Production in the Southern Ocean

“…The general meridional distribution of inferred ANCP and carbon export agrees well with patterns of high diatom productivity in the Southern Ocean (Balch et al, ; Rosengard et al, ). Recent in situ flux observations carried out during two research cruises across the Southern Ocean found that POC export out of the euphotic zone correlates well with the export of biogenic silicate (BSi) (Rosengard et al, ). That study suggests that diatom‐rich communities induce high export of labile POC aggregates, which can drive the high rates of remineralization in the mesopelagic zone inferred from our analysis of oxygen drawdown.…”

“…Despite the evidence for strong iron limitation, large diatom blooms occur in Southern Ocean waters (Brzezinski et al, ; Quéguiner et al, ), and sediment trap studies show large opal fluxes beneath these blooms (Honjo et al, ). Diatoms constitute a large fraction of the autotrophic biomass in the Southern Ocean, particularly in the vicinity of the polar front (PF) (∼ 50°S) (Balch et al, ; Rosengard et al, ). This phytoplankton group is characterized by a silica cell wall (opal), which makes them potentially denser than the fluid around them and promotes their sinking toward deeper sections of the water column (Miklasz & Denny, ; Smayda, ).…”

Importance of coccolithophore‐associated organic biopolymers for fractionating particle‐reactive radionuclides (234Th, 233Pa, 210Pb, 210Po, and 7Be) in the ocean

“…Additionally, the calcification in coccolithophores can play an important role in ballasting for aggregates of organic matter [ Ziveri et al ., ; Chow et al ., ] and thus the export of organic materials to the deep ocean. Previous studies [e.g., Francois et al ., ; Rosengard et al ., ] on the biological pump in the ocean also demonstrated a positive correlation between particulate inorganic carbon (i.e., biogenic calcite) and the fraction of exported organic matter that reaches the deep ocean (i.e., particulate organic carbon (POC) transfer efficiency). These field observations all pointed out the “ballasting effect” of biogenic calcite, serving as a proxy for organic matter transport out of the ecosystems in which coccolithophores predominate.…”

The effect of sample drying temperature on marine particulate organic carbon composition

“…The results of this sample treatment comparison highlight several characteristics of POC composition (bulk C/N, δ 13 C, δ 15 N, and thermal stability) that are not compromised by oven‐drying at 56°C or air‐drying at high flow and room temperature when the particle samples are dried within 24 h following collection. As this approach to sample treatment is common in the field (e.g., Buesseler et al ; Bishop and Wood ; Bishop et al ; Lam et al ; Rosengard et al ), these findings imply that a vast repository of marine particle samples collected at sea remain useful tracers of POC composition and cycling in the water column, even if originally unintended for such research questions at the time of collection. These particle samples collected and processed by such drying treatments may still be used for bulk organic analysis and RPO, the latter of which will expand the empirical applications of this method to understanding marine POC dynamics in the water column.…”

“…Still, the majority of particulate matter collected at sea is not intended for organic analysis. Samples for bulk elemental analysis are often dried in a heated oven, and samples for inorganic analysis are sometimes air‐dried, and then stored at room temperature before analysis (e.g., Buesseler et al ; Bishop and Wood ; Bishop et al ; Lam et al ; Rosengard et al ), rather than being freeze‐dried. The tremendous labor, financial cost and scientific potential of sampling marine particles warrants an investigation into how much POC integrity is sacrificed when different sample drying methods do not meet optimal standards of organic geochemical analysis (Wakeham and Volkman ).…”