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Abstract Marine fish precipitate carbonates in their intestines that they subsequently excrete as part of an osmoregulatory strategy. While fish carbonates are proposed to be volumetrically significant to the global carbonate budget, no study has presented direct evidence of fish carbonates in the open ocean. Here we examine sediment trap material collected by the Oceanic Flux Program (OFP) in the North Atlantic and observe the episodic occurrence of enigmatic blue particles since 1992. The blue particles are comprised of calcite with unusually high magnesium content (up to 46 mol%) with distinctively depleted δ¹³C and enriched δ¹⁸O compared with calcite produced by common marine calcifiers. Based on the mineralogical, isotopic, and textural similarities between the blue particles and fish carbonates, we propose that the blue particles are produced by pelagic fish. Our data suggest that fish modify their intestinal fluids to create a concentrated, highly supersaturated,¹³C depleted solution capable of precipitating calcite with high magnesium content and low δ¹³C. Collectively, our data imply that fish carbonate production is an open‐ocean phenomenon, opening up the possibility that fish contribute to the production, dissolution, and export of carbonates globally. 

Abstract. Ocean alkalinity enhancement (OAE) is a carbon dioxide (CO2) removal approach that involves the addition of alkaline substances to the marine environment to increase seawater buffering capacity and allow it to absorb more atmospheric CO2. Increasing seawater alkalinity leads to an increase in the saturation state (Ω) with respect to several minerals, which may trigger mineral precipitation, consuming the added alkalinity and thus decreasing the overall efficiency of OAE. To explore mineral formation due to alkalinity addition, we present results from shipboard experiments in which an aqueous solution of NaOH was added to unfiltered seawater collected from the surface ocean in the Sargasso Sea. Alkalinity addition ranged from 500 to 2000 µmol kg−1, and the carbonate chemistry was monitored through time by measuring total alkalinity (TA) and dissolved inorganic carbon (DIC), which were used to calculate Ω. The amount of precipitate and its mineralogy were determined throughout the experiments. Mineral precipitation took place in all experiments over a timescale of hours to days. The dominant precipitate phase is aragonite with trace amounts of calcite and magnesium hydroxide (MgOH2, i.e., brucite). Aragonite crystallite size increases and its micro-strain decreases with time, consistent with Ostwald ripening. The precipitation rate (r) in our experiments and those of other OAE-related calcium carbonate precipitation studies correlate with the aragonite saturation state (ΩA), and the resulting fit of log10(r) = n × log10 (ΩA−1) + log10 (k) yields a reaction order n=2.15 ± 0.50 and a rate constant k=0.20 ± 0.10 µmol h−1. The reaction order is comparable to that derived from previous studies, but the rate constant is 1 order of magnitude lower, which we attribute to the fact that our experiments are unseeded compared with previous studies that used aragonite seeds which act as nuclei for precipitation. Observable precipitation was delayed by an induction period, the length of which is inversely correlated with the initial Ω. Mineral precipitation occurred in a runaway manner, decreasing TA to values below those of seawater prior to alkalinity addition. This study demonstrates that the highest risk of mineral precipitation is immediately following alkalinity addition and before dilution and CO2 uptake by seawater, both of which lower Ω. Aragonite precipitation will decrease OAE efficiency because aragonite is typically supersaturated in surface ocean waters. Thus, once formed, aragonite essentially permanently removes the precipitated alkalinity from the CO2 uptake process. Runaway mineral precipitation also means that mineral precipitation following OAE may not only decrease OAE efficiency at sequestering CO2 but could also render this approach counterproductive. As such, mineral precipitation should be avoided by keeping Ω below the threshold of precipitation and quantifying its consequences for OAE efficiency if it occurs. Lastly, in order to be able to quantitatively determine the impact of mineral precipitation during OAE, a mechanistic understanding of precipitation in the context of OAE must be developed. 

Limestone microporosity is ubiquitous and extensively developed in most Phanerozoic limestones. From an economic perspective, microporosity is important because it contributes substantially to the carbonate pore system, which can host significant volumes of water and hydrocarbons. Therefore, determining the presence and distribution of limestone micropores is necessary for accurate hydrocarbon estimations, reservoir characterization, and fluid flow simulations. From an academic standpoint, microporosity is important because its genesis is intimately linked with the mineralogical stabilization of metastable sediments, a fundamental process in carbonate diagenesis. Many types of micropores contribute to what has been referred to as microporosity, but the vast majority is hosted among low-magnesium calcite (LMC) microcrystals that are present in limestone matrix and allochems. Geochemical, textural, and mineralogical data from natural settings and laboratory experiments indicate that LMC microcrystals are diagenetic in origin. More specifically, these data support a diagenetic model of mineralogical stabilization that involves dissolution of precursor sediments dominated by aragonite and high-magnesium calcite (HMC) minerals, and precipitation of LMC microcrystal cements. The stabilization process is inferred to take place in the meteoric, marine, and burial diagenetic realms. Although it has not been directly observed, carbon and oxygen isotopes, as well as trace element data suggest that LMC microcrystals form during burial diagenesis in marine-like fluids. Evidence suggests that porosity is not generated during this dissolution-precipitation process, but rather inherited from the precursor sediments. The final arrangement of the micropores in a limestone, however, depends on the precise diagenetic pathway. LMC microcrystals exhibit a range of microcrystalline textures that are classified on the basis of crystal morphology and size. The three main textural classes - granular (framework), fitted (mosaic), and clustered - have been recognized across a wide range of ages, depositional settings, burial depths, and precursor types, and are characterized by distinct petrophysical properties, such as porosity, permeability, and pore-throat size. Observations from modern sediments also support the hypothesis that LMC microcrystals develop from aragonite and HMC dominated lime mud. The origin of lime mud has been extensively studied but still highly debated. Of particular interest to the discussion of microporosity are proposed secular variations in the dominant mineralogy of carbonate sediments through the Phanerozoic. Microporous limestones comprised of LMC microcrystals are equally abundant during times of aragonite seas and calcite seas, which suggests that no special mineral precursor is required. Microporous textures are also observed in deep marine chalks where micropores are hosted between chalk constituents. Unlike shallow marine limestones, deep marine sediments start out as mostly LMC therefore mineralogical stabilization is not a significant process in chalk diagenesis.