The production and preservation of fish-derived carbonates in shallow sub-tropical marine carbonate provinces

• Main Author: Salter, Michael A.
• Published: Manchester Metropolitan University 2013
• Subjects: 639.2
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.594483

Recent studies have demonstrated that marine bony fish (teleosts) precipitate inorganic calcium carbonate in their gut as a by-product of osmoregulation, subsequently excreting it into the open water column as loosely aggregated millimetre-scale pellet. These studies have primarily focused on physiological aspects of this process and the fate of resulting carbonates in pelagic settings, which is likely to be dissolution at depth. However, the implications of such carbonate production in shallow tropical and sub-tropical carbonate provinces has also begun to be considered, and it is thought that fish may contribute significant quantities of morphologically distinctive mud-grade high-Mg calcite in certain habitat settings. However, most studies of carbonate mud in modern subtropical settings do not report particles that are obviously fish-derived, and questions concerning fish-derived carbonate characteristics and post-excretion stability thus arise. The present work therefore provides more detailed characterisation of the carbonate products of an expanded range of Caribbean fish species and determines their short-term preservation potential in a sub-tropical carbonate province (the Bahamas). Following collection of carbonates from 22 fish species (all starved), precipitates were characterised using scanning electron microscopy and a suite of complimentary chemical analysis techniques. The form in which these carbonates are likely to be incorporated into surface sediments was investigated by disaggregating pellets and characterising the liberated particles by performing detailed image-based grain size analyses, with additional experiments demonstrating the likely rate and extent of pellet disaggregation by placing them in agitated seawater. Finally, short-term preservation potential was determined in a series of experiments whereby excreted precipitates were exposed to surface seawater and shallow subsurface porewater conditions for several months. Results indicate that, at the point of excretion, fish-derived carbonates are morphologically and mineralogically more varied than previously thought, although most morphotypes (e.g., ellipsoids, dumbbells, spheres, rhombohedra) are seemingly unique in shallow sub-tropical marine settings. ii High-Mg calcite, typically containing 20–35 mol% MgCO3, is the dominant product of about half the species investigated, but Mg calcite with lower MgCO3 contents (in the range 2–20 mol%) is also common, as is aragonite, which can represent up to 27 wt% of carbonates excreted by some species. In addition, amorphous magnesium carbonate (AMC) and magnesium-rich amorphous calcium carbonate (ACC), both of which are strongly hydrated, represent the dominant precipitation products of some species, with a hydrated crystalline phase (monohydrocalcite) occasionally accompanying the latter. A non-carbonate phase, brucite, is nearly ubiquitous as a volumetrically minor phase. Where analysed, all of these phases are further found to differ from other carbonate sediments in the Bahamas with regard to their stable carbon and oxygen isotope compositions; a consequence of the important role of metabolic HCO3 - in the precipitation process. Detailed grain size analyses indicate that most morphotypes are released from pellets upon disaggregation as individual mud-grade particles that retain their distinctive forms. However, extreme agitation can result in polycrystalline forms releasing their fibre-like components; these particles being less distinctive than their parent forms. In contrast, some particles are intergrown and do not disaggregate beyond particle clusters that are up to fine sand sized. Moreover, excreted pellets do not necessarily always disaggregate, with moderate agitation in seawater resulting in a significant proportion of carbonate being retained as intact pellets, albeit smaller (typically fine sand sized) and more well-rounded than initial pellets (typically fine to coarse sand sized). In quiescent settings pellet diminution is less extensive, and it is thus hypothesised that a significant proportion of fish-derived carbonates excreted in the Bahamas is preserved as sand-grade pellets. Despite the highly distinctive nature of fish-derived carbonates, particles that can be attributed to production by fish nevertheless remain elusive in studies of Bahamian surface sediments. It is thus necessary to invoke post-excretion processes of dissolution and/or recrystallisation to explain the apparent disparity between production rates and occurrence as sedimentary particles. Indeed, it is demonstrated here that AMC, brucite, and large quantities of ACC undergo complete dissolution in seawater within a few days of excretion, with the remaining ACC apparently crystallising to form fine sand-grade (50–200 μm diameter) polycrystalline calcite spheres over similar timescales. iii Monohydrocalcite, also undergoes complete dissolution or alteration (to calcite) during 3 month exposures to artificial seawater, and is predicted to alter in a similar manner in natural settings. Conversely, anhydrous crystalline phases remain largely unchanged after porewater and seawater exposures lasting several months, although two important post-excretion processes are observed. Firstly, high-Mg calcite ellipsoids appear to undergo partial dissolution (with preferential loss of MgCO3) and possible recrystallisation, and, based on these observations, it is predicted that longer exposure times will result in changes being pervasive, possibly obscuring the piscine origin of initial crystals. Secondly, carbonate pellets containing minor amounts of aragonite at the point of excretion appear, in some cases, to stimulate post-excretion growth of abundant aragonite needles that are morphologically similar to aragonite needles that dominate Bahamian carbonate muds. It is further apparent that these processes are inhibited in uncleaned samples, possibly due to surface adsorption of organic compounds, but the evidence after 3 month exposures indicates that inhibiting factors might eventually be overcome. Based on these results and modelling of carbonate excretion across shallow platform areas of the entire Bahamian archipelago, it is predicted that about 18 % of excreted carbonates will dissolve after excretion, while a further 53 % may alter beyond recognition within a very short period (perhaps on the order of years). Moreover, characterisation of carbonates produced by three species of normally feeding fish indicates they produce only amorphous carbonates, despite producing crystalline phases when starved. This difference, attributed to the inhibition of CaCO3 crystallisation by dietary phosphate, indicates that loss of fish-derived carbonate to dissolution might be considerably higher than 18 % under normal natural circumstances. The sedimentary significance of fish-derived carbonates thus remains enigmatic, but results presented herein indicate that they follow very different preservation pathways depending on their excreted form, and that they may make previously unrecognised contributions to: i) the carbonate sand fraction (as peloids); and ii) aragonite needle muds (as crystals grown post-excretion).