Global Trends in the Distribution of Biogenic Minerals in the Ocean

Abstract

Dissolved inorganic carbon in the surface ocean is fixed into organic matter by phytoplankton through photosynthesis, creating the biomass that fuels the marine food web and driving air-sea exchange of carbon dioxide (CO 2 ). A portion of this biologically produced particulate matter sinks into the deep ocean, a process known as the biological carbon pump (BCP, Volk & Hoffert, 1985). Most particulate organic carbon (POC) is only slightly denser than seawater (Alldredge & Gotschalk, 1988) and does not sink readily on its own. Therefore, processes that create particles dense enough to sink out of the surface ocean are critical for the BCP to function, either through the association of POC with dense mineral phases (Klaas & Archer, 2002), or through the formation of larger particle aggregates (McCave, 1975). The size, mineral content, and porosity of biological material all influence the efficiency of vertical POC transport, and these characteristics are directly related to the biological communities that produce and consume this material.Because they are significantly denser than seawater, the biologically produced minerals biogenic silica (bSi) and calcium carbonate (CaCO 3 , also known as particulate inorganic carbon or PIC) provide ballast for the BCP. In addition to its ballasting role, PIC formation decreases alkalinity and raises seawater pCO 2 (Frankingouille et al., 1994;Humphreys et al., 2018). The counteracting effects of CO 2 drawdown by enhancing ballast, and

Biological export production controls upper ocean calcium carbonate dissolution and CO 2 buffer capacity

“…The positive correlation between the vertical POC attenuation and the CaCO 3 attenuation efficiency shown in Fig. 4 is dominated by small (1 to 53 μm) size classes where the majority of suspended POC and CaCO 3 exist (34). This implies coccolith calcite and potentially other…”

“…On the other hand, the data from sediment traps (15,20,32,33) and large-volume pumps (34)(35)(36) indicate that the connection between the vertical attenuations in POC and CaCO 3 fluxes (or concentrations) within the upper twilight zone might be more robust than our conservative estimate, leaning toward the upper limit of the uncertainty range (i.e., the steepest slopes inferred from The factors responsible for the biogenic dissolution of sinking CaCO 3 in the supersaturated upper water beneath the highly productive surface ocean are still not clearly understood. As constrained in this study, the CaCO 3 to POC molar ratio in the exported particles from the euphotic zone tends to be lower in subpolar eutrophic regions compared to low-latitude oligotrophic regions (fig.…”

“…small biogenically produced carbonates as the dominant substrate for shallow dissolution, whereas larger organisms such as pteropods and foraminifera sink very quickly due to their large size (34). All these potential drivers emphasize the complex interplay between twilight zone biogeochemical processes and upper ocean CaCO 3 dissolution.…”

“…Further independent support for the tight association between POC remineralization and shallow CaCO 3 dissolution comes from the suspended particle concentration data collected using largevolume pumps (34)(35)(36), although a direct comparison is made difficult because we do not explicitly simulate CaCO 3 concentrations in our model (see Materials and Methods). The data along the eastern equatorial Pacific (35) (chosen based on the best quality control; see Materials and Methods) exhibit a large range of 26 to 88% in the vertical attenuation of CaCO 3 concentration within the upper 300 m (Fig.…”

“…An important aspect of this study is that we assess the model performance using the global observational compilations of sediment trap-based CaCO 3 flux data ( 15 , 20 , 32 , 33 ) and large-volume pump–based CaCO 3 concentration data ( 34 – 36 ), both measured within the ocean’s upper twilight zone (upper 300 m). As shown below, the observed relationships between the vertical attenuations of CaCO 3 and particulate organic carbon (POC) fluxes or concentrations provide crucial constraints on the biogenic control of vertical CaCO 3 transfer efficiency above 300 m that is only weakly constrained by the observed tracer concentrations alone.…”