Tracking single coccolith dissolution with picogram resolution and implications for CO ₂ sequestration and ocean acidification

Abstract

Coccoliths are micrometer scale shields made from 20 to 60 individual calcite (CaCO 3 ) crystals that are produced by some species of algae. Currently, coccoliths serve as an important sink in the global carbon cycle, but decreasing ocean pH challenges their stability. Chalk deposits, the fossil remains of ancient algae, have remained remarkably unchanged by diagenesis, the process that converts sediment to rock. Even after 60 million years, the fossil coccolith crystals are still tiny (<1 μm), compared with inorganically produced calcite, where one day old crystals can be 10 times larger, which raises the question if the biogenic nature of coccolith calcite gives it different properties than inorganic calcite? And if so, can these properties protect coccoliths in CO 2 challenged oceans? Here we describe a new method for tracking dissolution of individual specimens, at picogram (10 −12 g) resolution. The results show that the behavior of modern and fossil coccoliths is similar and both are more stable than inorganic calcite. Organic material associated with the biogenic calcite provides the explanation. However, ancient and modern coccoliths, that resist dissolution in Ca-free artificial seawater at pH > 8, all dissolve when pH is 7.8 or lower. Ocean pH is predicted to fall below 7.8 by the year 2100, in response to rising CO 2 levels. Our results imply that at these conditions the advantages offered by the biogenic nature of calcite will disappear putting coccoliths on algae and in the calcareous bottom sediments at risk. marine carbon cycle | ocean buffer capacity | dissolution rate | atomic force microscopy | single particle dissolution

Assessing ocean alkalinity for carbon sequestration

“…However, based on these measurements, the volumetric surface area of carbonate minerals in the mixed layer could be between 10 −6.3 and 10 −3.2 m 2 L −1 . Furthermore, the majority, if not all of the PIC in the surface ocean, is produced biologically, which has been shown to considerably less reactive than synthetic counterparts (see the supporting information [ Cubillas et al , ; Hassenkam et al , ; Honjo and Erez , ; Keir , ; Walter and Hanor , ]), possibly due to a protective organic layer on the mineral surface [ Godoi et al , ]. Figure suggests that a precipitation rate of around 10 −5 mol m −2 h −1 would be expected for seawater (Ω calcite = 3–4) seeded with carbonate minerals.…”

“…However, based on these measurements, the volumetric surface area of carbonate minerals in the mixed layer could be between 10 À6.3 and 10 À3.2 m 2 L À1 . Furthermore, the majority, if not all of the PIC in the surface ocean, is produced biologically, which has been shown to considerably less reactive than synthetic counterparts (see the supporting information [Cubillas et al, 2005;Hassenkam et al, 2011;Honjo and Erez, 1978;Keir, 1980;Walter and Hanor, 1979]), possibly due to a protective organic layer on the Figure 12. The relationship between saturation state and precipitation rate in natural and synthetic phosphate free seawater at 25°C [Burton and Walter, 1987;Lopez et al, 2009;Mucci, 1986;Mucci et al, 1989;Mucci and Morse, 1983;Mucci, 1989, 1995].…”

A compaction front in North Sea chalk

“…It has long been recognized that chalk rheology is significantly affected by organic components [Risnes, 2001, and references therein], but these studies have been made largely in the context of chalk wettability. The evidence published by Hassenkam et al [2011] implies that the onset of dissolution is dramatically modified by the organic material associated with chalk particle surfaces. At the stress and temperature conditions that are relevant for chalk in the North Sea, the chalk rheology is expected to be largely controlled by pressure solution.…”

The mutual interplay between calcification and coccolithovirus infection

“…It accounts for up~50% of calcite raining down on marine sediments, with the other 50% derived from foraminifera (Broecker and Clark, 2009). Export of CaCO 3 is facilitated by its high density (2.7 g cm −3 ), inherent protective organic coatings (Hassenkam et al, 2011), association with particulate organic matter [marine snow, transparent exopolymeric particles and faecal pellets (Pedrotti et al, 2012;Collins et al, 2015)] and supersaturation in the upper water column (Westbroek et al, 1993;Balch et al, 2007;Berelson et al, 2007;Broecker and Clark, 2009). While there are no coccolith-specific estimates, the total PIC sinking-flux below 2000 m may be as much as 50 Tmol C year −1 (Berelson et al, 2007), with associated CaCO 3 :POC ratios (or 'rain ratios') having important implications for POC transport to the deep ocean and biological pump efficiency (Armstrong et al, 2002;Klaas and Archer, 2002;Ridgwell et al, 2009).…”