Size fraction analysis of fish-derived carbonates in shallow sub-tropical marine environments and a potentially unrecognised origin for peloidal carbonates

Highlights

• Break-up of carbonate pellets produced by marine bony fish is investigated.

• Pellets can remain intact and may contribute to sedimentary pellets and peloids.

• Rapid pellet break-up releases component particles in agitated settings.

• Particle size ranges from clay to fine sand grade: different depositional fates.

• Relevant to surface sediments in shallow sub-tropical marine provinces

Abstract

Marine bony fish are now known as primary producers of calcium carbonate. Furthermore, within the shallow sub-tropical platform settings of the Bahamas, this production process has been shown to occur at rates relevant to carbonate sediment production budgets. Fish excrete these carbonates as loosely aggregated pellets which, post-excretion, exhibit a range of distinctive crystal morphologies and have mineralogies ranging from low (0–4 mol% MgCO₃) to high (4–40 mol% MgCO₃) Mg-calcites, aragonite and amorphous carbonate phases. Here we provide the first quantitative assessment of the size fractions of the carbonates produced by a range of tropical fish species, and document the extent of post-excretion carbonate pellet break down under a range of physical agitation conditions. Specifically, we document the morphologies and size fractions of: i) intact pellets at the point of excretion; ii) intact pellets after agitation in seawater; and iii) the particles released from pellets post-disaggregation. Results indicate that fish-derived pellets initially fall within the very fine to very coarse sand fractions. Exposure to conditions of moderate seawater agitation for 30 days results in significant pellet diminution; 66% of initial pellet mass being released as individual particles, whilst 34% is retained as partially intact pellets that are smaller (fine sand-grade) and more rounded than initial pellets. In contrast, pellets exposed to very gently agitated conditions for up to 200 days show little change. Where pellet disaggregation does occur, particles are commonly released as individual clay- and silt-grade crystals. However, some morphotypes (e.g., polycrystalline spheres) can be intergrown and are released as strongly cohesive particle clusters falling within the coarse silt to fine sand fractions. Only very vigorous agitation may disaggregate such particles, resulting in the release of their component clay-grade crystals. We conclude that fish-derived carbonates may thus contribute not only to the mud-fraction of marine carbonates, but also to the fine sand fraction as intergrown particles, and to the fine to coarse sand fractions as intact and partially intact pellets. These experimental data indicate that hydrodynamic regimes local to sites of excretion will influence the generation of carbonates with different size fraction ranges. Rapid pellet disaggregation is more likely in high energy settings, hypothesised to result in redistribution of liberated mud-grade particles to lower energy platform-top settings and/or off-platform. In contrast, pellets excreted in lower energy settings are more likely to be preserved intact, and are thus proposed as a previously unrecognised source of pelletal and peloidal carbonate sediments.

Introduction

Micrite and microspar are common components of carbonate successions in the geological record, and are usually considered to result from the diagenetic alteration of fine-grained skeletal or abiotically precipitated carbonates (Lasemi and Sandberg, 1984). However, in most cases clear evidence for such an origin is enigmatic at best, and thus a range of known contemporary sources of mud-grade carbonate are usually invoked to explain mud production, these including: 1) mechanical and/or biological break down of skeletal carbonates, such as calcareous green algae and calcareous seagrass epiphytes (Lowenstam and Epstein, 1957, Land, 1970, Nelsen and Ginsburg, 1986, Gischler and Zingeler, 2002, Gischler et al., 2013); 2) biogenically-induced precipitation, such as that associated with microbial activity and blooms of unicellular algae; a process that has been linked to ‘whiting’ events (Greenfield, 1963, Robbins and Blackwelder, 1992, Yates and Robbins, 1995, Yates and Robbins, 2001); and 3) abiotic precipitation, which has also been linked with ‘whiting’ events (Shinn et al., 1989, Milliman et al., 1993, Morse et al., 2003). In addition, recent studies have identified teleost (bony) fish as important contributors to global marine carbonate production (Wilson et al., 2009) and as a potentially important source of mud-grade carbonate, especially in sites of high fish biomass (Perry et al., 2011).

The physiological processes underpinning carbonate production in fish have been described in detail elsewhere (see Walsh et al., 1991, Wilson et al., 2002). In brief, marine teleosts regulate their hydration levels by absorbing ingested seawater through epithelial cells in their intestines. Prior to absorption, dissolved Ca^2 + (and some Mg^2 +) is removed from this seawater via precipitation of Mg calcite (along with smaller quantities of other carbonate phases); this process occurring within intestinal fluids under conditions of highly elevated CaCO₃ saturation caused by secretion of metabolically-derived HCO₃⁻ into the intestine. The resulting carbonates are excreted into the open water column as generally sand-grade pellets of loosely aggregated mud-grade particles; the latter occurring with a diverse range of distinctive morphological forms (morphotypes) that vary with species (Perry et al., 2011, Salter et al., 2012).

Initial grain size measurements of these morphotypes, although not exhaustive, indicate length ranges of < 0.5 to > 50 μm, with different morphotypes occurring within reasonably narrow ranges (Perry et al., 2011, Salter et al., 2012). Given that the overall size range spans two orders of magnitude, and that differences in settling and entrainment potential must apply, it is predicted that different particle morphotypes should have different transport and depositional fates. Since the genetic origins of carbonate grains in the rock record are frequently used as indicators of palaeo-environment, palaeo-climate, and palaeo-ecology (e.g., Flügel, 2004), it is important that the potential transport and subsequent deposition of these grains is more fully understood. This paper presents an initial consideration of these issues through detailed analysis of the grain sizes of carbonates produced by a range of common Caribbean fish species; these data being used to inform a conceptual model concerning the post-excretion re-distribution of these crystals on and around shallow sub-tropical carbonate platforms, using the Bahamas as an example.

Furthermore, previous studies regarding the sedimentary significance of fish-derived carbonates have done so only with respect to the mud fraction (Perry et al., 2011, Salter et al., 2012); such studies assuming that excreted pellets, which disaggregate readily at pin prick when dry, eventually break down and release their component mud-grade particles individually. However, two alternative possibilities exist that have received no attention thus far, these being that: i) pellets remain intact after excretion; and ii) some particles released upon pellet disaggregation are strongly bound to other particles such that morphotypes are not actually released as individual particles, but rather as larger particle clusters. With regard to the former, existing descriptions of fish-derived carbonate pellets are remarkably similar to those given for numerous sedimentary pellets of presumed faecal origin from the surface sediments of shallow platform settings in the Bahamas (Illing, 1954, Ginsburg et al., 1958, Newell et al., 1959, Purdy, 1963, Reijmer et al., 2009). Such pellets have been found to occur with varying degrees of cohesive strength (Illing, 1954); a phenomenon that has been attributed to different degrees of induration. Thus, carbonate pellets initially similar to fish-derived pellets not only have the potential to be preserved intact in surface sediments, but can also undergo cementation; this process increasing their preservation potential and possibly resulting in the conversion of pellets to peloids (Rankey and Reeder, 2010). This paper therefore also investigates the possibility that a proportion of fish-derived carbonates may represent a hitherto unrecognised source of sand-sized carbonate pellets and peloids.