Selective Preservation of Coccolith Calcite in Ontong‐Java Plateau Sediments

Abstract

Dissolution of calcite in deep ocean sediments, which is required to balance global marine CaCO 3 production and burial fluxes, is still a poorly understood process. In order to assess the mechanisms of dissolution in sediments, we analyzed four multicore tops taken along a depth transect on the Ontong-Java Plateau. These cores were taken directly on the equator, and span water column calcite saturation states from ∼0.93 to ∼0.74, allowing us to assess the effect of dissolution on carbonate sediment composition. The top 2 cm of each multicore was sectioned and sieved to separate coccolith from foraminiferal calcite, and the %CaCO 3 , 13 C, Δ 14 C, and Mg/Ca were evaluated. The mass ratio of coccoliths to foraminifera increases by a factor of 3 as a function of water depth, reflecting the preferential dissolution of foraminifera. Carbon isotope ( 13 C and Δ 14 C) data suggest that most dissolution takes place at the sediment-water interface and primarily affects foraminifera. Mg/Ca analyses indicate that the Mg content of the entire foraminiferal assemblage decreases as a function of dissolution. In contrast, coccolith dissolution takes place within the sediments, rather than at the interface. Together these two processes cause coccoliths to be up to 1,000 radiocarbon years younger than foraminifera from the same depth horizon. Despite this within-sediment coccolith dissolution flux, sediments below the calcite saturation horizon remain enriched in coccolith calcite. Combined with global seafloor hypsometry and calcium carbonate content, this enrichment suggests that globally, coccoliths may outweigh foraminifera in deep ocean sediments by a factor of 1.8.

Shallow Calcium Carbonate Cycling in the North Pacific Ocean

“…Our single planktic foraminiferal assemblage dissolution rate is broadly consistent with the sparse and foraminiferal data from the North Atlantic and the Pacific (Fukuhara et al., 2008; Honjo & Erez, 1978, Figure 3). The larger spread in foraminiferal dissolution rates at a given 1− Ω makes it difficult to fit a curve to these data, and this range in reactivity may be related to foraminiferal calcite Mg content (Subhas, McCorkle, et al., 2019). In the absence of more foraminiferal data, we rely on the observation that foraminifera appear to dissolve ∼60% (1.6×) faster than coccoliths per unit mass (Chiu & Broecker, 2008; Subhas et al., 2018; Subhas, McCorkle, et al., 2019).…”

“…The larger spread in foraminiferal dissolution rates at a given 1− Ω makes it difficult to fit a curve to these data, and this range in reactivity may be related to foraminiferal calcite Mg content (Subhas, McCorkle, et al., 2019). In the absence of more foraminiferal data, we rely on the observation that foraminifera appear to dissolve ∼60% (1.6×) faster than coccoliths per unit mass (Chiu & Broecker, 2008; Subhas et al., 2018; Subhas, McCorkle, et al., 2019). Multiplying the near‐ and far‐from‐equilibrium E. huxleyi k mass values by 1.6 does a reasonable job of approximating the sparse foraminiferal data (Figure 3, yellow line, Table 2).…”

“…Foraminifera also demonstrate a far‐from‐equilibrium n ∼ 2. They appear to dissolve faster than coccoliths normalized by both mass and by surface area, suggesting that foraminifera require their own specific k values (Gehlen et al., 2005; Subhas et al., 2018; Subhas, McCorkle, et al., 2019). Aragonite dissolves more slowly than biogenic calcite, and also has the lowest n ∼ 1.8 (Dong et al., 2019).…”

Microbial ecosystem responses to alkalinity enhancement in the North Atlantic Subtropical Gyre

“…Tubes were capped, evacuated, and filled with 4 mL of 10% phosphoric acid via syringe to convert all CaCO 3 to CO 2 . Samples were then run on a Picarro-Automate autosampler system capable of measuring [CO 2 ] and δ 13 C of CO 2 (Subhas et al, 2019). Concentrations of CO 2 were converted to a mass of CaCO 3 using a standard curve of in-house Iceland Spar calcite; there was not enough CO 2 to measure δ 13 C on these samples.…”

Evolution of calcite microcrystal morphology during experimental dissolution

“…Although the discussion above focuses on calcite microcrystal dissolution in burial settings, our experimental results are broadly applicable to other natural environments where calcite dissolution may take place. For example, calcite dissolution may occur within the water column (Milliman et al 1999), at the sedimentwater interface (Subhas et al 2019), within sediments during early diagenesis (Malone et al 2001), burial diagenesis (Lambert et al 2006), and potentially as a result of CO 2 sequestration in deep carbonate reservoirs (Pokrovsky et al 2009). Additionally, and perhaps most importantly, carbonate dissolution in the ocean is expected to become more pervasive due to the falling saturation state with respect to carbonate minerals as a result of rising CO 2 concentrations (i.e., ocean acidification) (Kleypas et al 1999;Morse et al 2006).…”